Journal of Colloid and Interface Science235,227–234 (2001)doi:10.1006/jcis.2000.7367, available online at http://www.idealibrary.com on

Physicochemical Investigations on the Interaction of Surfactantsand Salts with Polyvinylpyrrolidone in Aqueous Medium

P. R. Majhi,∗ S. P. Moulik,∗,1 Susan E. Burke,† Marianne Rodgers,† and R. Palepu†,1∗Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700032, India; and†Department of Chemistry,

St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5

Received September 23, 1999; accepted December 4, 2000

Microcalorimetric investigations have been carried out onthe interaction of the surfactants sodium cholate, sodium deoxy-cholate, tetradecyltrimethylammonium bromide, cetyl(hexadecyl)trimethylammonium bromide, and p-tert-octylphenoxy polyoxy-ethylene ether (Triton X-100) and the salts potassium iodide,sodium benzoate, sodium bromide, and sodium salicylate with theneutral polymer polyvinylpyrrolidone (PVP). The enthalpy of di-lution of the surfactants has been measured in the absence andpresence of the polymer and the results are compared to deter-mine the effect of PVP on the micellization of the surfactantsand the energetics of the process. As well, the micellization ac-tivity of the surfactants in the presence of the polymer has beenstudied by conductometric and fluorimetric methods. The enthalpyof dilution of the salts has been measured to provide an under-standing of the nature and magnitude of their interaction withPVP. C© 2001 Academic Press

Key Words: microcalorimetry; thermodynamics; micelles; poly-mers; fluorimetry; surfactants.

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INTRODUCTION

Surfactant–polymer interactions are important in such areaoil recovery, colloid stability, surface modification, wetting, athe physiological transport and metabolism of lipids and thinteraction with proteins (1–8). Polyvinylpyrrolidone (PVP),potential substitute for serum albumin in the blood, is knoto interact with anionic surfactants, particularly sodium docyl sulfate (SDS). More extensively, the interaction of aniosurfactants, including SDS, with neutral polymers such as Ppolyethylene oxide, polyoxyethylene glycol, polyoxypropyleglycol, polyvinyl alcohol, poly(1,3-dioxalane), polyacrylamidmethylcellulose, and ethyl hydroxyethyl cellulose can be foin the literature (1, 7, 9–14). In contrast, the interactioncationic surfactants with neutral polymers, expected to be mless than that with anionic surfactants, has received little attion. Weak interactions of cationic surfactants with hydropbic polymers, viz. polypropylene oxide, polyvinyl methyl ethand hydroxypropyl cellulose, have been reported (19–21). A

1 To whom correspondence may be addressed. E-mail: spmcssju@vsnl.or rpalepu@stfx.ca.

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there is a recent report that cetylpyridinium chloride can foa second micelle along with the normal one by interacting whigh-molecular-weight PVP and polyethylene glycol (PE(18). Tetradecylpyridinium bromide has been reported to inact with PVP at a pH of 11.3 by way of deprotonation of Pin the alkaline medium (22).

It has been shown in the literature (14) that the presence ofhas considerable influence on ionic surfactant–neutral polyinteractions. The iodide ion can specifically bind to the ptein serum albumin under the isoelectric condition. Ion bindto biopolymers results in the reorganization of the functiogroups constituting the structure of the macromolecules25). Neutral carbohydrate molecules (glucose, sucrose, maamylose, etc.) are also known to interact with salts (24, 25)

In surfactant–polymer studies, experimental methods succonductivity, viscosity, surface tension, dialysis, NMR, neutscattering, fluorescence, and ultrasonic absorption are mostmonly employed. As well, microcalorimetry has been succfully used in recent years to derive thermodynamic informaon surfactant interactions with macromolecules includingdrophobically modified neutral polymers (5, 15–17).

