Overbased detergent particles: Experimental and molecular modelling studies

Overbased detergents (OD) are oil-soluble nanoparticles consisting of a calcium carbonate core and surfactant outer shell. They are important additives in automotive and marine engine oils. Langmuir trough (LT) and molecular dynamics (MD) simulations have been carried out to characterise the interaction between OD particles made from a range of surfactants. We also explore the factors that determine the shape of the particles, in particular by the level of overbasing (amount of calcium carbonate in the core), surfactant type and the presence of stearate co-surfactant. Clint and Taylor (J. H. Clint and S. E. Taylor, Colloids Surf., 1992, 65, 61) derived expressions for the particle size, contact angle and interparticle interaction between spherical particles at the air/water interface in a Langmuir trough surface pressure experiment. We extend this mathematical treatment to encompass the more general case of oblate and prolate ellipsoidal particles. On reinterpreting the experimental data, the derived expressions suggest that the largest deviations from spherical symmetry occur for the phenate and calixarate particles without co-surfactant. This is supported by molecular modelling studies also reported herein. The evidence from the combined LT and MD studies is that the co-surfactant makes the particles more spherical, by disrupting any tendency of the surfactants to self-assemble in an ordered fashion around the surface of the carbonate core.