Ionic interactions from the mixing of NaCl with the acetate, nitrate, perchlorate, and sulfate salts of guanidinium in water

Abstract

Ionic interactions have been investigated from the experimental isopiestic osmotic coefficient measurements on the mixing of NaCl with several guanidinium (Gn) salts, like CH₃COOGn, GnNO₃, GnClO₄, and Gn2SO₄, up to high ionic strengths. Analysis of osmotic coefficient data by the Pitzer theory offers valuable information on the mixing of ions of like and unlike charges of hydrophilic and hydrophobic nature. The mixing effects arising out of symmetrical and unsymmetrical mixing of ions are computed by the Pitzer theory in these systems. The excess Gibbs free energies of mixing, Δ_mG^E have been analyzed by the Friedman theory of cluster integral expansion. The Δ_mG^E values display very interesting features with respect to the ionic strength fraction of the acetate, nitrate, and perchlorate salts of guanidinium. The minima in the Δ_mG^E values are noted in the NaCl-rich mixtures, and the addition of these guanidinium salts slowly enhances Δ_mG^E, passing through zero to positive values. The mixing of NaCl with Gn₂SO₄ offers negative Δ_mG^E throughout the mixture composition. Although binary interactions are nearly absent in the NaCl-CH₃COOGn mixtures, the ternary and quaternary interactions are noted to be important in the mixtures of NaCl with other guanidinium salts. The Δ_mG^E values of the mixture containing Na⁺, Gn⁺, Cl^-, and SO₄^2- ions (where binary interactions are important) can be estimated by Young's cross square rule (YCSR) with confidence. The YCSR is not obeyed when NaCl is mixed with CH₃COOGn, GnNO₃ and GnClO₄, where ternary and quaternary interactions are dominant over the binary interactions.