Interactions of Na-salts and I-Propanol in 1-Propanol-Na-Salt-H2O systems: Toward an understanding the Hofmeister series (IV)

The excess chemical potential of 1-propanol (1P), mu(E)(1P), was evaluated in ternary 1P-Na-salt(S)-H2O at 25 degrees C. The counter anions of the Na-salts studied are SO42-, F-, Cl-, I-, and ClO4-. The effect of the anion on mu(E)(1P) follows the Hofmeister ranking, in that the more kosmotropic ions make the mu(E)(1P) value more positive. We then evaluate the effect of the Na-salt (S) on mu(E)(1P) the 1P-S interaction in terms of excess chemical potential, at a semi-infinite dilution. The results indicate that the 1P-S interaction in terms of excess chemical potential is unfavorable (repulsive) for all of the ions studied. The degree of repulsive interaction decreases in the order of the Hofmeister ranking from the kosmotropic to the chaotropic end. Namely, salting-out samples make the excess part of the chemical potential of 1P more unfavorable, while the salting-in counterparts make it less unfavorable. From earlier calorimetric studies on the same ternary systems, the enthalpic 1P-S interaction function, H-1P-S(E) was calculated. Hence, the entropy analogue, S-1P-S(E) was also obtained, and a detailed thermodynamic signature of 1P-S interactions became available. This revealed that both H-1P-S(E) and S-1P-S(E) decrease from the kosmotropic ion to the middle of the ranking (Cl-), whereupon they turn to increase toward the chaotropic end. Hence, the build up of unfavorable 1P-S interactions in Hofmeister salts (signified by mu(E)(1P)) relies on a pronounced enthalpy-entropy compensation, which must be accounted for in attempts to understand the molecular mechanisms underpinning Hofmeister effects.