Hosting Ability of Mesoporous Micelle-Templated Silicas toward Organic Molecules of Different Polarity

Abstract

Spin probe water solutions were adsorbed onto differently treated micelle-templated silicas (MTS) of different pore sizes to analyze the hosting ability of the MTS surface toward different organic molecules. The MTS synthesis was performed at 388 K by self-assembly of inorganic silica and micelles of cetyltrimethylammonium bromide (CTAB) to which different amounts of 1,3,5 trimethylbenzene (TMB) were added at different TMB/CTAB ratios to modify the pore size:  40, 65, and 80 Å pore diameter were obtained for TMB/CTAB ratio = 0, 2.7, and 13, respectively. As-synthesized MTS, calcined MTS, and octyldimethyl(C8) grafted MTS were used. These MTS were characterized by means of nitrogen sorption isotherms and TEM as homoporous silica with regular and reproducible structure. Different spin probes (nitroxides) were taken as models for different types of organic molecules, namely, neutral and charged molecules and surfactants. The computer aided analysis of the electron paramagnetic resonance (EPR) spectra of these probes provided information on the hosting ability of the differently treated solid surface in respect of the different structure and hydrophilicity of the probes. The spectral analysis allowed the depiction of the probable distribution and location of the different probes at the differently treated silica surfaces. For the as-synthesized MTS, void space became available for the probe adsorption in vicinity of the surface when TMB was used in the synthesis and then evaporated. For the calcined MTSs, the hydrophobic sites at the solid surface, namely, siloxanes, increased by increasing the TMB content in the synthesis mixture. The binding of the EPR probe with the surface of these MTS is favored when both hydrophilic and hydrophobic interactions occur, as found with surfactant probes bearing both a hydrophilic and a hydrophobic moiety. For the C8-grafted MTSs, the results provided a proof of the quality of grafting:  the surface is largely hydrophobic and favors self-aggregation of the surfactant probes, led by chain−chain interactions