Experimental and molecular modelling studies of overbased detergent particles

Langmuir trough, Langmuir-Blodgett (LB) film and ellipsometry experiments, and molecular modelling, were carried out to characterise a series of overbased detergent (OD) systems made with several surfactant types, including the first application of these approaches to a new class based on calixarates. Systems were synthesised with low, mid and high levels of basicity, which on the TBN (total base number) scale were 150, 250 and 350, respectively. The diameters determined from the Langmuir trough measurements were 3.4, 3.5 and 3.6 nm, respectively. These are consistently higher than those of phenate particles with the same basicities. The phenate and calixarate particles self-assembled into multilayer LB films on a stearic acid-coated silicon wafer. Film thicknesses were determined by ellipsometry. Differences in the estimated particle diameter for the phenates and calixarates by these two methods suggest that the OD particles are, in general, and to varying extents, non-spherical. In the phenate case the deposited LB film thickness was a strong function of the Langmuir film deposition surface pressure in the region where the monolayer is essentially close-packed, supporting the discoid shape for these particles, which was first predicted by our previous molecular modelling studies. The molecular modelling results suggest that the sulfonate systems are the most spherical, followed probably by the salicylates. Preliminary results on the calix[6]arates suggest that they form prolate ellipsoidal particles, capped at both ends with the surfactant molecule. The experiments and simulations are consistent in indicating that the particle shape and internal atomistic structure inside the inorganic core region of the particle are quite sensitive to the surfactant type.