A STUDY OF THE AGGREGATION BEHAVIOR OF HEXYLTRIMETHYLAMMONIUM BROMIDEIN AQUEOUS-SOLUTION

The self-association of n-hexyltrimethylammonium bromide (C(6)TAB) in aqueous solution has been examined as a function of temperature and el ectrolyte concentration. The critical micelle concentration (CMC) and the degree of counterion binding (beta) were determined by conductivit y measurement at temperatures over the range 288.15-318.15 K. Ultrasou nd 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) a nd static light scattering to obtain the aggregation number and the de gree of counterion binding in water at 298.15 K. The enthalpy change o n micellization in water was measured by microcalorimetry, Apparent ad iabatic compressibilities were calculated from a combination of densit y and ultrasound velocity measurements. Changes in the thermodynamic p roperties 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-alkyl trimethylammonium bromides it has been shown that the CMC of C(6)TAB i n water is lower than that predicted from the linear relationships bet ween CMC and the number of carbon atoms in the alkyl chain. Similarly, the standard Gibbs energy of micellization is less negative than pred icted, and the degree of counterion binding is much lower than for oth er C(n)TABs. It is suggested that the anomalous behavior of C(6)TAB 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 t he micellar components to the aqueous environment. (C) 1998 Academic P ress.