We apply a recent optical technique to investigate fundamental water adsorption/desorption phenomena on submicrometer Stöber silica sphere surfaces of varying hydrophilicity. Thermally annealed (partially dehydroxylated) silica colloidal crystals are used as test systems for the sensitivity of their photonic properties to water. The allocation of physisorbed water on the crystal spheres is inferred in situ during water desorption by simple optical spectroscopy. Silica dehydroxylation and water desorption were found to have dissimilar effects on the water distribution, which is therefore unique for each hydrophilicity and hydration state. Physisorbed water in hydrophilic (fully hydroxylated) compacts tended to accumulate between adjacent spheres forming large necks, whereas it distributed more uniformly (small necks) in hydrophobic (dehydroxylated) ones. Counterintuitively, water films on the spheres surface were released faster upon desorption in the case of hydrophilic crystals. With this exception, water desorption was identical irrespective of silica hydroxylation or water content. Remarkably, the separation between spheres in the nonclose-packed crystals exclusively depended on water content and not on hydrophilicity. These results are compatible with water transport from the spheres surface to the necks, which is gradually hindered in hydrophobic crystals. Our method revealed extreme accuracy allowing us to measure nanometer-scale changes like thin surface water films (from 5 to 0 nm) or slight sphere shrinkage upon annealing of less than 2% (4 to 7 nm), which are hardly discernible with other techniques like dynamic light scattering or electron microscopy. © 2012 American Chemical Society.

F.G.-G. was supported by the JAE Postdoctoral Program from the CSIC. This work was partially supported by EU FP7 NoE Nanophotonics4Energy Grant No. 248855; the Spanish MICINN CSD2007-0046 (Nanolight.es), MAT2009-07841 (GLUSFA), and Comunidad de Madrid S2009/MAT-1756 (PHAMA) projects.

Peer Reviewed