Study of solvation consequences of α-amino acids in aqueous ionic liquid solution probed by physicochemical approach

doi:10.1016/j.fluid.2013.03.030

Fluid Phase Equilibria

Volume 352, 25 August 2013, Pages 7-13

https://doi.org/10.1016/j.fluid.2013.03.030 Get rights and content

Highlights

•
l-Valine is more associated in aqueous Bu₄PBF₄ solution than the other amino acids.
•
Solute–solvent interaction predominates over the solute–solute interactions in all the studied solutions.
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The overall order of solute–solvent interaction is found to be glycine < l-alanine < l-valine.

Abstract

The apparent molar volume (ϕ_V), viscosity B-coefficient, molal refraction (R) and adiabatic compressibility (ϕ_K) of glycine, l-alanine, and l-valine have been studied in 0.001, 0.003, 0.005 mol dm⁻³ aqueous tetrabutylphosphonium Tetrafluoroborate (Bu₄PBF₄) solutions at 298.15 K from the values of density (ρ), viscosity (η), refractive index (n_D) and speed of sound (u) respectively. The limiting apparent molar volumes ( $ϕ_{V}^{0}$ ), experimental slopes ( $S_{V}^{*}$ ) are obtained from the Masson equation and have been interpreted in terms of solute–solvent and solute–solute interactions respectively. Jones–Dole equation were employed to analyze the viscosity data and the interpretation of the derived parameters A and B have also been carried out in terms of solute-solute and solute–solvent interactions in the solutions respectively. Molal refractions (R) have been determined with the help of Lorentz–Lorenz equation. Limiting apparent molar adiabatic compressibilities ( $ϕ_{K}^{0}$ ) of three amino acids at infinite dilution were evaluated and discussed.

Introduction

Amino acids are molecules that contain an amine group, a carboxylic acid group and a side-chain varying between different amino acids. They play a key role in biochemistry, where the term usually refers to α-amino acids. One of the most important function is to serve as the building blocks of proteins, which are linear chains of amino acids. Amino acids are very much important in nutrition and are commonly used in nutrition supplements, fertilizers, food technology and industry.

Ionic liquids have unique intrinsic properties, such as negligible vapour pressure, large liquid range, ability of dissolving a variety of chemicals, high thermal stability, large electrochemical window and their potential as ‘designer solvents’ and ‘green’ replacements for volatile organic solvents [1], [2], [3] used in reactions involving inorganic and bio-catalysis etc. They are also utilized as heat transfer fluids for processing biomass and as electrically conductive liquids in electrochemistry (batteries and solar cells) [4], [5], [6]. In the modern technology, the application of the salt is well understood by studying the ionic solvation or ion association. Ionic association of electrolytes in solution depends on the mode of solvation of its ions [7], [8], [9], [10], which in turn depends on the nature of the solvent/solvent mixtures. Such solvent properties as viscosity and the relative permittivity have been taken into consideration as these properties help in determining the extent of ion association and the solvent–solvent interactions. The non-aqueous system has been of immense importance [11], [12] to the technologist and theoretician as many chemical processes occur in these systems. The volumetric, viscometric and interferometric behavior of solutes has been found to be very useful in elucidating the various interactions occurring in solutions. Studies on the effect of concentration (molality), the apparent molar volumes of solutes have been extensively studied to obtain information on solute–solute, solute–solvent, and solvent–solvent interactions [13], [14], [15], [16], [17].

In view of the above and in continuation of our studies, we have performed a systematic study on the density, viscosity, refractive index and ultrasonic speed of some amino acids in aqueous Bu₄PBF₄ solutions at 298.15 K and we have attempted to report the limiting apparent molar volume ( $ϕ_{V}^{0}$ ), experimental slopes ( $S_{V}^{*}$ ), and viscosity B-coefficients, molar refraction (R) and limiting apparent molar adiabatic compressibility ( $ϕ_{K}^{0}$ ) for the cited amino acids in aqueous Bu₄PBF₄ solution (Table 1).

Section snippets

Source and purity of samples

Bu₄PBF₄ of puriss grade was procured from Sigma–Aldrich, Germany and was used as purchased. The mass fraction purity of Bu₄PBF₄ was ≥ 0.99. The amino acids Glycine (S.D. Fine Chemicals, > 0.99), l-alanine (S.D. Fine Chemicals, > 0.985), and l-valine (Loba Chemie, India, > 0.99) were used for the present study and were used as such without further purification. Bu₄PBF₄ was recrystallized twice from aqueous ethanol solution and dried under vacuum at T = 348 K for 6 h. Thereafter, it was stored over P₂O₅

Density calculation

Apparent molar volumes (ϕ_V) were determined from the solution densities using the following equation [25]. $ϕ_{V} = \frac{M}{ρ} - \frac{1000 (ρ - ρ_{o})}{m ρ ρ_{o}}$ where M is the molar mass of the solute, m is the molality of the solution ρ_o and ρ are the densities of the mixture and the solution respectively. The limiting apparent molar volume $ϕ_{V}^{0}$ was calculated using a least-square treatment to the plots of ϕ_V versus √m using the Masson equation [26]. $ϕ_{V} = ϕ_{V}^{0} + S_{V}^{*} \sqrt{m}$ where $ϕ_{V}^{0}$ is the limiting apparent molar volume at infinite dilution

Conclusion

The values of the limiting apparent molar volume ( $ϕ_{V}^{0}$ ), viscosity B-coefficients and limiting partial isentropic compressibility ( $ϕ_{K}^{0}$ ) indicates the presence of strong solute-solvent interactions which increases with the increase in the number of carbon atoms of the studied amino acids and with increase of mass fraction of Bu₄PBF₄ in the aqueous mixture. The refractive index and the molar refraction values suggest that l-Valine molecules are more tightly packed in the solution leading to higher

Acknowledgement

The authors are grateful to the UGC supported Major research project, Ref. No. RP/5032/FCS/2011 New Delhi for sanctioning a research fellowship and financial assistance to carry on this research work.

One of the authors, Prof. M.N. Roy thanks the University Grant Commission, New Delhi for supporting this work through one time grant award under Basic Scientific Research via the grant-in-Aid No. F.4-10/2010 (BSR).

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Study of solvation consequences of α-amino acids in aqueous ionic liquid solution probed by physicochemical approach

Highlights

Abstract

Introduction

Section snippets

Source and purity of samples

Density calculation

Conclusion

Acknowledgement

Thermochim. Acta

J. Chem. Thermodyn.

Chem. Rev.

Pure Appl. Chem.

Chem. Rev.

Chem. Soc. Rev.

Phys. Chem. Chem. Phys.

J. Phys. Chem. B

J. Solution Chem.

J. Phys. Chem. B

J. Solution Chem.

J. Indian Chem. Soc.

Nonaqueous Solution Chemistry

Molecular Acoustics

J. Chem. Soc., Faraday Trans. 1

Aust. J. Chem.