Supramolecular assemblies for extracting organic compounds

Supramolecular assemblies forextracting organic compoundsSoledad Rubio, Dolores Perez-Bendito

Supramolecular assemblies have been used largely in analytical extractionand concentration schemes. Their ability to form a phase with regions ofdifferent polarities, acidities and viscosities, where solutes of very differentstructure can be solubilized, has encouraged this use. At present, thebasis, characteristics and scope of extraction techniques, such as cloudpoint, are well established. However, they have not been accepted inroutine analysis, despite the demonstrated advantages of surfactant-medi-ated separations over other conventional extraction techniques. Thisarticle deals with the challenges in making these extraction techniquescompetitive and explores the potential for using other phenomena andmaterials where supramolecular assemblies are involved, such ascoacervation, hemimicelles/admicelles and amphiphilic-templated meso-porous silicas, for the extraction/concentration of organic compounds.# 2003 Published by Elsevier B.V.

1. Introduction

Supramolecular chemistry is a highlyinterdisciplinary ¢eld of science coveringthe chemical, physical, and biologicalfeatures of the chemical species ofgreater complexity than moleculesthemselves; they are held together andorganized by means of intermolecular,non-covalent, binding interactions [1].Recent developments in Supramolecularchemistry are spectacular, as measuredby the increasing number of researchgroups that joined the ¢eld and the workof which is reported in a vast range ofpublications, books, journals and meet-ings [2^4].Supramolecular chemistry comprises

two broad areas covering: supermoleculesthat result from the intermolecular asso-ciation of a receptor (i.e. macrocyclicpolyethers, cyclodextrins, etc) and itssubstrate (s); and, supramolecular assem-blies, polymolecular entities that resultfrom the spontaneous association of alarge unde¢ned number of componentsinto a speci¢c phase (i.e. micelles, micro-emulsions, vesicles, etc).

For years, analytical chemistry hastaken advantage of supermolecules andsupramolecular assemblies in the devel-opment of new analytical methods. Thereare few areas in this discipline in whichthe analytical capabilities of supramole-cular systems have not been investigated.Quantitative description of supramole-cular chemical systems interactions andtheir e¡ects have largely been studied byphysical chemists [5^8] and this know-ledge has been essential in obtaining themaximum advantage from the appli-cation of supramolecular systems in che-mical analysis. Fig. 1 shows the maincontributions of these entities to analy-tical processes on the basis of the di¡erente¡ects observed from supramolecular sys-tems and the interactions of their chemi-cal components.Molecules making up supramolecular

systems contain a hydrophilic and ahydrophobic part and/or proton donor oracceptor sites. As a result, they provideregions of di¡erent polarities, acidities,and viscosities, which determine the sub-strate solubilization site and are respon-sible for the microenvironmental e¡ectsthat impinge on chemical systems. Solu-bilization of substrates in supramolecularsystems has largely been exploited inextraction processes and separation tech-niques (Fig. 1). These systems have beentreated as a pseudophase, in whichsolutes partition from the bulk solventphase. Extraction and concentrationschemes based on substrate solubilizationin the supramolecular pseudophase, suchas cloud-point extraction (CPE), micel-lar-enhanced ultra¢ltration (MEUF),surfactant-assisted transport of solutesacross liquid membranes and reversemicellar extractions, have been exten-sively developed in recent years and their

Soledad Rubio,Dolores Perez-Bendito*

Department of Analytical

Chemistry,

Facultad de Ciencias,Edificio Anexo Marie Curie,

Campus de Rabanales,

E-14071-Cordoba, Spain

*Corresponding author.

Fax: +34-957 218644;

E-mail: [email protected]

Trends Trends in Analytical Chemistry, Vol. 22, Nos. 7+8, 2003

470 0165-9936/03/$ - see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0165-9936(03)00706-4

basic features and practical applications established[9^14]. Similarly, supramolecular assemblies, suchas micelles, microemulsions and vesicles, have beenused as mobile phases in a variety of liquid chromato-graphic techniques, mainly reverse-phase liquidchromatography (LC) [9,10,14^17], and as liquidpseudostationary phases in electrokineticcapillary chromatography [18^21], providing newpowerful separation techniques for many neutral andcharged compounds. Enhanced selectivities, lowercosts, and increased safety are some of the advantagesassociated with the use of these aqueous eluents. In thiscontext, it is worth noting the use of cyclodextrins andtheir derivatives or modi¢ed crown ethers for theenantiomeric separation of a wide number of analytes[22^24].In addition to extraction processes and separation

techniques, many other areas of analytical chemistryhave successfully used supramolecular systems (Fig. 1).Thus, the increased reagent concentrations within thesupramolecular pseudophase as compared with bulksolvent causes acceleration of chemical reactions ¢nd-ing important applications in kinetic- and equilibrium-based determinations [25,26].However, it has been largely established that the dif-

ferent microenvironments present in supramolecularsystems alter quantum e⁄ciencies, change the physicalproperties of sample solutions aspirated in atomic spec-trometry, modify chemical pathways and rates, stabilize

reactants, intermediates, transition states, and pro-ducts, change dissociation constants, etc [5].On the basis of these microenvironment e¡ects, many

surfactant-modi¢ed procedures and newmethodologicalapproaches have been developed in spectral analysis[14,27,28], electroanalytical chemistry [27,29,30],and kinetic analysis [25,26]. In the last few years, thee¡ects of solubilized solutes on the characteristic para-meters of supramolecular assemblies (i.e. criticalmicelle concentration) have also been exploited foranalytical measurements. A new analytical approach,namely the mixed aggregate method (Fig. 1), has beenproposed; it is based on measuring the CMC (criticalmicelle concentration) value of mixtures of amphiphilicsubstances, one of which is the analyte [31].This article deals with the challenges which face the

strategies developed so far for the analytical extractionof organic compounds using micelle-mediated phaseseparations and explores the potential of hemimicelles/admicelles and amphiphilic-templated mesoporous sili-cas for surfactant-mediated solid phase extractions.The pressure to decrease organic solvent usage in

laboratories has encouraged the requirement fororganic solvent-free procedures for which supramole-cular assemblies are excellent candidates. However,these entities have not so fare been accepted as extrac-tion systems in routine analytical procedures, despitethe demonstrated advantages of surfactant-mediatedseparations, when compared with conventional extrac-

Figure 1. Effects and analytical applications derived from supramolecular systems and chemical component interactions.

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tions [9^14]. Underlined below are the main reasonsfor this non-acceptance and the problems thatresearchers need to overcome with surfactant-mediated extraction schemes.

2. Micelle-mediated phase separations

Among the surfactant-mediated schemes developed forthe extraction/concentration of organic compounds,the micelle-mediated phase separation (mainly CPE)technique has received most attention from research-ers. Initially reported by Watanabe et al. [32^34], CPEhas been extensively investigated.Published reviews o¡er the phase-separation beha-

vior of neutral (non-ionic and zwitterionic) surfactantsas a function of the temperature, the type of appli-cations developed and the advantages/limitations ofseparations based on the cloud-point phenomenon[9^14,35,36]. As a result, parameters a¡ecting extrac-tion e⁄ciencies and pre-concentration factors achievedby the CPE technique are well-known, and thatfacilitates the implementation of the CPE technique innew analytical processes.Aqueous solutions of ionic surfactants, proteins,

synthetic polymers and some microemulsions canalso undergo separation into two liquid layers. Thisseparation may be initiated in a number of di¡erentways. Examples are changing parameters, such aspH, or adding a second substance, such as aconcentrated aqueous ionic salt solution or an organicsolvent.This separation into two liquid layers can also be

induced in systems having two dispersed hydrophiliccolloids of opposite electric charges. This phenomenon,termed coacervation, has scarcely been exploited foranalytical purposes [37^39]. Thus, alkyltrimethyl-ammonium surfactants have been known to undergocoacervation in the presence of sodium chloride, undersaturated conditions, and 1-octanol [37]. On the otherhand, anionic surfactants, such as per£uoro-octanoate[38] and alkyl sulfates, sulfonates and sulfoccinates[39], have undergone pH-induced coacervation.Although only a few applications have been reported sofar, coacervation can greatly extend the scope ofmicelle-mediated phase separations because the num-ber of ionic amphiphilic systems undergoing thisphenomenon.At this point, it is worth noting that, although

coacervation and cloud point involve basically thesame phase-separation phenomenon, most authorshave di¡erentiated between these two processes[10,13,14]. Thus, it is commonly accepted that cloudpoint refers to the phase separation of neutral sur-factants induced by the temperature, whereas‘‘coacervation’’ is reserved for the phase separation

of ionic amphiphiles induced by other parameters, asspeci¢ed above.The primary focus of this section is to discuss the dif-

ferent steps involved in cloud-point and coacervationseparations, to compare both techniques in terms ofanalytical and operational features and to state some ofthe challenges which both cloud-point and coacerva-tion separations face in order to become usefulapproaches in routine analysis.

