Stabilization, electrostatic diffuse-layer interactions

A quantitative treatment of the effects of electrolytes on colloid stability has been independently developed by Deryagen and Landau and by Verwey and Over-beek (DLVO), who considered the additive of the interaction forces, mainly electrostatic repulsive and van der Waals attractive forces as the particles approach each other. Repulsive forces between particles arise from the overlapping of the diffuse layer in the electrical double layer of two approaching particles. No simple analytical expression can be given for these repulsive interaction forces. Under certain assumptions, the surface potential is small and remains constant the thickness of the double layer is large and the overlap of the electrical double layer is small. The repulsive energy (VR) between two spherical particles of equal size can be calculated by ... 

At low concentrations of dissolved organic matter (DOM) (<0.01 mg of C/L) and at low ionic strength (10-3 M), the hematite particles are positively charged at this pH and are stabilized electrostatically by interacting diffuse layers with characteristic (Debye) lengths of 10 nm. As the ionic strength is increased to 10 1 M at these low DOM concentrations, the diffuse layers are compressed to 1 nm, and attractive van der Waals forces promote attachment in classical Derjaguin-Landau-Verwey-Overbeek (DLVO) destabilization by what has been termed double-layer compression. 

The speculation continues here into estuarine waters, bounded in this analysis at both inlet and outlet by waters that can, in the presence of suitable NOM, yield stable colloids. Diffuse layer thicknesses in these waters are small, on the order of 1 nm at 7 = 0.1. There can be sufficient salt in these waters to prevent classical DLVO electrostatic stabilization this is the conventional view. There may also be insufficient salt to form a thick layer of adsorbed NOM by screening of intra- and intermolecular repulsive interactions of the molecules of NOM. The result would then be a region of ionic strength or salinity in an estuary within which colloidal particles have a minimum stability and a maximum sticking probability. This possibility is shown by the proposed relationship between a and ionic strength shown in Figure 12. 

The electrostatic interaction between diffuse layers of ions surrounding particles is one of the most thoroughly theoretically developed factors of colloid stability. The theory of electrostatic factor is, essentially, the basis for the quantitative description of coagulation by electrolytes. This theory was developed in the Soviet Union by B.V. Derjaguin and L.D. Landau in 1935 -1941, and independently by the Dutch scientists E.Verwey and T. Overbeek, and is presently known by the initial letters of their names as the DL VO theory [44,45]. The DLVO theory is based on comparison of molecular interaction between the dispersed particles in dispersion medium and the electrostatic interaction between diffuse layers of ions, with Brownian motion of particles taken into account (in the simplest version of theory this is done on a qualitative level). 

The repulsive forces arise from the electromagnetic interactions of the charged layer surrounding the particles, the so-called electrical double layer. On the surface of the particles, a charged layer may be formed due to selective adsorption of ions. This part of the double layer is immobile and consists of tightly adsorbed ions in direct contact with the particle surface. In the solution adjacent to the particle, a second layer, in which the ions are more diffusely distributed, penetrates into the liquid. This part of the double layer is termed the diffusion layer. The extent of this diffusion layer depends on the electrolyte concentration increasing electrolyte concentration causes this diffuse double layer to shrink closer in to the particle, so that the electrostatic potential falls off more quickly with distance. The process by which the particles are stabilized by the repulsive forces of the electrical double layers is known as electrostatic stabilization. 

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

The details of the influence that electrostatic surface forces on the stability of foam films is discussed in Section 3.3. As already mentioned, the electrostatic disjoining pressure is determined (at constant electrolyte concentration) by the potential of the diffuse electric layer at the solution/air interface. This potential can be evaluated by the method of the equilibrium foam film (Section 3.3.2) which allows to study the nature of the charge, respectively, the potential. Most reliable results are derived from the dependence foam film thickness on pH of the surfactant solution at constant ionic strength. The effect of the solution pH is clearly pronounced the potential of the diffuse electric layer drops to zero at certain critical pH value. We have named it pH isoelectric (pH ). As already mentioned pH is an intrinsic parameter for each surfactant and is related to its electrochemical behaviour at the solution/air interface. Furthermore, it is possible to find conditions under which the electrostatic interactions in foam films could be eliminated when the ionic strength is not very high. 

Iv) Sol stability. The minimum in the stability of electrostatically stabilized sols as a function of pH, pAg, etc. does not give the p.z.c. but rather the i.e.p. because particle interaction is governed by the diffuse part of the double layer. 

Many properties of disperse systems are related to the distribution of charges in the vicinity of the interface due to the adsorption of electrolytes. The adsorption of molecules is driven by the van der Waals attraction, while the driving force for the adsorption of electrolytes is the longer-range electrostatic (Coulomb) interaction. Because of this, the adsorption layers in the latter case are less compact than in the case of molecular adsorption (i.e., they are somewhat extended into the bulk of the solution), and the discontinuity surface acquires noticeable, and sometimes even macroscopic thickness. This diffuse nature of the ionized adsorption layer is responsible for such important features of disperse systems as the appearance of electrokinetic phenomena (see Chapter V) and colloid stability (Chapters VII, VIII). Another peculiar feature of the adsorption phenomena in electrolyte solutions is the competitive nature of the adsorption in addition to the solvent there are at least two types of ions (even three or four, if one considers the dissociation of the solvent) present in the system. Competition between these ions predetermines the structure of the discontinuity surface in such systems -i.e. the formation of spatial charge distribution, which is referred to as the electrical double layer (EDL). The structure and theory of the electrical double layer is described in detail in textbooks on electrochemistry. Below we will primarily focus on those features of the EDL, which are important in colloid... 

RhGOlogy. Flow properties of latices are important during processing and in many latex applications such as dipped goods, paint, and fabric coatings. Rheology is used to characterize the stability of latices (45). For dilute, nonionic latices, the relative latex viscosity is a power-law expansion of the particle volume fraction. The terms in the expansion account for flow aroimd the particles and particle-particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle-particle interactions must be considered (46). A relative viscosity relationship for concentrated latices was first presented in 1972 (47). A review of empirical relative viscosity models is available (46). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rheological Measurements). 

In the case of polymer/clay nanocomposites, alkylammonium exchange species influence the affinity between the polymer and the clay surface. For example, it was reported that clays treated with dialkyl dimethylammonium halides, in particular with two chains of about 18 carbon atoms, have a surface energy similar to poly(olefins) such as polypropylene (PP) and polyethylene (PE) [27]. Polar polymers as polyamides (PA) have been recommended to get better interactions as reported by Toyota [8]. The alkyl chain length is related with increase in interlayer space required for the intercalation of polymer chains. Because of the nonpolar nature of their chains, they reduce the electrostatic interactions between the silicate layers and lower the surface energy of the layered silicates. As a consequence, an optimal diffusion of the polymer to dissociate the stacked clay layers, that is, an exfoliation process, can be obtained. Despite the compatibility of MMT modified by long alkyl chain quaternary ammonium with hydrophobic polymers (PE and PP), conventional alkylammonium ions show low thermal stability, that is, an onset decomposition temperature is close to 180°C.This poor thermal stability could limit their use in the preparation of PLS with matrices processed at high temperatures such as PA, poly(ethylene terephthalate) (PET), and poly(ether ether ketone) (PEEK) [30]. 

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