Liquid Mixtures of Xenon with Fluorinated Species: Xenon + Sulfur Hexafluoride

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

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.

IR spectroscopic study of the displacement of an SF6 monolayer on graphite by Xe

We report a study of displacement by xenon of a monolayer of sulphur hexafluoride initially condensed on a graphite surface. Earlier work showed that, below 112 K, Xe displaces SF₆ almost completely in a first-order transition. Working at higher temperatures, we show that this system has a simple eutectic-like phase diagram, at least for SF₆ not too dilute. In our experiment, both adsorbates are in equilibrium with their respective vapors in a cold cell. In our infrared reflection-absorption spectroscopy measurements, the SF₆ coverage on the surface is monitored by the frequency shift due to dynamic dipole coupling of the collective mode of the strong SF₆ ν₃ vibrational resonance. Simulations relate this frequency shift to the SF₆ areal density. Below T ≈ 134 K, with increasing Xe pressure, a small amount Xe dissolves in the solid SF₆ monolayer preceding its displacement by a solid predominantly Xe monolayer in a first-order transition. Above 134 K, there is a weaker first-order transition to a mixed liquid monolayer, followed by continuous increase in Xe concentration. If the initial SF₆ monolayer is near its melting line, the melting transition on adding Xe appears to become continuous.

Phase behavior of dipolar associating fluids from the SAFT-VR+D equation of state

The statistical associating fluid theory for potentials of variable range plus dipole (SAFT-VR+D) is extended to study associating dipolar fluids. In the SAFT-VR+D approach dipolar interactions are taken into account through the use of the generalized mean spherical approximation to describe a reference fluid of dipolar square-well segments. This enables the effect of the dipolar interactions on the thermodynamics and structure of fluids to be explicitly described. Predictions for the thermodynamic properties and phase behavior of dipolar associating square-well monomers with one, two, and four association sites are considered and compared with new isothermal-isobaric and Gibbs ensemble Monte Carlo simulation data. The results show that the SAFT-VR+D equation provides a good description of the phase behavior of dipolar associating fluids. Additionally we have applied the new theoretical approach to study the vapor pressure and saturated liquid density of water.