Emulsifying agent
Emulsifying Agents are the substances added to an emulsion to prevent the coalescence
of the globules of the dispersed phase. They are also known as emulgents or emulsifiers.
They act by reducing the interfacial tension between the two phases and forming a stable
interfacial film. The choice of selection of emulsifying agent plays a very important role
in the formulation of a stable emulsion. No single emulsifying agent possesses all the
properties required for the formulation of a stable emulsion therefore sometimes blends
of emulsifying agents have to be taken.
Criteria For The Selection Of Emulsifying Agents
An ideal emulsifying agent should posses the following characteristics:
1. It should be able to reduce the interfacial tension between the two immiscible
liquids.
2. It should be physically and chemically stable, inert and compatible with the other
ingredients of the formulation.
3. It should be completely non irritant and non toxic in the concentrations used.
4. It should be organoleptically inert i.e. should not impart any colour, odour or taste
to the preparation.
5. It should be able to form a coherent film around the globules of the dispersed
phase and should prevent the coalescence of the droplets of the dispersed phase.
6. It should be able to produce and maintain the required viscosity of the
preparation.
Classification of Emulsifying Agents
Emulsifying agents can be classified as:
Natural emulsifying agents from vegetable sources
These consist of agents which are carbohydrates and include gums and mucilaginous
substances. Since these substances are of variable chemical composition, these exhibit
considerable variation in emulsifying properties. They are anionic in nature and produce
o/w emulsions. They act as primary emulsifying agents as well as secondary emulsifying
agents (emulsion stabilizers). Since carbohydrates acts a good medium for the growth of
microorganism, therefore emulsions prepared using these emulsifying agents have to be
suitable preserved in order to prevent microbial contamination. E.g. tragacanth, acacia,
agar, chondrus (Irish Moss), pectin and starch.
Natural emulsifying agents from animal source
The examples include gelatin, egg yolk and wool fat (anhydrous lanolin). Type A gelatin
(Cationic) is generally used for preparing o/w emulsion while type B gelatin is used for
o/w emulsions of pH 8 and above. Lecithin and cholesterol present in egg yolk also act as
emulsifying agent. They show surface activity and are used for formulating o/w
emulsions. However they are used only for extemporaneous preparation and not for
commercial preparation as it darken and degrade rapidly in unpreserved systems. Wool
fat is mainly used in w/o emulsions meant for external use. They absorb large quantities
of water and form stable w/o emulsions with other oils and fats.
Semi-synthetic polysaccharides
Includes mainly cellulose derivatives like sodium carboxy methyl cellulose, hydroxyl
propyl cellulose and methyl cellulose. They are used for formulating o/w type of
emulsions. They primarily act by increasing the viscosity of the system. e.g., methyl
cellulose, hydroxypropyl cellulose and sodium carboxy methyl cellulose.
Synthetic emulsifying agents
This group contains surface active agents which act by getting adsorbed at the oil water
interface in such a way that the hydrophilic polar groups are oriented towards water and
lipophillic non polar groups are oriented towards oil, thus forming a stable film. This film
acts as a mechanical barrier and prevents coalescence of the globules of the dispersed
phase. They are classified according to the ionic charge possessed by the molecules of the
surfactant e.g., anionic, cationic, non-ionic and ampholytic.
Anionic Surfactants
The long anion chain on dissociation imparts surfaceactivity, while the cation is inactive.
These agents are primarily used for external preparations and not for internal use as they
have an unpleasant bitter taste and irritant action on the intestinal mucosa. e.g., alkali
soaps, amine soaps, metallic soaps, alkyl sulphates and phosphates and alkyl sulphonates.
Cationic surfactants
The positive charge cations produced on dissociation are responsible for emulsifying
properties. They are mainly used in external preparations such as lotions and creams.
Quaternary ammonium compounds such as cetrimide, benzalkonium chloride and
benzethonium chloride are examples of important cationic surfactants. These compounds
besides having good antibacterial activity are also used in combination with secondary
emulsifying agents to produce o/w emulsions for external application.
