Up to now, micelles composed of different surfactants (mixed micelles) are rarely studied with molecular methods. This is in contrast to their importance for pharmaceutical or industrial applications, where it is of great interest to predict the partition behavior for a large set of solutes (screening) within mixed micelles. This work is focused on molecular simulations of phase equilibria in mixed surfactant systems, because mixtures of different types of surfactants (nonionic or ionic) in aqueous solution can change the partition behavior of solutes tremendously. The extension of COSMO-RS for anisotropic phases, named COSMOmic, is computationally efficient and can be used as a screening tool for finding adequate surfactant systems for a specific extraction task. However, it needs micellar structures as an input. Therefore, molecular dynamics (MD) simulations of the self-assembly of pure Brij35 (polyethylene glycol dodecyl ether) and mixtures either with CTAB (cetyltrimethyl ammonium bromide) or SDS (sodium dodecyl sulfate) at different concentrations are performed. The micelles from the self-assembly MD simulations are used to predict the partition behavior of various solutes between micelle and bulk water with COSMOmic. In this way, various micelles of different size and composition are investigated and structural influences on partition equilibria of solute molecules like ephedrine, acetone, toluene, coumarin, isovanillin, ferulic acid, vanillic acid, syringic acid, and phenol are analyzed. For the first time, the self-assembly of pure Brij35 and the mixtures of Brij35/CTAB and Brij35/SDS is studied on an atomistic scale. Significant influences of atomic structure and composition of mixed micelles on partition equilibria are elucidated. The findings of this detailed analysis are in good agreement with experimental data and likely to improve the knowledge and understanding of mixed micellar extraction processes and can pave the way for more practical applications in the future.