EMF and Microcalorimetry Studies Associated with the Binding of the

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Langmuir 1999, 15, 5474-5479

EMF and Microcalorimetry Studies Associated with the Binding of the Cationic Surfactants to Neutral Polymers S. M. Ghoreishi, G. A. Fox, D. M. Bloor, J. F. Holzwarth,† and E. Wyn-Jones* Division of Chemical Sciences, University of Salford, Salford M5 4WT, U.K. Received November 3, 1998. In Final Form: April 26, 1999 EMF measurements on binary mixtures of the cationic surfactants tetradecyltrimethylammonium bromide and tetradecylpyridinium bromide and a range of nonionic polymers have been carried out. The existence of polymer/surfactant complexes was found for those polymers whose microstructures contain side groups with hydrophobic character. Isothermal titration microcalorimetry measurements were also taken to obtain more information about the polymer/cationic surfactant complexes. An examination of the dependence of the monomer surfactant concentration following the binding showed that “free” micelles are formed in solution before the polymers become “saturated” with bound surfactant. In some cases these free micelles occur in solution before the critical micellar concentration of the pure surfactantsan effect which is thought to occur due to an excess counterion concentration in the microenvironment at the vicinity of the complex. Finally a comparison of the present data with the binding behavior of these polymers with sodium dodecyl sulfate (SDS) allows us to identify the roles of electrostatic and hydrophobic interactions that take place to allow the formation of stable polymer/surfactant complexes.

Introduction It is now generally accepted that the interactions of cationic surfactants with neutral polymers are much reduced in comparison to those of anionic surfactants.1-9 Indeed, some synthetic polymers are known only to interact with anionic surfactants and show little or no affinity to cationics.1 However, if the polymers display evidence of some hydrophobic nature, or indeed are hydrophobically modified, then in some cases they have been found to bind cationic surfactants.5-8 In fundamental studies carried out on polymer/surfactant systems, the following critical concentrations referring to the total amounts of added surfactant are of prime importance: (i) T1 signaling the onset of binding; (ii) T2 signaling the saturation of the polymer with bound surfactant; (iii) Tf signaling the formation of free micelles. In addition, binding isotherms representing (i) the amount of bound surfactant as a function of monomer surfactant concentration and (ii) the amount of bound counterions are also important in order to understand the behavior of the systems. Although the importance of the above parameters in relation to binding studies has long been recognized,1 it is only recently that quick, reliable, and effective experimental methods have been developed to determine these * To whom correspondence should be addressed. † Fritz Haber Institut der Max Planck Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. (1) Goddard, E. O. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabliam, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Hayawaka, K.; Kwak, L. C. T. Cationic Surfactants. Surfactant Sci. Ser. 1991, 37, 189. (3) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (4) Morol, Y.; Akisada, H.; Saito, M.; Matuura, R. J. Colloid Interface Sci. 1977, 61, 233. (5) Carlsson, A.; Lindman, B.; Watanabe, T.; Shirahama, K. Langmuir 1989, 5, 1250. (6) Brackman, L. C.; Engberts, J. B. F. N. Chem. Soc. Reviews 1993, 22 (2), 85. (7) Bloor, D. M.; Mwakibete, H. K.; Wyn-Jones, E. J. Colloid Interface Sci. 1996, 178, 334. (8) Hoffrnann, H.; Huber, G. Colloids Surf. 1989, 40, 181. (9) Robb, I. P. Anionic Surfactants. Surfactant Sci. Ser. 1981, 11, 109.

