Phase transitions of soluble surfactants at a liquid-vapor interface

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 controversial. 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 versus surface coverage, whose instantaneous configurations display two-phase coexistence, along with correlation functions indicating a transition from gaseous to liquidlike behavior.