Forces between two glass surfaces with adsorbed hexadecyltrimethylammoniumsalicylate

Forces have been measured for hexadecyltrimethylammonium salicylate (C(16)T ASal) layers on glass beads. During the inward process, hydrophobic attract ion occurred at lower adsorption of C(16)TASal and electrostatic repulsion interactions happened at higher adsorption. While the jump-in phenomenon wa s observed for solutions of concentrations below the critical micelle conce ntration (cmc = 0.15 mM), the step-in phenomenon was characteristic for sol utions at the cme and above the cmc, suggesting the push-out of adsorbed C( 16)TASal layers and/or inserted micelles. The remarkable pull-off phenomeno n on the outward process occurred for all solutions, indicating a strong in teraction between C(16)TASal molecules. For aqueous 0.15 mM C(16)TASal solu tions of various NaSal concentrations, on the inward process, the electrost atic repulsive interaction decreased with adding NaSal. This is due to the electrostatic shielding by salt excess. The height of the force wall on the inward process reached a maximum at 0.01 M NaSal, but the interlocking bet ween molecules on two surfaces during the outward process was minimized at 0.1 M NaSal. These tendencies, which are different from that of the electro static repulsion interaction, imply the strong cohesion between adsorbed C( 16)TASal layers.