Synergistic Effect of Cationic Surfactant on Surface Properties of

Surface tension measurements have been made on aqueous solutions of anionic monoesters of an alternating copolymer of maleic acid and styrene, MAS-n ...
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Langmuir 2000, 16, 6884-6890

Synergistic Effect of Cationic Surfactant on Surface Properties of Anionic Copolymers of Maleic Acid and Styrene A. F. Olea,*,† C. Gamboa,† B. Acevedo,† and F. Martinez‡ Departamento de Quı´mica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile, and Departamento de Quı´mica, Facultad de Ciencias Fı´sicas y Matema´ ticas, Universidad de Chile, Santiago, Chile Received February 22, 2000. In Final Form: May 22, 2000 Surface tension measurements have been made on aqueous solutions of anionic monoesters of an alternating copolymer of maleic acid and styrene, MAS-n ,with n ) 0-10, in the presence of alkyltrimethylammonium surfactants. It was found that the surface activity of these mixtures is increased relative to that observed for the pure components. This synergistic effect was ascribed to the formation of a complex, and its dependence of the complex structure was studied. The standard free energy of adsorption per methylene group was found to be -1.4 kJ mol-1 for complexes formed with MAS. This value decreases for MAS-n, becoming equal to zero for longer linear alkyl chains. However, all the calculated values of the standard free energy were negative and lower than those obtained for the pure surfactant. These results indicate that the main contribution to this change of free energy is the neutralization of the negative charge by a hydrophobic cation.

Introduction Interaction between polyelectrolytes and ionic surfactants in aqueous solutions has been the subject of a number of studies and reviews, mainly because of the fundamental and technological importance of their mixtures.1-14 Depending on the surfactant concentration, these systems can be treated as the binding of surfactant molecules to a polyelectrolyte chain or the interaction of a polymer molecule with micelles.15-17 Systems formed by cationic surfactants and anionic polymers have been extensively studied in the whole †

Departamento de Quı´mica, Facultad de Ciencias. Departamento de Quı´mica, Facultad de Ciencias Fı´sicas y Matema´ticas. ‡

(1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255. (3) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, 1993. (4) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; p 203. (5) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; p 395. (6) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 29, 7412. (7) Babak, V.; Lukina, I.; Vikhoreva, G.; Desbrieres, J.; Rinaudo, M. Coll. Surf. A: Physicochem. Eng. Asp. 1999, 147, 139. (8) Babak, V. G.; Anchipolovskii, M. A.; Vikhoreva, G. A.; Lukina, I. G. Colloid J. 1996, 58, 145. (9) Babak, V. G.; Rinaudo, M.; Desbrieres, J.; Vikhoreva, G. A.; Michalski, M. C. Mendeleev Commun. 1997, 149. (10) Babak, V. G.; Vikhoreva, G. A.; Anchipolovskii, M. A. Mendeleev Commun. 1995, 2, 73. (11) Benrraou, M.; Zana, R.; Varoqui, R.; Pefferkorn, E. J. Phys. Chem. 1992, 96, 1468. (12) Binana-Limbele, W.; Zana, R. Macromolecules 1987, 20, 1331. (13) Binana-Limbele, W.; Zana, R. Macromolecules 1990, 23, 2731. (14) De Oliveira, V. A.; Tiera, M. J.; Neumann, M. G. Langmuir 1996, 12, 607. (15) Li, Y.; Dubin, P. L. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; Vol. 578, p 320. (16) Mizusaki, M.; Morishima, Y.; Yoshida, K.; Dubin, P. L. Langmuir 1997, 13, 6941. (17) Yoshida, K.; Morishima, Y.; Dubin, P. L.; Mizusaki, M. Macromolecules 1997, 30, 6208.

