Electrostatic, double layer repulsion

O. Mondain-Monval, F. Leal-Calderon, J. PhUlip, and J. Bibette Depletion Forces in the Presence of Electrostatic Double-Layer Repulsion. Phys. Rev. lett. 75, 3364 (1995). 

Fig. 1.6 DLVO interactions showing the energetics of colloidal particles as a competition between electrostatic double-layer repulsion and van der Waals attractions. The primary minimum is due to strong short-range electron overlap repulsion (shown in Figure 1.4... 

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. 

In the case of Brownian diffusion and interception, particle capture is enhanced by London attractive forces and reduced by electrostatic double layer repulsive forces. 

The process of cell deposition in the presence of repulsive forces may be considered as a two-step sequence. First the cells move, primarily under the action of gravity, to a region very near to the surface. In order to move closer to the surface the particle must experience the energy barrier formed by the electrostatic double-layer repulsions and London attraction. Diffusion of cells over the energy barrier is the second step of the process. If the deposition rate is much smaller than the sedimentation rate the second step... 

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. 

Sample Preparation. The polystyrene spheres to be used should be monodis-perse with a particle radius i of about 50 nm, although any size in the range i = 30 to 100 nm is suitable. Such nanospheres are available commercially as aqueous latex suspensions with 1% to 10% PS by weight. A small amount of this latex suspension should be diluted 100- to 1000-fold. Using a microsyringe, take 0.1 mL from the PS stock, deliver this into a rinsed dilution bottle, and then add 10 mL of a hltered lO-mM solution of NaCl or other 1 1 electrolyte. The purpose of this electrolyte is to partially suppress coulombic interactions (electrostatic double-layer repulsion) that can influence the diffusion constant and lead to R values that are artificially high by —10%. The electrolyte solution should be prepared from distilled water and stored at room temperature. Before use, it must be hltered through a suitable membrane (0.1-jum pore size) to remove dust particles. Avoidance of dust is cracial, and capped dilution bottles should be used. 

For similar micellas concentrations, the effect of electrolyte will be more severe for the anionic surfactant as it will suppress the electrostatic double-layer repulsive forces acting between the micelles. For nonionic surfactant, the repulsive force between micelles is a steric force rather than an electrostatic force, such that electrolyte has less of an effect. 

The effect of electrostatic double layer repulsion upon particle deposition. (From Ref. (7).)... 

In addition to the Laplace capillary pressure, three additional forces can operate at surfactant concentration below the cmc, namely electrostatic double layer repulsion Tj1, van der Waals attractions and steric (short-range) forces... 

The simple attractive van der Waals term (Va) and the simple electrostatic (double-layer) repulsive interaction energy (Vr) are now summed. Silica sol particles in the 10-100 nm radius size range, for salt concentrations of 0.1 M and greater and for pH values of 5 or more above the pHiep of silica are now considered. The isoelectric point is taken as pH 2-3. Thus for 25-nm-radius particles the variation of total energy Vt (= Va + Vr) with particle separation is attractive at all separations, and the sols are therefore expected to be unstable over this entire pH range above 5 as shown by the theory line in Figure 4 and in the inset in Figure 4. 

Forces between DNA Double Helices. The repulsion and attraction of DNA is the molecular interaction most studied to date. In simple salts, repulsion is again exponential with decay rates of 2.8 3.3 A (II). Unlike forces between polysaccharides, the coefficient of the force depends on the type of cationic counterion, even though electrostatic double layer repulsion is low enough to suggest that the helix is largely neutralized by ion association. 

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... 

The total interaction between any two surfaces must also include the van der Waals attraction, which is largely insensitive to variations in electrolyte concentration and pH, and so may be considered as fixed for any particular solute-solvent system. Further, the van der Waals attraction wins out over the double-layer repulsion at small distances, since it is a power-law interaction, whereas the double-layer interaction energy remains finite or rises much more slowly as 0. This is the theoretical prediction that forms the basis of the so-called Derjaguin-Landau-Verwey-Over-beek (DLVO) theory (illustrated in fig. 7.2) [15]. In the DLVO theory, the interaction between two particles is assumed to consist of two contributions the van der Waals attraction and the electrostatic double-layer repulsion. At low salt concentration, the... 

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is commonly used to describe interactions of charged surfaces across liquids [8, 9]. The DLVO theory models the interparticle interactions by superposing van der Waals attractions and electrostatic double layer repulsion forces. The direct force measurements have confirmed this theory down to surface separations of few nanometers [10]. 

It has been well established that four types of forces can operate in aqueous film layers at low surfactant concentrations (< CMC) (a) the Laplace capillary pressure, (b) the electrostatic double-layer repulsion, Hei, (c) the van der Waals interactions rivdw and (d) the short-range hydration or structural repulsive forces caused by steric hindrance in oriented and packed layers, n t. Initially, the Deryaguin disjoining pressure, TI, encompasses two of these contributions, as follows ... 

As summarized in the next sections, the micropipet technique has been used to measure adhesive interbilayer interactions based on these attractions that are enhanced by depletion flocculation produced by nonadsorbent polymer, and are attenuated by short-range hydration repulsion, thermal undulations and electrostatic double-layer repulsion energies [14,15,22,25]. 

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. 

Two types of black films can be distinguished [754, 755] common black films, which are stabilized by electrostatic double-layer repulsion, and Newton black films, which are stabilized by short-range forces. Common black films are 6-30 nm thick. Newton black films basically consist of a bilayer of surfactant. They have a defined thickness of 4—5 nm. For lipids, it is slightly larger [756]. If we consider a Newton black film formed from an oil phase in water, we are left with a lipid bUayer that still contains some oil molecules. Such films are extremely good electric insulators and are used in electrophysiological studies of membrane proteins [757]. 

For a rough estimation ofthe film thickness, we can assume that the film thickness is determined only by the electrostatic double-layer repulsion. We apply Eq. (7.19) and set the disjoining pressure equal to the external or hydrostatic pressure Pext = Ile(h). This leads to a film thickness of... 

For example, dispersions of colloidal silica at alkaline conditions are stabilized by adding certain salts although the electrostatic double-layer repulsion decreases... 

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