Ionic viscosity

Fig. 3. Variation of the apparent molal volumes and the ionic viscosity coefficient B across the lanthanide series.

The ionic viscosity enhancing substances are usually much more sensitive to pH-changes than the non-ionic. However, the addition of electrolytes, and also of acids and bases, affect thickeners as a result of increasing ionic strength or influencing the zeta potential (see Sect. 18.4.2). 

Bentonite and colloidal aluminium magnesium silicate are of mineral origin and do not exhibit all colloidal characteristics. They exhibit rheological properties such as becoming more fluid under stress but they are not always transparent. The colloidal systems of silicates have thus characteristics transitional to the coarser disperse systems. They hardly dissociate and are generally not split into ions. Therefore they are categorised (Table 23.14) under non-ionic viscosity enhancers, despite the fact that some dissociation occurs. 

Another quantity of interest is the ionic viscosity -coefficient. Equation 2.29, that in aqueous solutions describes the effect of the ion on the structure of the solvent water. Some values of in nonaqueous solvents have been compiled by Jenkins and Marcus [9] and are reproduced in Table 5.7. It should be noted that practically in all the solvents (except light and heavy water), all the Brj values are positive and the ions appear to enhance the structure of the solvent. However, the splitting of the... 

TABLE 5.7 The Ionic Viscosity B-coefficients in Nonaqneons Solvents at 25°C... 

With the knowledge now of the magnitude of the mobility, we can use equation A2.4.38 to calculate the radii of the ions thus for lithium, using the value of 0.000 89 kg s for the viscosity of pure water (since we are using the conductivity at infinite dilution), the radius is calculated to be 2.38 x 10 m (=2.38 A). This can be contrasted with the crystalline ionic radius of Li, which has the value 0.78 A. The difference between these values reflects the presence of the hydration sheath of water molecules as we showed above, the... 

From equation A2.4.38 we can, finally, deduce Walden s rule, which states that the product of the ionic mobility at infinite dilution and the viscosity of the pure solvent is a constant. In fact... 

Eactors that could potentiaHy affect microbial retention include filter type, eg, stmcture, base polymer, surface modification chemistry, pore size distribution, and thickness fluid components, eg, formulation, surfactants, and additives sterilization conditions, eg, temperature, pressure, and time fluid properties, eg, pH, viscosity, osmolarity, and ionic strength and process conditions, eg, temperature, pressure differential, flow rate, and time. 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

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. 

Polyoxymethylene Ionomers. Ionic copolymers have been prepared from trioxane and epichlorohydrin, followed by reaction with disodium thioglycolate (76). The ionic forces in these materials dismpt crystalline order and increase melt viscosity (see Acetalresins). 

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

Water-Based Muds. About 85% of all drilling fluids are water-based systems. The types depend on the composition of the water phase (pH, ionic content, etc), viscosity builders (clays or polymers), and rheological control agents (deflocculants or dispersants (qv)). 

High molecular weight polyacrylamides are used as viscosity builders in freshwater muds (53) or as bentonite extenders. The ionic nature of the polyacrylamide may range from nonionic to anionic (30% hydrolyzed) depending on the situation. Molecular weights ranging from >3 x 10 are used for this purpose. Polymer concentrations of 0.7—2.8 kg/m (0.25—1.0 Ib/bbl) are used depending on the appHcation. 

The physical picture in concentrated electrolytes is more apdy described by the theory of ionic association (18,19). It was pointed out that as the solutions become more concentrated, the opportunity to form ion pairs held by electrostatic attraction increases (18). This tendency increases for ions with smaller ionic radius and in the lower dielectric constant solvents used for lithium batteries. A significant amount of ion-pairing and triple-ion formation exists in the high concentration electrolytes used in batteries. The ions are solvated, causing solvent molecules to be highly oriented and polarized. In concentrated solutions the ions are close together and the attraction between them increases ion-pairing of the electrolyte. Solvation can tie up a considerable amount of solvent and increase the viscosity of concentrated solutions. 

Latex Types. Latexes are differentiated both by the nature of the coUoidal system and by the type of polymer present. Nearly aU of the coUoidal systems are similar to those used in the manufacture of dry types. That is, they are anionic and contain either a sodium or potassium salt of a rosin acid or derivative. In addition, they may also contain a strong acid soap to provide additional stabUity. Those having polymer soUds around 60% contain a very finely tuned soap system to avoid excessive emulsion viscosity during polymeri2ation (162—164). Du Pont also offers a carboxylated nonionic latex stabili2ed with poly(vinyl alcohol). This latex type is especiaUy resistant to flocculation by electrolytes, heat, and mechanical shear, surviving conditions which would easUy flocculate ionic latexes. The differences between anionic and nonionic latexes are outlined in Table 11. 

Low ionic impurity levels are imperative. In order to reduce the coefficient of thermal expansion of the final mol ding, and hence minimise stresses on the encapsulated siHcon chip, the highest possible filler loading is desired. This has to be balanced against the need to maintain as low a melt viscosity as possible to minimise the possibiHty of damage to the device during the encapsulation process. 

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