Aggregation Behavior of Common Motor Oil Additives

The aggregation behavior of three common motor oil additives, calcium dinonylnaphthalene sulfonate (CaDNNS), zinc dinonyldithiophosphate (ZDDP) and polyisobutenyl succinimide of tetraethylene pentamine (PIB-TEPA) in n-decane were examined using light scattering, osmometry and viscometry techniques. The experimental data were analyzed using two phenomenological models of association, the continuous association model which assumes the coexistence of aggregates of all sizes including the monomers, and the closed association model which assumes the coexistence of aggregates of only a single size with the monomers. The closed association model was found to describe the experimental data well for CaDNNS and PIB-TEPA while both association models fitted the data for ZDDP comparably, but somewhat poorly. The analysis of experimental data led to the conclusion that CaDNNS micelles were composed of 13 and 11 monomers, PIB-TEPA micelles had 19 and 9 monomers, and ZDDP aggregates had 4 and 3 monomers, at 25°C and 65°C, respectively. At 25°C, the CaDNNS and the ZDDP aggregates were ellipsoidal in shape and it was not possible to discriminate between oblate and prolate ellipsoids. The micelle sizes assuming both shapes have been estimated. The PIB-TEPA micelle was spherical with a diameter of 6.4 nm. For all three additives, the solutions were dominated by aggregates at solute concentrations larger than 0.02 gm/cm³ (20 kg/m³), while below this concentration, the presence of non-aggregated monomers was important to account for.