Measurements on Refractive Index, Density, Surface Tension, and Viscosity of Deep Eutectic Solvents Based on Matrine and Fatty Acids (n-Octanoic acid, n-Decanoic acid, and Lauric acid) at Various Temperatures and Mole Fractions

Abstract

Deep eutectic solvents (DESs) have been widely explored in recent decades as a new generation of green solvents. When using active pharmaceutic ingredients (APIs) to construct DESs, therapeutic DESs (THEDES) were obtained. A series of THEDESs based on fatty acid (FA) have been developed and shown excellent bio-activities, which displayed great potential applications in fields like wound healing, transdermal drug delivery etc. Physical properties, like density, and viscosity etc., are of great importance in practical implementation scenarios, thus, deep insights on the thermodynamic properties are highly desirable. In this work, physical properties, i.e. refractive index, surface tension, density, and viscosity, for a series of matrine and FA DESs were precisely measured. The influence of temperature, HBD composition and alkyl chain length on these properties were further discussed. It has been suggested by the results herein that all explored physical properties displayed a decline tendency with temperature, which may be ascribed to the decrease of H-bonding interaction with temperature. Besides, for each DES, increasing the HBD composition resulted in a decrease of all measured physical properties due to the decrease of molar weight of DES. As to the change of physical properties with alkyl chain length, the conclusions were much complex. The refractive index enhanced with alkyl chain length, while density and surface tension showed obvious decline with alkyl chain length. Moreover, variance of alkyl chain length caused only slight increase of viscosity that may be resulted from the increase of molar weight of DES.