Toward Understanding the Hofmeister Series. 1. Effects of Sodium Salts of Some Anions on the Molecular Organization of H₂O

Abstract

We studied a detailed thermodynamic behavior of 1-propanol (abbreviated as 1P) in mixed solvents of aqueous Na 2 SO 4, NaOOCCH 3, and NaClO 4, and NaSCN. We measured the excess partial molar enthalpy of 1P, H 1P E, in these mixed solvents at various initial salt concentrations. We then evaluated what we call the enthalpic interaction, H 1P - 1P E = (∂ H 1P E /∂ n 1P ), where n 1P is the amount of 1P. The composition dependence of H 1P - 1P E changes in a characteristic manner on addition of a specific salt. This induced change in the behavior of H 1P - 1P E is used to elucidate the effect of the salt on the molecular organization of H 2 O. Na + ion seems to hydrate a number less than seven or eight molecules of H 2 O, leaving the bulk H 2 O outside the hydration shell unperturbed. SO 4 2- also hydrates a number less than 26 H 2 O molecules with a concomitant increase in the degree of fluctuation of the bulk H 2 O. Na 2 SO 4 as a whole hydrates the total of 26 H 2 O. CH 3 COO - ion modifies H 2 O in a similar manner as an alcohol. Cl - ion hydrates a number less than seven or eight H 2 O molecules leaving the bulk H 2 O in the same state as in pure H 2 O. NaCl as a whole hydrates the total of seven or eight molecules of H 2 O. ClO 4 - and SCN - participate in hydrogen bonding with the hydrogen bond network of H 2 O keeping its connectivity intact. However, they reduce the degree of fluctuation inherent in liquid H 2 O. Thus, each ion has its own unique manner of modifying H 2 O, except that ClO 4 - and SCN - modify H 2 O in almost the same way. Therefore, it seems fair to state that the phenomenological net result manifested as the Hofmeister series is due to multifacetted factors working in the holistic manner, and a single or a small number of parameters is not sufficient to explain its effect.