Colloidal systems interparticle forces

In this group of disperse systems we will focus on particles, which could be solid, liquid or gaseous, dispersed in a liquid medium. The particle size may be a few nanometres up to a few micrometres. Above this size the chemical nature of the particles rapidly becomes unimportant and the hydrodynamic interactions, particle shape and geometry dominate the flow. This is also our starting point for particles within the colloidal domain although we will see that interparticle forces are of great importance. 

We have already introduced the idea that the primary particles of a dispersed system tend to associate into larger structures known as aggregates. The nature of the interparticle forces responsible for this aggregation is one of the most examined areas of colloid science. We defer our discussion of the aggregation (or coagulation) process until Chapter 13, but a few remarks about aggregates — the kinetic units that result from that process —and how their dimensions are represented quantitatively are in order at this time. 

The question to be discussed is whether saturation of the electric field (asserted by Proposition 2.1) implies saturation of the interparticle force of interaction. Consider for definiteness repulsion between two symmetrically charged particles in a symmetric electrolyte solution. In the onedimensional case (for parallel plates) the answer is known—the force of repulsion per unit area of the plates saturates. (This follows from a direct integration of the Poisson-Boltzmann equation carried out in numerous works, primarily in the colloid stability context, e.g., [9]. Recall that again in vacuum, dielectrics, or an ionic system with a linear screening, the appropriate force grows without bound with the charging of the particles.)... 

J. W. Goodwin, Rheological properties, interparticle forces and suspension structure, in D.M. Bloor and E. Wyn-Jones (Eds.), The Structure, Dynamics and Equilibrium Properties of Colloidal Systems. NATO ASI Series C 324, Kluwer, The Netherlands, 1990, pp. 659-679. 

A third important advantage of colloidal systems over atomic ones is that the interparticle forces can be varied readily via the electrolyte concentration and surface charge density on the particles. In general, the interparticle potential used in ordering studies is not the DLVO potential, because the separation between particles is significantly larger than the range of van der Waals forces, and this term is usually dropped. Instead a screened Coulomb potential is used, usually referred to as the Yukawa potential,... 

Perram, C.M., Nicolau, C., and Perram, J.W., Interparticle forces in multiphase colloid systems the resurrection of coagulated sauce beamaise. Nature, 270, 572-573, 1977. 

Under these conditions of highly-divided matter, the force due to gravity exerted on each individual particle may become extremely weak compared with interparticle forces (to be described in Sect. 3.3), to such an extent that a dispersion of these particles in a liquid may not spontaneously separate out under gravity. For this reason, some authors consider that a system comprising dispersed objects in a continuous medium should be called a colloidal system when the size of the objects is such that no rapid phase separation occurs, either through sedimentation or creaming (when the density of the particles is less than that of the suspension liquid). 

In most natural colloidal systems and in particular the aqueous solutions, the repulsive interparticle forces are related to the presence of ionised species on the surface of the particles. These surface charges may have various origins ... 

Hard sphere colloidal systems do not experience interparticle inta-actions until they come into contact, at which point the interaction is infinitely repulsive. Such systems represent the simplest case, where the flow is affected only by hydrodynamic (viscous) interactions and Brownian motion. Hard spha-e systems are not often encountered in practice, but model systems consisting of Si02 spheres stabilized by adsorbed stearyl alcohol layers in cyclohexane (56,57) and polymer latices (58,59) have been shown to approach this behavior. They serve as a useful starting point for considering the more complicated effects when interparticle forces are present. 

Additional interparticle forces exist in colloidal systems. They can be derived from a potential because they depend only on interparticle distance. Hence they act as springs and as such can cause pronounced elastic effects. However, the spring force depends on interparticle distance, and the springs even rupture when the particles move too far apart. This results in a highly nonlinear material response. During flow, the potential forces will affect the interparticle distances and consequently the frictional forces and the viscosity. We review each of these forces briefly in Sections 10.4.1-10.4.4. More thorough discussion can be found in Russel et al. (1989). 

While the colloidal method may appear straightforward, its success depends on an understanding of interparticle forces and how to manipulate them. Although such forces have a strong theoretical base, verified through direct surface force measurements the choice of the best surface active agent to use to control the forces is still a matter of trial and error for most ceramic systems. 

In many ceramic systems it is not possible to create a stable suspension simply by controlling pH. Large additions of acid or base can result in dissolution of the particles, or provide a too high ionic strength. Hence, addition of suitable polymeric dispersants is commonly used to create colloidally stable suspensions. These polymeric additives can induce an interparticle repulsion that prevents coagulation. Upon the close approach of two particles covered with adsorbed polymer layers, the interpenetration of the polymer layers give rise to a repulsive force, the so-called steric stabilization (10). There are some simple requirements for steric stabilization of colloidal suspensions, as follows ... 

Aggregation involves adhesion between colloidal particles, and a detailed consideration of interparticle attraction and bonding has been written by Visser (222) with 295 references. Special attention is given to immersed systems where London-van der Waals force and electric double layer repulsion as well as ionic attraction between surfaces of opposite charge are considered. 

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