Influence of temperature, water content and type of organic acid on the formation, stability and properties of functional natural deep eutectic solvents

Abstract

Natural deep eutectic solvents (NADES) have received considerable attention of biochemical and food industry due to their low toxicity and cost, remarkable solubilisation ability, negligible volatility at room temperature and good selectivity. In this sense, the purpose of this work was to synthesize various NADES based on several natural components (lactic acid (La), glucose (G), choline chloride (C) and citric acid (Ca)). In order to direct future applications, the NADES were characterized with respect to their physical (density), rheological, surface and thermal properties. The influence of temperature, type of organic acid, and water content in each of these systems were also evaluated. The samples were compared in two groups: with and without water (W) (LaGW(5:1:3) and LaG(5:1)), and with different organic acids (CLa(1:1) and CCaW(1:1:1)). Viscosity and density were inversely affected by the temperature for all samples; the water content in the LaG(5:1) and LaGW(5:1:3) samples resulted in differences in viscosity. Density values ranged from 1.13 g mL−1 (CLa(1:1) at 323 K) to 1.30 g mL−1 (CCaW(1:1:1) at 293 K). There was no significant difference between LaG(5:1) and LaGW(5:1:3), but there was a significant difference between CLa(1:1) and CCaW(1:1:1). The Ostwald-de-Waele rheological model indicated that only CCaW(1:1:1) presented a distinct change in rheological behaviour in response to an increase in temperature: it presented Newtonian behaviour at 298 K and pseudoplastic behaviour at 333 K. The Arrhenius model showed a good fit for the effect of temperature on the apparent viscosity of all NADESs. The apparent viscosities of the NADESs were compared to the surface tension and were directly proportional: the surface tension increased with an increase in viscosity. The thermal characterisation revealed a glass transition for NADES LaG(5:1), LaGW(5:1:3) and CCaW(1:1:1) at temperatures below 220.03 K, confirming their formation and stability. Based on the results, the proposed NADES may be expected to be potential green solvents in the diverse fields of food, pharmaceuticals and chemistry technology.