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Langmuir 1987, 3, 372-377
of the mixture dominate the overall isotherm shape in this case. The fraction of site with S < 1 is more for the isotherm of curve c which results in an S-shaped isotherm at large XI. The cumulative fraction of sites having S less than unity [ f ( l )can ] be obtained by using eq 2a, 4a, and 4b as r Distribution r ( n + 1, a ) f(1) = 1 (84 n! Uniform Distribution
where r(n + 1, a)is an incomplete F function.* Equations 8a and 8b were used to calculate f ( 1 )for curves b and c of Figure 5. They were 0.278 and 0.286 for curve b and 0.385 and 0.336 for curve c, using the r and the uniform distributions, respectively. Thus, both distributions provided very similar f ( 1 ) values for isotherms b and c of Figure 1 despite their different shapes. In summary, this study shows the following: (a) Adsorbent heterogeneity plays a significant role in determining the shape of the surface excess isotherm for adsorption of binary liquid mixtures. (b) Simple mathematical models based on the concept of a site selectivity distribution on a heterogeneous surface can be adequately used to account for adsorbent heterogeneity. The detailed structure of the distribution function is not critical. The choice of the model, therefore, (8) “Handbook of Mathematical Functions”; Abramowitz, M., Stegun, A., Eds. Applied Mathematics Ser. 55; National Bureau of Standards, U.S. Government Printing Office: Washington, DC, 1972.
1
3
2
4
5
SOlP
Figure 5. r and uniform site selectivity distribution functions: (solid line) correspondingto isotherms c of Figure 1; (dashed lines) corresponding to isotherms b of Figure 1. depends on its mathematical simplicity. (c) The effect of adsorbent heterogeneity is more pronounced when the mean of the site selectivity distribution is low. An S-shaped isotherm for adsorption of an ideal liquid mixture of equal adsorbate sizes can be caused by adsorbent heterogeneity. (d) The effect of adsorbent heterogeneity on the heat of immersion of a liquid mixture is much less pronounced than that on the surface excess isotherm. Consequently, the heat of immersion cannot be used to evaluate the degree of adsorbent heterogeneity.
Counterion Binding on Mixed Micelles: Effect of Surfactant Structure James F. Rathman and John F. Scamehorn* School of Chemical Engineering and Materials Science, Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019 Received April 11, 1986. I n Final Form: October 28, 1986 Previous work has shown that an electrostatic model can be used to accurately predict the fractional counterion binding on mixed ionic/nonionic micelles. New results show that this model can be successfully applied to a wide variety of surfactant mixtures. In particular, it is shown that mixed micelles containing nonethoxylated nonionic surfactants such as phosphine oxides, amine oxides, and sulfoxides at appropriate pH conditions exhibit binding behavior similar to that of mixtures containing polyethoxylates. The following ionic surfactants were studied in mixtures with a polydisperse nonylphenol polyethoxylate as the nonionic surfactant: dodecylpyridinium chloride, hexadecylpyridinium chloride, sodium dodecyl sulfate, and sodium octylbenzenesulfonate. The electrostatic model describes the binding for these systems very well. The following nonionic Surfactants were studied in mixtures with the cationic surfactant hexadecylpyridinium chloride: a monodisperse dodecyl polyethoxylate alcohol, two polydisperse nonylphenol polyethoxylates, dodecyldimethylphosphine oxide, dodecyldimethylamine oxide, and decyl methyl sulfoxide. The electrostatic model accurately describes the binding on all systems, although the model is slightly less accurate for the sulfoxide mixtures. These results are consistent with the concept that ionic/nonionic surfactant interactions and nonidealities of mixing in micelles are primarily of electrostatic origin, with specific chemical interactions, if present, being of secondary importance. Introduction The association of counterions with mixed micelles composed of ionic and nonionic surfactants is of great
* To whom correspondence should be addressed. 0743-7463/87/2403-0372$01.50/0
