Molecular Dynamics Simulations of Micellization in Model Surfactant/CO2 Systems
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Date
2003-04-22
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Abstract
Discontinuous molecular dynamics simulations are performed on surfactant (H[subscript n] T[subscript m])/solvent systems modeled as a mixture of single-sphere solvent molecules and freely-jointed surfactant chains composed of <i>n</i> slightly solvent-philic head spheres (H) and m 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 mole fraction, packing fraction and temperature on the phase behavior of a surfactant/scCO₂ system. 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 micelle size distribution of the equilibrated surfactant/scCO₂ system. The phase diagram for the surfactant/scCO₂ system 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 block for the surfactant. The location of the micelle-unimer transition is strongly affected by the head-solvent attraction but only weakly affected by the tail-tail and tail-solvent attractions. Both micellization and phase separation upon decreasing the temperature are found in our simulations.
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micelle, supercritcal carbon dioxide, surfactant
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Degree
MS
Discipline
Chemical Engineering
