LIGHT-SCATTERING STUDY OF A SURFACE-INDUCED PHASE-TRANSITION IN ALKANE FLUIDS

The surfaces of normal liquid alkanes (chain lengths between 15 and 18 carbon atoms) have been studied by light scattering from thermally ex cited capillary waves. These fluids have recently been reported to exh ibit a surfarce-induced phase transition at temperatures (T(k)) near, but distinct from, their melting points. In all cases the propagation of the capillary waves showed abrupt discontinuities at this transitio n. While the data above T(k) are consistent with predictions based on the known properties of the liquids, below this temperature they sugge st the presence of a structured surface layer which is viscoelastic in nature. Below T(k) both elastic and viscous parts of the surface exce ss transverse shear modulus are negative, indicating that the surface layer responds more easily to shear normal to the surface plane than d oes the bulk fluid. The light scattering thus suggests the existence o f a surface layer which is much more ordered than the bulk fluid. This is consistent with the negative surface excess entropy density found in the new phase.