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 a function of the distance from the network center.

Smectite clay minerals reduce the acute toxicity of quaternary alkylammonium compounds towards potentially pathogenic bacterial taxa present in manure and soil.

Quaternary alkylammonium compounds (QAACs) are a group of cationic surfactants which are disinfectants with numerous industrial and agricultural applications and frequently released into the environment. One recent hypothesis is that bacteria present in soil will be protected from acute toxic effects of QAACs in the presence of expandable layer silicates due to interlayer sorption. We therefore studied bacterial growth kinetics with high temporal resolution and determined minimal inhibitory concentrations (MICs) of two QAACs, benzyldimethyldodecylammonium chloride (BAC-C12) and didecyldimethylammonium chlorid (DADMAC-C10), for eight strains of different bacterial taxa (Escherichia coli, Acinetobacter, Enterococcus faecium, Enterococcus faecalis, and Pseudomonas fluorescens) in relation to QAAC sorption to smectite and kaolinite. The MICs of BAC-C12 and DADMAC-C10 were in the absence of smectite and kaolinite in the order of 10 to 30 µg mL-1 and 1.0 to 3.5 µg mL-1 for all strains except the more sensitive Acinetobacter strain. For all tested strains and both tested QAACs, the presence of smectite increased apparent MIC values while kaolinite had no effect on MICs. Sorption curves without bacteria showed that smectite sorbed larger amounts of QAACs than kaolinite. Correcting nominal QAAC concentrations employed in toxicity tests for QAAC sorption using the sorption curves explained well the observed shifts in apparent MICs. Transmission electron microscopy (TEM) demonstrated that the interlayer space of smectite expanded from 13.7 ± 1 Å to 19.9 ± 1.5 Å after addition of BAC-C12. This study provides first evidence that low charge 2:1 expandable layer silicates can play an important role for buffering QAAC toxicity in soils.

On the underestimated impact of the gelation temperature on macro- and mesoporosity in monolithic silica.

The preparation of monolithic SiO2 with bimodal porosity using a special sol-gel procedure ("Nakanishi process") generally shows a pronounced sensitivity towards several physico-chemical parameters of the initial solution (concentrations, precursors, pH, temperature, etc.). Thus, temporal and spatial variations of these parameters during the sol-gel reactions can affect the final meso- and macropore space with respect to the pore size distributions and homogeneity. In this study we thoroughly examine the sol-gel reaction in terms of the impact of temperature accuracy and homogeneity during the gelation and their effect on meso- and macropore space. The in-depth characterization of the macroporosity in monolithic SiO2 rods, prepared by utilizing a highly homogeneous and accurate temperature profile, shows that a decrease of only 1.5 °C during the reaction doubles the mean size of the macropores in the analyzed temperature ranges (22.0-28.0 °C and 33.5-36.5 °C). Rheological measurements of the gelation points and the viscosity of the starting solutions prove that a higher reaction rate is the main reason for this marked temperature-sensitivity. Furthermore, the mesoporosity is affected to a surprising extent by the applied small temperature differences during the gelation reaction. This phenomenon is shown to be mainly caused by the temperature-dependent differences in macropore and skeleton dimensions and an inhomogeneous distribution of mesopore sizes within the skeleton. In essence, our study reveals that the impact of temperature on the formation of meso- and macroscale dimensions during the sol-gel process has been underestimated so far. The impact of a poor temperature homogeneity during monolith synthesis is exemplarily demonstrated by the application of monolithic silica capillary columns in HPLC.