Scheme 1

concentration range. In dilute aqueous solution the surfactant binding is highly cooperative,18,19 and it has been found that micelle-like clusters are formed at a surfactant concentration known as the critical aggregation concentration (cac).20 Often, the cac values are several orders of magnitude lower than the critical micelle concentration, cmc, in the absence of polymer. These aggregates have been found to be smaller21 and larger20 than the polymer-free micelles. It is also known that because of the strong electrostatic attraction between these two species the interaction starts at very low concentration forming surfactant-polyelectrolyte complexes.1,15 These complexes show a synergistic effect on the surface tension and stability of emulsions and foams.8,22 It has also been shown that anionic polyelectrolytes exhibiting very low surface active form activite complexes in the presence of extremely low concentration of cationic surfactant.6,22 The aim of the present work is to study the effect of structure of the polyion and surfactant on the surface properties of complexes formed with oppositely charged surfactants. Therefore, a series of monoesters of an alternating copolymer of maleic acid and styrene, MASn, with n ) 0-10, were used as anionic polyelectrolyte (see Scheme 1). The hydrophobicity of MAS-n copolymers is given by the styrene group and an alkyl side chain of (18) Hayakawa, K.; Fukutome, T.; Satake, I. Langmuir 1990, 6, 1495. (19) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (20) Chu, D. Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (21) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (22) Babak, V. G.; Vikhoreva, G. A.; Lukina, I. G. Colloids Surfaces A: Physicochem. Eng. Asp. 1997, 128, 75.

10.1021/la000250t CCC: $19.00 © 2000 American Chemical Society Published on Web 07/27/2000

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increasing size. The cationic surfactants are the tetraalkylammonium bromides, CnTAB, with n ) 10-16. In this paper we report the measurements of surface tension of aqueous solutions of CnTAB in the presence of completely neutralized MAS-n, as a function of the structure of the MAS-n/CnTAB complex. Experimental Section Materials. Two kinds of poly(co-maleic anhydride-styrene), MAS, were used: one purchased from Aldrich, and the other polymerized by using 2,2′-azobis(isobutyronitrile) as initiator, according to a standard method. The average molecular weights were 350 000 and 110 000, respectively. The latter value was determined by viscometric measurements and gel permeation chromatography using poly(styrene) samples as standard. The esterification reactions were carried out in tetrahydrofuran solution at 65 °C in the presence of 4-(dimethylamino)pyridine as catalyst. Alcohols from methanol to 1-dodecanol (Aldrich) were used as received. A detailed description of the characterization of these copolymers has been given elsewhere.23 Decyltrimethylammonium bromide, C10TAB (Eastman), dodecyltrimethylammonium bromide, C12TAB (Aldrich), tetradecylammonium bromide, C14TAB (Aldrich), and cetyltrimethylammonium bromide, C16TAB (Sigma), were purified by successive recrystallizations from acetone. All aqueous solutions were prepared in distilled and deionized water. Surface Tension Measurements. The surface tension γ was measured using the ring method with a Kruss-D8 balance. The aqueous solutions were thermostated at 25 °C by circulating water. Sets of measurements were made by adding small aliquots of a concentrated surfactant solution at constant polymer concentration (1 g/L), and by diluting a mixture at constant polymer/surfactant ratio. Each measurement took 5-30 min to reach the equilibrium. Viscosity Measurements. The viscosity data were obtained in a Ubbelohde type dilution viscosimeter at very low flow rate (water flow time ) 318 s at 25.0 °C). Fluorescence Probing. The concentrations used were 1 g/L and 2 µM for the copolymer and pyrene, respectively. Fluorescence emission spectra were obtained on a SLM/Aminco SPF-500C spectrophotofluorimeter. The ratio III/I corresponds to the ratio of intensities of peak three (λ ) 384 nm) to peak one (λ ) 373 nm).

Results and Discussion In aqueous solution the structure and equilibrium properties of the MAS-n copolymers are mainly determined by the conformation adopted by the polymer chain. Neutralization of the carboxylic groups increases the electrostatic repulsion and a conformational transition from a compact to a random coil configuration is expected. This transition has been observed for copolymers with n e 4, whereas for copolymers with longer alkyl side chain a hydrophobic microdomain is formed over the whole range of pH.23 Thus, the sodium salts of MAS-n, with n e 4, can be considered an amphiphile where each repetitive unit possess a hydrophilic headgroup and a hydrophobic tail. For the highest values of n the number of heads and tails is low, because of the formation of hydrophobic aggregates. Considering these structural features, it is expected that these copolymers exhibit a surface activity which depend on the side alkyl chain length. This fact has already been demonstrated in previous work for the air-water24 and octane-water25 interfaces, and it has also been shown that the surface activity is caused by the alkyl side chains, (23) Olea, A. F.; Acevedo, B.; Martinez, F. J. Phys. Chem. B 1999, 103, 9306. (24) Rios, H. E.; Rojas, J. S.; Gamboa, C.; Barraza, R. G. J. Colloid Interface Sci. 1993, 156, 388. (25) Rios, H. E.; Aravena, M. H.; Barraza, R. G. J. Colloid Interface Sci. 1994, 165, 259.

