Use of the surface forces apparatus to directly measure the influence of self-assembled monolayers on the adhesion and deformation of rough solids

A contact mechanics approach is used to study the role of model sub-boundary lubricants on the deformation and adhesion between rough solids. Self-assembled monolayers of n-octadecanethiol and 1-hydroxylunicosanethiol adsorbed to microscopically rough gold are brought into contact with molecularly smooth mica using a surface forces apparatus. Deformations at the interface and concurrently those in the bulk were measured while controlling applied loads. The effect of the SAMs is deduced by comparing the behavior of these systems to that of uncoated gold as previously put forward. Most significantly, the presence of a self-assembled monolayer sharply defines the strength of adhesion as measured by precise pull-off forces for given loading conditions. As compared to uncoated gold, the coated gold also exhibits a lesser amount of microdeformation under equivalent loading. Moreover the microdeformations are reversible upon relief of the load. This elastic regime is used to estimate the compressibility of each self-assembled monolayer. While the deformations at the interface are recoverable, deformations of the bulk are not. The hysteresis of the bulk is attributed to the monolayer; the more adherent system of 1-hydroxylunicosanethiol and mica exhibits greater hysteresis than contact between n-octadecanethiol and mica.