With this background, we have undertaken a detailed stusing conductometric, fluorometric, and microcalorimemethods, of the interaction of several anionic and cationicfactants (sodium cholate, sodium deoxycholate, cetyltrimetammonium bromide, and tetradecyltrimethylammonium bmide), as well as a nonionic surfactant (p-tert-octylphenoxypolyoxyethylene (9.5) ether or Triton X-100), and salts (NaKI, sodium salicylate, and sodium benzoate) with a modately high-molecular-weight PVP in aqueous medium. We hrecently studied the interaction of the salts with PVPing conductometric and surface tension methods (7). In ation, we have investigated to what extent the enthalpy oflution of the surfactant and the salt solutions is affectedthe presence of the PVP in order to obtain informationthe nature of the interaction they undergo with the polymSupporting data have been obtained from the conductbehavior of the surfactants and the fluorescence behavipyrene and pyrene-l-carboxyaldehyde (PCA) during the mlization of the surfactants in the absence and presence opolymer.

0021-9797/01 $35.00Copyright C© 2001 by Academic Press

All rights of reproduction in any form reserved.

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228 MAJHI

MATERIALS AND METHODS

Materials

All the surfactants used (sodium cholate (NaC), sodideoxycholate (NaDC), cetyltrimethylammonium bromi(CTAB), tetradecyltrimethylammonium bromide (TTAB), anp-tert-octylphenoxy polyoxyethylene ether (Triton X-100were the same as those used earlier (26). The salts NKI, sodium salicylate (NaSl), and sodium benzoate (NaBwere A.R.-grade compounds from SRL (India). The polympolyvinylpyrrolidone (mol wt 40,000) was a product of Sigm(USA). The weight-average molecular weight of the sampletermined by us using the SLS (static light scattering) method (was 38,000. The polydispersity of the sample was not availa

The fluorescent probe pyrene-1-carboxyaldehyde frAldrich was recrystallized several times from ethanol. Tpyrene (99% optical grade) from Aldrich was purified by rpeated recrystallization, followed by sublimation.

The measurements with NaC and NaDC were taken in phphate buffer solution at pH 8.2. The strength of the counterNa+ in the buffer medium was 0.1 M. The water used in tsample preparation in this study was doubly distilled.

Microcalorimetric Measurements

Microcalorimetric measurements were made using an ITCstrument from Microcal (Northampton, MA) that can record hflow as low as 10µcal. For the titration experiment, 1.325 mof PVP solution of the desired concentration was placed incalorimeter cell, into which temperature-equilibrated aquesurfactant or salt solution to the maximum extent of 200µLwas successively injected for a preset number of additionreported earlier (26–29). The allowed time interval betweensuccessive injections was 4 min. The heat flow per injecand the corresponding enthalpy change per mole of the injecwere obtained using Microcal Origin software. Measuremefollowing this procedure were performed at several PVP ccentrations.

All measurements were carried out at a temperature of 30±0.001 K.

Conductivity Measurements

The conductivity measurements were performed with a CD83 bridge (Radiometer, Copenhagen) operating at 1000 Ha cell with a cell constant of 0.978 cm−1. In this procedure,10 mL of solvent (water or aqueous PVP solution) was plain a thermostated jacketed beaker at 298 K and surfactant stion of known concentration was progressively added usingautomatic Brinkman 250 burette. The specific conductancemeasured after each addition. The CMC values were estimfrom the break point in the conductance–concentration plot

Fluorescence Measurements

Fluorescence measurements were carried out to determthe CMC and the polarity of the water–micelle interface. Tw

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fluorescent probes, namely pyrene and PCA, were emploin this study. The pyrene emission spectrum is a well-knotool used in the determination of micropolarity. The polaritythe microenvironment is monitored using the pyrene emissspectrum, specifically the ratio,I1/I3, of the relative intensitiesof the (0, 0) and (0, 3) bands. The ratio decreases sharply aonset of micellization, as a result of the preferential resideof pyrene in nonpolar regions of the surfactant micelles (3Thus, the CMC values can be obtained from the plots ofI1/I3

as a function of surfactant concentration.The pyrene fluorescence measurements were made us

Perkin–Elmer MPF 66 fluorescence spectrophotometer inrange of 360 to 500 nm after the probe was excited at 340The excitation and emission slit widths were maintained atand 2.0 nm, respectively.