interest due to the development of applications which take advantage of the synergistic behavior of such systems. Despite this, there are still very few reported data in this area. It has been shown that the fractional counterion binding on binary mixed ionic/nonionic surfactant micelles can be 0 1987 American Chemical Society
Counterion Binding on Mixed Micelles
modeled by an electrostatic approach to predict how binding varies with composition.' Data presented in this paper show that this electrostatic model can be successfully used to describe counterion binding for a wide variety of both ionic and nonionic surfactant structures. These results support the assertion that electrostatics are the major factor in determining binding; surfadant structure has only secondary effects. The effect of ionic surfactant structure on fractional counterion binding on micelles of a single ionic surfactant has been studied in detail. It has been shown that the binding increases as the surfactant alkyl chain length is increased.l" One NMR study on the effect of ionic surfactant hydrophilic groups has shown that, even though the mechanism of binding of sodium cations to anionic surfactant micelles may vary with the type of head group, the degree of binding may not be significantly different.5 Previous studies of counterion binding on ionic/nonionic mixtures have used only polyethoxylated nonionic surfactants.'Y6-l2 It is generally agreed that the main reason counterion binding decreases upon the addition of nonionic surfactant is the decrease in the surface charge density on the micelle. Some researched have suggested that the binding on mixed micelles containing ethoxylated nonionic surfactants is further reduced due to the long polyether chains shielding the counterions from the ionic surfactant head groups. Other investigators maintain13J4that the highly polar polyethoxylate chain can associate with cations in solution (usually H+or Na+) to form an oxonium ion, giving the chain a slight positive charge. This "crown ether" effect would be expected to enhance the binding of anionic counterions to cationic/nonionic micelles while decreasing the binding of cations on anionic/nonionic micelles. Recent s t ~ d i e s ' ~on J ~ the effect of nonionic surfactant structure on mixed-micelle formation suggest that, depending on the length of the alkyl and polyethoxylate chains of the nonionic surfactant, it is possible for two types of mixed ionic/nonionic micelles to coexist in solution. Counterion binding measurements can help to .test the validity of these proposed phenomenon. One method of determining whether or not the nonionic polyethoxylate chain uniquely effects counterion binding is to compare bindings for such mixtures to bindings for mixed systems in which the nonionic surfactant is not ethoxylated. Counterion bindings on systems containing phosphine oxide, amine oxide, and sulfoxide surfactants in their nonionic forms are presented here. To our knowledge, this is the first time the binding on ionic/ (1) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984,88,5807. (2) Charbit, G.; Dorion, F.; Gaboriaud, R. J. Colloid Interface Sci. 1985,106, 265. (3) Sepulveda, L.; Cortes, J. J. Phys. Chem. 1985, 89, 5322. (4) Satake, I.; Tahara, T.; Matura, R. Bull. Chem. SOC.Jpn. 1969,42, 319. (5) Gustavsson, H.; Lindman, B. In Colloid Interface Sci. [Proc. Int. Conf.],50th 1976,2, 339-55. (6) Meguro, K.; Akasu, H.; Ueno, M. J . Am. Oil Chem. Soc. 1976,53, 145. (7) Hall, D. G.; Price, T. J. J. Chem. Soc., Faraday Trans. 1 1984,80, 1193. (8) Meyer, M.; Sepulveda, L. J. Colloid Interface Sci. 1984, 99,536. (9) Abe, M.; Tsubaki, N.; Ogino,K. Colloid Polym. Sci. 1984,262,584. (10) Tokiwa, F.; Aigami, K. Kolloid Z. Z. Polym. 1970,239, 687. (11) Tokiwa, F.; Moriyama, N. J.Colloid Interface Sci. 1969,30, 338. (12) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday SOC. 1964, 60,986. (13) Nishikido, N. J. Colloid Interface Sci. 1977, 60,242. (14) Kurzendorfer, C. P.; Schwuger, M. J.; Lange, H. Ber. Bunsen.Ges. Phys. Chem. 1978,82,962. (15) Abe, M.; Tsubaki, N.; Ogino, K. J . Colloid Interface Sci. 1985, 107,503. (16) Ogino, K.; Tsubaki, N.; Abe, M. J. Colloid Interface Sci. 1985, 107,509.
