Molecular simulations of micellar assemblies under temperature and pressure extremes

Surfactant micelles potentially face extreme temperatures and pressures in a range of oil production processes. Technologically, pressure-dependent micelle assembly can impact surfactant efficacy in applications such as the dispersant remediation of deep-sea oil spills where the pressure dramatically varies from the sea floor to the water surface. The pressure dependence of hydrophobic species transfer thermodynamics is mirrored as maxima in the critical micelle concentrations of nonionic, ionic and polymeric surfactants, with accompanying changes in the sign of the surfactant volume of assembly from positive to negative with increasing pressure.;In this dissertation, we report on the pressure effects on the volumes of assembly for ionic and nonionic surfactants using molecular dynamic simulation. These simulations verify that changes in the sign of the volume of micellization associated with pressure reentrant assembly result from the enhanced compressibility of surfactants in assemblies compared to monomers, which can be largely attributed to the hydrocarbon core of the micelles. The headgroup and tailgroup contributions to the volumes of micellization are analyzed through Kirkwood-Buff theory based on the proximal distribution of water.;Next, we investigate the impact of pressure on the uptake of argon, a model nonpolar gas, by anionic and nonionic surfactant micelles. The micelle/solute interactions are quantified via molecularly detailed potentials-of-mean force and solubility enhancement coefficients, which describe the response of gas solubility to increasing micelle concentration. An analytical liquid drop model is proposed to describe the variation in argon solubility within micelles with increasing pressure.;Finally, we extend the study to examine the temperature effect and combined temperature and pressure effect on the gas solubilization for anionic and nonionic micelles. Enthalpy of micellization changing with temperature for nonionic surfactant is determined directly from molecular simulation. Constant values of critical micelle concentration on pressure vs. temperature plane are predicted based on the simulations covering a broad temperature and pressure range.