Thermodynamics of confined perfluoropolyether films on amorphous carbon surfaces determined from the time-dependent evaporation kinetics

Abstract

The thermodynamics of ultrathin (≤10 angstroms) perfluoropolyether (PFPE) films in contact with amorphous carbon surfaces (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 molecular weight of 4000 g mol-1 and a fractionated (Mw/Mn = 1.05) Demnum sample of average molecular weight 2200 g mol-1. Data is also presented for the evaporation of a fractionated (Mw/Mn = 1.08) sample of the hydroxyl-terminated Fomblin Zdol (Mw = 2100 g mol-1). Evaporation of the nonfunctionalized PFPEs from amorphous carbon follows nonclassical, first-order desorption kinetics having a rate constant that varies inversely with time. Evaporation of the functionalized Fomblin Zdol is also nonclassical; however, the time dependence 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 of the PFPE film. An analytic expression for the dependence of the surface free energy on film thickness and temperature is derived from the time-dependent evaporation rate. In the Fomblin Z03+CHx system, reasonable agreement is found between the functional form of the thickness-dependent surface free energy change determined from the evaporation kinetics and that obtained from previous contact angle measurements. The temperature dependence of the free energy is used to derive expressions for the entropy and the attractive potential energy of the confined liquid film. In the case of an ultrathin, completely wetting fluid, the magnitude of the attractive potential energy increases, and the film entropy decreases, with decreasing film thickness.