The behavior of hydroxide and hydronium ions at the hexadecane–water interface studied with second harmonic generation and zeta potential measurements

By probing the electric potential at the hexadecane–water interface with second harmonic generation and the zeta potential at the surface of a hexadecane droplet in its emulsion, we show that hydronium ions don't have a specific affinity to the oil–water interface although hydroxide ions do. The observed apparent affinity of the hydronium ions to the hexadecane–water interface is more likely a result of the electro-static attraction effect. The adsorption free energy of the hydroxide ions at the oil–water interface was estimated to be −8.3 kcal mol−1. This study provides more experimental evidence for understanding the behavior of hydronium and hydoxide ions at the oil–water interface.