Effect of surfactant conformation on the structures of small size nonionic reverse micelles: a molecular dynamics simulation study.

We used constant pressure (P=0.1 MPa) and temperature (T=298 K) molecular dynamics simulations to study the structures and dynamics of small size reverse micelles (RMs) with poly(ethylene glycol) alkyl ether (CmEn) surfactants. The water-to-surfactant molar ratio was 3, with decane as the apolar solvent. We focused on the effect of the two possible imposed conformations (trans vs gauche) for the surfactant headgroups on RMs structures and water dynamics. For this purpose, we built up two RMs, which only differ by their surfactant headgroup conformations. The results obtained for the two RMs were compared to what is known in the literature. Here, we show that the surfactant headgroup conformation affects mainly the water-related properties such as the water core size, the area per surfactant headgroup, the headgroup hydration, and the water core translational diffusion. The properties computed for the RM with the surfactant in trans conformation fit better with the experimental data than the gauche conformation. We further show that the surfactant hydrophilic headgroup plays a crucial role in the micellar structures, favors the entrapment of the micellar water, and reduces strongly their diffusion compared to the bulk water.