Effect of surface chemistry of silica nanoparticles on contact angle of oil on calcite surfaces in concentrated brine with divalent ions

Hypothesis: For an oil droplet on calcite with an intervening brine film, the water contact angle theta(w) may be reduced markedly (greater water wetness) with surface modified silica nanoparticles (NP). Modification with cationic, anionic, and nonionic ligands may be used to control the nanoparticle adsorption and interactions at the oil-brine and brine-calcite interfaces to influence the rate and degree of reduction in theta(w).Experiments: The colloidal stability at 25 degrees C was determined in concentrated divalent brine (8 wt% NaCl and 2 wt% CaCl2) with dynamic light scattering, and the NP adsorption was determined on calcite. The NP adsorption at the oil-brine interface was characterized with the elastic dilational modulus. theta(w) was measured for model decane-stearic acid droplets and crude oil droplets on calcite from 25 to 80 degrees C.Findings: The fastest rate and greatest extent of reduction in theta(w) for grafted ligands followed the order: cationic quaternary trimethylamine > sulfonate > methyl phosphonate > gluconamide. New mechanisms for reduction in theta(w) were demonstrated on the basis of changes in interactions from NP adsorption at each interface. The greatest efficacy for the cationic NPs results from the weakest adsorption on calcite, steric repulsion at the three-phase contact line and the greatest desorption of carboxylate surfactants from the calcite. (C) 2020 Published by Elsevier Inc.