UltraSimplicity

1.5. Stabilising Emulsion

Coalescence and creaming are constantly working to ruin emulsions however, there are several ways in which emulsions can be stabilised. These include electrostatic repulsion and the addition of additives called emulsifiers and stabilisers [3].

1.5.1. Electrostatic Stabilisation

As mentioned in Section 1.4.4 droplets in emulsions can carry electrostatic charges and these charges are dependent on acidic surface groups and the acidity of the surrounding water phase. Charged oil droplets will repel each other since alike charges repel, this prevents two droplets sticking together or coalescing.

1.5.2. Polymeric Stabilisation

Polymers [4], including natural polymers such as proteins and gums, can be used to stabilise emulsions. Some of these polymers contain certain polar chemical groups, for example alcohols, that can adsorb onto the surface of the oil droplet. Gum Arabic is an example of a polysaccharide gum that can adsorb to oil droplets.

The polymer molecules can either adsorb at a single point or at several points and the adsorbed layer prevents two droplets from coalescing by forming a barrier (Figure 1.9). Molecules that are attached at more than one point form a strong protective barrier around the droplets that can withstand vigorous agitation without becoming detached. These polymer barriers stabilise the emulsion once it has been formed but don't usually make formation easier.



Figure 1.9: Adsorption of polymers at the oil/water interface.

Polymer molecules that can not adsorb onto the oil droplets but are soluble in water can stabilise emulsions by increasing the viscosity [5] of the water phase. Polysaccharide gums such as xanthan gum (but not gum Arabic) can be used to this end and are known as hydrocolloids. For coalescence and creaming to happen the water phase must easily flow around the oil droplets so that intimate contact between the droplets is possible. A more viscous water phase will flow less easily and sometimes cause water to be trapped between droplets that are trying to coalesce (Figure 1.10). Even small quantities of xanthan gum cause a large increase in the viscosity of the water phase.



Figure 1.10: Coalescence of oil droplets in water phase.

1.5.3. Small Particles

Very small solid particles can be added to emulsions to act as a stabiliser. The effectiveness of these particles depends on how they interact with both liquids and the interface between them. Like polymeric stabilisation, the particles form a physical barriers around the oil droplets. While small particles can help stabilise an emulsion they probably will not make its initial formation easier.

1.5.4. Surfactants

The name surfactant is short for "surface active agents" and is given to molecules that have an effect on the surface energy of the oil/water interface. These chemicals not only increase the stability of emulsions but also make them substantially easier to form in the first place. Surfactants can achieve this because they act as a 'bridge' between the oil droplets and the water, reducing the surface energy of the interface and thus reducing the amount of energy needed to create new oil/water surfaces (see Section 1.4.2).

Surfactants are molecules that are polar at one end and non-polar at the other, thus one end of the molecule dissolves in the oil droplet and the other end dissolves in the water phase (Figure 1.11).



Figure 1.11: The action of surfactants bringing the oil/water interface.

Oil and water can be made to form a stable emulsion by adding soap as a surfactant. Soap often contains sodium stearate which has a long non-polar "tail" which dissolves in oil and a polar "head" which dissolves in the water (Figure 1.12). Any molecules with polar and non-polar parts will have some ability to act as an emulsifier. For example alcohol will stabilize an emulsion to some extent, although emulsifiers with longer "tails" and highly soluble polar "heads" tend to work best.



Figure 1.12: Sodium stearate. {Top Of Page} 1.5.5. Weighting Agents

Oil-in-water emulsions will eventually separate because most oils are less dense than water and will eventually float to the surface. This process is called creaming (see Section 1.4.3) and can be delayed if the oil droplets are very fine. Creaming will not happen if the oil phase has the same density as water.

Weighting agent are oils that, unusually, have a density greater than the density of water. Thus the overall density of the oil phase can be increased by add such a substances. The closer the density of the two phases, the more stable an emulsion will be.