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Adsorption and capillary condensation of an organic fluid in a periodic mesoporous silica ͑SBA-15͒ are studied by in situ synchrotron diffraction. Powder diffraction patterns resulting from the two-dimensional hexagonal packing of the cylindrical pores of SBA-15 are collected as a function of vapor pressure during continuous adsorption and desorption of the fluid ͑perfluoropentane C 5 F 12 ͒, using a specially designed sorption cell. Seven diffraction peaks with systematic changes of the intensity are resolved as the adsorbed film thickness increases along the adsorption isotherm. The integrated intensities of the diffraction peaks are analyzed with a structural model involving four levels of electron density ͑dense silica matrix, microporous corona around the pores, adsorbed film, and core space of the pores͒. The model provides quantitative information about the structure of the evacuated specimen, the filling of the corona, and the growing thickness of the liquid film with increasing pressure. A very good fit of the data by this model is found for relative pressures up to p / p 0 Ϸ 0.5, but the fit of the high-indexed diffraction peaks becomes poor close to the capillary condensation pressure ͑p / p 0 Ϸ 0.68͒. Tentatively, this fact may be attributed to a deviation of the liquid film structure from the simple flat geometry close to the phase transformation, presumably caused by density fluctuations.

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).