Computational and experimental study of the effect of PEG in the preparation of damascenone-imprinted xerogels.

Macromolecules, such as polyethylene glycol (PEG), have been frequently used in the preparation of xerogels, mainly with the purpose of tuning the meso- or macroporosity. However, PEG has never been applied in the context of the preparation of molecularly imprinted xerogels for small molecules. Thus, we decided to conduct a computational and experimental study of the incorporation of PEG into formerly studied sol-gel mixtures for the preparation of damascenone-imprinted xerogels. Computationally, two types of pregelification models were studied, one representing the initial mixture (SI3/SIPA:5:3 models) and the other representing the same mixtures after considerable solvent loss (SI3/SIPA:40:1 models). The latter ones were particularly prolific in providing clear effects of the PEG. In the SI3:40:1 model (containing SI3 units of Si(3)O(3)(OH)(6) mimicking the final xerogels backbone), a prohibitive instead of a promoting effect of PEG on the template-SI3 association was observed. PEG was found to interpose the SI3 aggregates, turning them smaller and more disperse. In agreement with that, a much higher porosity and surface area were found for the corresponding xerogel prepared with PEG, while no appreciable improvement of the imprinting efficiency could be observed. In the SIPA:40:1 model (containing both SI3 and SIPA units; SIPA, Si(3)O(3)(OH)(5)C(3)H(6)NHC(6)H(5), representing the introduction of the organic functional group into the xerogel network), the interactions related to the network structuring were not significantly affected. This was due to the fact that the SIPA units themselves had a dispersive effect on the silica network; the PEG molecules were "pushed" into the aqueous/methanolic continuum, and their presence was somewhat redundant. Accordingly, both prepared SIPA-xerogels (with PEG or not) exhibited higher porosity compared to SI3-xerogels. Although the simulation results were not conclusive about the effect of PEG on the template-functional group association, experimentally it was clear that the imprinting effect was not improved with PEG.

Molecular dynamics simulations of pregelification mixtures for the production of imprinted xerogels.

A series of molecular dynamics (MD) simulations of different pregelification mixtures representing intermediate stages of the sol-gel process were set up to gain insight into the molecular imprinting process in xerogels, namely, to assess the template-gel affinity and template self-aggregation. The physical plausibility of the parametrization was checked, confirming the reliability of the simulations. The simulated mixtures differed in the water/methanol ratio (1:3, 5:3, and 5:1) and in the absence/presence of an organic functional group (phenylaminopropyl-) in the silicate species. The simulation results, expressed mainly by the radial distribution functions and respective coordination numbers, showed that the affinity of the template molecule, damascenone (a hydrophobic species), for the gel backbone would not be attained without the tested functional group, phenylaminopropyl-. The affinity, related to the capability to trap the template within the gel network, was derived mostly from the hydrophobic interaction. It was also inferred from MD simulations that lower water contents (methanol-richer mixtures) would facilitate a better dispersion of both the functional group and the template within the final gel, therefore favoring the imprinting process. From the experimental counterparts of the simulated mixtures, a series of imprinted and nonimprinted xerogels were obtained. There was only one xerogel exhibiting the imprinting effect, namely, the one containing the organic group obtained at the lower water/methanol ratio (1:3), in agreement with predictions from the MD simulations. Such congruence demonstrates the ability of MD simulations to provide information regarding the fine aspects of molecular interactions in pregelification mixtures for imprinting.

Synthesis of glycylglycine-imprinted silica microspheres through different water-in-oil emulsion techniques.

Sol-gel molecularly imprinted materials (MIMs) are traditionally obtained by grinding and sieving of a monolith formed by bulk polymerization. However, this process has several drawbacks that can be overcome if these materials are synthesized directly in the spherical format. This work aimed at the development of two efficient methods to prepare spherical glycylglycine-templated silica ("whole-imprinted" and surface-imprinted) through a combination of sol-gel and emulsion techniques. The synthesis of the microspheres was optimized regarding emulsion and sol-gel parameters. Imprinting efficiency of the prepared materials was studied by solid phase extraction and flow microcalorimetry. The particles prepared with glycylglycine and functional monomer, in basic medium (using cyclohexane as non-polar continuous medium) presented the highest imprinting factor - 2.5 - and the respective surface-imprinted material presented an imprinting factor of 1.5. The results of flow microcalorimetry confirmed the action of different mechanisms of glycylglycine adsorption: entropically-controlled interactions were present for the "whole-imprinted" material, indicating adsorption inside small imprinted pores; enthalpically-controlled interactions were observed for the surface-imprinted material, a behaviour more compatible with a template/surface-only interaction. Globally, the two approaches allowed for a successful imprinting effect which was more extensive for the "whole-imprinted" material, whereas the surface-imprinting feature confers to the surface-imprinted xerogel advantages regarding mass transfer kinetics. Overall, the spherical particles obtained by both approaches presented characteristics, such as sphericity, mesoporosity, easy/fast accessibility to imprinted sites, important indicators that these materials may be candidates for stationary phases for efficient, selective chromatographic separation.