Microstructure variations with concentration of propylene glycol–water solution probed by NMR

The solution microstructure variations with concentration of propylene glycol (PG)–water mixture were investigated using NMR technique. PG has an apparent critical point at around χPG = 0.3, this biphasic behavior of alkyl protons in PG–water mixture is different from the monotonous increase or decrease of other alcohol-water mixtures. At water-rich region, water molecules are in the vicinity of PG, forming weak C–H⋯O H-bonds with PG alkyl protons and strong O–H⋯O H-bonds with PG hydroxyls. PG gradually aggregates in the order of CH3, CH and CH2 with PG concentration increasing. At PG-rich region, the solution forms regions enriched in either hydrocarbons or hydroxyl groups, which results in the formation of microheterogeneous solution, where water is expelled from alkyl tail and accumulated in the region of PG hydroxyls head. In addition, the T1 and NOE results of PG aqueous solutions also support the weak hydrogen bond and microheterogeneous structural variations with concentration at molecular level. These results offer not only new insights into the mechanism of the outstanding capability of PG as cryoprotectant, but also provide possible reason of the anomalous thermodynamic behavior in the PG–water mixture.