Characterization of Repulsive Forces and Surface Deformation in Thin Micellar Films via AFM

Abstract

Here we examine how the force on an AFM tip varies as it approaches micellar surfactant films, and use this information to discern the film's surface structure and Young's Modulus.Rows of wormlike hemi-micelles were created at a graphite interface using 10 mM sodium dodecyl sulfate (SDS). We found that the repulsive force on a silicon nitride tip as it approached the surface was exponential, with a decay length of 2.0±0.1 nm. The addition of Na 2 SO 4 was found to cause a change in this behavior, with a clear split into two exponential regions at concentrations above 1 mM. We also observed that the range of these forces increased with added salt from ~15 nm in pure SDS to ~20 nm at a Na 2 SO 4 concentration of 1.34 mM. These forces were inconsistent with electrostatic repulsion, and were determined to be steric in nature.We show that the behavior at higher salt concentrations is consistent with the theory of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 polyelectrolyte brushes in the osmotic regime. From this, we hypothesize the presence of micellar brushes at the surface that behave similarly to adsorbed polymer chains. In addition, the Young's modulus of the film was taken from data near the interface using Sneddon's model, and found to be 80±40 MPa. Similar experiments were performed with 10 mM dodecylamine hydrochloride (DAH) solutions in the presence of added magnesium chloride. The decay length for the pure DAH solution was found to be 2.6±0.3 nm, and the addition of 1.34 mM of MgCl 2 caused this to increase to 3.7±0.3 nm. No decay length splitting was observed for DAH. We conclude that the behavior at the surface resembles that of an uncharged polymer brush, as the ionic and surface charge densities are much lower for DAH than for SDS.

Revealing Factors Governing Self-Assembly Morphology of Fatty Acid on Graphene Synthesized by Surfactant-Assisted LPE: A Joint MD, SAPT(DFT), and Experimental Study

“…The 3D AFM image (Figure c) reveals some regions which are higher than the hemicylandrical structures. For these areas, several structure can be considered: (i) the adsorbed micelles on the hemicylandrical surface, (ii) two-point wormlike, and (iii) single-point wormlike . The image of AFM and size distribution histogram are presented in Figure .…”

Nanomechanics of self-assembled surfactants revealed by frequency-modulation atomic force microscopy

“…2b). 12,14,15,17,[20][21][22]43 Although Δf linearly increased as the distance was decreased in this regime, the Hertz theory, which predicts a power law of 3/2, 44 can be fitted to the force versus distance curve converted from the Δf curve, which provides the Young's modulus of the micelles, as discussed later. The boundary between the EDL and Hertz regimes can be estimated from the crossover point between the fitted linear line and exponential curve.…”

“…In the gel phase, since the molecules in the micelles are not equilibrated with those in the bulk solution, the micellar hemifusion causes such a large P tip due to the irreversible structural change, which likely to cause a polymer-brush-like force profile. 16,17 Micelle hetero-nanostructure…”

“…To date, static-mode atomic force microscopy (SM-AFM) has successfully revealed micelles’ structures at the nanoscale level. 11–19 However, jump-to-contact has prevented an accurate analysis of their formation-and-disruption processes. 12,14,15,20–22…”

“…To date, static-mode atomic force microscopy (SM-AFM) has successfully revealed micelles' structures at the nanoscale level. [11][12][13][14][15][16][17][18][19] However, jump-to-contact has prevented an accurate analysis of their formation-and-disruption processes. 12,14,15,[20][21][22] Over the past decade, we have initiated the visualization of 3D hydration structures [23][24][25][26][27][28][29][30][31] and surface charge distributions 28,[30][31][32] at the solid-liquid interface using ultra-low noise frequency-modulation AFM (FM-AFM).…”