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Distribution of colloidal particles

Shaking out cannot be used to transform an aquosoi into an organosol. If an aquosoi is shaken with an organic liquid the particles as a rule assemble at the interface because that is their position of minimal free surface energy. Only in very exceptional cases can a distribution of colloidal particles between two liquid layers be observed... [Pg.62]

Fulda and Tieke [77] studied the effect of a bidisperse-size distribution of latex particles on the structure of the resulting LB monolayer. For this purpose, a mixed colloidal solution of particles la and lb was spread at the air-water interface. Particles la had a diameter of 434 nm, particles lb of 214 nm. The monolayer was compressed, transferred onto a solid substrate, and viewed in a scanning electron microscope (SEM). In Figure 10, SEM pictures of LB layers obtained from various bidisperse mixtures are shown. [Pg.224]

Smoluchowski, who worked on the rate of coagulation of colloidal particles, was a pioneer in the development of the theory of diffusion-controlled reactions. His theory is based on the assumption that the probability of reaction is equal to 1 when A and B are at the distance of closest approach (Rc) ( absorbing boundary condition ), which corresponds to an infinite value of the intrinsic rate constant kR. The rate constant k for the dissociation of the encounter pair can thus be ignored. As a result of this boundary condition, the concentration of B is equal to zero on the surface of a sphere of radius Rc, and consequently, there is a concentration gradient of B. The rate constant for reaction k (t) can be obtained from the flux of B, in the concentration gradient, through the surface of contact with A. This flux depends on the radial distribution function of B, p(r, t), which is a solution of Fick s equation... [Pg.80]

In subsequent sections of this chapter, we discuss further the distinction between macromolec-ular colloids and multiphase dispersions (Section 1.3), the use of the term stability in colloid science (Section 1.4), the size and shape of colloidal particles, the states of aggregation among particles, and the distribution of particle sizes that is typical of virtually all colloidal preparations (Section 1.5). The fact that particles in the colloidal size range are not all identical in size also requires a preliminary discussion of statistics, which is the subject of Section 1.5c and Appendix C. [Pg.2]

The Peclet number compares the effect of imposed shear (known as the convective effect) with the effect of diffusion of the particles. The imposed shear has the effect of altering the local distribution of the particles, whereas the diffusion (or Brownian motion) of the particles tries to restore the equilibrium structure. In a quiescent colloidal dispersion the particles move continuously in a random manner due to Brownian motion. The thermal motion establishes an equilibrium statistical distribution that depends on the volume fraction and interparticle potentials. Using the Einstein-Smoluchowski relation for the time scale of the motion, with the Stokes-Einstein equation for the diffusion coefficient, one can write the time taken for a particle to diffuse a distance equal to its radius R, as... [Pg.176]

For studying the stability of colloidal particles in suspension (Chapter 13) or for determining the potential at the surface of particles (Chapter 12), one often needs expressions for potential distributions around small particles that have curved surfaces. Solving the Poisson-Boltzmann equation for curved geometries is not a simple matter, and one often needs elaborate numerical methods. The linearized Poisson-Boltzmann equation (i.e., the Poisson-Boltzmann equation in the Debye-Hiickel approximation) can, however, be solved for spherical electrical double layers relatively easily (see Section 12.3a), and one obtains, in place of Equation (37),... [Pg.511]

It would lie far beyond the aim of this chapter to introduce the state-of-the art concepts that have been developed to quantify the influence of colloids on transport and reaction of chemicals in an aquifer. Instead, a few effects will be discussed on a purely qualitative level. In general, the presence of colloidal particles, like dissolved organic matter (DOM), enhances the transport of chemicals in groundwater. Figure 25.8 gives a conceptual view of the relevant interaction mechanisms of colloids in saturated porous media. A simple model consists of just three phases, the dissolved (aqueous) phase, the colloid (carrier) phase, and the solid matrix (stationary) phase. The distribution of a chemical between the phases can be, as first step, described by an equilibrium relation as introduced in Section 23.2 to discuss the effect of colloids on the fate of polychlorinated biphenyls (PCBs) in Lake Superior (see Table 23.5). [Pg.1174]

