The Mobility of Water Molecules through Hydrated Pores

Abstract

To achieve a more exact understanding of the water transport in natural channels, a series of non-natural structures have been developed. They have been studied by far-infrared spectroscopy, solid-state nuclear magnetic resonance, differential scanning calorimetry, thermogravimetric analysis, and variable-temperature powder X-ray diffraction to examine the behavior of water at the molecular level. Water in these predominantly nonpolar pores can be metastable, with filling and emptying occurring upon changes in solvent conditions. The water contained in these pores exhibits a dynamics that might be controlled, since it depends on the structural features of the monomers that form the pore “skeleton”. We have observed changes in the pore diameter depending on the selected isomer. This provokes at a given temperature differences in the arrangement and dynamics of the contained water. The water dynamics increases with both temperature and pore diameter in a process that is reversible over a temperature range specific for each structure. Beyond this particular temperature threshold, the pore water can be irreversibly evacuated, and at this point a decrease of the dynamics is observed. The slower dynamics of the remaining water in partially evacuated pores is probably due to the increased interaction with the inner-pore surface owing to a concomitant narrowing of the pore. These findings not only highlight the need for the presence of freely moving water inside the pore to sustain its permeability by water, but also point to the decrease in the dynamics of the remaining water in partially evacuated pores.