Synthesis, characterisation and chromatographic evaluation of pentafluorophenyl and phenyl bonded silica phases prepared using supercritical carbon dioxide as a reaction solvent

Pentafluorophenyl and phenyl silica stationary phases offer alternative selectivity compared to alkyl bonded C18 and C8 stationary phases, through other interactions such as π–π interactions, dipole–dipole and hydrogen bond interactions. Pentafluorophenyl and phenyl silica bonded stationary phases were efficiently prepared in sc-CO2 specifically pentafluorophenyl propyl (PFPP), pentafluorophenyl (PFP), phenyl propyl (PP) and phenyl (P) silica stationary phases. The bonded phases were characterised by elemental analysis, thermogravimetric analysis (TGA), BET, and by solid-state NMR spectroscopy. Chromatographic performance of the supercritical fluid generated phases was also investigated using the Neue test. The authors present results which demonstrate that pentafluorophenyl and phenyl stationary phases can be prepared successfully under supercritical conditions of 100 °C, 414 bar in a reaction time of 1 h with surface coverage comparable to traditional organic solvent based methods. Chromatographic results reveal that the pentafluorophenyl propyl (PFPP) phase provides superior separation performance for Neue test solutes despite having a lower ligand density (C: 5.67%, 2.2 μmol/m2) compared to the phenyl propyl (PP) analogue having the highest ligand density (C: 6.67%, 2.5 μmol/m2). The difference chromatographic performance is attributed to the polarity of the CF bond in PFPP phase. Moreover, as the alkyl chain length decreases, the hydrophobic interaction also decreases, and the PFPP phase (with a propyl linkage) provides better separation compared to the PFP phase.

► Functionalisation of silica to pentafluorophenyl (PFPP, PFP) and phenyl phases (PP, P) in sc-CO2 within 1 h.
► Chromatographic tests more informative than spectroscopic and CHN evaluation.
► Chromatographic results reveal PFPP phase superior to PP and PFP.
► Residual silanol activity confirmed as a contributing property of fluorinated phases.