Publication type: Article in scientific journal
Type of review: Not specified
Title: Probing hydrophobic interactions using trajectories of amphiphilic molecules at a hydrophobic/water interface
Authors: Honciuc, Andrei
Schwartz, Daniel K.
DOI: 10.1021/ja900607g
Published in: Journal of the American Chemical Society
Volume(Issue): 131
Issue: 16
Pages: 5973
Pages to: 5979
Issue Date: 2009
Publisher / Ed. Institution: American Chemical Society
ISSN: 0002-7863
Language: English
Subjects: Adsorption; Diffusion; Hydrophobic and hydrophilic interactions; Kinetics; Silicon dioxide; Surface properties; Surface-active agents; Temperature; Thermodynamics; Trimethylsilyl compounds; Water
Subject (DDC): 540: Chemistry
Abstract: Individual molecules of fluorophore-labeled alkanoic acids with various chain lengths, BODIPY-(CH(2))(n)-COOH (abbreviated as fl-Cn), were observed to adsorb and move at the methylated fused silica-water interface as a function of temperature using total internal reflection fluorescence microscopy. The statistical analysis of squared-displacement distributions indicated that the molecular trajectories were consistent with a diffusive model involving two intertwined modes. The slower mode, typically responsible for <50% of the molecular diffusion time, had a diffusion coefficient of <0.005 mum(2)/s and could not be distinguished from the apparent motions of immobilized molecules because of the limitations of experimental resolution. The faster mode exhibited diffusion coefficients that increased with temperature for all chain lengths, permitting an Arrhenius analysis. Both the effective activation energies and kinetic prefactors associated with the fast-mode diffusion coefficients increased systematically with chain length for fl-C2 through fl-C10; however, fl-C15 did not follow this trend but instead exhibited anomalously small values of both parameters. These observations were considered in the context of hydrophobic interactions between the adsorbate molecules and the methylated surface in the presence of water. Specifically, it was hypothesized that fl-C2, fl-C4, and fl-C10 adopted primarily extended molecular conformations on the hydrophobic surface. The increases in activation energy and entropy with chain length for these molecules are consistent with a picture of the transition state in which the molecule partially detaches from the surface and exhibits greater conformational freedom. In contrast, the small activation energy and entropy for fl-C15 are consistent with a scenario in which the surface-bound molecule adopts a compact/globular conformation with limited surface contact and conformational flexibility.
Fulltext version: Published version
License (according to publishing contract): Licence according to publishing contract
Departement: Life Sciences and Facility Management
Organisational Unit: Institute of Chemistry and Biotechnology (ICBT)
Appears in collections:Publikationen Life Sciences und Facility Management

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