Effects of Long-chain Molecules on the Population and Orientation of Organic Molecules at the Air/Water Interface

The effects of pentaand hexadecanoic acids on the population and orientational structure of p-nitrophenol, at the airlwater interface at a bulk pH of 2, have been studied by using the technique of second-harmonic generation. The results indicate that, with increasing fatty acid at the interface, (1) the population of p-nitrophenol at the airlwater interface first increases and then decreases, (2) the long axis of p-nitrophenol is oriented increasingly toward the surface normal, and (3) the behavior of p-nitrophenol is not sensitive to the monolayer liquid-expanded-liquid-condensed phase transition. This indicates that the p-nitrophenol is adsorbed near the polar group of the fatty acid and thus is a good probe of the lipid head group region.