parameters.7,10-14 This versatile approach has been further developed at Salford by combining measurements involving surfactant selective electrodes (EMF) and isothermal titration calorimetry (ITC).13,15,16 In comparison with anionic surfactant/neutral polymer systems, there is very little information available concerning quantitative binding characteristics of cationic surfactants. As a result, we describe here our EMF and ITC measurements on mixtures of tetradecyltrimethylammonium bromide (TTAB) and tetradecylpyridinium bromide (TPB) with a range of water-soluble nonionic polymers. Experimental Section TTAB was a Sigma product recrystallized (3 times) from ethanol before use. TPB was synthesized using a method described previously.7,14 The polymers together with their sources and molecular weights when available are listed in Table 1. EHEC and HM-EHEC were purified by dialysis and freeze-drying, and PVME was purified by dialysis. The remaining polymers were used as received. EMF Measurements. The surfactant membrane electrodes selective to TTAB and TPB were constructed in the laboratory7,14 and used to determine the concentrations of monomer surfactant and counterions, respectively, by measuring their EMF relative to a commercial sodium (Corning 476211) reference electrode and also a bromide ion (Corning solid-state ISE 30-35-00) selective electrode. The cells used for these measurements and the procedures to calculate the respective monomer concentrations have been described elsewhere.14 Isothermal Titration Calorimetry (ITC). The microcalorimeter used in this work was the Microcal ITC instrument. In (10) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (11) Takisawa, N.; Brown, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2099. (12) Wan-Badhi, W. A.; Wan-Yunus, W. M. L.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (13) Bloor, D. M.; Wan-Yunus, W. M. L.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (14) Palepu, R.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1990, 86, 1535. (15) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778. (16) Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E.; Langmuir 1995, 12, 4476.

10.1021/la9815502 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/03/1999

Binding of Cationic Surfactants to Neutral Polymers

Figure 1. Plot of the TTAB electrode (reference Na+) as a function of total TTAB concentration for the TTAB/polymer systems in 1 × 10-4 mol dm-3 NaBr: ([) pure TTAB; (9) TTAB + PVI (0.5% w/v); (/) TTAB + PVP/PVI (0.5% w/v); (+) TTAB + PVPy-N-O (0.1% w/v); (x) TTAB + PVP (0.5% w/v). Table 1. Sources and Molecular Weight of Polymers Used in This Work polymer

abbreviation

methylcellulose hydroxypropyl cellulose ethylhydroxy ethyl cellulose

MC HPC EHEC

hydrophobically modified EHEC Hydroxybutylmethyl cellulose hydroxypropylmethyl cellulose hydroxyethyl cellulose poly(vinylpyrrolidone) poly(vinylpyrridine nitrogen oxide) polyvinyl pyrrolidone/poly(vinylimidazole) copolymer pyrrolidone poly(allylamine) poly(vinylimidazole) poly(propylene oxide) poly(vinyl methyl ether)

HM-EHEC HBMC HPMC HEC PVP PVPy-N-O

mol wt

Langmuir, Vol. 15, No. 17, 1999 5475

Figure 2. Plot of the TTAB electrode (reference Na+) as a function of total TTAB concentration for the TTAB/polymer systems in 1 × 10-4 mol dm-3 NaBr: ([) pure TTAB; (9) TTAB + PPO (0.5% w/v); (/) TTAB + PVME (0.5% w/v); (+) TTAB + EHEC (0.5% w/v); (x) TTAB + PVP (0.5% w/v).

supplier

>50 000 Aqualon 100 000 Aldrich 120 000 Berol Nobel AB, Sweden 120 000 Berol Nobel AB, Sweden 120 000 Aldrich >50 000 Aqualon 250 000 Aldrich 40 000 Unilever 200 000 Unilever

PVP/PV1

70 000 Unilever

PPAA PV1 PPO PVME

Unilever 40 000 Unilever 1 000 Aldrich 27 000 Aldrich

the ITC experiment, one measures directly the energetics (enthalpy changes) associated with processes occurring at constant temperature. Experiments were carried out by titrating micellar cationic surfactant usually containing 0.5% (w/v) polymer. An injection schedule (number of injections, volume of injection, and time between injection) is set up using interactive software and this schedule automatically carried out with all data stored to disk. After each addition, the heat released or absorbed as a result of the various processes occurring in the solution is monitored by the calorimeter. In the present work, we present the results of the ITC experiments in terms of the enthalpy per injection (∆Hi) as a function of cationic surfactant concentration.13,16-19 The measurements were taken at 298 K.

