Characterization of Phase Separation in Mixed Surfactant Films by Liquid Tapping Mode Atomic Force Microscopy

Two-dimensional phase separation in Langmuir films comprising mixtures of charged and neutral surfactants transferred onto graphite was observed by liquid tapping mode atomic force microscopy (LTM-AFM). Resolution of the different phases was based on the difference in surface charge density, which resulted in a difference in tip-sample electrostatic interaction and a consequent apparent difference in height despite the identical tail lengths of the two surfactants. Exploration of the effects of temperature and subphase ionic strength showed that phase separation tended to occur at low temperature and high ionic strength, while at high temperature and low ionic strength the monolayer was homogeneous. The method offers a sensitive new approach to identifying distinct phases in Langmuir films.