As stated earlier, emulsions are thermodyNamically unstable systems and hence they have a tendency to attain a thermodynamically stable state by way of coalescence in the dispersed phase and ultimately separate into two phases. The formation of globules of the dispersed phase is associated with increased surface free energy.
V is the volume of the dispersed phase in ml, d is the mean diameter of globules, AF is the increase in surface free energy in ergs and y is the interfacial tension (dynes/cm) between the dispersed phase globules and the dispersion medium. If 100 ml of oil is to be dispersed as 1 um globule, in water when Yc/m = 50 dynes/cm, this would require an energy input of
The system attempts to overcome excess surface free energy by coalescence of the dispersed globules. An emulgent adsorbed as a monolayer at the interface lowers the surface tension. Thus, if an emulgent can reduce y-from 50 to 5 dynes/cm it will reduce the surface free energy from 7.2 to 0.72 cal. Similarly, if interfacial tension is reduced to 0.5 dynes/cm then the surface free energy would be reduced to one hundredth.
The ability to readily form a film around each dispersed globule is one of the most important requirements of an efficient emulgent. The film can be a monolayer, a multilayer, or a particulate one. A good film should not thin out and rupture when sandwiched between the globules, and if broken, it should reform readily. Hence a film should possess some degree of surface elasticity.
The dispersed phase globules are known to acquire an electric charge during the process of emulsification. The globules gather two charged layers around themselves called Helmholtz double layer as shown in Fig.
globule surface and a point in the bulk of liquid where positive and negative charges exist, or as Zeta potential, which is the difference in charge potentials between a point on the fixed charge layer on the lobule to the point where both positive and negative charges exist as above. Zeta potential can be measured by a zeta meter and helps in the reduction of the flocculation behavior of the system. Dispersed phase systems having a zeta potential value higher than 0.25 mV are likely to be stable because the intensity of the charge on the particles repels them from one another.
Emulsions having an internal phase of less than 5% behave like Newtonian fluids while concentrated emulsions show nonNewtonian flow characteristics. The consistency behavior of an emulsion depends upon several factors like the volume of the dispersed phase, size and size distribution of the globules, concentration and nature of emulgents, volume of the external phase, and the presence of other stabilizing agents. The emulsions having electrically charged globules, usually of o/w type, exhibit electroviscous effect i.e. an increase in viscosity due to particle-particle interaction.
The concentration of emulgent:
The concentration of an emulgent should be adequate for a particular system. If the concentration is less, a condensed film will not be formed around the dispersed phase globules and hence would not prevent coalescence. More than just desired concentration of the emulgent contributes very little in terms of increased viscosity. The aim, in practice, is to use the minimum amount of the emulgent(s) to produce a stable emulsion.
Mechanism: Emulgents form monomolecular, multimolecular, or particulate films around the dispersed globules.
Surfactant type of emulgents stabilize an emulsion by forming a monolayer of adsorbed molecules or ions at the interface and reducing interfacial tension. In the example described earlier if the emulgent could reduce the interfacial tension from 50 to 1 dyne/cm then the surface free energy would be reduced to one-fiftieth of that calculated earlier. Apart from reducing the interfacial tension, an €mulgent also prevents the coalescence between the globules by forming @ coherent monolayer around them. Such a film should be flexible so that it is capable of reforming rapidly if broken. The presence of a Surface charge due to an emulgent also helps in stabilizing the system by Causing repulsion between adjacent particles.
In modern-day practise, the combination of emulgents is preferred over Single emulgents. The combination consists of a predominantly.
hydrophilic emulgent in the aqueous phase and a hydrophobic agent; the oily phase to form a complex film at the interface. If used individually sodium cetyl sulfate and cholesterol separate from the emulsion but the combination of the two yields a complex film producing an excellent emulsion. A combination of sodium cetyl sulfate and oily alcohol) does not form a closely packed or condensed film and hence a poo, emulsion results. Suitable combinations of a hydrophilic Tween and a lipophilic Span are widely used to formulate satisfactory emulsions.