Results and Discussion The EMF data show that no binding was observed between TTAB and the polymers PVP, PVI, PVPy-N-O nor with the copolymer PVP/PVI as shown by the data in Figure 1 where the EMF of TTAB with and without the polymers are, to all intent and purposes, the same. On the other hand, binding takes place between the cationic surfactants and PVME, PPO, PPAA, and the celluloses. Typical EMF data for a TTAB electrode plotted as a function of added TTAB are shown in Figure 2 for pure TTAB and also the surfactant in the presence of some of the polymers. The data for the pure surfactants show a distinct break as expected at the cmc. Below the cmc, the (17) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (18) Thuresson, K.; Nystrom, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (19) (a) Brackman, J. C.; Van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988, 4, 1266. (b) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Irlam, K. D.; Engberts, J. B. F. N.; Kevelan, J. J. Chem. Soc., Faraday Trans. 1998, 94, 259.

Figure 3. Plot of the EMF, monomer surfactant (m1), and ITC data of TTAB as a function of total surfactant concentration in 0.5% EHEC (1 × 10-4 mol dm-3 NaBr): (9) pure TTAB; ([) TTAB + EHEC.

surfactant electrode performs under Nernstian conditions in the sense that the slope is acceptable at 59 mV per decade. Figures 3 and 4 display the electrode, ITC, and surfactant monomer concentration as a function of total surfactant concentration for the systems TTAB/EHEC and TPyB/PVME, respectively. It is clear from the data shown in Figures 2-4 that when the polymers are present the EMF data deviates from Nernstian behavior at surfactant concentrations which are less than the cmc of the pure surfactant. This behavior is attributed to the surfactant binding to the polymer. For the polymer solutions, the deviation of the EMF from the ideal Nernstian behavior is a fairly well-defined break as is shown in Figures 2-4. The break point is attributed with the onset of binding and is usually denoted by T1 or cac, the critical aggregation concentration. Once binding starts, the surfactant elec-

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Ghoreishi et al.

Table 2. Results for Binding between TTAB, TPyB, and Polymers in 1 × 10-4 mol dm-3 NaBr surfactant/polymer TTAB/PPO (0.5%) TTAB/PVME (0.5%) TTAB/EHEC (0.5%) TTAB/HMEHEC (0.5%) TTAB/HPC (1%) TTAB/HEC (1%) TTAB/HBMC (1%) TTAB/HPMC (1%) TTAB/MC (1%) TTAB/PPAA (0.1%) TTAB/PVI (0.5%) TTAB/PVP (0.5%) TTAB/PVPy-N-0 (0.1%) TTAB/PVP/PVI (0.5%) TPyB/PPO (0.5%) TPyB/PVME (0.5%) TPyB/EHEC (0.5%) TPyB/HMEHEC (0.5%)

T1 (mol dm-3)

Tf (mol dm-3)

Tf - m1 (mol dm-3)

0.00012 0.00013 0.00060 0.00012 0.0014 0.0018 0.0018 0.0016 0.0018 0.0020 no binding no binding no binding no binding 0.00013 0.00020 0.00013 0.00013

0.003 0.003 0.004 0.004 0.003 0.004 0.004 0.004 0.003 0.004 no binding no binding no binding no binding 0.0019 0.0018 0.0026 0.0037

0.0015 0.0014 0.0014 0.0019 0.0005 0.0006 0.0006 0.0004 0.0005 0.0016 N/A N/A N/A N/A 0.0009 0.0008 0.0013 0.0022

Figure 4. Plot of the EMF, monomer surfactant (m1), and ITC data of TPyB as a function of total surfactant concentration in 0.5% PVME (1 × 10-4 mol dm-3 NaBr): (9) pure TTAB; ([) TTAB + PVME.

trode monitors the monomer surfactant concentration (m1) as a function of total surfactant concentration (Cl); these data are also in Figures 3 and 4. When the polymer becomes fully saturated with bound surfactant at concentrations well in excess of the cmc, it is normally found for anionic surfactants that the EMF data with and without the polymer merge, indicating that the polymer does not bind any further surfactant and no longer has any influence on m1.12,13,15 Unfortunately in the present work and also in previous electrode studies of polymers binding to cationic surfactants,7,8 we have not been able to observe any point at which the EMF data merge after binding is completed. We believe that part of this problem may be due to the electrode not responding ideally at higher surfactant concentration. Certainly we have found that the high surfactant concentration response of the

