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.