Hydrated lyophilic colloids form multimolecular films around globules of dispersed oil. However, their use has declined due to the availability of a larger number of synthetic surfactants possessing good emulsifying properties. A hydrated colloid does not cause any appreciable lowering of surface tension and then; effectiveness depends upon their ability to form strong, coherent multimolecular films. Their tendency to increase the viscosity of the continuous phase enhances the stability of the emulsion.
Solid particulate films:
The emulgents forming particulate films are small solid particles that are wetted to some degree by both aqueous and non-aqueous liquid phases. They are concentrated at the interface where they produce a film around the dispersed globules thus preventing coalescence. If the particles are too hydrophilic they form o/w emulsions and if too hydrophobic they form w/o emulsions.
Chemically emulgents can be classified as synthetic, natural, and finely dispersed solids. This classification can also be correlated with the classification described earlier based on the mechanism of action. Thus, most of the emulgents forming monomolecular films are synthetic organic materials. Most of the emulgents that form multimolecular films are organic and are derived from a natural source. Powders, which form particulate films are invariably inorganic synthetic emulgents. These may be anionic, cationic, or non-ionic depending on the charge possessed by the surfactant.
This class includes the agents in which the surfactant io? bears a negative charge e.g. monovalent, polyvalent, and inorganic soaps: sulfates and sulphonates. Soaps are formed from various fatty acids containing preferably from 12 to 18 carbon atoms.
The alkali soaps including sodium, potassium, and ammonium salts of Jauric, myristic, palmitic, stearic, and oleic acids are water-soluble and form o/w emulsions. The metallic soaps like calcium or magnesiu® salts of fatty acids are water-insoluble and tend to form w/o emulsions: Organic soaps are prepared by the interaction of amino hydroxy components with fatty acids, for example
Organic soaps form o/w emulsions. They are practically neutral in H and represent a better balance between hydrophilic and hydrophobic groups.
Sulfated alcohols such as sodium lauryl sulfate form o/w emulsions. Sulphonates have a higher tolerance to calcium ions and are not
hydrolyzed as readily as sulfates. Sodium dioctyl sulphosuccinate is the frequently used sulphonate. ;
The surface activity of the cationic emulgents resides in the positively charged cation. They are commonly used in Jotions and creams due to their remarkable bactericidal properties. These are weak emulgents. Quaternary ammonium compounds are the group of cationic most extensively employed as emulgents. An example is cetylpyridinium bromide.
The undissociated surfactants are not susceptible to pH changes and the presence of electrolytes, unlike anionics and cationic. They contain a proper balance of’ hydrophilic and lipophilic groups. Non-ionic emulgents include glyceryl esters, fatty acid esters of sorbitol and their polyoxyethylene derivatives, polyethyleneglycol esters, and ethers.
A glyceryl ester (e.g. glyceryl monostearate) is too hydrophobic and hence it is widely used as an auxiliary emulgent.
Sorbitan fatty acid esters e.g. sorbitan monopalitsmitate (Span 40) are non-ionic and oil-soluble, promoting w/o emulsions. The polyoxyethylene sorbitan fatty acid esters e.g. polyoxyethylene sorbitan monopalmitate (Tween 40) are hydrophilic, water-soluble derivatives and promote o/w emulsion formation. “
Polyethylene glycol esters such as PEG 400 monostearate are also widely used to prepare emulsified lotions and creams.
Natural emulgents: Acacia is by far the most widely used emulgent followed by gelatin, lecithin, and cholesterol. Other natural materials are employed chiefly as auxiliary emulgents and stabilizers.
Acacia and gelatin form interfacial multilayers whereas lecithin and cholesterol form interfacial monomolecular layers. Acacia gum is soluble in water and forms o/w emulsions. It is colorless, odorless, and Produces consistency enough to prevent coalescence although it does not impart high viscosity. Acacia emulsions are stable over a wide pH range but the emulgent being a carbohydrate, a preservative is necessary for the formulation.