Acid Neutralization Rates—Why Total Base Number Doesn’t Tell the Whole Story: Understanding How the Colloidal Structure of Overbased Detergents Influences Acid Neutralization Rates

Neutralization of acidic contaminants in engine lubricating oil is an important topic for engine manufacturers. Often, the deterioration in total base number (TBN) and increase in total acid number (TAN) during engine test operation is used as an indication of oil lifetime. This is clearly an oversimplification given that no consideration is given to how corrosive the acid is, and how effective the base is at neutralizing different acids. The work detailed here will explore how the presence of inorganic acids can be combated by lubricant additives, such as overbased detergents, through rapid neutralization. To achieve this, stopped-flow UV/visible spectroscopy has been used to measure the reaction kinetics between an overbased detergent and sulfuric acid containing water-in-oil (w/o) microemulsion droplets. The key structural properties of overbased detergents that contribute to effective acid neutralization will be explored. To probe this, the core dimensions of a series of overbased detergents were measured using small-angle neutron scattering (SANS). The results indicate that the detergent core size strongly influences the acid neutralization rate of a detergent.