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Floc fragments

Floc fragment

Reaction 6

Secondary adsorption of polymer

Floc fragment

Restabilised floc fragment

• Be inert, nontoxic and free from incompatibilities

Among the alternatives are sodium carboxy-methylcellulose, microcrystalline cellulose, aluminium magnesium silicate (Veegum), sodium alginate (Manucol) and sodium starch (Primojel). Pregelatinised starches, Primojel and Veegum, are promising alternatives to compound tragacanth powder.

7.4.4 Suspension rheology

In deriving an equation for the viscosity of a suspension of spherical particles, Einstein considered particles which were far enough apart to be treated independently. The particle volume fraction 0 is defined by volume occupied by the particles (7 29) total volume of the suspension

The suspension could be assigned an effective viscosity, n *, given by n* = n0(i + 2.50)

where n0 is the viscosity of the suspending fluid. As we have seen, the assumptions involved in the derivation of the Einstein equation do not hold for colloidal systems subject to Brownian forces, electrical interactions and van der Waals forces. Brownian forces result from 'the random jostling of particles by the molecules of the suspending fluid due to thermal agitation and fluctuation on a very short time scale'.

A charged particle in suspension with its inner immobile Stern layer and outer diffuse Gouy (or Debye-Huckel) layer presents a different problem from that arising with a smooth and small nonpolar sphere. In movement such particles experience electroviscous effects which have two sources: (a) the resistance of the ion cloud to deformation, and (b) the repulsion between particles in close contact. When particles interact, for example to form pairs in the system, the new particle will have a different shape from the original and will have different flow properties. The coefficient 2.5 in Einstein's equation (7.30)

applies only to spheres; asymmetric particles will produce coefficients greater than 2.5.

Other problems in deriving a priori equations result from the polydisperse nature of pharmaceutical suspensions. The particle size distribution will determine n. A polydisperse suspension of spheres has a lower viscosity than a similar monodisperse suspension.

Structure formation during flow is an additional complication. Structure breakdown occurs also and is evident particularly in clay suspensions, which are generally flocculated at rest. Under flow there is a loss of the structure and the suspension exhibits thixotropy and a yield point. The viscosity decreases with increasing shear stress (Fig. 7.33).

Addition of electrolytes to a suspension decreases the thickness of the double layer and reduces electroviscous effects, an effect reflected in the reduced viscosity of the suspension.

7.4.5 Nonaqueous suspensions

Many pharmaceutical aerosols consist of solids dispersed in a nonaqueous propellant. Few studies have been published on the behaviour

Figure 7.33 A plot of shear stress t against shear rate D for plastic and pseudoplastic suspensions. As n = t/D, the slope of the line represents the viscosity at each rate of shear; in both the plastic and pseudoplastic systems the viscosity at level A is greater than that at level B.

Modified from J. C. Samyn, J. Pharm. Sci., 50, 517 (1961).

Figure 7.33 A plot of shear stress t against shear rate D for plastic and pseudoplastic suspensions. As n = t/D, the slope of the line represents the viscosity at each rate of shear; in both the plastic and pseudoplastic systems the viscosity at level A is greater than that at level B.

Modified from J. C. Samyn, J. Pharm. Sci., 50, 517 (1961).

of such systems, although their sensitivity to water is well established. Low amounts of water adsorb at the particle surface and can lead to aggregation of the particles with each other or to deposition on the walls of the container, which adversely affects the product.

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