Colloidal dispersions particle aggregation

The increase in size of water consumption and decrease of water quality due to man s impact make the problem of water purification and treatment from different contaminants, such as suspended and colloidal disperse particles, more acute. The efficiency of dispersion precipitation can be greatly improved with the use of flocculants - high-molecular compounds that have the ability to adsorb themselves on disperse particles and form fast precipitating aggregates [1], Both natural and synthetical water soluble polymers can be used as flocculants. The most commonly used and the most active of these polymers are polyacrylamide (PAA) and some of its derivatives. 

In the 1940s, a scientific theory describing the delicate balance of colloidal dispersion and aggregation was developed by Derjaguin and Landau and by Verwey and Overbeek. The theory is usually called the DLVO theory for short. In this theoretical description, a potential V is used to describe the different interactions between colloidal particles in solution and we can simply write... 

Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

This simple thermodynamic picture is substantially altered if we introduce dissimilar particles into our dispersion. The various interactions now depend on the nature of the two particles, relative to the solvent, and can either favour dispersal or aggregation. Again, this could be the basis for a natural control mechanism as the number and composition of the colloidal building blocks evolve, subtle changes in the interactions could switch a dispersion from stable to unstable. 

The physicochemical forces between colloidal particles are described by the DLVO theory (DLVO refers to Deijaguin and Landau, and Verwey and Overbeek). This theory predicts the potential between spherical particles due to attractive London forces and repulsive forces due to electrical double layers. This potential can be attractive, or both repulsive and attractive. Two minima may be observed The primary minimum characterizes particles that are in close contact and are difficult to disperse, whereas the secondary minimum relates to looser dispersible particles. For more details, see Schowalter (1984). Undoubtedly, real cases may be far more complex Many particles may be present, particles are not always the same size, and particles are rarely spherical. However, the fundamental physics of the problem is similar. The incorporation of all these aspects into a simulation involving tens of thousands of aggregates is daunting and models have resorted to idealized descriptions. 

The introduction of long-range interaction forces between colloidal particles can produce well dispersed or aggregated systems under... 

It should be realized, at the outset, that colloidal solutions (unlike true solutions) will almost always be in a metastable state. That is, an electrostatic repulsion prevents the particles from combining into their most thermodynamically stable state, of aggregation into the macroscopic form, from which the colloidal dispersion was (artificially) created in the first place. On drying, colloidal particles will often remain separated by these repulsive forces, as illustrated by Figure 1.1, which shows a scanning electron microscope picture of mono-disperse silica colloids. 

Fig. 9.4.23 Dispersibility of colloidal systems of a kind of metals in various organic liquids. er. Relative dielectric constant of liquids A, electron affinity disp, dispersion (O) floe, flocculation ( ) upon stirring, the suspension becomes turbid then particles slowly sediment) coag, coagulation ( immediately after stirring of the suspension, particles aggregate again to sediment). ( ) Boundary between disp and floe ( ) boundary between Hoc and coag. Broken lines divide each region, (a) Hexane, (b) benzene, (c) diethyl ether, (d) ethyl acetate, (e) letrahydrofuran. (0 dichloroethane. (g) benzyl alcohol, (h) 2-butanol, (i) butanol, (j) acetone, (k) ethanol. (From Ref, 23.)...

The facts that we have explicitly included the intraparticle interference function P[Q) in the analysis of scattering intensities and that it is accessible experimentally allow us to characterize colloidal dispersions structurally in more detail than we have been able to so far. In order to understand this, we need to understand clearly what we mean by small or large values of 6 or s and how they affect the behavior of P(6). This will also help us to understand how (and why) it is possible to combine light scattering with x-ray or neutron scattering to study structures of particles and their aggregates. 

TJhe aggregation of particles in a colloidal dispersion proceeds in two distinct reaction steps. Particle transport leads to collisions between suspended colloids, and particle destabilization causes permanent contact between particles upon collision. Consequently, the rate of agglomeration is the product of the collision frequency as determined by conditions of the transport and the collision efficiency factor, the fraction of collisions leading to permanent contact, which is determined by conditions of the destabilization step (2). Particle transport occurs either by Brownian motion (perikinetic) or because of velocity gradients in the suspending medium (orthokinetic). Transport is characterized by physical parame-... 

A most important physical property of colloidal dispersions is the tendency of the particles to aggregate. Encounters between particles dispersed in liquid media occur frequently and the stability of a dispersion is determined by the interaction between the particles during these encounters. 

Colloids are similar to solutions in that they consist of one phase uniformly dispersed in a second phase. There are many common examples of colloids, including milk, blood, paint, and jelly. However, colloids are not true solutions because the particles in the dispersed phase are not the size of molecules or ions. The particles in a colloid range in size from 10 to 200 nm. The dispersed particles might be supersized molecules (e.g., proteins) or aggregates of ions. While these particles are typically too small to be discerned, even by a microscope, they are much larger than, say, a sodium ion, which has a diameter of about 0.1 nm. 

---