Surface equilibration

US studies can produce informative free energy landscapes but assume that degrees of freedom orthogonal to the surface equilibrate quickly. The MD time needed for significant chain or backbone movement could exceed the length of typical US simulations (which are each typically on the nanosecond timescale). However, in spite of this caveat, US approaches have been very successful. One explanation for this success lies in the choice of initial conditions US simulations employ initial coordinates provided by high-temperature unfolding trajectories, which themselves have been found to yield predictive information about the nature of the relevant conformational space. 

This seems to be inconsistent with ascribing the increase in work function upon chemisorption to surface enrichment. AES, however, measures a concentration averaged over several outer layers and since enrichment should occur by inversion between the outer layers, enrichments based on AES data are a lower limit to the actual surface composition. Moreover, another point of dispute might be the surface equilibration, which could be an extremely slow process for a low-index plane. 

All experiments using lipid membranes employed equal weight ratios of the phospholipid or oxidized phospholipid, and one of the steroids. For the trough experiments, 2 mg of phospholipid and 2 mg of steroid were dissolved In a total of 5 ml of solvent. Hexane was used as the solvent In most Instances, but sma1 1 quantities of chloroform were necessary as a secondary solvent for complete dissolution of the steroid dlol and trlol species. Approximately 0.1 ml of solution was slowly spread on the aqueous sub-phase (consisting of pure water or 0.1 M KC1) In the trough, and the solvent was allowed to completely evaporate before compression experiments were Initiated. Compression was performed slowly In all cases to allow surface equilibration and each sample solution was Investigated at least four times to ensure reproducibility. 

Figure 29 also shows that increasing the H2S level above 13 ppb results in little, if any, further adsorption of H2S by Ni, although the methanation activity continues to decline (Fig. 28). Apparently, the amount and rate of sulfur adsorption on a Ni surface equilibrated with 13-ppb H2S in H2 is too...

For physisorption on/in porous solids, transport into mesopores and micropores often limits the rate of adsorption. Two-stage equilibria are frequently observed the more accessible outer surfaces equilibrate rapidly and remain in equilibrium with the ambient phase, acting as a source for slower transport of the adsorbate into the interior of the solid. Establishment of cmnplete equilibrium can be a slow process. 

In adsorption microcalorimetry, surface equilibration depends not only on the chosen probe molecule but also on the adsorption temperature. It is worth mentioning that the literature contains some controversial articles on this subject [29]. 

Gases B, C, and D experience single-site adsorption on the catalyst, and surface coverage is described by the classic Langmuir isotherm 0, = KiPi v. Dual-site chemical reaction on the surface equilibrates on the time scale of the adsorption of reactant A. Hence,... 

Physical adsorption of inert gases on metals is studied at temperatures between 10 and 78 K. At too high a temperature the adsorbed layer boils off the surface. At too low a temperature an adsorbed gas molecule does not migrate on the surface after striking it the surface structure is random and does not anneal to reflect the energetics of adsorbate-substrate interaction. In the temperature range for which surface equilibration occurs, the structure of the surface layer is independent of the inert gas adsorbed and of the metal surface exposed. Whether Xe is adsorbed on graphite,... 

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