Abstract

The orientational dynamics of a surfactant at both liquid/solid and liquid/liquid interfaces are characterized and compared. Acridine orange, a fluorophor, accumulates at water/hydrocarbon interfaces. Using fluorescence anisotropy measurements, the orientational relaxation is characterized with resolution on the picosecond time scale. At the interface of water and a monolayer of C₁₈ alkyl chains, the molecules reorients in-plane in 0.4 ns, with little hindered rotation. The molecule is strongly hindered from out-of-plane rotations: it is restricted to orientations within ±13° of the plane of the surface. Small amounts of alcohol widen the distribution to ±20°. At the interface of water and n-hexadecane, which is a liquid at room temperature, the in-plane reorientation is slightly faster: 0.3 ns, and it is unhindered. Compared to the liquid/surface interface, the molecule is even strongly hindered from out-of-plane orientation: it is restricted to only ±4° of the plane of the interface. The interpretation is that the surface of water is much smoother in contact with the hydrocarbon liquid than it is with the hydrocarbon monolayer. The effect of alcohols on the smoothness of the water/hexadecane interface is being investigated experimentally.

© 1992 Optical Society of America