Nanomechanical Characterization of Micellar Surfactant Films via Atomic Force Microscopy at a Graphite Surface

Abstract

In this work, we study the mechanical properties of sodium dodecylsulfate (SDS) and dodecylamine hydrochloride (DAH) micellar films at a graphite surface via atomic force microscopy (AFM). Breakthrough forces for these films were measured using silicon nitride cantilevers, and found to be 1.1 ±0.1 nN for a 10 mM DAH film and 3.0 ±0.3 nN for a 10 mM SDS film. For 10 mM SDS films, it was found that the addition of 1.5 mM of NaCl, Na 2 SO 4 , or MgCl 2 produced a 50-70% increase in measured breakthrough force. Similar results were found for 10 mM DAH films when NaCl and MgCl 2 were added. A model was developed, based upon previous work on lipid films and CMC data gathered via spectrofluorometry measurements, to produced the observed differences. The pH of the bulk solution was varied for both 10 mM SDS and DAH films, and was found to have little effect on breakthrough force.

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

“…Equation (12) implies that the measured decay length should increase with surface density, which is consistent with our observations for DAH in the presence Some attempts were also made to estimate the Young's modulus of the DAH film using equations (10) and (11), but we were unable to obtain reliable data about the strain at breakthrough. It appears that the DAH films are much less sturdy than their SDS counterparts at these salt concentrations, and are unable to resist the loading force of the tip for a long enough time to yield usable data.…”

“…The addition of 2 mM of Na 2 SO 4 would cause a decrease in this length to 2.8 nm, which is still significantly different from our ߣ ଶ . Using a tip charge density of -10 mC/m 2 consistent with aged silicon nitride, 15 a ߪ ௦ of -60 mC/m 2 for our surface, 12 a tip radius of 6 nm, and a Debye length of 3.3 nm we arrive at a total force at the surface of 0.2 nN after using equation (2). This is much smaller than our observed forces of 3.0 ± 0.3 nN, even though the tip charge density we are using is conservatively large.…”

“…In the salted regime, the concentration of ions in the bulk exceeds that in the brush, which leads to screening of the charges along the chain and a general contraction of the film. In our previous work, 12 we used modified Grahame equations to determine that the counterion concentration was roughly 1.5 M at the hemi-micellar surface and 8-10 mM in the bulk. This implies that we should be looking at the osmotic regime, as our concentration of ions near the surface should always be greater than in the bulk due to the presence of a diffuse double layer.…”

“…24 As salt is added to solution the critical micelle concentration (CMC) drops significantly, indicating that micellar structures are becoming more thermodynamically stable. 12,25 This increases the likelihood of observing larger micellar structures at the surface and results in a larger observed micellar brush length. It may also indicate that the reason equation (7) 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 17 low salt concentrations is that the micellar brush length falls below some critical value beneath which the theory breaks down.…”

“…For DAH the surface charge density is much lower than it is for SDS, and the counterion density at the surface 24 is only expected to be around 0.2 M even at the highest salt concentration. 12 When compared with the expected 1.5 M counterion concentration for SDS it becomes clear that the effects of osmotic swelling will be limited for DAH, and behavior will resemble that of an uncharged polymer.…”