Non-ionic surfactants
They are the class of surfactants widely used as emulsifying agents. They are extensively
used to produce both oil in water and water in oil emulsions for internal as well as
external use. The emulsions prepared using these surfactants remain stable over a wide
range of pH changes and are not affected by the addition of acids and electrolytes. They
also show low irritancy as compared to other surfactants. E.g. glyceryl esters such as
glyceryl monostearate, propylene glycol monostearate, macrogol esters such as polyoxyl
stearates and polyoxyl-castor oil derivatives, sorbitan fatty acid esters such as spans and
their polyoxyethylene derivatives such as tweens (polysorbates). iv) Ampholytic
surfactants: These are the substances whose ionic charge depends on the pH of the
system. Below a certain pH, these are cationic while above a defined pH, these are
cationic. At intermediate pH these behave as zwitterions. e.g. lecithin.
Finely Divided Solids
This group consist of finely divided solids having balanced hydrophilic lipophillic
properties. They accumulate at the oil/water interface and form a coherent interfacial film
around the droplets of dispersed phase globules and prevent coalescence. If the solid
particles are preferentially wetted by oil, a w/o emulsion is formed while if wetting is
done by water then o/w emulsion is seen. e.g., bentonite, aluminium magnesium stearate,
attapulgite, colloidal anhydrous silica and hectorite. The emulsions formed using finely
divided solids are stable and less prone to microbial contamination.
Classification of emulsifying agents
Synthetic emulsifying agents (Surfactants):
This group contains surface active agents which act by getting adsorbed at the oil water
interface in such a way that the hydrophilic polar groups are oriented towards water and
lipophillic non polar groups are oriented towards oil, thus forming a stable film. This film
acts as a mechanical barrier and prevents coalescence of the globules of the dispersed
phase. They are classified according to the ionic charge possessed by the molecules of the
surfactant e.g., anionic, cationic, non-ionic and ampholytic.
: They may be subdivided into anionic, cationic and nonionic surfactants.
i. Anionic Surfactants : The long anion chain on dissociation imparts surface activity,
while the cation is inactive. These agents are primarily used for external preparations and
not for internal use as they have an unpleasant bitter taste and irritant action on the
intestinal mucosa. e.g., alkali soaps, polyvalent soaps (metallic soaps), organic soaps,
sulphated alcohols and alkyl sulphonates.
a. Monovalent soaps : E.g. potassium, sodium, ammonium salts of lauric and oleic acid .
They are soluble in water and are good o/w emulsifying agents.
Disadvantages :
They have disagreeable taste and are irritating to the GIT.. So they are useful only for
external use emulsions.
They have a high pH . They get ppted below pH 10 because the unionized fatty acid is
formed which has a low aqueous solubility. So emulsions formed with alkali soaps are
not stable at pH less than 10.
b. Polyvalent soaps : The calcium, magnesium and aluminum salts of fatty acids (
metallic soaps) are water insoluble and give w/o emulsion.
c. Organic soaps : Triethanol amine soaps of fatty acids give o/w emulsion. They are
used for external use emulsions. They are less alkaline as compare to monovalent soaps .
They can act till pH 8.00
d. Sulfated alcohols : They are neutralized sulfuric acid estrs of such fatty alcohols as
lauryl and cetyl alcohol. They can be used as auxiliary emulsifying agents.
e. Sulfonates : In these compounds the sulfur atom is connected directly to the carbon
atom, giving the general formula
CH3(CH2)n CH2SO3 – Na+
e.g. sodium lauryl sulphate , dioctyl sulphosuccinate.
ii. Cationic Surfactants: The positive charge cations produced on dissociation are
responsible for emulsifying properties. They are mainly used in external preparations
such as lotions and creams. Quaternary ammonium compounds such as cetrimide,
benzalkonium chloride and benzethonium chloride are examples of important cationic
surfactants. These compounds besides having good antibacterial activity are also used in
combination with secondary emulsifying agents to produce o/w emulsions for external
application.
