Nuclear Magnetic Resonance Spectroscopy Investigation on Ultralow Melting Temperature Behavior of Dimethyl Sulfoxide-Water Solutions

On the basis of the molecular understanding that DMSO has a trigonal pyramidal geometry with one highly polarized sulfinyl group and two hydrophobic methyl groups, we used NMR technology to rationalize why DMSO can act as antifreeze for aqueous solutions. A series of DMSO-water, DMSO-methanol, and acetone-water binary solutions with various molar ratios were tested in specified low temperature conditions. The freezing test results indicated that only DMSO-water solutions with the DMSO-water molar ratio (n) in a specific range can form apparent ultralow freezing temperature solutions. Among all DMSO-water solutions, the apparent freezing temperature lower than -130 °C was obtained for n values of 0.25, 0.30, and 0.35, respectively. Multinuclear NMR chemical shifts, H diffusion experiment results, and viscosity measurements suggested that molecular rearrangement resulted in the formation of the water-core DMSO-shell aggregation unit in DMSO-water solutions. The weak methyl-methyl London dispersion forces among water-core DMSO-shell aggregation units can explain the apparent ultralow freezing temperatures of these DMSO-water solutions.