Flocculated emulsions

In protein-stabilized systems, flocculation typically occurs when the interaction free energy between a pair of protein-covered droplets becomes appreciably negative at some separation. This may arise for one of a number of possible reasons. 

Structures of some flocculated colloidal systems are observed to have a fractal character similar to that generated in computer simulation models. The number of particles within a floe of radius rf is given by 

The parameter k is the ratio rt,/ru it can be obtained by fitting to experiment. The use of the Krieger-Dougherty equation in conjunction with eqns. (5.15) to (5.17) is valid only a) low floe volume fractions where floes do not interact strongly or become connected together in the flow. In this approach the system may exhibit 

3 Suspensions Flocculated by Polymer Bridging and Cross-linking 

Whenever an emulsion contains non-adsorbing entities of much smaller size than the droplets, there is the possibility of flocculation taking place as a result of an attractive depletion interaction between the droplets. Historically, the reversibility of depletion flocculation to dilution was first demonstrated from observations of the creaming of natural rubber in the presence of water-soluble polymers. 

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Fig. 5. (a) In a flocculated emulsion the droplets are aggregated but separated by a thin film. Coalescence means that the thin film between droplets bursts and the aggregates become single droplets, (b) The result is an emulsion with a wide distribution of droplet sizes but without aggregates. 

Fig. 18. The repulsion force from adsorbed particles is greater than the van der Waals force between flocculated emulsion droplets under certain...

Figure 3.6. Microscopic image of a totally flocculated emulsion (water droplets of diameter 0.51 am in a mixture of dodecane and IPM).

There seems to be a sort of analogy here with the arrested phase separation of a protein-stabilized depletion-flocculated emulsion containing a thermodynamically incompatible hydrocolloid like xanthan gum (Moschakis et al., 2005 Dickinson, 2006b). 

Figure 8.6. Emulsion stability contours (log of emulsion stability in seconds) for 50/50 (mass) C02-water (0.01 M NaCl) systems with 10.7 mM PFPE COO NH4+ (g/mol = 2500) surfactant. w/c emulsions O (c/w) emulsions. Dotted line indicates the phase boundary of the surfactant in C02 cross-hatched region indicates highly flocculated emulsions (Lee et al., 1999b).

Dickinson, E. and Pawlowsky, K. 1996. Effect of high-pressure treatment of protein on the rheology of flocculated emulsions containing protein arxl polysaccharide. J. Agric. Food Chem. 44 2992-3000. 

Figure 11. Flocculated emulsions exhibit shear thinning. At increasing shear rates, the floes are first deformed and then partiaiiy and finaiiy compietely disrupted.

In this paper we present three techniques for the study of thin middle phase films adsorbed on emulsion droplets. Film thicknesses have been measured by small angle X-ray scattering, contact angles of adjacent droplets have been measured in flocculated emulsions, and much direct evidence for such films has been observed visually in the spinning drop interfacial tensiometer. 

If the free polymer concentration is increased above a certain limit, phase separation may occur and the flocculated emulsion droplets may cream or sediment faster than in the absence of the free polymer. 

Wessel and Ball " and Kanai et al. studied in detail the effects of shear rate on the fractal structure of flocculated emulsion drops. They showed that the size of the floes usually decreases with the increase of the shear stress often the floes are split to single particles at high shear rates. As a result, the viscosity decreases rapidly with the increase of the shear rate. 

Interesting effects are observed when a dispersion contains both larger and smaller particles the latter are usually polymer coils, spherical or cylindrical surfactant micelles, or microemulsion droplets. The presence of the smaller particles may induce clustering of the larger particles due to the depletion attraction (see Section 5.4.S.3.3, above) such effects are described in the works on surfactant-flocculated and polymer-flocculated emulsions. Other effects can be observed in dispersions representing mixtures of liquid and solid particles. Yuhua et al. ° have established that if the size of the solid particles is larger than three times the size of the emulsion drops, the emulsion can be treated as a continuous medium (of its own average viscosity), in which the solid particles are dispersed such treattnent is not possible when the solid particles are smaller. 

The stability of w/c emulsions, defined as the time required for the volume of the emulsion to settle from 100% to 90% based upon visual observation, has been measured for PFPE-COO"NH4" surfactants with molecular weights ranging from 667 to 7500 [17]. Figure 2.4-10 shows the stability of emulsions formed by the above microfluidizer for equal weights of water and CO2 and 1.3 wt% of 2500g/mol PFPE-COO NH4. For each experiment where non-flocculated emulsions were present during shear, the specific conductivity was less than O.lpS/cm, indicating water droplets in a CO2 continuous... 