Quaternary ammonium compounds (cetyl trimethyl ammonium bromide)
CH3(CH2)14 N+ (CH3)34Br
They have marked bactericidal activity. activity. So they are useful for anti infective
products such as skin lotions and creams. The pH of an emulsion prepared with cationic
emulsifier is in pH 4 -6 range. This is the range of normal pH of skin. So They are
suitable for skin. They are comparatively weak emulsifying agents, so they are used
along with auxiliary emulsifying agents such as cetostearyl alcohol. They are
incompatible with anionic surfactants.
Nonionics :
They are the class of surfactants widely used as emulsifying agents. They are extensively
used to produce both oil in water and water in oil emulsions for internal as well as
external use.
Advantages : They are not susceptible to pH change and presence of electrolytes.
They also show low irritancy as compared to other surfactants.
Most commonly used nonionics are
glyceryl esters ,
polyoxyethylene glycol esters and ethers
sorbitan fatty acid esters (spans)
polyoxyethylene derivatives of sorbitan fatty acid esters (Tweens or polysorbates)
Polyoxyethylene / polyoxypropylene block polymers (Poloxamers)
Glyceryl esters : e.g. glyceryl mono stearate. It is too lipophilic to be used as a primary
emulsifying agent. It is used as auxiliary emulsifying agent.
. CH2OOC C17H35
CHOH
CH2OH
Sorbitan fatty acid esters : e.g. sorbitan mono oleate. They are oil soluble nonionic
surfactants.and give w/o emulsions.
HO OH
O CH(OH) CH2-OOC-R
Polyoxyethylene derivatives of sorbitans fatty acids : They are hydrophilic and give o/w
emulsion.
OH(C2H4O)n (OCH2)nOH
CH2-OOC-R
O CH(O CH2)nOH
Polyoxyethylene/polyoxypropylene block poymers , also known as poloxamers consist
of combined chains of oxyethylene with oxypropylene where the oxyethylene portions
imparts hydrophilicity and oxypropylene portion imparts lipophilicity. The molecules are
synthesized as long segments of hydrophilic portions combined with long segments of the
hydrophilic portions, with each portion referred to as block. They are used in the
formulation of i/v emulsions and can impart structures to vehicles and interfacial films.
). iv) Ampholytic surfactants: These are the substances whose ionic charge depends on
the pH of the system. Below a certain pH, these are cationic while above a defined pH,
these are cationic. At intermediate pH these behave as zwitterions. e.g. lecithin.
2. Semi-synthetic polysaccharides
Includes mainly cellulose derivatives like sodium carboxy methyl cellulose, hydroxyl
propyl cellulose and methyl cellulose. They are used for formulating o/w type of
emulsions. They primarily act by increasing the viscosity of the system. e.g., methyl
cellulose, hydroxypropyl cellulose and sodium carboxy methyl cellulose
3. Natural emulsifying agents. :
i. Natural emulsifying agents from vegetable sources
These consist of agents which are carbohydrates and include gums and mucilaginous
substances. Since these substances are of variable chemical composition, these exhibit
considerable variation in emulsifying properties. They are anionic in nature and produce
o/w emulsions. They act as primary emulsifying agents as well as secondary emulsifying
agents (emulsion stabilizers). Since carbohydrates acts a good medium for the growth of
microorganism, therefore emulsions prepared using these emulsifying agents have to be
suitable preserved in order to prevent microbial contamination. E.g. tragacanth, acacia,
agar, chondrus (Irish Moss), pectin and starch.
Acacia : It is a carbohydrate gum which is soluble over a wide pH range. It can be used
as emulsifying agent in the following ratio to prepare primary emulsions :
Type of oilRatio of oil : gum : water for primary emulsionFixed oil 4 : 1: 2Mineral
oil3 : 1 : 2Volatile oil2 : 1 ; 2Oleo gum resin1 : 1 : 2
Tragacanth, pectin and starch are used as auxiliary emulsifying agents.
ii. Natural emulsifying agents from animal source
The examples include gelatin, egg yolk and wool fat (anhydrous lanolin).
