From nano-seggregation to mesophases: probing the liquid structure of perfluoroalkylalkanes with 129Xe NMR spectroscopy

Abstract

In this work we demonstrate that pure perfluoroalkylalkane diblock molecules are not isotropic liquids and self organize forming domains at the nanometric scale. ¹²⁹Xe NMR spectra were obtained as a function of temperature for seven liquid perfluoroalkylalkanes, covering a range of relative lengths of the hydrogenated and fluorinated segments. The results support the presence of domains richer in the hydrogenated groups, in which xenon is preferentially dissolved. The average local concentration within the xenon coordination sphere is estimated to be 0.05 mole fraction higher in hydrogenated groups than the stoichiometric proportion. Atomistic molecular dynamics simulations support this analysis and allow a detailed analysis of the liquid structure. Furthermore, ¹²⁹Xe NMR spectra in perfluorohexylhexane (F6H6) and perfluorohexyloctane (F6H8) obtained as a function of temperature, clearly detect the existence of two distinct environments in the fluid, one richer in hydrogenated groups and another richer in fluorinated groups, consistent with the formation of mesophases. It is important to stress that nano-segregation is this case observed in liquids interacting exclusively through dispersion forces, unlike most common examples of segregation which are determined by hydrogen bonding and polarity. Given the simple molecular structure and interactions of the studied PFAA, we believe that the present results can have a general impact in understanding the early mechanisms of segregation, phase separation and self-assembly.