Acidic and Electrosurface Properties of Binary TiO2-SiO2 Xerogels Using EPR of pH-Sensitive Nitroxides

Abstract

The binary xerogels TiO2–SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO2–SiO2 xerogels applications. This research investigates acid–base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO2–SiO2 xerogels, by the EPR of stable pH-sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge associated with surface electrical potential (SEP) was equal to –196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO2 content. Тhe local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution.

4-Dialkylamino-2,5-dihydroimidazol-1-oxyls with Functional Groups at the Position 2 and at the Exocyclic Nitrogen: The pH-Sensitive Spin Labels

“…requires nitroxides with a high sensitivity to acidity changes within a broad range of pH. A good example of such a spin probe is two-pK a nitroxide 1 , which was successfully used in numerous studies [ 8 , 26 , 27 , 28 , 29 , 30 , 31 ]. A covalent attachment of similar nitroxides to the surface of a catalyst or a sorbent may provide a useful method for studies of the near-surface layer in these materials.…”

“…An analysis of these data gives information about the acidic centers in the material and the local surface electrostatic potential. Recently, EPR studies using pH-sensitive spin probes were successfully used for the investigation of binary TiO 2 -SiO 2 xerogels [ 8 ].…”

Characterization and Fabrication of Xerogel Polymeric Nanocomposites and Multifunctional Applications

“…Biopolymeric X-Gs are made up of differing physically, chemically, mechanically, and biologically a liated properties, relative to varying parameters, such as precursoral materials, solvent phase, as well as drying conditions [36]. Recently, X-G have been versatily fabricated from biopolymeric substrates including pectin, cellulose, alginate, and chitosan [37][38][39][40]. Properties of these precursoral materials include non-toxicity and biocompatibility, which position them appropriate for numerous biomedical uses, like biosensing, wound mending, drug delivery, tissue scaffolding, and dressing applications [37].…”

Insights into the Role of Biopolymer-Based Xerogels in Biomedical Applications

“…The intermediate porosity of xerogels is highly preferable in sustained drug release; the high porosity of aerogels may lead to fast drug release, which is not desirable in some cases, such as cancer drug therapy [ 19 , 20 ]. Several research papers have been recently published regarding the fabrication and characterization of biopolymeric xerogels for different medical and biomedical applications [ 21 , 22 , 23 ], which are increasing by the day. However, to the best of our knowledge, a limited number of review papers on the applications of biopolymeric xerogels in biomedical applications have been published [ 24 , 25 ].…”