The production of stable hydrophobic surfaces by the adsorption of hydrocarbon and fluorocarbon...

Colloids and Surfaces

A: Physicochemical and Engineering Aspects 182 (2001) 285–298

The production of stable hydrophobic surfaces by theadsorption of hydrocarbon and fluorocarbon carboxylic

acids onto alumina substrates

M.E. Karaman, D.A. Antelmi, R.M. Pashley *Department of Chemistry, The Faculties, The Australian National Uni6ersity, Canberra, ACT 0200, Australia

Received 10 July 2000; accepted 20 November 2000

Abstract

The adsorption of various carboxylic acids onto alumina substrates has been studied as a model system for thestudy of the long range hydrophobic interaction. The adsorption was studied using classical adsorption isotherms,contact angle measurements and colloid probe atomic force microscopy. Carboxylic acids form esters with thehydroxyl groups present on the alumina surface and produce surfaces with a range of hydrophobicities and stabilitiesdepending on the acid’s functionality. Hydrocarbon carboxylic acid adsorption was found to be adequately describedby the Langmuir adsorption model and produced stable surfaces of varying hydrophobicities which resisteddissolution by water. By comparison, hydrophobic surfaces produced with fluorinated acids adsorbed from aqueoussolution were not always stable, although their stability was greatly improved when the chemisorption was carried outin a non-aqueous solvent at elevated temperatures. Adsorption of these fluorinated carboxylic acids produced themost hydrophobic surfaces. Interaction forces between alumina surfaces coated with adsorbed propionic acid were asexpected from DLVO theory with no evidence of hydrophobic forces. © 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Carboxylic acids; Alumina substrates; Hydrophobic surfaces

www.elsevier.nl/locate/colsurfa

1. Introduction

The adsorption of molecules to the surface ofparticulate solids has been a subject of consider-able research. This is particularly so in the case ofalumina owing to its great commercial importance

in areas such as catalysis, ceramics, and a com-mon stationary phase in chromatography. Theadsorption of carboxylic acids onto alumina isespecially strong and can be due to physical ad-sorption or chemisorption [1]. Chemisorption canoccur via a surface esterification type reactionwith a hydrated alumina surface as shown in Fig.1. The high reactivity of the hydrated aluminasurface towards carboxylic acids, among othermolecules, is thought to arise from the enhanced

* Corresponding author. Tel.: +61-2-62492631; fax: +61-2-62499614.

E-mail address: [email protected] (R.M. Pash-ley).

0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0927 -7757 (00 )00821 -9

M.E. Karaman et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 182 (2001) 285–298286

acidity of the hydroxyl group as compared to ahydrated silica surface for instance, which doesnot show such reactivity towards the carboxylategroup [2].

Much work has been done on the speciationand equilibria of Al (III) with various ligands.These include the hydroxy-substituted carboxylicacids such as lactic acid [3], salicylic acid [4], citricacid [5,6] and the dicarboxylic acids oxalic [7] andphthalic acid [8]. In these studies the mono- andpolynuclear ternary Al3+ –L–OH− complexeswere often identified.

Marklund et al. [9] studied the complexation inAl (III) systems with the saturated fatty acidspropionic and acetic acids and found that theligand dissociation constants for the reaction2Al3+ +HLlAl2(OH)2L3+ +3H+ was aboutKe=10−4.6 or 2.5×10−5 M, corresponding to astandard bond energy of about +10.6 kT.

In water treatment processes natural water istreated with chemicals such as aluminium sul-phate or ferric chloride to induce coagulation ofsuspended particulate matter. The untreated watertypically contains a heterogeneous mixture of in-organic colloidal materials, such as aluminosili-cates, silica and oxides of iron and manganesewhich are usually heavily coated with dissolvedorganic materials, typically present at levels of1–20%. The surface characteristics of the col-loidal material is influenced by this adsorption.The dissolved organic materials can consist ofphenols, fatty acids, sterols, sugars, hydrocarbons,ureas, amino acids, porphyrins and polymers. Thepolymeric organic impurities can be polypeptides,lipids, polysaccharides or humic acids [10]. The

humic substances are removed by their adsorptionto the hydrolysed aluminium hydroxide floc in aligand exchange reaction:

RCOO− +HO–Al�lRCOO–Al�+OH−

Alumina is a useful potential substrate for theproduction of stable substrates of varying hydro-phobicities. A long range attractive force isknown to exist between hydrophobic particles (the‘hydrophobic interaction’) [11,12], although itsorigin is still strongly debated [13–19]. The hydro-phobic interaction has been previously studiedmostly by surfaces prepared by surfactant adsorp-tion [20–23] (equilibrium adsorption from solu-tion and Langmuir–Blodget deposition) and forhydrophobic plastics [24]. However, surfactantcoated surfaces are often unstable due to thephysisorbed nature of the adsorption, and plasticsurfaces appear to produce only relatively weakhydrophobic forces [24]. By comparison, the car-boxylate/alumina system offers a range of hydro-phobic substrates with the enhanced stability dueto a chemisorption process (mechanism as shownin Fig. 1). Stable substrates with a range of hydro-phobicities can be used to investigate the mecha-nisms involved in the hydrophobic interaction. Inthis work the degree of hydrophobicity and thestability of a wide range of carboxylic acids ad-sorbed onto alumina are examined.

