  Understanding non-equilibrium phenomena, such as growth, and connecting them
to equilibrium phase behavior is a major challenge, in particular for complex
multicomponent materials. We use X-ray reflectivity to determine the surface
roughness of binary mixtures of several prototypical organic compounds. By
analyzing the roughness as a function of composition, we find a systematic
behavior depending on the bulk phase behavior in terms of intermixing,
co-crystallization or phase separation. Supported by kinetic Monte Carlo
simulations, we provide evidence that the growth behavior can be rationalized
by a lowered step edge barrier in the mixed films which is induced by reduced
in-plane crystallinity.
