The stability of suspensions

The equation that we gave in 6.4.2 for the velocity of a cloud of spheres moving in a liquid under the influence of gravity, is sometimes applicable to the stability of non-flocculated suspensions or emulsions, viz. 

Apis the density difference between the suspended and the continuous phases, 

Here the overall velocity V is the same as that of the interface between the concentrating suspension and the clear liquid left behind. 

Exercise consider what happens if salts are added to water to above their solubility limit, bearing in mind the effect of crystal shape. 

Sources of well-known polymeric material vary considerably, for instance 

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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],... 

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. 

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],... 

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... 

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). 

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. 

The results collected in this review are focused to demonstrate the advantages of electro-optics for investigation of the electrical properties of anisometric particles in dilute suspensions containing polyelectrolytes. Results on the structure of the adsorbed macromolecules and the stability of suspensions containing polyelectrolytes will also be discussed. 

The stability of finely dispersed suspension is determined on the optical density. The time of the suspension optical density conservation defines the stability of suspension. The activity of suspension is found on the bands intensity changes by means of IR and Raman spectra. The intensity increasing testify to transfer of NS surface energy vibration part on the molecules of medium or composition. The line speading in spectra testify to the growth of electron action of nanocomposites with medium molecules. Last fact is confirmed by x-ray photoelectron investigations. 

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 role of electrostatic repulsion in the stability of suspensions of particles in non-aqueous media is not entirely clear. In attempting to apply DLVO theory. 

Another parameter of current use in pharmaceutical practice for the characterization of the stability of suspensions is the flocculation ratio, or the extent of flocculation, p, defined as (106) ... 

Gmix is positive and the interaction is repulsive. The G,tux value increases very rapidly with a decrease in h when the latter is less than 18. This explains whey graft copolymers, such as Atlox 4913 or Hypermer CG-6(a) are ideal for the stabilization of suspensions in aqueous media. For the stabilization of dispersions in non-aqueous media, such as water-in-oil (W/0) emulsions, the stabilizing chain has to be soluble in the oil phase (normally a hydrocarbon oil). In this case, poly(hydroxystearic acid) (PHS) chains are ideal. An ABA block of PHS-PEO-PHS (Arlacel PI35, from UNIQEMA) is ideal for the stabilization of W/0 emulsions. 

The interfacial behaviour of surfactant-polymer mixtures, utilized for example in the stabilization of suspensions, depends on a complex interplay between different pair interactions. Addition of a polymer can either remove surfactant from a surface or enhance its adsorption, and vice versa, depending on the relative stability of the polymer-surfactant complexes in solution and at the interface. 

Abstract. The stability of suspensions/emulsions is under consideration. Traditionally consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. It is shown that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems. 

The particle size determines to a large extent the dissolution rate and later in the use of the final product may determine the bioavailability of poorly soluble compounds. The stability of suspensions and the homogeneity of powder mixtures may also be influenced. For a detailed description of the importance and reduction of particle size we refer to Sect. 29.2. If a substance does not have the required degree of fineness for the intended process then it will be necessary to bring it to that degree. 

Of the heterocyclic nitrogen-containing compounds, a-phenylindole is used to stabilize latex polymers and copolymers of vinyl chloride [99]. Melamine has foimd use for the stabilizing of suspension polymers, where a-phenylindole proves inactive [63]. Derivatives of morpholine [274], symmetrical triazine [275], pyrazole [276], as well as derivatives of imidazole, imidazoline, oxazole, oxazoline, thiazole, and thiazoline [277] are recommended in the patent literature for stabilization. 

The important effects in the stabilization of suspensions with uncharged polymer chains are summarized in Fig. 4.26 (45). 

In order to illustrate the above comments on the stability of suspensions and show the predictive power of the DLVO and extended-DL VO models, let us show... 

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