Extended DLVO theory: Electrostatic and non-electrostatic forces in oxide suspensions

According to classical DLVO theory all ions of background salt solution with the same ionic charge should result in the same effective force between colloidal particles. However, the relative effectiveness of different ions in influencing forces between ceramic oxide surfaces follows either a reversed Hofmeister sequence or a direct Hofmeister sequence depending on the type of oxide and if the pH is above or below the isoelectric point (iep). This ion specificity is inexplicable in classical double layer theory that deals only with pure electrostatic forces acting between the ions and the colloidal particles. A theoretical explanation is given here. At, and above, biological salt concentrations other, non-electrostatic (NES) ion specific forces act that are ignored in such modeling. In this overview we present the basic theory for the double layer near a single oxide surface and for the extended DLVO forces between oxide colloidal particles that accounts for these NES forces. We will demonstrate that ion specificity can be understood to a large degree once NES forces are included consistently in the non-linear theory.