Study on the interactions of tetradecylpyridinium dl-mandelic acid and cetylpyridinium dl-mandelic acid with some small biomolecules in aqueous solution

Interactions of two active pharmaceutical ingredient ionic liquids (API-ILs) tetradecylpyridinium dl -mandelic acid ([TetPy][MAN]) and cetylpyridinium dl -mandelic acid ([CetPy][MAN]) with some small biomolecules have been studied by using conductometric, density and UV–vis spectroscopic techniques. The chosen small biomolecules are glycine, l -alanine, l -valine, l -leucine and glycylglycine (AAGG). The critical micelle concentration ( cmc ) of [TetPy][MAN] and [CetPy][MAN] in aqueous AAGG solution is calculated from the measured conductance values, thus yielding some thermodynamic parameters of the micellization, such as the standard Gibbs energy change ( Δ G m 0 ) $(\mathrm{\Delta }{G}_m^0)$ , the standard enthalpy change ( Δ H m 0 ) $(\mathrm{\Delta }{H}_m^0)$ and the standard entropy change ( Δ S m 0 ) $(\mathrm{\Delta }{S}_m^0)$ . On the basis of the experimental densities of ternary solutions of (AAGG + API-ILs + water), apparent molar volume at infinite dilution ( V 2 , φ 0 $V_{2,\phi }^0$ ), apparent molar expansibility at infinite dilution ( E φ 0 ) $E_{\phi }^0)$ , transfer partial molar volume ( Δ t V 0 ${\mathrm{\Delta }}_{\mathrm{t}}{V}^0$ ) and hydration number ( n H $n_{\mathrm{H}}$ ) of AAGG in aqueous API-ILs solution were calculated. The dominant hydrophobic-hydrophilic and hydrophobic-hydrophobic interactions were involved in the investigated systems. The binding constant ( K b ) $(K_{\mathrm{b}})$ between AAGG and API-ILs has been calculated from UV–vis spectroscopic data. Effect of some factors including concentration, temperature, molecular structure, etc. on the aforementioned parameters and the existing solvent–solute interaction has been discussed.