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Monolayer lubricant films on magnetic disk surfaces consist of bonded and mobile molecules whose different adsorption states may result in different adhesion properties, which directly impact the stability of ultra-low flying head-disk interfaces. By measuring the adhesion force and the surface coverage as a function of bonding ratio, we investigated the contributions of the bonded and mobile molecules to the adhesion of monolayer perfluoropolyether Zdol4000 and Z03 films. The experimental results, which agree with our theoretical results, indicate that bonded lubricant molecules decrease adhesion by increasing the surface coverage, whereas the ability of mobile lubricant molecules to reduce adhesion depends on their adsorption strength with the substrate. Mobile molecules with polar end groups tend to adsorb strongly on a disk, thereby increasing the surface coverage and decreasing the adhesion force.