Ionic surfactant films imaged by atomic force microscopy

Forces acting on atomic force microscope (AFM) tips responsible for image formation are measured during scanning of films of ionic surfactant molecules adsorbed from aqueous solutions onto hydrophilic substrates. Near the critical micelle concentration mica substrate images show aggregate regions at the interface. Force versus distance measured curves show that patches form a thicker structure than the formed at partially covered regions, in agreement with the fact that at patches the adsorbates are perpendicularly oriented to the substrate plane. However, AFM topographic images registered at low scanning speed (5 mum/s) show that these patched regions appear as holes, forming inverted images. In AFM imaging of soft structures, as surfactants or biological material, inverted in contrast with images may be observed when there is a specific tip penetration through each scanned layer. This penetration is adjusted by changing the tip force set point, consequently different topographic profiles are obtained. The precise force set point to obtain the correct contrast in scanned images is obtained by the analysis of the force versus distance curves that show the normal to the scanned plane structure profile. Adsorption patterns as a function of time may be conveniently monitored and the adsorption rate may be determined. (C) 2001 Published by Elsevier Science B.V.