Figure 1. Surface tension of aqueous solutions of dodecyltrimethylammonium bromide at 25 C in (b) absence, and presence of (0) MAS-3s, (O) MAS-2, (2) MAS-6.

which adopt a perpendicular orientation at the interface. However, at the polymer concentration used in this work, 2-4 mM in a monomolar basis (one monomole contains 1 mol of each monomer), the surface tension of their aqueous solutions is reduced by no more than 7 mN/m. Synergistic Effect on Surface Activity of Surfactant-Copolymer Solutions. The addition of very small amounts of cationic surfactant to an aqueous solution of MAS-n copolymers produces an abrupt decrease of γ. Figure 1 shows the variation of the surface tension as a function of the logarithm of C12TAB concentration for different copolymers. For comparison the curve for the pure surfactant is also included. It is noted that the surface activity of C12TAB is greatly enhanced by the presence of the anionic copolymers. In the absence of surfactant, these copolymers reduce the surface tension of water by 3-6 mN m-1. Similar results have been obtained for all CnTAB, and the shape of these curves is determined only by the structure of the copolymer. For those MAS-n copolymers with n e 4, γ decreases sharply with CnTAB concentration until it reaches a steady value; further increase of concentration produces a new decrease of the surface tension. The curves obtained for copolymers with alkyl side chains longer than butyl, show two ranges of concentration in which γ changes at different rate. A similar behavior is observed for copolymers with substituted alkyl side chains, but shifted to much lower concentration. The surfactant concentrations at which the first break point is observed are similar to the critical aggregate concentration determined by fluorescence probing methods. A study of the nature and properties of the aggregates formed by the interaction of CnTAB and MAS-n copolymers is currently being carried out. In this paper we will focus on the surface activity shown by these systems in the low range of concentration, i.e., below the cac. For all polyion-surfactant mixtures, most of the gamma decrease is produced at surfactant concentrations below the cac. In those cases where n e 4 in the MAS-n copolymers, this change is produced at a surfactant/monomer ratio of 1/100. Thus, it is obvious that the surface tension reduction is produced by a new species

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Figure 2. (b) Relative viscosity and (9) fluorescence intensity ratio III/I of aqueous solutions of MAS (1 g/L) as a function of cetyltrimethylammonium bromide concentration.

formed by the polyanion/surfactant interaction. For the coupled carboxymethylchitin-C14TAB, it has been proposed that a nonstoichiometric complex is responsible of the surface activity at very low surfactant concentration.8 The existence of this complex cannot be detected by viscometry or fluorescence probing but its presence is clearly demonstrated by the surface tension results. Figure 2 shows the variation of relative viscosity and I3/I1 as a function of C14TAB concentration for an aqueous MAS solution. The decrease of viscosity has been ascribed to a conformational change, induced by the surfactant, that takes the polymer from a random-coil to a more compact conformation.21 Similar results have been obtained by using a fluorescence probing technique.11,20 The changes of fluorescence parameters, such as intensity, lifetime or spectra vibrational structure, have established the formation of hydrophobic microdomains at the cac.26 Thus, both viscosity and fluorescence measurements report on the surfactant concentration at which micelle-like clusters are formed. On the other hand, the lowering of surface tension monitors the first stages of the association process, when just a few surfactant molecules have been bound to the polymer backbone. Thus, the interaction between polyanions and cationic surfactants begins as a binding (26) Winnik, F. M In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; p 368.

process, and results in the formation of a very active surface complex. Thermodynamic of Adsorption. The Gibbs adsorption equation for a dilute aqueous solution, containing a mixture of a cationic surfactant and a surface active polyanion, can be written as

dγ ) -ΓS dµS - ΓP dµP - ΓC dµC

(1)

where γ is the surface tension of the solution, Γi and µi are the surface excess concentration and chemical potential of each species present in the interface, and the indices S, P, and C stand for surfactant, polymer, and complex, respectively. At equilibrium, µInt ) µbulk and

µInt ) µbulk ) ν+ µ+0 +ν- µ-0 + νRT ln f( m( (2) where the µ0’s are the standard chemical potentials of the ions, ν gives the total number of ions, f( is the mean ion activity coefficient, and m( denotes the mean ionic molality. In the case of completely dissociated electrolyte of the 1:1 type, ν ) 2 and m( ) m. The polyelectrolyte concentration is expressed on a monomer unit basis and it is assumed that each carboxylic group is also completely dissociated. In very dilute aqueous solutions the molality may be replaced by the molar concentration, and the activity coefficient of the surfactant can be considered

