Electrochromatographic behavior of silica monolithic capillaries of different skeleton sizes synthesized with a simplified and shortened sol-gel procedure

Silica monolithic capillaries (SMCs) were synthesized by a sol-gel process. First, a simplification of the synthesis was proposed by replacing the calcination and the drying steps which can have tremendous effects on chromatographic and physical properties, by a single water or methanol 2 h washing step. The efficiency of such a washing step was demonstrated and the comparison of the chromatographic and electrochromatographic properties between calcined and washed SMCs has shown that such a modification did not impair retention, efficiency, and stability of the monolith. This simplified procedure was carried out to synthesize SMCs with two different skeleton sizes. These capillaries were evaluated in electrochromatography and present high efficiencies (H = 5 µm) at least equal to the best ones reported in the literature. Furthermore, the influence of the skeleton size on the EOF of the second kind (EOF-2) was investigated with unmodified SMCs used under various experimental conditions including electrical field strength and buffer concentration. The ionic strength of the mobile phase and the applied electrical field that enable this EOF-2 were related to the size of the skeleton which was tuned by the synthesis conditions

MCM-41 silica monoliths with independent control of meso and macroporosity

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Influence of the hydrothermal treatment on the chromatographic properties of monolithic silica capillaries for nano-liquid chromatography or capillary electrochromatography

In the last decade, silica monolithic capillaries have focused more and more attention on miniaturized sepn. techniques like capillary electrochromatog. (CEC), nano-liq. chromatog. (nano-LC) and chip electrochromatog. owing to their unique chromatog. properties and their simplified prepn. compared with packed columns. They were synthesized according to a sol-gel multi-step process that includes, after a gelation step at 40° giving the macropores network and the silica skeleton, a post-gelation step (hydrothermal treatment at 120° in basic medium) that allows to tailor the mesopores and finally a calcination or a washing step to remove remaining polymers. To reduce the synthesis time, the no. of synthesis steps and above all the temp. synthesis, to adapt the synthesis of such silica monoliths in polymeric microsystem devices, the authors extensively studied the influence of the hydrothermal treatment and its duration on textural (pore size distribution) and chromatog. properties (retention, efficiency) of in situ-synthesized capillary monoliths in nano-LC and CEC. This study was performed on pure silica and octyl chains grafted silica monoliths. Untreated monoliths show small pores (<6 nm), whereas hydrothermally treated monoliths exhibit medium and large mesopores (8-17 nm). The hydrothermal treatment at 120° was not necessary for pure silica monolithic capillaries dedicated to normal phase liq. chromatog. or hydrophilic interaction liq. chromatog. (HILIC) and electrochromatog.: the suppression of the hydrothermal treatment did not impair efficiencies in CEC and in nano-LC but contributed to increase in retention factors. Minimal plate heights of .apprx.5 mm in CEC and 6 mm in nano-LC were obtained with or without hydrothermal treatment with bare silica. In the same way, the hydrothermal treatment was not necessary for grafted silica monoliths only dedicated to CEC. However, the hydrothermal treatment becomes essential before grafting to preserve the efficiency of the monolithic silica capillaries dedicated to nano-LC: in this particular case, the suppression of the hydrothermal treatment leads approx. to a loss of a factor two in efficiency

Characterization of mesoporous silica and its pseudomorphically transformed derivative by gas and liquid adsorption

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Controlling the morphology of mesostructured silicas by pseudomorphic transformations: a route towards applications

International audienceMicelle-templated silicas (MTS) such as MCM-41 and MCM-48 feature unique textural properties owing to their uniform distribution of mesopores with tunable sizes. MTS synthesis is relevant to unique self-assembly processes between surfactants and inorganic matter. The properties of MTSs have been explored in view of applications in fields as diverse as catalysis, chromatography, sensing, photonics, optics, drug delivery, etc. The aim of this contribution is to review, and to highlight by new results, a synthesis strategy we have developed since 2002 to control the particle morphology of MTSs at the micro- to millimeter scale, a key step for transferring these materials from the state of beautiful artworks to applicable products. It is based on the concept of pseudomorphic suynthesis. éseudomorphism is well known in the mineral world. It allows preparation of a mineral with a morphology which is not related to its crystallographic symmetry group. The reuslting mineral assumes the outward crystal habit of a different mineral. This principle occurs at a nonconstant matter content by using a mineralization solution that exchanges anions (or cations) with an existing (preshaped) solid body, and allows the new structure to precipitate while maintaining the existing morphology. The comcept of pseudomorphic transformation is now applied to amorphous preshaped silica particles to produce MTSs with the same morphology, using an alkaline solution to dissolve the silica and repricipitate it around surfactant micelles into the ordered MTS structures. MTSs with hexagonal and cubic symmetry, different pore sizes, and controlled morphology have been synthesized. The new pseudomorphs have been successfully used as supports in chromatography, a very demanding application in terms of particle size and morphology

Diffusion Properties of Hexane in Pseudomorphic MCM-41 Mesoporous Silicas Explored by Pulsed Field Gradient NMR

International audiencePulsed field gradient (PFG) NMR is a powerful tool to examine diffusion of adsorbates in porous systems. The use of mesoporous silicas with uniform particle sizes allowed us to demonstrate the possibilities of this technique. In particular, we confirmed that, in the Mitra mathematical approach of diffusion, the surfaceto-volume ratio is related to the geometry of the whole particle and not of a single pore. Hexane diffusion measured by PFG-NMR was efficient to study innovative materials like pseudomorphic MCM-41 mesoporous silicas presenting different pore topologies. The thorough analysis of the diffusion data allows monitoring the extension of the restricted diffusion domain. This method gives quantitative information on diffusion processes in bimodal pore systems and permits to gain insight into the internal structure of the pseudomorphic materials at different synthesis times. For a simple pore geometry, it is observed that the diffusion coefficient increases with the pore size. However, when materials possess a bimodal pore system (as for the intermediate materials of the pseudomorphic: transformation), the diffusion can either decrease or increase depending on the connectivity of the secondary large mesopores with the main mesoporous channels. By PFG-NMR it was possible to detect the rearrangement of the mesoporous network of MCM-41 with synthesis time and to confirm the time necessary for the ordered mesoporous channels of MCM-41 to run through the whole particle. This type of measurement can nicely complement usual characterization techniques (N-2 adsorption, SEM, TEM, etc.) in order to give a better picture of diverse porous materials

Optimization of the properties of macroporous chromatography silica supports through surface roughness control

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Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography

Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 m could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC