No abstract

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.

The interfacial and nonaqueous solution properties of model engine oil antisludge dispersants have been characterized. Straight chain C8-C18 mono, bis, and tris succinimides were synthesized with polar headgroups of different dimensions and varying nitrogen content. These molecules were found to be poorly soluble in water, as confirmed by log P calculations (P is the partition function between octan-1-ol and water.) Vapor pressure osmometry measurements carried out in acetone and toluene revealed that the mono succinimides show evidence of association into clusters of about three molecules with no noticeable chain length dependence, whereas the bis and tris compounds are essentially unassociated in these solvents. Surface pressure vs area curves from a Langmuir trough exhibit a characteristic plateau at low surface coverage, especially for short chain lengths and mono-substituted succinimides. This feature we attribute to a more compact packing of the headgroups under pressure. For the same lateral pressure the area occupied increases in the order: mono, bis, and tris, correlating well with the number of succinimide moieties in the headgroup. Ellipsometry performed on these molecules deposited on silicon wafers showed that the molecules generally formed excellent close-packed Langmuir-Blodgett films. The succinimides are extremely effective at binding to polar substrates, which is consistent with their practical use. For example, solutions of the various compounds deposited on TLC plates generally showed little movement, and there was the maximum amount of separation in those cases where the solvent had a natural affinity for the headgroup or tail.