SOLVENT BEHAVIOR IN HYDROPHOBIC SILICA NANOTUBES AND NANOTUBE MEMBRANES
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The development of template-synthesized silica nanotubes has created a unique opportunity for studying confined fluids by providing nanometer-scale containers in which, the inner diameter (i.d.) and surface chemistry can be systematically and independently varied. An interesting question to be answered is: Do solvents wet nanometer-scale tubes in the same way that they wet ordinary capillaries? To answer this question, we have conducted studies to explore the wettability of the hydrophobic interiors of individual nanotubes. In these studies, single nanotubes with i.d.'s of either 30 or 170 nm were investigated over a range of water/methanol mixtures. These studies provide a direct route for comparing wetting phenomena in nanotubes with conventional macroscopic theories of capillarity. Our observations reveal four important aspects of wetting in the sub-200 nm regime; (i) observation of a sharp transition between wetting and non-wetting conditions with increasing methanol concentration, (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 methanol mole fraction for the transition and (iv) the reversibility of the observed wetting. The single nanotube measurements were complemented with membrane transport experiments that corroborate our conclusions. The first two aspects conform to conventional capillarity (Young-Laplace), but the latter two do not. The variation between the predicted and the experimental values may be associated with either our reliance on macroscopic values of contact angles and surface tensions in the Young-Laplace equation, or to liquid phase instability within the hydrophobic pore.