Structural and physicochemical characterisation of binary glutamine-based deep eutectic solvents

Abstract

Amino acid-based deep eutectic solvents (AADES) are a new subclass of natural deep eutectic solvents with prospective applications especially in the energy and pharmaceutical industries. In this study, binary DESs were formulated, for the first time, from glutamine and lactic acid by the heating and stirring method. Subsequently, the effect of varying molar ratios of the DES components on their physicochemical, structural and thermal properties was evaluated. In this regard, the refractive index, water content, conductivity, viscosity and pH of the formulated DESs were analysed. Furthermore, the thermal stabilities of the DESs were determined using differential scanning calorimetry and thermogravimetry analysis, while the hydrogen bond interactions within the solvents were elucidated using nuclear magnetic resonance (1H and 13C NMR) and Fourier transform infrared spectroscopy (FTIR). Results showed that the densities (1.25–1.29 g/cm3), refractive indices (1.45–1.48), conductivity (0.19–0.51 mS cm−1), water contents (3.88–7.87 %), viscosities (867.34–8712.50 mPa·s) and pHs (1.85–3.37) of the DESs were significantly influenced by lactic acid. The NMR and FTIR spectra profiles confirmed the interactions of the hydrogen bond forming groups within the DESs. In addition, the solvents showed stability at room temperature up to 100 °C, after which decomposition set in until 400 °C; however, they formed no crystals between −40 °C and 100 °C. The glass transition phase of the DESs was also observed at −20 °C which highlights the potential application of the solvents in cryopreservation. Thus, this study presents valuable insights into the proposed utilisation of these GL-based DESs as sustainable solvents in industrial applications.