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-hexadecylamine and palmitic acid), each dissolved in tetradecane, were confined between two mica surfaces and investigated using a surface forces apparatus modified to study shear nanorheology. These two solutions showed similar nanorheological 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 formed a weakly adsorbed single layer at each mica surface, disrupting the layered structures found in neat n-tetradecane. In dynamic experiments at low shear amplitude, the storage modulus G(') exceeded the loss modulus G(") at low frequencies; above some characteristic frequencies G(") increased such 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 than for unfunctionalized n-alkanes at similar loads. Together, these results show that the addition of a small amount of polar alkane component, by disrupting the layered structures that would have been formed in the neat n-alkane, is effective in suppressing static friction and reducing kinetic friction in the boundary lubrication regime.