Phase behavior in model homopolymer/CO2 and surfactant/CO2 systems: discontinuous molecular dynamics simulations

Discontinuous molecular dynamics simulations are performed on surfactant (HmTn)/solvent systems modeled as a mixture of single-sphere solvent molecules and freely jointed surfactant chains composed of m slightly solvent-philic head spheres (H) and n solvent-philic tail spheres (T), all of the same size. We use a square-well potential to account for the head-head, head-solvent, tail-tail, and tail-solvent interactions and a hard-sphere potential for the head-tail and solvent-solvent interactions. We first simulate homopolymer/supercritical CO2 (scCO2) systems to establish the appropriate interaction parameters for a surfactant/scCO2 system. Next, we simulate surfactant/scCO2 systems and explore the effect of the surfactant volume fraction, packing fraction, and temperature on the phase behavior. The transition from the two-phase region to the one-phase region is located by monitoring the contrast structure factor of the equilibrated surfactant/scCO2 system, and the micelle to unimer transition is located by monitoring the aggregate size distribution of the equilibrated surfactant/scCO2 system. We find a two-phase region, a micelle phase, and a unimer phase with increasing packing fraction at fixed temperature or with increasing temperature at fixed packing fraction. The phase diagram for the surfactant/scCO2 system in the surfactant volume fraction-packing fraction plane and the density dependence of the critical micelle concentration are in qualitative agreement with experimental observations. The phase behavior of a surfactant/scCO2 system can be directly related to the solubilities of the corresponding homopolymers that serve as the head and tail blocks for the surfactant. The influence of surfactant structure (head and tail lengths) on the phase transitions is explored.