Theoretical modeling of micellization and solubilization in ionic surfactant systems

Abstract

Ionic surfactants constitute a very important class of surfactants, both from an academic as well as from a commercial viewpoint. In most commercial applications involving ionic surfactants, the selection of the surfactant is based on its aqueous bulk solution properties, including its micellization characteristics and its micellar solubilization characteristics (that is, the capacity of the surfactant micelles to encapsulate hydrophobic solutes in their interior, leading to a significant increase in the aqueous solubility of the hydrophobic solutes). Accordingly, the development of comprehensive, predictive, molecularly-based theories that relate the bulk solution properties of an ionic surfactant to its chemical structure should facilitate significantly the rational design, selection, and optimization of commercial surfactant formulations. The first major contribution of this thesis was the development of a molecular-thermodynamic theory of micellization of ionic surfactant-electrolyte systems, which takes into account the possibility that counterions (inorganic or organic, monovalent or multivalent) released by the ionic surfactant polar heads and by any added electrolytes can bind onto the charged micelle surface. By minimizing the free energy of micellization, various micellar solution properties were predicted quantitatively in the context of this theory, including : (i) the degree of binding of each counterion species onto the charged micelle surface, (ii) the micelle surface electrostatic potential, (iii) the critical micelle concentrations (CMC's), and (iv) the optimal shapes and average sizes of the micelles formed in solution.

(cont.) The micellization theory was applied to surfactant solutions containing monovalent counterions (specifically, alkali metal ions), multivalent counterions (specifically, A13+ and Ca2+), and organic counterions having pendant hydrophobic groups that penetrate into the micelle core (specifically, the salicylate ion). For all the surfactant systems considered, the quantitative predictions made compared well with the relevant experimental results available in the literature. The second objective of this thesis was to study the effect of the surfactant tail (chain) molecular structure on the surfactant micellar solution properties, for surfactants having more complex tail structures than linear alkyl tails, including: (i) branched akyl tails, (ii) alkylbenzene tails, and (iii) fluorocarbon tails. For surfactants with branched alkyl tails and alkylbenzene tails, a single-chain mean-field theory of chain packing was combined with suitable Rotational Isomeric State (RIS) models describing the chain torsional conformations, and the chain conformational characteristics as well as the packing free energy were predicted. Although the micellar solution properties of these surfactants were not investigated, the predicted packing characteristics can be viewed as a valuable "first-step" in the development of a comprehensive, predictive, molecularly-based micellization theory for these surfactants. In the case of fluorocarbon surfactants, in addition to modeling chain packing within the micelle core using suitable RIS models for the fluorocarbon chains, the remaining free-energy changes associated with micelle formation were modeled as well ...