Phase transitions of soluble surfactants at a liquid-vapor interface

Although medium chain length insoluble amphiphiles are well-known to form g aseous and liquid expanded phases on an air/water interface, the situation for the soluble case is controversial. We perform molecular dynamics simula tions of model surfactant molecules dissolved in a bulk liquid solvent in c oexistence with its vapor. Our results indicate a transition in both solubl e and insoluble surfactants: a plateau in surface tension versus surface co verage, whose instantaneous configurations display two-phase coexistence, a long with correlation functions indicating a transition from gaseous to liq uidlike behavior.