Multistep adsorption of perfluoropolyether hard-disk lubricants onto amorphous carbon substrates from solution

Abstract

We have used a quartz crystal microbalance (QCM) to study the solution-phase adsorption of the perfluoropolyether lubricants Fomblin ZDOL and Demnum SA from perfluorohexane and perfluorobutylmethyl ether onto model carbon-coated hard-disk surfaces. The validity of the QCM results and the applicability of the well-known Sauerbrey equation have been verified through combined QCM and surface plasmon resonance experiments. The adsorption isotherms are relatively simple and exhibit a Langmuirlike behavior. However, the plateau region is sometimes ill-defined, and the adsorption process is strongly dependent on the history of the system. The adsorbed mass at a particular concentration shows a pronounced dependence on the equilibration time between successive concentration changes, even though the adsorption kinetics appears to have equilibrated. In the first few seconds, the adsorption process is diffusion limited, with the adsorbed mass quickly increasing to a maximum before decaying to the equilibrium value. To interpret these results, we present an adsorption model involving a site-specific adsorption process, fast initial adsorption, and surface reorganization that is able to qualitatively reproduce the observed kinetic and equilibrium behavior.