Direct Measurements of the Thermodynamic Properties of Surfactants

Abstract

The thermodynamics of micellization can be derived from a study of the influence of various factors, such as temperature, pressure and electrolytes, on the critical micelle concentration (CMC). A more promising approach is to measure directly the thermodynamic functions of surfactants over a wide concentration range. In principle such data can be used for the study of (a) monomer-monomer interactions in the pre-micellar region, (b) changes in the thermodynamic functions during micellization, (c) micelle-micelle and micelle-counterion interactions in the post-micellar region. With modern instrumentation most functions can now be measured down to at least 0.01 molal. The interpretation of heat capacity, volume, expansibility and compressibility does not present much problem but most calorimetric determination of heats and entropies of micellization of ionic surfactants in the literature are doubtful since solute-solute interactions in the pre-and post-micellar regions were not taken into account correctly. With some systems, such as sodium decanoate, there are two breaks in the free energy curve, and the difference in free energy between these two breaks corresponds approximately to that which can be calculated from the corresponding enthalpy and entropy of micellization. Finally similar studies with aqueous organic mixtures, such as alcohols, amines and alkoxyethanols, indicate that transitions very similar to micellization can occur in the water-rich region, suggesting that these solutions also exist as microphases at high concentrations.