Adsorption isotherm flocculation

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

Polymer adsorption is important in the flocculation and stabilization of colloidal sols and has been reviewed by Vincent et al. (1) and Tadros (2). Polyvinyl alcohol (PVA) has been used in these studies because of its practical application in textiles, adhesives, and coatings. The adsorption of PVA has been studied on silver iodide by Fleer (3) and Koopal (4), and on polystyrene (PS) latex particles by Garvey (5). The adsorption isotherms reported by these workers extend up to 600 ppm PVA. The adsorption at... 

These results confirm the existence of weak or labile floes at partial PVA coverage, particularly with the high-molecular-weight fully-hydrolyzed Vinol 325 and Vinol 350. In contrast, the partially-hydrolyzed Vinol 523, which is comparable in molecular weight to the Vinol 325, gave an adsorption isotherm with little scatter, indicating the absence of flocculation. Partially hydrolyzed PVA shows specific interactions with polystyrene surfaces (mentioned below), and the absence of flocculation in this case is consistent with the theory proposed by Clark and Lai (14) for bridging flocculation. 

SEM images of the monodispersed minerals are shown in Figure 1. Unfortunately it was not possible to obtain kaolinite particles in the desired size range (10-20 pm) due to flocculation occuring in the sedimentation process. It was not desirable to chemically treat the surface of the kaolinite to prevent this, as this would affect later results. To this end, the kaolinite was used untreated, as received, with the smaller size fraction shown below. Monodispersed minerals will allow direct comparison in HPAM adsorption isotherms. 

Churaev, Nikologorodskaya, and co-workers (33) investigated the Brownian and electrophoretic motion of silica hydrosol particles in aqueous solutions of an electrolyte at different concentrations of poly(ethylene oxide) (PEO) in the disperse medium. The adsorption isotherms of PEO on the surface of silica particles were obtained. The thickness of the adsorption layers of PEO was determined as a function of the electrolyte concentration and the pH of the dispersed medium. The results can be used in an analysis of the flocculation and stabilization conditions for colloidal dispersions of silica (with non-ionogenic water-soluble polymers of the PEO type). 

The adsorption of nonionic cellulose derivatives (HPMC 2910, HEC, and HPC) and their stabilizing effect on suspensions of polystyrene latices and of the antirheumatismal ibuprofen have been examined by Law and Kayes [117, 118]. Adsorption isotherms (see Fig. 20 for HPMC and ibuprofen) are of a sigmoidal type rapid initial adsorption, then a plateau corresponding to monolayer adsorption and finally a further increase in adsorption. At the same time, a gradual reduction in the sedimentation volume (decreased flocculation) is observable, while the redispersibility tends to be optimum (minimum number of revolutions necessary) at values of HPMC corresponding to a monolayer surface coverage. 

Interpretation of the adsorption isotherms indicated that for the acidic silica surface the basic resin molecules interacted strongly with the surface and adopted a parallel orientation, thus making little contribution to preventing flocculation. On the other hand, for the predominantly alumina-coated su rface, the resin only made contact with the surface with its limited number of acid groups, the rest of the molecule being extended into the medium and providing a steric barrier to flocculation. 

Figure 10.34. Adsorption isotherms for differently treated silica particles (at pH 5.6). The adsorption density of PEO increases with temperature up until 700°C and then decreases (after ref. (6)). ( Reprinted from International Journal of Mineral Processing, 58, S. Mathur, P. Singh and B. M. Moudgil, Advances in selective flocculation technology for solid-solid separations, Page 216, Copyright (2000), with permission from Elsevier Science )...

The results from adsorption isotherms indicate that the dispersions may be considered as a two-component system. The relevant case is an A-B solvant system with A-A and B-B repulsions but A-B attractions. The general behavior of such systems features a phase separation region where concentrate A-B phase separates from a dilute one, and no separation when there is a sufficient excess of either A or B (5). This is easy to visualize in the present case indeed saturation of the particles surfaces with polymer should lead to a steric repulsion between panicles, and similarly saturation of the macromolecules with bound particles may also results in objects which are unable to bind to each other. Macroscopic flocculation is known to occur with surfaces which are covered with 1/3 to 1/2 of their saturation cover, hence this is also considered with the two-component description. 

From these two successive experiments it can be concluded that (i) flocculation can be induced by macromolecules (ii) this flocculation is a reversible process. The reversibility is due to the low energy of interaction between both components. This last point was obvious from the adsorption isotherm (Figure 2), where significant amounts of free polymer remain in equilibrium with unsaturated surfaces. 

Flocculation and deflocculation of montmorillonite (Veegum) suspensions by a non-ionic surfactants has been studied by Ohno et al. [21]. Adsorption isotherms of the surfactant on to the clay showed points of inflexion due to bimolecular adsorption. Maximum flocculation occurred when the surface was covered by a layer of surfactant oriented with the alkyl chain pointing out into the aqueous phase. The deflocculation which occurs on further addition of surfactant is due to the hydrophilization of the surface by the second layer of surfactant molecules. The viscosity of the clay suspension reaches a maximum at the point of maximum monolayer coverage and then falls on further addition of surfactant. Fig. 9.9... 

A major advantage of the simple model described in this paper lies in its potential applicability to the direct evaluation of experimental data. Unfortunately, it is clear from the form of the typical isotherms, especially those for high polymers (large n) that, even with a simple model, this presents considerable difficulty. The problems can be seen clearly by consideration of some typical polymer adsorption data. Experimental isotherms for the adsorption of commercial polymer flocculants on a kaolin clay are shown in Figure 4. These data were obtained, in the usual way, by determination of residual polymer concentrations after equilibration with the solid. In general, such methods are limited at both extremes of the concentration scale. Serious errors arise at low concentration due to loss in precision of the analytical technique and at high concentration because the amount adsorbed is determined by the difference between two large numbers. 

Coethite As(V) adsorption on synthetic goethite primarily for a study of impact on flocculation and electrokinetics. No isotherms. Final pH varied but not defined Matis et al. (1999)... 

Also MCM-41 and HMS obtained by flocculation are mesoporous materials (Figures 7, 8). Adsorption and desorption of N2 and Ar on MCM-41 follow a Type IV isotherm without hysteresis loop and with a sharp inflection at low relative pressure, corresponding to a narrow pore size distribution. 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

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