T2/(ITC) (mol dm-3) 0.08 0.03 0.03

0.08 0.04 0.04

electrode is dependent on the origin of the modified poly(vinyl chloride) used for the membrane and also on high salt concentration. To gain more information about the binding behavior of cationic surfactants with neutral polymers we also carried out ITC measurements in which experiments were performed by titrating micellar cationic surfactants containing 0.5% w/v polymer injected into a bulk solution containing 0.5% w/v polymer. The ITC data were measured only for a few of the polymers listed in Table 2. In previous ITC data on polymer/surfactant systems very distinctive changes in the enthalpy profile during the titration occurs and these can be identified with various critical concentrations associated with the binding.13,16-19 For each polymer/surfactant system studied the enthalpy change per injection (expressed in [kcal mol-1] injectant) is plotted against surfactant concentration (Figures 3 and 4) and compared with the EMF data described above. There are different events which occur in the ITC cell following an injection and which make a contribution to the measured ∆Hi value. These include the dissociation of surfactant micelles from the injectant, dilution effects, aggregation of bound surfactant, conformational changes in the polymer, etc. Although the individual contributions from these specific effects have not been satisfactorily resolved, there is progress currently made to understand these interactions.19b In the EMF data, the onset of binding T1 occurs when the EMF data with and without the polymer starts deviating and is clearly defined, and a vertical line corresponding to this concentration is shown in Figures 3 and 4. In each of the ITC experiments this concentration corresponds to the beginning of the pronounced increase in the ∆H data leading to the well-defined maximum shown in the figures. It is interesting to note that in reference to previous work reported on polymer systems T1 was assigned to the maximum in the enthalpy titration curve.9,16-19 This is not correct as we have shown here by comparing the complementary EMF and ITC data. We now consider the surfactant concentration denoted T2 at which the polymer becomes fully saturated with bound surfactant. As stated earlier, this concentration cannot be found in the EMF experiments but T2 can be measured from the ITC data when the enthalpies per injection with and without the polymer merge after binding and is indicated by the vertical line T2 in Figures 3 and 4. The concentration T2 is reached when the corresponding enthalpy per injection with and without the polymer is the same. When this occurs, the polymer no longer interacts with surfactant and the injections merely correspond to a dilution of micelles into a solution

Binding of Cationic Surfactants to Neutral Polymers

Figure 5. Plot of the EMF of the TPyB electrode (reference Br-) as a function of log(C1C2)1/2 concentration for the TPyB/ polymer systems in 1 × 10-4 mol dm-3 NaBr: (s) pure TPyB; (9) TPyB + EHEC (0.5% w/v); ([) TPyB + HM-EHEC (0.5% w/v). (C2 ) C1 + (1 × 10-4) mol dm-3).

Figure 6. Plot of TPyB counterion dissociation (R) as a function of total TPyB concentration for the TPyB/polymer system (0.5% w/v) in 1 × 10-4 mol dm-3 NaBr: ([) TPyB + PVME; (9) TPyB + PPO.

containing free micelles. Another noteworthy observation in this comparison between ITC and EMF data is that the maximum in the EMF data and hence monomer surfactant concentration for the polymer/surfactant system almost exactly corresponds to the pronounced maximum in the ITC enthalpy profile. This is indicated by the vertical line between T1 and T2 in Figures 3 and 4. The EMF experiments also indicate that once binding starts at T1 a small amount of bromide counterions also start binding as is shown in Figure 5. This observation is often used as evidence that surfactant aggregates are formed on the polymer. Typical variation of the degree of counterion binding as a function of surfactant concentration in the binding region is shown in Figure 6. During the initial binding stages the bound surfactant aggregates probably grow in number and size, and this surfactant association process is accompanied by an increase in the monomer surfactant concentration. This is the normal behavior expected for any binding process. When the monomer concentration reaches a maximum and starts decreasing with the addition of more surfactant, such a behavior is indicative of the formation of free micelles in solution.7,13,15 For example, the maximum in the monomer concentration found from the EMF data for the pure surfactant in the absence of polymer corresponds to the critical micellar concentration and once this is reached the monomer concentration decreases with further added surfactant (Figures 2-4). Micellization of ionic surfactants at low salt is the only aggregation process that we are aware of in which the monomer concentration decreases with increasing surfactant concentration. On this basis we conclude that in the polymer/surfactant systems the surfactant concentration Tf corresponding to the maximum in m1 (Figures 3 and 4) signals to the formation of free micelles in the solution and as further surfactant is added

