Diffusion of interacting micelles: A general treatment at low salt concentrations

The influence of chain length and electrolyte on the adsorption kinetics of cationic surfactants at the silica–aqueous solution interface

The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica-aqueous solution interface have been investigated using optical reflectometry. The effect of added electrolyte, the length of the hydrocarbon chain, and of the counter- and co-ions has been elucidated. Increasing the length of the surfactant hydrocarbon chain results in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicate that above the cmc micelles are adsorbing directly to the surface and that as the chain length increases the hydrophobicity of the surfactant has a greater influence on the adsoption kinetics. While the addition of 10 mM KBr increases the CTAB maximal surface excess, there is no corresponding increase for the addition of 10 mM KCl to the CTAC system. This is attributed to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) have little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption is increased in the presence of electrolyte. Slow secondary adsorption is seen over a range of concentrations for CTAC in the absence of electrolyte and importantly in the presence of LiCl; the origin of this slow adsorption is attributed to a structural barrier to adsorption.

A Study of the Aggregation Behavior of Hexyltrimethylammonium Bromide in Aqueous Solution

The self-association of n-hexyltrimethylammonium bromide (C6TAB) in aqueous solution has been examined as a function of temperature and electrolyte concentration. The critical micelle concentration (CMC) and the degree of counterion binding (beta) were determined by conductivity measurement at temperatures over the range 288.15-318.15 K. Ultrasound velocity measurements were used to obtain the CMC in water and in a range of concentrations of electrolyte (0.1 to 0.6 mol kg-1 NaBr) and static light scattering to obtain the aggregation number and the degree of counterion binding in water at 298.15 K. The enthalpy change on micellization in water was measured by microcalorimetry. Apparent adiabatic compressibilities were calculated from a combination of density and ultrasound velocity measurements. Changes in the thermodynamic properties on micellization were determined by applying the mass action model; good agreement was found between experimental and theoretical enthalpy changes. From comparison with the properties of other n-alkyltrimethylammonium bromides it has been shown that the CMC of C6TAB in water is lower than that predicted from the linear relationships between CMC and the number of carbon atoms in the alkyl chain. Similarly, the standard Gibbs energy of micellization is less negative than predicted, and the degree of counterion binding is much lower than for other CnTABs. It is suggested that the anomalous behavior of C6TAB is a consequence of the more highly organized core of the aggregates of very low aggregation number (3-4) and the high degree of exposure of the micellar components to the aqueous environment. Copyright 1998 Academic Press.

Photon correlation spectroscopy of surface active cationic drugs

Photon correlation spectroscopy (PCS) has been used to examine the aggregation in aqueous NaCl solution of a series of antidepressant and antihistamine drugs (hydrochlorides of imipramine, clomipramine, amitriptyline, butriptyline, protriptyline, doxepin, dothiepin, iprindole, diphenhydramine, bromodiphenhydramine, orphenadrine) propranolol hydrochloride and propantheline bromide. Critical micelle concentrations were measured by surface tension and PCS. Micellar sizes were investigated as functions of drug structure and drug and NaCl concentration. Generally, antidepressants formed the largest micelles. We propose that the antidepressants aggregate in a similar fashion to the phenothiazines by stacking with size increasing by addition of single monomers to stacks and by addition of more stacks to the aggregate.

Cholesterol monohydrate dissolution rate studies in aqueous micellar sodium chenodeoxycholate solutions

The dissolution rate of cholesterol monohydrate in various concentrations of sodium chenodeoxycholate (1) was significantly influenced by the addition of strong electrolytes. The mass transfer resistances decreased with increasing electrolyte concentrations and attained an asymptotic minimum value predicted and experimentally established for the convective/diffusion-controlled situation. Reduction of the interfacial barrier to dissolution was many times more sensitive to Mg2+ than to Na+ at equimolar concentrations. Cholesterol monohydrate solubilities increased nonlinearly with increasing 1 in 0.01 M phosphate buffer at pH 8.0 and was not influenced by the presence of strong electrolytes. Measured diffusion coefficients gave supporting evidence that the effective micellar size remained the same within the various experimental systems up to 116 mM chenodeoxycholate. The experimental findings indicated that the interfacial barrier is electrostatic in character. They are consistent with the phenomenon of diffusion of negatively charged micelles toward a negatively charged cholesterol monohydrate surface and the subsequent collision complex transfer of cholesterol molecules at the crystal surface. The results and mechanistic interpretations are also in accord with the previous model studies on cholesterol monohydrate dissolution in the presence of mixed micelles composed of nonionic and ionic surfactants.