2.1. Basis of cloud-point/coacervation extractionsCloud-point/coacervation extractions consist of threesteps, namely:

� solubilization of solutes in aqueous micelles;� separation of a surfactant-rich phase from theaqueous bulk induced by proper conditions; and,

� recoveryof the surfactant-richphase for analysis.

Substrate solubilization in micelles has traditionallybeen treated in terms of a two-phase process [40^42].Solubilizate distribution between two extreme sites isconsidered in this model. The two sites correspond to ahydrophobic ‘‘dissolved state’’ in the micellar interiorand a more polar ‘‘adsorbed state’’ at the micellar waterinterface. Solubilization is a dynamic process; substrateresidence times in aqueous micelles are of the order of103^105/s.There has been no uniformity in de¢nitions of par-

tition coe⁄cients or equilibrium constants that maybe used to represent the solubilization of solutioncomponents by surfactant micelles [7]. For analyticalpurposes, one of the commonest procedures has beento calculate a distribution coe⁄cient (D) representingthe degree of analyte partitioning from the aqueous tothe surfactant-rich phase. This coe⁄cient is given by:

D ¼ A½ �s= A½ �w

where [A]s and [A]w are the ¢nal analyte concentrationin the surfactant-rich phase and in the aqueousphase, respectively. Alternatively, it is possible torelate solubilization to equilibrium constants for thereaction:

surfactant micelleð Þ þ organic solute aqueousð Þ ¼

surfactant-organic solute micelleð Þ

where the transfer of the solute into the micelle is trea-ted as a binding phenomenon, for which [7,10] theequilibrium constant is:

Ks ¼ organic solute in micelle½ �=

surfactant in micelle½ � organic solute in bulk½ �


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The brackets indicate molar concentrationsmeasured with respect to the entire volume of solution.Quantitative descriptions of micellar solubilization

equilibria for analytical reagents and comparison ofequilibrium constants and distribution constants incloud-point methodology have been reported [43].The following step in the cloud-point/coacervation

process is the separation of the surfactant-rich phasecontaining the micelles from the aqueous bulk, whichhas been attributed to a decrease of the solubility of theamphiphile in water or to a sharp increase in themicelle aggregation number [13,14]. Typical phasediagrams for non-ionic, zwitterionic and anionic sur-factants are shown in Fig. 2. The temperature at whichthe phase separation occurs for non-ionic and zwitter-ionic amphiphiles is a function of the surfactant con-centration. However, the critical hydrochloric acidconcentration required for the phase separation of alkylsulfonate micelles does not depend on the temperatureover the range 10^80�C or on the surfactant concen-tration, at least up to 10% w/v. All phase diagrams canbe modi¢ed by the presence of organic additives, so, in

the optimization process of cloud-point/coacervation-based separations, the matrix e¡ect on the phasediagram should be always taken into account.The separation of the enriched surfactant phase from

the bulk solution presents some practical problems.Thus, the extraction of an aliquot for chromatographicor spectral analysis is generally carried out with aHamilton microsyringe. Because of the viscosity ofthe surfactant-rich phase, this process is troublesome.However, the actual volume of the surfactant-richphase is usually calculated from measurement of theheight that this phase ¢lls in the cylindrical tube inwhich the extraction process is performed. This methodcauses low precision in analytical measurements,especially when low volumes are obtained. To achievemore accurate measurements, solutions for calibrationare generally processed according the same procedureas for samples [37]. An easier procedure for both totalseparation of the surfactant-rich phase from theaqueous phase and accurate analytical measurementsconsists of cooling the extracted phases in an ice bath. Inthis way, the surfactant-rich phase becomes gelatinous,

Figure 2. Typical phase diagrams for (a) non-ionic (Triton X-114), (b) zwitterionic (3-(nonyldimethylammonio) propyl sulfate, C9-APSO4), and (c) anionic

(sodium dodecane sulfonate, SDoS) surfactants in aqueous solutions. L denotes homogeneous liquid region; L-L, two liquid phases region; E, emulsionregion; and, S, liquid-solid phases region.



dense enough to be separated from the aqueous phaseusing a simple spatula [45]. At room temperature, thegelatinous phase changes into a liquid that can then bediluted to give an accurate volume in a 1^2 ml standard£ask.

2.2. Coacervation versus CPEsThe use of coacervates in analytical separations is prac-tically unexplored. However, rare applications descri-bed recently have demonstrated their usefulness asextraction systems [37^39,44^46]. Essentially, separa-tions analogous to those described for the cloud-pointphenomenon should be possible. In this section, we willdiscuss both techniques in terms of extraction e⁄-ciencies, pre-concentration factors, some experimentalparameters and applicability in order to make theirimplementation easy. Method development in cloud-point/coacervation extractions is mainly related to theproperties of the analyte of interest, the required trace-level concentration and the nature of the matrix. Simi-larly, it is important to consider the compatibility of thesurfactant-rich phase with the chromatographic tech-nique and detectionmode used.

2.2.1. Extraction e⁄ciencies. Extraction e⁄ciencies fun-damentally depend on the equilibrium constant for theincorporation of the solute to the micellar phase (Ks).For a given surfactant system, there is a threshold valueof the binding constant that ensures the quantitativerecovery of the analyte in the surfactant-rich phase.Thus, for chlorophenols it has been demonstrated thatcomplete recovery can be achieved by cloud point whenthe solute-micelle binding constant is higher than ca.1000/M [47]. The equilibrium constant depends on dif-ferent parameters [48] related to the solute, such ashydrophobicity, the capacity to form hydrogen bridges,molar refraction, and dipolarity. Similarly, binding

constants are also in£uenced by other factors, such asthe structure of the surfactant, the presence of salts ororganic additives, and temperature.In general, the extraction e⁄ciency in coacervation,

similarly to that in CPE, increases with the hydro-phobicity of the analyte. Thus, the percentage of ana-lyte extracted in a surfactant-rich phase of dodecylsulfonate increases with the number of chloro sub-stituents for phenol derivatives, alkyl substituents forphthalic esters and the number of ring-condensationsfor polyaromatic hydrocarbons (PAHs), respectively[44].With respect to the structure of surfactants, as a gen-

eral rule, non-ionic micelles are more e¡ective in bind-ing organic solutes than ionic micelles for surfactantswith the same hydrocarbon chain length [13]. So, froma theoretical point of view, the e⁄ciency achieved forthe extraction of organic compounds should be higherby using CPE than coacervation-based separations.However, the results obtained to date do not alwayscon¢rm this general rule (see e.g. Table 1), suggestingthat many other factors a¡ect the e⁄ciency of micelle-mediated phase separations. For example, it is knownthan the solubilization capacity of non-ionic micellesrapidly increases as the temperature is raised to nearthe cloud point [13]. Similarly, the distribution coe⁄-cient for analytes in alkyl sulfonate micelles increasesas a function of hydrochloric acid concentration [44].These phenomena have been explained on the basis of awater-content reduction in the surfactant-rich phaseresulting in increased hydrophobicity, which involvesmore a⁄nity for organic compounds. These and manyother factors can modify the ability of micelles to solubi-lize and a¡ect their capacity for extraction. Anyway, itis important to stress that, independently of the type ofsurfactant used, extraction e⁄ciencies increase withhydrophobicity and amount of surfactant.

Table 1. Recoveries obtained for different organic compounds from spiked water samples using the phase separation behaviour of different charge-type

surfactant micelles

Compound
Recovery, %
Non-ionic [49, 50]
Anionic [39, 46] (c) Cationic [37] (d)
4-Chorophenol
87.10.6(a) 72.72.4 97.34.1 2,4-Dichlorophenol 95.70.2(a) 79.01.0 2,4,5-Trichlorophenol 97.10.8(a) 90.22.7 99.15.8 Pentachlorophenol >99.9(a) 99.03.0 114.112.6 Benzo[b]fluoranthene 62–105(*)(b) 106.49.6 Benzo[a]pyrene 100–105(*)(b) 98.07.7 Fluoranthene 75–95(*)(b) 10311.2 Benzo[k]fluoranthene 100–110(*)(b) 1004.7 Benzo[gih]perylene 90–105(*)(b) 96.96.6
(*) Recoveries dependent on the concentration level spiked. Cloud-point extraction using:(a) 1% poly(oxyethyleneglycol) monooctyl ether;(b) 0.1% Triton X-114;(c) 1% sodium dodecane sulphuric acid; and,(d) 0.5% cetrimide (a surfactant mixture composed mainly of tetradecyltrimethyl ammonium bromide.



