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.
