Exploring Dynamics and Intermolecular Interactions in Binary Mixtures of Reline and DMSO: An Investigation Using Nuclear Magnetic Resonance and Infrared Spectroscopic Techniques.

Reline, which is composed of choline chloride and urea in a molar ratio of 1:2, is the first and most extensively studied deep eutectic solvent (DES). In certain applications, reline is blended with organic solvents, dimethyl sulfoxide (DMSO) in most cases, to gain improved properties. Therefore, it is crucial to have a profound understanding of the impact of DMSO on the dynamics and structures of the species in the binary mixtures. In this study, neat reline and ten reline/DMSO mixtures, with DMSO molar fraction ranging from 0.1 to 0.95, were investigated primarily through a combined approach utilizing nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopic techniques. Based on our investigation, we probed a significant transition of the binary mixtures from large molecule solutions or viscous liquids to nonviscous small-molecule solutions at a DMSO molar fraction of 0.7. Specifically, upon analyzing the self-diffusion coefficient, ¹H T₁ and ¹H T₂, we observed a notable increase in the molecular mobility of the species within the reline/DMSO system, particularly when the DMSO molar fraction exceeded 0.7. Drawing upon the FT-IR findings, we suggest that the enhanced molecular mobility, as evidenced by NMR analysis, is correlated with the disruption of molecular hydrogen-bonding interactions involving the -NH₂ and -OH groups. Furthermore, based on 1D ¹H, 1D ¹⁵N, and 2D ¹H-¹H COSY spectra, it was revealed that the interaction between urea and choline remains relatively stable until the DMSO fraction exceeds 0.7, whereupon it exhibited a notable weakening as the DMSO fraction increases from 0.7 to 0.95. In the meantime, DMSO molecules predominantly engage in hydrogen bond interactions with urea and choline when the DMSO molar fraction exceeds 0.7. Our results align well with previous molecular dynamics (MD) simulation studies and provide profound insights into the significant transition in the reline/DMSO mixture system.