2.2.2. Pre-concentration factors. Very few systematicstudies have been reported so far on determining theparameters in£uencing the concentration factorachieved by coacervation-based separations. In a studyinvolving extraction of phenol by alkylsulfonates,based on their acid-induced phase separation, it wasfound that, in contrast with the behavior of non-ionicsurfactants, the concentration factor increased withthe alkyl chain length of the alkylsulfonate [44]. Thisbehavior was explained on the basis of the lower hydro-chloric acid concentration required for two-phaseseparation as more hydrophobic anionic surfactantsare used with extraction, and that involves less dilutionof the samples in the acidi¢cation process.It was also found that, as for neutral surfactants, the

concentration factor decreased in proportion to thetotal amount of surfactant added. The concentration ofthe added hydrochloric acid also in£uenced the volumeof the surfactant-rich phase; this decreased as the acid-ity of the medium increased. However, the volume ofthe surfactant-rich phase obtained for the coacervationof cetrimide, induced by sodium chloride/1-octanol,was found to be highly dependent on the amount ofcosurfactant (1-octanol) added [37]. Thus, the surfac-tant-rich volumes went from 2.20 ml to 0.29 ml as the1-octanol added went from 5 mL to 8 mL. Extraordinaryconcentration factors (e.g. 104) have been reported forthe extraction of porphyrins with £uoro, carboxylato,trimethylamino and methylpyridyl groups based on thepH-induced coacervation of per£uoro-octanoate [38].However, in some systems high apparent pre-concen-tration factors have been observed when the signal ofanalytes is sensitized by the presence of surfactant.Because of the disparity of data, more studies are neces-sary in order to obtain comprehensive knowledge of theparameters a¡ecting concentration factors in coacerva-tion-based separations.Cloud-point/coacervation processes involve both

extraction and concentration of analytes, so onealways has to consider extraction e⁄ciency versusconcentration factor in method development. Gen-erally, a compromise has to be achieved with regard tohydrophobicity and the amount of surfactant, in orderto obtain adequate trace-level concentration andextraction e⁄ciency. The equilibrium constant for theincorporation of the solute to the micellar phaseprovides information and guidelines for the selection ofsurfactants.

2.2.3. Nature of the matrix. The nature of the matrix isa fundamental aspect to be considered in method devel-opment. Organic and inorganic components of thematrix can modify the cloud-point and coacervationphase diagrams and alter the surfactant-rich phasevolume. These e¡ects can be specially important in theextraction of solid samples with a high proportion of

organic matter (e.g. most environmental samples). Forthis reason, quantitation methods using external cali-bration curves should be used only after a careful checkon the e¡ect of the matrix on recoveries and pre-con-centration factors.

2.2.4. Experimental parameters. With respect to experi-mental parameters, it is worth noting those related tothe phases separation. As in CPE, centrifugation isrequired in coacervation-based extractions in order tospeedup the separationof the twophases. Low recoveriesare frequently obtained in CPE because the temperaturegoes down during centrifugation [13]. Sometimes, thesystem becomes monophasic. Coacervation processesdo not generally depend on temperature, so recoveriesare not a¡ected in the centrifugation step.However, the e⁄ciency of non-ionic surfactant-

mediated extractions has been reported to depend onthe time taken for analytes to interact with micellesand to get into their core [49]. Equilibration times of20^30 min, at temperatures above the cloud point, arefrequently used before centrifugation. In the coacerva-tion-based separations reported to date, the resultsobtained have been mixed. Thus, in acid-induced anio-nic phase separations, the extraction e⁄ciency of thesesurfactants was found to be independent of the equili-bration time [44], so about 2^5 min was required forphases to mix and that results in very rapid separation.However, in chloride/1,octanol-induced cetrimidephase separations, equilibration times of 20^30 minwere proposed [37].Lastly, one aspect to comment on is that, in coacerva-

tion processes, the surfactant-rich phase separates outabove rather than below, as it generally does in cloud-point separations. This behavior facilitates removal of thesurfactant-richphase fromtheaqueousphase foranalysis.

2.2.5. Scope of analytical applications. The application ofcoacervation and cloud-point phenomena in organicanalysis has fundamentally addressed the extraction/concentration of hydrophobic analytes because of theirhigh solubility in the surfactant-rich phase. Althoughsome polar organic compounds strongly adsorb at themicelle-water interface and this permits their extrac-tion, usually only hydrophobic analytes bind tomicelles with su⁄cient strength to achieve quantitativerecovery. For analytes that possess an acidic or basicmoiety, both cloud-point [49] and coacervation[37,44] separations provide higher extraction e⁄-ciencies for the un-ionised form. The parameter indu-cing phase separation can also limit the types ofcompound that can be extracted. Thus, surfactantswith cloud point at high temperatures cannot beapplied to thermally labile compounds, while acid-induced coacervation separations are not suitable forextracting organics that undergo acid hydrolysis.



2.2.6. Practical analytical aspects.When the surfactant-rich phase is subjected to chromatographic analysis,selection of the structure of the surfactant is essential,since its elution may interfere in the detection of theanalytes of interest. The most commonly used non-ionic surfactants, the polyoxyethylene alkylphenols,possess aromatic rings and high retention times thatpreclude chromatographic analysis of the more polarcompounds (Fig. 3a) [50].The use of non-aromatic ionic [37,39] or zwitterionic

surfactants [51], which feature low retention times anddo not absorb above 210 nm, avoids this problem(Fig. 3b).Additionally, non-ionic surfactants are commercially

available as a mixture of homologues and oligomers. Asa result, their use is not recommended when massdetection of the surfactant-rich phase is involved.

2.3. Some development prioritiesFor these approaches to become suitable for routineanalysis, several challenges need to be faced. Althoughthe compatibility of the surfactant-rich phase obtainedin the extraction process and liquid chromatographyhas been largely exploited in organic analysis [35], thisphase must be treated before introduction into a gaschromatograph in order to remove the surfactant [52].Compatibility between CPE and capillary electro-

phoresis has also been demonstrated [53^55],

although rarely used. O¡-line coupling of the surfac-tant-rich phase and LC, gas chromatography (GC) orcapillary electrophoresis (CE) has always been used, sothe on-line coupling of cloud-point/coacervationextraction with these separation techniques is con-sidered a priority. To date, the only attempt reportedto automate the CPE technique was based on a £owinjection analysis (FIA) system where the surfactant-rich phase was retained by a collection column ¢lledwith cotton [56]. Chemiluminescence detection of theanalyte was used in order to avoid the high scatteringof surfactant aggregates. No coupling of this systemto chromatographic or electrophoretic techniques wasreported.However, comparative studies of cloud-point/

coacervation-based separations with those used ino⁄cial analytical methods are necessary in order todetermine when the use of these extraction tech-niques is really an advantageous alternative in termsof extraction e⁄ciencies and operational features. Inour opinion, these studies and automation are a pre-requisite step for these techniques to become acceptedin routine analysis.As commented upon above, application of cloud-

point/coacervation-based separations in organic ana-lysis has focused mainly on hydrophobic compounds.The extension of these extraction techniques to med-ium and highly polar compounds as well as to speci¢cgroups of compounds is highly desirable. For this, thesynthesis of functionalized surfactants that permitspeci¢c interactions between analytes and the surfac-tant aggregates should be undertaken.An area that needs more extensive investigation is

the application of these techniques to solid samples.Traditional extraction methods, including Soxhlet andsonication, are both time-consuming and solvent-con-suming. They often yield dirty extracts that requireextensive clean-up steps in order to achieve accuratedeterminations. The results obtained to date with bothcloud-point [35] and coacervation [45]-based separ-ations show that surfactants without speci¢c inter-actions with analytes behave like organic solvents;lower recoveries are obtained for the more hydrophobiccompounds because of their higher adsorption to thematrix. The e⁄ciency of the extraction usually dependson parameters such as temperature, surfactant concen-tration and stirring. The fact that the surfactant-richphase does not generally require additional clean-up orpre-concentration steps, that the extraction time doesnot take longer than about 1 h and that the extractionis carried out in an aqueous medium render these tech-niques a serious alternative to Soxhlet or sonicationextractions. However, more rigorous studies are neededto determine their real potential for this application.An interesting area to be explored is the use of cloud-

point/coacervation-based separations for applications

Figure 3. Chromatograms for several PAHs [Pyrene (Py), benz[a]-

anthracene (BaA), benzo[b] fluoranthene (BbF), benzo[k] fluoranthene

(BkF), benzo[e] pyrene (Beep)] contained in (a) a Triton X-100-rich phase,and (b) a sodium dodecane sulfonate (SDoS)-rich phase.



of environmental analysis in the ¢eld. Two properties ofthese techniques make them specially attractive for thispurpose:

� their simplicity of operation, which is carried outwithout special extraction equipment;

� and the low volume of the surfactant-rich phaseobtained in the extraction process, which facil-itates its transport to the laboratory in 1^2 mlvials.

