Ultrafast Orientational Dynamics of Nanoconfined Benzene
- PDF / 81,586 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 59 Downloads / 242 Views
0899-N05-03.1
Ultrafast Orientational Dynamics of Nanoconfined Benzene Xiang Zhu1,2, Richard A. Farrer1 and John T. Fourkas1,2 1 Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02467, USA 2 Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA ABSTRACT Optical Kerr effect spectroscopy has been used to study the orientational dynamics of benzene and benzene-d6 confined in nanoporous sol-gel glasses. Orientational diffusion was found to be inhibited considerably in confinement due to the strong wetting of benzene on silica. The orientational dynamics of benzene-d6 were found to be affected less than those of benzene, which is in agreement with the somewhat larger contact angle of benzene-d6 on silica. Comparison of our results to Raman data for confined benzene-d6 suggests that this liquid is considerably more organized at the pore surfaces than in the bulk.
INTRODUCTION The behavior of liquids changes profoundly upon confinement in a region with dimensions that are comparable to the size of a few molecules. Over the past decade, we have studied how liquid structure and dynamics are affected in such environments [1, 2] by employing optical Kerr effect (OKE) spectroscopy [3-5] to probe the orientational dynamics of liquids confined in nanoporous sol-gel glasses [6]. In OKE spectroscopy, an ultrafast, linearly-polarized pump pulse creates a small net alignment in a liquid composed of molecules with anisotropic polarizabilities. A probe pulse measures the induced birefringence as a function of delay time, which allows for orientational diffusion to be measured in real time as the liquid returns to an isotropic orientational state. Our past studies have revealed some general themes in the orientational dynamics of liquids confined in sol-gel glasses. In non-associated liquids, we generally observe one relaxation component with an orientational decay that is identical to that of the bulk liquid and a second component that relaxes more slowly (by as an order of magnitude or more) [1, 2]. The former component is attributed to molecules in the centers of the pores and the latter to molecules at the pore surfaces. For liquids that wet the pore surfaces weakly, the thickness of the surface layer with inhibited dynamics is generally less than the diameter of a molecule [7, 8]. We believe that this effect is indicative of surface dynamics that depend upon the orientation of the molecules relative to the pore wall. In contrast, for associated liquids such as water, there is no bulk-like relaxation in confinement [9]. While there is a clear signature of inhibited surface relaxation, the dynamics in the pore centers are also considerably slower than those of the bulk liquid, even when the pore diameter is as large as 100 Å. This behavior is a reflection of the highly cooperative nature of the orientational relaxation in such liquids. Benzene is a non-dipolar molecule that does not form hydrogen bonds. As a result we might expect its dynamics in confinement to resemble t
Data Loading...
