Electrostatic, double layer stabilization

In Chapter 5 we learned that, in water, most surfaces bear an electric charge. If two such surfaces approach each other and the electric double layers overlap, an electrostatic double-layer force arises. This electrostatic double-layer force is important in many natural phenomena and technical applications. It for example stabilizes dispersions.7... 

Roughly 60 years ago Derjaguin, Landau, Verwey, and Overbeek developed a theory to explain the aggregation of aqueous dispersions quantitatively [66,157,158], This theory is called DLVO theory. In DLVO theory, coagulation of dispersed particles is explained by the interplay between two forces the attractive van der Waals force and the repulsive electrostatic double-layer force. These forces are sometimes referred to as DLVO forces. Van der Waals forces promote coagulation while the double layer-force stabilizes dispersions. Taking into account both components we can approximate the energy per unit area between two infinitely extended solids which are separated by a gap x ... 

One of the central questions in the stability of foams is why are liquid films between two adjacent bubbles stable, at least for some time In fact, a film of a pure liquid is not stable at all and will rupture immediately. Formally this can be attributed to the van der Waals attraction between the two gas phases across the liquid. As for emulsions, surfactant has to be added to stabilize a liquid film. The surfactant adsorbs to the two surfaces and reduces the surface tension. The main effect, however, is that the surfactant has to cause a repulsive force between the two parallel gas-liquid interfaces. Different interactions can stabilize foam films [570], For example, if we take an ionic surfactant, the electrostatic double-layer repulsion will have a stabilizing effect. 

The classic 1948 Verwey-Overbeek text is well worth studying even today. E. J. W. Verwey and J. Th. G. Overbeek, Theory of the Stability ofLyophobic Colloids (Dover, Mineola, NY, 1999 originally published by Elsevier, New York, 1948). In 1967 Verwey told me that their studies were done in secret while Nazi soldiers controlled the Philips Laboratories where he and Overbeek pretended to do assigned work. Because they could publish nothing during the war, the world was eventually blessed with a coherent monograph that has defined much of colloid research ever since. This text is especially valuable for its sensitive, systematic treatment of electrostatic double layers. 

The combined effect of attraction and repulsion forces has been treated by many investigators in terms borrowed from theories of colloidal stability (Weiss, 1972). These theories treat the adhesion of colloidal particles by taking into account three types of forces (a) electrostatic repulsion force (Hogg, Healy Fuerstenau, 1966) (b) London-Van der Waals molecular attraction force (Hamaker, 1937) (c) gravity force. The electrostatic repulsion force is due to the negative charges that exist on the cell membrane and on the deposition surface because of the development of electrostatic double layers when they are in contact with a solution. The London attraction force is due to the time distribution of the movement of electrons in each molecule and, therefore, it exists between each pair of molecules and consequently between each pair of particles. For example, this force is responsible, among other phenomena, for the condensation of vapors to liquids. 

It is well-known that free films of water stabilized by surfactants can exist as somewhat thicker primary films, or common black films, and thinner secondary films, or Newton black films. The thickness of the former decreases sharply upon addition of electrolyte, and for this reason its stability was attributed to the balance between the electrostatic double-layer repulsion and the van der Waals attraction. A decrease in its stability leads either to film rupture or to an abrupt thinning to a Newton black film, which consists of two surfactant monolayers separated by a very thin layer ofwater. The thickness of the Newton black film is almost independent of the concentration of electrolyte this suggests that another repulsive force than the double layer is involved in its stability. This repulsion is the result of the structuring of water in the vicinity of the surface. Extensive experimental measurements of the separation distance between neutral lipid bilayers in water as a function of applied pressure1 indicated that the hydration force has an exponential behavior, with a decay length between 1.5 and 3 A, and a preexponential factor that varies in a rather large range. 

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. 

Adsorption of enteric viruses on mineral surfaces in soil and aquatic environments is well recognized as an important mechanism controlling virus dissemination in natural systems. The adsorption of poliovirus type 1, strain LSc2ab, on oxide surfaces was studied from the standpoint of equilibrium thermodynamics. Mass-action free energies are found to agree with potentials evaluated from the DLVO-Lifshitz theory of colloid stability, the sum of electrodynamic van der Waals potentials and electrostatic double-layer interactions. The effects of pH and ionic strength as well as electrokinetic and dielectric properties of system components are developed from the model in the context of virus adsorption in extra-host systems. 

