| Abstract
| - The development of template-synthesized silica nanotubes has created a unique opportunityfor studying confined fluids by providing nanometer-scale containers in which the inner diameter (i.d.) andsurface chemistry can be systematically and independently varied. An interesting question to be answeredis the following: do solvents wet nanometer-scale tubes in the same way they wet ordinary capillaries? Toanswer this question, we have conducted studies to explore the wettability of the hydrophobic interiors ofindividual nanotubes. In these studies, single nanotubes with i.d.'s of either 30 or 170 nm were investigatedover a range of water/methanol mixtures. These studies provide a direct route for comparing wettingphenomena in nanotubes with conventional macroscopic theories of capillarity. Our observations revealfour important aspects of capillary wetting in the 30−170 nm regime, a size range where the application ofthe Young−Laplace theory has not been experimentally investigated for hydrophobic pores. They are (i)a sharp transition between wetting and nonwetting conditions induced by addition of a cosolvent, (ii)invariance of this transition between nanotubes of 30 and 170 nm pore diameter, (iii) failure of the Young−Laplace equation to accurately predict the cosolvent's (methanol) mol fraction where the transition occurs,and (iv) reversibility of the observed wetting. The first two aspects conform to conventional capillarity (Young−Laplace), but the latter two do not. These measurements were complemented with ensemble experiments.The difference between theory and experiment is likely due to reliance on macroscopic values of contactangles or to liquid-phase instability within the hydrophobic pore.
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