Hierarchically organized (hybrid) silica monoliths for the application as stationary phases in HPLC
FacultiesFakultät für Naturwissenschaften
LicenseStandard (Fassung vom 01.10.2008)
This work deals with the sol-gel processing by employing diol-modified silanes as precursors towards hierarchically organized (hybrid) silica monoliths. With regard to their application as stationary phases in high performance liquid chromatography (HPLC), an important aspect is the hierarchical structuring with defined pore size domains in the mesoscopic and the macroscopic range. In order to avoid limitations of the diffusion pathways and the mass transfer within the monolithic column, a homogeneous co-continuous macroporous framework is desired, while an integrated mesoporous network allows for the separation of the chemical compounds. The sol-gel synthesis in combination with the employment of a structure-directing agent is a promising approach towards the synthesis of materials with integrated advanced functionalities as it allows for the design of the hierarchical pore size domains and can be performed in aqueous media at moderate temperatures and pressures. Adjusting the synthesis parameters, such as the choice and ratio of the structure-directing agent, e.g. Pluronic P123 or polydimethylsiloxane-based co-polymers (Tego Glide 440 and LA-S 687), the concentration of the acid catalyst, the gelation and aging temperature, etc., allows for a deliberate control of the final pore architecture of the silica monoliths with respect to the desired morphology on the different length scales. As in this work, diol-modified silanes are employed as precursors for the formation of the silica network, the influence of the chemical nature of the released diol on the phase separations in the mesoscopic and macroscopic range and hence on the resulting structural properties are studied. Besides well-defined pore size domains, the application of silica materials as stationary phases in HPLC, e.g. in the Reversed Phase (RP) mode, requires tailored surface properties, such as hydrophobic pore walls, which can be achieved by the incorporation of functional organic groups. Hybrid inorganic-organic silica monoliths have been synthesized via two different approaches. First by the direct synthesis approach at which the organic-modified silica network is generated by co-condensation of the ethylene glycol-modified methyl- or phenylsilane (MeGMS, PhGMS) with the ethylene glycol-modified silane (EGMS). The second approach is the post-synthetic functionalization of the silica surface via silylation reactions with chlorotrimethylsilane (TMCS) or chlorododecyldimethylsilane and via endcapping (by combining both silylation agents) resulting in a hydrophobic surface by simultaneous preservation of the hierarchical pore architecture.
Subject HeadingsHPLC [GND]
High performance liquid chromatography [LCSH]
Porous materials [LCSH]