Gelatin :
It is a protein .It has two isoelectric points, depending on the method of preparation. Type
a gelatin derived from acid treated precursor , has an isoelectric point between pH 7 and
9. Type B gelatin obtained from an alkaline precursor has an isoelectric point around pH
5. . Type A gelatin acts best as an emulsifier around pH 3 where it is -vely chaged: On the
other hand type B gelatin suitable as emulsifier at pH 8 where it is –vely charged.
Type A gelatin (Cationic) is generally used for preparing o/w emulsion while type B
gelatin is used for o/w emulsions of pH 8 and above.
Lecithin :
It is an emulsifier obtained from both plant (soyabean) and animal ( e.g. egg yolk)
sources and is composed of phosphatides. Although the primary component of most
lecithins is phosphatidyl choline . but it also contains phosphatidyl srine, phosphatidyl
inositol, phosphatdylethanloamine and phosphatidic acid.. It imparts a net –ve charge to
dispersed particles. They show surface activity and are used for formulating o/w
emulsions. Lecithins are good emulsifying agents for naturally occurring oils such as soy,
corn, or safflower. Purified lecithin from soy or egg yolk is used for i/v emulsions.
cholesterol :
It is a major constituent of wool alcohols , obtained by the saponification and
fractionation of wool fat. It forms w/o emulsion. It is because of cholesterol that wool fat
absorbs water and form a w/o emulsion It is also present in egg yolk.
Wool fat
It is mainly used in w/o emulsions meant for external use. They absorb large quantities of
water and form stable w/o emulsions with other oils and fats.
Finely dispersed solids :
They form particulate films around the dispersed droplets, producing emulsions which
are coarse grained but stable. Colloidal clays like bentonite, veegum are the examples of
finely divided solids used as emulsifying agents.
Bentonite : It is a gray, odorless and tasteless powder which swells in the presence of
water to form a suspension with a pH of about 9. Depend on the order of mixing, both
o/w or w/o emulsion can be formed with bentonite. For o/w emulsion, bentonite is first
dispersed in water and allowed to hydrate to form magma. Then oil phase is gradually
added with constant agitation. To prepare w/o emulsion, bentonite is first dispersed in oil
and then water is added gradually.
Veegum : Used as stabilizer in concentration of 1% for cosmetic lotions and creams.
Prepared with anionic or non ionic emulsifying agents.
Selection of emulsifying agent :
Criteria For The Selection Of Emulsifying Agents
An ideal emulsifying agent should posses the following characteristics:
Choice of emulsifying agent
Choice of emulsifying agent depends upon
i. Shelf life of the product
ii. Type of emulsion desired
Cost of emulsifier.
Compatibility
Non toxicity
Taste
Chemical stability.
An emulsifying agent suitable for a skin cream may not be acceptable for oral preparation
or i/v preparation due to its toxicity
HLB method for section of emulsifying agent :
The selection of surfactant to be used as emulsifying agent can be done by Griffin’s
method. It is based on balance between hydrophilic and lipophilic portion of the
emulsifying agent .
Griffin developed the system of Hydrophilic liophilic balance (HLB) of surfactants. The
HLB value of the emulsifier can be found from the literature or determined
experimentally or can be computed if the structural formula of the surfactant is known. It
is defined as the mol % of hydrophilic group divded by 5. A completely hydrophilic
molecule (without any non polar group has an HLB value of 20. The molecules that are
water soluble have high HLB value; those which are oil soluble have low HLB value.
Each surfactant is given a value between 0-18 It is used in the rational selection of
combination of nonionic emulsifiers. If an o/w emulsion is required, formulator should
choose an emulsifier with an HLB value in the range of 8-18. Emulsifier in the range of
4-6 can be used for w/o emulsions. Griffin also evolved a series of “required HLB
values” by a particular material if it is to be emulsified in the form of o/w or w/o
emulsion. The required HLB values for a particular oil will differ depending upon
whether o/w or w/o emulsion is required.