Langmuir, Vol. 15, No. 17, 1999 5477

both free micelles and polymer bound surfactant aggregates are formed simultaneously from Tf until T2 is reached. Since Tf also corresponds to the maximum in the ∆Hi plot, we conclude that for cationic surfactants in the absence of added salt the maximum in ∆Hi found in the ITC experiment can also be used to detect the onset of the formation of free micelles in solution. For all the systems, the maximum in the EMF data for the surfactant/polymer is always slightly less than the corresponding maximum for the system without the polymer. This statement however introduces a paradox in a sense that for these polymer/surfactant systems free micelles start occurring in solution at a total surfactant concentration less than the critical micellar concentration of the pure surfactant. This also means that the monomer concentration in each of these systems behaves in exactly the same way. For polymer/TTAB mixtures the degree of micellar dissociation calculated as described previously12,13 decreases in the following order of polymers: PVME > PPO > HMEHEC > EHEC. The order for the TPyB system is the following: PVME > PPO > EHEC > HMEHEC. The differences in the orders of the polymers for the two surfactants are probably caused by different surfactant/polymer interactions which affect the packing of the headgroups of the bound surfactant aggregate. This in turn will change the charge density at the surface of the aggregate. The consequence of these results is that for a particular polymer/surfactant system at a fixed surfactant concentration the concentration of free Br- counterions in solution increases for the reverse order of the above polymers. This in turn produces an effect which generates the same results as the addition of salt to a surfactant, i.e., a reduction in the surfactant monomer concentration which is in equilibrium with the polymer/surfactant complex. In addition the argument involving the counterions is very much the same as that used to explain why the monomer concentration of an ionic surfactant decreases with increasing concentration once micelles are formedsa result which is clearly shown in the EMF data. The numbers which emerge from estimates of the ionic strength are very similar in all these situations. We now turn our attention to the ITC data. The enthalpy per titration increases rapidly after T1 and becomes more and more endothermic until a maximum is reached at Tf. Between Tf and T2 the ∆Hi’s decrease rapidly as the process becomes more exothermic. For surfactants this latter type of behavior of ∆Hi’s is normally associated with a highly cooperative process and is consistent with the formation of free and bound micelles after Tf. On the other hand in the region T1-Tf where only binding of surfactant to the polymer takes place this process is endothermic and also apparently cooperative in the sense that there is a steep change in the ∆Hi’s. During this process it is likely that aggregates are formed on the polymer since a small degree of counterion binding takes place in this region. An examination of Figure 6 shows that the degree of micellar dissociation decreases from a high value of ∼0.8 for the bound aggregate at T1 to a limiting value of ∼0.4 at T2. This decrease in R allows the headgroups of the bound aggregate to pack closer together as the binding proceeds from T1 to T2. In the concentration range T1-Tf these R values are an average of the high value for the bound aggregate and the value of 0.2 which was evaluated for the pure micelles. We now turn our attention to the occurrence of free micelles in these polymer/ surfactant system at concentration below the cmc of the pure surfactant. The shift in the maximum found for the monomer concentration and hence the concentration at which free micelles occur also

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Ghoreishi et al. Table 3. Comparison of the Binding Results for SDS and TTAB with the Nonionic Polymersa

Figure 7. Plot of bound TTAB per gram polymer, (C1 - m1)/Cp as a function of monomer TTAB concentration for the system TTAB/polymer in 1 × 10-4 mol dm-3 NaBr: (9) TTAB + PPO (0.5%); ([) TTAB + PVME (0.5% w/v); (×) TTAB + PPAA(0.1% w/v).

