Pore size gradient effect in monolithic silica mesopore networks revealed by in-situ-SAXS-physisorption.

In disordered mesopore networks the size distribution and connection between adjacent pores control desorption. How network characteristics can be extracted from corresponding physisorption isotherms is still a matter of research. To elucidate this, we study krypton physisorption (117.8 K) in the mesopore networks of "Nakanishi"-type monolithic silica. Combining physisorption in scanning acquisition mode with synchrotron-based in-situ-SAXS provides complementary information on pore filling states. These data reveal a mean pore size gradient, in which pores grow smaller towards the material's network center. This structural motif cannot be derived through conventional isotherm analysis. But it is clearly exposed through scanning desorption curves, which do not quite converge, but merge individually with the main desorption isotherm before the lower hysteresis closing point. Hence, our findings provide the basis to build advanced models for analyzing scanning isotherms and extracting network characteristics through new descriptors, such as pore size and connectivity distributions as function of network center distance.