Interaction of cationic hydrophobic surfactants at negatively charged surfaces investigated by atomic force microscopy

Atomic force microscopy was used to study the adsorption of the surfactant octadecyl trimethyl ammonium chloride (C₁₈TAC) at a low concentration (0.03 mM) to negatively charged surfaces in water. Atomic force microscopy tips were functionalized with dimethyloctadecyl(3-tripropyl)ammonium chloride (C₁₈TAC-si) or N-trimethoxysilylpropyl-N,N,N-trimethylammomium chloride (hydrophilpos-si) to facilitate imaging of the adsorbed surfactant without artifacts. Tapping mode images and force measurements revealed C ₁₈TAC patches, identified as partial surfactant bilayers or hemimicelles. The forces controlling the adsorption process of the C ₁₈TAC to a negatively charged surface were investigated by measuring the forces between a C₁₈TAC-si or a hydrophilpos-si tip and a silica surface in the presence of varying concentrations of either NaCl or NaNO ₃. Screening of forces with an increasing NaCl concentration was observed for the C₁₈TAC-si and hydrophilpos-si tips, proving an electrostatic contribution. Screening was also observed for the hydrophilpos-si tip in NaNO₃, whereas a long-range attraction was observed for the C₁₈TAC-si tip for all NaNO₃ concentrations. These results indicate that screening of the forces for the C₁₈TAC-si tip depended on the type and/or size of the anion, possibly due to a different probability of the anions to enter the silane layers. The interaction of C₁₈TAC patches with C₁₈TAC-si tips in the presence of NaCl and the interaction of the patches with hydrophilpos-si tips in either NaCl or NaNO ₃ were repulsive and independent of the number of force curves measured, indicating a stable, positively charged C₁₈TAC patch. However, the forces measured between the patches and a C₁₈TAC-si tip in NaNO₃ depended on the number of force curves measured, indicating a change in patch structure induced by the first interaction.