2. Materials and methods

The water used in this study was producedfrom tap water fed into a Memtec Krystal

Fig. 1. Schematic of the esterification mechanism between hydroxyl groups on the alumina surface and carboxylic acids.

M.E. Karaman et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 182 (2001) 285–298 287

Kleen™ unit that uses a three stage purificationprocess (prefilter, reverse osmosis and activatedcharcoal) before being distilled, collected andstored in a positive pressure, dust-free laminarflow cabinet to prevent air borne contamination.Since this study involved Al2O3, the distilled,deionised water was collected directly into cleanpolyethylene bottles to eliminate any silica con-tamination. The polyethylene bottles used in thepreparation and storage of solutions were rinsedwith 10% NaOH and copious volumes of distilledMillipore water before use.

2.1. Contact angle in6estigation of carboxylic acidadsorption on alumina

Two types of alumina substrates were used forthese investigations: undoped alumina (a-Al2O3)obtained in wafer form from Silica Source Tech-nology, Temple, Arizona USA and 99.99% purealuminium sheeting obtained from Ullrich NoyesMetal, Sydney, Australia. AFM imaging indicatedthe aluminium sheeting was locally smooth to 4nm RMS but was pitted to some extent over largeareas. The aluminium sheeting having both highpurity and high reflectivity (a mirror like finish)was commercially available from this source forX-ray work. The aluminium surface readily oxi-dises producing a hydrophilic a-alumina surfacelayer. Typically, the surface oxide layer was foundto be 10–20 nm thick by electron ablation X-rayphotoelectron spectroscopy (EAXPS). The sap-phire (a-alumina) or 99.99% pure mirror-like alu-minium sheeting was cut to the appropriate size.The aluminium sheeting was washed with diethylether and ethanol to remove organic contami-nants, however, the sapphire substrates did notrequire any solvent treatment.

The final treatment of the substrates beforecontact angle investigations was either UV irradi-ation or plasma treatment. In the first case, sam-ples were UV irradiated (l 254 nm) for 1–1.5 h inthe presence of water vapour in a laminar flowcabinet. Plasma treatment was performed using amodified Harrick Plasma Cleaner (Ossining, NewYork) for 30 s at a power of 30 W and partialpressures of Argon and Water of 0.01 and 0.06Torr, respectively. Both treatments cleaned the

surface of contaminants and promoted surfacehydroxylation as seen by the reduction of thecontact angle to 0° for both substrates.

Equilibrium advancing and receding contactangles were measured inside a sealed glass cellfitted with a teflon syringe. A small beaker con-taining water was placed at the bottom of the cellproviding a vapour saturated environment. Adroplet was slowly forced onto the substrate viathe syringe and allowed to equilibrate whilst ob-serving the three phase line (TPL) through amicroscope. The droplet was of significant size toensure that the presence of the syringe needle didnot effect the angle at the TPL. As the volume ofthe droplet was increased the maximum angle justbefore the TPL moved forward was taken as theequilibrium advancing contact angle (uA). Theequilibrium receding angle (uR) was measured bywithdrawing liquid from the droplet via the sy-ringe and recording the angle just before the TPLbegan to move backwards. The advancing andreceding angles were measured by way of a go-niometer fitted to the microscope eyepiece. Thisprocess was carried out well away from the edgesof the substrate (eliminating any possible edgeeffect), and was repeated on a number of differentregions on each substrate. Knowledge of both theadvancing and receding angles across a substrateallowed conclusions to be drawn about the homo-geneity of the adsorbed layer.

2.2. Adsorption of per-fluorinated carboxylic acidsto sapphire

Per-fluorinated carboxylic acids of the formCnF2n−1OOH were obtained from Sigma–Aldrich and used as received. A homologous se-ries was studied where n=3, 8, 10, 12, and 14.Aqueous solutions of per-fluoropropionic acid(n=3) and per-fluorooctanoic acid (n=8) weremade to 0.01 M. The longer chained acids (n=10,12 and 14) were only slightly soluble in water andsaturated solutions were therefore prepared. Thesaturation concentration was determined poten-tiometrically by titrating against standardisedHNO3 to be 1.6×10−3, 6×10−4 and 3×10−4

M for n=10, 12 and 14, respectively. Solutions ofthe per-fluorinated acids in 1,1,2-trichlorotrifl-


uoroethane (Sigma–Aldrich) were all made to0.03 M.