Langmuir, Vol. 3, No. 3, 1987 373
nonethoxylated nonionic systems has been measured and modeled. The ability to accurately predict counterion binding on mixed micelles is important for properly modeling many of the phenomenon associated with surfactant mixtures. For example, Hall et al." have shown that the common assumption that regular solution theory can be used to describe mixture cmc data is invalid since it implicitly assumes that the fractional binding decreases linearly with decreasing mole fraction of ionic surfactant in the micelle, in contrast to experimental observations.
Experimental Section Materials. Highly pure hexadecylpyridinium chloride monohydrate (CPC), obtained from HEXCEL Specialty Chemicals, was used as received. Dodecylpyridinium chloride (DPC), from Pfaltz and Bauer, was recrystallized 3 times from petroleum ether and ethanol. Sodium dodecyl sulfate (SDS) from Fisher Scientific Co. was recrystallized from ethyl alcohol and water. Sodium octylbenzenesulfonate (OBS) from Fairfield Chemical Co. was recrystallized once from methanol and once from distilled water. All ionic surfactants other than CPC were stored in a desiccator in vacuo. The polyethoxylated nonionic surfactants were used as received. NP(EO),,, (trade name IGEPAL (20-660, GAF Corp.) and NP(EO),, (trade name T D E T N-14, Triton) are polydisperse nonylphenol polyethoxylates with an average of 10 and 14 mol of ethylene oxide per mol of nonylphenol, respectively. DE(EO)6, trade name BL-GSY, from Nikkol Chemicals, Japan, is a dodecyl polyethoxylate having six ethylene oxide groups per molecule; DE(EO)6 is monodisperse. Dodecyldimethylphosphine oxide (DDPO) was obtained from Proctor and Gamble Co. in its pure solid form and was used as received. Dodecyldimethylamine oxide (DDAO), trade name Ammonyx LO, from ONYX Chemical Co., was received in water/oil solution. The liquid was removed by freeze-drying and the residual solid was recrystallized twice from ethyl acetate. The highly hygroscopic precipitate was dried and stored in vacuo. Decyl methyl sulfoxide (DMS), from Fairfield Chemical Co., Inc., was recrystallized twice from petroleum ether, dried, and stored in vacuo. At high pH (pH >7.0), DDPO, DDAO, and DMS exist in nonionic form,'* while at low pH, protonation of the oxygen results in a cationic form. This study is concerned only with the nonionic (high pH) behavior of these surfactants. The sodium chloride was Fisher reagent grade and the water was distilled and deionized. The necessary pH adjustment for several of the systems studied was accomplished by using Baker and Adamson reagent grade potassium hydroxide. Measurements. The methods for determining counterion bindings and critical micelle concentrations (cmc) have been described earlier.' Measurements on mixtures containing DDPO, DDAO, and DMS were made at high pH to ensure that these surfactants were present only in nonionic form. The pH was maintained a t 9.8-10.0 and was monitored by using Markson pH meter, Model 6102. Dropwise additions of 0.5 M KOH were used to obtain the desired pH and all sample and calibration solutions were set t o the same pH. The chloride electrode exhibited no interferences due to pH in the range 2.0-12.0. With the exception of DMS, the purity of the surfactants was confirmed by HPLC analysis and/or the absence of minima in the surface tension curves. At the conditions used in this study, the solubility of DMS is less than its cmc at the temperature and added electrolyte concentrations used in this study; thus, it was not possible t o obtain a cmc for pure DMS. Precipitation problems for the CPC/DMS mixtures were encountered only when the mole fraction of CPC was less than approximately 0.3.
