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 excited capillary waves. These fluids have recently been reported to exhibit a surface-induced phase transition at temperatures (${\mathit{T}}_{\mathit{k}}$) near, but distinct from, their melting points. In all cases the propagation of the capillary waves showed abrupt discontinuities at this transition. While the data above ${\mathit{T}}_{\mathit{k}}$ are consistent with predictions based on the known properties of the liquids, below this temperature they suggest the presence of a structured surface layer which is viscoelastic in nature. Below ${\mathit{T}}_{\mathit{k}}$ both elastic and viscous parts of the surface excess transverse shear modulus are negative, indicating that the surface layer responds more easily to shear normal to the surface plane than does the bulk fluid. The light scattering thus suggests the existence of 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.