Flocculation DLVO theory

Suspension formulations are prepared to achieve a controlled flocculation (DLVO theory, ref), which will allow ease of redispersion upon shaking. A number of physicochemical phenomena may occur to disrupt the stability of the product. Moisture ingress to the container (and association with the drug particles) will lead to hydrolysis for molecules that are susceptible to this mechanism of degradation. Because of the inert hydrophobic nature of propellant, this can be controlled to some extent. The presence of moisture will also give rise to interactions between particles, which may result in irreversible aggregation. Related forms of aggregation may result... 

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

A number of chapters have been overhauled so thoroughly that they bear only minor resemblance to their counterparts in the first edition. The thermodynamics of polymer solutions is introduced in connection with osmometry and the drainage and spatial extension of polymer coils is discussed in connection with viscosity. The treatment of contact angle is expanded so that it is presented on a more equal footing with surface tension in the presentation of liquid surfaces. Steric stabilization as a protective mechanism against flocculation is discussed along with the classical DLVO theory. 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

The results of the present Investigation of particle formation mechanism by mecms of electron microscopy correspond to the data of Huxm and Chong (26) who proved, by using the DLVO theory of stability (27), that very small particles (with diameter less tEan 20 nm) formed during polymerization of vinyl acetate are less stable, so that their flocculation with larger particles Is possible Immediately after they... 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

Combination of the attractive van der Waals potential - -1/ and the repulsive double-layer potential ( gives rise to tiie famous DLVO theory of colloid stability. Depending on salt, the net potential can be such as to induce flocculation directly, pose a barrier to flocculation, or lead to a stable suspension. 

Wu, W., Giese, R.L, and Van Oss, C.J. Stability vs. flocculation of particle suspension in water-correlation with the extended DLVO approach for aqueous system, compared with DLVO theory, Colloidd Surfaces B Biointerfaces, 14, 47,1999. 

For polyelectrolytes, attachment resistance is not restricted to densely covered surfaces, because long-range electrostatic interactions come into play. Obviously, a polyelectrolyte chain will be repelled by a surface that carries a (net) charge of like sign this situation is analogous to that of colloidal particles in water, for which the DLVO theory is the generally accepted way to calculate the interaction, and the Von Smoluchowsky-Fuchs theory [23,24] provides the framework to calculate the resistance in the rate of (slow) flocculation. 

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of charged colloids (58) predicts a substantial decrease in stability against flocculation with decreasing particle radius. Most likely the newly formed... 

Sedimentation in emulsions may also be accompanied by the aggregation of emulsion droplets, referred to as flocculation. Flocculation leads to an increase in the effective size of settling aggregates, and as a result, leads to a higher sedimentation velocity. In dilute finely dispersed emulsions in which electrostatic stabilization is of primary importance, the major laws governing flocculation are close to those of coagulation of hydrosols, and are given by the DLVO theory (see Chapter VII, 4). In such systems flocculation may be reversible. 

The above theoretical rationalisation of the Schultze-Hardy rule was claimed as an early success of the DLVO theory. However, among the approximations and assumptions made in reaching equations (9.4) and (9.6) is that the surface potential is high. On the other hand, it is known that flocculation occurs at relatively low surface potentials. In this case the simple theory suggests that the c.c.c. is proportional to z rather than to specific adsorption of ions) may to some extent restore agreement with the Schultze-Hardy rule. 

Whether the colloidal particles encountering each other will flocculate (or coalesce) will generally depend on the net interaction resulting from the combined attractive van der Waals forces and repulsive electrostatic forces resulting from the overlap of the electric double layers. This theory of colloid stability, in considerably more detail than given here, is known as the Derjaguin, Landau, Verwey, Overbeek (DLVO) theory of colloid stability (Hiemenz 1986, Verwey c Overbeek 1948). 

The flocculation process is less investigated with respect to emulsions, especially water-in-oil emulsions. According to the DLVO theory [48] the aggregation stability of a disperse system is determined by the sum of energy of ion-electrostatic repulsion (Uj) and Van-der-Waals attraction (Um)... 

These forces originate from entirely different sources and therefore may be evaluated separately. The interplay of (i) and (ii) forms the basis of the classical theory of flocculation of lyophobic dispersions, flrst proposed by Derjaguin and Landau in Russia and independently by Verwey and Overbeek in the Netherlands and hence now known as the DLVO theory. The interplay of (i) and (iii) is commonly termed steric stabilization , and much has been written on this protective mechanism, although a workable understanding has developed only during the last two decades. 

Given the appropriate potential energy diagrams from the DLVO theory, the stability ratio may be calculated by graphical or numerical integration and then compared with experimental values of W=kyk, the ratio of the experimental rate constants for rapid and slow flocculation. Such a comparison is a severe test of the applicability of theory to experiment, and the observed deviations, although often not appreciable, reflect the assumptions and approximations which are necessary in the calculation of the potential energy terms. An advanced treatment of these issues will be found in Russel et al.- . 

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of charged colloids [58] predicts a substantial decrease in stability against flocculation with decreasing particle radius. Most likely the newly formed nuclei are not yet stabilized, and stability sets in only after a certain radius is obtained. After this size is reached, particles grow through monomer addition either reaction- or diffusion-limited, but with an overall rate still depending on the hydrolysis. 

In order to understand the retention and flocculation problems at hand, we need to discuss the forces prevailing between the components as well as how these can be manipulated. The colloidal interactions prevalent in such systems can be described by using the classical DLVO theory, which is presented below. Subsequently, a short description of polymer adsorption and the effect that this has on the interactions between surfaces is given. This is followed by a short presentation of flocculation and retention mechanisms in papermaking systems. 

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