ADSORBATE REORIENTATION AT A WATER (OCTADECYLSILYL)SILICA INTERFACE

Mathematical relations are derived to allow experimental study of adsorbate reorientation at a surface/liquid interface. It is shown from these relations that experiments can be devised to probe separately the rotations in the plane of the surface and the rotations away from the plane of the surface. This new technique is applied to the study of acridine orange interacting with an (octadecyisilyl)silica surface in equilibrium with an aqueous solution. Frequency-domain spectroscopy is used to characterize the fluorescence anisotropy decays of acridine orange. The experiments reveal that acridine orange is a strongly hindered rotor with respect to out-of-plane rotational motions but is able to rotate slowly in the surface plane. The interpretation is that acridine orange resides at the interface of water and octadecylsilane.