Paper SS+AS-ThA8
Desorption: Out of the Vacuum, into the Liquid (Invited Talk)

Thursday, November 10, 2016, 4:40 pm, Room 103C

The first paper on thermal desorption John Yates published with Ted Madey was entitled: Nitrogen Desorbs with Complex Kinetics. Thermal Desorption Spectroscopy, in more and more sophisticated experimental set-ups including flow reactors, became then a prominent techniques in John´s Laboratory to study simple and complex surface chemical reactions. In this short report, we look further into “complex kinetics” and find that the theoretical framework of a thermally activated process can not be applied to the detachment of particles in a micro-fluidic shear-flow experiment.

The theoretical description of the desorption process treats bond breaking as a thermally activated process, which can be described by the Arrhenius equation. The activation energy and the pre-exponential factor can be determined from a series of TPD experiments with either varying coverage or at constant coverage, using different heating rates. Here we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated, as in thermal desorption experiments, as a thermally activated process to determine activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data, obtained in phosphate-buffered saline solution, results in an activation energy of detachment around 20 kJ/mol. This value is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We hence conclude, that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction), and not physical detachment as would be the case in a gas desorption experiment.