EFFECT OF WETTING ON THE REORIENTATION OF ACRIDINE-ORANGE AT THE INTERFACE OF WATER AND A HYDROPHOBIC SURFACE

Acridine orange strongly adsorbs to the interface between water and the hydrophobic monolayer of n-octadecylsilane covalently bonded to silica. The orientational dynamics of acridine orange were studied by frequency-domain fluorescence anisotropy for both in-plane and out-of-plane rotational motions. It was found that, at a coverage of approximately 60 angstrom2 per alkyl chain, adsorbed acridine orange is virtually unable to rotate out of the surface plane but is able to rotate in an unhindered fashion in the plane of the surface. An out-of-plane orientational distribution having a standard deviation of 15-degrees was attributed to roughness of the interface, which is significantly greater than the roughness of the underlying silica substrate. Perturbations on the orientational distribution by short-chain alcohols, which wet the interface, were investigated. It was found that the presence of adsorbed alcohol increases the out-of-plane interfacial roughness but has little effect on the in-plane orientational behavior of acridine orange.