Theory The localized adsorption model for counterion binding, as discussed in a previous paper,' was used to model all mixture data presented here. It is important to discuss (17) Hall, D.; Huddleston, R. Colloids Surf. 1985, 13, 209. (18) Funasaki, N. J. Colloid Interface Sci. 1977, 62, 189.
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374 Langmuir, Vol. 3, No. 3, 1987
the assumptions and limitations of this model. The model is based on the assumption that the Poisson-Boltzmann equation describes the electrical potential in the diffuse double layer surrounding the charged micelle. The micelle is treated as a planar surface and micelle-micelle interactions are assumed to be negligible. Several counterion binding and "dressed micelle" models, which also incorporate the Poisson-Boltzmann equation, have been developed for single surfactant ionic and have attempted to specifically include effects such as interactions between head groups, variations in the dielectric constant, intermicellar interactions, and micelle structure. While such considerations are indeed important, the extension of these models to mixed ionic/nonionic systems would be quite formidible. The model used in this work is limited to conditions for which the Poisson-Boltzmann equation is applicable. Specifically, the surfactant and/or added salt concentrations must be low enough to be able to neglect micellemicelle interactions so that the continuum double-layer theory is ~ o r r e c t . ~ It l - is ~ ~also probably less accurate for 2:1 electrolytes; e.g., in systems containing divalent counterions, solvation forces can be quite large and cannot be neglected. The surface area per ionic surfactant head group in the mixed micelle, a, is given by eq 17 in ref 1. The main assumption made here is that the decrease in the surface charge density, which is inversely proportional to a, is due only to the replacement of ionic surfactant by nonionic surfactant as the nonionic content in the micelle is increased; i.e., it is assumed that the shape of the micelle is not a function of composition. This assumption will be discussed in more detail later. The value of a at any given composition is a function of the specific areas of pure ionic and nonionic micelles, aI and aN,respectively. The method described by T a n f ~ r dwas ~ ~used to estimate aI and aN. The third parameter needed to apply this model, in addition to aI and aN,is the fractional counterion binding on the pure ionic surfactant micelle. Once this value has been experimentally determined, the electrostatic model predicts the degree of binding at any given composition for the mixed ionic/nonionic micelle. Notice that no mixture parameters are required; this model gives a priori predictions of mixture bindings. A conversion factor necessary for obtaining consistent units was inadvertently omitted in the original paper: the right-hand side of eq 10 and 12 in ref 1should be divided by 9 X lo9 J m/C2. All previous results are still valid since this factor was correctly included in all calculations.
Results and Discussion A. Correction for Monomer Concentration, For univalent surfactant and counterion, the fractional counterion binding is equal to the ratio of the bound counterion concentration to the concentration of ionic surfactant in the micelle. The latter quantity is calculated by subtracting the ionic monomer concentration from the total ionic surfactant concentration. The values of the cmc for each surfactant in the presence of 0.03 M added NaCl are listed in Table I. The cmc's of DDPO and DDAO are (19) Gunnarsson, G.; .Jonsson, B.; Wennerstrom, H. J . Phys. Chem. 1980,84, 3114. (20) Beunen, J. A.; Ruckenstein, E. J . Colloid Interlace Sci. 1983,96, 469. (21) Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1983, 87, 5025. (22) Evans, D. F.; Mitchell. D. J.; Ninham, B. W. J. P h p . Chem. 1984, 88, 6344. (23) Chao, T.;Sheu, E.; Chen, S. J . PhJs. Chem. 1985, 89, 1395. (24) Chao, Y.;Sheu, E.; Chen, S. J. Phjs. Chem. 1985,89, 4862. ( 2 5 ) Tanford, C. J . Ph?s. Chem. 1972, 76, 3020.