Adsorption of the acids from either aqueous ornon-aqueous solutions was performed by placinga sapphire substrate in a high pressure reactionvial with 5 ml of per-fluorinated carboxylic acidsolution. The adsorption time was 2 h and per-formed at room temperature as well as at 60°C.Upon removal of the substrates from the solu-tions they were rinsed in distilled water and blowndry with compressed nitrogen. The contact angleswere reproducible within 2° across a single sub-strate, and within 10% on different substratestreated using similar adsorption conditions. Thelater variation may be due to differences in rough-ness between substrates, as substrates used werereused after cleaning in nitric acid and plasmatreated.

2.3. Carboxylic acid adsorption from solution

The alumina powder for adsorption isotherminvestigations was purchased from ICN (Cat c02084) in neutral form, having a particle size50–200 mm and a specific surface area of 150 m2

g−1. The acids used were acetic (glacial) \99.7%(M&B) wt. ml−1 1.048–1.051 g at 20°C, propi-onic \98% unilab (Ajax) wt. ml−1 0.922 g at20°C and valeric acid 99+% (Aldrich) wt. ml−1

0.939 at 20°C and were all used as purchased. Theadsorption isotherms were determined by weigh-ing approximately 1 g of Al2O3 powder (neutral)into stoppered 250 cm3 Erlenmeyer flasks. To thefirst flask, 100 cm3 of a 0.2 M carboxylic acidstock solution was added via pipette, the flask wasthen shaken periodically over the next 30 minthen allowed to stand for 1 h at room tempera-ture, before being filtered through a fine sinter tocompletely remove all the alumina powder fromsolution. The equilibrium free acid concentration(C) was determined by taking a 25 cm3 aliquot ofthe filtrate and titrating it against a standardisedNaOH solution using phenolphthalein as the indi-cator. This procedure was repeated using 0.03,0.07, 0.10 and 0.15 M carboxylic acid stocksolutions.

The calculated number of carboxylic acidmolecules per gram of alumina (N) was plotted

against the corresponding equilibrium acid con-centration (C) as well as the C/N versus C. Thesurface area per gram of alumina was determinedknowing the area occupied per molecule for agiven (acid) adsorbate.

2.4. Colloid probe atomic force microscopy

A sphere-on flat geometry was used as this isthe most practical geometry for the measurementof surface forces and also allows quantitativeanalysis. Alumina flats were prepared by cuttingundoped alumina (a-Al2O3) wafers (Silica SourceTechnology) to the appropriate size and glueingthem onto magnetic stainless steel stubs withAraldite™ epoxy resin. The sapphire wafer wasfound to be quite smooth having a RMS rough-ness of around 0.5 nm rms.

Alumina ‘colloid probes’ were prepared by at-taching 20 mm aluminium shot (obtained fromJohnson Matthey) using Epicote 1009 resin tocommercial Si3N4 cantilevers (Digital Instru-ments) using a micro manipulator. The alu-minium shot, which readily oxidises, provided ahydrophilic a-alumina-like surface of spherical ge-ometry. Typically the surface oxide layer wasfound to be 10–20 nm thick by electron ablationX-ray photoelectron spectroscopy (EAXPS).

The mounted flat substrate and alumina probewere UV irradiated for 1 h in the presence ofwater vapour to remove any organic contamina-tion just prior to AFM setup. The UV irradiationin the presence of water vapour and oxygen pro-duces ozone and hydroxyl radicals which cleanthe alumina surfaces rendering them hydrophilic.All AFM measurements were carried out using acustom built AFM built in this laboratory [25].The photo diode signal was digitised using MacA-DIOS II/16 card. The instrument was controlledusing Superscope IIE software. Once forces wereobtained in a given solution the cantilever waswithdrawn from contact and the fluid cell flushedthoroughly with the next solution. The solutionwas left to equilibrate at least 15 min beforemeasurements recommenced. At the end of theexperiment the colloid probe [26] was examined ina Cambridge S360 Scanning Electron Microscopeto obtain an accurate estimate of the probe ra-


Table 1Advancing (uA) and receding (uR) contact angles measured for water contacting carboxylic acid layers adsorbed onto alumina fromaqueous solutions

u°R (°) Valeric acid concentration (M) u°A (°)Propionic acid concentration (M) u°R (°)u°A (°)

0 10−610−3 2237 010−2 55 17 10−5 17 510−1 58 20 10−4 16 0

10−3 22 510−2 45 22Neat 99+% 52 10

dius, thus allowing direct comparison betweenrepeat experiments. The observed force was scaledwith the radius of the probe (R) as a function ofseparation distance (D) fitted using a numericalsolution to the Poisson–Boltzman equation for a1:1 electrolyte using the method of Chan et al. [27]with a Hamaker constant (Al2O3/H2O/Al2O3) of5.3×10−20 J.

