Nonstoichiometric interactions of long-chain ammonium salts in organic solvents

Several phenomena observed in solutions of long-chain substituted ammonium salts in nonpolar or slightly polar organic diluents cannot be reconciled with the ideal associated mixture approach commonly used. These phenomena include liquid-liquid immiscibilities and apparent average molecular weights of the aggregated salt that decrease with increasing concentrations. Nonspecific interactions between the solute and the solvent can account for them. These interactions are a plausible alternative to the assumption of the presence of higher oligomers (octa-, dodeca-, docosamers, etc.), besides a simple dimer or trimer, required to interpret osmometric data by the ideal association model. An expression has been derived which can fit the nominal molal activity coefficient log γ_A as a function of the ammonium salt molality m_A, for such systems as trilaurylammonium tetrahaloferrates in benzene and tetraisopentylammonium thiocyanate in chloro aromatic solvents recently studied, with a single interaction parameter α and one association constant β_j.