Nanoscale Clustering of Alcoholic Solutes in Deep Eutectic Solvents Studied by Nuclear Magnetic Resonance and Dynamic Light Scattering

Abstract

It was previously shown that water distributes heterogeneously in deep eutectic solvents (DESs). The aim of this study was to see whether this behavior was common to other hydrogen bonding compounds and determine when a solute formed an emulsion or a homogeneous solution. Pulsed field gradient (PFG) and nuclear Overhauser effect (NOE) nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) are employed to probe the phase behavior of glucose, 1-pentanol, and phenol in Ethaline (ethylene glycol/choline chloride, 2:1). By comparing the measured values of self-diffusivities to those calculated assuming Stokesian behavior, it was discovered that glucose forms a homogeneous solution, whereas 1-pentanol forms a heterogeneous mixture. A change in aggregate size of 10 wt % phenol in Ethaline is observed at 313 K. Above 313 K, aggregation is enhanced, and larger aggregates are formed, suggesting that melting of phenol affects the phase behavior. {1H,1H}-NOESY NMR showed that glucose interacts strongly with Ethaline, while with 1-pentanol and phenol, this interaction is weaker. DLS experiments further indicated the heterogeneity and homogeneity. Results suggest that solid solutes that are capable of strong hydrogen bonding are soluble in DESs, whereas liquid solutes form heterogeneous nanophases because of the weaker solute–solvent interactions and density difference.