The viscosity of polyelectrolyte solutions

The obvious effect of changing the salt concentration in a solution of a flexible polyelectrolyte is that the latter will expand in low salt concentration, as a result of the mutual repulsion of the charges along the chain, and contract in high salt concentrations, as discussed briefly in Chapter 2. In any particular solution the size of the polyelectrolyte coil will be such that the energy 

15% hydrolysed ---------HPAM, 25% hydrolysed --------HPAM, 35% hydrolysed) (after 

It is convenient to have a general correlation or data bank of the solution viscosity of polymers as functions of concentration, shear rate and the level of salinity (NaCl) or hardness (Ca ). Correlations for these quantities have been presented for HPAM by French et al. (1981). Auerbach (1985) has presented similar correlations for the concentration/viscosity relationship for commercially available xanthans with varying levels of pyruvate in different salinity brines. 

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The concentration dependence of the viscosity of polyelectrolyte solutions has been discussed by several authors [Cohen et al., 1988 Cohen and Priel, 1990 Borsali et al., 1992, 1994 Antonietti et al., 1997]. Several groups [Borsali et al 1992, 1994 Antonietti et al., 1997] have used the mode-mode coupling approximation of Hess and Klein [1983]. In the weakly charged polyelectrolyte limit, the latter formulation leads to an expression for the time-dependent viscosity of the form [Borsali et al., 1992]... 

Usually, flocculating agents are received as solids or thick liquids. They are dissolved or diluted to low concentrations because the viscosity of polyelectrolyte solutions can be quite high. This can make it difficult to wet and disperse the material properly. The result of improper dispersion is the formation of lumps that dissolve extremely slowly and that are not effective in the process. A supplier reports that dilute solutions of a flocculant often give superior results and that multiple-point addition of the flocculant can improve its contact with the brine [91]. The polymers are sensitive to shear, and the agitation process must be chosen with care. Turbulence at the addition point(s) should provide good dispersion but not break the floes. 

The viscosity of polyelectrolyte solutions varies dramatically with ionic strength and pH. The theory and experimental situation for these solutions is outlined in Section 10.4... 

In 1987 Witten and Pincus [71] presented a theory for the viscosity of polyelectrolyte solutions which was derived for concentrations near the overlap concentration. Again, at the limit Cp > c, the Fuoss law ti, oc c was obtained. Later Rabin [72] derived a similar relation (for the viscosity of poly-electrolyte solutions) on the basis of the theory by Hess and Klein. With some bold simplifications Rabin arrived at... 

The viscosity of xanthan solutions is also distinct from that of flexible polyelectrolyte solutions which generally shows a strong Cs dependence [141]. In this connection, we refer to Sho et al. [142] and Liu et al. [143], who measured the intrinsic viscosity and radius of gyration of Na salt xanthan at infinite dilution which were quite insensitive to Cs ( > 0.005 mol/1). Their finding can be attributed to the xanthan double helix which is so stiff that its conformation is hardly perturbed by the intramolecular electrostatic interactions. In fact, it has been shown that the electrostatic persistence length contributes only 10% to the total persistence length even at as low a Cs as 0.005 mol/1 [142]. Therefore, the difference in viscosity behavior between xanthan and flexible polyelectrolyte... 

Because of the polyelectroly tic nature, pectin solutions need to be made in excess of salt, usually in 0.05 0.1 M sodium chloride or phosphate, and use the same solvent for dilution (isoionic dilution) (Pals and Hermans, 1952). This is because, unlike neutrol polymers, the viscosity of dilute solution of polyelectrolytes displays unique dependence on concentration. As shown in Figure 9.5, the qsp of sodium pectate exhibits a maximum in pure water and low concentration of salt, a phenomenon caused by the so-called electroviscous effect. When the salt concentration is... 

Polyelectrolytes raise the viscosity of aqueous solutions thus acting as thickeners, and the magnitude of the effect increases with the polymer s molecular weight. Naturally occurring gums... 