The fluorescence studies with PCA were performed inAminco Bowman Series 2 luminescence spectrometer by eing the sample at 356 nm and recording the emission specin the range of 400–580 nm. The slit widths were 4 nm for bexcitation and emission spectra. This fluorophore is solubiliat the micellar interface and the maximum of the emission sptrum is red-shifted when the solvent polarity is increased. Thfrom the emission intensity, the CMC and an estimate of theelectric constant of the micellar interface can be obtained (In the study the fluorescence of the PCA has been measurobtain the CMC of the surfactants.

The preparation of the solution with the probe for the florescence measurements was carried out as follows. Initan appropriate volume of solution of the probe in ethanol wevaporated in a volumetric flask to form a thin film of the probThe solution of the polymer was then added to the evaporresidue and stirred overnight. The resulting solution containg the probe at a concentration of∼1× 10−6 M was used asa solvent for the preparation of solutions containing differconcentrations of the surfactant. All solutions were stirred foleast 2 to 3 h for complete homogenization.

The fluorometric measurements were made at 298 K.

RESULTS AND DISCUSSION

Behavior of Surfactants

The heat of dilution measurements were performed onsurfactants NaC, NaDC, CTAB, TTAB, and TX-100 in the preence of different concentrations of PVP. The data for NaCTTAB are shown in Figs. 1 and 2, respectively, along withdifferential plots of the enthalpy–concentration profiles. In pA of these figures, the CMC point and the mode of estimatof the standard enthalpy change for the micellization proc(1H o

m) are also shown. The method of data analysis andthermogram representations for micelle formation employmicrocalorimetry are well documented in the recent literatby us (26–29) and by others (32, 33). Experimentally, a c

ineocentrated (above CMC) solution of a surfactant was added suc-cessively to water or to an aqueous PVP solution and the heat

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FIG. 1. Microcalorimetric results on NaDC–PVP system at 303 K.(A) Dpendence of the enthalpy of dilution on [NaC] at three [PVP] as shown ininset. CMC point and estimation of1H◦m are indicated by arrowheads.(B) Arepresentative derivative plot of the enthalpy data in aqueous medium focorrect location of the CMC as shown in (A).

flow was determined. Initially, the surfactant solution was extsively diluted and demicellized and dilution of the monomerscurred. The resultant heat change was detected in the caloter. Subsequently, at higher solution concentrations, close toCMC, demicellization did not take place, and the micelles obecame diluted with the associated monomers. The enthchange tended to level off. The difference between the hethe peak point and the start of the leveling tendency is accoufor by the standard enthalpy change of the micellization (1H o

m)process (26, 28, 32, 33). A sharp difference in the heat chais therefore observed and this appears prominently in the diential plot. In the presence of PVP, a heat component relatethe interaction of the surfactant and the PVP affects the proand this is reflected in the changing pattern of the enthalconcentration profile.

It was observed that while the initial heat changes for the dtion of NaC and NaDC are variable, those for CTAB, TTAB, aTX-100 vary little. This reflects the way in which the micelladilution, the demicellization, and the dilution of the monome

contribute to the total heat of the overall dilution process. In tformer (NaC and NaDC), the overall process is endothermic

LPYRROLIDONE INTERACTION 229

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both the absence and the presence of PVP but the magnof the enthalpy increases with increasing concentration ofpolymer. In the cases of CTAB and TTAB, although the intial increment in the enthalpy is small, the magnitude increawith increasing PVP concentration. An initial increment in ethalpy in the presence of PVP has also been observed for TX-

