Electrostatic interactions stabilizing colloids

The stability of colloids can also be dramatically altered by inclusion of polymeric materials. If the polymer interacts favourably with the particle surfaces, i.e. it adsorbs, then both an increase and a reduction in stability is possible, via modification of the electrostatic interaction of the polymer is charged or a reduction in the van der Waals attraction. 

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. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

Figure 2. Charge separation and stabilization of photoproducts in organized environments a) Application of the electrostatic interactions with charged Si02 colloids, b) Use of hydrophobic-hydrophilic interactions in water-in-oil microemulsions.

Here a mixture of sterically stabilized colloidal particles, solvent, and free polymer molecules in solution is considered. When two particles approach one another during a Brownian collision, the interaction potential between the two depends not only on the distance of separation between them, but also on various parameters, such as the thickness and the segment density distribution of the adsorbed layer, the concentration and the molecular weight of the free polymer. The various types of forces that are expected lo contribute to the interaction potential are (i) forces due to the presence of the adsorbed polymer, (ii) forces due to the presence of the free polymer, and (iii) van der Waals forces. It is assumed here that there are no electrostatic forces. A brief account of the nature of these forces as... 

The classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory (Derjaguin and Landau, 1941 Yerwey and Overbeek, 1948) states that the stability of a colloidal system essentially depends on two independent interactions between colloidal particles van der Waals attractions and electrostatic repulsion ... 

The ODN adsorption onto cationic microgel poly(N-isopropylacrylamide) particles was reported to be dramatically affected by the salinity of the incubation medium [9] as illustrated in Fig. 6. The observed result was related to (i) the reduction in attractive electrostatic interactions between ODN molecules and the adsorbent and (ii) the drastic effect of ionic strength on the physico-chemical properties of such particles [17, 27]. In fact, the hydrodynamic size, the swelling ability, the electrokinetic properties, and the colloidal stability are dramatically affected by pH, salt concentration, and the medium temperature [27]. 

With polymers that have ionizable groups, adsorption of a polymer will alter the charge of the surface altering the electrostatic interaction energy and also provide steric protection for the colloid, because the ionized groups will give better than theta conditions for the poisoner in an aqueous solution. This type of polymer stabilization is called electrosteric stabilization because both the electrostatic and the steric play a role in stabilization. The equations for this total interaction are simply the sum of electrostatic and steric terms as well as the van der Waals attraction. 

In the theory developed by Derjaguin and Landau (24) and Verwey and Overbeek (25.) the stability of colloidal dispersions is treated in terms of the energy changes which take place when particles approach one another. The theory involves estimations of the energy of attraction (London-van der Walls forces) and the energy of repulsion (overlapping of electric double layers) in terms of inter-oarticle distance. But in addition to electrostatic interaction, steric repulsion has also to be considered. 

The stability of colloidal systems consisting of charged particles can be essentially explained by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1-7]. According to this theory, the stability of a suspension of colloidal particles is determined by the balance between the electrostatic interaction and the van der Waals interaction between particles. A number of studies on colloid stability are based on the DLVO theory. In this chapter, as an example, we consider the interaction between lipid bilayers, which serves as a model for cell-cell interactions [8, 9]. Then, we consider some aspects of the interaction between two soft spheres, by taking into account both the electrostatic and van der Waals interactions acting between them. 

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