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.

Larger voids in mechanically stable, loose packings of 1.3µm frictional, cohesive particles: Their reconstruction, statistical analysis, and impact on separation efficiency.

Lateral transcolumn heterogeneities and the presence of larger voids in a packing (comparable to the particle size) can limit the preparation of efficient chromatographic columns. Optimizing and understanding the packing process provides keys to better packing structures and column performance. Here, we investigate the slurry-packing process for a set of capillary columns packed with C18-modified, 1.3µm bridged-ethyl hybrid porous silica particles. The slurry concentration used for packing 75µm i.d. fused-silica capillaries was increased gradually from 5 to 50mg/mL. An intermediate concentration (20mg/mL) resulted in the best separation efficiency. Three capillaries from the set representing low, intermediate, and high slurry concentrations were further used for three-dimensional bed reconstruction by confocal laser scanning microscopy and morphological analysis of the bed structure. Previous studies suggest increased slurry concentrations will result in higher column efficiency due to the suppression of transcolumn bed heterogeneities, but only up to a critical concentration. Too concentrated slurries favour the formation of larger packing voids (reaching the size of the average particle diameter). Especially large voids, which can accommodate particles from>90% of the particle size distribution, are responsible for a decrease in column efficiency at high slurry concentrations. Our work illuminates the increasing difficulty of achieving high bed densities with small, frictional, cohesive particles. As particle size decreases interparticle forces become increasingly important and hinder the ease of particle sliding during column packing. While an optimal slurry concentration is identified with respect to bed morphology and separation efficiency under conditions in this work, our results suggest adjustments of this concentration are required with regard to particle size, surface roughness, column dimensions, slurry liquid, and external effects utilized during the packing process (pressure protocol, ultrasound, electric fields).

Mass transport properties of second-generation silica monoliths with mean mesopore size from 5 to 25nm.

The morphology of silica monoliths determines their mass transport properties. While eddy dispersion can be related to the size and structural heterogeneity of the macropores, longitudinal diffusion and trans-skeleton mass transfer resistance are influenced by the physical appearance of the mesopore space. We used two small analytes (uracil, naphthalene) and a large one (lysozyme) to characterize the column performance of a set of six second-generation analytical silica monoliths with systematically varied mean mesopore size from 5.5 to 25.7nm. Within this set of sample columns, the mean macropore size was conserved at about 1.2µm. Longitudinal diffusion and trans-skeleton mass transfer resistance were derived from peak parking experiments. Both contributions to the overall plate height are affected by the structural hindrance in the mesopores, which increases with smaller mesopore size. For the weakly retained naphthalene, this effect is counteracted by surface diffusion, which increases with the surface area of the mesopore space. Additivity of individual plate height contributions allows for the determination of eddy dispersion by subtraction of the other terms from the overall plate height curves. Column performance for lysozyme is limited by mass transfer resistance, which increases strongly with smaller mesopore size until lysozyme becomes totally excluded from the smallest (5.5nm) pores.

Morphological analysis of disordered macroporous-mesoporous solids based on physical reconstruction by nanoscale tomography.

Solids with a hierarchically structured, disordered pore space, such as macroporous-mesoporous silica monoliths, are used as fixed beds in separation and catalysis. Targeted optimization of their functional properties requires a knowledge of the relation among their synthesis, morphology, and mass transport properties. However, an accurate and comprehensive morphological description has not been available for macroporous-mesoporous silica monoliths. Here we offer a solution to this problem based on the physical reconstruction of the hierarchically structured pore space by nanoscale tomography. Relying exclusively on image analysis, we deliver a concise, accurate, and model-free description of the void volume distribution and pore coordination inside the silica monolith. Structural features are connected to key transport properties (effective diffusion, hydrodynamic dispersion) of macropore and mesopore space. The presented approach is applicable to other fixed-bed formats of disordered macroporous-mesoporous solids, such as packings of mesoporous particles and organic-polymer monoliths.

