Pore size distribution of MCM-41-type silica materials from pseudomorphic transformation - A minimal input data approach based on excess surface work

Abstract A collection of porous silica based materials have been synthesized by pseudomorphic transformation of silica-gel, commonly used in chromatography, to an ordered MCM-41 type material. The modified materials exhibit a bimodal pore size distribution (4 nm of MCM-41 and 20 nm for silica gel) with slit type geometry of the unmodified silica material and a cylindrical pore geometry of the MCM-41 type structure of the fully transformed material. Based on the Derjaguin-Broekhoff-de Boer (DBdB) theory and the previously published Excess Surface Work (ESW) approach we propose a method to derive the pore size distribution directly from the experimental isotherm without the need for a reference isotherm. Combining the ESW and the disjoining pressure approach an expression for a critical width is derived. This in turn relates the critical width to the relative pressure in a range where capillary condensation occurs. The method is simple, yet it provides information comparable to the standard NLDFT (non-local density functional theory) approach. Due to the absence of an interaction model the method is applicable to cases where interaction parameters are not available. We demonstrate the utility of the method comparing the results of characterization of the bimodal biphasic silica materials with the commonly used NLDFT and the BJH (Barrett-Joyner-Halenda) approach.