Carbon dioxide cleaning and separation systems

A separation method includes (a) providing a heterogeneous separation system, the heterogeneous cleaning system comprising CO2 in a first phase and an oil in a separate second phase; (b) entraining a material to be separated in the second phase; (c) wholly or partially solubilizing the second phase in the first phase to produce a separation system in which said material to be separated is insoluble; and then (d) separating the material from the system. The separating step is preferably followed by the step of (e) recovering at least a portion of the oil. The system is useful in a variety of applications, including cleaning (particularly metal cleaning), polymerization, extraction, coating, and particle formation and treatment.