Assessment suspension stability

Sedimentation ratio is often used to assess suspension stability. Byron reported the sedimentation ratios for a 1% sodium fluoresein suspension formulation with different amounts of surfactant (sorbitan trioleate) after standing for 20 days at room temperature. The suspension formulation with the lowest sedimentation ratio had thebest-flocculated system. However, all formulations were easily redispersible one complete revolution of the container was sufficient to produce a homogeneous dispersion. There was no clear difference in the times taken to reach apparent sedimentation equilibrium. Physical stability of the formulation was determined according to ... 

Although high performance liquid chromatography (HPLC) is the preferred technique for assessing the stability of formulations, spectrophotometry can also be used. Girona et al. (1988) used this technique for assessing the stability of an ampicillin-dicloxacillin suspension. 

Figure 5 shows the calculated potential energy of interaction Vt of AI2O3 particles (t/ = 0.25 pm, A = 4.5 x 10 J, and 0.01 M ionic strength) as a function of the surface-to-surface distance of separation for various conditions of potential in an aqueous suspension. Note that the height of the potential energy barrier increases quite sharply as the potential becomes larger than a certain critical value ( 30 mV in Fig. 5). Therefore, the potential is a very good index of the magnitude of the repulsive interaction between colloid particles. Because of this, measurements of potential are most commonly used to assess the stability of a given colloidal sol.

Compared to emulsions and foams discussed earlier, assessment of the stability of suspensions is relatively straightforward in most cases. Bottle or centrifuge tests are commonly used. Samples of the suspension components, and the suspension stabilizer or destabilizer to be tested, if any, are mixed in bottles or centrifuge tubes in a specified way, then let stand or centrifuged at a specified -force level. After a defined period of time, the suspensions are examined. For this, a timescale appropriate to the process under consideration has to be set. 

The stability of colloids will be determined by the charge. The zeta potential obtained by some electrophoretic method is used to assess the stability. Pagnoux et al. [134] carried out a study on the temperature dependence of the zeta potential for two alumina samples. They found that an increase in temperature led to higher zeta potentials and would therefore eause a more stable suspension. Contrary to this, Tari et al. [135] reported a trend toward an xmeharged surface state with temperature for an alumina suspension and concluded that an enhaneement of the coagulation rate with increasing temperature is to be expected. 

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... 

Investigations of the rheological properties of disperse systems are very important both from the fundamental and applied points of view (1-5). For example, the non-Newtonian and viscoelastic behaviour of concentrated dispersions may be related to the interaction forces between the dispersed particles (6-9). On the other hand, such studies are of vital practical importance, as, for example, in the assessment and prediction of the longterm physical stability of suspensions (5). 

Thermal stabilities were assessed by the time-dependent change of melt viscosity at a constant temperature and shear rate (290°C, 50 s"1 respectively). Figure 6.4 shows that three ofthe six resins showed a significant drop in viscosity as a function of time at 290°C. The average decrease in viscosity for Kel-F 6050, Alcon 3000, and an experimental suspension is 37%. 

It is important to characterize the physicochemical properties of the suspensions well, so that the PK data can be interpreted appropriately. Typical characterization of the drug substance includes purity, residual solvents, aqueous solubility pro Lie (pH 2, FaSSIF), crystallinity (XRPD/DSC), particle size, pl and logP. For solution formulations at various stages of discovery studies, dose analysis is essential, and for efLcacy assessment and toxicology studies, chemical stability for the... 

Physical stability is assessed by placing samples of the elastic liposome suspension into vials that are flushed with nitrogen and sealed. The vials are stored under varied conditions such as light protected or exposed, refrigerated and at room temperature. At different time periods (e.g. 10, 20, 30 days and monthly up to 6 months) the samples are analysed for particle size and residual drug content. 

Several classes of formulations of disperse systems are encountered in the chemical industry, including suspensions, emulsions, suspoemulsions (mixtures of suspensions and emulsions), nanoemulsions, multiple emulsions, microemulsions, latexes, pigment formulations, and ceramics. For the rational preparation of these multiphase systems it is necessary to understand the interaction forces that occur between the particles or droplets. Control of the long-term physical stability of these formulations requires the application of various surfactants and dispersants. It is also necessary to assess and predict the stability of these systems, and this requires the application of various physical techniques. 

Two main procedures can be applied for the characterisation of suspensions and assessment of their stability (such as flocculation). The first method depends on the measurement of particle size distribution and the rate of flocculation and/or Ostwald ripening after dilution of the suspension with the dispersion medium, while the second procedure depends on measurement of the state of suspension without dilution, using rheological techniques. As both methods are described in detail in Chapters 19 and 20, only a summary will be provided here. 

Application of Rheological Techniques to Assess and Predict the Physical Stability of Suspensions... 

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