(558a) Complementary Use of Simulations and Molecular-Thermodynamic Theory to Model Micellization and Micellar Solubilization

Micellar solubilization is relevant to many industrial, pharmaceutical, and biological processes and applications. Because it is such a broadly applicable phenomenon, gaining a fundamental understanding of the factors that drive surfactant and solubilizate self-assembly is of great practical relevance. Frequently, a highly specific set of surfactant/solubilizate solution characteristics is required for a given application. These characteristics typically include: the critical micelle concentration (CMC), the extent of solubilization, and the shape and size of the surfactant/solubilizate micellar aggregates.

Theoretical work relying on a thermodynamic model of self-assembly (referred to as the molecular-thermodynamic approach) permits prediction of solution properties for relatively simple surfactants and solubilizates where it is possible to identify a priori what equilibrium position each component will adopt in a self-assembled micellar aggregate. Unfortunately, for many surfactants possessing more complex chemical structures, it is not clear a priori how the system components will assemble and locate themselves within micellar aggregates.

An alternative to molecular-thermodynamic modeling is to use computer simulation methods to study the self-assembly of surfactants and solubilizates in solution. However, an atomistic-level description of micelle formation is computationally challenging because of the size and density of the micellar aggregates.

We have recently implemented a combined computer simulation/molecular thermodynamic approach to model micellization and micellar solubilization. This approach permits modeling of more complex surfactant/solubilizate systems using less computational time than has been possible to date. In this approach, molecular dynamics simulations are used to simulate a surfactant or solubilizate at an oil/water interface (modeling the micellar core/water interface) to determine the local environment of each portion of the molecule. After identifying the hydrated and the unhydrated atoms, molecular-thermodynamic modeling is performed to predict: (i) the free-energy change associated with forming a micellar aggregate, (ii) the critical micelle concentration (CMC), and (iii) the optimal shape and size of the micellar aggregate.

Our results indicate that a combined computer simulation/molecular thermodynamic modeling approach can be used to extend the applicability of molecular-thermodynamic theory to significantly more complex surfactants and solubilizates than has been possible to date.