  A mean-field theory is presented which describes the basic observations of
recent experiments revealing rich wetting behaviour of n-alkane/methanol
mixtures at the liquid-vapour interface. The theory, qualitative and in part
heuristic, is based on a microscopic lattice-gas model from which a Cahn-Landau
approach is distilled. Besides the physics associated with the short-range
components of the intermolecular interactions, effects of the long-range tails
of the net van der Waals forces between interfaces are also taken into account.
Including weak long-range forces which favour wetting in the theory does not
visibly alter the critical wetting transition for the nonane/methanol mixture,
in contrast with the generic expectation of first-order wetting for such
systems, but in good agreement with experiment. For decane/methanol weak
long-range forces bring the transition very close to the prewetting critical
point, leading to an adsorption behaviour closely reminiscent of short-range
tricritical wetting, observed experimentally for alkane chain length between
9.6 and 10. Finally, for undecane/methanol the transition is clearly of first
order. First-order wetting is also seen in the experiment.
