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Volumetric Investigations on Molecular Interactions of Glycine/l-alanine in Aqueous Citric Acid Solutions at Different Temperatures

Poonam Patyar, Gurpreet Kaur, Tarnveer Kaur
Journal of Solution Chemistry 2018, 47 (12): 2039-2067
Apparent molar volumes <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>ϕ</mml:mi> <mml:mi>V</mml:mi> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> of glycine/l-alanine in water and in aqueous citric acid (CA) solutions of varying concentrations, i.e. (0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) mol·kg-1 were determined from density measurements at temperatures T  = (288.15, 298.15, 308.15, 310.15 and 318.15) K and at atmospheric pressure. Limiting partial molar volumes <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:mo>(</mml:mo> <mml:msubsup> <mml:mi>ϕ</mml:mi> <mml:mi>V</mml:mi> <mml:mtext>o</mml:mtext> </mml:msubsup> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> and their corresponding partial molar volumes of transfer <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>Δ</mml:mi> <mml:mtext>tr</mml:mtext> </mml:msub> <mml:msub> <mml:mi>ϕ</mml:mi> <mml:mi>V</mml:mi> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> have been calculated from the <mml:math xmlns:mml=""&gt; <mml:msub> <mml:mi>ϕ</mml:mi> <mml:mi>V</mml:mi> </mml:msub> </mml:math> data. The negative <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:msub> <mml:mi>Δ</mml:mi> <mml:mtext>tr</mml:mtext> </mml:msub> <mml:msub> <mml:mi>ϕ</mml:mi> <mml:mi>V</mml:mi> </mml:msub> </mml:mrow> </mml:math> values obtained for glycine/l-alanine from water to aqueous CA solutions indicate the dominance of hydrophilic-hydrophobic/hydrophobic-hydrophilic and hydrophobic-hydrophobic interactions over ion/hydrophilic-dipolar interactions. Further, pair and triplet interaction coefficients, i.e. <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>V</mml:mi> <mml:mtext>AB</mml:mtext> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mspace/> <mml:mtext>and</mml:mtext> <mml:mspace/> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>V</mml:mi> <mml:mtext>ABB</mml:mtext> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> along with hydration number <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>n</mml:mi> <mml:mtext>H</mml:mtext> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> have also been calculated. The effect of temperature on the volumetric properties of glycine/l-alanine in water and in aqueous CA solutions has been determined from the limiting partial molar expansibilities <mml:math xmlns:mml=""&gt; <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>∂</mml:mi> <mml:msubsup> <mml:mi>ϕ</mml:mi> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mtext>o</mml:mtext> </mml:msubsup> <mml:mo>/</mml:mo> <mml:mi>∂</mml:mi> <mml:mi>T</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> <mml:mi>p</mml:mi> </mml:msub> </mml:math> and their second-order derivative <mml:math xmlns:mml=""&gt; <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:msup> <mml:mi>∂</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:msubsup> <mml:mi>ϕ</mml:mi> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mtext>o</mml:mtext> </mml:msubsup> <mml:mo>/</mml:mo> <mml:mi>∂</mml:mi> <mml:msup> <mml:mi>T</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> <mml:mi>P</mml:mi> </mml:msub> </mml:math> . The apparent specific volumes <mml:math xmlns:mml=""&gt; <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>ϕ</mml:mi> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> for glycine and l-alanine tend to approach sweet taste behavior both in the presence of water and in aqueous CA solutions. The <mml:math xmlns:mml=""&gt; <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>ϕ</mml:mi> </mml:msub> </mml:math> values for glycine/l-alanine increase with increase in concentration of CA at all temperatures studied. This reveals that CA helps in enhancing the sweet taste behavior of glycine/l-alanine which also supports the dominance of hydrophobic-hydrophobic interactions.


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