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′ ≈ 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.