Structure of aqueous and non-aqueous electrolyte solutions

The structure of water is especially important for studying the properties of water and aqueous solutions associated with the transfer phenomenon. This is due to the fact that the structure determines the conditions for the translational movement of the liquid. In the article show that water has its own structure, which partially changes with the appearance of a charged particle. At the same time, an ionic zone is formed around the ion, which has a quasi-crystalline structure, in which water, under the influence of a charge, as well as under the influence of other reasons, takes on a special orientation and acquires special properties.

Thus, in water in which the electrolyte is dissolved, two structures appear: the own structure of water (H2O)w and the structure of water in the zone of (H2O)i ions. Statistical probability in this case requires the appearance of a certain amount of water of an intermediate structure, which is usually called destructured water (H20)d. A balance is established between the structures

which at a given temperature will shift in one direction or another, depending on the nature of the dissolved substances, as well as on their concentration.

To understand the structure of the aqueous solutions of electrolytes that we are discussing, it should be taken into account that there are vacant places in the structure of water and that they can be filled not only with water molecules, but also with other particles, in particular, atoms and molecules of neutral substances. It was found that the structural features of solutions are determined not so much by the interaction between particles spreading over relatively large distances, but by interactions, albeit weak, but rapidly decreasing with distance. The study of the structural features of aqueous solutions of electrolytes showed that the structure of dilute solutions of electrolytes is determined only by the structure of water. Note that to date, a complete quantitative theory of concentrated electrolyte solutions has not yet been created. This is explained to some extent by the fact that the derived formulas do not satisfy some general requirements arising from thermodynamics and modern ideas about the nature of electrolyte solutions. The structure and structure of solutions, as well as their properties, depend mainly on the nature of the solvent and solute, its concentration, temperature, pressure and interaction of ions with solvent molecules, i.e. from solvation. The phenomenon of solvation is determined by all types of interactions in solutions: ion-solvent, solvent-solvent, ion-ion. Studies of aqueous solutions by X-ray diffraction, optical, electrochemical, thermodynamic and other methods show that the hydration of ions is accompanied by disturbances in the ordered structure of water. The water surrounding the ion is called hydration water, and the number of molecules that make up this environment is characterized by the hydration number.

Numerous experimental data have established that in the process of hydration, ions, according to their effect on water molecules in solution, fall into two groups.

The first group includes, as a rule, large singly charged mono- and polyatomic ions (K+, Cs+, Cl-, Br-, J-, ReO4-, JO4-, etc.), for which the effects of disordering the structure of water are predominant. The second group includes multiply charged and small singly charged mono- and polyatomic ions (Al3+, Mg2+, Li+, PO43-, SO42-, etc.), for which the effects of ordering the structure of water are predominant.

In the work, based on the IR spectra of 31 electrolytes (14 cations and 11 anions), it was established that all cations contribute to the formation of the structure of water and, according to the strength of their influence, a sequence is formed (H+>Li+>Cs+>Rb+>K+>Na+). All anions destroy this structure, i.e. The concentration of free water molecules here is much higher than in pure water. The most active agents in this case are PF-, ClO4-, and NO3-. Halogen ions, according to the strength of their destructive effect, form the sequence J->Br->Cl->F-.

Ions of the first group cause the same disturbances in the ordered structure of water as an increase in temperature, ions of the second group cause the opposite effect. The behavior of ions of the first group is unusual and they are characterized by negative hydration. This phenomenon is that near ions, water molecules become more mobile than in pure water. Ions of the second group, which are characterized by positive hydration, reduce the mobility and Brownian rotation of nearby water molecules. In general, ions in an electrolyte solution have the same effect on solvent molecules as an increase in internal pressure in a liquid. This idea was used by Gibson to derive the following expression for the isothermal compressibility of an electrolyte solution:

where xi and x^ are the mole fractions of the solvent and dissolved molecules, respectively, V2 -is the partial molal volume of the solute, V_m -is the molar volume of the solution, p - is the external pressure on the solution, A and B are constants. The term p_e represents the additional effective internal pressure caused in the solvent by the presence of dissolved ions.

The structure of solutions of electrolytes in water retains the main structural features of the solvent. However, the coordinating influence of ions on neighboring solvent molecules also makes a large contribution to the structure. Taking these two circumstances into account allowed the authors of the work to propose a structural formula to describe the structure of electrolyte solutions:

where к - is the coordination number of the ion, R - is the structural branching factor, equal to 3 for water, N_n is the number of solvent molecules in the solvation shell with number n.

The structure of solutions, in particular, its structure depends significantly on temperature. As the temperature rises and the concentration of the dissolved electrolyte increases, the hydrogen bonds between water molecules gradually break down, and the translational and rotational movements of the molecules in it become easier. The structure of the water is destroyed. The interaction of water molecules with ions changes to a lesser extent. Even near the K+ and Cs+ ions, the movement of water molecules turns out to be slower than in the mass of water. In this case, negative hydration turns into positive. The discovered picture corresponds to the relationship existing between the structural state of the solvent and the solvation of ions in solutions. Structural pressure also has a certain influence on the structure of aqueous solutions of electrolytes. Structural pressure is determined by external pressure. In their action, ions with positive hydration increase the structural pressure of the solution, and ions with negative hydration decrease it. With this approach, the change in the structural pressure of the solution acts in the direction opposite to the structural temperature. An increase in external pressure also leads to the destruction of the water structure, and, consequently, to a decrease in the destructive effect of ions. One of the most important characteristics of the structure of solutions is the coordination number - the number of solvent molecules that make up the immediate environment of the ion. The coordination number of ions depends on the electrolyte concentration, temperature and other factors. We know much less about the details of the structure of non-aqueous solvents and their solutions than about water. Although recently physicists have been paying a lot of attention to their study using a variety of methods, modeling their structures at least to the extent that has been done for water is still impossible, with the exception of liquid NH3 and HF. Apparently, here there is nothing more than a chain and ring association and the forces acting at a short distance do not play such a leading role as in water. It seems to us that in the division into aqueous and non-aqueous solutions, the leading factors are the unique structure of water and the specificity of short-range forces in it.

However, when dividing into subgroups, this factor fades into the background and the chemical characteristics of liquid molecules and the details of their interactions with ions become decisive.

Conclusion

In conclusion, we note that for the construction of a full-fledged theory of solutions, a fruitful solution to the following issues is very important: development of the problem of the liquid state of matter, study of solutions based on aqueous, nonqueous and mixed solvents, theoretical and experimental study of solutions in a wide range of concentrations in terms of solute and solvent composition, study of solutions in a wide range of temperatures and pressures. There is still a lot of serious work to be done in this direction. Therefore, the need to know the thermophysical properties of liquid solutions in a wide range of state parameters leads to their experimental study.

electrolyte structure water solution

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