Sucre Cumana

Research Topic:

Silica monoliths, silica aerogels
High Performance Liquid Chromatography (HPLC)
Supercritical Fluid Chromatography (SFC)

Description of the topic:

Beds packed with spherical particles has been used in liquid chromatography for decades. Over the years the packing material has undergone several modifications to allow separations, which would not be possible by any other separation process. Due to the great capability of chromatography to separate several systems, the current efforts lie in making the process easier, faster, cheaper and more efficient. The separation efficiency of packed beds has been improved by reducing the size of the beads which composes the packing; however the resulting increment in surface area available for the adsorption-desorption process comes along with an increase in the pressure required to pump the mobile phase. It imposes a technical limitation for the minimal particle size which can be used.

Silica monoliths were a real breakthrough as they were introduced around two decades ago as a novel stationary phase. They have made possible to carry out faster separations with lower pressure drops through the column. The mean feature of these materials is their higher porosity for similar surfaces area when compared with conventional packed beds.

Silica monoliths are synthesized by the sol-gel method, in which the hydrolysis and polycondensation of an alkoxysilane occurs in the presence of a water soluble polymer (these are indeed hybrid polymer-silica monoliths). This polymer induces a phase separation in the system and two phases are formed, a silica phase and a solvent phase. The silica phase becomes the skeleton and the space filled by the solvent phase becomes the pores of the monolith after drying.

The most relevant feature of silica monoliths is their bimodal pore structure, which is constituted by mesopores (pores with diameter between 2 and 50 nm) and macropores (openings exceeding 50 nm in diameter). The mesopores exist on the surface of the silica skeletons and provide sufficient surface area at the time that macropores offer a network through which the mobile phase can flow. Conveniently, while the macroporous structure is formed through phase separation and gelation during hydrolysis and polycondensation of an alkoxysilane in the presence of organic additives, the mesoporous structure is tailored by solvent exchange and aging.

Despite the advantages offered by these monoliths, the main obstacle to their broad application in chromatographic separations is their high price, which is related to their difficult production process. Normally the monoliths are prepared by pouring the sol into molds, and after removing the gelated samples from them, different treatments are applied, but basically washing, solvent exchange, temperature treating and drying. After this sequence the gels are cut in the desired size and shape and afterwards encased into a column for being used. This last step is one critical point of the monolith production.

The aim of the work:

The production of silica monoliths in-situ is the starting point of this work. It includes the preparation of the piece directly in the column and avoiding the drying and cladding steps, which constitute the main drawbacks of their current production. This would make the implementation of monoliths in chromatographic separations much easier and cheaper. Further on it would be useful in the field of bioseparation, where the sterile conditions require one-way systems. In situ preparation would allow a fast in-field procedure for this.

Aside from the idea of employing monoliths to increase efficiencies and to decrease analysis times, resides the possibility of reducing the dimensions of the chromatographic systems to reach the same effects. Miniaturized techniques are environmentally friendly, reduce expenses and, among other advantages, allow coupling separation techniques to each other and to various detection techniques. Therefore the use of capillaries as columns for HPLC separations would lead to advantages like enhanced performance and sensitivity and at the same time savings in consumable materials.

Supercritical extraction allows converting silica gels into aerogels. The second part of this work intends the use of silica aerogels as stationary phase for SFC separations. This has never been put into practice, but it is expected that all the advantages which these gels would bring to HPLC, may be present when working with supercritical fluids as mobile phase.

Since the bioseparation is targeted, a further aim of this work will include the study of the possibility of the immobilization of proteins in the pores of the monoliths. This would improve bioselective adsorption and then subsequent recovery of a compound from the immobilized compound. This technique has an enormous importance for the purification of biological mixtures.