Molecular dynamics simulations are used to study the formation of gaseous and liquid expanded phases of surfactants on a liquid/vapor interface. Both insoluble and soluble surfactants are considered, modeled as freely jointed chains in a monatomic solvent with appropriate Lennard-Jones interactions. For both insoluble and soluble cases our results indicate that the surface tension as a function of coverage shows a plateau close to the clean interface value until a critical surface concentration, beyond which the surface tension lowers steeply. For the soluble case, we also detail a complete Gibbs construction for the surface excess and report an adsorption isotherm. Snapshots of the positions of the molecules, together with analysis of correlation functions, show that these model surfactants assemble from solution and exhibit coexistence in the plateau of isolated surfactants and liquid aggregates. We have identified this coexistence as a gas-liquid phase transition, in agreement with recent experimental evidence in the soluble case.