  Although medium chain length insoluble amphiphiles are well known to form
gaseous and liquid expanded phases on an air/water interface, the situation for
the soluble case is less clear. We perform molecular dynamics simulations of
model surfactant molecules dissolved in a bulk liquid solvent in coexistence
with its vapor. Our results indicate a transition in both soluble and insoluble
surfactants: a plateau in surface tension vs. surface coverage, whose
instantaneous configurations display two phase coexistence, along with
correlation functions indicating a transition to gaseous to liquid-like
behavior.
