Volume suspensions

Figure 5.2. Effect of height on sedimentation of a 3 per cent (by volume) suspension of calcium carbonate...

Streptokinase (250,000 units, Kabi, Stockholm, Sweden) was dissolved in 0.1 M trishydroxymethylmethane hydrochloride pH 7.4 (2.5 ml) and a solution of 0.1 M p-amidinophenyl p -anisate in dimethylsulphoxide (0.25 ml) added. A slight cloudiness resulted. To this mixture was added 0.5 ml of a solution of human lys-plasminogen (8.99 mg/ml in the above buffer) [Kabi, Stockholm] and the solution was thoroughly and rapidly mixed. After standing on ice for 15 minutes, the solution was stored on ice until use. After approximaely 2 hours 0.4 ml (40,000 units) of the material was diluted with 3.0 ml of a slurry of L-lysine-sepharose 4B (a 33% wet wt/volume suspension in the above buffer) and stood at 0°C for 1 hour. The gel was filtered on a glass sinter... 

An early comparison of US and dielectrophoretic separations revealed the lower size limits of microparticles (0.65 pm for single particles and 14 nm for particle ensembles) manipulated by dielectrophoresis to be similar to those for ultrasonic fields (0.25 pm in intermediate volume suspensions to 40 nm in microchamber assemblies). Unlike US-assisted separations, dielectrophoretic separations require either very low volumes to avoid heating in salt-containing suspensions or desalination prior to separation in the field [111]. 

Problem 6.3(a) (Worked Example) At what critical shear rate do you expect the onset of shear thinning in a 40% (by volume) suspension of hard spheres of radius 1 tim in water at room temperature (Hint use the data of Fig, 6-4, plus scaling principles.)... 

For a given volume fraction, the smaller particle dispersions gave higher viscosity. This may be due to more particle-particle interactions because of the larger interfacial area. Parkinson et al., varied the fraction of small particles (0.1 micron) in 117, by volume suspensions. Increasing the concentration of the 0.1 micron... 

Another widely used particle size analyser is based on the forward scattering of laser light through a dilute (< 1 % by volume) suspension of crystals retained in a small ( 10mL) agitated cell. The resulting Fraunhofer diffraction pattern is detected and translated, by means of the instrument software, into a particle size distribution (BS ISO 13320, 2000). 

Estimate the hindered settling velocity of a 25% (by volume) suspension of 200pm glass beads in an inelastic carboxymethyl cellulose solution (n = 0.8 and m = 2.5Pa S") in a 25 mm diameter tube. The density of glass beads and of the polymer solution are 2500 kg/m and 1020kg/m respectively. 

The efihient ftom a ftictoiy consists of constant flow of 3 m /h of a 3% by volume suspension of solids in water. Before the liquid efiluent can be discharged to river it is necessary to fdter out the solids. The Alters available can handle 0.6 m /hr of suspension and therefore the efOuent must be concentrated in a large tank, eflbctive dimensions 6x4x3m deep, which may be subdivided and used as a batch settler. Batch settling data for the effluent is as follows ... 

Fig. 9 Dynamic surface tension of mixtures between a highly surface-active model suspension and solutions (1 g/L) of modified natural polymers at the same mixing ratio (volume suspension volume polymer) of 10 1 dynamic surface tension, was measured by bubble pressure tensiometer (t = 60 s)...

To find out the kinetic stability of the pore-generating agent distribution in the suspension volume, suspensions with variable AIO(OH) content were evaluated. For the stability evaluation, the modified Andreasen s method was employed. The suspension with the pore-generating agents (range 3wt% - 5 wt %) was poured into a cylinder (15.10 m diameter with height level of 8.10 m). After 1 and after 3 hours, 3,5. lO l of suspension... 

Marlow and Rowell discuss the deviation from Eq. V-47 when electrostatic and hydrodynamic interactions between the particles must be considered [78]. In a suspension of glass spheres, beyond a volume fraction of 0.018, these interparticle forces cause nonlinearities in Eq. V-47, diminishing the induced potential E. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

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]... 

Although the remainder of this contribution will discuss suspensions only, much of the theory and experimental approaches are applicable to emulsions as well (see [2] for a review). Some other colloidal systems are treated elsewhere in this volume. Polymer solutions are an important class—see section C2.1. For surfactant micelles, see section C2.3. The special properties of certain particles at the lower end of the colloidal size range are discussed in section C2.17. 

