Cationic-Surfactant-Coated Mica Surfaces below the Critical Micellar Concentration: 1. Patchy Structures As Revealed by Peak Force Tapping AFM Mode

The morphology and structure of the self-assembled surfactant aggregates at the solid-liquid interface remain controversial. For the well-studied system of cationic cetyltrimethylammonium bromide (C(16)TAB) adsorbed onto the opposite negatively charged, atomically smooth mica surface, a variety of surface aggregates have been previously reported: AFM imaging pointing to cylinders and surface micelles as opposed to mono/bilayer-like structures revealed by neutron and X-ray reflectometry, NMR, spectroscopic techniques, and numerical simulations. To reconcile with the latter results, we revisit the morphometry of the C(16)TAB-coated mica surfaces using the recent peak force tapping (PFT-AFM) mode that allows fragile structures to be imaged with the lowest possible applied force. The evolution of the structural organization at the mica-water interface is investigated above the Krafft boundary over a wide concentration range (from 1/1000 to 2 cmc) after long equilibration times to ensure thermodynamic equilibrium. A complex but fairly complete picture has emerged: At very low concentrations, the C(16)TAB surfactants adsorb as isolated molecules before forming small clusters. Above 1/140 cmc, monolayer-like stripes are formed. As the concentration is increased, a connected network of these patches progressively covers the mica substrate. Above 1/80 cmc, bilayer-like patches build on top of the underlying monolayer, and ultimately a complete bilayer (at about half the cmc) covers the entire mica substrate. Thanks to the less invasive PFT-AFM imaging mode, our observations not only agree with the theoretical predictions and numerical simulations but also reconcile, at last, the direct observations by means of the AFM imaging technique with the results obtained with other techniques.