Emulsions are widely used in many applications, the following of which are worth mentioning:
- food, which emulsions are by far the most widely used in systems such as mayonnaise, salad creams, beverages, etc.;
- cosmetic and personal care products, such as hand creams, lotions, sunscreens, hair sprays, etc.;
- agrochemical, for formulation of many herbicides, insecticides, plant growth regulators, etc.;
- industrial, rolling oils, lubricants, treatments.
- consumer, cleaner, polishing, treatments, coatings etc.
In the following we give a description of the nature of emulsions.
Chemical nature of emulsions
Emulsions are a class of disperse systems consisting of two immiscible liquids. The liquid droplets (the disperse phase) are dispersed in a liquid medium (the continuous phase). Several classes may be distinguished: oil-in-water (O/W), water-in-oil (W/O) and oil-in-oil (O/O).
The latter class may be exemplified by an emulsion consisting of a polar oil ( e .g. propylene glycol) dispersed in a non-polar oil (paraffinic oil), and vice versa.
To disperse two immiscible liquids one needs a third component, namely the surfactant or emulsifier. The choice of the surfactant is crucial formation of the emulsion and its long term stability.
Choice of the surfactant
Several surfactants and their mixtures are used for the preparation and stabilisation of oil-in-water (O/W) and water-in-oil (W/O) emulsions. A summary of the most commonly used surfactants is given below.
The most common cationic surfactants are the quaternary ammonium compounds with the general formula RxN+X–, where N+ is the ammonium cation, R represents alkyl groups, and the counter ion X- is usually Cl-. These quaternaries are made by reacting an appropriate tertiary amine with an organic halide or organic sulphate. Another class of cationics are those based on pyridinium salts such as lauryl pyridinium chloride.
Several other anionic surfactants are commercially available, such as sulphosuccinates, isethionates (esters of isothionic acid with the general formula RCOOCH2−CH2−SO3Na) and taurates (derivatives of methyl taurine with the general formula RCON(R’)CH2−CH2−SO3Na), sarchosinates (with the general formula RCON(R’)COO Na), and these are sometimes used for special applications.
The most common nonionic surfactants are those based on ethylene oxide, referred to as ethoxylated surfactants. Several classes can be distinguished: alcohol ethoxylates, alkyl phenol ethoxylates, fatty acid ethoxylates, monoalkaolamide ethoxylates, sorbitan ester ethoxylates, fatty amine ethoxylates and ethylene oxide–propylene oxide copolymers (sometimes referred to as polymeric surfactants). Another important class of nonionics are the multihydroxy products such as glycol esters, glycerol (and polyglycerol) esters, glucosides (and polyglucosides) and sucrose esters. Amine oxides and sulphinyl surfactants represent nonionics with a small head group.
These are surfactants containing both cationic and anionic groups. The most common amphoterics are the N-alkyl betaines, which are derivatives of trimethyl glycine (CH3)3NCH2COOH (described as betaine). An example of betaine surfactant is lauryl amido propyl dimethyl betaine C12H25CON(CH3)2CH2COOH. These alkyl betaines are sometimes described as alkyl dimethyl glycinates. The main characteristics of amphoteric surfactants is their dependence on the pH of the solution in which they are dissolved.
The simplest type of a polymeric surfactant is a homopolymer, which is formed from the same repeating units, such as poly(ethylene oxide) or poly(vinyl pyrrolidone). These homopolymers have little surface activity at the o/w interface, since the homopolymer segments (ethylene oxide or vinylpyrrolidone) are highly water soluble and have little affinity to the interface. Clearly, homopolymers are not the most suitable emulsifiers. A small variant is to use polymers that contain specific groups that have high affinity to the surface. This is exemplified by partially hydrolysed poly(vinyl acetate) (PVAc), technically referred to as poly(vinyl alcohol) (PVA). These partially hydrolysed PVA molecules exhibit surface activity at the O/W interface.
The chemical nature of the polymer surfactants is important for the choice of the surfactants. This is illustrated in the third images, which shows that an emulsifier with unsaturated alky chain such as oleate (ethoxylated sorbitan monooleate) is more suitable for emulsifying an unsaturated oil. An emulsifier with saturated alkyl chain (stearate) is better for emulsifying a saturated oil.
Surfactant mixtures, e.g. ionic and non-ionic, or mixtures of non-ionic surfactants, can be more effective in emulsification and stabilization of the emulsion. Nonionic polymers, sometimes referred to as polymeric surfactants, e.g. pluronics or PEO−PPO−PEO, are more effective in stabilisation of the emulsion, but they may suffer from the difficulty of emulsification (to produce small droplets) unless high energy is applied for the process. Polyelectrolytes such as poly(methacrylic acid) can also be applied as emulsifiers. Mixtures of polymers and surfactants are ideal in achieving ease of emulsification and stabilisation of the emulsion. Lamellar liquid crystalline phases that can be produced using surfactant mixtures are very effective in emulsion stabilisation. Solid particles that can accumulate at the O/W interface can also be used for emulsion stabilisation. These are referred to as Pickering emulsions, whereby particles are partially wetted by the oil phase and partially by the aqueous phase.
Classification of emulsions
O/W and W/O macro/emulsions: they usually have a size range of 0.1-5 μm with an average of 1-2 μm. These systems are generally opaque or milky due to the large size of the droplets and the significant difference in the refractive index between the oil and water phases.
Nano-emulsions: usually have a range of sizes 20-100 nm. Like macroemulsions, they are only kinetically stable. They can be transparent, translucent or opaque, depending on the size of the drop, the difference in the refractive index between the two phases and the volume fraction of the dispersed phase.
Double and multiple emulsions: these are emulsions of emulsions, W/ O/W and O/W/O systems. They are usually prepared using a two-step process. For example, a multiple W/O/W emulsion is prepared by forming a W/O emulsion, which is then emulsified in water to form the final multiple emulsion.
Mixed emulsions: these are systems composed of two dispersed droplets that do not mix in a continuous medium.
Micellar emulsions: they usually have sizes ranging from 5 to 50 nm. They are thermodynamically stable and strictly speaking should not be described as emulsions. A better description is “swollen micelles” or “micellar systems”.