Interactions of divalent cations with tetrameric acid aggregates in aqueous solutions

Abstract Interactions in aqueous solutions between a tetrameric naphthenic acid in sodium salt (BP10Na 4 ) and divalent Mg 2+ and Ca 2+ cations were investigated by isothermal titration calorimetry (ITC) and dynamic light scattering (DLS). Colloidal aggregation of the tetrameric sodium naphthenates to vesicles and micelles enhances its binding with divalent cations. In vesicle systems, Ca 2+ was found to bind first with carboxylate groups located at the external water/lipid interface of vesicles. The cations then diffuse through the membrane to interact with the inner carboxylate groups. Higher Ca 2+ concentration and higher salinity gradients enhance cation diffusion. Under solution chemistry conditions where micelle dominates, Ca 2+ reacts only with outer carboxylate groups of BP10Na 4 without any significant cation diffusion through the aggregates. The binding of calcium with sodium naphthenate is an entropy-driven process where entropy gain comes from the dehydration of both cations and carboxylate groups of BP10Na 4 . The affinity of Mg 2+ to BP10Na 4 is much weaker than Ca 2+ due to its stronger hydration and hence larger size of hydrated magnesium ions than hydrated calcium ions.