Surfactant micelles in aqueous solution: critical resolution in AFM
It’s a widely spread idea that performing AFM in liquids is a rather complicated research approach. Actually, many tasks related to the investigation of molecular structures and com¬plexes in a liquid environment claim high demands on the SPM equipment.The picture below shows an AFM scan of surfactant molecules dynamically self-assembled into parallel, hemicy¬lindrical micelles on a hydrophobic graphite surface.
Sample: 10 mM aqueous solution of cetyltrimethylammonium bromide surfactant in contact with HOPG.
AFM equipment: NTEGRA Prima , Constant force imaging mode in liquid cell using a very low set point ("soft contact mode"– force ~100pN).
AFM probe: CSG01, 0.03 N/m
Author: Hannes Schniepp, The College of William and Mary, Williamsburg, VA, USA.
Observation of the dynamically self-assembled molecular structure in this case was a chal¬lenge. The imaging mode used is the “soft contact mode” – scanning with constant force using a very low set point. The vertical force of the probe needs to be kept as close as possible to the set point, in this case ~ 100 pN. If the force exceeds this value, the native molecular assembly will be disturbed (the periodicity of the micelles is ~5 nm.). For lower forces, the con¬trast vanishes.Note that the vertical topography modulation is only 0.1-0.2 nm,but still signifi¬cantly above the noise level. This example shows also that NTEGRA nanolaboratory provides enough sensitivity and stability to study dynamic molecular structures in an aqueous environ¬ment.
