Prevention of Creaming or Sedimentation

Clearly, if Ap = 0, v = 0 however, this method is seldom practical. Density matching, if possible, only occurs at one temperature. 

Since the gravity force is proportional to R, then if R is reduced by a factor of 10, the gravity force is reduced by 1000. Below a certain droplet size (which also depends on the density difference between oil and water), the Brownian diffusion may exceed gravity and creaming or sedimentation is prevented. This is the principle of formulation of nanoemulsions (with size range 20-200 nm) that may show very little or no creaming or sedimentation. The same applies for microemulsions (size range 5-50 nm). 

These are high-molecular-weight polymers, either natural or synthetic, such as xanthan gum, hydroxyethyl cellulose, alginates, and carrageenans. In order to understand the role of these thickeners, the gravitational stresses exerted during creaming or sedimentation should be considered  

Simple calculation shows that is in the range 10 to 10 Pa, which implies that for prediction of creaming or sedimentation it is necessary to measure the viscosity at such low stresses. This can be achieved by using constant stress or creep measurements. 

As discussed above, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at a relatively long distance of separation between the droplets. However, by adding small amounts of electrolyte, such minima can be made sufficiently deep for weak flocculation to occur. The same applies to stericaUy stabihsed emulsions, which show only one minimum, but whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or the addition of a nonsolvent for the chains (e.g., an electrolyte). 

Stress = mass of drop x acceleration of gravity = nR Apg To overcome such stress one needs a restoring force, Restoring force = Area of drop x stress of drop = 4reR 7p Thus, the stress exerted by the droplet 7p is given by, 

Simple calculation shows that Tp is in the range Pa, implying that 

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Several advantages of lamellar liquid crystalline phases in cosmetics can be quoted (i) they produce an effective barrier against coalescence (ii) they can produce gel networks that provide the right consistency for application as well as prevention of creaming or sedimentation (iii) they can influence the delivery of active ingredients both of the lipophilic and hydrophilic types (iv) since they mimic the skin structure (in particular the stratum corneum) they can offer prolonged hydration potential. 

A circulating tank is recommended, the motion being required to prevent skinning, creaming and sedimentation. Screening or filtration is also provided to retain air bubbles, and any coagulum. The tank contents need to be maintained at a constant temperature and the tank is equipped with a lid to prevent evaporation of water when not in use. 

The above-described thickeners satisfy the criteria for obtaining very high viscosities at low stresses or shear rates. This can be illustrated from plots of shear stress a and viscosity tj versus shear rate y (or shear stress), as shown in Figure 10.25. These systems are described as pseudoplastic or shear thinning. The low shear (residual or zero shear rate) viscosity tj(0) can reach several thousand Pa s, and such high values prevent creaming or sedimentation [24, 25]. 

The use of a rheology modifier such as HEC or xanthan gum that produces a viscoelastic solution which prevents not only creaming or sedimentation but also entrapment of the oil droplets in between the suspension particles, or the suspension particles in between the emulsion droplets. 

Sedimentation or creaming is prevented by the addition of thickeners that form a three-dimensional elastic network in the continuous phase. If the viscosity of the elastic network, at shear stresses (or shear rates) comparable to those exerted by the particles or droplets, exceeds a certain value, then creaming or sedimentation is completely eliminated. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

A shelf-life of several months at ambient temperature requires particularly low creaming and sedimentation in the package, which is facilitated by a low fat content (10 or 12%) and optimized processing conditions, mainly heat treatment and homogenization. Coffee creams with >15% fat need chilled storage to prevent irreversible creaming. 

The above-described thickeners produce a three-dimensional gel network by the overlap of the polymer coils of H EC or the double helices of xanthan gum. Apart from their effect in reducing creaming and sedimentation by producing a high residual viscosity (at low shear rates), these polymers will also prevent the... 

A beverage emulsion is a concentrate added to sugar and carbonated water to make soda and fruit drinks. The oil-in-water emulsion provides flavor as well as opacity in products such as orange soda. Traditionally, gum arabic has been used to stabilize these emulsions. Interfacial starch derivatives (Section 20.4.2) are used to prevent creaming (phase separation), sedimentation, and loss in flavor and opacity, where desired, both in the concentrate and in the finished beverage. The concentrate is made by homogenizing the oils with an equal amount of the solubilized lipophillic starch, citric acid, sodium benzoate and color. A fine emulsion, typically 1 micrometer or less, is required for stability and for opacity, where desired. 

Mixtures of surfactants and polymers are very common in many industrial formulations. With many suspension and emulsion systems stabilized with surfactants, polymers are added for several reasons, e.g. as suspending agents ( thickeners ) to prevent sedimentation or creaming of these systems. In many other systems, such as in personal care and cosmetics, water-soluble polymers are added to enhance the function of the system, e.g. in shampoos, hair sprays, lotions and creams. The interaction between surfactants and water-soluble polymers furnishes synergistic effects, e.g. enhancing the surface activity, stabilizing foams and emulsions, etc. It is, therefore important to study systematically the interaction between surfactants and water-soluble polymers. 

These thickeners produce very high viscosities at low shear rates or shear stresses and hence they overcome the stresses exerted by the sedimenting or creaming particles of droplets. Generally, these high molecular weight polymers produce non-Newtonian flow with a yield stress that prevents separation on storage. 

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