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(2v) symmetry) covalently attached to silica surfaces that have been silanized with n-propyltrimethoxysilane (C(3)-TMOS) or 3,3,3-trifluoropropyltrimethoxysilane (CF(3)-TMOS) when the interface is subjected to pure supercritical carbon dioxide (scCO(2)). The in-plane BODIPY 494/503 rotational reorientation dynamics are assessed by using the Debye-Stokes-Einstein expression. As the scCO(2) density increases the local microviscosity surrounding the tethered BODIPY 494/503 molecule decreases. The terminal group (CH(3) versus CF(3)) 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(2).
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