Physical and Theoretical Chemistry

Biography

Biography: Elena G. Kovaleva

Abstract

Statement of the Problem: Many solid-phase materials , for instance, porous and nanostructural objects as well as the systems of specific functionality are widely used in aqueous solutions as promising heterogeneous catalysts of various reactions, can serve as suitable carriers for catalytically active organic and bioorganic groups and enzymes, and also be adsorbents of large and small molecules. The properties of solid-phase materials are affected therefore both the chemical nature of the solution and some specific conditions arising in the phase and on the surface of these materials.  Surface electrical potential (SEP) is among the most important surface characteristics of these materials. At present, there is no method for measuring SEP of hydrated porous and nanostructured materials. The purpose of this study is to develop the method for measuring SEP of different solid phase hydrated materials by EPR of pH sensitive nitroxides (NR) as spin probes and labels. Methodology & Theoretical Orientation: A variety of nitroxides with the range of pH-sensitivity from 2 to 8.5 pH were incorporated into nanoporous and nanostructured inorganic oxides as powders and membranes and a diversity of organo-inorganic hybride materials both by adsorption from aqueous solution and through covalent binding technique. pH-dependent parameters of EPR spectra were measured through monitoring pH in a bulk solution and inside materials. Findings: The negative and positive values of Stern potential  for the positively and negatively charged surfaces were measured from the characteristic shifts of the EPR titration curves for the “slow-motional” NR located in the  material near-surface about those for a bulk solution. Stern layer thickness for mesoporous silicas  was determined from the near-surface electrical potential profile using a model of  practically cylindrical nanosized hydrated channels of the mesoporous silicas with channel diameters ranging from 2.3 to 8.1 nm. Conclusion & Significance: An unique technique for measuring the near-surface (Stern) potential as well as Stern layer thickness and surface charge  based on EPR of pH-sensitive nitroxides as spin probes and labels,  have been developed for a wide range of hydrated porous and nanostructured materials with a great potential for adsorption processes and heterogeneous catalysis.