In-Depth Study of Coating Multimodal Porosity Using Ellipsometry Porosimetry in Desorption Scanning Mode

Ellipsometry porosimetry provides full adsorption/desorption isotherms and is among the most accurate techniques to probe porosity in thin films. Here we address a novel technique based on desorption scanning for a precise assessment of the porosity features revealing the size distribution of interconnections between the various populations of pores in the coatings. This investigation was performed on model (multimodal) mesoporous silica films prepared by evaporation-induced self-assembly in the presence of micellar templating agents, before and after pore network modification through chemical attack in NH4F. The systems were also analyzed by grazing incidence small-angle X-ray scattering to confirm the multimodal periodic organization and homogeneity of the porosity. Conventional pore-blocking effect could not explain by itself the unconventional behaviors observed in the desorption scans. We propose a new desorption mechanism, addressed as pore transit desorption, to justify the experimental features and provide a full description of the porous network complexity. Based on these, we discuss several model cases of porosities combining up to two populations of pores and three populations of interconnections. Overall, interpretations made in this work can be generalized and transposed to any mesoporous material.