Flocculation continuous phase

If there is particle—particle interaction, as is the case for flocculated systems, the viscosity is higher than in the absence of flocculation. Furthermore, a flocculated dispersion is shear thinning and possibly thixotropic because the floccules break down to the individual particles when shear stress is appHed. Considered in terms of the Mooney equation, at low shear rates in a flocculated system some continuous phase is trapped between the particles in the floccules. This effectively increases the internal phase volume and hence the viscosity of the system. Under sufficiently high stress, the floccules break up, reducing the effective internal phase volume and the viscosity. If, as is commonly the case, the extent of floccule separation increases with shearing time, the system is thixotropic as well as shear thinning. 

The sequence, flocculation — coalescence — separation, is compHcated by the fact that creaming or sedimentation occurs and that this process is determined by the droplet size. The sedimentation velocity is monitored by the oppositely directed forces which form the buoyancy and the viscous drag of the continuous phase on the droplet ... 

Viscosity Increase. The flocculation rate of an emulsion is iaversely proportional to the viscosity of the continuous phase and an iacrease of the viscosity from 1 mPa-s (=cP) (water at room temperature) to a value of 10 Pa-s (100 P) (waxy Hquid) reduces the flocculation rate by a factor of 10,000. Such a change would give a half-life of an unprotected emulsion of a few hours, which is of Httle practical use. 

Stability implies a resistance to change, and may be defined qualitatively in those terms. In the specific case at hand, stability is defined as resistance to molecular or chemical disturbance. This requirement recognizes that a flocculated dispersion may be more stable than a peptized dispersion from the standpoint of its future behavior. A physically stable dispersion is one which will not undergo molecular replacements at the interface between the dispersed solid and the continuous phase. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

We first consider emulsion droplets submitted to attractive interactions of the order of ks T. Reversible flocculation may be simply produced by adding excess surfactant in the continuous phase of emulsions. As already mentioned in Chapter 2, micelles may induce an attractive depletion interaction between the dispersed droplets. For equal spheres of radius a at center-to-center separation r, the depletion... 

Rgure 3.8. Volume fraction of n-alkane at the onset of flocculation as a function of n-aUcane chain length (C H2 +2). The continuous phase is made of a mixture of n-aUcane, dodecane, and SMO (1 wt%). Glycerol droplets (5% in volume) have a diameter of 0.38 pm. T = 65°C. (Adapted from [13].)... 

As drops of this dispersed phase collect near the separation interface, they will flocculate into a closely packed mass which can best be described by the term liquid-liquid foam. Each drop is surrounded by a thin film of the continuous phase. The film between two adjacent drops can rupture and the two combine by coalescence in the foam layer. Only those drops near the general phase boundary can coalesce into the general drop phase layer. The residence time in the flocculation zone can be many minutes, and considerable mass transfer may occur there. 

Viscosity. This parameter can be monitored by standard rheological techniques. The rheological properties of emulsions, reviewed by Sherman (1983), can be complex, and depend on the identity of surfactants and oils used, ratio of disperse and continuous phase, particle size, and other factors. Flocculation will generally increase viscosity thus, monitoring viscosity on storage will be important for assessing shelf-life. 

Flocculation. Flocculation means an aggregation of emulsion droplets but, in contrast to coalescence, the films of the continuous phase between the droplets survive. Hence, the process may be partially reversible. Both processes, flocculation and coalescence, speed up the creaming of an emulsion due to the increase of the drop size. The process of flocculation is even more important for dispersions of solids than for emulsions because in this case a coalescence is not possible. 

A prolific variety of composite latex particles appears in both the open and patent literatures. The subject has been reviewed (1,2) by several authors. Composite implies the presence of at least two dissimilar components either of which could, in principle, constitute the major component by volume. Some features of composite particles, which retain colloidal stability during preparation and subsequent storage, that is where the product is a dispersion in which flocculation, aggregative, and coalescence processes are largely absent so long as the continuous phase remains, will be described here. There are alternative and important processes for preparing composite particles which give flocculated particles readily separated from the liquid diluent phase and dried for use as powder. 

Increased depletion attraction. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet. This attractive force increases as the concentration of colloidal particles increases, until eventually, it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (68-72). This type of droplet aggregation is usually referred to as depletion flocculation (17, 18). 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

It seems that coalescence process is very delayed by the emulsifier. In order to investigate the flocculation process, a study through an optical microscopy of the droplets and flocks has been employed. Unfortunately, a little difference in the contrast (brown against pale yellow) between droplets and continuous phase has not permitted a good determination of mean droplets dimensions. Probably, mean diameter should be few microns (i-5). 

Sherman concluded that in the 0/W systems a small fraction of the continuous phase was immobilized by the dispersed phase either by attractive forces or by flocculation. Therefore, the apparent volume fraction was greater than the actual volume fraction of that component. The equations he used to describe the viscosities of these emulsions further extended the approach of Mooney and took account of the particle diameter. 

These diagrams demonstrate that the SMNs should be well dispersed in the continuous phase, monodis-perse, and small in size. This can be visualized if one assumes that the SMNs in the diagrams are 5 nm particles. Thus, there would be four layers of particles between the dispersed phase regions. If one replaces the 5 nm particles with 20 nm particles, then a single 20 nm particle will essentially take up the same space (linearly) as four 5nm particles. Replacing the 5nm particles with 20 nm particles makes the path to coalesce or flocculation less tortuous. Also, at equal weights of particles, there are fewer 20 nm particles than 5 nm particles. It is well known that the viscosity... 

This is achieved by the addition of free (nonadsorbing) polymer in the continuous phase [26]. At a critical concentration, or volume fraction of free polymer,, weak flocculation occurs as the free polymer coils become squeezed-out from between the droplets. This is illustrated in Figure 10.27, which shows the situation when the polymer volume fraction exceeds the critical concentration. 

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