A film one molecule thick. It is often referred to as a monolayer. Films that form at surfaces or interfaces are of special importance. Such films may reduce friction, wear, or corrosion, or may stabilize emulsions, foams, and solid dispersions. Thin films on water surfaces can reduce evaporation losses, though the spreading of thin films on such surfaces can represent a serious environmental problem. In some instances, the film spreads by itself and is essentially insoluble in the substrate (an insoluble monolayer). In other cases, the film molecules have a low substrate solubility and concentrate at the surface (a Gibbs monolayer).