Transport and Adsorption Phenomena in Mesoporous Silica

The analysis and control of transport phenomena in fluidic nanopores and nanochannels is important in applications such as biochemical analysis, power generation and environmental protection. A unique aspect of nanofluidics is that the relevant length scale is comparable to the range of various surface and interfacial forces in liquids (such as electrostatic, van der Waals and steric interactions). Thus, to obtain an adequate description of transport phenomena in nanospace, it is necessary to understand the discreteness of molecules, especially when the size decreases to 2 nm. Micelle-templated mesoporous silicas (MPSs) possess highly ordered structures such as 2D hexagonal and 3D cubic structures and pores within the 2–50 nm range. In particular, 2D hexagonal films that generally have pore channels parallel to the surface plane have been widely synthesized by using various types of template molecules. If the pore channels of such materials are aligned in a certain direction, these materials can be employed for various purposes such as the fabrication of oriented nanowires, optoelectronic devices, recording media, selective separations, and nanofluidic systems. 3D cubic structures give large surface areas and become good candidates for highly efficient catalysts and sensors. Advances in the synthesis, measurement and analysis of nanotubes and nanochannels have allowed ion and liquid transport to be routinely examined and controlled in spaces with dimensions that range from 10 to 100 nm. The ability to now explore transport and adsorption phenomena in confined spaces of around 2 nm offers a range of possibilities. We have investigated several unique transport and adsorption phenomena in mesopores measuring a few nanometers in diameter, including nonlinear I–V curves of ionic current passing through MPS thin films filled with aqueous solutions, humidity-dependent adsorption rate of water into MPS, and the reduction of melting and freezing temperature of water in MPS.