PHYSICOCHEMICAL CHARACTERIZATION OF OIL-SOLUBLE OVERBASED PHENATE DETERGENTS

A combined experimental and chemical modelling investigation of model overbased calcium carbonate detergents is presented. Colloidal dispersions of phenate-stabilised calcium carbonate with varying ratios of each component were synthesised under laboratory conditions and characterised using gravimetric analysis, spectroscopy and surface monolayer studies (Langmuir trough). Interpretation of the experimental data was assisted using ab initio GAUSSIAN92 and molecular dynamics modelling simulations. The chemical species in the product were found to consist of CaCO3 (from C-13 solid-state NMR) and calcium phenate, the stabilising surfactant (from IR and UV spectroscopies supported by GAUSSIAN92 calculations). The microcolloidal state of the overbased system was confirmed by Langmuir-trough sizing of the particles. The particle size increases with particle carbonate content (basicity) measured by the so-called TBN number, increasing from 2.5 to 3.2 nm diameter through 150 to 300 TBN, respectively. By extrapolation to zero-TBN we estimated the phenate-shell diameter and therefore the average number of CaCO, molecules included within the colloidal particle at each TBN (2 to 16, respectively). The interparticle interaction between the particles can be represented quite well using the inverse power analytic form, V(r) = epsilon(sigma/r)(-n) where n approximate to 20. Computer simulation studies were performed in order to gain a better understanding of the structure of the phenate molecules and hence of their role in the overbased system. The computations indicate that a major structural transformation takes place in the phenate molecule for n-alkyl chains in excess of C-8. This involves a realignment of the adjacent alkyl chains so that they point essentially in opposite directions which causes the phenate molecule to present a flatter structure. It is possible that this conformational rearrangement is an important factor in facilitating the formation of an effective stabilising shell.