Dispersion forces colloid stability

Like other colloidal dispersions, the colloidal stability of microgels is determined by the balance between the van der Waals attractive forces (V ) and the repulsive forces (Vg) applied on... 

The use of surface charge to provide colloid stability to particles dispersed in dilute electrolytes in aqueous solution, or even in media of intermediate polarity, is an effective means of stabilising particles against van der Waals forces of attraction. Figure 3.16 shows typical potential... 

Our objective in this chapter is to establish the quantitative connections between interparticle forces and colloid stability. Before we consider this it is instructive to look at the role of interaction forces in a larger context, that is, the relation between interparticle forces and the microstructure of dispersions and the factors that determine such a relation. These aid us in appreciating the underlying theme of this chapter, namely, the manipulation of interparticle forces to control the properties of dispersions. 

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

In this discussion of colloid stability we will explore the reasons why colloidal dispersions can have different degrees of kinetic stability and how these are influenced, and can therefore be modified, by solution and surface properties. Encounters between species in a dispersion can occur frequently due to any of Brownian motion, sedimentation, or stirring. The stability of the dispersion depends upon how the species interact when this happens. The main cause of repulsive forces is the electrostatic repulsion between like charged objects. The main cause of attractive forces is the van der Waals forces between objects. 

Several repulsive and attractive forces operate between colloidal species and determine their stability [12,13,15,26,152,194], In the simplest example of colloid stability, dispersed species would be stabilized entirely by the repulsive forces created when two charged surfaces approach each other and their electric double layers overlap. The overlap causes a coulombic repulsive force acting against each surface, which will act in opposition to any attempt to decrease the separation distance (see Figure 5.2). One can express the coulombic repulsive force between plates as a potential energy of repulsion. There is another important repulsive force causing a strong repulsion at very small separation distances where the atomic electron clouds overlap, called Born repulsion. 

There are some qualitative difficulties when the specific ion effects are explained via the dispersion forces of the ions. Particularly the anions, for which the dispersion coefficients / , are large, affect the double layer interactions. However, experiments on colloid stability [6] or colloidal forces [11] revealed strong specific ion effects especially for cations. Furthermore, the ions which affect most strongly the solvating properties of the proteins are those from their vicinity, since they perturb mostly the structure of water near the proteins. However, the van der Waals interactions of ions predict that the cations remain in the vicinity of an interface, and the anions are strongly repelled, while Hofmeister concluded that anions are mainly responsible for the salting out of proteins. 

Ninham and coworkers [12,13] very recently proffered an alternative explanation. They proposed that the shortcoming of traditional DLYO theory in predicting particle stability has arisen from its ab initio decomposition of forces into van der Waals and double-layer components. More specifically, they suggested that not accounting for dispersive interactions between colloid surfaces and dissolved ions is primarily responsible for the reported discrepancies in the traditional colloidal-stability modeling approach. 

Based on Equations 9.1 and 9.2, colloidal stability (maximum dispersion) depends on maximum R, which describes the maximum repulsive energy between two planar colloidal surfaces. It also appears from these equations that Rf is controlled by or CECV. However, in addition to the repulsive force, there is an attraction force (Af) between soil colloidal particles. The force of attraction (van der Waals force) between two particles separated by a distance of 2d (d = particle diameter) is described by... 

The interaction potentials described in previous sections for adsorbing homopolymer and terminally anchored layers in good solvents clearly indicate the ability of polymers to stabilize colloidal dispersions against flocculation due to van der Waals dispersion forces. Indeed, the practice preceeded the analyses by centuries in some cases and decades in others, since the use of adsorbing polymers dates to ancient times, and block copolymer stabilizers emerged from industrial laboratories in the 1960s (Napper, 1983). 

Chernoberezhskii, Y.M., Golikova, E.V., and Malinovskaya. L.V.. Surface forces in thin films and colloid stability, Nauka, Moscow. 1974. cited after Golikova, E.V.. Chernoberezhskii, Yu, M., and loganson, O.M., On a correlation between the aggregation stability and integral electro surface characteristics of oxide dispersions. Kolloid. Zh., 62, 596, 2000. 

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... 

---