3. Results and discussion

3.1. Contact angle measurements

By measuring the contact angles of a-aluminasubstrates treated with a variety of carboxylicacids (both aryl and alkyl carboxylic acids ofquite differing functionality) the resulting hydro-phobicity could be used to monitor the adsorptioncharacteristics of the adsorbate.

Sapphire wafers placed into a 10−1 M propi-onic acid solution (pH 2.83) gave an equilibriumadvancing contact angle of 58° and equilibriumreceding angle of around 20°. After several daysin water the advancing angle was found to be 58°and the receding angle around 22°, which suggeststhat the esterified surface was resistant to hydroly-sis. The advancing angle increased to 70° and thereceding angle decreased to zero after the sub-strate was soaked in 0.5 M HCl.

Substrates treated with 10−6 M valeric acidwere found to have an equilibrium advancing andreceding angle of 20–22° and zero respectively. Asthe valeric acid concentration was increased to10−5 M (pH 4.4) the equilibrium advancing anglewas found to be 16–20° and the equilibrium

receding angle increased to 5–6°. At 10−4 M (pH3.8) the advancing angle 16–22°, receding angleappeared about zero. At 10−3 M (pH 3.3) theadvancing angle ranged between 19 and 24° andthe receding angle around 3–5°. In 10−2 M solu-tion (pH 2.9) the advancing angle and receding40–45 and 22°, respectively. In neat 99+% va-leric acid the advancing and receding angles werefound to be 52 and 10°, respectively.

Comparing the contact angles obtained for pro-pionic and valeric acids (refer Table 1), it appearsthe shorter chained (C3) propionic acid gaveslightly higher advancing and receding contactangles than the longer chained (C5) valeric acid. Itis possible that, as the valeric acid adsorbs, itimposes steric packing constraints resulting in alower adsorption density and thus lower contactangles than its shorter chain counterpart. Alterna-tively, valeric acid may adsorb in multilayers, thusreducing the contact angle.

The contact angles given by treating sapphiresubstrates with several other carboxylic acids wereinvestigated. Malonic (HOOCCH2COOH) andglutaric (HOOC(CH2)5COOH) acids were chosenfor investigation because they are ‘dioic’ acids andcould potentially bridge across adjacent aluminasites producing a hydrocarbon like surface layer.Malonic acid treatment was found not to producevery hydrophobic surfaces (30–35°) even in con-centrated solutions (1 M). This may be becauseeven if both carboxyl groups on the malonic acidmolecule react with the alumina surface the resul-tant ester would have two planar carbonyl groupsparallel to the surface per adsorbate. This wouldgive twice as many hydrophilic groups per hydro-phobic group across the substrate surface.


Alternatively, if only one carboxylate groupwas to adsorb, and the other prevented fromreacting, then the resulting surface layer wouldappear more hydrophilic. However, this possibil-ity is unlikely given the advancing contact anglewas found to be around 30–35°. In comparison,the longer chained glutaric acid was thoughtcould react with the alumina surface producing ahydrocarbon-like bridged surface layer with thetwo carbonyls buried beneath the hydrophobicsurface layer. In 10−1 M glutaric acid the advanc-ing angle was found to be approximately 30°,which suggests that only one carboxyl group permolecule reacted with the a-alumina surface, pos-sibly as a result of steric repulsion and conforma-tional strain constraints, preventing the secondcarboxyl group from reacting.

Thioglycolic acid (thiol acetic acid) was investi-gated as it was thought the thiol substituentwould impose an increased hydrophobicity. Sap-phire substrates treated with 1 M thioglycolic acidwere found to have an equilibrium advancingangle of about 70°. This angle remained the sameeven after immersion in 0.5 M HCl for 24 h withgentle heating thus suggesting the ester remainedresistant to hydrolysis. Substrates placed into neat95% thioglycolic acid were found to have signifi-cantly lower advancing contact angles (less than5°), suggesting a multilayer adsorption. Alu-minium substrates treated in a 10−4 M thiogly-colic acid were found to have equilibriumadvancing and receding contact angles of 40 and3°, respectively. Substrates treated in 1 M thiogly-colic acid gave similar contact angles to thatobserved for sapphire. Untreated aluminium sub-strates were silver and shiny in appearance butafter treatment appeared to have iridescent purpleand green patches on their surfaces. A similareffect was also found to occur with the adsorptionand esterification of methacrylic acid. This effectcould perhaps be due to the formation of rela-tively thick films producing interference patternswhich could also account for the extremely lowcontact angles observed. The inertness of theseadsorbed multilayers were examined by soakinginto diethyl ether, followed by sequential rinsingwith distilled water, ethanol, water, ethanol andthen drying under nitrogen. Even after this proce-

dure, the coloured patches remained unchanged,suggesting the esterified thioglycolic acid multilay-ers were robust and inert.

