Reversible Solubilization of Pyrene by a Gas Switchable Surfactant Investigated by Molecular Dynamics Simulation

The reversible solubilization behavior of pyrene by a CO2/N-2 switchable surfactant (named N'-dodecyl-N,N-dimethyl-acetamidinium bicarbonate (DDAB)) was investigated with molecular dynamics (MD) simulations. We first individually simulated the aggregation of the inactive surfactant N'-dodecyl-N,N-dimethyl-acetamidines (DDA) and effective surfactant DDAB in water. Detailed structural properties analysis showed that DDAB molecules aggregated into a micelle, while the aggregation of DDA molecules was considered to be an oil droplet that was separated from the water phase. MD simulations revealed that pyrene molecule was solubilized in the interior hydrophobic region of the micelle as expected. Pyrene was adsorbed on the surface of the oil droplet which is due to the dense packing of DDA molecules inside the oil droplet. The simulated release process showed that the solubilized pyrene in the interior was squeezed out when the micelle was changed to an oil droplet. Reduced density gradient (RDG) function was used to study the weak interactions and explore the molecular driving force behind the reversible solubilization. The results demonstrated that repulsion effects of water molecules on the DDA headgroups play an important role on the pyrene release. Because of the persistent molecular motion of DDA molecules into the droplet center, pyrene was finally repelled out of the oil droplet. Our study provided a molecular mechanism into the reversible solubilization of a gas-controlled switchable surfactant. This is expected to be useful for surfactant-enhanced remediation (SER) experiments.