Further, the surfactant-rich phase is highly pre-servative for analytes, so it can be kept in the ¢eld,under room temperature conditions, for extensive mon-itoring campaigns. We have checked that PAHs arestable for at least 15 days in acid-induced alkyl sulfo-nate-rich phase surfactants (unpublished data).One ¢nal aspect to consider in this section relates to

coacervation-based separations. There is wide scope toexplore these, considering that di¡erent amphiphilicmolecules, such as surfactants, proteins, and syntheticpolymers, undergo coacervation processes. As men-tioned above, to date very few systems have found ana-lytical applications. Many other systems undergoingsimple or complex coacervation have been describedwith the purpose of being used in encapsulation pro-cesses [57], physico-chemical studies [58], etc. There-fore, the prospects for research in this area are vast andexciting.

3. Surfactant-mediated solid-phase extractions

Solid-phase extraction (SPE) is the most popularsample-preparationmethod [59]. It has undergone con-siderable development in the last 10 years, with manyimprovements in format and automation, and theintroduction of new phases. Its acceptance in routineanalysis, as an alternative to liquid^liquid extraction(LLE), is increasing in di¡erent areas, mainly environ-mental monitoring, where several o⁄cial methods foranalysis of organic compounds in drinking waterand wastewater use SPE as the sample-preparationmethod. In this section, we will discuss the potential ofhemimicelles/admicelles and amphiphilic-templatedmesoporous silicas to be used as sorbents materials inSPE.

3.1. Hemimicelles/admicellesIonic surfactants adsorb on the surface of metal oxides,such as alumina, silica, titanium dioxide and ferric oxy-hydroxides, forming aggregates, termed hemimicellesand admicelles, that are much like micelles and that canbe used to solubilize organic molecules. The processin which the analytes are partitioned between the bulksolution and the hemimicelles/admicelles aggregates is

generically named adsolubilization, a phenomenon akinto micellar solubilization in bulk aqueous solutions.In recent decades, a large number of studies have

been published dealing with the adsorption of ionic sur-factants on opposite-charged surfaces. There is generalagreement that the surfactants assemble at the solidsurface if a critical concentration is exceeded, but thereis a controversy about the type of aggregate formed[60].

3.1.1. Surfactant-adsorption isotherms. Important stud-ies have been performed to clarify ionic surfactant-adsorption isotherms on metal oxides [61^66]. Gen-erally, they can be divided into four linear regions whenpresented on a double logarithmic scale (see Fig. 4).Two types of isotherms can be distinguished, accordingto the type of surface on which the surfactant adsorp-tion is carried out, namely:

� constant charge surfaces, on which adsorptionof surfactants is not accompanied with deproto-nation or protonation (Fig. 4a); and,

� constant potential surfaces, on which the sur-factant adsorption is accompanied by release oruptake of protons in the vicinity of the adsorp-tion sites (Fig. 4b).

In region I of isotherms, called the Henry’s Lawregion, surfactants adsorb as monomers, without inter-action between them, through electrostatic attractions.In region II, micelle-like aggregates of adsorbed surfac-tants are formed as a result of cooperative interactionsbetween the long-chain hydrocarbon tails of the surfac-tants. It is commonly accepted that these aggregates,termed hemimicelles, are monolayers adsorbing headdown on an oppositely charged surface (Fig. 4c). Forconstant charge surfaces (Fig. 4a), the slope of the iso-therm decreases in this region, since increasing surfac-tant adsorption leads to surface-charge compensation,which results in reduced electrostatic attraction. Forconstant potential surfaces (Fig. 4b), the surface chargeis not compensated by the adsorption of isolated surfac-tant molecules and, as a result, the slope of the adsorp-tion isotherm does not decrease but increases in thesecond region. The region I/region II transition hasbeen termed the critical hemimicelle concentration(HMC) [67], analogous to the CMC.The beginning of the region III coincides with surface

saturation by the adsorbed surfactant. Hydrophobicinteractions of the non-polar chain-groups of the sur-factants are the driving forces for the formation ofaggregates in this region, which are termed admicellesand may have the structure of spheres or bilayers [68](Fig. 4c). The outer surface of the admicelle is ionicin nature and is composed of the ionic head group ofthe surfactant. The admicellar regions are therefore



hydrophilic in nature. Above the CMC, micellizationstrongly competes with adsorption and the coveragedensity remains nearly constant with increasing sur-factant concentration in solution (region IV) (Fig. 4c).

3.1.2. Factors a¡ecting hemimicelle/admicelle formation.The formation of monolayers and bilayers of surfac-tants at solid-liquid interfaces is basically in£uenced bythe same parameters that a¡ect to aqueous micelles for-mation. Thus, parameters a¡ecting the CMC of surfac-tants, such as addition of electrolyte, changes in thelength and degree of branching of the hydrophobicmoiety of the surfactant, and presence of organic addi-tives, will cause variation in the HMC (the region II/region III transition) and the region III/region IV tran-sition at about the some rate and in the same directionas the CMC [69]. Similarly, formation of mixed ionic-non-ionic hemimicelles and admicelles, equally tomixed aqueous micelles, have been reported, permit-ting the adsorption of non-ionic surfactants on sorbentsin which they do not adsorb at all by themselves [70].

However, pH is considered a di¡erential parameterfor the formation of micellar aggregates in aqueousmedia and at solid-liquid interfaces. Thus, hydrogenand hydroxyl ions have basically an electrolytic e¡ecton the formation of aqueous micelles [31], while theygovern the charge density on the sorbent surface andtherefore the extent of surfactant adsorption. It isimportant to keep in mind that the surface charge ofmineral oxides depends on the pH of the solution; belowa particular pH, a metal oxide possesses a positive sur-face charge, whereas, above that pH, it has a negativesurface charge. As a result, the pH is a very powerfultool for manipulating the concentration of hemi-micelles/admicelles at a solid/liquid interface.It has been pointed out that the type of counterion

has an important e¡ect on the adsorption of the surfac-tant [71]. For example, the adsorption of cetyl-pyridinium salicylate on a silica/water interface isconsiderably larger than that of the chloride salt.It has been suggested that the magnitude of thecounterion binding could be related to the in£uence

Figure 4. Schematic adsorption isotherms of ionic surfactants on (a) constant charge surfaces and (b) constant potential surfaces, and

(c) structures proposed for the surfactant adsorbed in each region.



of the counterions on the so-called water structurebecause both e¡ects follow the same lyotropic ion series[72].

3.1.3. Potential of hemimicelles/admicelles for extraction/concentration of organic compounds. The adsolubilizationphenomenon has been applied in very di¡erent areas,such as pharmacy, soil remediation, wastewater treat-ment, and thin-¢lm formation [7]. In analytical processes,theapplicationsdeveloped todatehave focusedon:

� admicellar-enhanced chromatography [14],where themain problem found has been the slowelution of the surfactant from the support, thusgiving di¡erent retention performances withtime;

� the pre-concentration of heavy metals fromaqueous samples, based on the formation ofcomplexes with chelating agents previouslyadsolubilized [73^76]; and,

� the adsolubilization of hydrophobic chelatespreviously formed in the bulk aqueous solution[77,78].

To our knowledge, hemimicelles/admicelles have notbeen used as sorbents for the SPE of organic compoundsprior to chromatographic techniques [79] despite theirgreat potential in this ¢eld.In this section, we will discuss the usefulness of

adsolubilization for the extraction/pre-concentration oforganic compounds on the basis of the results obtainedin the application of hemimicelles/admicelles to theremoval of organic contaminants from aqueousstreams in wastewater treatment.