A contact angle investigation was carried out tofind whether an acrylic acid would esterify to thealumina surface and whether the unsaturateddouble bond would open up and bridge acrossadjacent groups producing a robust, high contactangle hydrophobic surface. Alumina surfacestreated with 10−4 M methacrylic acid for 1 h theadvancing and receding contact angles found tobe 30 and 5°, respectively. This procedure wasrepeated with 0.1 M methacrylic acid and theadvancing contact angle found to be between 30and 40°. The substrate was placed back into solu-tion and gently warmed to 77°C for approxi-mately 0.5 h, it was then removed from solution,dried and contact angle determined. The equi-librium contact angle was found to be signifi-cantly lower and hydrophilic (B5°). Thissuggested the gentle warming in the presence ofmethacrylic acid had either hydrolysed the esteror had polymerised adsorbed multilayers at thealuminium surface rendering it hydrophilic. Thetreated substrate was placed under the microscopeand as observed for thioglycolic acid, green andpurple patches appeared evident, again consistentwith the adsorption of relatively thick multi-layers(probably several microns).

The hydrophobic properties of aromatic substi-tuted carboxylic acids were tested by adsorptionof mandelic acid (a-hydroxy phenylacetic acid)and a-toluic acid (phenylacetic acid). Substratestreated in 1 M mandelic acid gave equilibriumadvancing and receding angles of approximately75 and 45°, respectively. Similar advancing andreceding angles were found at 10−1 M, whereasthe advancing angle at 10−2 M was slightly lowerat 70°. Even though the absorbate contained anaryl group the contact angles did not indicate ahighly hydrophobic surface and this is possiblydue to the presence of a hydroxyl moiety. Sub-strates treated in (1.5%) phenyl acetic acid solu-tion gave equilibrium advancing and recedingangles of approximately 75 and 2°, respectively.When substrates were heated in the phenylaceticacid solution containing dilute hydrochloric acidthe advancing angle decreased to 15°. It appears


that acid hydrolysis of the coating occurred onheating in aqueous acid solution.

The advancing and receding contact anglesmeasured for the per-fluorinated carboxylic acidsadsorbed to sapphire from aqueous and non-aqueous solution at 60°C are summarised in Ta-bles 2 and 3. The adsorption concentrations werechosen so as to obtain monolayer coverage, fol-lowing the adsorption isotherms measured by Laiet al. [28]. In general, the contact angle increasedas the chain length increased in agreement withthe overall trends observed by Zisman [29]. Themajor difference between adsorbing from aqueousand non-aqueous solution occurred in the stabilityof the adsorbed layers to dissolution by the testliquid (water). The results show that the contactangle was significantly reduced after 24 h of con-tact for the per-fluorinated carboxylic acids oflength n=8, 10 and 12 adsorbed from aqueoussolution. However, the contact angle for theshortest (n=3) and the longest (n=14) acid re-mained constant indicating no dissolution hadoccurred. A possible explanation is that the sur-face esterification reaction may not have pro-ceeded sufficiently towards formation of thesurface ester present for the intermediate chainlengths, given the large excess of water. In thiscase the monolayers would appear to have beenphysisorbed due to acid–base interactions andtherefore be susceptible to dissolution. Fig. 2 indi-cates that the degree of dissolution may be linkedto relative solubility of the acid homologues. Theshorter chained homologues dissolved to a largerextent than the longer ones, with the exception ofper-fluoropropionic acid which showed no sign ofdissolution at all. Adsorption of carboxylic acids

has previously been noted to be strongest whenthe solution pH is close to the pKa of the acids[30]. The resulting pH of the per-fluorinated car-boxylic acid solutions fell in the pH range 2–3. Asuccessful surface esterification may thereforehave been favoured for propionic acid given thatit had the lowest pKa in the homologue series,closest to the solution pH. Similar trends, in termsof contact angle and adsorbed layer stability, wereobserved upon performing the aqueous adsorp-tion at ambient temperature.

