Mechanism of acid neutralization by overbased colloidal additives in hydrocarbon media

Stopped-flow kinetic studies have been employed to study the mechanism of a cid neutralization for the reaction between overbased commercial detergent additives, used as crankcase lubricants, and aqueous strong acids (e.g., HC l and H2SO4) dispersed in the form of water-in-oil microemulsion droplets. Three water-soluble pH indicators, including methyl orange, 4-nitrophenyl-2 -sulfonate, and nile blue have been used to monitor the change in pH accomp anying the neutralization reaction. This process represents a model reactio n involving communication between a nanometer-sized colloidal particle and a similar-sized aqueous droplet with the reactants solubilized inside the r espective colloidal species-dynamic processes in such systems have rarely b een studied previously in a systematic way. The state of the system at the end of the neutralization reaction was monitored by the techniques of small angle neutron scattering (SANS) and ultracentrifugation. The rate was stud ied as a function of the initial concentrations of overbased additive and d roplets and the initial acid concentration. The effect of temperature and t he nature of the hydrocarbon oil were also systematically investigated, and in addition, the surfactant stabilizing the droplet was also changed. Thro ugh these studies, it was found that the model reaction provides fundamenta l information concerning the mechanism of acid neutralization by detergent additives. It is found, rather surprisingly, that the mechanism involves ba se transfer from the particle into the water cores of the microemulsion dro plets, where neutralization of the solubilized acid occurs.