Reorganization and crystallite formation in Langmuir-Blodgett films

The atomic force microscope (AFM) was used to monitor the reorganization and approach to equilibrium of Langmuir-Blodgett monolayers and multilayers of cadmium fatty acid salts submerged under aqueous subphase for extended periods. We have found that uniform, constant thickness monolayer and multilayer films are unstable to bilayer step defects that originate at isolated sites and quickly spread to cover the entire film (30 min for cadmium arachidate). Although the network of defects originally appears random and meandering, within 10 h features with straight edges begin to form and eventually (48 h submerged) the entire film comprises high islands with straight edges aligned with sixfold symmetry. The direction of island edges are explicitly shown to correspond to a particular molecular lattice direction by comparison of molecular resolution images with lower magnification images. The driving force behind the reorganization is the formation of headgroup-headgroup interfaces stabilized by the strong interaction of cadmium ions at the cost of substrate-headgroup and water-headgroup interfaces.