Thermodynamic studies of ternary systems: The effects of various univalent cations in aqueous tetrabutylammonium chloride mixtures and the importance of experimental data in computer modeling

Aqueous ternary systems involving tetraalkylammonium cations have complex interactions. These systems exhibit strong hydrophobic interactions, increase the hydrogen bonding of water, and have a high degree of ion association. Thus, not all thermodynamic models can accurately predict solution behavior. To properly analyze systems that exhibit complex behavior, it is important to utilize a model that can account for all interactions occurring in solution.;We compared the thermodynamic models of Harned's rule, the Reilly and Wood equations, Pitzer formalism, Scatchard-Rush-Johnson formalism, and Scatchard-Rush-Johnson formalism as modified by Leifer and Wigent (SRJ-LW), and found that SRJ-LW formalism is the only model which accounts for all interactions occurring in systems exhibiting complex behavior.;Osmotic coefficient data were obtained for the systems LiCl-(n-Bu) 4NCl-H2O and Me4NCl-(n-Bu)4NCl-H 2O at 25°C utilizing a gravimetric isopiestic technique. The systems were analyzed using SRJ-LW formalism, the theory of co-spheres developed by Gurney and extended by Desnoyers and co-workers, and the concept of solvent structure enforced ion pairing discussed by Diamond. The osmotic coefficients, the activity coefficients of each electrolyte, the total excess free energy, and the contributions to the excess free energy due to pair, triplet, and higher order multiplets were calculated over a large range of total ionic strengths and the entire ionic strength fraction range.;The studies illustrate the importance of all ions in determining the thermodynamics of the system. They also indicate that the solvent structure making and/or breaking effect of all ions in solution, the hydrophilic nature and hydration of the ions, the ability and likelihood for the ions to form stable multiplets, and the relative sizes of the ions all influence the thermodynamic properties of the system.