Single Molecule Studies of the Hydrophobic Interaction At the Solid-Liquid Interface

The interactions between adsorbate molecules and hydrophobic surfaces are of significant interest due to their importance in a variety biological and separations processes.  While macroscopic observations of hydrophobic interactions are often reported (e.g. using AFM or SFA), it is challenging to extrapolate ensemble-averaged measurements to molecular-level phenomena.  Using total internal reflection fluorescence microscopy to image individual molecules at the solid-liquid interface, we directly observe dynamic behavior, including surface diffusion and adsorption/desorption kinetics, that are due to non-specific interactions.  Since these phenomena are examined at the single molecule level, we are able to identify unique diffusive modes/mechanisms and characteristic residence times of multiple molecular populations on chemically homogeneous surfaces without relying on models to quantify these behaviors.  The data presented here show a trend towards a stronger probe-surface interaction with increasing alkyl chain length monolayers despite these monolayers having the same water contact angle.  On longer (16-18 carbon) alkylsilane monolayers our probe molecule, fluorescently labeled dodecanoic acid, has significantly slower diffusion and exhibits far fewer desorption mediated “flights” than on short (4-6 carbon) monolayers.  Additionally, probes on the longer chain surfaces are much more likely to exhibit extended surface residence times as opposed to short transient surface visits.  We hypothesize that this increase in interaction is due to an increase in the monolayer’s ability to entangle the fatty acid probe.