Stability of soft colloidal particles in a salt-free medium.

The stability of a salt-free dispersion containing soft spherical colloidal particles is investigated theoretically. Here, a particle comprises a rigid core and an ion-penetrable membrane layer; the ionic species in the liquid phase come solely from those dissociated from the functional groups in the membrane layer. We show that, similar to the case of a salt-free rigid dispersion, the total energy, which comprises the electrical energy and the van der Waals energy, is always positive far away from the surface of a particle and does not have a secondary minimum. Both the Derjaguin approximation for the estimation the electrical energy of two spheres and the criteria for the critical coagulation concentration in the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory are inapplicable. If the molar concentration of the fixed charge in the membrane layer exceeds ca. 0.1 M, the stability of a dispersion remains roughly the same. The maximum allowable particle concentration for a stable dispersion for the case of soft particles is lower than that for the case of the corresponding rigid particles.