Zeta-potential adsorbed polymer

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

For Gg (b), a reasonable (although not strictly correct) procedure is to replace the Stern potential in one of the standard equations for Gg by the zeta potential of the polymer-coated particles this assumes that the plane of hydrodynamic shear corresponds to the periphery of the adsorbed layer. 

Effect of adsorbed polymer on the double-layer. Because of the presence of adsorbed train segments, the double layer is modified. The zeta-potential, , is displaced because the adsorbed polymer displaces the plane of shear. The parameters for describing adsorbed polymers are the fraction of the first layer covered by segments, 0, and the effective thickness, A, of the polymer layer, The insert gives the distribution of segments over trains and loops for polyvinyl alcohol adsorbed on silver iodide. Results obtained from double layer and electrophoresis measurements. 

The amount of polymer adsorbed on each sample was measured by pressure filtration through a 0.1 m filter, followed by analysis of the filtrate for residual polymer by gel permeation chromatography with refractive index determination. Particle zeta potentials were measured by taking a small sample of the solids from the centrifuge and re-suspending them in the supernatant prior to analysis in a Malvern Instruments Zetasizer . The concentration of all other types of ions in the supernatant was analysed by ICP atomic emission spectroscopy. 

Figure 7.7 Zeta potentials (calculated from electrophoretic mobility data) relating to particles of different ionogenic character plotted as a function of pH in acetate-veronal buffer at constant ionic strength of 0.05 mol dm 3, (a) Hydrocarbon oil droplets, (b) Sulphonated polystyrene latex particles, (c) Arabic acid (carboxylated polymer) adsorbed on to oil droplets, (d) Serum albumin adsorbed on to oil droplets...

Chibowski, S., Zeta potential and thickness of a polymer adsorbed layer in the system dispersed sohd-electrolyte, Pol. J. Chem., (H. 1137, 1993. 

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. 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

Water-soluble cellulose derivatives themselves adsorb onto solid particles and may for instance affect the suspension properties of these insolubles. The mechanisms involved are quite complex and depend on the polymer concentration. At low concentrations macromolecules influence the electrophoretic mobility and the flocculation of the particles. At higher concentrations, surface coverage by the adsorbed polymer is sufficient to prevent particle-particle interaction and thus to stabilize the suspension sterically. As an example, the effect of NaCMC (among other polymers) on the zeta potential, flocculation and sedimentation properties of sulfadimidine has been investigated by Kellaway and Najib [115,116],... 

For all cellulose derivatives tested, a reduction of the zeta potential of the suspensions with increased polymer concentration is observed (Table 20). The effect of the molecular mass differs depending on the derivative concerned. For HEC and HPC, the amount adsorbed and the area per molecule decreased as the molecular mass increased, indicating a flatter adsorption conformation. For HPMC, the adsorption increased as the macromolecular chains became longer. Adsorption was maximum for the more hydrophobic HPC. 

When a polyelectrolyte adsorbs on an oppositely charged surface the adsorbed amount compensates, or slightly over-compensates, the surface charge [6], Adsorption of cationic polymers continues even after the zeta potential is reversed [7], This is the case for cationic starch. 

Adsorbed polymers will slowly coUapse as the loops and tails become less extended. The measured zeta potential of the fibre will fall as the effective radius of charge interaction collapses. Hydrogen bonds form between adsorbed starch molecules and fibre surfaces... 

For the extreme case of parallel orientation the only eifects on flocculation are (a) the modification of the attractive force between particles which is likely to be small in most cases, and (b) the effect on the zeta potential which could be significant depending on the chemical character of the surface and polymer. When the polymer chains are extended into the liquid phase there is a major effect which arises when particles approach sufTiciently close for the adsorbed layers to interact. 

Similar to the neutral polymers, organic solvents can adsorb at the interface between the capillary wall and the electrolyte solution, through hydrogen-bonding or dipole interaction, thus increasing the local viscosity within the electric double layer. Organic solvents may also influence the zeta potential by affecting the ionization of the silanol... 

The thickness of the adsorbed polymer layer may also be calculated from electrophoretic mobility measurements by calculating the zeta-potential at the plane of shear of the particle. This potential corresponds to the potential at the periphery of the adsorbed layer. By also measuring the zeta-potential of the bare particle and assuming a value of 4 A for the Stern layer, use of Eq. 15 allows the calculation of be, the electrokinetic thickness of the adsorbed layer (13). 

Figure 5.9 (a) Force versus distance curves for alumina at different pH values calculated from Equations (5.8) and (5.11) with parameters as detailed in Franks et al. (2000). At pH 9 the van der Waals attraction dominates. As pH is decreased the range and magnitude of the EDL repulsion increases as zeta potential increases (see Figure 5.8). At very small separation distances the van der Waals attraction always dominates the EDL repulsion, (b) Force versus distance curves for silica particles interacting with an adsorbed polymer (Zhou et al, 2008). Upon approach, the adsorbed polymer provides a weak steric repulsion. Upon separation (retraction) the polymer creates a strong long range attraction because chains are adsorbed on both surfaces. The van der Waals only interaction is shown for comparison...

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