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constant. Thus, at constant temperature and pressure dµi ) 2RT d(ln Ci). Therefore, eq 1 becomes

dγ ) -2RT [ΓS d ln CS + ΓP d ln CP + ΓC d ln CC] (3) In our experiments the monomer concentration CP was kept constant and 1-3 orders of magnitude higher than the surfactant concentration. On the other hand, all surfactant molecules adsorbed in the surface must be forming a complex with a polymer chain. Thus, under these conditions the change of surface tension is produced only by the increase of surface excess concentration of the complex. It follows that

ΓC )

(

)

δγ 1 4.606RT δlogCC

(4)

where CC denotes the concentration of surfactant molecules bound to the negatively charged carboxylic groups of the copolymer. If just one surfactant molecule is bound by each carboxylic group, then CC is equal to the surfactant concentration Cs and finally we have

ΓC )

(

)

1 δγ 4.606RT δlogCS

(5)

Thus, the complex surface concentration can be obtained from the slope of a plot of γ against log Cs. Surface Area and Efficiency of Adsorption. To compare the effects produced by the adsorption at the liquid-gas and liquid-liquid interfaces, two parameters have been defined: the effectiveness of adsorption at an interface Γm, which corresponds to the maximum concentration that a surfactant can attain at the interface, and the efficiency of adsorption pC20, given by the negative logarithm of the bulk phase concentration of surfactant required to reduce the surface tension of the solvent by 20 mN m-1.27 It has also been shown that when the surface tension of the pure solvent is reduced by this amount Γ is close to its saturation value. Thus, the maximum value of ΓC was obtained from the slope of a plot of γ versus log CS at the concentration of surfactant C20, and the area per molecule in a saturated surface was calculated from the relation

aSm ) 1016/NΓm

(6)

Table 1 list the values of pC20 and the area per molecule in the region of surface saturation, aSm, for the different mixtures of copolymers and CnTAB. In general, the values of aSm are much greater than the 35 A2 measured for surfactant-free solutions of MAS-n (n ) 6, 8, 10, 12), and similar to those found for maleic acid-olefin copolymers.28 In the latter system each monomeric unit has two carboxylate groups and one alkyl side chain of variable length (n ) 12-18). The area per repetitive unit was found to increase from 87 to 256 with growing chain length, and this effect has been ascribed to a configuration where the chains orient parallel to the surface. The data collected in Table 1 show that it is not possible to establish a clear relationship between the area occupied by a complex molecule and the length of the alkyl group of surfactant or copolymer. However, the am values of copolymers (27) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley: New York, 1989. (28) Barraza, R. G.; Rios, H. E. J. Colloid Interface Sci. 1999, 209, 261.

Table 1. Effectiveness of Adsorption, Γm in Mol cm-2, Area Per Molecule at Saturation Concentration, asm in A°2, for Complexes Formed by MAS-n Copolymers and Alkyltrimethylammonium Bromides DeTAB

