Emulsions water droplets attraction

As an emulsion passes through an electrical field, the small water droplets are polarized and then stretched due o ihe polar attractions. This polarization greatly increases the speed and forte of impact of the parlt-< les upon one another as they speed toward the attracting electrode Because of the weakened him due to surface stretching and because of the greater collision force due to increased peed th< droplets unite more readily Ihe elec ire. fu-ld works better on a loose-red emulsion. irnl -. irrot tolerate... 

Water droplets in an organic continuous phase show very low electric repulsion potentials. This means that a pure w/o-emulsion cannot be stable, because the total potential energy is only of the attraction mode. 

Equation (12.10) also shows that when > 0.5 - that is, when the solvency of the medium for the chains becomes poor - Gj j will be negative and the interaction will become attractive. Thus, it is important to ensure that the solvent used to prepare the W/O emulsion is a good solvent for the PHS chains, otherwise flocculation of the water droplets (perhaps followed by their coalescence) may occur. Fortunately, the PHS chains are soluble in most hydrocarbon solvents used in most formulations. The condition = 0.5 is referred to as 0-solvent, and this denotes the onset of a change from repulsion to attraction. Thus, to ensure steric stabilisation by the above mechanism it must be ensured that the chains are kept in better than 0-solvent. 

The transparent visual aspect of nano-emulsions with droplet sizes below 200 nm makes them especially attractive for application in cosmetics. Apart from the appearance, similar to that of microemulsions, other advantages of nano-emulsions for cosmetic applications are their kinetic stability, a droplet size that can be controlled, and the possibility to achieve improved active delivery. For all these reasons, nano-emulsions have attracted increasing interest in the cosmetic field, as reflected by the papers [8,10,106] and numerous patents [19-21,23,24,27,107-111] that have appeared in the last few years. Oil-in-water nano-emulsions with a droplet size lower than 100 nm have been described in patents as hair- and skin-care [19,21,23,24,107-109], makeup [110], and sunscreen [20,111] formulations. 

Many different types of interaction can induce reversible phase transitions. For instance, weak flocculation has been observed in emulsions stabilized by nonionic surfactants by increasing the temperature. It is well known that many nonionic surfactants dissolved in water undergo aphase separation above a critical temperature, an initially homogeneous surfactant solution separates into two micellar phases of different composition. This demixtion is generally termed as cloud point transition. Identically, oil droplets covered by the same surfactants molecules become attractive within the same temperature range and undergo a reversible fluid-solid phase separation [9]. 

Those systems in which the emulsifier carries a charge would impart specific characteristics to the emulsion. A double layer will exist around the oil droplets in an O/W emulsion. If the emulsifier is negatively charged, then it will attract positive counterions while repelling negative charged ions in the water phase. The change in... 

In the case of biopolymer molecules residing in the space between colloidal particles or droplets, the force associated with the deep energy minimum at contact is often referred to as the depletion force because the intervening biopolymer species are depleted from the narrow gap between the pair of neighbouring particles. This attractive interparticle interaction underlies the phenomenon of reversible depletion flocculation in oil-in-water emulsions (see equation (3.41) in chapter 3). 

Mayonnaise and ice cream both have a yield stress, which is produced in these foods by droplets and air bubbles that must deform if flow is to occur. Another food with a yield stress is mustard, it is neither an emulsion nor a foam, but a suspension or paste, containing particles 30 pm or so in diameter that attract each other and form a weak network (Gerhards and Schubert 1993). Mustard is made by simply grinding mustard seeds, together with vinegar, salt, spices, and water, into a mash. The grinding releases oils that impart to mustard its distinctive flavor. The rheology of particulate suspensions is covered in Chapter 6. 

Fig. 4 The total potential energy of interaction Vt as a function of distance of surface separation H for two similar oh droplets in an oil-in-water emulsion. (A) Electrostatic stabilization by a monolayer of ionic surfactant. (B) Steric stabilization by a monolayer of non-ionic surfactant. V van der Waals attractive force Vr electrostatic repulsive force Vs steric repulsive force.

Consider the mixture resulting from vigorous shaking of salad oil (nonpolar) and vinegar (polar). Droplets of hydrophobic oil are temporarily suspended in the water. In a short time, however, the very polar water molecules, which attract one another strongly, squeeze out the nonpolar oil molecules. The oil then coalesces and floats to the top. If we add an emulsifying agent, such as egg yolk, and shake or beat the mixture, a stable emulsion (mayonnaise) results. 

We have compared these theoretical predictions of the low-frequency modulus to experimental measurements on compressed emulsions and concentrated dispersions of microgels [121]. The emulsions were dispersions of silicone oil (viscosity 0.5 Pas) in water stabilized by the nonionic surfactant Triton X-100 [102, 121]. The excess surfactant was carefully eliminated by successive washing operations to avoid attractive depletion interactions. The size distribution of the droplets was moderately polydisperse with a mean droplet diameter of 2pin. The interfacial energy F between oil and water was 4mJ/m. The contact modulus for these emulsions was thus F 35 kPa. The volume fraction of the dispersed phase was easily obtained from weight measurements before and after water evaporation. Concentrated emulsions have a plateau modulus that extends to the lowest accessible frequencies, from which the low-frequency modulus Gq was obtained. Figure 11 shows the variations of Gq/E"" with 0 measured for the emulsions against the values calculated in the... 

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