Nano-segregation in mixtures of hydrogenated and fluorinated alcohols: Using 129Xe NMR spectroscopy to measure local composition

Abstract

Binary liquid mixtures of n-alkanols and 1H,1H-perfluoroalcohols were investigated, combining 129Xe NMR spectroscopy, atomistic molecular dynamics (MD) simulations and the measurement of excess molar volumes. All mixtures display very large and positive excess molar volumes, which are accurately reproduced by the MD simulation results. The 129Xe NMR spectra confirm the existence of segregation between hydrogenated and fluorinated chains and thus the existence of hydrogenated-rich and fluorinated-rich nanodomains. The degree of segregation gradually increases with the chain length of both alcohols but is lower than that observed in equivalent mixtures of alkanes + perfluoroalkanes. The average local composition within xenon’s coordination sphere was calculated from the 129Xe NMR spectra and found to differ up to ±0.03 mol fraction from the nominal composition of the mixtures. For all mixtures, the xenon’s enrichment curves are S-shaped: at low concentration of the fluorinated alcohol, xenon atoms are located predominantly in more fluorinated environments (than the nominal concentration of the mixture), while at higher concentrations, they are found in environments that are more hydrogenated. MD simulations confirm the existence of segregation between hydrogenated and fluorinated chains and the tendency of xenon atoms to locate near end groups, CH3 and particularly CF3, but not OH. Local compositions computed from the simulation runs reproduce the experimental S-shaped curves with remarkable accuracy, validating the used molecular models and methodology and adding enhanced consistency to the whole study. The MD simulations were further used to analyse the interstitial space in the mixtures, in particular the existence of voids with the adequate volume to accommodate xenon atoms and its dependence on the mixture composition. The results indicate that the location of xenon atoms results from a balance between the degree of segregation between hydrogenated and fluorinated chains and the existence of voids with the adequate volume.