The stability of the layers adsorbed from non-aqueous solution was dependent on temperature.If the adsorption was performed at ambient tem-perature, the layers could be dissolved from thesurface upon immersion in water. The degree towhich dissolution occurred, as indicated by thechange in contact angle after periods of soakingin pure water, was not reproducible and no cleartrend was apparent. However, the per-fluorinatedmonolayers were stable for all of the homologuestested when the adsorption was performed fromthe non-aqueous solvent at higher temperatures(60°C). No significant difference in the contactangle was observed when measured after immers-ing the substrates in pure water for periods of 24h in this case as shown in Fig. 2 (compare emptyand filled squares). The non-aqueous conditionsappeared to provide a more suitable environmentfor surface esterification than the aqueous mix-tures. Since the water produced in the reactionwas not miscible in 1,1,2-trichlorotrifluoroethane,the equilibrium of the surface esterification wasdriven towards the formation of the surface esterupon the removal of condensed water. In order toobtain reliable surface esterification in non-

Table 2Contact angles for fluorocarbon carboxylic acids adsorbed from aqueous solution onto sapphire (at 60°C)

Advancing angle (°)No. carbons Receding angle (°)

InitialAfter 24 hInitial After 24 h

35 283 77735360 25938

97 5910 58 2232747612 107

118 115 81 7614


Table 3Contact angles for fluorocarbon carboxylic acids adsorbed from non-aqueous solution onto sapphire (at 60°C)

No. carbons Receding angle (°)Advancing angle (°)

After 24 h InitialInitial After 24 h

3 83 78 38 39858 84 41 36

90 3888 42109012 88 49 46

10514 75104 72

aqueous conditions, elevated temperatures werenecessary nevertheless.

3.2. Adsorption from solution

The results obtained for the adsorption of ace-tic and propionic acids are summarised in Table4. Given the nature of the chemisorption processthe data was examined using a simple Langmuiradsorption model. Three main assumptions weremade in this model, firstly, it is assumed that alladsorbed species interact only with the adsorptionsites on the adsorbent and not with each other[31], secondly, all adsorption sites are equivalentand thirdly the adsorption is limited to only amonolayer.

The Langmuir isotherm adequately describesthis adsorption from solution, knowing (C) theequilibrium acid concentration and (N) the num-ber of moles of acid per gram of adsorbent(alumina):

CN

=C

Nm

+1

KNm

(1)

where Nm is the number of adsorption sites (inmoles) per gram of adsorbate and K the equi-librium constant.

The number of moles of acid adsorbed pergram of alumina (N) was plotted against theequilibrium acid concentration (C), for acetic andpropionic acids (refer Fig. 3). In this analysis it isassumed that both the ionised and the unionisedacid can adsorb equally well and so the totalmolar concentration was used in Eq. (1).

Over the concentration range studied most ofthe acid in solution is in the unionised form. The

observed adsorption increased with increasing ad-sorbate concentration. An estimate of Nm wasobtained by plotting C/N against C. When C/Nwas plotted against C, straight lines (refer Fig. 4)having a correlation coefficient of 0.97 and 0.99were obtained for acetic acid and propionic acid,respectively. This high correlation suggested theLangmuir model provided an adequate descrip-tion of the adsorption process. Knowing the slopeof these lines (1/Nm) and the y intercepts (1/KNm),allowed Nm the maximum number of adsorptionsites per gram of adsorbate and the equilibriumconstant K to be estimated (summarised in Table4).

The ion adsorption behaviour of metal oxidesarises from the acid–base surface hydroxylgroups. The protonation and deprotonation ofthese surface hydroxyl groups causes the oxidesurface to develop an electrical charge promotingadsorption [32]. Tamura et al. [32] modelled theion exchange reaction on metal oxides (MnO2,TiO2, Fe3O4 and Al2O3), considering the acid andbase hydroxyl sites on the metal as independentspecies located in different layers [33–35] (referFig. 5). This surface adsorption is affected bylateral interactions between species localised in thesolid/solution interphase, geometric interactionssuch as the steric hindrance due to the size of theadsorbing ion at the hydroxyl surface sites [32]and by the local charge on the surface ion pairs.

The cation and anion exchange takes place atthe acid (–OH(a)) and base (–OH(b)) hydroxylsites on the metal oxide surface as shown in Fig.5. The acid and base surface hydroxyls are equalin number and considered as independent species.The anion and cation exchange reactions at theacid and base surface hydroxyl sites are given by:


OH(a)+M+l –O− –M++H+

–OH(b)+H2O+X−l –OH2+–X− +OH−

The radius of a hydroxide ion is 1.45 A, andmaximum density of hydroxyl groups in onelayer with closest packing for metal oxides hasbeen reported to be 1.95×10−5 mol m−2 corre-sponding to a site area of 8.5 A, 2. For aluminathe surface hydroxyl site area varies in the liter-ature depending on the method used but is typi-cally around 10 A, 2 [32].

