In the idealized model, the chemical components of the  and  bulk phases remain unchanged except when approaching the dividing surface.  The total moles of any component (Examples include: water, ethylene glycol etc.) remains constant in the bulk phases but varies in the surface phase for the real system model as shown below.
In the real system, however, the total moles of a component varies depending on the arbitrary placement of the dividing surface.  The quantitative measure of adsorption of the -th component is captured by the surface excess quantity.  The surface excess represents the difference between the total moles of the -th component in a system and the moles of the -th component in a particular phase (either  or ) and is represented by:
where  is the surface excess of the -th component,  are the moles,  and  are the phases, and  is the area of the dividing surface.
 represents excess of solute per unit area of the surface over what would be present if the bulk concentration prevailed all the way to the surface, it can be positive, negative or zero. It has units of mol/m.