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 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.

Anion Selection Criteria for Water‐in‐Salt Electrolytes

“…Strong chaotropes on the other hand are only weakly hydrated. [ 45–48 ] Due to their higher charge density, kosmotropic anions also have a stronger tendency than chaotropic anions to form ion pairs with solution cations, resulting in a more intact hydrogen‐bonding network. [ 41 ] In other words, kosmotropic anions compete with water for a place in the cation solvation shell, thus freeing water molecules from this crucial trap, which is at least in part responsible for the high overpotentials toward water electrolysis in water‐in‐salt electrolytes.…”

Effect of an “Ionic Liquid” Cation, 1-Butyl-3-methylimidazolium, on the Molecular Organization of H2O

“…We see that the mixing schemes described above obtained from the third derivative quantities in our earlier paper 18 are consistent in the IL-rich region with conclusions by some spectroscopic stuies. 19 Namely, we study a ternary system, 1-propanol (1P)-[bmim]Cl-H 2 O. We determine the third derivative thermodynamic quantity of 1P in the system and use its behavior as a probe to learn about the effect of [bmim]Cl on the molecular organization of H 2 O.…”

Production of Zn(l-Met)2 Chelate with Improved Flowability by Reactive Crystallization

“…As summarized in Table , the recoveries increase in the order of NO 3 – < CH 3 COO – < Cl – . Because the preferential hydration of hydrophobic solutes by water molecules increases in the order of Cl – < CH 3 COO – , CH 3 COO – , it has a stronger salting-out effect than that by Cl – . Interestingly, the recovery results do not correlate directly with salting-out behavior.…”

“…Because the preferential hydration of hydrophobic solutes by water molecules increases in the order of Cl − < CH 3 COO − , CH 3 COO − , it has a stronger salting-out effect than that by Cl − . 23 Interestingly, the recovery results do not correlate directly with salting-out behavior. This may be due to the additional water molecules originating from the dissolution of Zn(CH 3 COO) 2 dihydrate.…”