Thermodynamic Studies of Ionic Hydration and Interactions for Amino Acid Ionic Liquids in Aqueous Solutions at 298.15 K

Abstract

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.

Density and sound speed study of hydration of 1-butyl-3-methylimidazolium based amino acid ionic liquids in aqueous solutions

“…Thus, large values for hydration numbers as compared those obtained from electrostriction indicates that compressibility data yields the hydration numbers due to both the electrostriction as well as secondary hydration shell which may be due to different contributions including hydrophobic hydration. This observation is in harmony with the recent hydration number estimated using osmotic data and found that the cooperative H-bonding is responsible for the observed high hydration numbers which have been supported further by analysis of excess partial molar entropy data of 1-ethyl-3-methylimidazolium based amino acid ionic liquids [35]. a Data from reference [56].…”

“…The [35] and has comparatively less effect of hydrophobic ion-ion association. The sign and magnitude of deviation parameter B V is related to the nature of ion-ion interactions including the solvent mediated solutesolute cosphere overlap and further the negative magnitude of B V is generally attributed to the existence of hydrophobic interactions whose strength is more if the value of B V becomes more negative [50][51][52][53].…”

“…All the solutions were prepared on molality basis using fresh doubly quartz distilled water and converted to molarity scale whenever required using density data. [Ile] were synthesized using the method reported earlier [35]. Only the source of impurity in the synthesized AAILs is the possibility of presence of bromide ions from [Bmim][Br], unreacted [OH] À ions from [Bmim][OH] and water remaining after drying AAILs.…”

“…Aqueous solutions of some of these and similar ionic liquids are studied well through density, surface tension, heat and osmotic pressure measurements [26][27][28][29][30][31][32][33][34]. Using osmotic and activity coefficient studies we reported earlier that the hydration of [Emim] based AAILs is cooperative in nature wherein partial entropic and enthalpy-entropy compensation analysis further reflected that hydrophobic interactions play an important role in hydration of amino acid anion of AAIL [35]. In present work, we studied the volumetric properties at different temperatures between (293.…”

The force field for imidazolium-based ionic liquids: Novel anions with polar residues

“…[1][2][3][4][5][6][7][8] Due to versatile chemical nature and low melting points, ILs constitute excellent solvents for a variety of exciting applications. [5,[9][10][11][12][13][14][15][16][17][18][19][20] The physical chemical properties of ILs are important for capturing toxic gases, separation applications through biphasic systems, solubilization of nanoscale entities, such as nanotubes and fullerenes. [1,12,13,[21][22][23] Amino acid (AA) ionic liquids (AAILs) are a relatively new research field.…”