Stability, of suspension

In order to quantify the sedimentation of suspended particles, the ratio R of sedimentation layer volume (l/j) to total suspension volume may be used. A measure of sedimentation may also be obtained from the height of the sedimented layer (h ) in relation to the initial height of the suspension (/iq). 

In a completely deflocculated system the particles are not associated pressure on the individual particles can lead in this layer to close packing of the particles to such an extent that the secondary energy barriers are overcome and the particles become irreversibly bound together. In flocculated systems (where the repulsive barriers have been reduced) particles settle as floes and not as individual particles. The supernatant clears but, because of the random arrangement of the particles in the floes, the sediment is not closely packed and caking does not readily occur. 

In flocculated or concentrated suspensions, zone settling occurs (Fig. 7.28). In the region 

A-B of Fig. 7.28(b) there is hindered settling of the particle interface at a constant rate at B-C a transitional settling occurs from C to D consolidation of the sediment occurs. 

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Stabilization of emulsions Stabilization of suspensions Stabilization of foams Control of crystal growth... 

Figure 6. Effect of magnesium aluminum silicate and cellu-losic hydrocolloids on stability of suspensions...

The stability of suspensions, emulsions, creams, and ointments is dealt with in other chapters. The unique characteristics of solid-state decomposition processes have been described in reviews by D. C. Monkhouse [79,80] and in the monograph on drug stability by J. T. Carstensen [81]. Baitalow et al. have applied an unconventional approach to the kinetic analysis of solid-state reactions [82], The recently published monograph on solid-state chemistry of drugs also treats this topic in great detail [83],... 

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). 

TGA results (Figure 6.5) confirm the improved stability of suspension polymers, especially when organic peroxides are used. 

In aqueous suspension, the stability is discussed in reference to the DLVO (Deryaguin-Landau-Verway-Overbeek) theory. Within this framework, all solid substances have a tendency to coagulate due to their large van der Waals attractive force. The coulombic repulsive force among colloidal particles more or less prevents this tendency. These two opposite tendencies determine the stability of suspensions. What kind of parameters are concerned in the present nonaqueous system, for which little is known about the stability This is an interest in this section. 

When suspensions are formulated to provide a stable system, the particle size becomes critical. Flocculated suspensions also require careful particle size control either in the process of manufacturing or in the starting material. Equally important is the crystal habit — the outward appearance of an agglomeration of crystals. Crystal structure can be altered during the manufacturing process, particularly if the product is subject to temperature cycling, and this can alter the stability of suspensions. 

In most cases, foam stabilities of suspensions at pH 2.5 and 5.0 indicated that 10% and 40% succinylated flours were not different from the nontreated sample. The pH 5.0 suspension containing flour that was treated with 80% succinic anhydride formed very stable foams. 

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. 

The stability of suspensions can also be increased by the adsorption of surfactants on the particles. Some guidelines include [193] ... 

The role of electrostatic repulsion in the stability of suspensions of particles in non-aqueous media is not yet clear. In order to attempt to apply theories such as the DLVO theory (to be introduced in Section 5.2) one must know the electrical potential at the surface, the Hamaker constant, and the ionic strength to be used for the non-aqueous medium these are difficult to estimate. The ionic strength will be low so the electric double layer will be thick, the electric potential will vary slowly with separation distance, and so will the net electric potential as the double layers overlap. For this reason the repulsion between particles can be expected to be weak. A summary of work on the applicability or lack of applicability of DLVO theory to non-aqueous media has been given by Morrison [268],... 

The adsorption of surfactants at the liquid/air interface, which results in surface tension reduction, is important for many applications in industry such as wetting, spraying, impaction, and adhesion of droplets. Adsorption at the liquid/liquid interface is important in emulsification and subsequent stabilization of the emulsion. Adsorption at the solid/liquid interface is important in wetting phenomena, preparation of solid/liquid dispersions, and stabilization of suspensions. Below a brief description of the various adsorption phenomena is given. 

