Density profile of water confined in cylindrical pores in MCM-41 silica

Abstract

It is shown that traditional estimates of the pore diameter in the porous silica material MCM-41-S15 (of order 15Å) are too small to allow the amount of water that is absorbed by these materials (around 0.5gH2O/g substrate) to occur only inside the pore. Either the additional water is absorbed on the surface of the silica particles and outside the pores, or else the pores are larger than the traditional estimates. In addition the low Q Bragg intensities from a sample of MCM-41-S15 porous silica under different dry and wet conditions and with different hydrogen isotopes are simulated using a simple model of the water and silica density profile across the pore. It is found the best agreement of these intensities with experimental data is shown by assuming the much larger pore diameter of 25Å(radius 12.5Å). Qualitative agreement is found between these simulated density profiles and those found in recent empirical potential structure refinement simulations of the same data, even though the latter data did not specifically include the Bragg peaks in the structure refinement. It is shown that the change in the (100) peak intensity on cooling from 300K to 210K, which previously has been ascribed to a change in density of the confined water on cooling, can equally be ascribed to a change in density profile at constant average density. It is further pointed out that, independent of whether the pore diameter really is as large as 25Å or whether a significant amount of water is absorbed outside the pore, the earlier reports of a dynamic cross-over in supercooled confined water could in fact be a crystallisation transition in the larger pore or surface water.