Kristof Hormann

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.

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.

Synthesis and morphological characterization of phenyl-modified macroporous–mesoporous hybrid silica monoliths

Compared with pure silica-based or organic-polymer monoliths, hybrid organic-silica monoliths offer the combined advantages of mechanically strong stationary phases, simpler preparation protocols, resistance to swelling and shrinking in many solvents and better pH stability. Comprehensive data on the systematic characterization of the pore space morphology of hybrid organic-silica monoliths and their connection to pure silica-based monoliths are still scarce in the literature. In this work, we adapted the general sol–gel procedure with phenyltrimethoxysilane and tetramethoxysilane as precursors to prepare phenyl-modified macroporous–mesoporous silica monoliths via spinodal decomposition involving poly(ethylene glycol). Effects of polycondensation temperature and poly(ethylene glycol) amount were investigated with respect to the corresponding macropore space morphology. We characterized the monoliths by thermogravimetric analysis and infrared spectroscopy (phenyl-modification), nitrogen physisorption and scanning electron microscopy (meso- and macropores) as well as confocal laser scanning microscopy for three-dimensional reconstruction of the macropore space morphology. The statistical analysis of a reconstruction by chord length distributions allowed us to assess the monoliths macropore space heterogeneity through a quantitative approach. Relying exclusively on image analysis, we provide an accurate and model-free description of the void space distribution. Complementary macroporosity profiles were recorded to identify macroscopic heterogeneities inside a monolith. Analyzed structural features are connected to key transport properties of the macropore space. Phenyl-modified monoliths from this work were compared with previous pure silica-based and hybrid organic-silica monoliths regarding the bulk homogeneity of the monoliths and the critical wall region in capillary column format. The comparison with a conventional C18-silica monolith demonstrated a selectivity tuning with the phenyl-modified silica monoliths by π–π-interactions between the stationary phase and aromatic analytes. Application of the phenyl-modified monoliths in capillary liquid chromatography reflected the selectivity behaviour of commercial phenyl-modified silica particles, but with the advantage of a higher separation efficiency for the monolithic stationary phase.

Larger voids in mechanically stable, loose packings of 1.3 μm 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.

Topological analysis of non-granular, disordered porous media: determination of pore connectivity, pore coordination, and geometric tortuosity in physically reconstructed silica monoliths

Gaining adequate knowledge on the morphology of porous media is critical to ensuring their continued success as support structures in applications that rely on efficient mass transport. The physical reconstruction of a porous medium provides the optimum basis for an accurate characterization of its morphology, yet the identification of meaningful descriptors is not straightforward, especially not for monolithic materials, whose continuous solid phase and open pore network resist the tessellation schemes applicable to granular media. In this work, we focus on a hardly investigated component of silica monolith morphology, namely the topology of the hydrodynamically accessible macropore space. We propose and apply suitable methods to determine pore connectivity, pore coordination, and geometric tortuosity in four silica monolith samples after physical reconstruction of their macropore space by confocal laser scanning microscopy. Pore connectivity is traced by medial axis analysis, whereas pore coordination is evaluated after compartmentalization of the open macropore space into individual pores and pore throats by a maximum inscribed spheres approach. The geometric tortuosity is determined by medial axis analysis as well as by a propagation method that maps the geodesic distance from the center point of a reconstruction to every other point in the pore space. The presented results provide a comprehensive description of silica monolith topology as well as quantitative data for the construction of pore network models. The proposed analysis methods are applicable to any porous material that can be physically reconstructed at the required resolution.