Structure of Water under Confinement in Periodic Mesoporous Organosilicas Investigated by X-ray Scattering

The effect of pore wall chemistry and pore diameter on the structure of confined water was studied by X-ray scattering on water confined in periodic mesoporous organosilicas (PMOs). A shift in the first structure factor peak at q approximate to 1.8 & Aring;(-1) reveals a variation in the density of the confined water depending on the hydrophilicity and pore size. Smaller and more hydrophilic pores induce a lower density in the water. In contrast to bulk water, the pair distribution functions (PDFs) of confined water show a splitting of the second-neighbor peak into either two, in the case of smaller and more hydrophilic pores, or three separate peaks, in larger and more hydrophobic pores. From the running coordination number, we conclude that a smaller and more hydrophilic confinement leads to a stronger developed tetrahedral network in confined water, while confinement in larger and hydrophobic pores gives tetrahedral arrangements that are bulk-like or even less pronounced than in bulk water.