Beyond DLVO theory

For large separations, the force between two solid surfaces in a fluid medium can usually be described by continuum theories such as the van der Waals and the electrostatic doublelayer theory. The individual nature of the molecules involved, their discrete size, shape, and chemical nature was neglected. At surface separations approaching molecular dimensions continuum theory breaks down and the discrete molecular nature of the liquid molecules has to be taken into account. 

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Additional influences on dispersion stability beyond those accounted for by the DLVO theory, like surface hydration and steric effects, have received considerable attention over the past several decades [194,278],... 

Figure 2.3a is purely a sketch. The exact interaction potential between n-butylammonium-substituted clay plates (or other charged colloidal particles) in solution must incorporate many effects, such as the size of the small ions and the molecular degrees of freedom of the solvent, that are beyond the scope of either the coulombic attraction theory or DLVO theory. However, whatever the complicated functional dependence, the curve must comprise two states of equal thermodynamic potential. Somehow, the valleys in VT, the total potential, must be of equal depth. As discussed previously, in Figure 2.3b we see that the DLVO theory can never account for this experimentally proved phenomenon. 

Theories of colloid stability based on electrostatics go way back beyond the DLVO theory, to the Gouy-Chapman theory of the electrical double layer proposed in the early 1910s and the Stem theory of counterion condensation proposed in 1924. There was much weighty speculation about the counterion distribution around colloidal particles throughout the 20th century, but nobody succeeded in measuring it until our work in 1997. This work is described in detail in Chapter 8. 

Values of e, n and ve and Hamaker constants for two identical types of a material in a vacuum, which are calculated from Equation (567) by taking e3 = 1 and 3 = 1, are given in Table 7.1. Unfortunately, the lack of material constants, such as the dielectric constant, as a function of frequency for most of the substances, and also the complexity of the derived formulae have hampered the general use of the Lifshitz model. However, Lifshitz theory made possible the advent of the first theories on the stability of hydrophobic colloids as a balance between London attraction and electrical double-layer repulsion. Later, these theories were further elaborated by Derjaguin and Landau, and independently by Verwey and Overbeek. The general theory of colloidal stability (which is beyond the scope of this book) is based on Lifshitz theory and has become known as the DLVO theory, by combining the initials of these four authors. 

Experimental Results. The DLVO theory, which is based on a continuum description of matter, explains the nature of the forces acting between membrane surfaces that are separated by distances beyond 10 molecular solvent diameters. When the interface distance is below 10 solvent diameters the continuum picture breaks down and the molecular nature of the matter should be taken into account. Indeed the experiment shows that for these distances the forces acting between the molecularly smooth surfaces (e.g., mica) have an oscillatory character (8). The oscillations of the force are correlated to the size of the solvent, and obviously reflect the molecular nature of the solvent. In the case of the rough surfaces, or more specifically biomembrane surfaces, the solvation force displays a mono tonic behavior. It is the nature of this solvation force (if the solvent is water, then the force is called hydration force) that still remains a puzzle. The hydration (solvation) forces have been measured by using the surface force apparatus (9) and by the osmotic stress method (10, II). Forces between phosphatidylcholine (PC) bilayers have been measured using both methods and good agreement was found. 

Polymers have been used to both stabihze and destabilize colloidal dispersions. Their use in stabilizing colloidal dispersions in nonaqueous liquids is particularly important because, owing to the low dielectric constants of such liquids, the concentration of ions is very low and the electrostatic stabilizing forces minimal. Since London-van der Waals forces are attractive, stabilization provided by polymers may be the only means to prevent flocculation. In contrast to electrostatic forces in DLVO theory, polymeric stabihzation forces often are significant at particle separations of a few tens of nanometers, well beyond the range of attractive forces. 

Boinovich, L. 2010. DLVO forces in thin liquid films beyond the conventional DLVO theory. Current Opinion in Colloid Interface Science 15, no. 5 297-302. doi 10.1016/j. cods.2010.05.003. 

Steric and Hydrodynamic Effacts. Additional influences on dispersion stability beyond those accounted for by the DLVO theory, like steric, sinface hydration, and hydrodynamic effects have received considerable attention over the past several decades (54). More generally, the stability of a dispersion can be enhanced (protection) or reduced (sensitization) by the addition of material that adsorbs onto particle surfaces. Protective agents can act in several ways. They can increase double layer repulsion if they have ionisable groups. The adsorbed layers can lower the effective Hamaker constant. An adsorbed film may necessitate desorption before particles can approach close enough for van der Waals forces to cause attraction. 

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