Formation, Growth, and Inhibition of Calcium Naphthenate Particles in Oil/Water Systems as Monitored by Means of Near Infrared Spectroscopy

A new experimental setup based on near infrared (NIR) spectroscopy has been utilized to monitor the formation, growth, and inhibition of calcium naphthenate particles in oil-water (o/w) systems under different experimental conditions. The naphthenic acids were dissolved in toluene and brought in contact with an aqueous solution. The reaction between the dissociated acid monomers and Ca2+ at the o/w interface was initiated by adding dissolved CaCl2 to the water phase. By using stirrers in both bulk phases, the formed particles were dispersed into the oil phase and the changes in optical density (OD) of the naphthenate solution were continuously monitored using a fiber optic NIR probe. Due to the particles present in the solution, the baseline of the NIR spectra was shifted upwards compared to the pure toluene phase, depending on the size and the number of particles detected by the probe. The formation and growth of naphthenate particles have been shown to depend on the naphthenic acid structure, concentrations, and pH of the aqueous phase. In addition, the presence of oil-soluble surfactants has been shown to cause a lowering of the particle volume. Possible mechanisms behind this effect are briefly discussed.