Structure of Alkylated Silica Surfaces: Quenching of Fluorescence from CovalentlyBound Pyrene

摘要

The influence of overlying solvent on the structure of alkylated silica surfacesis investigated. In these experiments, 3-(1-pyrenyl)propyldimethylchlorosilane (3PPS) was chemically bonded at low coverages to silica gel, which was further modified with monomeric C1, C8, and C18 ligands. Fluorescence lifetimes of 3PPS were measured to determine the differences in the interfacial polarity of these materials in methanol and methanol/water solution, Rates of quenching by mercuric chloride were also measured to correlate differences in polarity with the exposure of the immobilized probe to solvent. For C1-modified and bare silica, the results indicate an interfacial environment that is dominated by exposure to solvent. On C18-modified silica, access by the surface-bound probe to the quencher is minimal, indicating that the long alkyl chains limit exposure of the undertying silica to the solvent. The most interesting behavior was exhibited by C8 ligands, which produce an interfacial environment where the probe has limited contact with the solvent in methanol but becomes much more exposed in 75/25% methanol/water. This change in environment was attributed to the collapse of the alkyl ligands, driven by hydrophobic interactions that reduce their contact with the aqueous solution.