Contrary to most textbook descriptions, real fluid mixtures show composition and structural heterogeneities on a microscopic level. This micro-heterogeneity plays a major role in governing solubility and surface tension. In the Siepmann group, we use large-scale Monte Carlo simulations to investigate the thermophysical properties of multi-component fluid mixtures. An example of this can be seen with surfactants, which play an important role in many biological and technological processes. Common ionic or polar surfactants consist of a long hydrocarbon chain that is hydrophobic and a strongly hydrophilic head group. Hydrofluorocarbons (HFCs) of the form F(CF2)m(CH2)nH, abbreviated as FmHn, have been identified as a new primitive surfactant class. In recent multi-component simulations of the HFC surfactant system done in our group, strong segregation was found in the local composition of these mixtures. The figure below shows the spatial distribution of the local hydrocarbon (CH3 and CH2 segments) mole fraction for mixtures containing 9 mole-% solutions of F8H8 (left), F8H12 (middle), and F8H16 (right) surfactants in an n-perfluorooctane solvent. There is a strong preference for hydrocarbons segments in these mixtures to aggregate, and the coloring, which is scaled according to mole fraction as indicated in the color bar beneath the figure, shows the regions that are substantially enriched in hydrocarbon segments. These systems contain a total of 3540 molecules and the linear dimension of the periodic simulation boxes is about 100 Å.