The Behavior of Cyclohexane as a Sparingly Soluble Solute in Polyethylene Glycol

Polyethylene glycol (PEG) has been recognized as an environmentally friendly solvent in chemical synthesis. To better understand how PEG behaves as a chemical solvent, this study focuses on cyclohexane as a solute, which is only sparingly soluble in PEG200. Results from ¹H-NMR spectra and ¹H-NMR relaxation measurements indicate solubility to be below a cyclohexane mole fraction of 0.06. For this low concentration range, the concentration dependence of density, viscosity, and cyclohexane and PEG200 self-diffusion coefficients all display linear dependence with cyclohexane mole fractions. However, evaluation of the hydrodynamic radii of cyclohexane results in unrealistically low values, suggesting the presence of dynamical heterogeneity. Furthermore, very large negative excess molar volumes that become less negative with increasing temperature suggest a strong structural reorganization response of the PEG200 oligomers near the cyclohexane solutes. However, the presence of cyclohexane clusters or aggregates as a possible explanation is not supported by ¹H-NMR relaxation and dynamic light scattering results. Preliminary qualitative results from classical MD simulations further support the absence of aggregates at low concentrations, but they also reveal the progressive formation of pairs, groups, and ultimately patches of cyclohexane molecules with increasing cyclohexane concentration.