  We deeply investigate a simple model representative of the recently
synthesized Janus particles, i.e. colloidal spherical particles whose surface
is divided into two areas of different chemical composition. When the two
surfaces are solvophilic and solvophobic, these particles constitute the
simplest example of surfactants. The phase diagram includes a colloidal-poor
(gas) colloidal-rich (liquid) de-mixing region, which is progressively
suppressed by the insurgence of micelles, providing the first model where
micellization and phase-separation are simultaneously observed. The coexistence
curve is found to be negatively sloped in the temperature-pressure plane,
suggesting that Janus particles can provide a colloidal system with anomalous
thermodynamic behavior.
