Atomistic simulation of overbased detergent inverse micelles

We describe the results of atomistic simulations of oil-soluble micelles containing a calcium carbonate core, stabilized by either sulfonate or phenate surfactant molecules. Strong Coulombic forces between the ions provide the driving mechanism for the model Ca2+, CO32-, and surfactant molecules to arrange themselves into an inverse micelle structure, with the calcium carbonate in the core and the surfactant anions forming a stabilizing shell around this core. In contrast to conventional water-containing inverse micelles, these structures are quite rigid and show negligible fluctuation in shape with time. They are also relatively insensitive to temperature, explaining their effectiveness at elevated temperatures (similar to 650 K in engine oil) as slow release acid neutralizers and the ease with which they can be extracted from oil and subsequently redispersed. The shape and properties of the micelles are largely determined by the geometry of the individual surfactant molecules. The sulfonates consist of single alkyl chains, and the phenates, of double alkyl chains joined via sulfur-bridged aryl moieties. Structurally and dynamically the two classes are quite different. Simulations carried out in vacuo and in hydrophobic solvent show the sulfonate systems are spherical, whereas the phenate surfactants self-assemble into more rigid disk-shaped structures. The phenate micelles swell out to a certain extent when 'soaked' with a model hydrophobic solvent, enabling the alkyl chains to be more effective at covering the carbonate core. This arises from penetration of the solvent molecules in between the phenate alkyl chains, which open out.