Effects of monovalent and divalent cations on the rheology of organic acid laden interfaces

The focus of this work is to investigate the interfacial rheology of a fatty acid, stearic acid (SA), and a commercial mixture of naphthenic acid (CMNA), capable of mimicking the real naphthenic acids (NAs) found in crude oils. We aim to propose an acidic molecular interaction at the water-oil interface under different aqueous phase compositions, by changing its electrolyte content, adding monovalent salts (NaCl and KCl), divalent salts (CaCl2 and BaCl2), and, increasing the pH of the water subphase. The viscoelastic properties are measured using the pendant drop dilation and contraction technique known as drop shape analysis (DSA). Furthermore, we prepare water-in-oil emulsions to correlate interfacial properties with their stability. The experimental results reveal that SAs can form rigid and viscoelastic films upon low sinusoidal deformations and higher cation content. We find that the emulsions containing SAs are more stable in high pH environments, where we observe the presence of birefringence features in SA laden droplets. This outcome evidences the interplay between emulsion stability and mechanical resistance at hydrocarbon-high pH interfaces loaded with SAs.