Generally, one phase of emulsion is water or aqueous solution, which is called aqueous phase; The other phase is an organic phase that is immiscible with water, known as the oil phase.

1、 Classification

Three classification methods:

1. Classified by source: natural products and synthetic products;

2. Classified by molecular weight: low molecular weight emulsifiers (c10-c20) and high molecular weight emulsifiers (c thousands);

3. According to whether it can ionize in aqueous solution, it can be divided into ionic type (anions, cations, and anions and cations) and non-ionic type.

This is the most commonly used classification method.

 

2、 The function and principle of emulsifiers

The main function of emulsifiers is to reduce the surface tension of the two liquids being emulsified. Therefore, when surfactants are used as emulsifiers, one end of their hydrophobic group adsorbs onto the surface of insoluble liquid particles (such as oil), while the hydrophilic group extends towards the water. Surfactants are directionally arranged on the surface of liquid particles to form a hydrophilic adsorption film (interfacial film), so as to reduce the mutual attraction between droplets, reduce the surface tension between two phases, and promote mutual dispersion to form emulsions.

The concentration of surfactant has a direct impact on the strength of the interfacial facial mask. With high concentration, there are many surfactant molecules adsorbed on the interface, forming a dense and strong interface facial mask.

Different emulsifiers have different emulsification effects, and the amount required to achieve the optimal emulsification effect also varies. Generally speaking, the greater the molecular force of emulsifier that forms the boundary facial mask, the higher the film strength, and the more stable the lotion; On the contrary, the smaller the force, the lower the film strength, and the more unstable the emulsion.

When there are polar organic molecules such as fatty alcohol, fatty acid and fatty amine in the facial mask, the strength of the membrane is significantly improved. This is because emulsifier molecules interact with polar molecules such as alcohol, acid and amine in the interface adsorption layer to form a complex, which increases the strength of the interface facial mask.

The emulsifier composed of more than two surfactants is a mixed emulsifier. Due to the strong interaction between molecules, the interfacial tension is significantly reduced, the amount of emulsifier adsorbed on the interface is significantly increased, and the density and strength of the formed interfacial facial mask are increased.

During the formation of emulsion, the interfacial tension between oil and water is greatly reduced due to the participation of surfactants, and it becomes a stable emulsion. However, there is still oil-water interfacial tension in emulsion which cannot reach zero due to CMC or solubility restrictions. Therefore, lotion is a thermodynamic unstable system.

The interfacial tension between oil and water of micro emulsion is so low that it can not be measured. It is a thermodynamic stable system. This is mainly achieved by adding a second type of surfactant with completely different properties (such as moderately sized alcohols such as pentanol, hexanol, and heptanol, known as co surfactants), which can further reduce the interfacial tension to a very small level, even resulting in instantaneous negative values. This can be explained by Gibbs' adsorption equation for multi-component systems.

 

3、Type of emulsion

Type

Common emulsion , one phase is water or aqueous solution, and the other is organic matter that is insoluble with water, such as grease, wax, etc.emulsion formed by water and oil can be divided into three types:

(a) Oil in water type (O'W)
(e) Compound milk (W/O/W)
(b) Oil in water type (W/O)

(1) Oil/water (0/W) emulsion, oil dispersed in water. Oil is a dispersed phase (internal phase), and water is a continuous phase (external phase) oil in water emulsion, which can be diluted with water. Such as milk, soybean milk, etc.

(2) Water/oil (W/0) emulsion, water dispersed in oil. Water is a dispersed phase (internal phase) and oil is a continuous phase (external phase) of water in oil emulsion. This kind of emulsion can be diluted with oil. Such as artificial butter, crude oil, etc.

(3)Ring shaped emulsions, formed by alternating dispersion of water and oil phases layer by layer, mainly come in two forms: oil in water and oil in oil 0/W/0 (i.e. water phase with dispersed oil droplets suspended in oil phase and water in oil and water in water W/0/W (i.e. oil phase with dispersed water droplets suspended in water phase). This type of emulsion is rare and generally exists in crude oil.

 

Method of checking emulsion type

(1) Dilution method

Dilute the emulsion with the same liquid as the continuous phase. The water soluble emulsion is oil/water type, and the oil soluble emulsion is water/oil type.
For example, milk can be diluted by water, but cannot be miscible with vegetable oil. It can be seen that milk is O/W emulsion.

