What Determines the Size of Liquid Capillary Condensates Below the Bulk Melting Point?

Capillary condensation from vapor has been studied at temperatures below the bulk melting point T of the condensing substance using a surface force apparatus. Both mica and mica modified by self-assembly of a fluorinated surfactant (perfluoro-1H,1H,2H,2H-decylpyridinium chloride) have been used as substrate surfaces. The condensing liquids, cyclooctane and menthol, nearly wet (contact angle < 15 degrees) mica but show a high (similar to 60 degrees) contact angle on the fluorinated surface. As in previous studies with unmodified mica, we find that both cyclooctane and menthol condense as liquids below T-m and that the size of the condensates at solidvapor coexistence is limited and inversely proportional to the temperature depression below T-m or Delta T. A comparison of the size of the condensates between the fluorocarbon surfaces and the mica surfaces and the quantitative dependence of the size of the condensate on Delta T for cyclooctane lead us to conclude that the maximum condensate size is determined by the equilibrium between condensed, "supercooled" liquid and vapor, and is hence proportional to the surface tension of the liquid-vapor interface. From a consideration of the equilibrium between a liquid and a hypothetical solid condensate, it is concluded that a solid condensate does not usually form for kinetic reasons although two exceptions were found in earlier work.