Water Miscibility, Surface Tension, Density, and Dynamic Viscosity of Hydrophobic Deep Eutectic Solvents Composed of Capric Acid, Menthol, and Thymol

Abstract

Deep eutectic solvents (DESs) are rapidly emerging as environmentally benign and viable alternatives not only to common organic solvents but to relatively toxic ionic liquids as well. By judicious selection of the constituents and subsequent variation in their composition, the key physicochemical properties of the DESs can be easily tailored for a specific application. Impact of variation in constituents as well as their composition on important physical properties of natural DESs (NADESs) prepared using a common fatty acid n-decanoic acid (DA) and two naturally occurring monoterpenoids, thymol (Thy) and L(−)-menthol (Men), is assessed. Specifically, water miscibility, surface tension, density, and dynamic viscosity of 17 different DESs of three combinations of constituents, Men:DA, Thy:DA, and Thy:Men in four, four, and nine compositions, respectively, are measured. The water intake by the DESs under ambient conditions varies from 0.6 to 2.7 mol·kg–1 offering a means to gauge their hydrophobicity. Water miscibility is relatively lower for Men:DA DESs and higher for Thy:DA DESs. Surface tension values are within 28.37 mN·m–1 to 31.14 mN·m–1 under ambient conditions; they are higher for the DESs with the Thy constituent and vary systematically with DES composition. The densities of all the DESs are lower than that of water and decrease linearly with increasing temperature. While density does not vary with composition for Men:DA, it is found to increase linearly with an increase in Thy mole fraction for Thy:DA and Thy:Men DESs. The dynamic viscosity of the DESs are relatively lower as compared to the common tetralkylammonium salt- and metal salt-based DESs, and exhibit strong constituent and composition dependence. Rather than the Arrhenius-type expression, the temperature-dependence of the dynamic viscosity better conforms to the Vogel–Fulcher–Tammann (VFT) model with activation energy of the viscous flow varying systematically with the constituent and composition. Reported physical properties and their dependence on constituents/composition of the NADESs will enhance their utility and help establish them as novel alternate media in science and technology.

Fluorescence of pyrene and its derivatives to reveal constituent and composition dependent solvation within hydrophobic deep eutectic solvents

“…From the temperature-dependent Py-PDMS-Py behavior, it is revealed that, similar to that reported for the η bulk for these DESs, the Arrhenius empirical expression was not found to be satisfactory for η μ . 49 The Vogel–Fulcher–Tammann (VFT) relation exhibited a better correlation between η μ and temperature: 61–63 Here A , B , and T 0 are empirical constants. The correlation between ln η μ and 1/ T along with the best fit curves based on the VFT equation is presented in Fig.…”

“…The k q for the pyrene-nitromethane pair (the respective viscosities are obtained from ref. 49) varies between 1.6 (AE 0.0 1 ) Â 10 9 M À1 s À1 (for Thy : Men :: 1 : 2 at 293.15 K, Z = 64.9 mPa s) to 14.6 (AE 0.1 8 ) Â 10 9 M À1 s À1 (for Thy : DA :: 2 : 1 at 363.15 K, Z = 1.73 mPa s). These k q values are compared with those reported in other relevant solubilizing milieux with similar viscosities.…”

“…From the temperature-dependent Py-PDMS-Py behavior, it is revealed that, similar to that reported for the Z bulk for these DESs, the Arrhenius empirical expression was not found to be satisfactory for Z m . 49 The Vogel-Fulcher-Tammann (VFT) relation exhibited a better correlation between Z m and temperature: [61][62][63] ln Z…”

“…It is convenient to learn that, for the DESs investigated, E a,Zm follows a similar trend to E a,Z ; it is higher for Thy : Men as compared to that for DA : Men and Thy : DA DESs. 49 It again infers more hindrance to the ''microviscous flow'' in the cybotactic region of Py-PDMS-Py within the Thy : Men DES as opposed to the other two DESs due perhaps to the presence of a constituent with planar phenyl (Thy) with the other one having a cyclohexyl chair form (Men); a long chain alkyl acid instead of Thy or Men as the other constituent does not offer similar restrictions on the solute Py-PDMS-Py within the solubilizing sphere offered by the DES. The complex role of the constituents as well as the composition of DESs on the microfluidity sensed by a solute is clearly established, further highlighting the intriguing nature of these solvents.…”

High-Pressure Density of DL-Menthol/Octanoic Acid Eutectic Solvents

“…The viscosity and density of the DL-menthol/acetic acid (1:1 and 1:2) ESs were investigated by Kurtulbaş et al at 298.15 K using the densimeter and viscometer provided by Anon Paar. Deepika et al measured the viscosity and density of L-menthol/decanoic acid ESs at 1:3, 1:1, 3:2, and 3:1 molar ratios from 293.15 to 363.15 K at atmospheric pressure, and they also reported the surface tension of the solvents at 298.15 K. Rabhi et al investigated the densities of the DL-menthol/dodecanoic acid(1:2) ES from 293.15 to 373.15 K at 0.1 MPa. To the best of our knowledge, with respect to the DL-menthol/octanoic acid ESs, no high-pressure densities were found in the literature except for the 1:1 molar ratio …”