Note that the minimum flocculation degree value is observed at the same surfactant ratios at which the minimum interfacial tension value is achieved. This makes clear the previously known facts about the increase of the emulsion stability at low y. This occurs, first of all, due to an appreciable decrease in the flocculation degree up to its complete suppression. These phenomena result in an increase in the sedimentation stability both for non-flocculated emulsions (dispersity increase) and partly flocculated emulsions (decrease in flock size and flocculation degree). 

If we treat the free water droplets as one type of particle and the floes as the other type, Eq. (85) may be used to find the dielectric properties of a partially flocculated emulsion. 

There are two recent developments in the theory of acoustics which deserve to be mentioned here. The first one is a theory of acoustics for flocculated emulsions (21). It is based on EC AH theory, but it uses an addition an effective medium approach for calculating thermal properties of the floes. The success of this idea is related to the feature of the thermal losses that allows for insignifieant partiele -particle interactions even at high volume fractions. This mechanism of acoustic energy dissipation does not require relative motion of the particle and liquid. Spherical symmetrical oscillation is the major term in these kinds of losses. This provides the opportunity to replace the floe with an imaginary particle, assuming a proper choice of the thermal properties. 

The attenuation coefficient in the flocculated emulsion is lower at low frequencies and higher at high frequencies than that of the nonflocculated emulsions. The decrease in attenuation at low frequencies on flocculation as a result of the thermal overlap effects mentioned earlier, whereas the increase at high frequencies results from increased scattering of ultrasound by the floes. The same ultrasonic spectroscopy technique has been used to study the disruption of floes in a shear field (38). As the emulsions are exposed to higher shear rates the floes become disrupted and their attenuation spectra become closer to that of nonflocculated droplets. 

At low surfactant concentrations it is observed that an attraction dominates at short separations. The attraction becomes important at separations below about 12 nm when the surfactant concentration is 0.01 mM, and below about 6 nm when the concentration is increased to 0.1 mM. Once the force barrier has been overcome the surfaces are pulled into direct contact between the hydrophobic surfaces at D = 0, demonstrating that the surfactants leave the gap between the surfaces. The solid surfaces have been flocculated. However, at higher surfactant concentrations (1 mM) the surfactants remain on the surfaces even when the separation between the surfaces is small. The force is now purely repulsive and the surfaces are prevented from flocculating. Emulsion droplets interacting in the same way would coalesce at low surfactant concentrations once they have come close enough to overcome the repulsive barrier, but remain stable at higher surfactant concentrations. Note, however, that the surfactant concentration needed to prevent coalescence of emulsion droplets cannot be accurately determined from surface-force measurements using solid surfaces. 

Figure 22 The total number of constituent drops in a flocculating emulsion, decreases with time, t, because of a parallel process of coalescence. The curves are calcualted for the following parameter values initial number of constituent drops iiq = lO cm coalescence rate constant F = 10 s h Curve 1 is a numbeiical solution to Eq. (121) Curves 2 and 3 are the results predicted by the models of Bor-wankar et al. (194) and van den Tempel (193), respectively. The values of the flocculation rate constant are (a) ar= 10 " cmVs (b) ar=...

At very small separation distances, the value of Gr becomes very small indicating that the velocity of approach of the moving particle to the second particle or surface becomes small as well. Such an effect may be expected to become significant in contexts such as particle flocculation, emulsion stabihty, and particle deposition onto surfaces. In summary, these hydrodynamic effects will be repulsive for approaching particles or surfaces and attractive for receding systems. It can be seen from Table 4.6 that the effect is much greater in the case of sphere-plane approach than that of sphere-sphere interaction. 

Sr, and Zn) promote W/O emulsions which flocculate some of the flocculated emulsions are stable against coalescence, others are not. The stabilization of coalescence by some oleates of polyvalent metals was found to result from the formation of a thick film of complex hydrolysis products. This observation confirms the findings of Wasan et al. (17) that coalescence rates could be inversely related to interfacial viscosities or thickness of the interfacial film (0.7-0.3 ym). 

Emulsions were stored at 4, 25, and 45 °C and also submitted to a number of freeze-thaw cycles (24 h freezing at —20 °C followed by 24 h thawing at room temperature). During the storage time, different methods were used to analyze emulsion stability. In the case of pronounced instability or destabilization during storage, the modification of the emulsion was also observed by the naked eye. For study of flocculation, emulsions were stored in graduated cylinders and the water separation was measured by visual observation [10]. 

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