The Morton-Kaizerman-Altier (MKA) equation fails to describe experimental s
welling data of polystyrene particles with toluene in the absence of free o
r adsorbed surfactants. A modification of the MKA equation is possible by t
he consideration of a volume work leading to a swelling pressure in analogy
to macroscopic gels. With the modified equation a satisfactory description
of the experimental data is possible if a size dependence of all the three
interaction parameter of the particles with the swelling agent, the swelli
ng pressure, and the interfacial tension of the particles is considered. In
the case of particles with diameters below 50 nm and with high surface cha
rge densities an additional electrostatic repulsion contributes to the volu
me increase during swelling, which is not yet considered in the theory.