Molecular simulation of a dichain surfactant water carbon dioxide system. 1. Structural properties of aggregates

Molecular dynamics simulation of a dichain surfactant + water + carbon diox ide (solvent) system is performed to study the structural properties of rev ersed micelle-like surfactant aggregates formed in the system. The simulati ons use a detailed and realistic molecular model for the surfactant molecul e and explicit representation of the water and solvent molecules to enable quantitative comparisons with a prior experimental (small-angle neutron sca ttering) study. The results of the simulation are found to be in reasonable agreement with experimental values. The simulations show that the size and shape of the surfactant aggregates depends on their water-to-surfactant ra tio. A higher water-to-surfactant ratio results in larger and more spherica l aggregates. The two distinct tails of the surfactant molecule exhibit dif ferent conformations in carbon dioxide indicating contrasting CO2-philic be havior. The perfluoroalkane tails assume more extended conformation than th e alkane tails. The microstructure of the aqueous core reveals that the wat er molecules in the interfacial region are strongly oriented in response to the electric fields of the anionic headgroups and sodium counterions, whil e water near the center of the core approaches bulklike properties with the presence of a hydrogen-bonded network.