Characterization of Two Types of Silanol Groups on Fused-Silica Surfaces Using Evanescent-Wave Cavity Ring-Down Spectroscopy

摘要

Evanescent-wave cavity ring-down spectroscopy has beenapplied to a planar fused-silica surface covered with crystal violet (CV~(+)) cations to characterize the silanol groups indirectly. A radiation-polarization dependence of the adsorption isotherm of CV~(+) at the CH_(3)CN/silica interface is measured and fit to a two-site Langmuir equation to determine the relative populations of two different types of isolated silanol groups. CV~(+) binding at type I sites yields a free energy of adsorption of -29.9 +- 0.2 kJ/mol and a saturation surface density of (7.4 +- 0.5) X 10~(12) cm~(-2), whereas the values of -17.9 +- 0.4 kJ/mol and (3.1 +- 0.4) X 10~(13) cm~(-2) are obtained for the type II sites. The CV~(+) cations, each with a planar area of approx120 A~(2), seem to be aligned randomly while lying over the SiO~(-) type I sites, thereby suggesting that this type of site may be surrounded by a large empty surface area (>480 A~(2)). In contrast, the CV~(+) cations on a type II sites are restricted with an average angle of approx40 deg tilted off the surface normal, suggesting that the CV~(+) cations on these sites are grouped closely together. The average tilt angle increases with increasing concentration of crystal violet so that CV~(+) cations may be separated from each other to minimize the repulsion of nearby CV~(+) and SiOH sites.