Critical micelle concentration and self-aggregation of hexadecyltrimethylammonium bromide in aqueous glycine and glycylglycine solutions at different temperatures

Abstract

Conductivities, densities and ultrasonic speeds measurements of hexadecyltrimethylammonium bromide (HTAB) in aqueous solutions of glycine (Gly) and glycylglycine (Gly-Gly) have been made at various temperatures. The critical micelle concentration (CMC), the degree of ionization (β) of the micelles, standard free energy, enthalpy, and entropy of the micellization process (ΔGm°, ΔHm°, and ΔSm°) for the present systems were estimated at different temperatures. The CMC values of HTAB in aqueous Gly and Gly-Gly were also evaluated by density and ultrasonic speed measurements. Apparent molar volumes, (Vϕ), apparent molar volumes at infinite dilution, (Vϕ°), apparent molar compressibilities, (Kϕ), of HTAB in the pre- and post-micellar regions, and volume change on micellization (ΔVϕm) were also estimated. Large positive values of TΔSm° and small negative values of ΔHm° suggest that micellization process is driven primarily by entropy increase. The increase in ΔVϕm and Kϕ with rise in temperature is indicative of less compact micellar structure of HTAB in presence of amino acid additives. These data suggest that amino acids are solubilised probably in the palisade layer of the micelle.