ATOMISTIC SIMULATION OF OVERBASED DETERGENT INVERSE MICELLES

We describe the results of atomistic simulations of oil-soluble micell es 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 surf actant molecules to arrange themselves into an inverse micelle structu re, with the calcium carbonate in the core and the surfactant anions f orming a stabilizing shell around this core. In contrast to convention al water-containing inverse micelles, these structures are quite rigid and show negligible fluctuation in shape with time. They are also rel atively 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 mice lles are largely determined by the geometry of the individual surfacta nt molecules. The sulfonates consist of single alkyl chains, and the p henates, of double alkyl chains joined via sulfur-bridged aryl moietie s. Structurally and dynamically the two classes are quite different. S imulations carried out in vacuo and in hydrophobic solvent show the su lfonate systems are spherical, whereas the phenate surfactants self-as semble into more rigid disk-shaped structures. The phenate micelles sw ell out to a certain extent when 'soaked' with a model hydrophobic sol vent, enabling the alkyl chains to be more effective at covering the c arbonate core. This arises from penetration of the solvent molecules i n between the phenate alkyl chains, which open out.