text: The presence of an interface influences generally all thermodynamic parameters of a system. There are two models that are commonly used to demonstrate interfacial phenomena: the Gibbs ideal interface model and the Guggenheim model. In order to demonstrate the thermodynamics of an interfacial system using the Gibbs model, the system can be divided into three parts: two immiscible liquids with volumes  and  and an infinitesimally thin boundary layer known as the Gibbs dividing plane () separating these two volumes.
  The total volume of the system is:
  All extensive quantities of the system can be written as a sum of three components: bulk phase , bulk phase , and the interface . Some examples include internal energy , the number of molecules of the th substance , and the entropy .
  While these quantities can vary between each component, the sum within the system remains constant. At the interface, these values may deviate from those present within the bulk phases. The concentration of molecules present at the interface can be defined as:
  where  and  represent the concentration of substance  in bulk phase  and , respectively.
  It is beneficial to define a new term interfacial excess  which allows us to describe the number of molecules per unit area: