Langmuir 1989, 5 , 89-95 The final step may be slow and represents the path for passivation of the surfaces of the coinage metals.
Acknowledgment. This work was supported by the National Science Foundation under Grant No. CHE8516247. No' (TPP)Zn?14074-80-7; (TPP)Co, 14172-90-8; (17)Chin, D.-H.; LaMar, G. N.; Balch, A. L. J.Am. Chern. Soc. 1980, 102,4344.
89
(TPPIFe,16591-56-3;(TpP)Mn,31004-82-7;PhO-, 3229-70-7;.OH, 3352-57-6;HzO,7732-18-5;(C18TPP)FeOH,98715-91-4;CuOH, 12125-21-2;AgOH, 12258-15-0;AuOH, 12256-43-8;0', 14337-01-0; (Cu=O)-, 59700-47-9;(Ag==O)-,35366-11-1;(Au=O)-, 11722621-8; Cuz, 12190-70-4; Ag,, 12187-06-3; Au,, 12187-09-6; eo., 17778-80-2;03,10028-15-6;( C & T P P ) F d 113533-07-6; , CUOCU, 1317-39-1; AgOAg, 20667-12-3; AuOAU, 1303-57-7; CUOOCU, 80345-87-5;AgOOAg, 25455-73-6;AuOOAu, 117203-41-5; . C u d , 1317-38-0;*Ag==O,1301-96-8;.Au=O, 37043-69-9;OH-, 1428030-9; C U , 7440-50-8;Ag, 7440-224 Au, 7440-57-5;c , 7440-44-0; 02, 7782-44-7;Cu1(MeCN),(C104),14057-91-1;AgC104,7783-93-9.
Binding of Polyelectrolytes to Oppositely Charged Ionic Micelles at Critical Micelle Surface Charge Densities P. L. Dubin,* S. S. The, D. W. McQuigg, C. H. Chew,? and L. M. Gant Department of Chemistry, Indiana University-Purdue University at Indianapolis, Indianapolis, Indiana 46223 Received June 10, 1988. I n Final Form: August 12, 1988 The interaction of ionic/nonionic surfactant micelles with strong polyelectrolyteshas been studied by turbidimetric titrations. There exists a critical micellar fraction of ionic head groups, Y,, below which no interactions can be detected. At Y > Y,, the solute components form either soluble aggregates,complex coacervate, or amorphous precipitate, depending, in part, on the po1ymer:surfactant stoichiometry. The dependence of Y, on the square root of the ionic strength, PI2,appears as a linear phase boundary, which is rather insensitive to polymer concentration or total surfactant concentration. For several polyelecat values of trolyte/mixed-micelle systems, we have measured the surface potential of the micelle (qo) Y and I corresponding to the polyion-micelle complex formation phase boundary. In the system poly(dimethyldiallylammonium chloride)/sodium dodecyl sulfate, CI2EO6,the values of \komeasured from the pK, of the probe, dodecanoic acid, are constant (ca. -10.8 mV) along the phase boundary. For the same polymer-surfactant system, \Eo measured with the fluorescence probe hexadecylhydroxycoumarin increases with Y, from -18 to -24 mV along the phase boundary. For the system of reversed charge, sodium polystyrenesulfonate/dodecyldimethylamine oxide, in which the cationic surfactant head group itself acts as a potential probe, the surface potential varies from +12 to +43 mV along the phase boundary. The foregoing results, taken together with the absence of an effect of polymer chain length on Y,, suggest that critical conditions for complex formation are dictated by the cooperative adsorption of a relatively short sequence of polyion segments to the colloid surface. It appears likely that this process occups when a critical potential is reached at the locus of the closest approach of the polymer chain to the micelle surface. The measured potential at critical conditions increases with the distance between this steric boundary and the mean position of the solubilized probe.
