During the past few years, there has been a surge in interest and research in the arena of utilizing deep eutectic solvents (DESs) as green solvents. This manifested in applying DESs in a variety of industrial applications. Most of the reported work in this field was directed toward the choline chloride-based DES. Recently, the area of DES synthesis was widened by considering other quaternary ammonium and phosphonium salts. In this work tetrabutylammonium chloride (TBAC) is used as a salt for the synthesis of three different DES systems based on three different hydrogen bond donors (HBDs), namely, glycerol, ethylene glycol, and triethylene glycol. Screening tests for each DES system was performed to identify salt:HBD ratios that exhibit a minimum freezing point, and at least three such ratios were selected for each system. Physical properties including melting point, density, viscosity, surface tension, refractive index, conductivity, and pH were measured for the three DES systems at different temperatures ranging from (293.15 to 353.15 K). It is worth mentioning that this class of DES exhibits a wide range of properties that can be tailored toward specific chemical and other engineering applications.

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Abstract-Deep

In this work, a viscosity model for choline chloride-based deep eutectic solvents (DESs) was developed. In addition to temperature, the new model presented here considers the composition of the salt in the DES, which was not the case for all previously reported viscosity models. Two forms of the proposed model were optimized and fitted to viscosity experimental data. The developed model was tested using the experimental viscosity values for nine common choline chloride-based DES systems of different hydrogen bond donors. The performance of the model was evaluated using three different statistical indicators, namely average relative deviation, correlation coefficient (R 2 ) and standard deviation. In addition, a significance t-test was also conducted on each of the fitted DES viscosity data. The statistical indicators showed a very good agreement between the model-predicted viscosity and the experimental viscosity values for the DESs with relatively low viscosities (DES1, DES2, DES4 and DES5). For DESs with high viscosities (DES3, DES7, DES8 and DES9), the second model gave better predictions. Overall, this proposed model was capable of predicting DES viscosities for different salt compositions at different temperatures as supported by the statistical significance t-test and other statistical indicators.

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In this work, the molar heat capacity of type III deep eutectic solvents (DESs) for three quaternary salts was investigated in the temperature range of 298.15–353.15 K. Several hydrogen bond donors were used with the three salts to prepare the DESs. Melting and/or glass transition points of the DESs were also measured and presented. The results showed that the molar heat capacity for all systems increased almost linearly with temperature. In addition, the results showed that molar heat capacity is directly proportional to the molar mass of the DES. The molar heat capacity for all DES systems studied in this work ranged from 219.3 to 605.9 J·mol–1·K–1. The tetrabutylammonium chloride (TBAC)–urea DES system showed the highest molar heat capacity range of 590.1–605.9 J·mol–1·K–1 while the choline chloride (ChCl)–phenol system had the lowest range of 219.3–236.8 J·mol–1·K–1. The TBAC–urea DES system had the highest melting point (300.29 K) compared to all other DES systems in this work. Melting and/or glass transition points for all studied systems ranged from 224.8 to 300.29 K. This indicated that these solvents could be used in the liquid phase at room temperature for future applications.

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