3.1.3.1. Adsolubilization versus micellar solubilization.The partitioning of neutral and ionic organic com-pounds between sorbed surfactant aggregates (hemi-micelles and admicelles) and an aqueous phase hasbeen described using a pseudo-phase model, similar tosolubilization in aqueous micelles [80^82]. Adsolubili-zation is predominantly controlled by hydrophobicity ofadsolubilizates as well as microenvironmental proper-ties of surfactant adsorbed layers.For many systems, the partition coe⁄cient Pads of

neutral molecules is equal to the micellar solubilizationconstant Pmic. However, because of the di¡erent struc-tural characteristics of micelles and hemimicelles/admicelles, the distribution behavior of some neutralorganics di¡ers from that expected frommicellar solubi-lization [68,83]. Also, admicelles have been known toprovide more hydrophobic environments that conven-tional micellar media; they have hydrophobic proper-ties corresponding to 1-octanol or ethyl acetate [84].For solutes that may undergo protonation, adsolubi-

lization constants are systematically 2^3 times larger

than the corresponding micellar solubilization ones. Athermodynamic model has been presented for the inter-pretation of adsolubilization changes with pH for weakacids and bases [80].The adsolubilized amount of organic compounds

increases with surfactant adsorption, reaches a max-imum, and then decreases. It is generally observed thatthe decrease in adsolubilization occurs above the CMCof the surfactant because the adsolubilizate is parti-tioned between the surfactant-adsorbed layer andmicelles in solution. However, in some cases, thedecrease in adsolubilization begins below the CMC, andthat has been attributed to the di¡erential solubilizationability of the surfactant adsorption aggregates (hemi-micelles and admicelles) [85].

3.1.3.2. Adsolubilization-based SPE procedure. Anadsolubilization-based SPE sequence should compriseat least three steps [86]:

1. a packed bed containing mineral oxide should betreated with a surfactant solution below the CMCat an adequate pH to generate the hemimicelle/admicelle phase;

2. an aqueous stream containing the organic com-pounds and a constant low surfactant concen-tration should be passed through the bed in orderfor adsolubilization to occur; and,

3. when the target pollutants have been saturatedin the column, the pH of the system should bechanged to strip o¡ the surfactant bed andsimultaneously collect the concentrated pollu-tant solution.

The selected pH value in the ¢rst and third stepsdepends on both the point of zero charge (pzc) of themineral oxide and the nature of surfactant. Forexample, for systems involving aluminium oxide(pzc=9.1) and sodium dodecyl sulfate, the pH valueproposed for the ¢rst and third steps has been 3.8and 10.5, respectively [87]. The continuous supplyof a low surfactant concentration during the secondstep is necessary to maintain constant the amount ofsurfactant adsorbed on the mineral oxide. In this way,classical breakthrough curves for the organic com-pounds result.To date, the analytical applications developed for

extraction/pre-concentration of heavy metals usinghemimicelles/admicelles have utilized column formatsand o¡-line procedures [73,75^77]. However, on-linecoupling of adsolubilization-based SPE to LC seems verysimple using cartridge formats and automatic injectionof an aliquot of the ¢nal extract into the chromato-graphic system.An interesting feature of adsolubilization-based SPE

is that regeneration of the metal oxide surface should be



achieved simply by tuning the pH of the aqueous phase.No deterioration in the adsorption capacity of alumi-nium oxide has been observed once it is recycled [88] soit is possible to reuse the sorbent.

3.1.3.3. Scope of analytical applications. Surfactantaggregates on solid surfaces may incorporate ionic ornon-ionic solutes by a mechanism very similar to ionexchange or to solubilization, as it occurs in classicalmicellar solutions [82,83]. So, the simultaneous reten-tion of analytes with a wide range of polarities can becarried out, and that is important in environmentalmonitoring. This ability of hemimicelles/admicelleshas been used to remove simultaneously bothwater-insoluble organics and multivalent ions fromwastewater [89].The microenvironmental properties directly respon-

sible for the incorporation of ionic organics into hemi-micelles/admicelles can be modi¢ed by di¡erent factors,such as type of sorbent, pH, degree of saturation of thesorbent by the surfactant, and type of aggregate (hemi-micelle or admicelle) formed. So, the adsolubilizationphenomenon has potential as a highly versatile extrac-tion method and some degree of selectivity could beachieved for analytes. However, the scope of analyticalapplications could be greatly enhanced with the use offunctionalized surfactants.Hemimicelles/admicelles also have potential for

sampling of organic compounds in the ¢eld and sub-sequent analysis in the laboratory. These aggregatescan greatly facilitate the transport and storage ofsamples collected in remote sites. To con¢rm this poten-tial, studies should be undertaken to determine how thestability of analytes sorbed depends on the storage time,temperature, type of sorbent, pH, and sample matrix.

3.2. Amphiphilic-templated mesoporous silicasSupramolecular assemblies, mainly liquid crystalsformed from surfactants and block copolymers, havebeen used as templates for the synthesis of highlyordered, hierarchical inorganic materials. The result-ing mesoporous solids have unique properties desirablefor adsorption including a very high surface area (up to1600 m2/g), monodispersed pore diameters in therange 2^50 nm and a stereoregular arrangement ofchannels that mimics the liquid crystal formed by theamphiphilic used in their preparation. Since their dis-covery in 1992 by Mobil researchers [90,91], thesematerials have attracted wide interest, and spectacularprogress has been made in their synthesis and char-acterization [92]. Now, the scienti¢c community isfocusing its attention on the numerous applicationsthat stem from the new means of controlling the solidmorphology at the mesoscopic scale of length [93].In this section, we focus on the synthesis and

characteristics of amphiphilic-templated mesoporous

silicas, which are expected to have great potential assorbents in analytical separation processes.

3.2.1. Synthesis. The preparation of mesoporous sili-cates requires at least three ingredients in the appropriateamounts: a source of silica; an amphiphilic; and, a sol-vent (usually water). Other reagents, such as acids,bases, salts, expander molecules and cosolvents, mayalso be used. After the ordered organic-inorganic com-posite material is formed under the appropriate condi-tions, the amphiphilic is removed by calcination, solventextraction, etc, leaving theporous silicatenetwork.To date, three signi¢cant synthetic routes have been

discovered and produced three relevant kinds of mate-rial (Table 2). The ¢rst involves cationic surfactants asstructure-directing agents [90,91]. The synthesis maybe conducted in basic conditions, in which case it fol-lows an ionic assembly mechanism, schematicallyrepresented as S+I (S denotes surfactant and I inor-ganic material), or under acidic conditions, where it fol-lows a counter-ion mediated S+XI+ pathway (X is Cl

or Br). The M41S family of silicas, which is made up ofthe hexagonal MCM-41, cubic MCM-48 and lamellarMCM-50 members (Table 2), has been produced byusing cationic surfactants as templates.The second synthetic route was introduced by Pinna-

vaia’s group [94,95], who produced hexagonal meso-porous silicas (MSU-X) based on hydrogen-bondinginteractions between self-assembled non-ionic poly-ethylene oxide surfactants and neutral oligomeric silicaprecursors S0I0 (Table 2). These materials lack a reg-ular channel packing order and show a wormhole-likepore structure. However, they exhibit uniform channeldiameter over a range comparable to M41S materials,thicker pore walls, a higher degree of condensation,and therefore, a higher thermal stability.The third synthetic route involves amphiphilic di-

and tri-block copolymers as organic structure-directingagents [96^98] (Table 2). Highly ordered hexagonaland cubic mesoporous silica structures (genericallytermed SBA materials) with large (up to 50 nm) poreshave been synthesized via the S+XI+ mechanism usingcommercially available polyalkylene oxides, strongacidic conditions, and dilute aqueous polymer solu-tions. Silica wall thicknesses are typically in the range3^9 nm, which makes SBA more thermally and hydro-thermally stable than previous mesoporous structures.Copolymers interact weakly with the inorganic surface,so the surfactant separation from the composite, eitherby calcination or solvent extraction, is easier than inthe case of ionic surfactants.Important contributions towards the elucidation of

the mechanism of formation of amphiphilic-templatedmesoporous silicas have been made in recent yearsfor both, those based on electrostatic interactions[90,91,99^104] and thosewhere thewhole organization



process is driven by hydrogen bonding [94,105,106].While all these mechanisms have been supported bysome experimental results, they are neither exclusivenor de¢nitive, the actual mechanism of formationdepending on the synthesis conditions.

3.2.2. Characteristics of mesoporous silicas relevant foradsorption of organic compounds.Mesoporous silicas o¡erpotential for separations based on size exclusion andtargeted surface chemistry. In this ¢eld, the applica-tions developed to date have focused on:

� the use of mesoporous silicas as a packing mate-rial in LC [107^109];

� the removal of di¡erent pollutants in wastewatertreatments [110^114]; and,

� the separation of biological molecules in food orpharmaceuticals processing [115].

