Antifoams and defoamers are generally inert chemicals consisting of a liquid (e.g., silicone, mineral oils, or hydrophobic surfactants) and a hydrophobic solid (e.g., fatty acids, fatty alcohols, silica, or wax). Ultimately, the antifoam/defoamer you select should have a balance of compatibility and incompatibility. It should be compatible enough that it effectively disperses in the foaming medium.