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