These materials can separate all type of analytes(basic, acid and neutral) within acceptable retentiontimes and good peak shapes [107], and they are con-sidered ideal for chromatographic separation of largemolecules, such as proteins [108,109]. However, thefunctionalization of MCM- and SBA-type materials hasallowed for the e⁄cient separation of heavy metals[110^113] and toxic anions, such as arsenate [114]. Inmany cases, the mesostructured materials showremarkably high selectivity for the target pollutant inthe presence of competing species, and, because of theirlarge surface area, they are capable of quite high levelsof loading. To our knowledge, mesoporous silicas have

not been exploited for extraction/pre-concentration oforganic compounds in analytical processes.In this section, we will discuss the potential of

mesoporous silicas as sorbents in SPEon the basis on theirability to produce size exclusion- and surface chemistry-based separations, and their ¢t to more standardproperties such as thermal andmechanical stability.

3.2.2.1. Morphology control and pore size. A key issuefor the applicability of mesoporous silicas in size-exclu-sion-based separations is their pore accessibility, whichconditions mass transfer and pore blockages duringuse, and pore size, which determines size selectivity.Three-dimensional pore structures, such as MCM-48,

present better pore accessibility than unidirectionalstructures, such as MCM-41 or SBA-15, which havelong, non-intersecting pores that impose mass-transferlimitations in the less accessible parts of the channels.However, the wormhole-like structured materials, suchas MSU-X, with highly interconnected pores and smalldomain sizes, provide short di¡usion paths for the ana-lytes in the framework channels.The morphology of surfactant-templated mesoporous

silicas can be predicted by using the so-called packingfactor [116]

g ¼ V=aI

where V is the e¡ective volume of the hydrophobicchain, a is the mean aggregate surface area perhydrophilic head group, and I is the critical hydro-phobic chain length. The parameter g depends on the

Table 2. Types of mesoporous silicas obtained by supramolecular templating: abbreviations used in the literature and mesostructures formed



molecular geometry of the surfactant molecules, suchas the number of carbon atoms in the hydrophobicchain, the degree of chain saturation, and the size andcharge of the polar head group. In addition, the e¡ectsof solution conditions, including ionic strength, pH, co-surfactant concentration and temperature, are inclu-ded implicitly in V, a, and I. The morphology of the cor-responding mesophases is related to g, as shown inTable 3 [93].In addition, the surfactant/silica ratio is a critical

variable in the formation of mesostructures, and pro-ductswith di¡erentmorphology canbe obtained accord-ing to requirements byvarying this ratio [117,118].However, both the rate of adsorption and the capa-

city of the mesoporous material depend on the pore-to-solute size ratio. In separations based on size exclusion,size selectivity should be enhanced as the pore sizeapproaches the adsorbing solute size, but pore blocka-ges and di¡usion may limit adsorption kinetics andcapacity.The pore size can be tuned in three di¡erent ways, by:

1. changing the alkyl chain length of the surfactantused in the synthesis procedure;

2. adding expander molecules, such as 1,3,5-tri-methylbenzene [80] or linear hydrocarbons,[119] which dissolve in the hydrophobic regionof the micelles, thus increasing their size; or,

3. ageing a sample prepared at low temperature,e.g. 70�C, then heating it in its mother liquor athigh temperature, e.g. 150�C [120].

3.2.2.2. Pore surface characterization and functionali-zation. The nature, concentration and reactivity ofinternal surface hydroxyl groups determine the proper-ties of mesoporous materials, since these groups possessa⁄nities suitable for the physical adsorption of mole-cules, for example through hydrogen bonding, and mayalso be used as reactive points for the attachment oftethering functional groups.The surface properties of pore walls have been studied

by adsorption of polar and non-polar molecules and byusing nuclear magnetic resonance (NMR), Fouriertransform infrared (FTIR) and thermal gravimetric/di¡erential thermal analysis (TG/DTA) [91,121]. Theresults of these studies have revealed that the internal

surface of MCM-41 and MCM-48 are hydrophilic, andthat about 26^30% of the Si atoms carry OH groups.At least three di¡erent silanol groups have beendistinguished [122]: single; hydrogen-bonded; and,geminal silanol groups.Interior surface silanols readily undergo dehydr-

oxylation far below 600�C. Therefore, for electro-statically assembled materials, the calcination processtypically used to remove the surfactant from the frame-work depletes the surfaces of hydroxyl groups fororganosilane functionalization, and rehydration isnecessary for reconstruction of silanol groups [121]. Bycontrast, for mesoporous materials synthesizedthrough electrically neutral routes, the surfactantmolecules can be easily removed from the channels bysolvent extraction. This optimizes the number of sur-face hydroxyl groups that are available on the porewalls for organosilane functionalization [112].Recent studies have shown that numerous functional

groups, including amines, chlorides, thiols, carboxylicacids and phenyls, may be attached successfully to thesurface of mesoporous materials via tethering alkylchains [123^128]. In this way, highly selective SPEprocedures can be developed.Two routes have been developed to incorporate the

functional groups. In-situ functionalization involves theaddition of the target species, usually as the triethoxy-silane (EtO)3Si-X, where X is the functional group, dur-ing the preparation of the gel [127,129,130]. Theproduct from this route may not be calcined thermallyas the functional group is expected to decompose athigh temperatures. Therefore, the use of block copoly-mers as structure-directing agents is preferred, sincethese can be removed by solvent extraction. The order ofthe structure is often impaired because organic silanescan have only three connections with other Si species,whereas ordinary silicate can have four connectionssuitable for constructing a rigid three-dimensionalstructure.Post-synthesis functionalization involves the grafting

of the functional group onto a calcined sample[131^134]. Triethoxysilane with a suitable functionalgroup is generally used as reagent, although Grignardreagents that form stable Si-C bonds have been also pro-posed [135]. The post-synthesis route favors the forma-tion of well-ordered structures; however, the procedurerequires an extra step compared with in-situ prepara-tion and also requires three silanol hydroxyls in asuitable con¢guration for optimal binding. Furthermore,the diameter of the pores may be reduced as a result ofthe coating of an extra layer of Si-X species. Such pore-size reduction could be signi¢cant in those mesoporousmaterials with pore diameter lower than 2 nm and, asa consequence, the e¡ective binding of adsorbatespecies to these functional sites may become drasticallyrestricted [112].

Table 3. Surfactant packing parameter, g, expected structures and

examples for such structures

g
Expected structure Example
1/3
Cubic (Pm3n) SBA- 1 1/2 Hexagonal (p6) MCM- 41, SBA- 15 1/2–2/3 Cubic (Ia3d) MCM- 48 1 Lamellar MCM- 50


3.2.2.3. Structural stability. It is well known thanmesoporous silicas exhibit good thermal stability. Uponheating these materials up to 1000�K, the structuralintegrity can still be retained. By contrast, much lesswork has been performed on their mechanical stabilitywhen they are subjected to high pressure, which isimportant for applications involving on line coupling ofSPE to LC. In a recent investigation on MCM-41 [136],it was shown that application of mechanical pressurebetween 100 and 480 MPa caused gradual loses of theMCM-41 structural order as pressure increased, asre£ected by the broadness and reduction of di¡ractionpeaks in XRD, as well as by the decrease of BET(Brunauer, Emmett and Teller) surface area and porevolume in nitrogen adsorption-desorption. However,no special problems of mechanical stability have beenreported in the chromatographic separations developedto date by usingmesoporous silicas [107^109].Brie£y, the possibility of controlling the morphology,

pore size and surface reactivity of mesoporous silicasrender these materials highly attractive for developingselective SPE procedures.

4. Conclusions

Supramolecular chemistry has achieved a high level ofdevelopment stimulated by the wide research horizonsthat this interdisciplinary discipline o¡ers. Increasingknowledge of the formation and the characteristics ofsupramolecular assemblies, as well as of the synthesisof materials involving these aggregates, has openedup new perspectives on the resolution of practicalanalytical problems. This article has explored the greatanalytical potential for using cloud point and coacerva-tion as extractants and hemimicelles/admicelles andamphiphilic-templated mesoporous silicas as sorbentsin SPE. In our opinion, intensive research with analy-tical goals on these systems will lead to signi¢cant¢ndings that will improve existing methods for theextraction/pre-concentration of organic compounds.

Acknowledgements

The authors gratefully acknowledge ¢nancial supportfrom the Spanish CICyT (Project BQU2002-01017)

References

[1] J.M. Lehn, Supramolecular Chemistry: Concepts and Perspec-tives. A Personal Account, John Wiley & Son, New York, USA,1995.

[2] J.W. Steed, J.L. Atwood, Supramolecular Chemistry, JohnWiley & Son, New York, USA, 2000.

[3] J.L. Atwood, J.M. Lehn, J.E.D. Davies, D.D. MacNicol, F. Vogtle(Eds.), Comprehensive Supramolecular Chemistry,11-Volume Set, Pergamon Press, Oxford, UK, 1996.

