  Phase field models for two-phase flow with a surfactant soluble in possibly
both fluids are derived from balance equations and an energy inequality so that
thermodynamic consistency is guaranteed. Via a formal asymptotic analysis, they
are related to sharp interface models. Both cases of dynamic as well as
instantaneous adsorption are covered. Flexibility with respect to the choice of
bulk and surface free energies allows to realise various isotherms and
relations of state between surface tension and surfactant. Some numerical
simulations display the effectiveness of the presented approach.
