5 - Quantum mechanical forces in condensed media

Summary

Lifshitz theory and its extensions: an overview

Molecular recognition

The ideas behind the DLVO theory of long-range forces in colloidal particle interactions and of the electrical double layer that we have just outlined held centre stage in colloid science for at least 70 years. Quantitatively, as already remarked, agreement between experiment and theory was illusory, except at salt concentrations less than 10⁻² M, or at most 10⁻¹ M. It was illusory in the sense that while classical theory captured some essentials of the forces between ions and macromolecules, ion specificity was still missing. While the electrical double-layer forces decayed exponentially as predicted, the magnitude of the forces changed with a change in counterion or co-ion. Recall our typical examples. A change in counterion in a background electrolyte from Br⁻ to OAc⁻ could produce an increase in magnitude of the forces by a factor of 50 to 100! The same will be seen to occur with different surfaces with a change from Na⁺ to Li⁺. We will see much more of this specificity later. These differences depended on the nature of the charged interacting surfaces and, at the same electrolyte concentration, on the counterion. They occur even with a change in an apparently indifferent co-ion in an intervening electrolyte. The accommodation of such results could only be achieved by calling in more unquantified fitting parameters. This is unsatisfactory as predictability is lost. The attractive van der Waals forces were obscured by short-range effects due to solvent structure.