Examination of the curves in Fig. 1 reveals that the CMCnot affected by the presence of polymer, but the enthalpy chafor the process increases with increasing PVP concentrationTTAB (Fig. 2), a minor increase in the CMC has been observThe CMC of SDS has been reported by a number of woers (1, 11–14, 17) to increase with the concentration of Pand other interacting neutral polymers, and a critical aggretion concentration, CAC (for a polymer-induced aggregatioappears at a concentration well below the CMC. The presanionic surfactants, NaC and NaDC, show a marked contrathis behavior. However, it has been reported (15) that hydropbically modified polyacrylamide does not affect the CMCcyclic (mono)n-dodecyl sodium phosphate but does changeenthalpy of micellization. In these systems, it would appear tthe surfactant and polymer are not interacting directly, but t

FIG. 2. Microcalorimetric results on TTAB–PVP system at 303 K(A) Dependence of the enthalpy of dilution on [TTAB] at two [PVP] as showin the inset. CMC point and estimation of1H o

m are indicated by arrow heads

hein(B) A representative derivative plot of the enthalpy data in aqueous medium forthe correct location of the CMC as shown in section (A).

230 MAJHI ET AL.

TABLE 1Thermodynamic Parameters for the Micellization of NaC, NaDC, CTAB, TTAB, and Triton X-100 in the Presence of PVP at 303 K

Surfactanta PVP (wt%) 103×CMC −1Gom(kJ mol−1) −1Ho

m(kJ mol−1) 1Som(JK−1 mol−1)

NaC 0 0.32± 0.08 20.3± 0.69 1.0± 0.03 63.5± 2.180.5 0.32± 0.08 20.3± 0.69 1.1± 0.04 63.4± 2.151.5 0.32± 0.08 20.3± 0.69 1.5± 0.04 62.0± 2.152.5 0.32± 0.08 20.3± 0.69 1.8± 0.05 61.0± 2.11

NaDC 0 0.14± 0.033 22.3± 0.50 2.4± 0.04 65.6± 1.520.5 0.14± 0.033 22.3± 0.50 2.8± 0.04 64.3± 1.521.5 0.14± 0.033 22.3± 0.50 3.2± 0.05 62.9± 1.492.5 0.14± 0.033 22.3± 0.50 4.2± 0.05 59.5± 1.49

CTAB 0 0.019± 0.006 27.4± 0.70 14.3± 0.28 43.1± 1.391.5 0.019± 0.006 27.4± 0.70 17.0± 0.30 34.2± 1.32

TTAB 0 0.072± 0.022 24.0± 0.60 8.5± 0.20 51.2± 1.321.5 0.072± 0.022 24.0± 0.60 10.4± 0.22 44.9± 1.252.5 0.072± 0.022 24.0± 0.60 11.2± 0.20 42.3± 1.32

TX-100 0 0.0054± 0.0016 30.6± 0.45 −7.3± 0.18 105.7± 1.001.5 0.0054± 0.0016 30.6± 0.45 −5.0± 0.20 98.3± 1.10

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the endothermic enthalpy variation is caused by the competof surfactant and polymer for water molecules to satisfy theirdividual hydration requirements. Although not included in thstudy, microcalorimetric measurements at higher (alkaline)might prove interesting to verify whether PVP can interact wcationic surfactants by way of ionization of PVP at higher p(22).

The polyacrylamide polymer (PAA, mol wt 8000) has bereported not to affect the micellization nor the energetics ofprocess for dodecyltrimethylammonium bromide (DTAB) (1We have also not observed changes in CMC for CTAB and TTin PVP solution but have observed a change in the enthalpmicellization,1H o

m. Wanget al.(16) have observed that TTABdoes undergo interaction with PAA (mol wt∼200,000) as indi-cated by changes in both the CAC and the CMC, but the streof binding is lower than that of SDS.

For the calculation of the Gibbs energies of micellization,have used the equations

1Gom = (1+ f )RT ln XCMC [1]

d(1Go

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m, [2]

and (1H o

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m, [3]

where1Gom and1S o

m are the standard Gibbs energy change astandard entropy change of micellization, respectively;XCMC

is the mole fraction concentration at CMC;f is the counterionbinding of the micelles; andT is the absolute temperature o

the process. Thef values for NaC, NaDC, CTAB, and TTABused in the present calculation are 0.10, 0.18, 0.72, and 0

0.06; TX-100= 0.0054. (Ref. Moulik, S. P.,Current Science.71,368 (1996).

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respectively (34, 35), assuming for convenience thatf is notaffected in the PVP environment. This assumption may novalid for other water-soluble polymers (18).

It is seen from the results (Table 1), and illustrated in Fig.that1H o

m varies linearly with PVP concentration; the interceand slopes depend on the nature of the surfactant and the sficity of interaction of water molecules in competition with thpolymer. The1H o

m is thus affected by PVP and the changeactively reflected in the1S o

m. As −1Hom increases in magni

tude, the positive1S om for the micellization of the surfactan

decreases. In the presence of the polymer, the entropy-dprocess is constrained. In Fig. 4, this compensation effepresented with a fair correlation at a compensation temperaof 300 K vis-a-vis the experimental temperature of 303 K.may be mentioned here that according to Kevelamet al. (15)neither the CMC nor the1H o

m of dodecyltrimethylammoniumbromide has been found to be influenced by the presence ofbut hydrophobically modified PAA has shown a sizable effeThe absence of interaction between the cationic surfactanthe neutral polymer is often thought to be due to (i) the unvorable overlap of their hydration shells and (ii) the protectof the hydrophobic micellar interior by the bulky head groof the cationic surfactant. The hydrophobic interaction betwthe micelle and the polymer is thus reduced; for a favorableteraction, the polymer must be hydrophobically modified. Usmicrocalorimetric measurements, Wanget al.(16) have very re-cently reported that both hydrophobically unmodified and mified PAA can interact with both SDS and TTAB as indicatby both the CAC and the CMC, but the interaction with Sis stronger than that with TTAB. A good reason for the intaction observed by Wanget al. for TTAB could be that it ismore hydrophobic than DTAB. As well, the PAA they used

.73,of higher molecular weight (200,000) than that (8000) used byKevelamet al. (15). It is possible that DTAB may interact with

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SURFACTANT–SALT–POLYVINY

FIG. 3. Enthalpy of micellization in aqueous PVP medium and salt–Pinteraction. (A)1H o

m-[PVP] profiles for NaC, NaDC, CTAB, and TTAB a303 K. (B)1Ho

i -[PVP] profiles for NaSl, KI, NaBr, and NaBz at 303 K.

FIG. 4. Enthalpy–entropy compensation plot for the micellization of sufactants in aqueous PVP medium.

PYRROLIDONE INTERACTION 231

P

PAA at higher pH by deprotonation of the polymer in line witthe observation of Shirahamaet al. (22).

The microcalorimetric method shows that the CMC valuof the studied surfactants are on the whole independent (exminor increase for TTAB) of the presence of PVP although tenthalpy of micellization depends on PVP concentration. In viof the recent report (18) of two break points in the conductivplots for the cationic surfactant cetylpyridinium chloride (CPCin PVP and PEG of different molecular weights, and the oservation that PVP of molecular weight lower than 40,000 donot show the second break, we have carried out conductomric and fluorimetric studies to explore the interaction of NaDCTAB, and TTAB with PVP (mol wt 40,000). A representativconductance–concentration plot indicating two breaks is shoin Fig. 5. The first break is called the critical aggregation cocentration (CAC) and it is close to the conventional CMC ais attributed to the polymer free micelle formation. The secobreak may be attributed to the polymer-bound micellar agggates (Cp). Both these values decrease slightly with increasiPVP concentration. In the case of CTAB (graph not presentethe CAC value decreases slightly whereas theCp value remainsunchanged. The results are given in Table 2 along with the vues obtained by fluorescence. Graphical representations oresults obtained for the NaDC–PVP and TTAB–PVP systemsfluorimetry are given in Figs. 6 and 7, respectively. It is observthat NaDC exhibited only one break and the CMC increased wPVP concentration, contrary to microcalorimetric observatioThe magnitudes of the CMC obtained by the two methods weon average, in agreement with each other. Fluorimetry has ashown the possibility of two aggregation processes for TTAand CTAB (Table 3) in PVP medium. Experimental resul

r- FIG. 5. Conductance–concentration profiles for TTAB in aqueous PVPmedium at 298 K.

