N,N-Dimethylformamide-induced phase separation of hexafluoroisopropanol–water mixtures

Abstract

We found that addition of N,N-dimethylformamide (DMF) induces phase separation of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)–water mixtures. The phase diagram of a DMF–HFIP–water ternary system at 298 K showed that phase separation occurs in a closed-loop area in the water-rich mole fraction range of x_H2O > ∼0.4. To clarify the mechanism of DMF-induced phase separation of DMF–HFIP–water mixtures at the molecular level, small-angle neutron scattering (SANS) and ¹H and ¹³C NMR measurements were conducted on the mixtures with varying DMF concentrations along a volume ratio of HFIP to water of 1 : 1 (x^S_HFIP = 0.147). Additionally, the solvation structure of DMF in water and HFIP–water mixtures was elucidated by molecular dynamics (MD) simulations. The SANS results revealed that the inherent heterogeneity of HFIP–water mixtures is increased with increasing DMF concentration toward the lower phase separation concentration, but decreased when the DMF concentration further increased beyond the upper phase separation one. ¹H and ¹³C NMR measurements and MD simulations suggested that preferential solvation of the hydrophobic moiety of DMF by HFIP is the main driver of the phase behaviour of the DMF–HFIP–water system.