Table I. Critical Micelle Concentrations of Surfactants in the Presence of 0.03 M Added NaCl surfactant T,"C cmc, pM surfactant T,OC cmc, pM CPC 30 91 NP(E0)14 30 66 DPC 30 14220 DE(EO)6 30 67 SDS 30 2200 DDPO 30 100 OBS 40 4270 DDAO 30 250 NP(EO)lo 30 53 DMS 30 1800" NP(EO)lo 40 62
" Approximate solubility of DMS. Table 11. Summary of Model Parameters for Unbound Counterion Concentration = 0.03 M aT, nm2/ UN, nm2/ KR, system T,"C W,/RT niolecuie molecule m3/k;n01 CPC/NP(EO),n 30 -1.28 0.675 0.846 1.60 DPC'/NP(EO);, 30 -1.28 0.640 0.846 0.92 0.663 0.846 1.05 SDS/NP(EO)10 30 -2.72 0.846 1.42 OBS/NP(EO)lo 40 -2.73 0.849 CPC/DE(EO)e 30 -1.28 0.675 0.680 1.60 CPC/NP(EO)ld 30 -1.28 0.675 0.917 1.60 CPC/DDPO 30 -1.28 0.675 0.661 1.60 CPC/DDAO 30 -1.28 0.675 0.650 1.60 CPC/DMS 30 -1.28 0.675 0.674 1.60
significantly higher than the cmc's of the ethoxylated surfactants, even though they have similar alkyl groups-this is expected, since such oxides are considered somewhat more polar, and thus more water soluble, than other nonionics. However, it should be noted that the cmc of DDAO in Table I is much lower than values reported elsewhere,26indicating that the DDAO used here is significantly polydisperse. All binding measurements are reported for solutions having an average unbound counterion concentration of 0.03 M. For single surfactant solutions, the monomer concentration is approximately equal to the value of the cmc given in Table I, since the cmc's were measured in the presence of 0.03 M swamping electrolyte. For mixtures of surfactants, the total monomer concentration is approximately equal to the mixture cmc, which is calculated assuming regular solution theory adequately describes mixture cmc and compositions. The regular solution theory interaction parameters, WR, are listed for each binary system in Table 11. These have been determined experimentally from mixture cmc data or are a reasonable estimate. Due to the fact that the cmc's of the pure ionic surfactants are quite low, the monomer correction for all systems reported here was small and usually negligible; thus, any inadequecies of regular solution theory have little effect on the results. Also, the uncertainty in the cmc's of pure DDAO and DMS is only a minor problem because of the low cmc of pure CPC; in all mixtures, the concentration of the ionic monomer CPC is negligible. B. Counterion Binding Results. 1. Effect of Ionic Surfactant Alkyl Chain Length. In all figures presented here, the symbols represent experimentally determined bindings and the lines are the values predicted by the localized adsorption model. The counterion binding is observed to decrease quite slowly with increasing mole fraction of nonionic surfactant in the micelle. This gradual change in binding has been observed in previous studies on ionic/nonionic mixtures and is predicted by the binding model used in this work. The bindings for the CPC/NP(EO),, and DPC/NP(EO),, systems are shown in Figure 1. The data for the CPC/NP(EO),, mixture have been reported previously' and are repeated here for purposes of comparison. As (26) Herrmann, K. W. J . Phys. Chem. 1962,66, 295.
Langmuir, Vol. 3, No. 3, 1987 375
Counterion Binding on Mixed Micelles
-g
'
I
I
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:
0.0 0.0
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Figure 1. Effect of ionic surfactant alkyl chain length on
counterion binding.
