MOLECULAR-DYNAMICS SIMULATIONS OF A CALCIUM-CARBONATE CALCIUM SULFONATE REVERSE MICELLE

We have investigated the microscopic properties of a reverse micellar aggregate consisting of 102 calcium carbonate, 11 calcium (2-hexadecyl )benzenesulfonate, and 22 water molecules in nonpolar environments (CC l4, octane, and vacuum) using molecular dynamics simulations. The simu lated aggregate is a model for the micellar species in detergent addit ives to automotive and diesel lubricants. From our simulations we conc lude that the structure of the micelle is similar in both CCl4 and oct ane solvents, but the structure of the two solvents in the vicinity of the micelle is different. Both solvents are ordered by the micelle, b ut CCl4 to a larger extent. In solution, the aggregate is roughly sphe rical, with an average core diameter of 23 Angstrom, surfactant layer thickness of 9 Angstrom, and total diameter of approximately 41 Angstr om. The crystalline order of the initial core configuration is, for th e most part, maintained throughout the solution simulations. Conformat ional distributions of the hexadecane chains in solution suggest that the behavior of the middle parts of the surfactant tails is similar to that in aqueous micelles, while the ends of the chains resemble liqui d alkanes. The simulations reveal that approximately 25% of the micell e core surface is exposed to solution. The polar core, water molecules , and surfactant headgroups behave similarly in the vacuum and solutio n simulations, but the surfactant tails behave qualitatively different ly in vacuum: in the absence of solvent the hydrocarbon tails collapse onto the surface of the core, producing a much more compact micelle.