Analytical silica monoliths with submicron macropores: current limitations to a direct morphology-column efficiency scaling.

Shrinking the structural elements of a particulate bed or monolith (i.e., the particle or domain size) yields more efficient columns only when the homogeneity of the bed can be conserved in that process. We investigate this complex issue for a set of 2nd generation analytical silica monoliths with macropores reaching submicron dimensions using chromatographic methods, mercury intrusion porosimetry, scanning electron microscopy, and confocal laser scanning microscopy (CLSM), and present eddy dispersion simulations and a chord length distribution analysis for the CLSM-based physical reconstructions at macropore resolution. The combined results allow us to identify relevant morphological advances made from 1st to 2nd generation monoliths and additionally highlight the current limitations to a direct morphology-efficiency scaling with respect to the performance that can be accomplished in HPLC practice with these columns. Whereas the improvement in radial homogeneity from 1st to 2nd generation silica monoliths is represented by a dramatic increase in column efficiency, the further reduction of macropore size in the 2nd generation monoliths does not lead to the expected improvement of plate height data, although these monoliths realize submicron macropores at a simultaneously conserved bulk macropore space homogeneity and negligible radial heterogeneity. Our study implies that limitations to further improved column efficiency arise from the intrinsic border effects of the used 4.6mm i.d. analytical columns. This includes the sample distribution onto the monoliths and asynchronous sample collection through the endfittings at the column inlet and outlet, respectively. Only when these effects are reduced will additionally improved 2nd generation monoliths live up to column efficiencies, which are envisioned for them based on their morphological properties.

Comparison of first and second generation analytical silica monoliths by pore-scale simulations of eddy dispersion in the bulk region.

We present the first quantitative comparison of eddy dispersion in the bulk macropore (flow-through) space of 1st and 2nd generation analytical silica monoliths. Based on samples taken from the bulk region of Chromolith columns, segments of the bulk macropore space were physically reconstructed by confocal laser scanning microscopy to serve as models in pore-scale simulations of flow and dispersion. Our results cover details of the 3D velocity field, macroscopic Darcy permeability, transient and asymptotic dispersion behavior, and chromatographic band broadening, and thus correlate morphological, microscopic, and macroscopic properties. A complete set of parameters for the individual eddy dispersion contributions in the bulk was obtained from a Giddings analysis of the simulated plate height data. The identified short-range structural heterogeneities correspond to the average domain size of the respective monoliths. Our plate height curves show that structural improvements in the bulk morphology of 2nd generation monoliths play only a minor role for the observed improvement in overall column efficiency. The results also indicate a topological dissimilarity between 1st and 2nd generation analytical silica monoliths, which raises the question how the domain size of silica monoliths can be further decreased without compromising the structural homogeneity of the bed.

Morphology and separation efficiency of a new generation of analytical silica monoliths.

The heterogeneous morphology of current silica monoliths hinders this column type to reach its envisioned performance goals. We present a new generation of analytical silica monoliths that deliver a substantially improved separation efficiency achieved through several advances in monolith morphology. Analytical silica monoliths from the 1st and 2nd Chromolith generation are characterized and compared by chromatographic methods, mercury intrusion porosimetry, scanning electron microscopy, and confocal laser scanning microscopy. The latter method is instrumental to quantify morphological differences between the monolith generations and to probe the radial variation of morphological properties. Compared with the 1st generation, the new monoliths possess not only smaller macropores, a more homogeneous macropore space, and a thinner silica skeleton, but also radial homogeneity of these structural parameters as well as of the local external or macroporosity. The 66.5% reduction in minimum plate height observed between silica monoliths of the 1st and 2nd Chromolith generation can thus be attributed to two key improvements: a smaller domain size at simultaneously increased macropore homogeneity and the absence of radial morphology gradients, which are behind the considerable peak asymmetry of the 1st generation.