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nJRNALOFCOLLOID AND INTERFACESCIENCE 157,14-18 (1993)

C. MALTESHANDP. SOMASUNDARAN* Langmuir Centerfor Colloids and Interfaces. Henry Krumb School of Mines. Columbia University. New York. New York 10027

ReceivedJune II, 1992; a()!:ep1edOctober 2, 1992 has been largely neglected.In an earlier study, using steady , Dynamic fluorescence spectroscopy of micelle-solubilized py- statefluorescence spectroscopy. the authors haveshown that rene was used to measure the size of the sodium dodecylsulfate the affinity of the cation toward polyethylene glycol (PEG) (SDS) aggregate bound to polyethylene glycol (PEG). The had a significant effect on its interaction with SOS: cations measurements were carried out with very dilute PEG and SDS such as Na + and Cs+ which bind strongly to PEO affect concentrations, to ensure that there were no free surfactant ag- gregates in solution and that all aggregatespresent were bound interactions between PEO and SOS in a manner distinct from Li + and Mg2+ which bind weakly to PEO (10). Dubin to the polymer. Effect of salts of sodium, cesium, lithium, and magnesium of varying concentrations on the size of the bound et af. studied the effect of micelle counterion on the inter- aggregatewas determined. The affinity of the cation toward PEG action between PEO and SOS using dye solubilization and controlled the size of the SDS aggregatebound to the polymer. dynamic light scattering(. 11). It was proposedthat the bind- The stronger the binding between the cation and PEG, the ing force between PEO and SDS is controlled. among other smaller was the bound SDS aggregate. There was no effect of factors, by the simultaneous affinity of the cations for the either the anion or the salt concentration for a particular cation micelle and association with the polymer. Effects of Li +, on the size of the bound SDS aggregate. As the ionic strength Na +, and NH: were studied and it was observed that increased, the only parameter affected was the number of bound NH: appearedto be more tightly bound to the SOS micelle surfactant aggregates per polymer chain. Results are discussed than the other counterions studied, yet had the least effect in terms of simultaneous affinities of the cations toward the polymer and the micelle. c>1993 Academic Press, loc. on the polymer-micelle binding. It was therefore suggested that the weak interaction of NH: with PEO was more im- portant than its strong binding to the micelle in determining INTRODUCTION its influence on polymer-micelle complexation. In the pres- ent work. the aggregatesize of SDS bound to PEG was de- Interactions between polyethylene oxide (PEO) and s0- termined by studying the fluorescencedecay of the micelle- dium dodecylsulfate (SDS) have been investigated in the solubilized fluorescenceprobe pyrene. Similar studies have past using diverse techniques ranging from the traditional been undertaken earlier (7-9) but were conducted at high surfacetension (I), conductivity (2), and solubilization (3) surfactant and polymer concentrations and in some casesat to the novel nuclear magnetic resonance (4, 5), neutron surfactant concentrations for both polymer bound micelles scattering(6), and fluorescencespectroscopy (7-9). The re- and free micelles to be present together in solution. The sults obtained for the binding isotherm of SDS to PEa, size numbers obtained were an averageof the two speciespresent , of the SDS aggregatebound to PEa, and conductivity and and hence not a true indication of the SOSaggregate bound dye solubilization of the PEO-SDS complex do not depict to PEO. In the present investigation, the size of the SDS a vivid image of the nature of the PEO-SDS complex, but aggregatebound to PEO ( or polyethyleneglycol) was deter- a majority of the theories point in the same direction. As mined under conditions of dilute polymer and surfactant more advanced and specialized techniques are applied to concentration where there were no unbound or free aggre- characterizethis system,a clearer picture of the interaction gates.Changes in the size of the aggregatesize as a function is beginning to emerge. Effects of several variables on the of different salts and salt concentrations were determined. interactionsbetween sodium dodecylsulfateand polyethylene oxide have been determined, but the role of salts has not MATERIALS AND METHODS received much attention. Specifically, the effect of binding of the cationsto polyethyleneoxide ( PEO) on the subsequent Materials interactions betweenPEa and sodium dodecylsulfate(SDS) Polyethyleneglycol (PEG) standardof weight average . To whom COne5JM)ndenceshould be addressed. molecularweight of 5000 (polydispersity\ .03) from Sci-

