Small angle X-Ray scattering (SAXS) on calcium sulfonate dispersions: effects of friction on microstructure

Overbased calcium sulfonates were investigated using small angle X-ray scattering (SAXS). Different scattering intensity simulation and curve fitting programs were used to determine the sizes of the micelles and to characterize the interactions between them. SAXS results for diluted solutions were compared to electronic microscopy observations. The overbased calcium sulfonates appear as polydisperse micelles having an average calcium carbonate core radius of 2.0 nm with a standard deviation of 0.4 nm. In a concentrated system (33 wt% CaCO3), the micelles are subjected to steric repulsions. However, an attractive potential of less than kT between micelles diluted (11 wt% CaCO3) in a mineral oil has been obtained from the SAXS results. The 11 wt% CaCO3 dispersion was used to study the behavior of the overbased calcium sulfonates in friction experiments. Under severe friction conditions, a viscous deposit on the surfaces was recovered. According to the SAXS spectrum analysis, the viscous deposit contains more polydisperse particles (average micelle core radius of 2.2 nm with a standard deviation of 0.6 nm) which do not have same structure as those contained in the original additive. Wide angle X-ray scattering (WAXS) shows the presence of highly crystalline calcite.