Local Composition in a Binary Mixture on a One-Dimensional Ising Lattice

Expressions for the local composition in a binary mixture of particles placed on an Ising lattice are given in one dimension (1D) as a function of the average composition and of the interchange energy. The particles are supposed to interact through pairwise interactions between nearest neighbors. The problem is solved by using the classic analogy with the Ising spin problem. The result for the first neighbor is in agreement with an expression obtained from quasi-chemical theory, which is known to be exact in 1D. The composition for more remote neighbors is also determined. Besides, a model proposed in the literature to calculate approximately the local composition for the first neighbor in any dimension is examined. The solution to the problem in 1D provides insight into the model and a different interpretation of the thermodynamic contributions involved in the method

Thermodynamic Consistency in the Modeling of Speciation in Self-Complexing Electrolytes

Speciation in aqueous solutions of complexing electrolytes is an important topic in environmental, toxicological, and sometimes industrial issues. Its determination requires an estimation of the deviations from ideality that originate from association effects, and excluded volume and electrostatic interactions between the various species. Modeling of such solutions is often based on the use of commonly accepted values for the complex formation constants and the use of the Davies equation to compute the activity coefficients of the species in solution. In this work, it is shown that this treatment may result in a thermodynamic inconsistency when moderately concentrated solutions of multiply self-complexing salts are considered. This observation casts some doubt on the determination of the speciation in such solutions. Occurrence of this shortcoming is illustrated in the case of zinc and cadmium halides. This finding suggests that the description of deviations from ideality (besides association) should be improved. It also reveals that available complexity constants for some common salts might not always have optimum values in the literature

Local Composition for a Binary Mixture of Particles on a Three-Dimensional Ising Lattice

Results from Monte Carlo (MC) simulations are reported for a binary mixture of particles A and B placed at the nodes of a three-dimensional cubic Ising lattice, with pairwise nearest neighbor interactions between species <i>i</i> and <i>j</i>, <i>E</i>_<i>ij</i>. The local composition is studied as a function of composition and of the value of the interchange energy (δ = 2<i>E</i>_AB – <i>E</i>_AA – <i>E</i>_BB). The MC results are used to assess the accuracy of analytical theories proposed in the literature as a function of the interchange energy. The local composition is shown to obey an exact limiting law at high dilution of one component, thus providing a simple expression for the limiting activity coefficient. The activity coefficients of the species and the entropy per particle are computed from the MC data

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

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