MAS MAS-1 MAS-2 MAS-3 MAS-3s MAS-4 MAS-4s MAS-6 MAS-8 MAS-10 Surfact

pC20

aSm

3.78 2.88 2.74 3.82 4.40 2.81 4.45 3.04 3.30 3.47 2.08

126 152 295 156 162 100 175 161 124 149 96

DTAB pC20

aSm

4.26 4.07 4.08 4.04 5.11 3.21 5.1 3.04 3.13 3.56 2.21

54 137 210 167 92 149 58 123 117 170 69

TTAB

CTAB

pC20

aSm

pC20

aSm

4.74 4.48 4.56 3.89 5.00 3.26 5.4 3.14 3.50 3.62 2.82

64 262 156 200 56 174 83 160 246 200 54

4.95 4.74 4.81 4.26 5.08 3.28 5.6 3.65 3.13 3.65 3.47

74 180 188 165 79 149 58 188 256 333 79

carrying a branched alkyl chain (or no side chain) are definitely lower than those obtained for copolymers with linear alkyl side chains, and similar to the area per molecule in a saturated surface of a polymer-free surfactant solution. These results can be explained by assuming that the configuration adopted at the interface by a monomer-surfactant complex is mainly determined by a hydrophobic interaction between the alkyl chains of the surfactant and the side chain of the copolymer. This interaction pulls the whole monomeric unit up to the surface, bringing about a considerable increase of the area occupied. It is interesting to note that one methyl group is enough to produce this effect, and a further increase of side chain length does not change the surface configuration. In the case that the alkyl group is branched or there is not an alkyl side chain, this interaction does not exist and the area covered by the complex at the surface is determined only by the ionic group. On the other hand, the efficiency of adsorption seems to have the following relations with the complex structure. (i) For a given surfactant the values of pC20 decrease with increasing length of the linear alkyl group of the copolymer, while complexes formed by copolymers with branched alkyl groups or no side alkyl chain at all, exhibit higher efficiencies of adsorption. (ii) In copolymers with small alkyl groups, the efficiency increases with increasing size of the surfactant hydrophobic tail. (iii) For copolymers with alkyl groups larger than butyl, the efficiency does not depend on the length of the surfactant alkyl chain. Thus, the effect of the complex structure on the surface activity of these compound seems to be determined by its influence on the efficiency rather than on the effectiveness of adsorption. Standard Free Energy of Adsorption. The adsorption process can be considered as a distribution between the bulk and surface phases. To calculate a standard free energy of adsorption, associated with this equilibrium, it is necessary to define standard states for molecules distributing in these two phases. Various expressions have been proposed,8,27,29 but most of the data have been tabulated by using the relationship suggested by Rosen27

∆G0ad ) RT lnaπ - πASm

(7)

where aπ is the activity of surfactant molecules in the bulk of the solution at some fixed value of π () γ0 - γ) and ASπ is the molar area of the surfactant in a saturated surface. As mentioned above, this condition is reached when the surface tension of the solvent has been reduced (29) Rosen, M. J.; Aronson, S. Colloids Surfaces 1981, 3, 201.

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Figure 3. Plots of the change of the standard free energy of adsorption as a function of the number of methylene groups in the alkyl chain of the cationic surfactant. (b) C12TAB-CH3(CH2)nSO4- salts, (2) MAS-CnTAB, (9) MAS-1-CnTAB, (1) MAS-4-CnTAB, (0) CnTAB.

by 20 mN m-1. The activity of an ionic surfactant is equal to the product of its mean ion activity coefficient and its mean ionic molality, aπ ) f(m(, and by using the same considerations discussed to obtain eq 3, the activity can be replaced by the molar concentration. Thus, this expression can be rewritten as

∆G0ad ) - 2.303RT(pC20 + 1.30)

(10)

where aSm is the area in Å2 per molecule. On the other hand, Babak et al.8,22 have derived the following equation

All the ∆G0ad values, calculated by eq 10, for the system MAS-n/CnTAB were negative, and larger than those measured in pure surfactant or polymer solutions. These results indicate that the complexes are absorbed more strongly than their separate components. As pointed out by Rosen, the standard free energy of adsorption can be broken into the additive contributions from different groups.27 Similar free energy relations have been used in the study of the distribution of hydrophobic molecules between the aqueous and micellar phases.30-32 For the polymer/surfactant complexes, the standard free energy of adsorption at the interface may be written as

∆G0ad ) RT ln(C/π)|20

∆G0ad ) ∆G0W + nCS ∆G0CS + nCP ∆G0CP

∆G0ad ) 2.303RT logC20 - 6.023*20aSm ) - 2.303RTpC20 - 120.46aSm (8)

(9)

where C is the bulk molar concentration of the surfactant, π is the surface pressure, and the index indicates that these quantities are measured at π ) 20 mN m-1. It has also been shown that this relation provides the same numerical results as eq 8,8 and it has the advantage that does not need a complete γ-log C plot to determine ∆G0ad, but only the surfactant concentration required to decrease the surface tension by 20 mN m-1. This equation can be rewritten in terms of pC20 as

(11)

where ∆G0W denotes the contribution of the hydrophilic group; ∆G0CS and ∆G0CP are the incremental free energy per methylene group of the surfactant and polymer, respectively; nCS and nCP are the number of these groups (30) Tanford, C. The Hydrophobic Effect, 2nd; John Wiley: New York, 1988. (31) Sepulveda, L.; Lissi, E. A.; Quina, F. Adv. Colloid Interface Sc. 1986, 25, 1. (32) Gamboa, C.; Olea, A. F. Langmuir 1993, 9, 2066.

