On the structural features of triphenylmethylphosphonium:ethylene glycol-based deep eutectic solvents

This study presents a comprehensive investigation into the structural and physicochemical properties of binary mixtures based on triphenylmethylphosphonium halides (TPMP-X, where X = Br or Cl) and ethylene glycol (EG). By varying the molar ratio of the components, a detailed analysis was conducted to identify the eutectic composition and to understand the molecular interactions driving the formation of the eutectic. Experimental solid–liquid phase diagrams confirmed TPMPBr:EG and TPMPCl:EG mixtures are Deep Eutectic Solvents (DESs), with eutectic points observed at 1:15 and 1:11 M ratios, respectively. Rheological measurements and NMR diffusion studies revealed compositional trends in viscosity and molecular mobility, while 1H and 31P NMR spectroscopy highlighted the role of halide anions in modulating hydrogen bonding. UV-VIS spectroscopy further supported these findings by showing composition-dependent variations in optical band gap and Urbach energy, particularly in chloride-containing systems. Collectively, the results demonstrate that the eutectic point corresponds to a region of distinct molecular organization, characterized by enhanced structural coherence and unique electronic behavior. This work underscores the significance of accurate eutectic point determination for the rational design and application of phosphonium-based DESs.