Non-DLVO forces

Non DLVO forces in water deserve a special subchapter because they are important and far from being understood. They are important because water is the universal solvent in nature. Also, in more and more industrial processes water is used instead of organic solvent since it is harmless to the environment. 

Non-DLVO forces also occur when the aqueous medium contains surfactants, which form micelles, or polyelectrolytes. A discussion of the complex interaction is, however, beyond the scope of this book. We recommend Ref. [199],... 

Because the double layer force vanishes in the absence of surface charges, one expects the attractive van der Waals force to cause the coagulation of all neutral (or even weekly charged) colloids. The absence of such a behavior has been explained by the existence of an additional (non-DLVO) force, the hydration interaction, which is due to the structuring of water in the vicinity of hydrophilic surfaces. This chapter is devoted to the identification of the microscopic origin of the hydration force, and to the presentation of a unified treatment of the double layer and hydration forces, the Polarization Model. 

Attempts to utilize traditional DLVO approaches to quantify the Schulze-Hardy Rule have found limited and qualified success [23,59]. Although a qualitative agreement of the predicted dependence of ccc on counterion valence can be demonstrated, non-DLVO forces are typically ignored and analytical solutions of the DLYO equations predict unrealistically large ccc values [23,59]. 

Formation and stability studies of black foam films can be summarised as follows 1) surface forces in black foam films direct measurement of disjoining pressure isotherm DLVO- and non-DLVO-forces 2) thin foam film/black foam film transition establishing the conditions for the stability of both types of black films and CBF/NBF transition 3) formation of black foam films in relation to the state of the adsorption layers at the solution/air interface 4) stability of bilayer films (NBF) theory and experimental data. 

Non-DLVO Forces. Although DLVO theory worked very well for the electrolyte-induced coagulation of bitumen-in-water emulsions, it cannot be applied in some cases. 

It is customarily assumed that the overall particle-particle interaction can be quantified by a net surface force, which is the sum of a number of independent forces. The most often considered force components are those due to the electrodynamic or van der Waals interactions, the electrostatic double-layer interaction, and other non-DLVO interactions. The first two interactions form the basis of the celebrated Derjaguin-Landau-Verwey-Overbeek (DLVO) theory on colloid stability and coagulation. The non-DLVO forces are usually determined by subtracting the DLVO forces from the experimental data. Therefore, precise prediction of DLVO forces is also critical to the determination of the non-DLVO forces. The surface force apparatus and atomic force microscopy (AFM) have been used to successfully quantify these interaction forces and have revealed important information about the surface force components. This chapter focuses on improved predictions for DLVO forces between colloid and nano-sized particles. The force data obtained with AFM tips are used to illustrate limits of the renowned Derjaguin approximation when applied to surfaces with nano-sized radii of curvature. 

In many studies good agreement between experimental AFM curves and DLVO theory was reported. On the other hand, Ducker et al. [77] suggest existence of additional non DLVO forces. 

Ducker, W.A. et al.. Forces between alumina surfaces in salt solutions Non-DLVO forces and the implications for colloidal processing, J. Am. Ceram. Soc., 77, 437, 1994. 

Christenson, H. K., Non-DLVO forces between surfaces - solvation, hydration and capillary effects, J. Disp. Sci. Technol, 9, 171-206 (1988). 

With increasing electrolyte concentration the film thickness decreases down to the critical value Cei, cr — 2 X 10 mol dm [7j. At Cei> Cei.a- the remains constant, close to 16 nm. The left-hand part of the h C ) dependence indicates that there is an electrostatic component of disjoining pressure while the plateau indicates the existence of non-DLVO forces due to the steric interaction between the adsorbed polymer layers. Similar are the h,v(Cei) curves of foam films stabilized by A-B-A copolymers, non-ionic surfactants, non-ionic phospholipids, and so on [1—4, 33). 

All three types of thin liquid films from both ABA and AB polymeric surfactants are stabilized by DLVO-forces at low electrolyte concentrations and by non-DLVO-forces at higher electrolyte concentrations. The latter are steric surface forces of the type brush-to-brush and loop-to-loop interactions (according to de Gennes). These steric forces act in 0/W emulsion films as well, but there transitions to Newton black films (NBF) have also been established. A difference between foam and O/W emulsion films has been observed. The barrier in the ri(h) isotherm for an emulsion film is much lower and the transition to NBF can occur. The NBFs from polymeric surfactants are very stable, as are the emulsions obtained from the same solutions. Actually, two types of bilayer emulsion films are obtained, those stabilized by brush-to-brush or loop-to-loop steric interactions and the others - by short-range interactions, also steric, in a two-dimensional ordered system. The minor difference in the experimentally measured thickness (about 2 nm) is not sufficient to characterize the state of these films. 

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