Introduction The voluminous literature on polymer-surfactant interactions has been documented in a number of reviews.14 Predominantly, these studies deal either with complexes formed between polyelectrolytes and oppositely charged ionic surfactants below the critical micelle concentration (cmc) or with the association of nonionic polymers (e.g., poly(oxyethy1ene)) with micelles of ionic surfactants (e.g., SDS). Conspicuously absent are investigations of interactions between polyelectrolytes and ionic surfactants above the cmc. The reasons for this lacuna are straightforward: charged micelles, which do not interact with polyelectrolytes of like charge, associate so strongly with oppositely charged polyelectrolytes that irreversible phase separation usually results. We have, however, found that this interaction may be attenuated by reduction of the micelle surface charge density, most easily accomplished with mixed ionic/nonionic micelles."10 Such systems may provide a paradigm for the general phenomenon of Coulombic association between polyelectrolytes and oppositely charged colloids. Permanent address: Department of Chemistry, National University of Singapore, Singapore. f
0743-7463/89/2405-0089$01.50/0
Strong polyelectrolytes form complexes with oppositely charged mixed micelles." Complex formation appears to be a critical phenomenon, occurring abruptly upon decrease in ionic strength, I , or increase in micelle surface charge density, u. The second variable may be controlled via the mole fraction of ionic surfactant in ionic-nonionic mixed micelles, Y,5-9or by pH titration of micelles with weak acid or weak base head groups, such as alkylamine (1)Robb, I. D. In Anionic Surfactants: Physical Chemistry of Surfactant Action, Lucassen-Reynders, E. H., Ed., Marcel Dekker: New York, 1981;p 109. (2) Goddard, E. D. Colloids Surf. 1986,19, 255. (3)Goddard, E.D. Colloids Surf. 1986,19, 301. (4)Dawydoff, W.; Linow, K.-L.; Phillip, B. Acta Polyrn. 1987,38,307, 461. (5)Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983,95, 453. (6)Dubin, P. L.;Davis, D. D. Macromolecules 1984,17, 1294. (7)Dubin, P. L.; Rigsbee, D. R.; McQuigg, D. W. J.Colloid Interface Sci. 1985,105, 509. (8)Dubin, P.L.; Davis, D. D. Colloids Surf. 1985,13, 113. (9)Dubin. P. L.; Ripsbee, D. R.; Gan, L. M.; Fallon, M. A. Macromolecules 1988,21,2555. (10)Dubin, P. L.; Chew, C. H.; Gan, L. M. J. Colloid Interface Sci., in press. (11)A variety of studies have been carried out to confirm that association occurs uniquely between polyion and micelles and not between polyion and free surfactant (ref 5 and 21). Perhaps the most striking result is the absence of turbidity on addition to polymer of mixed micelle, just below Yc,regardless of the quantity of surfactant added. ~~
0 1989 American Chemical Society
90 Langmuir, Vol. 5, No. 1, 1989
oxides.10 At constant Z, a critical value of Y may be identified such that below Y, no observable interaction occurs between polyion and micelle. For Y > Y,, one observes the formation either of a second liquid phase, i.e., a "complex coacervate",12or of reversible "soluble" polyion-micelle complexes with dimensions comparable to or somewhat larger than those of the surfactant-free polymer.'js9 We have investigated a number of polyelectrolyte/surfactant systems including28 PDMDAAC/TX100/SDS,H PDMDAAC/C~~EOG/SDS,' PDMDAAC/TX100/CgCOONa,13 Q4VP/TX-l00/SDS,' NaPSS/TX100/DTMAB,'O NaPSS/DMDAO,'O and PAA/TX-100/ DTMAB.'" In many of these systems, it appears that Y, varies linearly with PI2,although this dependence is sensitive to changes in micelle dimensions over the range of Y and Z investigated.'O This dependence on Z1I2 suggests a dependence on the reciprocal Debye length, K-', while the significance of Y is presumably its relation to the micelle surface charge density, u. Indeed, if the mean head group area or the micelle curvature did not depend on micelle composition, u would vary linearly with Y. Phase boundary plots of Y, vs I or Y, vs P I 2 are found to be quite insensitive to the concentration of polymer or of total Surfactant! This observation suggesta that complexes are not primarily interpolymer, a speculation borne out by dynamic light scatteringe*@ as well as by limited viscosity measurements.6 Furthermore, for the system NaPSS/DMDAO, Y, is nearly independent of polymer chain length over the range 2 X lo4 < MW < 6 X lo6, which would also seem to preclude a role for interpolyion aggregation. Phase separation is thus seen as a consequence of localized electrostatic interactions between a flexible charged chain and an oppositely charged surface. Investigations of this phenomenon are of interest as a way to shed light on the general phenomenon of electrostatic association between polyions and oppositely charged particles. One may note, for example, that the "free sliding" or nonspecific binding of proteins to DNA,14which is one component of the gene recognition and gene expression process, is viewed as a primarily electrostatic process, readily suppressed by an increase in ionic strength. Another example of Coulombic polyion-colloid interaction is the flocculation of inorganic particles by polyelectrolytes, e.g., in water clarification. Also, complex formation between synthetic polyelectrolytes and proteins has received some attention as a route to stabilized or immobilized