Probing the Three-Dimensional Structure of DTAB Surfactants
應用說明
Atomic force microscopy (AFM) provides a unique direct-visualization tool to study the three-dimensional structure of adsorbed surfactants on solid surfaces. In the limited body of reported topography data, however, the heights of observed surfactant aggregates were significantly lower than expected values.
The reduced corrugation on an AFM topography image is usually due to the fact that the surface adsorbates are soft compared to the spring constant of the cantilever and the applied force is too high. The technique commonly used to image weakly adsorbed surfactant aggregates in contact mode AFM is to maintain a force within the pre-contact doublelayer electrostatic interaction range1, 2. Generally, the force is on the order of 101 to 102 pN. The difficulty encountered in obtaining uncompressed topography images on adsorbed surfactant aggregates seemed to suggest that such small forces were still too high for the soft surfactants.
Alternatively, MAC Mode from Keysight Technologies, Inc. enables precision control of the oscillation of a magnetically coated AFM cantilever and makes it possible to image with an average force well below 1 pN. This lower force minimizes the distortion of soft molecules. In our work, topography structures of cationic surfactant aggregates, dodecyltrimethylammonium bromide (DTAB), adsorbed on a negatively charged hydrophilic mica surface above the critical micelle concentration (cmc) have been studied under different imaging conditions.