expected, the binding for the pure ionic components is higher for the surfactant having the longer alkyl chain. Since micelle size is in large part determined by constraints on the packing of the alkyl chains, the aggregation number for CPC micelles is considerably higher than that of DPC; the larger charge concentration in the case of CPC results in a slightly higher fractional counterion binding. This trend is also observed for the mixtures, suggesting that the mixed CPC/NP(EO),o micelles are larger than the DPC/NP(EO)lo micelles. As the mole fraction of nonionic surfactant is increased, the difference between the two systems becomes less. Recalling that the localized adsorption model gives true a priori predictions of the mixture counterion binding, it is remarkable how well it describes the mixture data. 2. Different Types of Ionic Surfactant Alkyl Chains and Head Groups. OBS and SDS are two anionic surfactants having different alkyl groups and different hydrophilic head groups. As shown in Figure 2, despite these structural differences, the mixture binding for both systems is quite similar. The binding on OBS/ NP(EO)lomicelles decreases slightly more rapidly than on SDS/NP(EO),, micelles as mole fraction of nonionics increases. This trend is predicted by the model and is a consequence of the different aI values of the two surfactants. 3. Effect of Type of Polyethoxylated Nonionic Surfactant. Figure 3 shows bindings for the CPC/NP(EO),, and CPC/NP(EO)14systems. The increased length of the nonionic surfactant hydrophilic chain results in a decrease in the binding and this effect becomes more pronounced as the mole fraction of nonionic surfactant increases. At low mole fractions of CPC in the micelle, the localized adsorption model predicts slightly higher bindings than those that are observed experimentally. There could be several explanations for this, the most obvious being the difficulty in accurately estimating aN for polyethoxylate surfactants. The orientation and behavior of the ethylene oxide chain in the mixture, especially for the longer (EO),, chain, are uncertain, and the Tanford method of calculating aNmay no longer be accurate. Several NMR studies have shown that micelle size and shape vary with composition in ionic/nonionic surfactant mixtures.np28 If the change in micellar shape is significant, (27) Nilsson, P.; Lindman, B. J. Phys. Chem. 1984,88, 5391.
0.2
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MOLE FRACTION OBS OR SDS IN MICELLE
MOLE FRACTION CPC OR DPC IN MICELLE
Figure 2. Effect of type of anionic surfactant alkyl group on
counterion binding.
0.0 0.0
0.2
0 4
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MOLE FRACTION CPC IN MICELLE
Figure 3. Effect of increasingnonionic surfactant ethylene oxide number on counterion binding.
the surface charge density will change for two reasons: (1) the replacement of ionic surfactant by nonionic surfactant and (2) the change in micellar surface area due to the change in geometry. Since reason 2 violates one of the basic assumptions of the model presented here, the localized adsorption model would not be expected to work as well if the shape of the micelle changes drastically with varying composition. Perhaps the deviation between experimental and theoretical results for the CPC/NP(EO),( system is due to changes in the micelle shape which are more pronounced than in the CPC/NP(EO)lo mixtures. Comparison of results for the binary mixtures of CPC/DE(EO), and CPC/NP(EO)lo are given in Figure 4. The alkyl chain of the two nonionic surfactants is nearly the same length, though NP(EO)loincludes a benzene ring and DE(EO), does not. The localized adsorption model describes the data extremely well and, as was also seen in Figure 3, the binding is higher for mixtures containing the nonionic surfactant having fewer ethylene oxide groups. This trend supports the claim that the major effect of (28) Guering, P.; Nilsson, P.; Lindman, B. J . Colloid Interface Sci. 1985, 105, 41.
376 Langmuir, Vol. 3, No. 3, 1987
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Rathman and Scamehorn
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MOLE FRACTION CPC IN MICELLE
Figure 4. Effect of structure of ethoxylated nonionic surfactant on counterion binding on mixed micelles. 0.4 0.6 0.8 1 .o MOLE FRACTION IONIC SURFACTANT IN MICELLE Figure 6. Stern layer electrical potential calculated from the localized adsorption model for all systems.
