No abstract

We studied a detailed thermodynamic behavior of 1-propanol (abbreviated as 1P) in mixed solvents of aqueous Na2SO4, NaOOCCH3, and NaClO4, 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 H2O. Na+ ion seems to hydrate a number less than seven or eight molecules of H2O, leaving the bulk H2O outside the hydration shell unperturbed. SO4 2- also hydrates a number less than 26 H2O molecules with a concomitant increase in the degree of fluctuation of the bulk H2O. Na2SO4 as a whole hydrates the total of 26 H2O. CH3COO- ion modifies H2O in a similar manner as an alcohol. Cl- ion hydrates a number less than seven or eight H2O molecules leaving the bulk H2O in the same state as in pure H2O. NaCl as a whole hydrates the total of seven or eight molecules of H2O. ClO4 - and SCN- participate in hydrogen bonding with the hydrogen bond network of H2O keeping its connectivity intact. However, they reduce the degree of fluctuation inherent in liquid H2O. Thus, each ion has its own unique manner of modifying H2O, except that ClO4 - and SCN- modify H2O 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.

We use the thermodynamic behaviour of 1-propanol (1P) as a probe in ternary 1P–salt–H2O systems to elucidate the effect of a salt on the molecular organization of H2O. For salts, we have chosen CaCl2, NH4Cl, and (CH3)4NCl (TMAC). Having fixed the counter anion at Cl−, we compare here mainly the effects of chosen cations on H2O. Together with an earlier study on NaCl, we found that Ca2+, Na+, and NH4+ are hydrated by a number of H2O molecules and leave the bulk H2O away from the hydration shell unperturbed. The hydration numbers were found to be 6.4 ± 1.6, and 1.2 ± 0.4, for Ca2+ and NH4+, respectively with the hydration number 5.2 for Na+, the result of a simulation study, chosen as a reference. Thus, a salting out (also referred to as structure making, stabilizing, or kosmotropic) tendency would decrease in the order; Ca2+ > Na+ > NH4+. TMA+, on the other hand, showed a more hydrophilic characteristics than the probing 1P. Thus, TMA+ is rather chaotropic (salting in, structure breaking or destabilizing) within this methodology.