First measurements of the electrostatic double-layer force with the AFM were done in 1991 [9, 10]. The electrostatic double layer depends on the surface charge density (or the surface potential) and the ionic strength. A brief introduction to the theory of the electrostatic force is given in Chap. 4. The electrostatic double-layer force is in many cases responsible for the stabilization of dispersions. An AFM experiment can be regarded as directly probing the interaction between a sample surface and a colloidal particle (or the AFM tip). Since the AFM tip is relatively small, this interaction can be probed locally. The lateral spacial resolution can be of the order of few nanometers. 

As an illustration, Fig. 8a shows a typical DLVO-type disjoining-pressure isotherm Tl h) (see Refs 3,4 and 62 for more details). There are two points, h = hy and h = h2> at which the condition for stable equilibrium, Eq. (42), is satisfied. In particular, h = h-y corresponds to the so-called primary film, which is stabilized by the electrostatic (double layer) repulsion. The addition of electrolyte to the solution may lead to a decrease in the height of the electrostatic barrier, IIjjj j (3,4) at high electrolyte concentration it is possible to have < P, then the primary film does not... 

Below, we will describe the two most common methods of stabilizing a colloidal suspension, i.e. either by creating an electrostatic double-layer at the solid-liquid... 

Figure 10.4. Stabilization of oil-in-water emulsion with an ionic emulsifier by electrostatic double layer, and with a nonionic emulsifier by formation of hydrated layer. Adapted from Belitz and Grosch (1987).

The second interaction that is relevant for the stability is of electrostatic nature In the presence of inert salts (salts that are composed of ions that do not adsorb on the colloidal surface), an electrostatic double layer is formed around... 

Several types of surface forces determine the interactions across thin liquid films. In addition to the universal van der Waals forces, the adsorbed ionic surfactants enhance the electrostatic (double-layer) repulsion. On the other hand, the adsorbed nonionics give rise to a steric repulsion due to the overlap of hydrophilic polymer brushes. The presence of surfactant micelles in the continuous phase gives rise to oscillatory structural forces, which can stabilize or destabilize the liquid films (and dispersions), depending on whether the micelle volume fraction is higher or lower. These and other surface forces, related to the surfactant properties, were considered in Sec. VI. 

It seems contradictory that dense particles can grow from solution, resulting in a stable sol. Why aren t the polynuclear ions and polymers repelled by the electrostatic double layer that stabilizes the colloidal particles The answer is revealed in Fig. 5 the repulsive barrier increases with the size of the particles, so the nuclei may be unstable against aggregation until they reach a certain size. Kramer et al. [41] directly demonstrated that the rate of growth of titania particles was related to the electrostatic barrier. Particles grew to a diameter of 4nm in 24h at pH 9.7, but reached 50 nm in 4 h if salts were added to compress the double layer, and they reached 6 nm in 1 h if the pH was reduced to 7 (near the lEP of titania). Of course, if the repulsive barrier is too low, the particles do not form a stable sol. According to Her [13], silica sols made by hydrolysis of alkali silicate will precipitate if the salt concentration exceeds 0.3 N. 

The interaction parameter should be smaller than 1/2 to provide a repulsion. This requires the medium to be a good solvent for the free dangling polymer. Polymeric repulsion occurs only when polymeric stabilizer layers overlap. The thickness of these layers is often of the order of 10 nm. In contrast, electrostatic double layers can be much thicker if the ion concentration of the medium is low (eq. 10.4.4). Also, the polymer repulsion potential is quite steep. As a result, the total potential for polymerically stabilized systems (Figure 10.4.3) shows no deep primary minimum. A shallow minimum, similar to the secondary minimum for electrostatically stabilized systems, is possible. 

The electrostatic double-layer force dominates at relatively medium-large separations. However, when very far away from the surfaces (very large separations) and when the surfaces are brought very close to each other, the attractive van der Waals forces (may) overcome the repulsive forces and dominate the interactions. If vdW forces dominate the surfaces will be pulled into a strong adhesive contact (attraction -instability) whereas stability is obtained in the region where the repulsion forces dominate. 

For many systems, this is indeed observed and many aqueous dispersions and emulsions are efficiently stabilized by electrostatic double-layer forces. Electrostatic stabilization is, however, sensitive to salts. Ifthe salt concentration increases, both the range and the amplitude of electrostatic double-layer forces decrease. The dispersion... 

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