Figure 27-1 Measured particle-size distribution of colloids formed when FeS04 was oxidized to Fe31 in 10 4 M OH in the presence of phosphate (POJ ), silicate (SiOJ ), or no added anions. [From M. L Magnuson. D. A. Lytle. Figure 27-1 Measured particle-size distribution of colloids formed when FeS04 was oxidized to Fe31 in 10 4 M OH in the presence of phosphate (POJ ), silicate (SiOJ ), or no added anions. [From M. L Magnuson. D. A. Lytle.
To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. [Pg.36]

Micelles are formed by association of molecules in a selective solvent above a critical micelle concentration (one). Since micelles are a thermodynamically stable system at equilibrium, it has been suggested (Chu and Zhou 1996) that association is a more appropriate term than aggregation, which usually refers to the non-equilibrium growth of colloidal particles into clusters. There are two possible models for the association of molecules into micelles (Elias 1972,1973 Tuzar and Kratochvil 1976). In the first, termed open association, there is a continuous distribution of micelles containing 1,2,3,..., n molecules, with an associated continuous series of equilibrium constants. However, the model of open association does not lead to a cmc. Since a cmc is observed for block copolymer micelles, the model of closed association is applicable. However, as pointed out by Elias (1973), the cmc does not correspond to a thermodynamic property of the system, it can simply be defined phenomenologically as the concentration at which a sufficient number of micelles is formed to be detected by a given method. Thermodynamically, closed association corresponds to an equilibrium between molecules (unimers), A, and micelles, Ap, containingp molecules ... [Pg.133]

P(r) can be transformed into a distribution of the particle size as defined by the hydrodynamic radius Rh. But only for TDFRS, and not for PCS, a particle size distribution in terms of weight fractions can be obtained without any prior knowledge of the fractal dimension of the polymer molecule or colloid, which is expressed by the scaling relation of Eq. (39). This can be seen from the following simple arguments ... [Pg.34]

It is likely that the unusual effectiveness of the silver preparations described herein is due to the relationship between the surface properties/inner properties (i.e., oxide/ metal) of the particles and the size distribution of the particles. The smaller the average particle size, the greater the surface area and the greater the contribution of the particular surface chemistry. However, if the particles are excessively small there can be a loss of stability and/or other interactions that negatively affect the product. The sifver compositions of the instant invention are remarkabie because they are stable in essentially pure water without surfactants, etc. Also, the materials are essentially colorless while other colloidal silver preparations (particularly with larger particle sizes) usually show colors. These properties are a result of the exact manufacturing conditions as discussed above. [Pg.5]

However, TEM measurements performed on a Pt0 83Sn017/C (Figure 9.15) indicated that the increase of the metal loading on the carbon support led to the formation of a multimodal distribution of the particle size. Then, to overcome this problem, colloidal methods were also developed in our laboratory. [Pg.400]

The high precision of the disk centrifuge allowed the comparison of sedimentation velocities of colloidal particles with and without an adsorbed polymer layer, from which the hydrodynamic thickness of the adsorbed layer could be calculated (4). Here the disk centrifuge, giving complete size distributions, made the use of monodisperse samples unnecessary. [Pg.203]

Clearly, sedimentation FFF is a separation technique. It is an important member of the field-flow fractionation (FFF) family of techniques. Although other members of the FFF family (especially thermal FFF) are more effective for polymer analysis, sedimentation FFF is advantageous for the separation of a wide assortment of colloidal particles. Sedimentation FFF not only yields higher resolution than nearly all other particle separation techniques, but its simple theoretical basis allows a straightforward connection between observed particle migration rates and particle size. Thus size distribution curves are readily obtained on the basis of theoretical analysis without the need for (and uncertainties of) calibration. [Pg.216]


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See also in sourсe #XX -- [ Pg.40 ]




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