Abstract
Conductance data for Et4NBr, Et4NBF4, Bu4NBr, Bu4NBF4 in acetonitrile for the molar concentration range of 2·10-4–1·10-2 mol·dm-3 over the temperature range from 5 to 55 °C are reported. Limiting molar conductivities and ion association constants were determined by using the Lee-Wheaton equation for the symmetrical electrolytes. On the basis of the preliminary conductometric data analysis it was established that the closest approach parameter is almost independent from the temperature for all studied acetonitrile solutions. Therefore, the closest approach parameter was adopted as a sum of cation and anion radii for further conductometric data treatment. The limiting conductivities of Br-, BF4-, Et4N+ and Bu4N+ ions and the structure-dynamic parameter of ion-molecular interaction obtained from the experimental data on limiting molar conductivities were evaluated in the framework of the approach proposed by authors [Kalugin O. N., Vjunnik I. N. Limiting ion conductance and dynamic structure of the solvent in electrolyte solution. Zh. Khim. Fiz. (Rus.) 1991, 10 708-714]. Elongation of the alkyl radical of the tetraalkylammonium cation from Et4N+ to Bu4N+ leads to a significant increase in the structure-dynamic parameter, which indicates the dynamic structuring of the solvent near the tetrabutylammonium ion and increased solvophobic solvation of the Bu4N+ compared to Et4N+. Ion association constants are discussed in terms of competition between Coulomb and non-Coulomb forces in terms of short-range square-mound potential. An increase in the ion association constants in the sequence Bu4NBF4<Et4NBF4<Bu4NBr<Et4NBr was explained by the increase in the contribution of short-range ion-molecular interactions to the interionic attraction in addition to the electrostatic component. An increase in temperature enhances the ionic association due to both the electrostatic and short-range components.
Highlights
Non-aqueous electrolyte solutions are widely used in electrochemical energy storage devices, such as supercapacitors (SCs) with porous nano-carbon electrodes [1]
Giving the fact that solutions of Et4NBF4 are considered as model electrolyte solutions for SCs [14], within this research we focused on studying the influence of the temperature on the electrical conductivity of tetraalkylammonium salts R4NX (R = Bu, Et; X = BF4, Br) in acetonitrile
It is worth mentioning that obtained values (Table 1, 2) of limiting molar electrical conductivities and respective association constants for Et4NBF4, Bu4NBF4, Bu4NBr in acetonitrile are in good agreement with the data, previously obtained in our laboratory [23,24,25]
Summary
Non-aqueous electrolyte solutions are widely used in electrochemical energy storage devices, such as supercapacitors (SCs) with porous nano-carbon electrodes [1]. Exceptional attention was paid to quantitative characteristics of ion-ion and ion-molecular interactions when interpreting obtained conductometric data This data is important for systematization of already available data and to formulate new approaches to optimize properties of electrolyte solutions for SCs. An excess of tetrafluoroboric acid (up to pH=5) was added to the water solution of tetrabutylammonium hydroxide (ω=40%). Experimental densities of the solutions of Bu4NBF4, Bu4NBr and Et4NBF4 in acetonitrile were obtained for the concentration range of 0-1 mol kg-1 and fitted by equation (2). The approach is based on the application of non-linear least squares method to solve the overfilled system of the equations: concentration dependence of electrical conductivity ( theor ) (3), the mass action law (4), mass balance equation (5) and expression for mean ionic activity coefficient (6) for a set of k experimental points: theor. The Pethybridge modification [18] of Lee-Wheaton equation [19,20,21] was used for the concentration dependence of electrical conductance of diluted solutions: 1
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