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Entropy of H2O Wetting Layers Peter J. Feibelman*, and Ali Alavi
 

Summary: Entropy of H2O Wetting Layers
Peter J. Feibelman*, and Ali Alaviž
Sandia National Laboratories, Albuquerque, New Mexico 87185-1413 and Department of Chemistry,
UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
ReceiVed: January 6, 2004; In Final Form: April 26, 2004
Proton order is less constrained in a fully H-bonded, adsorbed H2O monolayer than in bulk ice. Thus, though
weakly, configurational entropy favors wetting by deposited H2O over formation of 3-D crystalline mounds.
A Pauling-type estimate yields a 0 K adlayer entropy of 1
/2kB ln(9/2), almost twice that of ice Ih. Thus, at
150 K, where periodic adlayers on metals are observed, residual entropy reduces their free energies relative
to a 3-D ice crystal by 4.5 meV/molecule, or 3% of the cost of the adlayers' broken H-bonds. A Debye
model implies that vibrational entropy contributes no more than another 4.4 meV/molecule to the free energy
preference for wetting, at 150 K. This information, though only a bound because of substantial uncertainty
in measured adlayer Debye temperatures, is nonetheless sufficient to conclude that T ) 0 K energies dominate
the free energy balance between wetting and mound formation.
I. Introduction
The need to count the low-energy configurations of a proton-
disordered, hydrogen-bonded crystal like ice Ih to evaluate its
configurational entropy has spawned the development of "ice
models" and studies of their subtle correlations. Though typically

  

Source: Alavi, Ali - Department of Chemistry, University of Cambridge

 

Collections: Chemistry