Flocculated suspensions preparation

For comparison, flocculated suspensions at pHiep and suspensions with 10 vol% were prepared and investigated by the same procedures. 

Consideration must be given to establishing good physical stability of a suspension. If the particles settle and eventually produce a cake at the bottom of the container, they must redisperse readily to achieve dosage uniformity. Viscosity-enhancing agents can be used to keep the particles suspended. Preparation of flocculated suspensions is not recommended because the larger floes may irritate the eye. 

Currently, suspensions prepared from micronised active substances are the only marketed dehvery system for nebulisation of poorly water soluble substances such as steroids and cyclosporine [53]. Several problems are inherent in nebulising micro-suspensions and they vary from non-optimised lung deposition for the active substance to heterodispersity of the active substance concentration in the aerosol droplets and poor compatibility with different types of nebulisers, particularly ultrasonic devices. Suspensions may also have poor stability and the two components (solid and liquid) tend to separate with time within the formulation by sedimentation or flocculation, depending on the particle density relative to that of the liquid. Several jet nebulisers can deliver suspensions quite effectively, even independently of the primary particle size [54], but ultrasonic devices may convert primarily the continuous phase into aerosol whereas vibrating mesh inhalers can be blocked by particles being larger than the pore diameter of the membrane. 

The -potential plays an important role in that it is widely used as a measure of the stability of colloidal suspensions. Suspensions prepared at pH values close to the isoelectric point (lEP) may flocculate fairly rapidly because the repulsion may not be sufficient to overcome the van dw Waals attraction. Farther away from the lEP, we should expect the rate of flocculation to be slower. In practice, for good stability, suspensions are often prepared at pH values comparable to those of the plateau regions of the -potential or electrophoretic mobility curve. For the data shown in Fig. 4.18, this corresponds to pH values of <5 or >7. 

Suspensions containing silane Z-6076 and titanate LICA 12, on the other hand, showed large peld stresses, higher viscosities and highly thixotropic flow behavior, indicating that toe suspension is highly flocculated. The suspension prepared with silane Z-6020 is also not as well dispersed as toe suspension with the zircoaluminate but the relatively low peld stress, lower viscosity and absence of thixotropy indicates a much improved dispersion as compared with the silane 2r 6076 and titanate LICA 12. 

Polymers are suspended as microparticles in the latex and interactions between these microparticles are prevented by the presence of adsorbed suspending agent and soap molecules. Blending results in a random suspension of dissimilar particles in the mixture of latexes, each unaffected by the other. Rate of flocculation depends entirely on the stabilizer and not on the polymer characteristics as such. Coagulated mass contains an intimate mixture of the polymers. Acrylonitrile butadiene styrene (ABS) polymers [23-25] may be prepared by this method. 

Flocculating agents can be simple electrolytes that are capable of reducing the zeta potential of suspended charged particles. Examples include small concentrations (0.01-1%) of monovalent ions (e.g., sodium chloride, potassium chloride) and di- or trivalent ions (e.g., calcium salts, alums, sulfates, citrates or phosphates) [80-83], These salts are often used jointly in the formulations as pH buffers and flocculating agents. Controlled flocculation of suspensions can also be achieved by the addition of polymeric colloids or alteration of the pH of the preparation. 

In Fig.7, the effect of the adsorption temperature of HPC in preparing the polymer-coated latices on the flocculation behavior of the system is demonstrated. It was found that both systems show almost the same behavior in flocculation, but evince a fairly different behavior in protection. The HPC-coated latex suspension treated at room temperature flocculated in a 1.2 Mol MgCl2 aqueous... 

Transmission electron microscopy micrographs (Fig. 13.24) also indicate an important characteristic of the supported particles. As in the case of suspensions, they are either aggregated or isolated. Support surface properties may he an important factor governing this aggregation. After deposition on the support, we observed that samples prepared from acidic hydrosols are characterized by the presence of aggregated particles constituting flocculates ranging from 10 to 200 nm, whereas samples prepared via basic hydrosols contain only isolated particles. The opposite was observed when hydrosols were concerned. These final states of the supported particles may be controlled... 

Electrokinetic Measurements. Electrophoretic mobilities were measured with a flat-cell apparatus manufactured by Rank Brothers, Cambridge, England. In addition, several mobility values were checked for accuracy with a Zeta Meter, New York. Mobilities were determined with a small volume of the suspension (approximately 25 cc) that had been prepared for the adsorption experiments. The pH of the solution was measured prior to determining the electrophoretic mobilities, which involved measuring the velocities of five to ten particles in each direction. An average value of the mobilities was recorded. Samples containing the flocculated particles were dipped into an ultrasonic bath for approximately one second prior to making the pH and mobility measurements. 

Step 3—In a separate step, styrene-acrylonitrile (SAN) resin is prepared by emulsion, suspension, or mass polymerization by free-radical techniques. The operation is carried out in stainless-steel reactors operated at about 75°C (167°F) and moderate pressure for about 7 hours. Tlie final chemical operation is the blending of the ABS graft phase with the SAN resin, plus adding various antioxidants, lubricants, stabilizers, and pigments. Final operations involve preparation of a slurry of fine resin particles (via chemical flocculation), filtering, and drying in a standard fluid-bed dryer at 121-132°C (250-270°F) inlet air temperature. 

Effect of Pyrite Particle Size on Dispersion. It was suspected that a lot of the apparently non-dispersed pyrite particles shown in Figure 2 was due to the settling of hoarse particles between 10 and 37 micrometers. Pyrite has a specific gravity of about 5 0, while that of coal is around 1.2-1.3 Therefore, a pyrite suspension having only particle size below 10 micrometers was prepared and tested. The results, which are also shown in Figure 2(b), showed that the minus 10 micrometer pyrite suspension remained very stable, with only 10 - 20% weight of the particles settled or flocculated. From these observations, it is believed that the selective dispersion of pyrite will be more effective for the smaller particle sizes. 

Suspensions are coarse dispersions of finely divided solids in a liquid. The solid particles have a mean particle size greater than 0.1 pm in diameter. Pharmaceutical suspensions are administered orally, topically, and parenterally and should avoid the following problems sedimentation, caking, flocculation, and particle growth. Physicochemical principles in the solid/liquid interface will be discussed in this section as they pertain to the preparation of good pharmaceutical suspensions. 

Using preformed sols instead of metal alkoxides as precursors is an attractive alternative in sol-gel preparation because recent advances in inorganic colloidal dispersions allow some control over the characteristics of the starting sols [11]. Often a colloidal suspension of sol particles is stabilized (i.e. prevented from flocculation) by pH adjustment. Thus, pH of the solution, which can be changed by the addition of either acid or base, is the single most important parameter in obtaining a gel from preformed sols. Other parameters that can impact on gel quality are the size and concentration of the starting sol particles. 

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