Study of the effect of electrolyte and temperature on the critical micelle concentration of dodecyltrimethylammonium bromide in aqueous medium

The effect of three organic electrolytes, viz. tetrapropylammonium bromide (C12H28NBr), tetrabutylammonium bromide (C16H36NBr), and tetrapentylammonium bromide (C20H44NBr) (0.01 mol . kg (1)) on the aggregation behaviour of a cationic surfactant, dodecyltrimethylammonium bromide (DTAB) (1.5 to 29.8) mmol . kg (1) in aqueous medium over a wide temperature range (288.15 to 318.15) K has been studied by conductivity measurements. From conductivity versus surfactant concentration plots, the critical micelle concentration (CMC) of DTAB has been determined, which shows that all the investigated electrolytes promote the aggregate formation of DTAB. The CMC values decrease to a certain minimum value around T = 298.15 K in all cases and then increase with further rise in temperature, displaying a typical U-shaped behaviour. Further, from the temperature dependence of CMC values, various thermodynamic parameters of micellization viz. standard free energy change (Delta G(m)(0)), standard enthalpy change (Delta H-m(0)) and standard entropy change (Delta S-m(0)) have been calculated. The values of Delta G(m)(0), Delta H-m(0) and Delta S-m(0) indicate that the electrolytes have arbitrated DTAB aggregation and micellization process is both enthalpy as well as entropy controlled. The results reveal the major role of hydrophobic interactions in the present (surfactant + electrolyte + water) ternary system. Fluorescence probe study has also been employed to affirm the CMC values obtained from conductivity measurements. (C) 2014 Elsevier Ltd. All rights reserved.