  Alkanes of medium chain length show an unusual wetting behavior on (salt)
water, exhibiting a sequence of two changes in the wetting state. When
deposited on the water surface at low temperatures, the liquid alkane forms
discrete droplets that are interconnected only by a molecularly thin film. On
increase of the temperature, there occurs a sudden jump of the film thickness
and, at this first-order transition, a mesoscopically thick layer of liquid
alkane is formed. Heating the system further leads to a divergence of the film
thickness in a continuous manner. While the long-range forces between substrate
and adsorbate are responsible for the critical transition, which occurs at the
temperature at which the Hamaker constant changes sign, it is primarily the
short-range components of the forces that bring about the first-order
transition. We calculate the Hamaker constant of the system and show how,
within a modified Cahn theory, accurate predictions of the first-order
transition temperatures can be obtained for n-alkanes (pentane and hexane) on
water and even on brine. Furthermore, we study the variation of the contact
angle as a function of temperature.
