Title: Mobile nanostructures (cybotactic groups) as a basis of generalised phenomenological model of aqueous electrolyte solutions

Authors: Andrey Yu. Efimov; Maria K. Khripun; Lubov A. Myund; Olga N. Pestova

Addresses: Department of Chemistry, Saint Petersburg State University, Universitetskii pr. 26, 198504 Petergof, Russia ' Department of Chemistry, Saint Petersburg State University, Universitetskii pr. 26, 198504 Petergof, Russia ' Department of Chemistry, Saint Petersburg State University, Universitetskii pr. 26, 198504 Petergof, Russia ' Department of Chemistry, Saint Petersburg State University, Universitetskii pr. 26, 198504 Petergof, Russia

Abstract: In this paper, we briefly review our researches into concentrated electrolyte solutions by means of a proposed phenomenological model. Electrolyte solutions are complex systems with a multitude of interactions. However, totality of interactions in solution is reflected in its constitution. The description of solution structure includes all parameters that define short-range ordering of particles, a full description of all possible coexistent vibrationally averaged structures (V-structures). We believe that information about the presence of the concentration ranges with different dominant structures and boundaries may be derived from the solubility polytherm (phase diagram) of the water-salt system. The boundary concentration corresponds to the eutectic composition (EC). In the range of pre-eutectic concentrations the dominant structure is the proper structure of water, and ion hydration is the major type of interaction between components. A crystalline hydrate or anhydrous salt crystallises at reduced temperatures from solutions with post-eutectic concentrations; hence, the dominant solute included V-structures are the structures of corresponding crystalline hydrate or quasi-crystalline fragments of anhydrous salt. For above-discussed spatial ordering of particles we used Stewart's term 'cybotactic group' (CG), which is a nanodimensional mobile V-structure that corresponds to the unit cell of a solid phase, which crystallises from solution under freezing. Analysis of concentration dependences of structural-sensitive properties (parameters) serve as information source about structural changes in solution, and 'special points' or singularities as the concentration boundaries of structural changes in solution correspond to eutectic and peritectics on solubility diagram. We proposed a new method of sounding to probe some peculiarities and differences in the solution structure. This method consists of monitoring of structural and dynamical features of new micro components introduced into the system under investigation. As such micro components one can use small additives of salts of transition and rare-earth metals. The structure of solution has a pronounced effect on the condition of complex-forming cations, which are presented as additives. If dominant structure (CG) has changed, there are alterations in structural and dynamical parameters of micro components, thus indicating that their behaviour 'obeys' the overall structural rearrangement in the solution. Such alterations of micro components can serve for probing some differences in the dominant structure of the supporting solution.

Keywords: aqueous electrolyte solutions; cybotactic groups; solubility diagrams; eutectic composition; crystalline hydrates; sounding method; phenomenological modelling; micro-heterogeneity; mobile nanostructures; nanotechnology; ion hydration.

DOI: 10.1504/IJNT.2016.074526

International Journal of Nanotechnology, 2016 Vol.13 No.1/2/3, pp.95 - 111

Available online: 04 Feb 2016 *

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