Amino acid ionic liquids are a special class of ionic liquids due to their unique acid-base behavior, biological significance, and applications in different fields such as templates in synthetic chemistry, stabilizers for biological macromolecules, etc. The physicochemical properties of these ionic liquids can easily be altered by making the different combinations of amino acids as anion along with possible cation modification which makes amino acid ionic liquids more suitable to understand the different kinds of molecular and ionic interactions with sufficient depth so that they can provide fruitful information for a molecular level understanding of more complicated biological processes. In this context, volumetric and osmotic coefficient measurements for aqueous solutions containing 1-ethyl-3-methylimidazolium ([Emim]) based amino acid ionic liquids of glycine, alanine, valine, leucine, and isoleucine are reported at 298.15 K. From experimental osmotic coefficient data, mean molal activity coefficients of ionic liquids were estimated and analyzed using the Debye-Hückel and Pitzer models. The hydration numbers of ionic liquids in aqueous solutions were obtained using activity data. Pitzer ion interaction parameters are estimated and compared with other electrolytes reported in the literature. The nonelectrolyte contribution to the aqueous solutions containing ionic liquids was studied by calculating the osmotic second virial coefficient through an application of the McMillan-Mayer theory of solution. It has been found that the second osmotic virial coefficient which includes volume effects correlates linearly with the Pitzer ion interaction parameter estimated independently from osmotic data as well as the hydrophobicity of ionic liquids. The enthalpy-entropy compensation effect, explained using the Starikov-Nordén model of enthalpy-entropy compensation, and partial molar entropy analysis for aqueous [Emim][Gly] solutions are made by using experimental Gibb's free energy data and literature enthalpy data. This study highlights that the hydrophobic interaction persists even in the limit of infinite dilution where the hydration effects are usually dominant, implying importance of hydrophobic hydration. Analysis of the results further shows that the hydration of amino acid ionic liquids occurs through the cooperative H-bond formation with the kosmotropic effect in contrast to the usual inorganic salts or hydrophobic salts like tetraalkylammonium halides.

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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 ionic hydration is crucial for understanding various phenomena such as osmolyte effects, ion transport, electrochemical processes, etc. Ionic liquids (ILs) from the biomaterials have great affinity for protein solubility and stabilization due to hydrophobicity and hydrogen bonding ability. To explore potential use of ionic liquids, it is essential to understand hydration of ionic liquids with detailed information on H‐bonding within the water of hydration. In this context, near‐infrared spectral investigation of H‐bonding in water of hydration of 1‐n‐alkyl‐3‐methylimidazolium bromides and 1‐ethyl‐3‐methylimidazolium based amino acid ionic liquids were made through simultaneous estimation of hydration number and hydration spectra of ILs in the spectral range of 7800‐5800 cm−1. Weak H‐bonding extist in the water of hydration for 1‐n‐alkyl‐3‐methylimidazolium bromides while strong cooperative H‐bonding exists in case of amino acid ionic liquids leading to higher hydration numbers. The results further demonstrate that the individual resolved spectral components can be used to analyse (qualitatively and quantitatively) different types of H‐bonded species at ionic surfaces. Along with all these, the effect of hydrophobicity on hydration behavior as well as on the nature and strength of H‐bonding within the hydration shell of ions are also probed and discussed.

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