Exploring Fluorous Affinity by Liquid Chromatography

Terms such as "fluorous affinity" and "fluorophilicity" have been used to describe the unique partition and sorption properties often exhibited by highly fluorinated organic compounds, that is molecules rich in sp3 carbon-fluorine bonds. In this work, we made use of a highly fluorinated stationary phase and a series of benzene derivatives to study the effect of one single perfluorinated carbon on the chromatographic behavior and adsorption properties of molecules. For this purpose, the adsorption equilibria of α,α,α-trifluorotoluene, toluene, and other alkylbenzenes have been studied by means of nonlinear chromatography in a variety of acetonitrile/water eluents. Our results reveal that one single perfluorinated carbon is already enough to induce a drastic change in the adsorption properties of molecules on the perfluorinated stationary phase. In particular, it has been found that adsorption is monolayer if the perfluoroalkyl carbon is present but that, when this unit is missing, molecules arrange as multilayer stack structures. These findings can contribute to the understanding of molecular mechanisms of fluorous affinity. (Figure Presented).

Fluorous phases have been largely employed to selectively recognize highly fluorinated compounds with significantly large F-moieties (made of at least 6 sp3 perfluorinated carbon, the so-called ponytail), on the basis of a sort of “like dissolves like” interaction, known in literature as “fluorophilicity” (Fluorous Phase Extraction, for instance, is based on this principle). In this study, we made use of a highly perfluorinated stationary phase (perfluorohexylpropylsiloxane-bonded silica) and a series of benzene derivatives to estimate, on the one hand, the effect of the alkyl-chain on the adsorption behavior of molecules and, on the other, to evaluate if the presence of one single perfluorinated sp3 carbon could affect the adsorption mechanism on a perfluorinated adsorbent. To this effect, the adsorption equilibria of α,α,α-trifluorotoluene, toluene and other alkylbenzenes have been studied by means of nonlinear chromatography, using acetonitrile/water mixtures as eluents. The adsorption isotherms were determined through an advanced numerical approach based on the simulation of peaks recorded under nonlinear conditions (Inverse Methods). The Equilibrium-Dispersive Model of chromatography has been used for this purpose. Our results have shown that even the presence of a single CF3 unit is enough to induce drastic change in the chromatographic retention mechanism. In particular, it was found that alkylbenzenes exhibit a multilayer adsorption behavior, which becomes more pronounced increasing the length of the alkyl-chain. On the other hand, α,α,α-trifluorotoluene molecules arrange themselves as to form surface monolayer coverage.