Molecular-thermodynamic approach to predict micellization, phase behavior and phase separation of micellar solutions. I. Application to nonionic surfactants

Abstract

We present a detailed description of a molecular‐thermodynamic approach which consists of blending a molecular model of micellization with a thermodynamic theory of phase behavior and phase separation of isotropic (surfactant–water) micellar solutions. The molecular model incorporates the effects of solvent properties and surfactant molecular architecture on physical factors which control micelle formation and growth. These factors include (i) hydrophobic interactions between surfactant hydrocarbon chains and water, (ii) conformational effects associated with hydrocarbon‐chain packing in the micellar core, (iii) curvature‐dependent interfacial effects at the micellar core–water interface, and (iv) steric and electrostatic interactions between surfactant hydrophilic moieties. The free energy of micellization g_mic is computed for various micellar shapes Sh and micellar‐core minor radii l_c. The ‘‘optimum’’ equilibrium values, l^*_c, Sh*, and g^*_mic, are obtained by minimizing g_mic with respect to l_c and Sh. The deduced ‘‘optimum’’ micellar shape Sh* determines whether the micelles exhibit two‐dimensional, one‐dimensional, or no growth. These results are then utilized in the thermodynamic theory to predict a broad spectrum of micellar solution equilibrium properties as a function of surfactant concentration and temperature. These properties include (1) the critical micellar concentration, (2) the micellar size distribution, (3) the critical surfactant concentration for the onset of phase separation, and (4) other thermodynamic properties such as the osmotic compressibility. The proposed molecular‐thermodynamic approach provides an excellent description of a wide range of experimental findings in aqueous solutions of nonionic surfactants belonging to the polyoxyethylene glycol monoether and glucoside families.