Normal and shear forces between charged solid surfaces immersed in cationic surfactant solution: the role of the alkyl chain length.
Using a surface force balance (SFB), we measured the boundary friction and the normal forces between mica surfaces immersed in a series of alkyltrimethylammonium chloride (TAC) surfactant solutions well above the critical micelle concentration (CMC). The surfactants that were used--C14TAC, C16TAC, and C18TAC--varied by the length of the alkyl chain. The structures of the adsorbed layers on the mica were obtained using AFM imaging and ranged from flat bilayers to rodlike micelles. Despite the difference in alkyl chain, all the surfactant solutions reduce the friction between the two mica surfaces enormously relative to immersion in water, and have similar friction coefficients (μ ≈ 0.001). The pressure at which such lubrication breaks down is higher for the surfactants with longer chain lengths and indicates that an important role of the chain length is to provide a more robust structure of the adsorbed layers which maintains its integrity to higher pressures.
Department of Materials and Interfaces, Weizmann Institute of Science , Rehovot 76100, Israel.
Gilad
G
Silbert
Nir
N
Kampf
Jacob
J
Klein