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

Abstract

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 (C16TAB) 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/bilayered-like structures revealed by neutron and X-ray reflectometry, NMR, spectroscopic techniques, and numerical simulations. In order to reconcile with the latter results, we revisit the morphometry of the C16TAB-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 cmc to 2 cmc) after long-equilibration times to insure thermodynamic equilibrium. A complex but fairly complete picture has emerged: At very low concentrations the C16TAB surfactants adsorb as isolated molecules before forming small clusters. Above 1/140 cmc, monolayeredlike stripes are formed. As the concentration is increased, a connected network of these patches covers progressively the mica substrate. Above 1/80 cmc, bilayered-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 agree not only 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.

Shaping Metallic Nanolattices: Design by Microcontact Printing from Wrinkled Stamps

“…For deeper investigations, further AFM modes were performed, such as phase contrast imaging, which depends on many material properties such as adhesion, stiffness, and friction (see tapping mode AFM results in Figure S12 in the Supporting Information). [ 56,57 ] The frequency‐modulated Kelvin probe force microscopy (FM‐KPFM), however, promises to provide a more pronounced contrast here. KPFM is very sensitive to even small differences in work functions of materials, which in turn depend on surface potential.…”

Imaging live bacteria at the nanoscale: comparison of immobilisation strategies

“…Such modes may also be used to acquire nanomechanical maps of the bacteria. 38 When using AFM, BL21 on PLL were well adhered, bacteria were smooth and resolution was high enough to see the porin lattice (with ∼7 nm periodicity in the outer membrane 4 ) cover- ing the surface. When imaging bacteria adhered with Cell-Tak™, unidentified aggregates approximately 10-20 nm high and 50-100 nm wide were observed on both BL21 and MG1655, making the samples unusable for high resolution studies.…”

Dramatic Slowing Down of Oil/Water/Silica Contact Line Dynamics Driven by Cationic Surfactant Adsorption on the Solid

“…The amount of CTAB adsorbed on silica increases with bulk concentration, or surfactant chemical potential, in three steps as follows: slow increase below Γ*, step-like increase above Γ*, and then saturation to a plateau value Γ ∞ of 2.4 μmol•m −2 similar to that measured in literature. 46,48−50 The shape of the isotherm results from the arrangement of the surfactants on the solid surface, and the present picture emerging from literature is the following: 51,52 isolated aggregates are first detected around a bulk concentration 10 −5 mol• L −1 . As bulk concentration further increases, monolayered-like patches of surfactants progressively form from these aggregates, grow, and finally connect to form a network, until a complete monolayer is obtained at larger concentration.…”

“…We show that: (i) Only electrostatically bound surfactants at the silica surface contribute to friction at the contact line, through contact line pinning on surfactants patterns. 51 (ii) Surfactant bound to silica through weaker interactions desorbs as the contact line recedes, without contributing to friction. (iii) The gain in affinity for oil of surfactant-covered silica has much less positive impact on the water dewetting dynamics than does heterogeneity-induced contact friction in a negative way.…”