Solutions of Alkylammonium and Bulky Anions: Description of Osmotic Coefficients within the Binding Mean Spherical Approximation

Abstract

The binding mean spherical approximation (BiMSA) is used to describe osmotic coefficients for aqueous solutions of salts containing alkylammonium cations or bulky anions. A total of 35 salt solutions is accurately described at 25°C over the whole concentration range, up to very high concentrations such as 20 mol·kg −1 for methylammonium chloride or 24 mol·kg −1 for ammonium thiocyanate. The ion diameters, the permittivity of solution, and the association constant are adjustable parameters within the BiMSA model. New diameter values are assigned to alkylammonium cations and bulky anions such as tetrafluoroborate, alkanesulfonates, methylsulfate, trifluoroacetate, or trifluoromethanesulfonate anions. Alkylammonium sizes are in reasonable agreement with literature values. Besides, association constants values obtained within the BiMSA model compare well with literature values when available. INTRODUCTIONThe mean spherical approximation (MSA) 1−4 is a statistical thermodynamic model for the representation of the thermodynamic properties of electrolyte solutions, regarding an ion i as a charged sphere of diameter σ i immersed in a solvent of permittivity ε. The MSA model uses parameters (ion sizes and relative solution permittivity) that have some physical meaning. For applications to real solutions, crystallographic values are taken for the halide anions radii and the size of a given cation at infinite dilution is the same for all salts containing this cation. 5 Within the binding MSA (BiMSA) version of the model, 6,7 an effective association constant is introduced, taking into account the pairing or association of unlike ions. This model has been shown to provide good description for deviations from ideality in aqueous solutions of salts such as alkali halides, nitrates, perchlorates, sulfates, 6−8 or, more recently, lanthanide salts 9,10 or mixtures of salts. 5,7,10 The BiMSA model has been widely used to describe monatomic cations. An attempt at describing salts containing a tetramethyl-, tetraethyl-, or tetrapropylammonium cation and a halide anion has previously been reported. 6 Nevertheless, the results obtained at the time are not satisfying, because despite an accurate description of the osmotic coefficients of tetraalkylammonium halide solutions, the values for the adjusted parameters, such as the cation diameter, were not plausible.Therefore, in order to extend the number of solutions described within the BiMSA model and in order to describe solutions containing large and polyatomic cations or anions, we apply here the BiMSA model to the description of aqueous solutions containing tetraalkylammonium salts with methyl, ethyl, or propyl alkyl chains and with the chloride, bromide, iodide, or nitrate anion. Because micelles might be formed

Aqueous solutions of ionic liquids. Description of osmotic coefficients within the Binding Mean Spherical Approximation

“…Note that in this study, the variation of ε W with temperature is taken into account. Eqs 10 and 11 have been used in several papers dealing with MSA and BiMSA [18][19][20][21][22]28 and are used here to provide a set of parameters compatible with those previously obtained. In eq 10, σ + (0) is the diameter of cation at infinite dilution.…”

“…They are slightly larger than that of the tetraethylammonium cation adjusted previously. 28 The temperatures used, the BiMSA parameters and the AARDs are collected in Table 3. The parameter values for the 13 systems studied here are collected in Table 4.…”

“…This model has shown to provide very good description of the thermodynamic coefficients of aqueous solutions of salts such as alkali halide, nitrate, perchlorate, sulfate [20][21][22] , mixtures of salts [23][24][25] or complex electrolyte systems 26 and recently, lanthanide salts 25,27 and concentrated solutions of bulky ions such as alkymammonium or alkylsulfate ions. 28 Besides, the MSA model uses a few parameters that have some physical meaning. For instance, the values for halide anions radii are taken equal to the ionic radius and the cation diameters at infinite dilution are not salt dependent.…”

“…28 We report here the description of aqueous solutions of ionic liquids based on imidazolium cations, that are cyclic quaternary ammonium cations.…”

“…For these solutions, ion diameters were assigned classic MSA values 23,24 in the case of simple anions or more recently determined values in the case of polyatomic anions. 28 To the best of our knowledge, this model has not yet been applied to describe aqueous ionic liquids solutions.…”