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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.