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| - Thermodynamics of Confined Perfluoropolyether Films on Amorphous Carbon SurfacesDetermined from the Time-Dependent Evaporation Kinetics
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| - The thermodynamics of ultrathin (≤10 Å) perfluoropolyether (PFPE) films in contact with amorphous carbonsurfaces (CHx and CNx) are derived from the time-dependent kinetics of film evaporation. Two nonfunctionalized PFPE structures were studied: a polydisperse (Mw/Mn = 1.4) Fomblin Z with an average molecularweight of 4000 g mol-1 and a fractionated (Mw/Mn = 1.05) Demnum sample of average molecular weight2200 g mol-1. Data is also presented for the evaporation of a fractionated (Mw/Mn = 1.08) sample of thehydroxyl-terminated Fomblin Zdol (Mw = 2100 g mol-1). Evaporation of the nonfunctionalized PFPEs fromamorphous carbon follows nonclassical, first-order desorption kinetics having a rate constant that variesinversely with time. Evaporation of the functionalized Fomblin Zdol is also nonclassical; however, the timedependence of the rate constant deviates substantially from that characteristic of the nonfunctionalized PFPEs.These evaporation kinetics result from the increase in the surface free energy that accompanies thinning ofthe PFPE film. An analytic expression for the dependence of the surface free energy on film thickness andtemperature is derived from the time-dependent evaporation rate. In the Fomblin Z03 + CHx system, reasonableagreement is found between the functional form of the thickness-dependent surface free energy changedetermined from the evaporation kinetics and that obtained from previous contact angle measurements. Thetemperature dependence of the free energy is used to derive expressions for the entropy and the attractivepotential energy of the confined liquid film. In the case of an ultrathin, completely wetting fluid, the magnitudeof the attractive potential energy increases, and the film entropy decreases, with decreasing film thickness.
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