Ordered nanoporous silica is seen to contract reversibly during the condensation and evaporation of fluids in the pores, forming a capillarity-driven actuation system. In situ x-ray diffraction is used to measure the strain of the ordered pore lattice in dependence on the relative vapor pressure of different fluids. Elastic moduli extracted from these strain isotherms are independent of the fluid used, and do therefore constitute real nanomechanical properties of the solid porous framework. The relationship between this pore-load modulus and the Young’s modulus of the silica walls is analyzed with simple analytical considerations and with finite element calculations.

By means of in situ small-angle x-ray diffraction experiments and semi-grand-canonical ensemble Monte Carlo simulations we demonstrate that sorption and condensation of a fluid confined within nanopores is capable of deforming the pore walls. At low pressures the pore is widened due to a repulsive interaction caused by collisions of the fluid molecules with the walls. At capillary condensation the pores contract abruptly on account of attractive fluid-wall interactions whereas for larger pressures they expand again. These features cannot solely be accounted for by effects related to pore-wall curvature but have to be attributed to fluid-wall dispersion forces instead.

Ordered and disordered pores in SBA-15 silica and the gradual filling of these pores by an adsorbed fluid (dibromomethane, CH2Br2) are investigated by in situ small-angle X-ray scattering. Adsorption into the microporous corona and film formation at the corrugated surface of the ordered cylindrical pores is described by two different geometrical models. The analytical form factor resulting from these models is used to fit the integrated intensities of up to 10 Bragg diffraction peaks from the mesopore lattice. Model fits for the evacuated sample yield the porosity caused by the ordered pores. From these results and the total porosity obtained by nitrogen sorption, we determine the contribution of the disordered porosity, which is nearly 20% for the present sample. The model fits also provide new insight into the adsorbate structure in the ordered pores at different stages of pore filling, while the analysis of diffuse scattering provides information about fluid adsorption in the disordered pores in the walls. It is shown that the filling of the wall porosity affects the evaluation of the adsorbed amount in the ordered pores and leads to a distinction between relative and absolute adsorbed amounts. Using absolute adsorbed amounts, the filling isotherm of the ordered pores and the overall pore filling isotherm can be derived and compared with direct adsorption measurements. On the basis of the results of the present study, a quantitative modeling of the pore morphology and fluid sorption in the ordered and disordered pore regions of SBA-15 is presented in a subsequent paper (part II, DOI 10.1021/jp810040k).