(2) Conductive method

The conductivity of water and oil differs greatly, and the conductivity of oil/water emulsion is hundreds of times larger than that of water/oil. Therefore, two electrodes are inserted in the emulsion and neon is connected in series in the loop, and the oil/water light is on.

(3) Staining method

Add 2-3 drops of oil-based or water-based dyes into the test tube, and judge the type of emulsion according to which type of dye can make the continuous phase evenly colored.

(4) Filter paper wetting method

Drop the lotion on the filter paper. If the liquid can expand rapidly and a small drop is left in the center, the lotion is oil in water; if the lotion drops do not expand, the oil in water type.

(5) Optical refraction method

The different refractive index of water and oil to light is used to identify the type of emulsion. If the emulsion is oil in water, the particles play a light collecting role, and only the left outline of the particles can be seen with a microscope; If the emulsion is water in oil, the particles play the role of astigmatism, and only the right outline of the particles can be seen with a microscope;

The main factors affecting the type of emulsion

(1)Phase volume:

The phase volume theory was proposed by 0stwald from a geometric perspective. The viewpoint is that assuming that the liquid beads of lotion are the same size and rigid spheres, the phase volume fraction of the liquid beads can only account for 74.02% of the total volume when they are most densely packed. If the phase volume integral number of the liquid beads is greater than 74.02%, the lotion will be deformed or damaged.

(a) Uniform droplet rich pile woven emulsion
(b) Uneven droplet dense stacking emulsion
(c) Non spherical liquid droplets require stacking and emulsion (unstable)

Take the O/W type emulsion as an example, if the phase integral number of oil is greater than 74.02%,emulsion can only form W/0 type, when the O/i type is less than 25.98%, and when the fraction is 25.98% -74.02%, it may form either 0/W or W0 type.

 

Molecular Structure and Properties of Emulsifiers - Wedge Theory

The wedge theory is based on the spatial structure of emulsifiers to determine the type of emulsion. The wedge theory suggests that the cross-sectional areas of hydrophilic and hydrophobic groups in emulsifiers are not equal. The molecules of emulsifiers are viewed as wedges, with one end larger and the other smaller. The smaller end of the emulsifier can be inserted into the surface of the droplet like a wedge and arranged in a directional manner at the oil-water interface. The hydrophilic polar end extends into the aqueous phase, while the lipophilic hydrocarbon chain extends into the oil phase, resulting in increased interfacial strength.

 

Influence of Emulsifier Material on emulsion Type

In addition to the influence of factors such as emulsion composition materials and emulsion forming conditions, external conditions also have an impact on the type of emulsion. For example, the hydrophilic and lipophilic nature of the emulsion wall is strong, and O/W emulsion is easy to form when the hydrophilic nature of the emulsion wall is strong, while W/0 emulsion is easy to form when the lipophilic nature of the emulsion wall is strong. The reason is that the liquid needs to maintain a layer of continuous phase on the wall, so that it is not easy to be dispersed into liquid beads when stirring. Glass is hydrophilic while plastic is hydrophobic, so the former is prone to form O/W emulsions while the latter is prone to form W/0 emulsions.

 

Theory of Aggregation Velocity of Two Phases

The coalescence speed theory starts from the influence of the coalescence speed of the two kinds of droplets that make up the emulsion on the emulsion, and judges that the coalescence speed of the two kinds of droplets depends on the coalescence speed of the two types of droplets when the emulsion, shark, and kill together cover the demand.

 

Temperature

An increase in temperature will lower the hydration degree of hydrophilic groups, thereby reducing the hydrophilicity of molecules. Therefore, the 0/w emulsion formed at low temperatures may transform into a W/0 emulsion upon heating up. This transition temperature is the temperature at which the hydrophilic and lipophilic properties of the surfactant reach an appropriate equilibrium, known as the phase transition temperature PIT.

However, when the concentration of emulsifier is large enough to overcome the influence of the wetting property of the emulsifier material, the type of emulsion formed only depends on the nature of the emulsifier itself and has nothing to do with the hydrophilicity and lipophilicity of the vessel wall.