Abstract
Important interfacial processes in disciplines ranging from medicine to the separations sciences occur over a wide range of pressures, temperatures, and time scales. In this paper we report a new high-pressure total internal reflection fluorescence (HP-TIRF) apparatus that allows rapid fluorescence measurements of sub-monolayers in contact with liquids and supercritical fluids between 293 K and 353 K and up to 250 bar with picosecond time resolution. We use the HP-TIRF system to study the in-plane rotational reorientation dynamics of the fluorescent probe BODIPY 494/503 (C<sub>2v</sub> symmetry) covalently attached to silica surfaces that have been silanized with <i>n</i>-propyltrimethoxysilane (C<sub>3</sub>-TMOS) or 3,3,3-trifluoropropyltrimethoxysilane (CF<sub>3</sub>-TMOS) when the interface is subjected to pure supercritical carbon dioxide (scCO<sub>2</sub>). The in-plane BODIPY 494/503 rotational reorientation dynamics are assessed by using the Debye–Stokes–Einstein expression. As the scCO<sub>2</sub> density increases the local microviscosity surrounding the tethered BODIPY 494/503 molecule decreases. The terminal group (CH<sub>3</sub> versus CF<sub>3</sub>) within the silane monolayer governs the onset and absolute magnitude of the observed viscosity changes. The results are explained in terms of the wellknown solubility of fluorine-containing species in scCO<sub>2</sub>.
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