Exploring the "friction modifier" phenomenon: nanorheology of n-alkane chains with polar terminus dissolved in n-alkane solvent

Dilute solutions of two polar end-functionalized linear alkanes (1-hexadecy lamine and palmitic acid), each dissolved in tetradecane, were confined bet ween two mica surfaces and investigated using a surface forces apparatus mo dified to study shear nanorheology. These two solutions showed similar nano rheological properties that differed from those observed for pure n-alkanes . In static measurements, a "hard wall", rather than an oscillatory force, was observed as a function of film thickness. The polar alkane component fo rmed a weakly adsorbed single layer at each mica surface, disrupting the la yered structures found in neat n-tetradecane. In dynamic experiments at low shear amplitude, the storage modulus G(') exceeded the loss modulus G(") a t low frequencies; above some characteristic frequencies G(") increased suc h that G(') approximate to G("), indicating significantly more energy loss through viscous modes at higher frequency. When the amplitude was varied at fixed frequency, no stick-slip was observed and the limiting value of the shear stress at high effective shear rate was an order of magnitude less th an for unfunctionalized n-alkanes at similar loads. Together, these results show that the addition of a small amount of polar alkane component, by dis rupting the layered structures that would have been formed in the neat n-al kane, is effective in suppressing static friction and reducing kinetic fric tion in the boundary lubrication regime.