Suspensions sediment volume

Suspensions are generally evaluated with respect to their particle size, electrokinetic properties (zeta potential), and rheological characteristics. A detailed discussion on the methods/techniques and relevant instrumentation is given in Sec. VII. A number of evaluating methods done specifically with suspension dosage forms, such as sedimentation volume, redispersibility, and specific gravity measurements, will be treated in this section. 

The sedimentation volume of a pharmaceutical suspension can be evaluated using simple, inexpensive, graduated, cylindrical graduates (100-1000 mL). It is defined as the ratio of the equilibrium volume of sediment, Vu, to the total volume of the suspension, V0. 

The degree of flocculation, / , is defined as the ratio of the sedimentation volume of the flocculated suspension, F, to the sedimentation volume of the suspension when deflocculated, F,yj. It is expressed as ... 

Great care must be exercised when using graduated cylinders because decreases in the diameter of small containers can produce a wall effect, which often affects the settling rate or ultimate sedimentation volume of flocculated suspensions. Such small containers have a tendency to hold up the suspensions due to adhesive forces acting between the container s inner surface and the suspended particles. 

JB Kayes. Pharmaceutical suspensions relation between zeta potential, sedimentation volume and suspension stability. J Pharm Pharmacol 29 199-204, 1977. 

The colloidal stability of silica Suspensions in the present work was assessed by sediment volumes and from the optical coagulation rate constant. In the first method, 50 mg of silica was dispersed in 5 cm3 polymer solution (concentration 10-2 g cm 3) in a narrow tube and the sediment height found at equilibrium. Coagulation rates of the same systems were found by plotting reciprocal optical densities (500nm, 1cm cell) against time. When unstable dispersions were handled, the coagulation was followed in... 

Adsorption of nonionic and anionic polyacrylamides on kaolinite clay is studied together with various flocculation properties (settling rate, sediment volume, supernatant clarity and suspension viscosity) under controlled conditions of pH, ionic strength and agitation. Adsorption and flocculation data obtained simultaneously for selected systems were correlated to obtain information on the dependence of flocculation on the surface coverage. Interestingly, optimum polymer concentration and type vary depending upon the flocculation response that is monitored. This is discussed in terms of the different properties of the floes and the floe network that control different flocculation responses. Flocculation itself is examined as the cumulative result of many subprocesses that can depend differently on system properties. 

Among the properties measured here, the settling rate is mainly a measure of the size of the floes and in later stages the compressibility of floes and floe networks, and the supernatant clarity is a measure of the size distribution of floes and size dependent capture of the particles and floes by the polymer. The sediment volume and the pulp viscosity on the other hand, are direct measures, not only of floe size and structure but also of adsorbed polymer layers. It is to be noted in this regard that it is this latter aspect which makes it possible to estimate the thickness of adsorbed polymer layers by measuring the viscosity of the medium and the suspension in the presence of polymers (20,21). This combination of effects is another reason one cannot always expect correlation between various flocculation responses. 

Polymers may show an optimum flocculation concentration which depends on molecular weight and concentration of solids in suspension. Overdosing with flocculant may lead to restabilisation(44), as a consequence of particle surfaces becoming saturated with polymer. Optimum flocculant concentrations may be determined by a range of techniques including sedimentation rate, sedimentation volume, filtration rate and clarity of supernatant liquid. 

MICHAELS, A. S. and Bolger, J. C. Ind. Eng. Chem. Fundamentals 1 (1962) 24. Settling rates and sediment volumes of flocculated kaolinite suspensions. 

Most of the cases concerned granules that were suspended in water before the administration. Consequently, the properties of the suspension thus formed were assessed. The parameters that attracted most attention were the relative sediment volume (volume of sediment/total volume) and the redi-spersability. See Chapter 5.3 for details on the suspensions. 

The most important parameters for the physical stability of suspensions are the relative volume of sediment (= volume of sediment/total volume) and the redispersability. They are tested after 1 -4 weeks have elapsed. 

Sediment in an aged dispersion may be collected and measured in a crudely quantitative test. To distinguish between a deflocculated, a flocculated, and an aggregated suspension, a weighed amount of solid is uniformly suspended in a small quantity of liquid, the suspension is transferred to a graduate cylinder, the volume of sediment during a stated period of time is measured, and the specific sediment volume (milliliters per gram) vs time is plotted. In... 

The sedimentation volume of mineral powders is also affected by the electric charge on the surface of the particles,4 just as the state of dispersion of suspensions and suspensoid colloids depends on the amount of this... 

This phenomenon has been applied to the present pesticidal suspension and the results obtained (using rheological and sediment volume experiments) are given in the present paper. 

Sediment volume and redispersion For sediment volume experiments a 50% w/v suspension was prepared using a 2% w/w of the polymer. 5g of the resulting suspension was added to 5 ml solutions of PEO to cover a wide concentration and the resulting suspension placed in stoppered cylinders and kept in constant temperature cabinets (25 1 C). The sediment height was followed with time for several weeks until equilibrium was reached. At this point the tubes were mechanically Inverted end-over-end and the number of revolutions required for redispersion was noted. 

In the literature, the reported use of zeta potential measurement for non-aqueous suspensions is relatively infrequent because non-aqueous suspensions only represent a small percentage of all medicated suspensions. Su and others evaluated the flocculation-deflocculation behavior of cefazolin sodium in non-aqueous media and the effect of surfactants as measured by zeta potential along with sedimentation and porosity measurements. A significant difference in zeta potential was observed when the particles were dispersed in peanut oil and ethyl oleate. The addition of lecithin reduced the zeta potential of cefazolin sodium, resulting in a deflocculated state accompanied by a decrease in sedimentation volume. The effect of surfactant... 

With some suspensions it was actually found that the sediment volume was directly proportional to the viscosity over a certain range, which confirms that Stokes Law can be applied here [370]. 

When micronized crospovidone of low bulk density is used in such suspensions, it is found beneficial in practice to combine it with other auxiliaries such as sodium citrate as an electrolyte, sugar, poloxamer or povidone, to increase the sediment volume. In the example given in Section 2.4.6.2 (Fig. 55), this is done by adding povidone K 90. A suspension of 7.5% of low-density micronized crospovidone with 5% povidone K 90 showed no further sedimentation after a 24-hour test. 

Fig. 86. Effect of the concentration of micronized crospovidone of low bulk density on the sediment volume of an amoxicillin suspension (Table 143)...

Again, the quantity of micronized crospovidone was determined from the relative sediment volume. As is evident from Fig. 87, no sedimentation was observed above a crospovidone concentration of 9% in the final suspension (= 29 g crospovidone in the sales product). After several weeks, it was still very easy to redisperse the suspension with a few rocking movements. 

---