哈工大高等热力学文献阅读报告(3)

well understood at temperatures where the desorption rate becomes appreciable.

Simple models can provide bounds on the values of the entropies of adsorbed molecules. A lower bound on the entropy is obtained by assuming that the adsorbed molecule remains localized in a potential well and that the molecular motions parallel

Getting ready to leave. The illustration

depicts an adsorbed methane molecule

moving across a typical potential energy

landscape encountered at the surface of a

crystalline solid. The Campbell-Sellers

correlation reveals that the molecule readily

traverses the in-plane barriers to motion at

temperatures where desorption becomes

important, and thereby executes nearly free

translational motion (black arrows) and

rotational motion (red arrow) within the

surface plane.

to the surface are highly restricted. The upper bound is given by the so-called ideal twodimensional gas model, in which the molecule-surface potential is fl at within the surface plane—the molecule can freely translate and rotate across the surface. Unfortunately, the limiting values of the desorption prefactor generally span a very wide range that also grows with increasing molecular size and temperature. For example, the limiting models predict prefactors that differ by a factor of about 250 for methane desorption near 65 K, with this difference increasing to nearly 5×104for n-butane desorption at 170 K .

Without a reliable estimate of the entropic contribution, the calculated desorption rate can have an uncertainty of several orders of magnitude. Previously, the available data were insufficient to establish a trend in the entropies of adsorbed molecules. To address this issue, Campbell and Sellers calculated the entropies of numerous adsorbed molecules by evaluating reported data obtained from measurements of equilibrium adsorption isotherms and thermal desorption rates. They found that the standard entropies of the adsorbed molecules Sad0 are linearly correlated with the standard entropies of the gaseous molecules Sg0 , and that the relation

Sad0= 0.70Sg0– 3.3R

accurately fits a large set of data, spanning entropy values over a range of ~50R. The data set includes different classes of molecules and surfaces, such as n-alkanes, methanol, and several small molecules adsorbed on magnesium oxide, the closepacked

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