[4] J.L. Atwood, G.W. Gokel (Eds.), Journal of SupramolecularChemistry, Elsevier, Oxford, UK, 2002.

[5] J.H. Fendler, Membrane Mimetic Chemistry, John Wiley &Son, New York, USA, 1982.

[6] J.H. Fuhrhop, J. Koning, Membranes and MolecularAssemblies: The Synkinetic Approach, The Royal Society ofChemistry, Cambridge, UK, 1994.

[7] S.D. Christian, J.F. Scamehorn (Eds.), Solubilization in Surfac-tant Aggregates, Surfactant Science Series 55, Marcel Dekker,New York, USA, 1995.

[8] M. Gratzel, K. Kalyanasundaram, Kinetics and Catalysis inMicroheterogeneous Systems, Surfactant Science Series 38,Marcel Dekker, New York, USA, 1991.

[9] D.W. Armstrong, Sep. Purif. Methods 14 (1985) 213.[10] W.L. Hinze, D.W. Armstrong (Eds.), Ordered Media in

Chemical Separations, ACS Symposium Series 342, AmericanChemical Society, Washington DC, USA, 1987.

[11] W.L. Hinze, Ann. Chim. (Rome) 77 (1987) 167.[12] J.F. Scamehorn, J.H. Harwell (Eds.), Surfactant-Based

Separation Processes, Marcel Dekker, New York, USA, 1989.[13] W.L. Hinze, E. Pramauro, Crit. Rev. Anal. Chem. 24 (1993)

133.[14] E. Pramauro, E. Pelizzetti, Surfactants in Analytical

Chemistry. Applications of Organized Amphiphilic Media,Elsevier, New York, USA, 1996.

[15] D.W. Armstrong, F. Nome, Anal. Chem. 53 (1981) 1662.[16] M.G. Khaledi, Trends Anal. Chem. 7 (1988) 293.[17] H. Watarai, J. Chromatogr. A 780 (1997) 93.[18] S. Terabe, K. Otsuka, T. Ando, Anal. Chem. 57 (1985) 39.[19] S. Terabe, K. Otsuka, D. Ichikawa, A. Tsuchiya, T. Ando,

Anal. Chem. 56 (1984) 113.[20] S. Terabe, N. Matsubara, Y. Ishihama, Y. Okada,

J. Chromatogr. 608 (1992) 23.[21] K.D. Altria, J. Chromatogr.A 892 (2000) 171.[22] V. Schurig, J. Chromatogr. A 906 (2001) 275.[23] S. Fanali, J. Chromatogr. A 875 (2000) 89.[24] B. Chankvetadze, Capillary Electrophoresis in Chiral Analysis,

John Wiley & Son, New York, USA, 1997.[25] D. Pe¤ rez-Bendito, S. Rubio, Trends Anal. Chem. 12 (1993) 9.[26] M. Lo¤ pez Carreto, S. Rubio, D. Pe¤ rez-Bendito, Analyst

(Cambridge, UK) 121 (1996) 33R.[27] G.L. McIntire, CRC Crit. Rev. Anal. Chem. 21 (1990) 257.[28] A. Sanz-Medel, M.R. Fernandez, E. Blanco, M.L. Ferna¤ ndez-

Sa¤ nchez, Spectrochim. Acta B 54 (1999) 251.[29] E. Pelizzetti, E. Pramauro, Anal. Chim. Acta 169 (1985) 1.[30] R.A. Mackay, J. Texter, Electrochemistry in Colloids and

Dispersions, VCH, New York, 1992.[31] D. Sicilia, S. Rubio, D. Pe¤ rez-Bendito, Anal. Chem. 67 (1995)

1872.[32] H. Ishii, J. Miura, H. Watanabe, Bunseki Kagaku 28 (1977)

252.[33] H. Watanabe, H. Tanaka, Talanta 25 (1978) 585.[34] H. Watanabe, N. Yamaguchi, H. Tanaka, Bunseki Kagaku 28

(1979) 366.[35] R. Carabias-Mart|¤ nez, E. Rodr|¤ guez-Gonzalo, B. Moreno-

Cordero, J.L. Pe¤ rez-Pavo¤ n, C. Garci¤ a-Pinto, E. Ferna¤ ndezLaespada, J. Chromatogr. A 902 (2000) 251.

[36] F.H. Quina, W.L. Hinze, Ind. Eng. Chem. Res. 38 (1999)4150.

[37] X. Jin, M. Zhu, E.D. Conte, Anal. Chem. 71 (1999) 514.[38] S. Igarashi, T. Yotsuyanagi, Microchim. Acta 106 (1992)

37.[39] I. Casero, D. Sicilia, S. Rubio, D. Pe¤ rez-Bendito, Anal. Chem.

71 (1999) 4519.



[40] L.K.J. Tong, M.C. Glessman, J. Am. Chem. Soc. 79 (1957)4305.

[41] K.L. Mittal (Ed.), Solution Chemistry of Surfactants, PlenumPress, New York, USA, 1979.

[42] P. Mukerjee, J.R. Cardinal, J. Phys. Chem. 82 (1978) 1620.[43] H. Hoshino, T. Saitoh, H. Takemoti, T. Yotsuyanagi, Anal.

Chim. Acta 147 (1983) 339.[44] D. Sicilia, S. Rubio, D. Pe¤ rez-Bendito, Anal. Chim. Acta 460

(2002) 13.[45] F. Merino, S. Rubio, D. Pe¤ rez-Bendito, J. Chromatogr. A 962

(2002) 1.[46] D. Sicilia, S. Rubio, D. Pe¤ rez-Bendito, N. Maniasso,

E.A.G. Zagatto, Anal. Chim. Acta 392 (1999) 29.[47] E. Pramauro, Ann. Chim. (Rome) 80 (1990) 101.[48] F.H. Quina, E.O. Alonso, J.P.S. Farah, J. Phys. Chem. 99

(1995) 11708.[49] R.P. Frankewich, W.L. Hinze, Anal. Chem. 66 (1994) 944.[50] C.G. Pinto, J.L. Pe¤ rez-Pavo¤ n, B. Moreno-Cordero, Anal. Chem.

66 (1994) 874.[51] T. Saitoh, W.L. Hinze, Anal. Chem. 63 (1991) 2520.[52] B. Froschl, G. Stangl, R. Niessner, J. Fresenius’, Anal. Chem.

357 (1997) 743.[53] J.W. Jorgenson, K.D. Lukacs, J. Chromatogr. 218 (1981) 209.[54] R. Carabias-Marti¤ nez, E. Rodri¤ guez-Gonzalo, J. Domi¤ nguez-

Alvarez, J. Herna¤ ndez-Me¤ ndez, Anal. Chem. 71 (1999) 2468.[55] R.S. Sirimanne, J.R. Barr, D.G. Patterson, J. Microcol. Sep. 11

(1999) 109.[56] Q. Fang, M. Du, C.W. Huie, Anal. Chem. 73 (2001) 3502.[57] B.F. Gibbs, S. Kermasha, I. Alli, C.N. Mulligan, Int. J. Food Sci.

Nutr. 50 (1999) 213.[58] G. Weiss, A. Knoch, A. Laicher, F. Stanislaus, R. Daniels,

Int. J. Pharm. 124 (1995) 87.[59] M.C. Hennion, J. Chromatogr. A 856 (1999) 3.[60] B. Dobias (Ed.), Coagulation and Flocculation, Surfactant

Science Series 47, Marcel Dekker, New York, USA, 1993.[61] M.R. Bohmer, L.K. Koopal, Langmuir 8 (1992) 2649.[62] M.R. Bohmer, L.K. Koopal, Langmuir 8 (1992) 2660.[63] L.M. Koopal, E.M. Lee, M.R. Bohmer, J. Colloid Interface Sci.

170 (1995) 85.[64] T.P. Goloub, L.K. Koopal, B.H. Bijsterbosch, M.P. Sidorova,

Langmuir 12 (1996) 3188.[65] K.T. Valsaraj, Sep. Sci. Technol. 24 (1989) 1191.[66] K.T. Valsaraj, Sep. Sci. Technol. 27 (1992) 1633.[67] J.H. Harwell, J. Hoskins, R.S. Schechter, W.H. Wade,

Langmuir 1 (1985) 251.[68] T. Behrends, R. Herrmann, Colloids Surf. [, B ?] 16 (2000) 15.[69] D. Bitting, J.H. Harwell, Langmuir 3 (1987) 531.[70] J.F. Scamehorn (Ed.), Phenomena in Mixed Surfactant

Systems, ACS Symp. Ser. 311, Amer. Chem. Soc, Washington,D.C., USA, 1986.