enzymesI5and as the basis of nonchromatographic protein separations.''j Polyelectrolyte-micelle complexes offer a useful paradigm for the phenomena cited above, inasmuch as the surface charge density of the colloidal component may be varied in a continuous fashion. Of particular interest is the critical nature of complex formation, which, as noted above, occurs abruptly with an increase in micelle surface charge or a decrease in ionic strength. This phase transition like behavior presumably corresponds to a discontinuity in the free energy of complex formation. Odijk has indeed predicted from theoretical considerations that (12)Bungenburg de Jong, H. G. Trans. Faraday SOC.1932,28, 27. (13)Dubin, P. L.; Th6, S. S. M.S. Thesis, Purdue University, 1988. (14)Shaner, S. L.; Melanqon, P.; Lee, K. S.; Burgess, R. R.; Record, M. T., Jr. Cold Spring Harbor Symp. Quant. Biol. 1983,47,463. (15)(a) Margolin, A. L.; Sherstuyk, J. F.; Izumrudov, V. A.; Zezin, A. B.; Kabanov, V. A. Eur. Polym. J.1985,146,625.(b) Kohjiya, S.; Maeda, K.; Ikushima, Y.; Ishihara, Y.; Yamashita, S. Nippon Kagaku Kaishi 1986,12, 2302. (c) Hua, J.; Liu, L.; Yin, R.; Fang, S.; Jian, Y. J. Appl. Polym. Sci. 1963,31,2667. (16)(a) Sternberg, M.; Hershberger, D. Biochim. Biophys. Acta 1974, 342,195. (b) Bernfield, P.;Nisselbaum, J. S.; Berkeley, B. J.; Hanson, R. W. J . Biol. Chem. 1960,235,2852.
Dubin et al. charged flexible chains will collapse on oppositely charged rigid rods as the ionic strength is reduced below some critical value." However, rigorous calculation of the total free energy for systems such as ours is a formidable task. But it is clear that measurement of the micelle surface potential at critical conditions must be prerequisite to even a zero-order treatment. For the foregoing reasons we have measured micelle surface potentials a t and near conditions for critical complex formation. For the system PDMDAAC/Cl2EO6/SDS we have employed amphiphilic acids as probes for micelle surface potential. From enhancement of the pKa of these acids one may determine the potential in the vicinity of the acid group. We have measured these pKa values by using either a weak acid fluorescent detergent probe solubilized in the micelle18or by potentiometric titration of an alkylcarboxylic acid c o s ~ r f a c t a n t . As ~ ~ we shall see, the position of the probe, relative to the ionic head groups and the distance of closest approach of the polyion segments, is a major consideration in such studies. Therefore, we have also measured the surface potential of DMDAO micelles, for which the micelle surface is more clearly defined. In this case the relevant phase boundary is obtained by turbidimetric pH titrations in the presence of the strong polyanion NaPSS.
Experimental Section Materials. SDS was purissima grade from Fluka (Happague, NY), ClzEOBwas from Nikko Chemicals Co. (Tokyo),CllCOONa was from Sigma (St.Louis, MO),DMDAOwas from Fluka Chemie AG (>97% pure), and Triton X-100was from Rohm & Haas (Springhouse, PA). ClPEOI2was kindly provided by Dr. J. E. Thompson, Proctor & Gamble Co., Cincinnati, OH. A sample of PDMDAAC, reported MW ca. 2 X l@(Trade name "Merquat loo"), was a gift from Calgon Corp. (Pittsburgh, PA) as were samples with the following reported molecular weights 1 X lo4, 5 X lo4, 2 X lo6,and 2 X lo6. NaPSS narrow MWD standards were from Pressure Chemical Co. (Pittsburgh, PA). The (HHC) was fluorescence probd 4-heptadecyl-7-hydroxycoumarin from Molecular Probes, Inc. (Eugene, OR). Methods. 1. Turbidimetric Titrations. Turbidimetric titrations of mixed micellepolyelectrolytesystems may be carried out in several ways. In 'type I" titrations, anionic surfactant is added to a mixture of nonionic surfactant and polymer; consequently, Y is increased at constantZ (and nearly constant polymer concentration and total surfactant concentration). "Type 11" titrations correspond to the addition of mixed micelles to polymer at constant Y and Z and provide information about the stoichiometry of complex formation. Last, polymer may be added to micellar solutions in "type 111" titrations. Type 1titrations were performed by adding 40 mM SDS, using a 2 . 0 - d Gilmont microburet,to a mixture of 1g L-' PDMDAAC and 20 mM C1&O8 at constant ionic strength (adjusted with NaC1) and at 25.0 0.1 "C. Turbidity was monitored with a Brinkmann PC 800 probe (2 cm) colorimeterat 420 nm. After correction for scattering of the micelles by subtraction of values obtained with a polymer-freesolution,the blank-correctedturbidity was plotted vs Y. A sharp increase in turbidity occurs at a well-defined surfactant composition, denoted as Y,. For DMDAO, a mixture of 50 mM surfactant and 0.50 g L-l NaPSS was titrated, in the desired NaCl solution, with 0.500 M HC1 until the abrupt appearance of turbidity at some critical pH. The relationship between pH, and @,,where @ is the degree of protonation of the micellar amine oxide head groups, was obtained from potentiometric titration as described below. 2. Potentiometric Titrations. pH titrations were carried out in polymer-free solution by using a temperature-compensated (17)Odijk, T. Macromolecules 1980,13,1542. (18)(a) Fernandez, M.S.; Fromhen, P. J.Phys. Chem. 1977,81,1755. (b) Fernandez, M. S. Biochim. Biophys. Acta 1981,646,23. (19)Siano, D. B.; Meyer, P.; Bock, J. J. Colloid Interface Sci. 1987, 11 7,544.
