Role of the hydration force in the stability of colloids at high ionic strengths

It is shown that the repulsion between colloidal particles or emulsion drop lets depends both on the surface charge density and on the surface dipole d ensity, the latter being a result of the presence of ion pairs on the surfa ce. For illustration purposes, one considers the case in which a surfactant , such as sodium dodecyl sulfate, is adsorbed on the surface of droplets or particles. As the concentration of electrolyte (NaCl) increases, the charg e on the surface decreases, and the number of ion pairs increases, because of the association-dissociation equilibrium. At relatively low salt concent rations, the repulsion due to the double layer is dominant and decreases as the electrolyte concentration increases. At relatively high electrolyte co ncentrations, the hydration repulsive force due to the ion pairs present on the surface becomes dominant. Consequently, as the salt concentration incr eases, the total repulsion decreases and passes through a minimum, after wh ich it increases. If the hydration repulsion is large, the emulsion or coll oidal system will remain stable at any electrolyte concentration. If the hy dration repulsion is small, the system will be stable only for sufficiently low electrolyte concentrations. At intermediate strength of the hydration repulsion, the stability depends on the size of the particles or droplets a nd the Hamaker constant. The rate of coagulation of particles of small radi i and small Hamaker constants reaches a maximum and then decreases with inc reasing ionic strength. For particles of large radii, the increase of the m aximum of the interaction energy with increasing electrolyte concentration can be so large (similar to 30 kT) that it can forbid the coagulation at th e primary minimum; however, the particles can aggregate at the secondary mi nimum, which is deep.