Colloid stability physical model

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. 

Physical model for colloid stability. Net energy of interaction for spheres of constant potential surface for various ionic strengths (1 1 electrolyte) (cf. Verwey and Overbeck). 

To predict the transport and fate of colloids in the subsurface, it is important to understand both the mechanism of particle deposition and that of remobilization in porous media. Experiments by MacDowell-Boyer (1992) and Monte Carlo simulations of Brownian particles near the surfaces of the media indicate that the secondary stability minimum (see physical model on colloid stability. Figure 14.10) can play an important role in the deposition and reentrainment of submicron particles at ionic strengths relevant to groundwater. 

Appendix 14.1 A Physical Model (DLVO) for Colloid Stability 867... 

APPENDIX 14.1 A PHYSICAL MODEL (DLVO) FOR COLLOID STABILITY... 

Clays of the montmorillonite family are lamellar aluminosificates [46] used in many industrial processes and in products such as paints, softeners, and composite materials [47]. They swell when brought into contact with water, which is due to the insertion of water molecules between the sheets. Complete exfoliation can be induced leading to dispersions of disk-like particles of 10 A thickness and 300-3000 A in diameter, depending on the variety of clay used. These clay platelets bear a rather large surface electrical charge so that electrostatic interactions between them must be considered and are actually responsible for the colloidal stability of these dispersions. These suspensions have been widely studied as model colloids and also because they form physical thixotropic gels. 

Suppose we have a physical system with small rigid particles immersed in an atomic solvent. We assume that the densities of the solvent and the colloid material are roughly equal. Then the particles will not settle to the bottom of their container due to gravity. As theorists, we have to model the interactions present in the system. The obvious interaction is the excluded-volume effect caused by the finite volume of the particles. Experimental realizations are suspensions of sterically stabilized PMMA particles, (Fig. 4). Formally, the interaction potential can be written as... 

This chapter will focus on the modeling of physical colloidal interactions and will present the conceptual and methodological approach to quantifying the physical aggregation process by a variety of techniques. Only brief consideration will be given to stability considerations, which are of utmost importance... 

The various flocculation models which are valid in the different regimes described above allow one to compute the particle/ particle collision rate for any given particle sizes, chemical and physical condition. From the magnitude of this collision rate, one can estimate a colloidal system s stability in cases (iv) and (v). However, in cases (ii) and (iii), both flocculation and creaming will be important in the colloidal breaking process. Consequently, in order to determine whether a colloidal system will be stable in these two cases, we have to determine the net rate of particle loss due to both creaming and flocculation. 

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