Yield value sedimentation

In figure 6 are presented some of the available 210Pb profiles in deep sea sediments. The data available to-date yield values ranging between (20-400) cm2/103 year for particle mixing coefficients, K [69]. The data also show that the mixing coefficients do not exhibit any systematic trend either with the sediment type or with sedimentation rate [69]. 

At pH 12, the disulfide and noncovalent bonds are both broken, and the monomer with a sedimentation constant of 1.45 Svedberg units is released. From frictional ratios, the monomer appears to exist as a coil with a diameter of 16 A and a length of 150 A. Analysis of the primary structure of K-casein (Loucheux-Lefebvre et al. 1978) suggests considerable secondary structure in the monomer. 23% a-helix, 31% /3-sheets, and 24% 0-turns. In contrast, other investigators, using several different approaches, obtained a-helix contents ranging from 0 to 20.8% (Bloomfield and Mead 1975). Circular dichroism spectra on the monomer indicated 14 and 31% for a-helix and / -sheet, respectively (Loucheux-Lefebvre et al 1978). An earlier study of the optical rotatory dispersion of the K-casein monomer yielded values for the a-helix content ranging from 2 to 16% (Herskovits 1966). 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

Creaming is the separation of dispersed droplets from the continuous phase under the action of gravity. The droplets do not coalesce when they touch. If the dispersed medium is less dense than the continuous phase, the emulsion will cream upwards. Conversely, for a more dense dispersed phase, the emulsion will cream downwards or sediment. Creaming will inevitably occur in any dilute emulsion in accordance with Stokes s law if the phases are not exactly equal in density. Creaming is, therefore, a trivial form of instability that can be avoided by addition of thickeners that confer plastic rheology, and, therefore, yield value to the aqueous phase. 

In sedimentation velocity experiments, the centrifnge is rnn at high speeds (up to -80,000 rpm), and the rate of sedimentation of the bonndary between the solvent and the sedimenting protein is measured to yield the sedimentation coefficient of the molecule (or molecules). This value is related to the size and shape of the molecule and is measnred in Svedberg units (denoted by S, and formally it is equal to 10 s). E.g., the 30S subunit of the ribosome has a sedimentation coefficient of 30 S. The Svedberg of Sweden invented the analytical ultacentrifnge, and he received the 1926 Nobel Prize in Chemistry for his work characterizing the hydrodynamic and mass properties of proteins. 

The above systems show a yield value , xp, and a high viscosity at low shear rates [residual or zero shear viscosity //(o)]. Providing xp is higher than a certain value (> 0.1 Pa) and //(o) > 1000 Pa s, no sedimentation occurs with most suspensions. 

The most common procedure for eliminating creaming or sedimentation is to use a thickener in the continuous phase, e.g. a high molecular weight polymers such as hydroxyethyl cellulose or xanthan gum. These thickeners produce a gel in the continuous phase that has a yield value (> 0.1 Pa) and a high zero shear viscosity (>1000 Pa s), thus preventing any creaming or sedimentation. 

Rheological measurements are used to investigate the bulk properties of suspension concentrates (see Chapter 7 for details). Three types of measurements can be applied (1) Steady-state shear stress-shear rate measurements that allow one to obtain the viscosity of the suspensions and its yield value. (2) Constant stress or creep measurements, which allow one to determine the residual or zero shear viscosity (which can predict sedimentation) and the critical stress above which the structure starts to break-down (the true yield stress). (3) Dynamic or oscillatory measurements that allow one to obtain the complex modulus, the storage modulus (the elastic component) and the loss modulus (the viscous component) as a function of applied strain amplitude and frequency. From a knowledge of the storage modulus and the critical strain above which the structure starts to break-down , one can obtain the cohesive energy density of the structure. 

The Zeleny (Zeleny 1947), sedimentation, and microsedimentation tests (AACC Methods 56-60,56-61A and 56-63) are based on a 5 min sedimentation volume of a fixed flour-water slurry stained with bromophenol and a weak acidic acetic-isopropanol solution (Figure 15.4). Samples containing high gluten yield higher sedimentation values. The values are also adjusted according to protein content. 

Although the stable sediment is the denser one, it flows for low rates of shear Hkc a (very viscous) NEWTONian liquid, whereas the flocculated sediment on the contrary often shows a pronounced yield value and non-NEWXONian behaviour. 

---