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 c harged and neutral surfactants transferred onto graphite was observed by li quid tapping mode atomic force microscopy (LTM-AFM). Resolution of the diff erent phases was based on the difference in surface charge density, which r esulted in a difference in tip-sample electrostatic interaction and a conse quent apparent difference in height despite the identical tail lengths of t he two surfactants. Exploration of the effects of temperature and subphase ionic strength showed that phase separation tended to occur at low temperat ure and high ionic strength, while at high temperature and low ionic streng th the monolayer was homogeneous. The method offers a sensitive new approac h to identifying distinct phases in Langmuir films.