If the area of the adsorbed molecule (s) onthe adsorbent surface is known (in m2 mol−1),then the specific area (A) of the adsorbent (inm2 g−1) is easily estimated assuming the adsor-bate forms a packed monolayer.

A=sNm (2)

The alumina powder used in these investiga-tions had a particle size in the micron sizerange, was found to be amorphous from X-raydiffraction analysis and according to the sup-plier (ICN) had a specific area of 150 m2 g−1.

In order for this powder (i.e. large particle size)to have such a high specific area the samplemust be quite porous. Assuming the area peradsorbed carboxylate is 21 A, 2 [35] then the ac-tual experimentally determined specific areawould only correspond to 7892 m2 g−1. Thesupplied value was most likely based on a nitro-gen adsorption isotherm and would include ac-cess to the fine pores on the alumina surface.The lower specific adsorption values obtainedfor carboxylic acid adsorption from solution, areconsistent with reduced access to these finepores. However, a possible alternative interpre-tation is that both acetic and propionic acidsadsorb on surface sites with an area per site ofabout 40 A, 2, which indicates that carboxylicacid adsorption forms half a monolayer on alu-mina (as illustrated in Fig. 6). This interpreta-tion is consistent with surface forcemeasurements (see later), which show that thesurface charge continued to increase via proto-nation of free aluminium hydroxide sites duringthe acid adsorption and esterification process.

Fig. 2. Advancing contact angles of water on monolayers of perfluorinated acids adsorbed from aqueous solutions (circles) and from1,1,2-trichlorotrifluoroethane (squares) at 60°C. Measurements were taken immediately after adsorption (solid symbols) and afterimmersing the substrates in pure water (at ambient temperature) for 24 h as a check of the adsorbed layers resistance to dissolution.


Table 4Values of Nm and K for acetic and propionic acids estimated from adsorption isotherms (Fig. 3)

1/KNm (g l−1) Nm (mol g−1)Acid K (l mol−1)1/Nm (g mol−1)

36.61Acetic 6.068×10−41648 45.0225.36 6.402×10−41562 61.59Propionic

3.3. Colloid probe atomic force microscopy

Typical forces measured between hydrophilic,clean sapphire flats and oxidised aluminium col-loid probes immersed in distilled water are givenin Fig. 7. The forces correspond to those expectedfrom DLVO theory for a surface potential of+18 mV, a surface charge density of 2.2×10−4

C m−2 which corresponded to an area/charge of740 nm2 e−1 and a Debye length of 60 nm. TheHamaker constant was assumed to be 5.3×10−20

J (and for the theoretical curves in Figs. 8 and 9).The observed forces appear to fall between thelimits of constant charge (upper line) and constantpotential (lower line). The forces also indicate thatthe surfaces are pulled into a primary minimumby strong van der Waals forces at short separa-tion distances with no evidence of any additionalshort range repulsive force. It should be notedthat surface force measurements do not identifythe sign of the surface charge for symmetricalsurfaces in symmetrical electrolytes (i.e. Z :Z elec-trolytes), however, at the pH of the water used inthese studies (5.7, due to dissolved CO2) the alu-mina surfaces will be positively charged, since thispH value is below the isoelectric point of about 7[35].

The addition of mM propionic acid (pH 3.7)reduced the double layer repulsion and led toattractive forces down to separations of about2–3 nm (see Fig. 8). The forces measured werestill more repulsive than those expected from vander Waals forces suggesting that there is still asignificant double-layer repulsive force. The ob-served forces were fitted with a constant surfacecharge (upper solid line) of 2.6×10−4 C m−2,which corresponds to an area/charge of 610 nm2

e−1, a constant surface potential (lower solid line)of +8 mV, and a Debye length of 22 nm. Theshort-range repulsive force observed at separa-

tions less than about 2–3 nm could be due tosurface hydration, surface swelling (i.e. the forma-tion of a surface gel layer) or surface roughness.

Increasing the propionic acid concentration to10 mM (pH 3.2) has the effect of increasing thestrength of the repulsive forces associated with adeveloping surface charge (Fig. 9). The observedforces were fitted with a constant surface charge(upper solid line) of 0.76×10−3 C m−2, whichcorresponds to an area/charge of 210 nm2 e−1, aconstant surface potential (lower solid line) of+14 mV, and a Debye length of 13 nm. As thepH was reduced, it appears the initial positive

Fig. 3. Adsorption of acetic and propionic acids to aluminashown as the number of moles of acid adsorbed per gram ofalumina vs. the equilibrium acid concentration.


