Theoretical and experimental investigations of electrostatic effects associated with ionic surfactant micelles

Ionic surfactants are the most commonly used type of surfactant, yet the complex electrostatic intramicellar and intermicellar interactions involved in micelle formation and micellar solution phase behavior of these surfactants are not well understood. In this thesis, both theoretical and experimental investigations were conducted in order to develop a better molecular-level understanding of the micellar properties of aqueous ionic surfactant solutions. The first major contribution of this thesis was to extend a molecular-thermodynamic theory of micellization and micellar solution phase behavior previously developed by our group for nonionic surfactants to describe and model the behavior of ionic surfactants. Analytical approximations to the Poisson-Boltzmann equation were used to calculate the electrostatic contribution to the free energy of micellization. To correct for the neglect of the finite size of the ions in the ion cloud, the model was modified to include a Stern layer, a region immediately surrounding the micelle surface from which the counterions are excluded. Including the Stern layer improved CMC predictions and provided some counterion specificity. In addition, a model for the fractional counterion binding was developed based on the Gibbs adsorption equation. In an effort to make the molecular-thermodynamic theory more accessible to both academic and industrial surfactant researchers, it was incorporated into a user-friendly computer program, known as program PREDICT. Program PREDICT is capable of predicting a wide range of micellar solution properties for a variety of ionic, nonionic, and zwitterionic surfactants. Examples of many of its predictive capabilities are presented and compared with experimental data. In addition, some fundamental predicted surfactant solution properties are correlated with industrially-relevant surfactant performance characteristics. The molecular-thermodynamic theory with a Stern layer model was found to underestimate micellar growth of typical ionic surfactants in the presence of added salt. This underestimation of micelle size is due primarily to the neglect of electrostatic intermicellar interactions. In an effort to address intermicellar interactions using a more rigorous, systematic approach, a new statistical-thermodynamic framework for micellar solutions based on the McMillan-Mayer theory of multicomponent solutions was developed. The framework was first implemented in the case of nonionic surfactant solutions exhibiting attractive and excluded-volume intermicellar interactions. It was demonstrated that repulsive excluded-volume intermicellar interactions encourage micelle formation and growth. In addition, this framework was used to make accurate predictions of several micellar solution properties, such as the CMC, the critical concentration for phase separation, and the osmotic compressibility. The McMillan-Mayer approach was then extended to model the behavior of ionic surfactant solutions which exhibit both excluded-volume and electrostatic intermicellar interactions. To model the electrostatic intermicellar interactions, the other micelles in solution were included as part of the diffuse ion cloud surrounding a central micelle. Two different approaches were used to calculate the electrostatic potential created by the micelle and the ion cloud. First, the Debye-Hickel approximation, which provides an analytical solution and can model both spherical and cylindrical micelles, was used to predict micellar growth. Indeed, it was found that electrostatic intermicellar interactions encourage micellar growth, yielding a significant improvement over the predictions of the simpler molecular-thermodynamic theory. The second approach involved the use of a modified Poisson-Boltzmann equation, which included the effect of the finite size of the charged solutes in the ion cloud. This approach represents a more accurate description of the electrostatic potential, but can only be used in the case of spherical micelles. Although quantitative accuracy is not yet achieved, both of these approaches represent a valuable "first step" in the development of a theory for electrostatic intermicellar interactions. In conjunction with the theoretical work, an experimental investigation into the properties of dodecyl ethoxy sulfate micelles with 1, 2, 4, or 6 EO groups was conducted. Light scattering and viscosity techniques were used to measure micelle shape and size at various salt concentrations and temperatures. The DLVO potential was used to quantify the effect of electrostatic and attractive intermicellar interactions on the light scattering data. It was found that the resulting equilibrium micelle shape and size is determined by an interplay between steric and electrostatic interactions among the surfactant heads at the micelle surface. Specifically, the surfactants with one and two EO groups exhibited one-dimensional growth in the presence of added salt, a common behavior for typical ionic surfactants. In contrast, the surfactants with four or six EO groups formed small spherical micelles over the entire range of salt concentrations and temperatures examined. This experimental investigation provided valuable insight into the molecular-level interactions which govern the equilibrium micelle shape and size. Thesis Supervisor: Daniel Blankschtein Title: Professor