In a colloidal dispersion, the colloid-colloid interaction potential is approximated as the interaction potential between two hard spheres. For two hard spheres of diameter of , the interaction potential as a function of interparticle separation is:
called the hard-sphere potential where  is the center-to-center distance between the spheres.
If both colloids and depletants are in a dispersion, there is interaction potential between colloidal particles and depletant particles that is described similarly by the hard-sphere potential. Again, approximating the particles to be hard-spheres, the interaction potential between colloids of diameter  and depletant sols of diameter  is:
where  is the center-to-center distance between the spheres. Typically, depletant particles are very small compared to the colloids so 
The underlying consequence of the hard-sphere potential is that dispersed colloids cannot penetrate each other and have no mutual attraction or repulsion.