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232 MAJHI

TABLE 2CMC Values of Surfactants Obtained by the Methods

of Conductometry and Fluorimetry at 298 Kc

Conductance Fluorescence

TTAB CTAB CMC(/mM) TTAB CTABPVP (wt%) (cac)(Cp) (cac)(Cp) NaDC (cac)(Cp) cac(Cp)

0 3.9 0.90 6.0b 4.0b 0.90b

0.05 3.9 0.90 6.0b 4.0b 0.90b

0.10 (3.8)(5.4) (0.99)(1.2) 7.1b (5.0)(7.8)a (1.0)(2.1)a

0.50 (3.6)(4.3) (0.89)(1.2) 7.8b (5.8)(10.0)a (1.5)(3.2)a

(4.8)(9.0)b

1.0 (3.2)(4.2) (0.84)(1.2)

a With PCA.b With pyrene.c CMC values by microcalorimetry for NaC, NaDC, CTAB, and TTAB a

18.2, 8.0, 1.05, and 4.0 mM, respectively, at 303 K. The CMCs by conductomand fluorimetry are accurate within±10 and±8%, respectively.

obtained on TTAB using both pyrene and PCA as probes, shin Fig. 7, exhibit two distinct aggregation regions. Similar rsults are also obtained for the CTAB–PVP system (graphshown).

The above results support the reports of Garcia-Mateoset al.(18) finding two aggregation processes for the cationic surtant CPC in neutral polymer solutions depending on the moular size of the polymer. From the nature of the conductanconcentration plot, the degree of counterion dissociationthe aggregates associated with polymer is less than thein agreement with the above authors. This supports polyminduced ion binding to micelles by the favorable orientatiof the polymer segments around them. The microcalorimemeasurements on a salt–PVP system (presented in thesection) indicate a favorable interaction of ions with PVSince1CMC(=Cp− CAC) is lower than CMC, the proces

FIG. 6. Dependence ofI1/I3 of pyrene on [NaDC] in aqueous PVP mediumat 298 K.

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TABLE 3Standard Enthalpy of Interaction (∆H o

i ) of the Saltswith PVP at 303 K

−1Hoi (kJ mol−1)

PVP (wt%) KI KBr NaBz NaSl

0.25 — — — 0.50± 0.020.50 0.33± 0.01 0.21± 0.01 0.13± 0.01 1.17± 0.061.00 — — 0.32± 0.04 —1.50 — 0.73± 0.04 0.43± 0.02 2.67± 0.142.50 2.17± 0.11 0.99± 0.05 0.52± 0.03 4.03± 0.26

of polymer-induced aggregation of the amphiphiles is energecally favorable compared to the normal micellization processthe aqueous medium. In this respect, at a constant PVP contration, the process is more favorable for CTAB than for TTABCTAB with two extra methylene groups is more hydrophobthan TTAB.

The results indicate that both CAC andCp values for TTABand CTAB in the presence of PVP, as well as the CMCare mildly affected by the PVP concentration as measuredthe conductance method. The fluorescence method has demstrated dependence of theCp on PVP concentration whereasthe CAC remains almost unaffected. The CMC of the surfactaNaDC has shown a dependence on PVP concentration thasimilar to the microcalorimetric findings. It is considered probable that NaC would show parallel behavior if examined bconductance and fluorescence.

It has been observed in the fluorimetric measurements tthe I1/I3 ratio is always higher in the presence of PVP. Thsuggests that the presence of PVP in the solution enhancesmicropolarity at the binding site environment.