Batch suspension reactors are, theoretically, the kinetic equivalent of water-cooled mass reactors. The major new problems are stabilization of the viscous polymer drops, prediction of particle size distribution, etc. Particle size distribution was found to be determined early in the polymerization by Hopff et al. (28, 29,40). Church and Shinnar (12) applied turbulence theory to explain the stabilization of suspension polymers by the combined action of protective colloids and turbulent flow forces. Suspension polymerization in a CSTR without coalescence is a prime example of the segregated CSTR treated by Tadmor and Biesenberger (51) and is discussed below. In a series of papers, Goldsmith and Amundson (23) and Luss and Amundson (39) studied the unique control and stability problems which arise from the existence of the two-phase reaction system. 

Betnovate scalp application is an aqueous suspension and contains carbomer, isopropyl alcohol, sodium hydroxide and purified water. Carbomer is a thickening agent and it is used to increase the stability of suspension/emulsion formulations. Isopropyl alcohol is often used in topical formulations. It may be used as a solvent or as a disinfectant (if >70% concentration). Sodium hydroxide would be used to adjust the pH of the formulation, specifically in this case... 

As mentioned earlier, polymerization techniques can also be used in the presence of nanotubes for preparation of polymer/CNT nanocomposite materials. In these, in-situ radical polymerization techniques of polymerization in the presence of CNT filler under or without applied ultrasound. Both new factors (presence of CNT and ultrasound) can affect reaction kinetics, stability of suspension or the size of prepared particles. For example, ultrasound waves can open C=C bond of monomer, which starts polymerization initiation. Thus vinyl monomers (styrene, methyl methacrylate or vinyl acetate) can be polymerized without addition of initiator, only by application of ultrasound. This is called sonochemical polymerization method (15,33,34). 

The numerous technological applications of adsorption from solution include liquid purification, the stabilization of suspensions, ore flotation, soil science, adhesion, liquid chromatography, detergency, enhanced oil recovery, lubrication, and last but not least, applications in the life sciences (e.g. adsorption by cell membranes, blood vessels, bones, teeth, skin, eyes, and hair). 

Gg (instantaneous modulus), Hg (residual viscosity) and G (shear modulus) all showed a rapid increase above 30g dm bentonite. This was attributed to the formation of a gel network structure in the continuous medium and the strength of such a gel increased with increase in bentonite concentration. The results could be qualitatively described in terms of the elastic floe model of Hunter and co-workers. Moreover, the settling characteristics of the structured suspensions were found to be consistent with the predictions from the rheological measurements. This demonstrates the value of rheological studies in predicting the longterm physical stability of suspension concentrates. 

Adsorption of surfactants has developed into a domain on Its own. So far we have only introduced the non-ionic part (sec. 2.7d). Abundant applications are found in detergency, flotation, enhanced oil recovery, drug administration and other pharmaceutical purposes, paints, cosmetics, ceramic materials and the stabilization of suspensions in general. 

D. Greiff, Stabilities of suspensions of influenza virus dried by sublimation of ice in vacuo to different contents of residual moisture and sealed under different gases. Appl. Microbiol. 20.-935-938 (1970). 

D. Grieff and W. A. Rightsel, Stabilities of suspensions of virus after freezing or drying by vacuum sublimation and storage. Cryobiology i(6) 432 44, 1967. 

Powder color, texture, particle size, fill weight, smell, case of reconstitution, reconstitution volume, viscosity, taste, color stability after reconstitution, and physical stability of suspension are critical blinding parameters. 

This is a useful guide to the physical stability of suspensions. If freeze—thaw cycling or elevated temperature exposures are chosen for physical stability testing, companion samples of a closely related marketed suspension should be included in the testing protocol for comparison purpose because pharmaceutical suspensions are not normally designed to withstand temperature extremes during storage (15-30°C optimum). 

Stabilization of suspensions by low-density micronized crospovidone as a hydrophilic polymer... 

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