Suspension problems

Additional difficulties are encountered by elderly patients if a medication is in the form of a suspension. Problems may occur because a patient cannot see, or disregards, the words Shake Well on the label or is not able to exert the amount of agitation necessary to provide a uniform suspension. Certainly, unevenly distributed amounts of active ingredients throughout a suspension may result in serious consequences for a patient in terms of either under- or overdosing. 

Van Ooteghem, M. M. M. (1993), Formulation of ophthalmic solutions and suspensions. Problems and Advantages, in Edman, P, I id., Biopharm.aceu.tics of Ocular Drug Delivery, CRC Press, Boca Raton, FL, pp. 27 42. 

The magnitude of the free-settling velocity has proven useful in characterizing solid suspension problems into easy, moderate, or difficult categories, as reported in Table iJ ... 

A simple one-dimensional diffusion sedimentation model works quite well for solids distribution. This model shows that large settling velocity solids at high concentration are much more difficult to homogenize than those with a low settling velocity at low concentration. Experimental data fit this model quite well and probably help explain the old rule of thumb, solids with a settling velocity less than 5 mm/s represent an easy suspension problem and if greater than 30 mm/s, a difficult one . ... 

Other high-shear impellers include the tapered blade ChemShear impeller and dispersing disks such as the Cowles impeller. These provide excellent shear, but far less flow than the RDT. They are used primarily in small scale batch applications where dispersion time is not critical. Pitched blade tmbines (PBT) are used when large density differences could lead to a suspension problem. They require higher speed to create the same drop size as the RDT, since they have a lower power number. The flow discharge angle for PBTs varies with Reynolds number and blade angle. 

Apart from chemical composition, an important variable in the description of emulsions is the volume fraction, outer phase. For spherical droplets, of radius a, the volume fraction is given by the number density, n, times the spherical volume, 0 = Ava nl2>. It is easy to show that the maximum packing fraction of spheres is 0 = 0.74 (see Problem XIV-2). Many physical properties of emulsions can be characterized by their volume fraction. The viscosity of a dilute suspension of rigid spheres is an example where the Einstein limiting law is [2]... 

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

The amount of metal required gives an indication of the water content. note 3. If the conversion takes longer, add some liquid ammonia to keep the volume of the suspension between 500 and 800 ml. iinte 4. The conversion of lithium and potassium into the alkali amides has never given problems. 

An aerosol is a suspension of either a solid or a liquid in a gas. Fog, for example, is a suspension of small liquid water droplets in air, and smoke is a suspension of small solid particulates in combustion gases. In both cases the liquid or solid particulates must be small enough to remain suspended in the gas for an extended time. Solid aerosol particulates, which are the focus of this problem, usually have micrometer or submicrometer diameters. Over time, solid particulates settle out from the gas, falling to the Earth s surface as dry deposition. 

A somewhat similar problem arises in describing the viscosity of a suspension of spherical particles. This problem was analyzed by Einstein in 1906, with some corrections appearing in 1911. As we did with Stokes law, we shall only present qualitative arguments which give plausibility to the final form. The fact that it took Einstein 5 years to work out the bugs in this theory is an indication of the complexity of the formal analysis. Derivations of both the Stokes and Einstein equations which do not require vector calculus have been presented by Lauffer [Ref. 3]. The latter derivations are at about the same level of difficulty as most of the mathematics in this book. We shall only hint at the direction of Lauffer s derivation, however, since our interest in rigid spheres is marginal, at best. 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Because in the most common bottom feed version the feed suspension enters the dmm from the bottom, fast settling slurries are not suitable for most rotary dmm filters nor are very fine slurries because of inevitable penetration problems. 

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

Acryhc and methacryhc nonaqueous dispersions (NADs) are primarily utilized by the coatings industry to avoid certain difficulties associated with aqueous dispersion (emulsion) polymers. Water as a suspension medium has numerous practical advantages, but also some inherent difficulties a high heat of evaporation, a low boiling point, and an evaporation rate that depends on the prevailing humidity. Nonaqueous dispersions alleviate these problems, but introduce others such as flammabihty, increased cost, odor, and toxicity. 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

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