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Structure and Ultrafast Dynamics in Deep Eutectic Solvents

Periodic Reporting for period 1 - STUDYES (Structure and Ultrafast Dynamics in Deep Eutectic Solvents)

Reporting period: 2019-09-01 to 2021-08-31

This project (STUDYES) is about creating a fundamental understanding of the physical and chemical properties of a novel class of designer solvents known as deep eutectic solvents (DES). The fundamental understanding is sought from a combined structural and dynamical viewpoint of these solvents using laser spectroscopies with very short pulses of light (on the order of femtoseconds; one femtosecond is a millionth of a billionth of one second).

A DES is a binary mixture of an organic salt (organic cation plus halide anion, example, choline chloride) and a hydrogen bond donor (example, ethylene glycol or urea), which when mixed in a specific molar ratio is found to melt at a much lower temperature than those of the individual constituents. The depression in melting point can remarkably exceed several tens of degrees Celsius which implies that an otherwise solid mixture of 1:2 choline chloride and urea at room temperature spontaneously forms a liquid!

Before STUDYES, the structural characterization of DESs primarily involved time-averaged measurements such as neutron diffraction. Dynamical studies of DESs were also limited to nanosecond timescales using fluorescence emission studies. Through STUDYES, the ultrafast molecular-scale mobility of DESs (reorientational timescales, vibrational lifetimes, etc.) and the effect of an external chemical agent such as water on the nascent DES molecular structures and ultrafast dynamics have been determined for the first time.

The scientific results of STUDYES are important for the society because of the central role that chemical solvents play in our daily lives. Solvents occupy a strategic place as common household items, laboratory reagents, and heavy-duty industrial materials. The choice of a suitable solvent is largely determined by the bulk physical and chemical properties which are closely intertwined with the fluctuating microscopic solvent structure, intermolecular interactions, and rotational dynamics. DESs have novel properties such as efficient drug dissolution, ionic conductivity, non-volatility, and thermochemical stability that find use in diverse chemical and industrial applications. These low-cost and environment-friendly reagents further hold the promise to replace the most common organic solvents in chemical research, namely, volatile organic compounds and ionic liquids. DESs are biocompatible, biodegradable, and usually negligibly toxic, and therefore extremely promising as sustainable green solvents for chemical and industrial applications.

The overall objectives of STUDYES are 1) understanding the dynamic molecular structures and intermolecular interactions in DESs 2) tying macroscopic properties such as DES viscosity with the ultrafast molecular-scale mobility; 3) investigating the effect of water on DES molecular structures and dynamical interactions. Our experiments show that the unique physicochemical properties of DESs are due to the presence of strong hydrogen bonds between the two components. The molecules in the solvent begin to tumble faster with increasing temperature which is attributed to a decrease in the viscosity of the medium. We also find that mixtures of water with DESs perturb the intermolecular interactions only at high levels of hydration (40% and above) and that the behaviours are characteristically different for the surface and the bulk of the solvent. Specifically, for one DES (1:2 mixture of choline chloride and urea), we find evidence for sequestration of water into microscopic cavities within the solvent cage in the bulk whereas the surface shows enhanced structural ordering before water-water interactions finally begin to dominate.
In order to study the hydrogen-bonded structures and ultrafast dynamics in DESs, we synthesized and characterized DESs made up of 1) choline chloride and urea 2) choline chloride and malonic acid. The heating and grinding methods were employed, respectively, since carboxylic acids decompose at high temperatures. Linear and non-linear infrared spectroscopic measurements were performed on these samples to gain insights into their structures and ultrafast dynamics, both for the liquid-air interface and in bulk.

Femtosecond mid-infrared (IR) pulses were used to determine the structural inhomogeneity and vibrational lifetimes of reporter IR modes in the synthesized DESs. The strength and lifetime of H-bonding interactions is accurately reflected in the frequency shifts of the vibrational modes and the linewidths. We used fs mid-IR pump-probe spectroscopy to investigate the intrinsic functional groups of DESs with strong IR absorption cross-sections, such as the carbonyl stretch of urea (HBD), the O–H stretch of choline chloride and malonic acid (HBD), as well as the N–H stretch of urea. These experiments made it possible to establish the temperature, functional group and viscosity dependences of molecular reorganizations in DESs.

We have also used polarization-resolved, mid-IR pump-probe spectroscopy on DESs containing isotopically diluted water to measure the rotational mobility of OD bonds in HDO molecules. A low concentration of OD groups is chosen to decouple the anisotropy decay from energy transfer among OD stretch vibrations. Hence, the anisotropy dynamics purely reflect the rotational mobility of the HDO molecules. We find that the rotational mobility of water molecules strongly depends on the local (hydrogen-bonding) environment. Complementary surface-specific sum frequency generation spectroscopy studies have shown that there is chemical speciation and restructuring of the liquid-air interface of an archetype DES with increasing levels of hydration.

Combining DES synthesis and cutting-edge ultrafast spectroscopies has enabled the first molecular-scale characterization of the structural dynamics of DESs and DES-water mixtures. The results of STUDYES (three planned publications - one to be submitted and two others in preparation) are expected to advance our fundamental mechanistic understanding of how the molecular-scale structural dynamics of DESs determine their macroscopic physicochemical properties such as the melting point, the viscosity, and ion mobility. This understanding will be crucial in the long run for the effective utilization of DESs for chemical and industrial applications.
Ultrafast mid-infrared spectroscopies have revealed the under-pinning mesoscopic nanostructures and dynamics of novel DESs. We expect that this new understanding will aid the predictability and tailoring of DES characteristics to develop versatile platforms in the future involving environmentally clean, green solvents.

The project has implemented a bottom-up approach to understanding the macroscopic properties of DESs. Cutting-edge ultrafast spectroscopies combining femtosecond temporal resolution and broadband spectral resolution have been used to determine the strength and dynamics of intermolecular interactions, the extent of homogeneous and heterogeneous domains, and the timescales for the local equilibrium and non-equilibrium dynamics, both in bare DESs and DES-water mixtures. The planned publications for this project (currently under preparation) will bridge the gap in understanding between the mesoscopic organization and the macroscopic response in this emerging class of solvents. The results are expected to expand the synthetic utility and chemical potential of DESs for solvent research in chemical laboratories and industries.
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