Physical stability assessment

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). 

Crowley, K.J. Zografi, G. Cryogenic grinding of indomethacin polymorphs and solvates assessment of amorphous phase formation and amorphous phase physical stability. J. Pharm. Sci. 2002, 91, 492-507. 

Clas, S.-D. Cotton, M. Moran, E. Spagnoli, S. Zografi, G. Vadas, E.B. Assessment of the physical stability of lyo-philized MK-0591 by differential scanning calorimetry. Thermochim. Acta 1996, 288, 83-96. 

The effect of environmental moisture on the physical stability of effervescent tablets in foil-laminate packages containing microscopic imperfections was examined. Physical stability, after storing at different RH and temperature conditions, was assessed by noting whether the tablet components reacted prematurely. 

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 ... 

The physical stability of a colloidal system is determined by the balance between the repulsive and attractive forces which is described quantitatively by the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. The electrostatic repulsive force is dependent on the degree of double-layer overlap and the attractive force is provided by the van der Waals interaction the magnitude of both are a function of the separation between the particles. It has long been realized that the zeta potential is a good indicator of the magnitude of the repulsive interaction between colloidal particles. Measurement of zeta potential has therefore been commonly used to assess the stability of colloidal systems. 

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. 

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

Th. E. Tadros, Control and Assessment of the Physical Stability of Pesticidal Suspension Concentrates , Chem. Ind. (London), 1980, 211. 

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