Fluctuation in adsorbed phases

摘要

A study has been made of fluctuations in adsorbed phase coverage based on a model which assumes localized adsorption for the first layer and nonlocalized mobile adsorption for the second layer. For a homogeneous surface the envelopes, extremum locations, and limiting knee point of the fluctuation are calculated. Anshyphen;shaped semiloop exists in the fluctuation, if the adsorption strengthcis larger than the critical valuecK=28.43. The molar fluctuation (fluctuation divided by coverage) has always one minimum. In the case of adsorption on a heterogeneous surface withshyphen;different patches there existsmaxima and minima in the fluctuation provided that lsqb;Mirsqb;ci/Miminus;1ciminus;1gsim;cK=2@qL8.43,i=2, 3,...,s, whereciandMiare the adsorption strength and the number of lattice sites for theith patch, respectively, and lsqb;Mirsqb;=Jsk=iMk. From these extrema in the fluctuation we can estimate the characteristic constantscirsquo;s andMirsquo;s of an adsorbed phase. The theory is in excellent agreement with the observed fluctuations of argon on rutile of 85thinsp;deg;K by Drain and Morrison and xenon on porous silver at 77.4thinsp;deg;K by Hobson.