0.o
eCPC/DDAO SYSTEM, PH=IO p 0.8
0.2
i;
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MOLE FRACTION CPC I N MICELLE
Figure 5. Counterion binding on mixed micelles composed of CPC and nonethoxylated nonionic Surfactants. increasing polyethoxylatechain length is to reduce binding by sterically hindering the association of counterions with the ionic surfactant head groups in the mixed micelle. If the crown ether effect were a dominant factor, then the binding on cationic/nonionic mixtures would be higher than predicted by the electrostatic model (which assumes total surface charge is due to the ionic surfactant only) since the polyether chain would presumably lend a positive charge to the micelle. It would also be expected that the bindings of CPC/NP(EO)14 would be higher than those of CPC/NP(EO)loat any given composition;however, the opposite is observed, so it appears that the steric effect of EO groups is much more influencial than oxonium ion formation. 4. Effect of Phosphine Oxide, Amine Oxide, and Sulfoxide Nonionic Surfactants. Results for counterion binding on mixed CPC/DDPO, CPC/DDAO, and CPC/ DMS micelles are presented in Figure 5. The localized adsorption model predicts the binding for all systems quite well. The experimental bindings for the CPC/DMS system are slightly higher than those predicted by the model, suggesting that the DMS in some way stabilizes the chloride binding. The binding for the CPC/DDPO system
is lower than those for the CPC/DDAO system at low mole fractions of CPC in the micelle, and the localized adsorption model does not predict this difference; this is most likely the result of inadequate knowledge of the area occupied by the nonionic surfactant head group in the mixed micelle. In general, though, such differences are small and it is remarkable that the variation of binding with micellar composition is essentially the same for the nonionic oxides as that observed for mixtures containing nonionic surfactants with bulky polyethoxylate head groups. Due to the uncertainties in the cmc's of the pure DDAO and DMS, the exact composition of these surfactants is not known; e.g., as mentioned, the DDAO is quite highly polydisperse. Fortunately, no such problems were encountered with DDPO, and since the binding behavior for mixtures containing each of these nonethoxylated nonionics is quite similar, all data in Figure 5 are believed to be reliable. Results for mixtures containing DDAO may be expected to vary somewhat depending on the original source of the surfactant. The absolute value of the electrical potential in the Stern layer, calculated from the localized adsorption model, is plotted for each system in Figure 6. The remarkable fact that all systems exhibit similar binding behavior is reflected in the shape of the electrical potential curves, which are all nearly alike. The electrical potentials calculated from this model have been used in a new electrostatically based model to describe mixed-micelleformation.29 The electrical potential for the DPC/NP(EO)lo system is greater than the other systems because the fractional binding is lower in comparison; since fewer of the ionic head groups in the micelle have an associated counterion, the net charge in the Stern layer is greater. The fact that the electrostatic model used in this study works so well for vastly dissimilar ionic and nonionic surfactants lends support to the basic premise that structural effects are secondary and that electrostatic considerations alone can in large part explain the nonidealities of mixed ionic/nonionic micelle formation. (29) Rathman, J. F.; Scamehorn, J. F. Langmuir 1986,2, 354.
Langmuir 1987,3, 377-382 Another argument against the crown ether effect can be made from the observation that there is no significant difference between the binding for systems containing an ethoxylated nonionic surfactant and systems containing a nonethoxylated nonionic surfactant. In addition, the binding of cationic counterions to anionic/nonionic micelles is essentially the same as the binding of anions to cationic/nonionic micelles. The insensitivity of counterion binding to all these factors suggests that there are no specific interactions between the counterions and the polyether chain of the ethoxylated nonionics. It is important to stress that this discussion of the crown ether formation is only speculative because binding is certainly not a direct method of quantifying this effect. There is no evidence in these results to support the idea that two types of micelles, one rich in ionic surfactant and the other rich in nonionic, coexist in solution for any of the systems studied. If a micellar phase rich in ionic surfactant were present, the fractional counterion binding would be expected to be essentially constant as the overall composition varied. The lengths of the alkyl and polyethoxylate groups of the nonionics used in this investigation are most likely not in the range in which the two separate micellar phases exist.15J6
Conclusions The localized adsorption model can be used to accurately predict the fractional counterion binding on mixed ionic/nonionic micelles. The model works well for surfactants of markedly different structure, including both ethoxylated
377
and nonethoxylated nonionics. The minimal effect of surfactant structure indicates that electrostatics is the dominant force in determining counterion binding.