[71] R. Bury, P. Favoriti, C. Treiner, Colloids Surf. 139 (1998) 99.[72] A.L. Underwood, E.W. Anacker, J. Colloid Interface Sci. 117

(1987) 242.[73] J.L. Manzoori, M.H. Sorouraddin, F. Shemirani, Talanta 42

(1995) 1151.[74] M. Hiraide, W. Shibata, Anal. Sci. 14 (1998) 1085.[75] J.L. Manzoori, M.H. Sorouraddin, A.M.H. Shabani, J. Anal.

At. Spectrom. 13 (1998) 305.[76] J.L. Manzoori, M.H. Sorouraddin, A.M.H. Shabani,

Microchem. J. 63 (1999) 295.[77] M. Hiraide, J. Iwasawa, H. Kawaguchi, Talanta 44 (1997)

231.[78] M. Hiraide, J. Hori, Anal. Sci. 15 (1999) 1055.[79] T. Saitoh, Y. Nakayama, M. Hiraide, J. Chromatogr. A 972

(2002) 205.[80] P. Favoriti, V. Monticone, C. Treiner, J. Colloid Interface Sci.

179 (1996) 173.

[81] I. Cherkaoui, V. Monticone, C. Vaution, C. Treiner, Int. J.Pharm. 176 (1998) 111.

[82] I. Cherkaoui, V. Monticone, C. Vaution, C. Treiner, Int. J.Pharm. 201 (2000) 71.

[83] C.L. Lai, E.A. O’Rear, J.H. Harwell, M.J. Hwa, Langmuir 13(1997) 4267.

[84] T. Saitoh, K. Taguchi, M. Hiraide, Anal. Chim. Acta 454(2002) 203.

[85] K. Esumi, M. Goino, Y. Koide, J. Colloid Interface Sci. 183(1996) 539.

[86] J.S. Smith, K.T. Valsaraj, Sep. Purif. Technol. 13 (1998)147.

[87] C.L. Lee, J.C. Lee, Chemosphere 47 (2002) 277.[88] P.M. Jain, J.S. Smith, K.T. Valsaraj, Sep. Purif. Technol. 17

(1999) 21.[89] K. Esumi, K. Yoshida, K. Torigoe, Y. Koide, Colloids Surf. 160

(1999) 247.[90] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck,

Nature (London) 359 (1992) 710.[91] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge,

K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard,S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc.114 (1992) 10834.

[92] L. Bonneviot, S. Giasson, S. Kaliaguine, M. Stocker,Microporous Mesoporours Mater. 44 (2001) ix.

[93] U. Ciesla, F. Schu« th, Microporous Mesoporous Mater. 27(1999) 131.

[94] S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science (Washington,DC) 269 (1995) 1242.

[95] E. Prouzet, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 36(1995) 516.

[96] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson,B.F. Chmelka, G.D. Stucky, Science (Washington, DC) 279(1998) 548.

[97] D. Zhao, Q. Huo, J. Feng, B.F. Chemelka, G.D. Stucky, J. Am.Chem. Soc. 120 (1998) 6024.

[98] C. Yu, Y. Yu, L. Miao, D. Zhao, Microporous MesoporousMater. 44 (2001) 65.

[99] J.C. Vartuli, C.T. Kresge, M.E. Leonowicz, S.B. McCullen,I.D. Johnson, E.W. Sheppard, Chem. Mater. 6 (1994)2070.

[100] J.S. Beck, J.C. Vartuli, G.J. Kennedy, C.T. Kresge, W.J. Roth,S.E. Schramm, Chem. Mater. 6 (1994) 1816.

[101] C.F. Cheng, Z. Luan, J. Klinowski, Langmuir 11 (1995)2815.

[102] J.S. Beck, J.C. Vartuli, Curr. Opin. Solid State Mater. Sci. 1(1996) 76.

[103] Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger,R. Leon, P.M. Petro¡, F. Schu« th, G.D. Stucky, Nature 368(1994) 317.

[104] Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng,T.E. Gier, P. Sieger, A. Firouzi, B. Chmelka, F. Schu« th,G.D. Stucky, Chem. Mater. 6 (1994) 1176.

[105] P.T. Tanev, T.J. Pinnavaia, Science (Washington, DC) 267(1995) 865.

[106] P.T. Tanev, T.J. Pinnavaia, Chem. Mater. 8 (1996) 2068.[107] M. Gru« n, A.A. Kurganov, S. Schacht, F. Schu« t, K.K. Unger,

J. Chromatogr. A. 740 (1996) 1.[108] K. Nakanishi, R. Takahashi, T. Nagakane, K. Kitayama,

N. Koheiya, H. Shikata, N. Soga, J. Sol.-Gel Sci. Technol. 17(2000) 191.

[109] K. Nakanishi, H. Minakuchi, N. Soga, N. Tanaka, J. Sol.-GelSci. Technol. 13 (1998) 163.

[110] X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, J. Liu,K.M. Demner, Science (Washington, DC) 276 (1997) 923.

[111] J. Liu, X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, M. Gong,Chem. Eng. Technol. 21 (1998) 97.



[112] L. Mercier, T.J. Pinnavaia, Environ. Sci. Technol. 32 (1998)2749.

[113] L. Mercier, T.J. Pinnavaia, Adv. Mater. 9 (1997) 500.[114] G.E. Fryxell, J. Liu, T.A. Hauser, Z. Nie, K.F. Ferris, S. Mattigod,

M.Gong,R.T.Hallen,Chem.Mater. 11 (1999)2148.[115] J.M. Kisler, A. Da« hler, G.W. Stevens, A.J. O’Connor,

Microporous Mesoporous Mater. 44 (2001) 769.[116] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, J. Chem. Soc.,

Faraday Trans. 72 (1976) 1525.[117] J.C. Vartuli, K.D. Schmitt, C.T. Kresge, W.J. Roth,

M.E. Leonowicz, S.B. McCullen, S.D. Hellring, J.S. Beck,J.L. Schlenker, D.H. Olson, E.W. Sheppard, Chem. Mater. 6(1994) 2317.

[118] P. Behrens, A. Glaue, C. Haggenmu« ller, G. Schechner, SolidState Ionics 101-103 (1997) 255.

[119] N. Ulagappan, C.N.R. Rao, Chem. Commun. (1996) 2759.[120] D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Garces,

M.M. Olken, N. Coombs, Adv. Mater. 7 (1995) 842.[121] H. Landmesser, H. Kosslick, W. Storek, R. Fricke, Solid State

Ionics 101-103 (1997) 271.[122] X.S. Zhao, G.Q. Lu, A.K. Whittaker, G.J. Millar, H.Y. Zhu,

J. Phys. Chem. B 101 (1997) 6525.[123] T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Nature

(London) 420 (1999) 867.

[124] A.M. Liu, K. Hidajat, S. Kawi, D.Y. Zhao, , J. Chem. Soc. Chem.Commun. (2000) 145.

[125] P.M. Price, J.H. Clark, D.J. Macquarrie, J. Chem. Soc., DaltonTrans. Vol. ? ((2000)) 101.

[126] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000)1403.

[127] C.M. Bambrough, R.C.T. Slade, R.T. Williams, Phys. Chem.Phys. 2 (2000) 3499.

[128] H.H.P. Yiu, P.A. Wright, N.P. Botting, J. Mol. Catal. B 15(2001) 81.

[129] F. Babonneau, L. Leite, S. Fontlupt, J. Mater. Chem. 9 (1999)175.

[130] A. Cauvel, G. Renard, D. Brunel, J. Org. Chem. 62 (1997)749.

[131] F. De Juan, E. Ruiz-Hitzky, Adv. Mater. 12 (2000) 430.[132] R. Anwander, I. Nagl, M. Widenmeyer, G. Engelhardt,

O. Groeger, C. Palm, T. Ro« ser, J. Phys. Chem. B 104 (2000)3532.

[133] C. Yoshina-Ishii, T. Asefa, N. Coombs, M.J. MacLachlan,G.A. Ozin, , J. Chem. Soc. Chem. Commun. (1999) 2539.

[134] X.S. Zhao, G.Q. Lu, J. Phys. Chem. B 102 (1998) 1556.[135] K. Yamamoto, T. Tatsumi, Microporous Mesoporous Mater.

44^45 (2001) 459.[136] N. Bai, Y. Chi, Y. Zou, W. Pang, Mater. Lett.ers 54 (2002) 37.



Supramolecular assemblies for extracting organic compounds

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