Aging emulsions

As an emulsion ages, droplet coalescence occurs and leads to increased... 

Emulsion aging rates increase with temperature. Aging rates in turbulent flow appear to become arrested after a certain point, generally being less than the rates observed in laminar flow. Aging rates are suppressed by increased surfactant concentration as a result of the anticoalescence action of the surfactant. 

Monitoring Emulsion Aging. The surfactants used in transport emulsions may gradually lose their ability to stabilize the oil droplets. As the oil droplets coalesce, a two-phase mixture is formed, and it remains pumpable with no significant change in effective viscosity. This process is referred to as emulsion failure. An alternative to this process is inversion of the emulsion, in which a water-in-oil emulsion is formed with a potentially very high viscosity. Proper selection of the surfactant formulation can prevent the occurrence of emulsion inversion. 

Indicators of emulsion aging that may be monitored include droplet size growth, viscosity decline, surfactant loss, and reduction of shear stability. 

In 1996, Pal studied the effect of droplet size and found it had a dramatic influence on emulsion rheology (62). Fine emulsions have much higher viscosity and storage moduli than the corresponding coarse emulsions. The shear thinning effect is much stronger in the case of fine emulsions. Water-in-oil emulsions age much more rapidly than oil-in-water emulsions. More recently, Lee et al. (63) and Aomari et al. (64) examined model emulsions and found that a maximum shear strain existed which occurred around 100s. ... 

P Sherman. Rheological change in emulsion aging. IV. 0/W emulsions at intermediate and low rates of shear. J Colloid Interface Sci 27 281,1968. 

The emulsion was cooled to 20° C. A dithiocarbamate salt or secondary amine was added and the emulsion aged for 8 h to peptize the polymer. The bulk Mooney viscosity of the polymer was thus reduced to levels suitable for rubber processing. 

In summary, analytical centrifugation can be successfully used for screening purposes in emulsion product development. Different samples can be directly compared within a short time. To predict the shelf-life for emulsions aging not only by creaming or sedimentation, calibration with a known system has to be carried out. 

Sherman [38] showed the influence of the particle size on the tendency to flocculate or to coalesce in water-in-oil emulsions. Aged emulsions after shearing (to break the network) showed an increase of viscosity to a steady value which depends on the emulsion age (Fig. 39). This phenomenon is correlated to the size of the particles and leads to the conclusion that the lower the particle size (especially in the submicronic range), the stronger the forces holding the particles together. The kinetics of the flocculation shows the same trend. 

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