Inhomogeneous Distribution of Water Adsorbed under Low Pressure in CrAPO-5 and SAPO-5:  An Interference Microscopy Study

Intracrystalline concentration profiles of water, adsorbed in large crystals of CrAPO-5 and SAPO-5 under equilibrium with water vapor at 1 and 20 mbar, were determined with use of interference microscopy. At lower pressure, the profiles reveal highly inhomogeneous distributions of intracrystalline water in both crystal types. This effect is attributed to structural heterogeneity of the crystals. Its possible relation to the progress of the crystal growth process is considered. Under the vapor pressure of 20 mbar, the water molecules are homogeneously distributed over the crystals. In this case, the pore volume is saturated with liquid-like water. The structural heterogeneity has been found to have a low or no influence on the final uptake level of the crystals.