Hydrogen Bonding in Liquid Water and in the Hydration Shell of Salts

Abstract

A near-IR spectral study on pure water and aqueous salt solutions is used to investigate stoichiometric concentrations of different types of hydrogen-bonded water species in liquid water and in water comprising the hydration shell of salts. Analysis of the thermodynamics of hydrogen-bond formation signifies that hydrogen-bond making and breaking processes are dominated by enthalpy with non-negligible heat capacity effects, as revealed by the temperature dependence of standard molar enthalpies of hydrogen-bond formation and from analysis of the linear enthalpy-entropy compensation effects. A generalized method is proposed for the simultaneous calculation of the spectrum of water in the hydration shell and hydration number of solutes. Resolved spectra of water in the hydration shell of different salts clearly differentiate hydrogen bonding of water in the hydration shell around cations and anions. A comparison of resolved liquid water spectra and resolved hydration-shell spectra of ions highlights that the ordering of absorption frequencies of different kinds of hydrogen-bonded water species is also preserved in the bound state with significant changes in band position, band width, and band intensity because of the polarization of water molecules in the vicinity of ions.

H‐Bonding in Water of Hydration: NIR Spectral Studies of Hydration Behavior of 1‐n‐Alkyl‐3‐metylimidazolium‐Based Bromide and Amino Acid Ionic Liquids at 298.15 K

“…The shift data are compiled in Table . In pure liquid water, component around 6300 cm ‐1 corresponds to ice‐like structure in liquid state with fully four H‐bonded water molecules having in‐phase vibrations . However, this band is not observed for water in the hydration shell as explained below.…”

“…As the present work is focused on the hydration studies of ionic liquids, only the Gaussian components for water in the hydration shell will be discussed in more detail. As noted earlier there are total five Gaussian components for water in the pure liquid state or in the bound state. Just like for water in the hydration shell of alkali and alkaline earth metal halides, the polarization of water around an ion affects the intensity and peak positions of individual Gaussian components but their order is preserved even in the hydration shell.…”

“…Such species absorb preferentially on the high frequency side (blue) of the spectrum while strong relatively stable H‐bonds absorbs at low frequency side (red) of the spectrum. Our recent work on analysis of H‐bonded network in pure liquid water emphasized that water consist five different types of H‐bonded species such as fully H‐bonded ice‐like, fully H‐bonded distorted ice‐like, steady state equilibrated doubly and triply H‐bonded species, singly H‐bonded species and NHBs absorbing at different unique frequencies . Furthermore, the optimum temperature of ∼316.5 K (43.5 °C) is observed at which equal concentration of fully H‐bonded species and sum of singly H‐bonded species and NHBs exists.…”

“…Hence, along‐with hydration properties of the molecules, H‐bonding in the hydration shell needs to be more understood to explain the properties of such solutions. Recently we investigated the hydrogen bonded network in pure liquid water and for water in the hydration shell of alkali and alkaline earth halide for which a method of simultaneous estimation of the spectrum of water in the hydration shell and hydration number of solutes was proposed . From the hydration study of alkali and alkaline earth halides, it is observed that the ordering of absorption frequencies of the different kinds of the hydrogen bonded species is also preserved in bound state.…”

“…The structure and dynamics of H-bonded network in water is more complex and studied extensively by many scientists with the help of Raman and IR spectroscopy and simulation techniques. [1][2][3][4][5][6][7][8][9][10][11][12][13] These studies reveal the presence of non-Hbonded configurations (NHBs) also known as dangling hydrogen bonds which are transient species with life-time of < 200 fs in liquid water. [1,2] Such species absorb preferentially on the high frequency side (blue) of the spectrum while strong relatively stable H-bonds absorbs at low frequency side (red) of the spectrum.…”