Binding of Polycarboxylic Acids to Cationic Mixed Micelles: Effects of Polymer Counterion Binding and Polyion Charge Distribution.

Mixed micelles of cetyltrimethylammonium chloride (CTAC) and n-dodecyl hexaoxyethylene glycol monoether (C12E8) bind to polyanions when the mole fraction of the cationic surfactant exceeds a critical value (Yc). Yc corresponds to a critical micelle surface charge density at which polyelectrolyte will bind to this colloidal particle. Turbidimetric titrations were used to determine Yc for such cationic-nonionic micelles in the presence of acrylic acid and acrylamido-2-methylpropane sulfonate homopolymers (PAA and PAMPS, respectively) and their copolymers with acrylamide, as function of pH, ionic strength, and polyelectrolyte counterion. In 0.20 M NaCl, Yc for PAA is found to be remarkably insensitive to pH, i.e., virtually independent of the apparent polymer charge density xiapp. On the other hand, the expected inverse relationship between Yc and xiapp is observed either for PAA when NaCl is replaced by TMACl (tetramethylammonium chloride), or when xiapp is manipulated using acrylic acid/acrylamide copolymers at high pH. The effective charge density of PAA is thus seen to be suppressed by specific sodium ion binding, indicating that the influence of salts on the interaction of polycarboxylic acids with colloidal particles may differ qualitatively from their effect on the analogous behavior of strong polyanions. Comparisons between homo- and copolymers of acrylic acid were carried out also to test the hypothesis that the "mobility" of charges on PAA at moderate pH (degree of ionization less than unity) could make this "annealed" polymer exhibit the behavior of a more highly charged one. The results, while consistent with this expectation, were obscured by the likely effect of copolymer sequence distributions. Copyright 1998 Academic Press.

Analysis of Polydispersity of Mixed Micelles of TX-100/SDS and C12E8/SDS by Capillary Electrophoresis

Capillary electrophoresis (CE) was used to study the polydispersity of TX-100/SDS and C12E8/SDS micelle systems. Compared with other techniques, CE provides more specific and graphically detailed information on the compositional polydispersity of such micellar systems. Clear evidence of the coexistence of micelles rich in either nonionic or anionic component has been obtained. This coexistence is explained by considering that the stability of mixed micelles is affected by several factors, including counterion binding, and steric shielding of the electrostatic repulsive interaction among the headgroups of SDS by the bulky EO groups of TX-100. Due to these two factors, micelles with different compositions may be energetically equivalent, leading to a broad distribution of micelle size and composition. In this study, we observe how the pattern of the distribution changes with the mole fraction of SDS. Electrophoretographic patterns also showed that broader compositional distribution of the original components produces a wider distribution of mobilities.

Cryo-TEM of Polyelectrolyte-Micelle Complexes

Cryo-TEM was carried out on samples containing polyelectrolyte-micelle complexes, formed by combining poly(diallyldimethylammonium chloride) (PDADMAC), a strong cationic polyelectrolye, with oppositely charged mixed micelles of sodium dodecyl sulfate (SDS) and nonionic Triton X-100 (TX100), in 0.40 M NaCl. Complexation appears to involve the formation of micelle-rich regions, presumably within the domains of polymer chains, without any particular organization or restructuring of the micelles. At polymer concentrations (CP)

Size-Exclusion Chromatography of Carboxyl-terminated Dendrimers as a Model for Permeation of Charged Particles into Like-Charged Cavities

In order to examine the permeation of small charged colloids into cavities of like charge, size-exclusion chromatography was carried out with carboxyl-terminated dendritic polymers on a porous glass stationary phase. Chromatographic partition coefficients KSEC were measured for solute diameters ranging from 2 to 8 nm, at neutral pH, and ionic strengths from 0.01 to 0.09 M, and were reported relative to those of noninteracting solutes of equal size (pullulan and Ficoll). The experimental results were compared to values calculated using the treatment of F. G. Smith III and W. M. Deen for charged spheres in cylindrical pores of like charge (J. Colloid Interface Sci. 78, 44 (1980); 91, 571 (1983)). The measured degrees of permeation were typically 20-100% larger than the calculated values, the discrepancies being greatest for the larger solutes at high ionic strength. It is shown that the tendency of the calculations to overestimate the repulsion are likely to arise from linearization of the Poisson-Boltzmann equation.