We refrain from thermodynamic quantification of the resulfor this secondary aggregation process until more data, includthe aggregation number and the size of the aggregates, ahand.

Behavior of Electrolytes

The binding of surfactant to polymer can be affected by tpresence of salt. Foxet. al. (17) have reported that NaBr cansignificantly decrease both the CAC and the CMC of SDSthe presence of PVP and a related copolymer. To studyphysico chemical behavior of the surfactant–PVP system,have investigated the enthalpy of dilution of the salts NaBKI, NaBz, and NaSl in the absence and presence of PVP. Tenthalpy of dilution results for KI and NaSI are depicted iFigs. 8 and 9. It is observed that the enthalpy of dilution (1Hd)is indeed affected by the presence of PVP, in the relative orNaBz<NaBr<KI <NaSl. We consider that the anions of thsalts are the effective interacting species and find that the intetion process is exothermic. In the absence of PVP, the enthaof dilution varies mildly with increasing salt concentration.

is initially slightly exothermic and then becomes slightly en-dothermic. The extrapolated enthalpy of dilution to zero salt

with

SURFACTANT–SALT–POLYVINYLPYRROLIDONE INTERACTION 233

FIG. 7. Dependence ofλmax of PCA andI1/I3 ratio of pyrene on [TTAB] in aqueous PVP medium at 298 K. (A) 0.1% PVP with PCA; (B) 0.5% PVPPCA; (C) 0.5% PVP with pyrene.

FIG. 8. Enthalpy of dilution of KI in aqueous PVP medium at 303 K. FIG. 9. Enthalpy of dilution of NaSl in aqueous PVP medium at 303 K.

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234 MAJHI

concentration for each PVP concentration can be taken to bstandard partial molar enthalpy of dilution (1Ho

d ) of the salt.The standard enthalpy of interaction of a salt with PVP,1Ho

i ,is then obtained from the relation

1H oi =

(1H o

d

)PVP−

(1H o

d

)water, [4]

where (1H od )PV P and (1H o

d )water represent the standard ethalpies of dilution of the salt in the presence and absencPVP, respectively.

The results are presented in Table 3 and their dependenwt% PVP is illustrated in Fig. 3B. It is seen in the figure thatdependence of1H o

i on PVP concentration is linear; the magntude of the slope gives a measure of the comparative strenginteraction. The order NaSl>KI >NaBr>NaBz is followed.The exothermicity of the1H o

i speaks in favor of bindingof the salt with the PVP as well as local organization of solventhe site of interaction. The different degrees of interaction ofanions Br−, I−, Bz−, and SI− presumably arise from the differences in their size and surface charge density. The salt binto PVP is thus distinct compared to the interaction of thefactants, where the micelle and the polymer segments comwith each other for hydration.

CONCLUSIONS

The microcalorimetric method demonstrates unchanCMC and increased enthalpy of micellization for NaC, NaDTX-100, TTAB, and CTAB in aqueous PVP medium, wherethe conductometric and fluorimetric methods indicatepresent of two types of aggregate formation for TTAB and CTin the presence of PVP, one close to that of the pure surfaand the other at a higher concentration. Both are slightly affeby the PVP concentration. The microcalorimetric techniqueunable to distinguish these two processes, perhaps due toenthalpy values or because they were not well separated fotection.

The PVP enhances the micropolarity at the pyrene bindsites in the micellar solution of the NaDC, TTAB, and CTAB

The salts KI, NaBr, NaSl, and NaBz interact with PVexothermally. The strength of interaction follows the ordNaSl>KI >NaBr>NaBz.

ACKNOWLEDGMENTS

The authors (P.M. and S.P.M.) acknowledge financial assistance from

Department of Science and Technology, New Delhi, India. R.P. is grateto NSERC for an Operating Research Grant and M.R. acknowledges a U

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Summer Research Fellowship from the St. F. X. University Council for Rsearch. We acknowledge Dr. J. P. Kratohvil (Editor) for his helpful suggestioWe also thank Mr. Louis Digout for his technical assistance.

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