Molecular dynamics simulation of a dichain surfactant + water + carbon dioxide (solvent) system is performed to study the structural properties of reversed micelle-like surfactant aggregates formed in the system. The simulations use a detailed and realistic molecular model for the surfactant molecule and explicit representation of the water and solvent molecules to enable quantitative comparisons with a prior experimental (small-angle neutron scattering) 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 ratio. A higher water-to-surfactant ratio results in larger and more spherical aggregates. The two distinct tails of the surfactant molecule exhibit different conformations in carbon dioxide indicating contrasting CO2-philic behavior. The perfluoroalkane tails assume more extended conformation than the alkane tails. The microstructure of the aqueous core reveals that the water molecules in the interfacial region are strongly oriented in response to the electric fields of the anionic headgroups and sodium counterions, while water near the center of the core approaches bulklike properties with the presence of a hydrogen-bonded network.

One of the primary reasons that supercritical carbon dioxide has thus far failed to achieve its full potential as an environmentally benign alternative to conventional industrial solvents is that few surfactants are known at present for use in CO2. As an initial step toward developing the molecular-level understanding needed to design new surfactants, we report the first molecular simulations of the self-assembly of dichain surfactants in supercritical carbon dioxide into stable, spherical aggregates. These aggregates exhibit the expected characteristics of reverse micelles with aqueous cores, consistent with earlier experimental findings, demonstrating the potential of molecular simulation for modeling such complex systems.

Supercritical fluids (SCFs) have great technological potential for minimizing the organic wastes associated with polymer manufacturing and processing. However, significant challenges remain for developing the same level of understanding of the behavior of polymers in SCFs as has been reached for polymers in traditional organic solvents. Small-angle neutron scattering was used to study the effect of pressure and temperature on the phase behavior of poly(dimethylsiloxane) (PDMS) in supercritical carbon dioxide (SC CO2). It was demonstrated that PDMS−SC CO2 solutions reproduce all main features of the temperature−concentration phase diagram for polymers in organic solvents. Moreover, because of their continuously adjustable solubility, SCFs exhibit novel effects, such as a pressure-induced transition to the ϑ point and to the good solvent domain, in addition to a polymer−solvent demixing at a lower critical solution pressure.