Cream concentrated emulsion

H.M. Princen, M.P Aronson, and J.C. Moser Highly Concentrated Emulsions. II Real Systems. The Effect of Film Thickness and Contact Angle on the Volume Fraction in Creamed Emulsions. J. CoUoid Interface Sci. 75, 246 (1980). 

Another process which leads to HIPE instability is gravitational syneresis, or creaming, where the continuous phase drains from the thin films as a result of density differences between the phases. This produces a separated layer of bulk continuous phase and a more concentrated emulsion phase. The separated liquid can be located either above or below the emulsion, depending on whether the continuous phase is more or less dense, respectively, than the dispersed phase. This process has been studied by Princen [111] who suggests that it can be reduced by a number of parameters, including a high internal phase volume, small droplet sizes, a high interfacial tension and a small density difference between phases. 

Highly concentrated emulsions are also evident in everyday applications. A classic example is mayonnaise, in which a large volume of vegetable oil is emulsified in a small amount of vinegar, using lecithin from egg-yolk as the emulsifier. In addition, HIPEs are most probably found in many cosmetic products, especially gels and creams. However, little information is available on products of commercial importance, so one can only speculate on their exact nature and composition. 

The process of emulsion droplets floating upwards under gravity or in a centrifugal field to form a concentrated emulsion (cream) quite distinct from the underlying dilute emulsion. This is not the same as the breaking of an emulsion. See also Sedimentation. 

Cream is a concentrated emulsion of milk fat in an aqueous phase the concentration depends on the type of cream. 

Creaming or sedimentation occurs when the dispersed droplets or floccules separate under the influence of gravity to form a layer of more concentrated emulsion, the cream. Generally a creamed emulsion can be restored to its original state by gentle agitation. This process, which inevitably occurs in any dilute emulsion if there is a density difference between the phases as a consequence of Stokes law, should not be confused with flocculation which is due to particle interactions resulting from the balance of attractive and repulsive forces. Most oils are less dense than... 

The increase in sensitivity over standard DSC instruments and the ability to study reactions occurring in solution directly mean that HSDSC may be applied to the study of a range of systems not open to study by standard DSC. Typical examples include the denaturation of proteins, phase changes in lipid bilayers, phase transitions in dilute polymer solutions, and changes in structure of creams and emulsions. Although there are many systems that have been studied using HSDSC, the discussion that follows will concentrate on systems of biological or pharmaceutical relevance. 

Using a theoretical model of creaming behavior in concentrated emulsions, Pinfield et al. (42) predicted a segre-... 

HM Princen. Highly concentrated emulsions I. Real systems. The effect of film thickness and contact angle on the volume fraction of creamed emulsions. J Colloid Interlace Sci 71 246, 1980. 

Creaming of emulsion n. Separation of an emulsion into two layers of different concentration. The more concentrated top layer, i.e., the layer containing the greatest number of dispersed droplets per volume, has a creamy appearance. Gentle agitation of the two layers often effects uniformity of the emulsion. 

Propylene glycol acts as a preservative in concentrations above 20 % of the aqueous phase. However, it is less active than sorbic acid and methyl hydroxybenzoate. It is most effective in combination with surfactants (especially hydrophilic) and other components of creams and emulsions. Cream Base DAC (Table 12.6) is an example. It contains 20 % of propylene glycol in the aqueous phase and macrogol 20 glycerol monostearate as hydrophilic surfactant. It is well preserved in this way. 

Coalescence requires that the molecules of liquid within two or more emulsion droplets coming into direct contact. Droplets therefore need to be in close proximity, which is for example the case in highly concentrated emulsions, flocculated emulsions, or creamed layers. In a subsequent step, a disruption of the interfacial membrane must occur to allow the liquid molecules to come into direct contact. The rate at which coalescence proceeds, and the physical mechanism by which it occurs, is thus highly dependent on the nature of the emulsifier used to stabilize the system. Improving the stability of an emulsion to coalescence may... 

Equation (14.18) only applies for an infinitely dilute emulsion. For a concentrated emulsion, the creaming or sedimentation rate is v reduced with increasing volume fraction of the emulsion. This can be empirically expressed as... 

A concentrated emulsion is kinetically stable and not diermodynamically stable. It is only a matter of time before system becomes physically unacceptable. The emulsion can eidier cream or sediment depending on the relative densities of the continuous and discontinuous phase. Flocculadon can accelerate this separation. Ostwald ripening can lead to huger drop size. Coalescence can occur where die emulsion drops come togedier and physically become one larger drop. This coalescence is referred to as oiling. In extreme cases diere can be a phase inversion wiiere die continuous phase becomes (he discontinuous phase. 

Creaming or sedimentation of emulsions with droplet sizes above 1 pm causes some experimental difficulty because of the change of the total amount of spins in the NMR active volume of the sample tube during the experiment. This can be accounted for by extra reference measurements with no gradient applied before and after each NMR scan at a particular value of d. In addition, such reference measurements may provide information on the creaming rate which is a useful characteristic of emulsions. Creaming or sedimentation is not a problem in the study of most food emulsions (such as low-calorie spreads), highly concentrated emulsions or viscous water-in-crude-oil emulsions [15]. 

X10" ms" for 10 pm droplets and 4.4 x 0 ms for 1 pm droplets. This means that in a 0.1 m container creaming or sedimentation of the 10 pm droplets is complete in 0.6 hour and for the 1 pm droplets this takes 60 hours. For moderately concentrated emulsions (0.2 < cj) < 0.1) one has to take into account the hydrodynamic interaction between the droplets, which reduces the Stokes velocity to a value v given by the following expression [97] ... 

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