The physicochemical and thermal properties of deep eutectic solvent with different hydrogen bond donor (alpha hydroxy acid and polyol)

Deep eutectic solvents (DESs) are a novel class of solvents that have gathered interest due to their unique properties particularly in its application of biomass fractionation. It is critical to study its properties from an engineering standpoint because of its importance in mass transfer, fluid mechanics, modelling, simulation and equipment design. The comparison of physicochemical and thermal properties of DESs has not been thoroughly investigated, particularly when a different functional group, such as alpha hydroxy acid (lactic acid, LA) or polyol (glycerol, Gly), is used as the hydrogen bond donor (HBD). Furthermore, understanding how molar ratios affect properties is crucial since they may be fine-tuned to meet the needs of specific applications. Thus, the purpose of this study is to determine the physicochemical properties (viscosity, density, refractive index and pH) and the thermal properties (freezing point and decomposition temperature) of DESs with different HBD functional groups (LA and Gly) at various molar ratios (1:2–1:10). The HBD's functional groups and molar ratio change significantly impacted the properties due to hydrogen bonding sites and strength, alkyl chain length and molecular weight and electrostatic and dispersion interactions. The correlation between DESs molar ratios and properties is not always linearly due to eutectic phenomena.Funding provided by: Ministry of Education, Malaysia*Crossref Funder Registry ID: Award Number: FRGS/1/2020/STG05/UNIKL/02/1Preparation of DES Choline chloride (>98% purity) was purchased from SigmaAldrich. LA (88%) and Gly were obtained from Vchem. DES was prepared by mixing ChCl with LA or Gly in molar ratios of 1:1– 1:10. The mixture was heated to 80°C and constantly stirred for 4 h until a stable and homogeneous liquid formed. The sample was cooled and kept at 30°C for 24 h. The DES synthesis is complete when a clear solution appears after 24 h. Physicochemical and Thermal Properties of DESs pH The pH of DESs was measured with a pH metre (Mettler Toledo AG 8603) as a function of temperature, ranging from 25°C to 70°C with a 5°C interval. Density The density of DESs was determined using a digital density metre (Mettler Toledo). Refractive Index The refractive index of DESs was determined using a digital refractometer (Atago 1211NAR-1T liquid). Each experiment was preceded by a calibration using deionised water. All measurements were obtained at room temperature. Viscosity A viscometer (Brookfield RVT) was used to measure the viscosity of DESs as a function of temperature, ranging from 25°C to 70°C with a 5°C interval. Decomposition Temperature The decomposition temperature of DESs was determined using a Thermogravimetric Analyzer (TGA)/Differential Scanning Calorimetry (Mettler Toledo). The crucible containing DES at a mass of approximately 10–30 mg was placed in the cell. The heating rate was fixed at 20°C/min over a temperature range of 30°C–900°C under nitrogen conditions. Freezing Temperature The freezing point was determined by adding the DESs to a 20-mL scintillation vial equipped with a thermocouple. A dry ice cooling bath was used to cool the vials. The temperature at which they started to freeze was recorded