Interactions of divalent cations with tetrameric acid aggregates in aqueous solutions

Interactions in aqueous solutions between a tetrameric naphthenic acid in sodium salt (BP10Na4) and divalent Mg2+ and Ca2+ 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, Ca2+ 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 Ca2+ concentration and higher salinity gradients enhance cation diffusion. Under solution chemistry conditions where micelle dominates, Ca2+ reacts only with outer carboxylate groups of BP10Na4 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 BP10Na4. The affinity of Mg2+ to BP10Na4 is much weaker than Ca2+ due to its stronger hydration and hence larger size of hydrated magnesium ions than hydrated calcium ions.

Graphical abstractTwo-step binding of calcium ions with BP10 vesiclesFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Transport mechanism of divalent cations through tetrameric acid BP10 membranes. ► A two-step binding process of Ca2+ with BP10 vesicles. ► Entropy-driven binding of Ca2+ with BP10 in vesicles. ► Stronger affinity of BP10 with Ca2+ than with Mg2+.