Wetting films (of thickness $t\le 14$ nm) of $t$-butanol in slits of mica have been studied as a function of the slit width $H$. A capillary condensation transition occurs when $H\approx 3t \mathrm{for} t>\mathrm{}5\mathrm{}$ nm. This is consistent with a mechanism of film thickening and coalescence due to van der Waals forces across the slit. van der Waals thickening cannot account for the transition with thinner $t<\mathrm{}5\mathrm{}$ nm) adsorbed films, as found previously with nonwetting liquids.

• Received 4 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.1821