Preparation of Suspensions

Suspensions can be formed either by nudeation or by subdivision and dispersion. The nudeation process requires a phase change, such as condensation of vapour to yield solid, or precipitation of a salt from a supersaturated solution. In the latter case a supersaturated solution must be formed. The supersaturation condition is then alleviated by condensation on nudei (which need not be composed of the same material) already present, or else by formation of nuclei with subsequent condensation. The nudei eventually grow to microscopic, or macroscopic, size. Additional details of this process are discussed elsewhere [49,320], 

Subdivision (dispersion) is also common in geologic systems. In this case dispersion involves weathering of parent rock (physical disintegration and/or chemical dissolution with re-precipitation) followed by suspension of the particles in liquid. The physical disintegration can be caused by a range of events [49]  

These are significant activities, that occur repeatedly over many cycles. According to Yariv and Cross during the geologic history of the earth more than 70% of weath- 

Another example of subdivision/dispersion is given by the classic disintegration of metals that occurs when two electrodes of the same metal are submersed and subjected to an electric potential that causes arcing (Bredig s arc). Here, metal melts at the electrode tips and becomes dispersed into suspension. Such suspensions can be stabilized by the presence of peptizing electrolyte. 

In industrial and laboratory settings the subdivision process more commonly involves the comminution of large particles or aggregates into smaller sizes, either dry with subsequent dispersion (size reduction to the order of a few pm) or directly in a slurry (size reduction to as small as a few tenths of pm). Examples of comminution machines include agitator ball mills, colloid mills, cutting mills, disk mills, homogenizers, jet mills, mechanical impact mills, ring-roller mills, and roll crushers. 

These are significant activities that occur repeatedly over many cycles. According to Yariv and Cross, during the geologic history of the earth more than 70% of weathered magmatic or metamorphic rocks have been reduced in grain size from diameters between 0.1 to several millimetres, to diameters of less than 20 pm [46]. 

Two main processes are utilised for the preparation of solid/liquid dispersions  

The role of surfactants in the preparation of suspensions using these two methods 

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Eor the preparation of suspensions and emulsions, coUoid mills and homogenizers, respectively, are used. Ultrasonic mills that utilize vibrating reeds in restricted chambers to reduce the particle size of the dispersed ingredients can also be employed (see Colloids Ultrasonics). 

A broad scope is documented for the preparation of suspensions of 2-alkenylpotassium lithium toV-butoxide complexes from unsaturated hydrocarbons by means of butyllithium/ potassium /erf-butoxidc (Schlosser Lochmann base LICKOR reagents )38-45,432 456. Examples are given in Section D.1.3.3.3.3.2.1. 

Uses (routes) Wetting agent (at 0.1 to 0.5%) in the preparation of suspensions. Most commonly used in water... 

Figure 7.7 Examples of (a) a colloid mill and (b) a ball mill used for the preparation of suspensions by the dispersion method. Figure (a) courtesy Premier Mill, Reading, PA Figure (b) courtesy Paul O. Abbe Inc., Little Falls, NJ.

Surfactants play a major role in the preparation of suspensions of polymer particles by heterogeneous nucleation. In emulsion polymerization, the monomer is emulsified in a nonsolvent (usually water) using a surfactant, whereas the initiator is dissolved in the continuous phase. The role of surfactants in this process is obvious since nucleation may occur in the swollen surfactant micelle. Indeed, the number of particles formed and their size depend on the nature of surfactant and its concentration (which determines the number of micelles formed). 

Preparation of Suspensions A standard 400 g leg aqueous pesticide suspension was prepared by a wet milling process using a Dyno mill (ex. Willy. A. Bachofen, Basle, Switzerland). 40g... 

Extemporaneous preparation of suspensions of drugs available commercially only in other dose forms is widely practised in hospital pharmacy, particularly for paediatric use. Dmgs such as acetazolamide, amiodarone and mercaptopurine are examples. In such formulations, alternatives to traditional suspending agents such as tragacanth should be examined. The ideal suspending agent should ... 

Various auxiliaries with different functions are used in suspensions or dry syrups and instant granules for the preparation of suspensions. These include thickeners, hydrophilic polymers as dispersing agents, sugars, surfactants, electrolytes, colorants,etc. [296]. 

In granules for the preparation of suspensions, povidone can also act as a binder (see Table 75). 

The other major application of polymeric dispersants is in the preparation of solid/hquid dispersions (usually referred to as suspensions). In general, two methods are used for the preparation of suspensions condensation and dispersion methods ... 

The preparation of suspensions and emulsions separately, allowing sufficient time for complete adsorption (equilibrium). 

Milling is a common technique for preparation of suspensions, while colloids are used to be directly synthesized in the solution. Complicated systems, e g., containing heterojunction, or inclusions of a catalyst are also prepared by chemical synthesis. Sometimes the semiconductor... 

Method of manufacture, although there is often a lack of important detail (e.g. clear protocols for the homogenisation step in the preparation of suspensions) impairing their reproducibility Special mention (if any) regarding its use... 

The preparation of sterile ophthalmic suspensions, as required by all official compendia, presents some difficulties since filtration as a sterilization method cannot be used. Even if heat sterilization is possible in some cases, it is to be avoided since it might cause partial dissolution of the drug at high temperature and separation of larger crystals on cooling. Therefore, aseptic methods are preferable. In some cases, final sterilization by y irradiation is possible. A detailed picture of different techniques for aseptic preparation of suspensions is given in Refs. 4.1 and 44. 

There are. however, many gums that have been (and still are) used in the preparation of suspensions some examples are (3,72) agar, alginates, guar gum. and xanthan gum. the latter being the most frequently found in i iarmaceutical and food suspensions. Figure 14 is a schematic representation of this polysaccha-... 

Bernhardt C (1988) Preparation of suspensions for particle-size analysis—methodical recommendations, liquids and dispersing agents. Adv Colloid Inter Sci 29 79-139... 

The laminar flow is disturbed more by operating with the open product as happens with the preparation of suspension-type eye ointments, than with eye drop preparation or by the mixing technique with coimected syringes for semisolid eye preparations (see Fig. 10.5). Consequently, a higher risk of contamination exists (see Sect. 31.3.2), which has to be accounted for in the risk assessment. 

In the preparation of suspension suppositories it is important that the used particles of the active substance are small and remain small (don t reagglomerate). Small particles being essential for a correct content and a sufficient content uniformity of the suppositories, dispersion of the active substance in the suppository base will usually be preceded by or combined with particle size reduction, see Sects. 29.2 and 29.3). Large primary particles should be ground and agglomerates should be broken up. If an active substance is not available in the required particle size, the coarse powder must be ground in a rough stone or porcelain mortar. Active substances kept in stock in the required primary particle size... 

For the preparation of suspension suppositories with a macrogol base the preparation process steps are compared with Sect. 11.5 as follows ... 

To obtain a good uniformity of content is the greatest challenge in the preparation of suspension suppositories and can only be achieved by a proper design of the (batch) preparation instruction and properly following it. So the outcome of determination of content uniformity is very relevant for the monitoring of the preparation process. 

Wetting means bringing a solid in close contact with a liquid. This process is of importance for example for the preparation of suspensions and the disintegration of a tablet in the gastrointestinal tract. 

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