Even when well defined model systems are used, colloids are ratlier complex, when compared witli pure molecular compounds, for instance. As a result, one often has to resort to a wide range of characterization teclmiques to obtain a sufficiently comprehensive description of a sample being studied. This section lists some of tire most common teclmiques used for studying colloidal suspensions. Some of tliese teclmiques are discussed in detail elsewhere in tliis volume and will only be mentioned in passing. A few teclmiques tliat are relevant more specifically for colloids are introduced very briefly here, and a few advanced teclmiques are highlighted. 

Altliough tire behaviour of colloidal suspensions does in general depend on temperature, a more important control parameter in practice tends to be tire particle concentration, often expressed as tire volume fraction ((). In fact, for hard- sphere suspensions tire phase behaviour is detennined by ( ) only. For spherical particles... 

Method(B). Add3g. (3ml.)ofbenzonitrileto50ml.of lo-volumes hydrogen peroxide in a beaker, stir mechanically and add i ml. of 10% aqueous sodium hydroxide solution. Warm the stirred mixture at 40° until the oily suspension of the nitrile has been completely replaced by the crystalline benzamide (45-60 minutes). Cool the solution until crystallisation of the benzamide is complete, and then filter at the pump and recrystallise as above. One recrystallisation gives the pure benza-mide, m.p. 129-130° yield of purified material, 2-2-5 S ... 

If no solid precipitate is obtained, an oil or an oily suspension, may be produced. Allow to stand, and then, if possible, separate the oil directly in a separating Tunnel and dry with solid KOH. If the volume of the oil is too small for such separation, extract with ether and then separate the ethereal solution, dry as before, filter, and distil off the ether. Distil the amine (if considered necessary) and identify. 

An interesting historical application of the Boltzmann equation involves examination of the number density of very small spherical globules of latex suspended in water. The particles are dishibuted in the potential gradient of the gravitational field. If an arbitrary point in the suspension is selected, the number of particles N at height h pm (1 pm= 10 m) above the reference point can be counted with a magnifying lens. In one series of measurements, the number of particles per unit volume of the suspension as a function of h was as shown in Table 3-3. 

Place 84 g. of iron filings and 340 ml. of water in a 1 - 5 or 2-litre bolt-head flask equipped with a mechanical stirrer. Heat the mixture to boiling, stir mechanically, and add the sodium m-nitrobenzenesulphonate in small portions during 1 hour. After each addition the mixture foams extensively a wet cloth should be applied to the neck of the flask if the mixture tends to froth over the sides. Replace from time to time the water which has evaporated so that the volume is approximately constant. When all the sodium salt has been introduced, boU the mixture for 20 minutes. Place a small drop of the suspension upon filter paper and observe the colour of the spot it should be a pale brown but not deep brown or deep yellow. If it is not appreciably coloured, add anhydrous sodium carbonate cautiously, stirring the mixture, until red litmus paper is turned blue and a test drop upon filter paper is not blackened by sodium sulphide solution. Filter at the pump and wash well with hot water. Concentrate the filtrate to about 200 ml., acidify with concentrated hydrochloric acid to Congo red, and allow to cool. Filter off the metanilic acid and dry upon filter paper. A further small quantity may be obtained by concentrating the mother liquid. The yield is 55 g. 

Sodium acetyllde. Replace the ammonia - addition tube by a wide tube reaching almost to the bottom of the flask (or use the device depicted in Fig. II, 7, 12, b) and pass acetylene (Fig. VI, 16, 1, c) into the suspension of sodamide in liquid ammonia maintain the bath temperature at about — 35° so that little ammonia is lost. Continue the passage of acetylene until a uniformly black liquid is formed (usually 4r-5 hours) (7). Carefully watch the wide gas entry tube if much solid collects inside this tube, remove it before the tube is completely blocked. Add liquid ammonia, if necessary, to restore the original volume (ca. 3 5 litres). 

If filtration is slow, the following procedure may be used. Place the fine suspension in a large evaporating dish and evaporate to dryness on a water bath. Dissolve the resulting sticky mass in the minimum volume of dilute alcohol (1 volume of water 3 volumes of methylated spirit about 200-260 ml.) and allow... 

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. 

Next we consider replacing the sandwiched fluid with the same liquid in which solid spheres are suspended at a volume fraction unit volume of liquid-a suspension of spheres in this case-the total volume of the spheres is also 0. We begin by considering the velocity gradient if the velocity of the top surface is to have the same value as in the case of the... 

Now we return to consider the energy that must be dissipated in a unit volume of suspension to produce a unit gradient, as we did above with the pure solvent. The same fraction applied to the shearing force will produce the unit gradient, and the same fraction also describes the volume rate of energy dissipation compared to the situation described above for pure solvent. Since the latter was Po, we write for the suspension, in the case of dv/dy = 1,... 

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