surface excess

Surface excess refers to the amount of a substance, like a dye or oil, that accumulates at the interface between two phases, such as a liquid and a gas or two liquids. It's the difference between how much of the substance is present in the entire system and how much is in the bulk regions away from the interface. Essentially, surface excess quantifies how much of the substance "prefers" to be at the surface, often because it interacts differently there than inside the bulk. This concept is important in understanding phenomena like emulsions, adsorption, and surface tension.