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Figure 4. Plots of the change of the standard free energy of adsorption as a function of the number of methylene groups in the polymer side chain of MAS-n-CnTAB complexes. (9) C16TAB, (2) C14TAB, (b) C12TAB.

attached to the tetraethylammonium group of the surfactant and the side chain of the copolymer, respectively. The complexes are assumed to be formed mainly by the electrostatic attraction between the carboxylate group of a monomer unit and the ionic group of the surfactant. This assumption implies that the hydrophilic group and consequently ∆G0W are the same for all the systems studied. Thus, the effect of surfactant and monomer structures on the surface activity of the complex can be studied by use of eq 11. In the absence of copolymer the standard free energy of adsorption of the CnTAB was found to be a linear function of nCS and the incremental free energy per methylene group was equal to 1.8 kJ mol-1, which compares quite well with values obtained for ionic surfactants.8,27 In Figure 3, the values of ∆G0ad obtained for the complexes are plotted against nCS at a fixed nCP. A plot of the standard free energy of adsorption of the system C12TAB-CH3(CH2)nSO4- is included for comparison.27,33 The ∆G0ad values of these cationic-anionic surfactant salts were calculated from the pC20 values reported in ref 33 by using eq 8. It is interesting to note that the lines corresponding to pure surfactants, MAS-CnTAB complexes, and C12TAB-CH3(CH2)nSO4- salts have similar slopes, i.e., 1.8, 1.4, and 1.6, respectively. For the other (33) Lange, H.; Schwuger, M. J. Kolloid Z. Z. Polym. 1971, 243, 120.

complexes this slope decreases with the presence of an alkyl group in the copolymer side chain, until it becomes parallel to the nCS axis (see curve for MAS-6 in Figure 3). It is also worth to note that the free energy changes for the surface adsorption of the polymer-surfactant complexes are larger than those obtained for the surfactants. The differences reach about 10-16 kJ mol-1 for MAS, MAS-3s, and MAS-4s; 7-10 kJ mol-1 for MAS-1, MAS-2, and MAS-3; and 4-6 kJ mol-1 for MAS-6, MAS-8, and MAS-10. The behavior shown by the C12TAB-CH3(CH2)nSO4salts and MAS, MAS-3s, and MAS-4s indicates that the driving force for the adsorption at the interface is the neutralization of the polymer charge by a hydrophobic counterion. In these compounds there is not an alkyl group pendent from the polymer chain (MAS) or it is a branched one (MAS-3s, MAS-4s) impeding any hydrophobic interaction with the alkyl group of the cationic surfactant. Thus, the incremental free energy of adsorption per methylene group is due exclusively to a destabilization of the complex in the bulk phase. For complexes in which the copolymers carry a side alkyl chain, the difference in ∆G0ad is smaller and it diminishes with increasing alkyl chain length. This effect can be assigned to a stabilization of the complexes in the bulk solution produced by a long-range hydrophobic interaction between the alkyl groups of the copolymer and

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the bound surfactant. In other words, the hydrophobic part of the surfactant interacts with alkyl side chains from distant monomers instead of its neighbors. When the alkyl group in the copolymer is long enough (butyl), this interaction does not depend on the length of the surfactant chain. Similar conclusions are reached by studying the effect of the side chain length, at nCS constant, on ∆G0ad. Figure 4 shows the curves obtained by plotting ∆G0ad against nCP for different surfactants. It is clear that the change of free energy of adsorption decreases with increasing length of the alkyl side chain, reaching a platteau with a butyl group in the side chain. Conclusions Highly surface-active complexes are formed in aqueous solutions of MAS-n copolymers and cationic surfactants. The enhanced surface activity is observed at very low concentrations of surfactant, i.e., as low as one surfactant molecule per polymer chain. The configuration adopted by these complexes at the surface depends on the polymer

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structure. Complexes formed by copolymers carrying a linear alkyl side chain occupy a larger surface area than those having branched alkyl groups, or without an alkyl side chain. The surface activity seems to be determined by the efficiency of adsorption as measured by the pC20 parameter. A relationship between this quantity and the free energy of adsorption has established that the synergistic effect on the surface activity is the consequence of a decrease of the free energy of adsorption produced mainly by neutralization of the charge in the copolymer by the hydrophobic cation. This effect is more pronounced in those complexes where the copolymer carries a branched side alkyl chain. This result could be of importance for the design of systems where enhancement of this property is desired. Acknowledgment. The authors are grateful to FONDECYT for financial support of this work under Grant 1990968. LA000250T