Colloidal suspensions forces

The behavior of colloidal suspensions is controlled by iaterparticle forces, the range of which rarely extends more than a particle diameter (see Colloids). Consequentiy suspensions tend to behave like viscous Hquids except at very high particle concentrations when the particles are forced iato close proximity. Because many coating solutions consist of complex mixtures of polymer and coUoidal material, a thorough characterization of the bulk rheology requires a number of different measurements. 

Look closely at the settling times in Table 1 - the times span from a fraction of a second to almost a lifetime A great deal of the suspended matter found in waste waters fall into the colloidal suspension range, so obviously we caimot rely on gravitational force alone to separate out the pollutants. 

The methodology discussed previously can be applied to the study of colloidal suspensions where a number of different molecular forces and hydrodynamic effects come into play to determine the dynamics. As an illustration, we briefly describe one example of an MPC simulation of a colloidal suspension of claylike particles where comparisons between simulation and experiment have been made [42, 60]. Experiments were carried out on a suspension of AI2O3 particles. For this system electrostatic repulsive and van der Waals attractive forces are important, as are lubrication and contact forces. All of these forces were included in the simulations. A mapping of the MPC simulation parameters onto the space and time scales of the real system is given in Hecht et al. [42], The calculations were carried out with an imposed shear field. 

J. T. Padding and A. A. Louis, Hydrodynamic interactions and Brownian forces in colloidal suspensions coarse-graining over time and length scales, Phys. Rev. E 74, 031402 (2006). 

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

Clay minerals have a permanent negative charge due to isomorphous substitutions or vacancies in their structure. This charge can vary from zero to >200cmol kg" (centimoles/kg) and must be balanced by cations (counter-ions) at or near the mineral surface (Table 5.1), which greatly affect the interfacial properties. Low counter-ion charge, low electrolyte concentration, or high dielectric constant of the solvent lead to an increase in interparticle electrostatic repulsion forces, which in turn stabilize colloidal suspensions. An opposite situation supports interparticle... 

Equation 3 Indicates that a semilog plot of flux against concentration should be a straight-line intercepting the horizontal axis at the gel concentration (C ). When the bulk-stream concentration (C ) equals the gel concentration (C ) there is no driving force for removal of solute from the membrlne. The gel layer increases in thickness until the flux is zero. Figure 10 provides experimental confirmation for a number of protein solutions and colloidal suspensions ( ). The Intercepts with the horizontal axis are reasonable values for the gel concentration. 

Leong, Y.K., Scales, P.J., Healy, T.W., Boger, D.V. (1995). Interparticle forces arising from adsorbed polyelectrolytes in colloidal suspensions. Colloids and Surfaces A Physicochemical and Engineering Aspects, 95, 43-52. 

The bitumen comes as a residue from the refining of conventional or heavy crude oil, or from natural deposits of oil (tar) sand. Bitumen, being a complex mixture of more than 1000 different molecules, is itself a colloidal suspension of asphaltenes in a continuous phase of saturated parrafins, aromatic oils and resins [774], Descriptions of different kinds of asphalts are given in Refs. [775,776], At low asphaltene concentration the suspension is Newtonian. Once the concentration increases above about 8 % v/v, however, the asphaltenes form a three-dimensional network and the suspension can become a viscoelastic gel [774]. The asphaltenes interact through van der Waals forces so that a bitumen containing 15% asphaltenes is solid at room temperature and liquid above about 60-100 °C. 

The DLVO-theory is named after Derjaguin, Landau, Verwey and Overbeek and predicts the stability of colloidal suspensions by calculating the sum of two interparticle forces, namely the Van der Waals force (usually attraction) and the electrostatic force (usually repulsion) [19],... 

In several cases, however, the DLVO-theory has shown to be inadequate, due to the occurrence of other inter-particle forces that may be present in colloidal suspensions. These phenomena are summed up below ... 

Stabilising a colloidal suspension implies that the total interparticle potential decreases with increasing inter particle distance. The different kinds of stabilisation all use some of the above-mentioned interparticle forces. 

Colloidal suspensions are often stabilized by the adsorption of polymers that are expected to exert additional configurational-steric repulsive forces. The additional, potentially significant van der Waals interactions between polymer coatings... 

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