Unveiling structural heterogeneities in aqueous solutions using dynamic light scattering

Abstract: To investigate the existence  of  molecular heterogeneties in mixtures of DMSO-water, the dynamics were measured with the method of dynamic light scattering (DLS). Three different compositions (20 mol %, 33 mol% and 60 mol%) were included in the study and measured at room temperature (295 K) and for one composition (33 mol %) also a temperature dependence (from 295 K down to 263 K) was measured. Measurements were done on samples both with and without nanoparticles acting as tracers for the DLS. The diffusion coefficients of DMSO in water was extracted from the analysis and the results from samples without nanoparticles are consistent with diffusion of DMSO molecules reported previously, except for the highest concentration,  and showed a clear Arrhenius behaviour with an activation energy of 26±1kJ/mol. The viscosity was extracted from the diffusioncoefficient of the nanoparticles in the solutions and followed an expected trend regarding the concentration as well as for the higher temperatures, but deviated for lower temperatures due to an unexpected drastic change in the diffusion coefficient around the temperature T= 273 K. The reason for the drastic change could be connected to a possible liquid-liquid phase separation in the DMSO-water  mixture. The hydrodynamical radii was estimated using Stokes-Einsteins equation and had a small but unsure concentration dependence so the size could only be confirmed to be around 0.43 nm at T= 295 K and increased with temperature up to 1.6 nm atT= 263 K, indicating clustering effects and supporting the theory of molecular heterogeneity in DMSO-water mixtures. This was a pilot study to aproposed x-ray experiment at NanoMax at MaxIV to capture the nanoscale fluctuations present in binary solutions.