Sedimentation volume technique

Therefore to determine the particle surface tension by using the sedimentation volume technique, the required basic procedures in the experiments can be summarized as follows ... 

Stability may be inherent or induced. In the latter case, the original system is in a condition of metastable or neutral eouilibrium. External influences which induce instability in a dispersion on standing are changes in temperature, volume, concentration, chemical composition, and sediment volume. Applied external influences consist of shear, introduction of a third component, and compaction of the sediment. Interfacial energy between solid and liquid must be minimized, if a dispersion is to be truly stable. Two complementary stabilizing techniques are ionic and steric protection of the dispersed phase. The most fruitful approach to the prediction of physical stability is by electrical methods. Sediment volumes bear a close relation to repulsion of particles for each other. 

Suspensions are generally evaluated with respect to their particle size, electrokinetic properties (zeta potential), and rheological characteristics. A detailed discussion on the methods/techniques and relevant instrumentation is given in Sec. VII. A number of evaluating methods done specifically with suspension dosage forms, such as sedimentation volume, redispersibility, and specific gravity measurements, will be treated in this section. 

Polymers may show an optimum flocculation concentration which depends on molecular weight and concentration of solids in suspension. Overdosing with flocculant may lead to restabilisation(44), as a consequence of particle surfaces becoming saturated with polymer. Optimum flocculant concentrations may be determined by a range of techniques including sedimentation rate, sedimentation volume, filtration rate and clarity of supernatant liquid. 

Several other techniques have been designed to monitor the sedimentation of suspensions, of which photosedimentation, X-ray sedimentation and laser anemometry are worthy of mention. The simplest sedimentation test is based on the visual observation of settling, whereby the turbidity of a suspension is estimated visually, or the height of the sediment and sediment volume are recorded as a function of time. Although this visual estimation of sedimentation is only qualitative, it is adequate for many practical situations. 

W.M. Last, J.P. Smol (2001). Tracking Environmental Change using Lake Sediments. Volume 1 Basin Analysis, Coring, and Chronological Techniques. Kluwer Academic Press, The Netherlands. 

Frequently, solid surfaces are not available in homogeneous, flat-plate form. Especially in pharmaceutical applications, solids are usually powdered and in this case the estimation of surface free energy is better made by other methods, different from contact angle measurement.The two main techniques used with these solids arc the sedimentation volume experiments and the so-called thin-layer wicking method. The former method (22,26) is based on (he measurement... 

Overall, wettability measurement of small particles is a difficult problem that is further aggravated in the case of heterogeneous surfaces. Some of these problems can result from the presence of patches of different composition in the same particle. It is considered that if these patches are below a critical size of 0.1 mm, the surface is homogeneous regarding its wettability. Several indirect techniques have been developed to measure the surface tension, and thus the wettability of small particles. In these techniques, the surface tensions of the particles are derived from thermodynamic models and include the advancing solidification front or freezing front, sedimentation volume, and particle adhesion techniques [44, 45]. 

The sedimentation volume method is a well-established technique for studying the suspension stability of powders in liquids. The behaviour of such systems is governed mainly by van der Waals and electrostatic interactions. It is important to note that the former interactions can be related to the interfacial tensions of the solid particles suspended in a liquid (61), whereas the effect of the latter interactions can be considered negligible, e.g. for polymer particles, particularly in non-aqueous media (62). When two solid particles are brought into contact and if only van der Waals forces are considered, the free energy of adhesion of two solid phases in a liquid can be determined as follows ... 

In recent years, several sedimentation measurement techniques have been developed [1]. All involve monitoring the variation of some physical or chemical property over a defined sensing volume. The derived information is then used in conjimction with an initial calibration in order to determine the settling behaviour of a given suspension. However, the majority are unreliable since the type of calibration they require is suspension-specific. Consequently, few devices are capable of continuous on-line operation and, therefore, operators still have to rely on direct sampling methods in which samples are extracted for analysis in order to approximate the dynamic behaviour of the settling process. This approach is itself prone to errors and evidently non-representative since suspension composition may change rapidly with time. 

Biomolecule Separations. Advances in chemical separation techniques such as capillary zone electrophoresis (cze) and sedimentation field flow fractionation (sfff) allow for the isolation of nanogram quantities of amino acids and proteins, as weU as the characterization of large biomolecules (63—68) (see Biopolymers, analytical techniques). The two aforementioned techniques, as weU as chromatography and centrifugation, ate all based upon the differential migration of materials. Trends in the area of separations are toward the manipulation of smaller sample volumes, more rapid purification and analysis of materials, higher resolution of complex mixtures, milder conditions, and higher recovery (69). 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Export processes are often more complicated than the expression given in Equation 7, for many chemicals can escape across the air/water interface (volatilize) or, in rapidly depositing environments, be buried for indeterminate periods in deep sediment beds. Still, the majority of environmental models are simply variations on the mass-balance theme expressed by Equation 7. Some codes solve Equation 7 directly for relatively large control volumes, that is, they operate on "compartment" or "box" models of the environment. Models of aquatic systems can also be phrased in terms of continuous space, as opposed to the "compartment" approach of discrete spatial zones. In this case, the partial differential equations (which arise, for example, by taking the limit of Equation 7 as the control volume goes to zero) can be solved by finite difference or finite element numerical integration techniques. 

Another method (EPA 3545, accelerated solvent extraction) has been validated using a variety of soil matrixes, ranging from sand to clay. In the method, conventional solvents such as methylene chloride (or a hexane-acetone mixture) are heated [100°C, (212°E)] and pressurized (2000 psi), then passed through the soil sample (this technique is also suitable for application to petroleum sludge and petroleum sediment). The method has the advantage of requiring smaller solvent volumes than is required by traditional solvent extraction techniques. 

Another technique consists of submitting the emulsion to centrifugation and determining the droplet volume fraction < / at the top (bottom) of the cream (sediment). The centrifugation typically takes several hours until the equilibrium volume fraction is achieved. After equilibration, if the droplets occupy a distance much less than that of the centrifuge lever arm, the spatial gradient in the acceleration can be neglected, and the osmotic pressure can be determined (see Fig. 4.1) ... 

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