Probing the Structure of Liquids with ¹²⁹Xe NMR Spectroscopy: n-Alkanes, Cycloalkanes, and Branched Alkanes

Abstract

The liquid organization of linear, branched, and cyclic alkanes was studied using atomic (129)Xe as a NMR probe. (129)Xe chemical shifts have been experimentally determined for xenon dissolved in a total of 21 alkanes. In order to allow the comparison of the different solvents at similar thermodynamic conditions, the measurements were performed over a wide range of temperatures, from the melting point of the solvent up to 350 K. The results were rationalized in terms of the density, nature, and organization of the chemical groups within xenon's coordination sphere. Additionally, molecular dynamics simulations were performed using established atomistic force fields to interpret and clarify the conclusions suggested by the experimental results. The analysis is able to interpret previous results in the literature for ethane and propane at very different experimental conditions.

From nano-emulsions to phase separation: evidence of nano-segregation in (alkane + perfluoroalkane) mixtures using 129Xe NMR Spectroscopy

“…As can be seen, the values for each solvent at the same T r become closer, but seem to have a larger dispersion than was observed for the n-alkanes. 11 This behaviour, already seen in other physical properties, may be related to differences in the structure of the solvent in the vicinity of xenon when the chain length increases. However, it should be considered that at least part of the effect might come from the presence of isomeric impurities in the used perfluoroalkanes; as was seen for alkanes, xenon dissolved in different isomers can have very different chemical shifts.…”

“…The experimental values were then corrected for the effect of the bulk susceptibility of the solvent and referenced to the chemical shift of gaseous xenon extrapolated to zero pressure, as discussed in more detail in previous work. 11 The calculated medium shifts are reported in Table 1 and shown in Fig. 1a.…”

“…Moreover, its intermolecular potential is similar to that of n-alkanes 10 and being a spherical particle, it should cause only a negligible structural perturbation when dissolved in a liquid and used as an NMR probe. In recent work on the liquid structure of alkanes, 11 an approach combining 129 Xe NMR and molecular dynamics simulations allowed us to show, among other effects, that xenon is preferentially solvated by the methyl groups of n-alkane molecules, suggesting that these liquids tend to create cavities near the ends of the molecules, an effect that is more pronounced for the longer n-alkanes. We have also demonstrated the utility of this approach when applied to more complex systems such as ionic liquids; 12 in this case, 129 Xe NMR was able to identify changes in the liquid structure along a homologous series of ILs and, in combination with MD simulations, to demonstrate differences in the mobility and accessibility of xenon to the polar and nonpolar regions of the liquid structure.…”

Self-association of oligothiophenes in isotropic systems

“…The difference is that the solvent becomes the oligothiophene itself and the greater or smaller tendency to self-associate will be reflected on a greater or smaller tendency of the molecules to form organized regions in the liquid, consisting of transient dimers or larger selfassociated aggregates, which on average brings up some heterogeneity and structuration at the nanoscale and influences the stability of the liquid phase. [25][26][27][28][29] For instance, a higher tendency to self-associate in solution can be reflected on an average state of the liquid phase that has a more significant statistical contribution from local ordered structures (e.g. dimers) and a weaker character of a purely randomized liquid.…”

“…On the other hand, self-association can lead to some degree of organization and structuration at the nanoscale in apparently isotropic media. The existence of nanostructuration in the liquid phase has already been reported in some systems, including ionic liquids and aliphatic and aromatic hydrocarbons, [25][26][27][28][29] thus making the concept of phase isotropicity only fully applicable on a larger scale.…”