Acknowledgment. Financial support for this work was provided by the Mobil Research and Development Corp., the Shell Development Co., DOE Contract 1985BC10845.000, the OU Energy Resources Institute, and the Oklahoma Mining and Minerals Resources Research Institute. We thank the Proctor and Gamble Co. for donation of the DDPO surfactant. Kevin Stellner, Steven Hendon, Terry Davis, and Ronda Huffines helped obtain the data presented here. Nomenclature a micellar surface area per charged hydrophilic group in micelle, m2/molecule a for a single-component ionic micelle, m2/molecule a1 micellar surface area per hydrophilic group for a aN single-componentnonionic micelle, m2 molecule cmc critical micelle concentration, kmol/m constant from localized adsorption model of counKB terion binding, m3/kmol R gas constant, 1.987 kcal/(kmol K) T temperature, K regular solution theory interaction parameter, WR kcal/ kmol electrical Stern layer potential, V YO Registry No. NP(EO),, 9016-45-9; DPC, 104-74-5; CPC, 123-03-5; SDS, 151-21-3; OBS,28675-11-8;DE(EO)B,3055-96-7; DDPO, 871-95-4; DDAO, 1643-20-5; DMS, 3079-28-5.
B
Spectroscopic Studies of Dye Solubilizates in Micellelike Complexes of Surfactant with Polyelectrolyte Katumitu Hayakawa,* Junko Ohta, Tamaki Maeda, and Iwao Satake Department of Chemistry, Faculty of Science, Kagoshima University, Korimoto-1, Kagoshima, Japan 890
Jan C. T. Kwak Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received May 2, 1986. I n Final Form: December 1, 1986 Absorption and fluorescence spectra of the cationic dyes rhodamine 6G (R6G),proflavin (PF),and acridine orange (AO) were measured in the presence of anionic polyelectrolytes, i.e., sodium salts of dextran sulfate (DxS) and poly(viny1sulfate) (PVS),with or without the cationic surfactant dodecyltrimethylammonium bromide (DTAB). The dimer bands of R6G and PF appear at 498 and 435 nm in aqueous solutions with excess DxS or PVS, respectively. Addition of DTAB induces a red shift of the bands indicative of dissociation of the dimer into the monomer. The monomer band maximum appears at a longer wavelength than that of aqueous dye. A similar red shift of the monomer band of the dyes is observed in micellar solutions of DTAB and sodium dodecyl sulfate. These results indicate that PF and R6G dissolve into DTA+-polyanion complexes in the monomeric form. The increase in fluorescence intensity of both dyes induced by DTAB addition also points at the solubilization of monomeric dye in the polymer-surfactant complex. DTAB addition to AO-DxS solution induces only a minor change in the absorption and fluorescence spectra, indicative of strong cooperative binding of A 0 with DxS.
Introduction dialysis,lPH solubility,7NMR? potentiometric titration,&'l and neutron scattering.12 Our studies of the binding of The interaction between polyelectrolytes and oppositely charged surfadants produces a special type of organization. (1) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, The formation of soluble polymer-surfactant complexes 37, 223. can be deduced from the marked changes observed in (2) Goddard,E. D.; Hannan, R. B. J. Colloid Interface Sei., 1976,55, many properties of the solution by the use of a variety of 73. techniques such as surface tension,lS2dye sol~bilization,~~~ (3) Murata, M.; Arai, H. J. Colloid Interface Sei. 1973, 44, 475. 0743-7463/S7/2403-0377$01.50/0 0 1987 American Chemical Society