follows the above trends for the different polymers. This shift accompanies the relative increase in the concentration of free Br counterions. We believe that the clue to the explanation for this unusual result is analogous to that proposed earlier by Holmberg et al. to explain the primary step in the formation of polymer/surfactant complexes.20 In the present systems the bound surfactant in the T1-Tf region is a “loose” aggregate which only binds a small amount of Br- counterions. However the positive charge density at the surface of these bound aggregates are likely to attract Br- counterions in such a way that the local concentration of Br- ions in the microenvironment of the complex is much higher than their bulk concentration. Thus any free surfactant ions in this enhanced ionic strength environment will form micelles at effectively lower surfactant concentration than the “normal” cmc, and we believe that these are the precursors to the “free” micelles which are formed in these systems. Typical binding isotherms showing values of (C1 - m1) per gram polymer in the binding region are compared in Figure 7. In Figure 7 [(C1 - m1)/Cp)] increases with m1 in the binding region as is expected for any system involving ligands binding to macromolecules. In the present work “free” micelles are formed at Tf before the polymer becomes saturated with bound surfactant at T2. As a result the binding isotherms have only been shown in the region T1-Tf, which is exclusively associated with the binding of surfactant to polymer and can be described as an equilibrium between monomer surfactant in the bulk solution and bound surfactant on the polymer. In the region T1-Tf the quantity C1 - m1, where C1 is the total added surfactant, represents the amount of surfactant bound to the polymer. On the other hand between Tf and T2 this quantity represents the amount bound surfactant plus surfactant in the “free” micelles and it is not possible to distinguish between these two quantities. In the binding isotherms Cp is defined in terms of grams per unit volume which is the conventional practice in this area. The shapes of the binding isotherms in Figure 7 are also similar to others reported in various adsorption processes involving surfactants.22b The values of C1 - m1 at Tf, i.e., Tf - m1, are listed in Table 2. In terms of molecular recognition the charged headgroups and the area between these headgroups on a (20) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundelof, L-O. Phys. Chem. 1992, 96, 871. (21) Dubin, P. L. J.; Gruber, J. H.; Xia, J. L.; Zhang, H. W. J. Colloid Surf. Sci. 1992, 148, 25-41. (22) (a) Ghoreishi, S. M.; Li, Y.; Warr, L.; Bloor, D. M.; Wyn-Jones, E. Langmuir, to be submitted for publication. (b) Dobias, E. Surfactant adsorption in mineral flotation. In Coagulation and Flocculation Theory and Applications; Dobias, E., Ed.; Marcell Dekker: New York, 1993; Vol. 47.

polymer

TTAB

HPMC HBMC HEC HPC MC EHEC HM-EHEC PPO PVME PPAA PVI PVP PVPy-N-O PVP/PVI

W/I W/I W/I W/I W/I S/I S/I S/I S/I S/I N/I N/I N/I N/I

T f - m1 (mol dm -3) 0.0004 0.0006 0.0006 0.0005 0.0005 0.0014 0.0019 0.0015 0.0014 0.0016

SDS

T f - m1 (mol dm-3)

W/I W/I W/I S/I S/I S/I S/I S/I S/I S/I S/I S/I S/I S/I

0.0007 0.0005 0.0008 0.0197 0.0224 0.018 0.013 0.051 0.039 0.021 0.0167 0.0445 0.01 0.011

a S/I, strong interaction, T - m < 1 × 10-3 mol dm-3; W/I, weak f 1 interaction Tf - m1 > 1 × 10-3 mol dm-3; N/I, no interaction.

