The mixing of K⁺, (nBu₄)N⁺, Mg²⁺, and Ca²⁺ with guanidinium cation in water: ionic interactions from a thermodynamic study

Abstract

Ionic interactions have been determined from the experimental osmotic coefficients, densities, and sound speeds for the mixing of guanidinium cation (Gn⁺) with K⁺, tetra-n-butylammonium cation (nBu₄)N⁺, Mg²⁺, and Ca²⁺ in aqueous solutions containing Cl^- at several ionic strengths. As Mg²⁺ and Ca²⁺ are more hydrophilic than K⁺, and (nBu₄)N⁺ is a strong hydrophobic cation, their mixing with another strong hydrophobic species, that is, Gn⁺, throws light on the importance of binary and ternary interactions involving two cations and one common anion. The excess free energy of mixing, Δ_mG^E, volumes, Δ_mV^E, and compressibility, Δ_mK^E calculated from independent osmotic coefficient, density, and sound speed measurements indicate strong binary interactions as a result of mixing of K⁺-Gn⁺ and (nBu₄)N⁺-Gn⁺ cation pairs. The mixing of Mg²⁺ with Gn⁺ and Ca²⁺ with Gn⁺ suggests the neutralization of opposing effects produced by the hydrophilic and hydrophobic cations. This situation, which is similar to that noted in an earlier study (Kumar, A. J. Phys. Chem. B 2000, 104, 9505) on the mixing of Na⁺ and Gn⁺, offers important information on the binary and ternary interactions present on the mixing of one hydrophilic or hydrophobic cation with another hydrophobic cationic species.