Molecular-Level Insights into the Microstructure of a Hydrated and Nanoconfined Deep Eutectic Solvent

Abstract

Despite the recent advancements in the field of deep eutectic solvents (DESs), their high viscosity often prevents practical applications. A versatile strategy to overcome this problem is either to add a co-solvent or to confine the DES inside a nanoscaled self-organized system. This work assesses the microstructures of a hydrated and nanoconfined DES comprising benzyltripropylammonium chloride [BTPA]Cl and ethylene glycol (EG). They act as a hydrogen-bond acceptor and a donor, respectively. The hydrogen bonding between [BTPA]Cl and EG in the DES (i.e., BTEG) and the molecular states of water in the hydrated BTEG were studied by Raman spectroscopy. The results show different hydrogen-bonding associations between water–water and water–BTEG or EG molecules. In addition, we investigated the confinement effects of BTEG in a Polysorbate 80 (Tween-80)/cyclohexane reverse micellar (RM) system. The results are compared with those of an ionic liquid-encapsulated RM system. The formation, bonding characteristics, and thermal stability of the RM droplets were studied by solubilization, dynamic light scattering, rheology, and Raman spectroscopy experiments. Furthermore, it is shown that hydrogen bonding between the DES and the surfactant leads to a stable RM system. Interestingly, the viscosity of the RM system is significantly lower than that of the neat DES suggesting that DESs have a much wider practical applicability in the form of RMs.

Microstructure of Deep Eutectic Solvents: Current Understanding and Challenges

“…Similar behavior of water acting as an intermediatory molecule to perturb the original microstructural arrangement due to its self-involvement in hydrogen bond network has also been observed in DES comprised of benzyltripropylammonium chloride [BTPA][Cl] and ethylene glycol (EG). 123 It is worth noting that degree of hydration determines how strongly the water molecules present inside the DES microstructure get bonded/ associated with other components.…”

How Hydration Affects the Microscopic Structural Morphology in a Deep Eutectic Solvent

“…Recently, it has been revealed through experiments as well as computer simulations that even a small amount of dilution of the intrinsically designed deep eutectic solvents (DESs) alters their microstructure. − The hygroscopic nature of various DESs has led to the emergence of an exciting subdomain for researchers, both in the fundamental study and technological applications. − The addition of water in these eutectic mixtures is found to exhibit a great impact on their electrochemical and biotechnology applications. − It has been reported that the kinetics of electrodeposition of metal ions is significantly improved when aqueous DESs are used . Moreover, the solubility of various natural products and enantioselectivity of compounds can be tailored by tweaking the amount of water in DESs. , Aqueous DESs have also contributed in the development of microextraction techniques. , While such applications are still a work in progress, unveiling the fundamental understanding of aqueous eutectic mixtures has attracted a lot of interest recently.…”

Miscible–Immiscible Transition of Deep Eutectic Solvents in Water Switched by CO2

“…First, the peak at 3392 cm –1 was mainly attributed to the O–H broadening and N–H stretching of MEA/4-methoxyphenol DES. This peak was blue-shifted from 3392 to 3398 for the bottom phase and red-shifted from 3392 to 3379 cm –1 for the top phase, respectively, which indicated that the hydrogen bonds between MEA and 4-methoxyphenol were partially destroyed . Second, compared with the neat MEA/4-methoxyphenol DES, three new peaks at 1624, 1534, and 1427 cm –1 , attributable to HCO 3 – , MEACO 2 – , and CO 3 2– stretching, , were generated in the top phase after CO 2 bubbling.…”

“…This peak was blue-shifted from 3392 to 3398 for the bottom phase and red-shifted from 3392 to 3379 cm −1 for the top phase, respectively, which indicated that the hydrogen bonds between MEA and 4-methoxyphenol were partially destroyed. 28 Second, compared with the neat MEA/4methoxyphenol DES, three new peaks at 1624, 1534, and 1427 cm −1 , attributable to HCO 3 − , MEACO 2 − , and CO 3 2− stretching, 33,34 were generated in the top phase after CO 2 bubbling. Third, two peaks at 1466 and 1444 cm −1 attributable to the skeleton vibration of the benzene ring in 4methoxyphenol disappeared.…”

“…Interestingly, this two-phase system was readily converted back to the homogeneous mixture by bubbling N 2 for 1 h at 65 °C. Very recently, a few groups reported experimental and theoretical evidence on the hydrated behavior of DESs, mostly based on choline chloride. − Their results indicated that the hydrogen bonds between the components of the DESs became successively weaker with the increase of water content, which implied that the nanostructure of DES was progressively damaged. By using neutron total scattering and empirical potential structure refinement, Edler et al further proved that the nanostructure of choline chloride/urea DES began to be disrupted when H 2 O content was greater than 51 wt %.…”