(500a) A Computer Simulation--Molecular-Thermodynamic Model For Self-Assembly

Goldsipe, A. C., Massachusetts Institute of Technology
Stephenson, B. C., Massachusetts Institute of Technology
Beers, K. J., Massachusetts Institute of Technology
Blankschtein, D., Massachusetts Institute of Technology

A hybrid model of surfactant self-assembly in aqueous solution was developed to leverage the advantages of atomistic molecular dynamics computer simulations (CS) and molecular-thermodynamic (MT) theories. This CS-MT model provides quantitative information about the changes in hydration that occur upon self-assembly, giving insight into the hydrophobic driving force for micellization. The model also allows the quantitative prediction of micellization properties for structurally complex surfactants that cannot be modeled with traditional MT theories. Because the simulations are conducted primarily to obtain information about changes in hydration, only two relatively short simulations are needed for each surfactant of interest: (1) a simulation of the surfactant in its fully hydrated monomeric state, and (2) a simulation of the surfactant in a micelle. Compared to traditional CS studies of micelle formation, the CS-MT model requires significantly less computation. Traditionally, each moiety in a surfactant is assumed to be completely hydrophobic or completely hydrophilic. The CS-MT model relaxes this assumption and characterizes the degree of hydrophobicity of each surfactant moiety via its change in hydration upon self-assembly. To predict micellization properties, the hydrophobic driving force for self-assembly is calculated from two terms: (1) the free-energy change associated with loss of water contacts upon self assembly (dehydration),and (2) the change in the hydration free energy for each surfactant moiety. Several aggregates were modeled using both the CS-MT model and the traditional MT model. In the simplest case, the free energy of aggregate formation of 15 different oil aggregates was found to differ by an average of 1% between the two models. Critical micelle concentration (CMCs) were also predicted for 15 nonionic, ionic, and zwitterionic surfactants, with varying degrees of structural complexity. For simple surfactants with moieties that can be clearly identified as hydrophobic and hydrophilic, both the CS-MT and the MT models generally provide accurate CMC predictions. However, for surfactants containing multiple hydrophobic or hydrophilic groups, the CS-MT model provides much more accurate CMC predictions. In particular, we modeled a homologous series of dodecyl alkyl dimethyl ammonium bromide surfactants, with an alkyl side chain containing 1, 2, 4, or 6 carbon atoms. For this homologous series, the CMCs predicted using the CS-MT model were remarkably close to the experimental CMCs, and the MT predictions were inaccurate. The CS-MT model is a very promising alternative to both CS and MT theories for the study of the self-assembly in aqueous solution of structurally complex surfactants.