In the presence of two phases ( and ), the surface (surface phase) is located in between the phase  and phase .  Experimentally, it is difficult to determine the exact structure of an inhomogeneous surface phase that is in contact with a bulk liquid phase containing more than one solute.  Inhomogeneity of the surface phase is a result of variation of mole ratios.  A model proposed by Josiah Willard Gibbs proposed that the surface phase as an idealized model that had zero thickness.  In reality, although the bulk regions of  and  phases are constant, the concentrations of components in the interfacial region will gradually vary from the bulk concentration of  to the bulk concentration of  over the distance x.  This is in contrast to the idealized Gibbs model where the distance x takes on the value of zero.  The diagram to the right illustrates the differences between the real and idealized models.