0021-9797193$5.00 14 Copyriaht e 1993 by Academic Press.Inc. AU riIJ1tsof rep~uction in any fonD reserved. SIZE OF SDS AGGREGATE BOUND TO PEO 15 entific Polymer Products was used as received. Sodium do- where (P] is the probe concentration, Cothe.tal surfactant decylsulfate (SDS ) of greater than 99% purity pun:based from concentration, CAC the critical aggregationsodium sulfate, sodium phosphate, cesium gation number of the SOSaggregate bound III"the polymer. chloride, lithium chloride, magnesium chloride. and mag- The solubility of pyrene in aqueous medil is extremely nesium sulfate were used as received. Pyrene crystals from low and for measurementsof the aggregate.cia the amount Aldrich Chemicals were recrystallized from ethanol of pyrene must be high enough to form exrimers. For. the dilute surfactant concentrations studied, the desiredpyrene Methods concentration was achieved using the pr~re described Time-resolved fluorescencespectra were recorded on a by Zana ( 13). Stock solutions of pyrene at mncentrations Photon Technology LS 100spectrophotometer according to in the range of 0.01 or 0.001 kmol/m3 werrfirst prepared the pulse method (12). The lamp used a 30:70 mixture of in ethanol. Weighed amounts of these solu1i>nswere then nitrogen:helium as the plasmagas. The following procedure transferred into vials for the desired conCe8trationof the j wasused to determinethe sizeor the bound aggregate.Pyrene probe. Ethanol was evaporatedby circulati~ nitrogen over monomer and excimer fluorescencekinetics have been ex- the solution. Fmally the flask was filled w-.tI die desired tensively studied in the literature. The formation of an ex- amounts of surfactant and polymer solutiol15md. the reac- cimer occurs when an excited state pyrene monomer (p.) tants dissolved using vortex stirring for 121b 15 h. Using forms a complex with a pyrene monomer in the ground state this technique all the pyrene was solubilizedmdthe desired (P). The photophysics ofpyrene monomer (Eq. I) and ex- P)'R'Deconcentrations were achieved. The fYTeneconcen- cimer formation (Eq. 2) can be representedas follows: tration was checked by measuring UV at.>rbance at 334 nm. To check whether all the pyrene was scWlbilizedin the hydrocarbon phase,steady state spectra were obtained for P + h" - p' - P + h" (I} some of the samples.From the ratio of the mtensitiesof the P + p' - (PP}c*.cimer- P + P + h... (2] first and third peaks, it was ensured that aD.e pyrene was in a hydrocarbon environment ( 15). Sinwater it was assumedt~ there was no the system as well as aggregatesize will affect the observed adsorption of the pyrene on the polyethyle.x glycol (PEG). decayprofiles ( 13). Under the conditions that the excitation The polymer concentration was kept fixed at 20 ppm for all pulse is narrow compared to the fluorescencelifetime and the experiments. that the probe moleculesare stationary in their host micelles during the time window measured,the monomer fluores- cenceintensity as a function of time is given by (14) RESULTS AND DISCUSSION I, = Ioexp[-km . t - n( - exP{kc-t})J., Interactions between polyethylene gly~ (PEG) and so- (3] dium dodecylsulfate (SDS) were investiga.m earlier using where It is the fluorescenceafter time t, 10is the fluorescence steady-statefluorescence spectroscopy of pyteneend-labeled intensity at time t = 0, n is the averagenumber of probes polyethyleneglycol (PyPEGPy) ( 10). Cha~ in the excimer formation of the pyrene-labeled polymer were used as an per aggregate,km is the rate constant of decayof an excited pyrene monomer, and ke is the excimer formation-dissocia- indication of interactions between the polymer and surfac- tion rate constant. At long times, the fluorescencedecay pro- tant. It was ensuredthat hydrophobic interxtions between files representthe decaydue to monomeric emission.There- the pyrene moiety and SDS were minimal and the changes fore Eq. [3] reducesto in the emission of pyrene were due to intemction between , the polymer and the surfactant. The ratio o(the excimer and monomer intensitieswas taken as a measureof polymer coi.l In (-1, )= -n-ko" t dimensionsand hencewas termed the coili~ index. A higher (41. 10 value of coiling index suggestspolymer coiling whereasa lower value suggestspolymer coil expansion.The schematic and extrapolation to I 0 givesn, the averagenumber of = behavior of the PyPEGPy in the presenceofSDS is shown probesin eachaggregate. For polymer-surfactantaggregation in Fig. 1. It was also observedthat the oveT:l11nature of the n is defined as interactions betweenPyPEGPy and SDS''-as not affectedby the salt type but the interactions occurred at lower SDS con- {Pl. N centrations as the ionic strength was increased.Details of n= JfL ~ [5] [Agg] Co- CAC the behavior of the polymer in the presenceof the surfactant 16 MAL TESH AND SOMASUNDARAN betweenSOS concentrations at the CAC and point B. This PyPEG?y:20 ppm was basedon the fact that in this region the surfacetension of the solutions remained constant. It can also be statedthat there are no free surfactant micelles at SOS concentrations in this region.Changes in the conformation of pyrene labeled polyethylene glycol (PyPEGPy) due to interaction with s0- dium dodecylsulfate(SOS) were determined in the presence of various saltsof sodium, cesium, lithium, and magnesium. Sodium dodecylsulfate(SOS) concentrations at CA C and B were noted and are listed in Table I along with the aggregate sizesdetermined at an SOS concentration at B. The size of the SOS aggregatebound to PEa is smaller for the cations that bind strongly to PEa (Cs+ and Na+) than for those that interact weakly with it (Li + and Mg +) 16. The affinity \; of the cations toward PEa follows the order ( 16) Cs > Na ~ Li > Mg. The size of the aggregateof SDS bound to PEG in the presenceof various salts follows the reverseorder: size in the presenceof Cs < Na < Li < Mg. For a given cation SOS CONCENTRATION. ~ there is no effect of either the anion or the salt concentration FIG. I. Typical behavior of pyrene end-labeled polyethylene glycol on the sizeof the bound aggregateas seenfrom Tables land (PyPEGPy) in the presenceof sodium dodecylsulfate(SDS) indicating the 2. Earlier studies(7, 8) showedthe sizeof the polymer bound critical aggregationconcentration (CAC) and the SDSconcentration at which PyPEGPyis most coiled (point B). SOS aggregateto increase with increase in ionic strength (like free surfactant micelles); these studies were usually conducted at surfactantconcentrations where both polymer- have been given earlier (10). The sodium dodecylsultate bound and free micelles existed and hencethe observedin- concentration at which the value of the coiling index in- creasein size could have been from an increasein the size creasesis called the critical aggregationconcentration (CAC) of the free micelles. In the caseof LiCI, MgCl2, and MgSO4 between PEG and SDS. The SDS concentration at which the sodium dodecylsulfate concentrations at B were nearly the pyrene-labeledpolyethylene glycol is most coiled (point equal to the critical micellization concentration (determined B) correspondsto the maximum amount of surfactantbound in the absenceof polyethylene glycol). The size of the SOS to the polymer as determined from surface tension mea- micelle was measured in these salt solutions using fluores- surements(10). It wasassumed that all binding of the sodium cencespectroscopy. The SOS micelle in the absenceof poly- dodecylsulfate(SDS) to polyethylene glycol (PEG) occurs mer was much larger than that bound to the polymer. Based

TABLE I Size of SDS AggregatesBound to PEG: Effect of Various Salts

[SOS)at B Conc. CAC. (seeFig. .\r Salt (M) (kmol/m3) (kmol/mJ) [50S] Studied N

Sodium chloride O.SO 2.96 ) ( 10-5 9.30 X 10-' 9.08x 10-' Sodium sulfate 0.01 6.93 ) ( 10-5 3.41 X 10-' 3.12X 10-' 0.10 2.78 ) ( 10-5 2.66 X 10-. 2.63 X 10-4

Sodium phosphate 0.01 1.23 ) ( 10-' 5.09 X 10-' 5.IOX 10-' , O.SO 7.20 ) ( 10-5 5.40 X 10-' 5.32 X 10-' Lithium chloride 0.01 I.SO) , 104 6.30 X 10-' 6.05 X 104 1.00 2.00 ) , 10-5 1.23 X 10-' 1.23 X 104 Cesium chloride 0.001 4.00 ) ( 10-' 7.06 X 10-. 7.08 X 10-4 0.01 1.37 ) , 10-' 4.08 X 10-' 4.97 X 104 Magnesiumchloride 0.005 4.68 ) , 10-5 4.45 X 10-' 4.SOx 104 1.000 1.36 ) , 10-5 1.09 X 10-' 1.01 X 10-4 Magnesiumsulfate 0.10 4.87 ), 10-5 2.04 X 10-. 1.99 X 104 1.00 7.22 ), Ir 1.26 X 10-' 7.11 X 10-' .From reference( 10). SIZE OF SDSAGGREGATE BOUND TO P£O 7

TABLE 2 micelle is important with smaller hydrated radii favoring Effect of Cationson Size of SDS AggregateBound to PEG greater interaction (21). For example, the larger hydrated Li + ion cannot approach the highly chargedsurface of the Cation SDS agregate size micelle as closely as the smaller Na + ion and cannot screen

