Evidence of the adsorption of hydroxide ion at hexadecane/water interface from second harmonic generation study

The electric potential and water molecular orientation at oil/water interfaces, as well as the origin of the interfacial potential, have been intensively investigated recently. In some reports, the purity of hexadecane and its influence to the origin of the negative charge at hexadecane/water interface was in debating. In the debate the reliability of sum frequency generation-vibrational spectroscopy in evaluating the influence of chemical impurities on the electric potential and molecular structure at the hexadecane/water interface was called into question. Here we revisited this system using another second order nonlinear spectroscopic technique, Second Harmonic Generation (SHG). The SHG results sensitively reflected the influence of the impurities in hexadecane. As a representative long-chain carboxylic acid, which is generally considered to be the principle impurity in hexadecane, oleic acid at a concentration of several micromolar caused a notable change in the SH intensity of the hexadecane/water interface. The presence of carboxylic-type impurities was confirmed in the as-received hexadecane with 99% purity. After purification with alumina columns, the concentration of the impurities was substantially reduced and the influence on the SH signal was barely detectable. The effect of the hydroxide ion on the structure of water molecules at the hexadecane/water interface was also confirmed, which strongly supports the adsorption of hydroxide ions at oil/water interfaces.