The highly nonlinear concentration dependence evident in Figure 1.9 makes determination of the intrinsic viscosity by extrapolation of tjsp/c data via Eqs. (1.24) and (1.25) impossible. Cohen and Priel [1990] report observing that linear concentration dependence is observed at concentrations substantially below Cmax. allowing determination of [ ]. Theoretical analysis [Nishida et al., 2001, 2002] suggests that the dominant contribution responsible for the appearance of the peak in the / sp/c versus c plots comes from the intermolecular electrostatic repulsions between polyions. Based on this idea, Nishida et al. [2002] propose a method to determine the intrinsic viscosity of polyelectrolyte solutions at very low ionic strength, by assuming additivity in the contributions of intra- and intermolecular interactions that is. 

It was proved that for molecular weights above 400,000 the shear effects becomes important even to the usual concentrations for the determination of intrinsic viscosity, [tj], at the specific shear gradients of capillary viscometers. The shear effects are higher in the case of polyelectrolytes solutions in good solvents and also in solutions containing rigid macromolecules. 

This section could have followed section 1 in this chapter. The reason for placing it here is that the concepts and derivations developed in section 2 are needed for deriving the viscosity equation for polymeric electrolytes (polymer solutions) and polymer melts. Similar to the previous two sections, the viscosity of polyelectrolytes and polymer melts without an external electric field are discussed first, and then the viscosity of those materials under an external electric field or having strong surface charges are focused upon thereafter. 

Polyelectrolytes are used to modify the interaction (i.e., to increase electrostatic repulsion) between particles when absorbed on a surface. They are used in hair conditioners to coat hairs and reduce static (conditioners also contain silicones to provide a smooth coating on the hair surface). Polyelectrolytes also modify the viscosity of aqueous solutions and are extensively used as emulsifiers and thickeners. Polyelectrolyte gels based on polyacrylates are used as super-absorbent gels (termed hydrogels) in baby nappies (diapers, US). The materials are able to swell up to many... 

It has been reported that the viscosities of aqueous solutions of a number of polyelectrolytes, such as sodium salts of poly(acrylic acid), carboxymethyl cellulose and copolymer of maleic and vinyl acetate, decrease rapidly when they are UV irradiated, due to chain scission reactions [1049]. 

Contrary to Eq. (2,33) the reduced viscosity of polyelectrolyte solutions is observed to decrease strongly with increasing concentration at low ionic strength. In the late 1940s this experimental fact inspired Fuoss and Strauss [67-70] to propose their famous empirical linearization of the reduced viscosity... 

The viscosity of sodium algiaate solutioas is slightly depressed by the additioa of moaovaleat salts. As is frequeatly the case with polyelectrolytes, the polymer ia solutioa coatracts as the ionic strength of the solution is increased. The maximum viscosity effect is obtained at about 0.1 N salt concentration. 

It was found earlier by experiment and theory that the viscosity intrinsic of polyelectrolyte solutions is nearly linear with the reciprocal square root of the ionic strength over a certain range, such as... 

The viscosity and non-Newtonian flooding characteristics of the polymer solutions decrease significantly in the presence of inorganic salts, alkali silicates, and multivalent cations. The effect can be traced back to the repression of the dissociation of polyelectrolytes, to the formation of a badly dissociating polyelectrolyte metal complex, and to the separation of such a complex fi"om the polymer solution [1054]. 

The viscosity of the oxidized polymer (VIII) was determined using DMF as a solvent. Chloroform was not a good solvent because it was too volatile and resulted in poor reproducibility. The reduced viscosities are plotted against polymer concentration (Figure 6). Polymer VIII behaved like a polyelectrolyte, the reduced viscosities increased sharply on dilution in a salt free solution. The addition of 0.01 M KBr did not completely suppress the loss of mobile ions however, at 0.03 M KBr addition a linear relationship between the reduced viscosities and concentration was established. 

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