micellar surface where the hydrocarbon chains are exposed to water provide two different binding sites for intermolecular attraction which allows the micelle to recognize different segments of polymers. Specifically the charged headgroups offer sites for electrostatic interaction with oppositely charged species whereas the hydrocarbon in contact with water in the area between the headgroups will recognize hydrophobic parts of polymers, and mutual contact results in a reduction of the overall hydrocarbon contact with water. If a typical synthetic water-soluble polymer has sufficient flexibility, one can envisage a configuration allowing ion-dipole association between the dipole of the hydrophilic group and the ionic headgroup of the surfactant and contact between the hydrophobic parts of the polymer and the hydrocarbon areas which are exposed to water at the periphery of the micelle. The overall result of either or both interactions is to diminish the unfavorable conditions governing the stability of the micelle, reducing its relative free energy and cmc by promoting the formation of micelles at lower SDS concentrations. To assess the impact of the above interactions we compare in Table 3 some binding results for the present polymers with the cationic (TTAB) surfactant and also the anionic (SDS) surfactant. The SDS/polymer data have been recently reported.22 The polymers PVI, PVP, and PVPy-N-O and the copolymer PVP/PVI bind very strongly to SDS but not to TTAB. This means that the primary driving force promoting binding between SDS and these polymers is electrostatic with all the polymers showing mildly cationic behavior because of the basic properties of the side group in the monomer units. The remaining polymers all bind the cationic surfactants and SDS albeit with different affinities. The attractive interaction between the cationic surfactants and these polymers must be associated with the hydrophobic effect. Indeed all the polymers, with the exception of those discussed above which do not interact with cationic surfactants, all possess various alkyl groups. This promotes interaction with a cationic micelle which leads to a mutual reduction of hydrocarbon contact with water as the polymer wraps itself around the micellar surface. This interaction is also present in the complexes of these polymers with SDS. However with SDS additional electrostatic attractive forces can also occur between the etheric oxygen atom of PPO, PVME, and possibly EHEC, HM-EHEC, MC, and HPC and the anionic headgroup. This interaction arises either due to the formation of the oxonium ion at the oxygen atom in the linkage or as proposed by Dubin21 the sodium

Binding of Cationic Surfactants to Neutral Polymers

cation forms a bridging link between the etheric oxygen atom and the sulfate headgroup of the anionic surfactant. The polymers HBMC, HPMC, and HEC bind very weakly to both SDS and the cationic surfactant suggesting that the interactions stabilizing these weak complexes are hydrophobic. It should also be pointed out that the extent of these interactions leading to the stability of the complex also depends on the flexibility of the polymer molecule and in particular whether local and segmental conformational changes in the polymer chain can adapt to the geometric constrains imposed on the polymer backbone by the binding sites on the micellar periphery, i.e., to ensure that the donor sites on the polymer can physically make contact with the attractive binding sites on the micellar surface. We now consider the mechanism of the process leading to the formation of polymer/surfactant complexes. The more plausible explanation still remains that of Holmberg et al.,20 who proposed the formation of micelles at concentrations lower than the cmc of the surfactant by assuming that the initial step in the surfactant/polymer association consists of a redistribution of monomer surfactant with preference for the polymer coil regions over the bulk solution. As a result, the local surfactant concentration in the coil regions reaches values of the order of normal cmc values long before the bulk concentration. This leads to the formation of free micelles which

Langmuir, Vol. 15, No. 17, 1999 5479

must interact immediately with the polymer to reduce their free energy so that they remain stable under the normal bulk conditions of the solution. This mechanism must take place at a time scale which is much shorter than the lifetime of the micelle. For SDS the slow relaxation time associated with the micelle formation/ dissolution process23 is related to the lifetime of the micelle and is of the order 1 × 10-4 s and can be taken as the upper limit of the time scale of the association of polymer with the micelle forming a complex at T2. Acknowledgment. S.M.G. thanks The Islamic Republic of Iran for the provision of a maintenance award, enabling this work to be carried out under the supervision of E. W.-J., at the University of Salford. G. A. F. thanks the University of Salford for the provision of a maintenance award. He also thanks J. H. for the opportunity to carry out the ITC experiments and the Deutscher Akademischer Austauschdienst (DAAD), for the provision of a short term research grant. We also thank Dr. Jonathan Warr, Unilever Research, Port Sunlight, U.K., for providing samples of polymers and also for helpful discussions. LA9815502 (23) Bloor, D. M.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 2, 1982, 78, 657-669.