Binding of Polyelectrolytes to Ionic Micelles
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0.6
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Langmuir, Vol. 5, No. I, 1989 91
T
Y
c
00 00
01
02
03
04
Y
Figure 1. Turbidimetric titrations of 1.0 g L-' PDMDAAC + 20 mM Cl&Os with 40 mM SDS at varying ionic strengths at 25.0 "C.
Orion 811pH meter with a Model 91-00-02combination electrode. To determine pK, and pKo of the micelle-solubilized probe, CllCOOH, this carboxylic acid was combined at a mole ratio of 1:20 with 20 mM ClZEO6,with or without SDS. Because of the low solubility of the carboxylic acid form, CllCOONa was added to the other components at pH 11 and 40 O C . After complete dissolution, the solution was cooled and titrated under Nzat 24 f 1 O C with 0.500 N HC1. Inflection points in the titration curve were observed at a = 1 and CY = 0, where a is the degree of neutralization of carboxylic acid. The mean value of pH + log [(l- .)/CY]between a = 0.2 and a = 0.8 was used to determine pK, (pKoin the case of SDS-freesolutions). The surface potential of the mixed micelle \ko was then calculated fromm pK, - pKo = -0.434e\ko/kT (1) where kT/e = 25.6 mV at 24 "C. Similar titrations were conducted for 50 mM DMDAO, following the method of Tokiwa et d." DMDAO was first dissolved in NaCl solution with the addition of slight excess base to produce a solution with pH ca. 9.5 and then titrated with 0.500 M HC1. Inflection points were observed at both 6 = 1and 0 = 0, at pH 9.0 and 2.9, respectively, in the blank-corrected titration curves. The former inflection could not be observed if the surfactantwas dissolved in NaCl alone; such titrations produced anomalous curvature at the extremes of pK vs 0 plots. It seems likely that the sample as provided is alrealy partially protonated, perhaps through adsorption of COz and moisture. 3. Fluorescence Titrations. Aliquots of stock HHC in ethanol (4.2 mg mL-') were added to a 20 mM micellar solution of ClzE06-with or without SDS-by using 1min of sonication to give a mole ratio of HHC to total surfactantof less than 1:750. The pH was adjusted to 12.5 with 5 M NaOH and then adjusted with HCl to the desired pH in the range pH 7-12.5. At this second point, the indicator is completely dissociated. Fluorescence spectra were measured with an SLM 8000C spectrofluorometer,with an excitation wavelength of 366 nm, at an emission wavelength of 450 nm. The fluorescence emission of the protonated form at 450 nm is zero. Consequently,the degree of dissociation CY at each pH is the measured fluorescence divided by the limiting value observed at high pH. The pK, of the micelle-solubilizedindicator was obtained as pH + log [(l - a ) / a ]with a precision of f0.03. Micelle surface potential at various values of Y and Z was obtained from the difference in pK, for micelles with and without SDS, i.e., by application of eq 1.
Results PDMDAAC/C12E06/SDS. 1. Turbidimetric Titrations. Turbidimetric titrations of PDMDAAC + C12E06with SDS a t varying ionic strengths are shown in Figure 1. The abrupt increase in turbidity corresponds to phase separation and occurs at a mole fraction of anionic surfactant Y,. (It is noteworthy that the substitution of (20) Tokiwa, F.;Ohki, K.J.Phys. Chem. 1966, 70,3938.
Figure 2. Dependence of Y , on ionic strength for PDMDAAC/Cl2EO6/SDS(from data of Figure 1).