Fig. 4. The equilibrium free acid normalised by the number ofmoles of acid adsorbed per gram of alumina plotted againstthe equilibrium concentration.

Fig. 5. Acid and base hydroxyl sites on a metal oxide andion-exchange reactions at the oxide/solution interphase:shaded circle, oxide ions; filled circle, metal ions; (a) acidhydroxyl sites; (b) base hydroxyl sites (redrawn from Ref.[30]).

Fluorinated carboxylic acids produced morehydrophobic surfaces (uA=105° for n=14) onalumina than their hydrocarbon counter-parts.The hydrophobic surfaces produced from adsorp-tion in aqueous solution were not always found tocharge on the alumina surface in water gradually

increased as more surface sites becameprotonated.

4. Conclusions

Hydrocarbon carboxylic acids were found toadsorb strongly onto aluminium and sapphire(a-alumina) substrates producing contact anglesup to 70°, which were found to be stable tohydrolysis. The contact angle of the dioic acids(e.g. malonic, glutaric) were much lower thanobserved for the monosubstituted acids (e.g. pro-pionic, valeric). This was probably because withthe monosubstituted acids, the carboxylate isbound directly to the alumina surface beneath ahydrocarbon surface layer, whereas, with the dioicacids it appears only one carboxylate was boundto the surface whereas the other carboxyl or car-bonyl is free to undergo hydrogen bonding.

Fig. 6. Schematic showing a partial monolayer adsorption ofcarboxylic acids to the alumina surface (modified and redrawnfrom Ref. [30]).


Fig. 7. Forces measured between a 20 mm aluminium colloidprobe against a sapphire (a-alumina) substrate in distilledsilica free water (pH 5.6). The observed force (F) was scaledwith the radius of the probe as a function of separationdistance fitted using a numerical solution to the Poisson–Boltzmann equation for a 1:1 electrolyte using the method ofChan et al. [14] with a Hamaker constant (Al2O3/H2O/Al2O3)of 5.3×10−20 J. The observed forces were totally repulsivewhich were fitted with a constant surface charge (upper solidline) 2.2×10−4 C m−2, a constant surface potential (lowersolid line) of +18 mV, and a Debye length (k−1) of 60 nmwhich corresponded to a background electrolyte of about3×10−5 M.

DLVO theory. The surfaces were found to pulltogether into a primary minimum by strong vander Waals forces at short distances with no evi-dence of any short range repulsion. However, atmM propionic acid the repulsion was reduced andan attractive force was observed down to separa-tions of about 2–3 nm. The force was morerepulsive than expected from van der Waals forcesalone, suggesting the presence of a significantdouble-layer repulsion. The short ranged repul-sion observed at distances less than 2 nm could bedue to surface hydration, surface swelling or sur-face roughness. As the concentration of propionicacid was increased to 10 mM the forces becamemore repulsive, suggesting that the surface haddeveloped a higher surface charge. This was most

Fig. 8. Forces measured between a 20 mm aluminium colloidprobe against a sapphire (a-alumina) substrate in 1×10−3 Mpropionic acid (pH 3.7). The observed force were fitted with aconstant surface charge of 2.6×10−4 C m−2, which corre-sponded to an area/charge of 610 nm2 e−1, a constant surfacepotential of +8 mV, and a Debye length (k−1) of 22 nm.

be stable to dissolution, however, their stabilitywas greatly improved when the chemisorption wascarried out in a non-aqueous solvent at elevatedtemperatures.

Hydrocarbon carboxylic acid adsorptionisotherms were found to be adequately describedusing the Langmuir model. Both acids studiedproduced adsorbed layers of about 0.5 of a fullypacked monolayer.

Interaction forces between alumina substrateswere measured using colloid probe AFM as afunction of propionic acid concentration. In waterthe forces between alumina surfaces agreed with


Fig. 9. Forces measured between a 20 mm aluminium colloidprobe against a sapphire (a-alumina) substrate in 1×10−2 Mpropionic acid (pH 3.2). The observed force were fitted with aconstant surface charge 0.76×10−3 C m−2, which corre-sponded to an area/charge of 210 nm2 e−1, a constant surfacepotential of +14 mV, and a Debye length (k−1) of 13 nm.

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likely due to a pH effect, that is as the propionicconcentration is increased residual aluminium hy-droxide surface sites protonate, thus generating astronger positive charge. The forces observed con-formed with DLVO theory, no hydrophobic at-traction was observed.

By studying a wide variety of fluorinated andhydrocarbon carboxylic acids of differing func-tionality, stable surfaces with different hydropho-bicities were obtained. These stable substratesmay be useful for the investigation of the hydro-phobic interaction using direct forcemeasurements.

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