Cesium-Cs+ 2O:t6 its charge as effectively. In such a case, PEG-SDS interaction will be favored more by Li + than Na + ions. The aggregate Sodium-Na+ 24:t 5 Lithium-Li+ 48 :t 12 size ofSDS bound to PEG is larger in lithium than in sodium Magnesium-Mg2+ 51 :t 13 salt solutions. This suggests that a simultaneous affinity of the cations towards the micelles as well as the polymer determines the size of surfactant aggregate bound to the on the sizesof the SDS micelle and the SDSaggregate bound polymer. to the polymer it wasensured that in the presenceof polymer there were no free aggregatesof SDS even though the con- SUMMARY centration was close to the CMC. i Measurement of size of the sodium dodecylsulfate (SDS) The number of aggregatesper polymer chain calculated aggregate bound to polyethylene glycol (PEG) depicted its from the values of nand pyrene concentration is found to strong dependence on the salt in solution. Moreover, the decreasewith increasein ionic strength and is independent cation and its affinity toward PEG were the factors that de'- of the type of salt added to the system(Fig. 2). The number termined the maximum size of the SDS aggregate bound to of ethylene groups associatedwith each surfactant aggregate the polymer. The anion type and the salt concentration did has been reported to increasewith increasein ionic strength not influence the bound surfactant size. The SDS aggregate ( 7 ). The presenceof polymer in the PEO-SDS system has size was large in solutions of lithium and magnesium which been proposed to decreasethe free energy of formation of do not bind strongly to polyethylene oxide, whereas in salts the micelle-water interface but increasethe steric and elec- of cesium and sodium, which bind strongly to PEO, the SDS trostatic repulsions at the micellar surface (17). Another aggregate was much smaller. Increase in ionic strength re- model for PEO-SDS complexation considers the polymer sulted in fewer bound SDS aggregates per polymer chain. It to replace the water of hydration around the polar groups is proposed that the size of the surfactant aggregatesin regions and the methylene groups of the surfactant ( 18). Increased of confined geometry (unlike free micelles in solution) is ionic 'strength will decreasethe free energy of micellization controlled by the nature of the interaction site and is inde- and this decreasemay be more than that induced by the pendent of the number or sites available. polymer. Hence the interactions betweenthe surfactant ag- gregatesand polymer chain will decreaseas the ionic strength 4 - _.~.~.: is increased.The decreasedinteraction may manifest itself ( VARIOUS SALTSor in the form of fewer aggregatesper polymer chain. In a study - SOOIUW on the interactionsbetween tetradecyl trimethyl ammonium - LITHIUW bromide (TT AB) and polyacrylic acid (PAA) it wasobserved t - CESIU\4 - ~AGNESIU~ that the size of the TT AB aggregatebound to PAA was in- 3 z dependentof polymer chargedensity as well as the amount :c [PEG: 5000W.W. :I: \ bound ( 19). Furthermore, in a study on the size of the SDS u I 20 ppm '" hemimicellesat the alumina-water interface,the aggregation Go... . -;;;. 2 number was the same at pH 3 and 6.5 (isoelectric point of ...... alumina is 8.3) and varied only with the amount adsorbed -c '" ... (20). Neither was there any effect observedof ionic strength ~ } '" '" on the sizeof the bound aggregateswhen determined in 0.03 00( ,., and 1.0 M sodium chloride solutions. These studies show 1 '" that the size of a surfactant aggregatein an area of restricted geometry (such as that bound to a polymer or adsorbedat ~ the solid/liquid interface) is independent of the number of --401 sites available for interaction. Any changein the number of o~ . . ...1 sites results only in a change in the number of aggregates 1C)-3 .10-2 10-1 10° present. IONIC STRENGTH Binding of cations to free sodium dodecylsulfatemicelles J- - -, FIG. 2 Number of sodium dodecylsulfateaggregates per polymer chain also needsto be considered: it has been demonstrated that as a function of ionic strength: salts of cesium, sodium. lithium, and mag- for inorganic ions, interaction of the hydrated ions with the ,nesium --, t. 18 MAL TESH AND SOMASUNDARAN ACKNOWLEDGMENTS 12. Lackowicz, J., "Principles of FluorescenceSpectroscopy," Vol. 51, Plenum, New York, 1983. Financial support from the National ScienceFoundation, Unilever Re- 13. 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Colloid Interface Sci. lOS, I (1985). 19. Kiefer, J. J., Somasundaran,P., and AnanthapadmaDabhan,K. P.. in 8, Zana, R., Lianos, P., and Jacques,L., J. Phys. Chern. 89,41 (1985). "Polymer Blends.Solutions and Interfaces"(I. Noda am D. N. Rubingh, Eds.). p. 423. Elsevier, New York, 1992. 9. van Starn,J., Almgren, M., and Lindblad, C., Prog. Colloid Polyrn. Sci. }. 84,13(1991). 20. Xu, Qun, and Somasundaran.P., unpublished results,Columbia Uni- 10. Maltesh, C., and Somasundaran,P., Langmuir 8, 1926 (1992). versity, 1992. II. Dubin, P. L., Gruber, J. H., Xia, J., and Zhang, H., J. Colloid Interface 21. Mukherjee, P., Mysels.K. J.. and Kapauan, P., J. Ph}'.1.Chem.11, 4166 Sci. 148, 35 (1992). IIQIi7)