ADSORPTION, WETTING, AND CAPILLARY CONDENSATION OF NONPOLAR FLUIDS INMICA SLITS

The adsorption behavior of n-pentane and cyclohexane in mica slits at room temperature has been studied as a function of chemical potential and gap width with multiple-beam interferometry. The measured film thi cknesses close to saturation for large slit widths (effectively isolat ed surfaces) range up to 7 nm with n-pentane (at a relative vapor pres sure of 0.9996) and 3 nm with cyclohexane (at a relative vapor pressur e of 0.995). The thickness of these adsorbed wetting films is slightly larger than that predicted by van der Waals theory. The difference ma y be accounted for by thermal fluctuations of the adsorbed liquid-vapo r interface. At smaller slit widths a capillary condensation transitio n occurs as the slit fills up with liquid. The separation at which thi s occurs is in good agreement with a film-thickening mechanism due to van der Waals forces across the gap only for the thickest films (t gre ater than or equal to 6 nm). For thinner films the capillary condensat ion transition occurs at larger than expected slit widths, and the dev iations are large for t less than or equal to 3 nm, We speculate that these larger than-expected condensation separations are related to a f luctuation-enhanced film thickness in this regime. The work demonstrat es the utility of measurements in a system consisting of a single slit -pore, without the complications of polydispersity and connectivity of pore networks. The results show that vapor adsorption isotherms can b e measured with multiple-beam interferometry, i.e., in the surface for ce apparatus.