Calorimetric determination of the thermodynamics of alcohol-surfactant mixed micelle formation: Temperature and concentration effects

Abstract

The thermodynamic parameters of surfactant/alcohol mixed micelles were determined in mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in a series of solvents composed of varying concentrations of n-propanol (C3OH), n-butanol (C4OH), and n-pentanol (C5OH). Conductivity studies were used to determine CMC values and the degrees of counterion binding. Isoperibol solution calorimetry was used to directly determine CMC values and enthalpy of micellization (ΔmicH). From these experimentally determined values, the Gibbs energy and entropy of micellization were calculated as a function of temperature and alcohol concentration. In general, CMC values decreased with increasing alcohol carbon chain length and concentration but increased with temperature. The effects of temperature, alcohol carbon chain length, and alcohol concentration on the energetics of these mixed micellar systems are explained in terms of the relative contributions to ΔmicH by alkyl chain interactions, hydrophobic effects, and electrostatic contributions.