Adhesion and friction experiments were carried out with a variety of surfactant monolayer-coated surfaces using the surface forces apparatus technique. The equilibrium and time-dependent adhesion and the interfacial or ''boundary'' friction of these surfaces were measured as a function of monolayer density and phase state, load, temperature, contact time, sliding speed, and relative humidity. The aim was to investigate the correlations between the friction and adhesion of these fairly ideal, molecularly smooth, and chemically homogeneous surfaces. The results show that friction and adhesion are not generally related if one only compares the magnitude of the friction force (or coefficient of friction) with the adhesion force (or adhesion energy). However, since friction is an energy-dissipating process, one might expect to find a relation between friction and adhesion hysteresis. This was confirmed by comparing the adhesion energy hysteresis during loading-unloading cycles with the friction forces measured under similar conditions. We find that-in spite of its great complexity and dependence on many parameters-the boundary friction force varies with temperature, sliding velocity, load, and relative humidity in highly systematic ways, which can be represented by a dynamic ''friction phase diagram''. We also derive a simple approximate equation relating boundary friction to adhesion energy hysteresis which satisfactorily accounts for the experimental results. The correlations we have observed between friction and adhesion hysteresis also appear to apply to other tribological and energy-dissipating systems, and it is proposed that a general and practical theoretical framework-similar to that used to describe energy-dissipating, such as viscous, forces in bulk polymer systems-can be developed with the capability of predicting the tribological performance of boundary lubricants under different experimental conditions.
