Molecular Dynamics Simulation of Emulsification/Demulsification with a Gas Switchable Surfactant

Molecular dynamics simulations were performed to study the reversible emulsification and demulsification processes of a hexadecane/water mixture by a CO2/N2-triggered surfactant named N′-dodecyl-N,N-dimethylacetamidinium bicarbonate (DDAB). Specifically, the simulations reproduced experimental observations, that is, the active form of the surfactant, DDA+, which can stabilize hexadecane/water emulsions in aqueous solution. Furthermore, when DDA+ surfactants were switched to DDA molecules, their inactive form, the surfactant-stabilized emulsion was spontaneously broken, and the oil phase was separated from bulk water. The potential of mean force (PMF) between emulsion and oil droplets showed that the repulsion effect among the DDA+ headgroups played an essential role in stabilizing emulsion. Besides, radial and spatial distribution functions were applied to investigate the hydration behavior of DDA and DDA+. The results demonstrated that the DDA+ headgroups could be fully hydrated with water molecules located around the three methyl groups in the headgroup. Once the headgroup was deprotonated, the headgroup changed into a nonplanar conjugation structure. Moreover, the hydrophilicity of DDA headgroups decreased significantly compared with DDA+. The reversible transformation of the hydrophilicity is the key for switchable surfactants that can be used as an emulsifier and a demulsifier simultaneously. The hydrophilicity of the surfactants was quantified by PMF between surfactants and water, which could be used to compare the hydrophilic difference between the inactive and active forms of the molecule and is expected to help design new switchable surfactants.