Fundamental to the utility of the HLB value concept is the fact that the HLB values are
algebraically additive. Thus calculations can be done to find the correct ratio of
combination of low HLB value and high HLB value surfactant for a particular oil for a
particular type of emulsion.
Example :
Liquid paraffin ( Required HLB 10.5) 15 gms
Emulsifying agents : 5 gms
(A) Sorbitan monooleate ( HLB 4.3)
(B) Polyoxyethylene 20 sorbitan mono oleate ( HLB 15.0)
Water
By allegation method , it can be found that (A) and (B) should be mixed in the ratio of
4.5 and 6.2 to get the required 10.5 HLB value. Because the formula calls for 5 gm of
emulsifying agent , the required weights are 2.1 and 2.9 gms. Respectively.
The formulator can chose a single emulsifying agent which can yield HLB value of 10.5.
But more often in case of o/w emulsions, stable emulsion can be produced by utilsing a
combination of a hydrophilic and hydrophobic surfactant. Such combination appears to
produce mixed interfacial phases of high surface coverage as well as of sufficient
viscosity to prevent creaming and promote stability. HLB values of combination may be
determined by taking weighted average of the individual surfactant HLB values. Many
combinations can be tried to choose the best emulsifying agent.
If the HLB value of oil is not known, it becomes necessary to determine the parameter.
Various blends are prepared to give a wide range of HLB mixture and emulsions are
prepared in a standard manner. The HLB of the blend used to give the best product is
taken to be the HLB of oil.
Theories of Emulsification
Change from A to B will increase the surface area of the phase A. If 1 cm3 of minearl oil
is dispersed into globules having a diameter of 0.01 micrometer in 1cm3 of water , how
much will be the surface area increased?
The surface area will become 600m2 ( greater than a basket ball court); the surface free
energy will increase by 8 calories. Soemulsions are thermodynamically unstable and the
droplets have tendency to coalesce.
Emulsifying agents are needed to decrease surface tension and to stabilise the droplets.
THEORY OF EMULSIFICATION :
When oil and water are mixed and agitated, droplets of different sizes are produced.
However, two immiscible phases tend to have different attractive forces for a molecule at
the interface. A molecule of phase A is attracted to phase A but is repelled by Phase B.
This produces interfacial tension between two immiscible liquids. (Interfacial tension at a
liquid is defined as the work required to create 1 cm2 of new interface.
A fine dispersion of oil and water necessitates a large area of interfacial contact. Its
production requires an amount of work equal to the product of interfacial tension and the
area change. Thermodynamically speaking , this work is the interfacial free energy
imparted to the system. A high interfacial energy favors a reduction of interfacial area,
first by making the droplets to get spherical shape( minimum surface area for a given
volume) and then by causing them to coalesce (decrease in number of droplets). This is
the reason for including the words “Thermodynamically unstable” in definition of opaque
emulsions. To make a stable emulsion droplets have to be stabilized so that they do not
coalesce.
Droplet Stabilisation : ( Mechanism of action of emulsifying agents)
Droplets can be stabilized by making use of emulsifying agents . Emulsifying agents
assist in the formation of emulsion by two mechanisms.
By lowering the interfacial tension And/or
Interfacial tension can be reduced by using surfactants.
By preventing the coalescence of droplets
i. By lowering the interfacial tension (Reduction in interfacial tension –
thermodynamic stabilization):
The increased surface energy associated with formation of droplets, and hence surface
area in an emulsion can be reduced by lowering of interfacial tension. Assuming the
droplets to be spherical
∆ F = 6 γ V/d
∆ F = energy input required
γ = interfacial tension
V = volume of dispersed phase in ml
d = mean dia of particles
If V= 100 ml of oil , d = 1 μm ( 10-4 cm) , γ o/w = 50 dynes / cm ,
∆ F = 6 x 50 x 100 / (1 x 10-4 ) = 30 x 107 ergs = 30 joules or 30 / 4.184 = 7.2 cal.
In the above example , addition of emulsifier which reduces γ from 50 to 5 dynes / cm
will reduce the surface free energy from 7.2 to 0.7 cal. Such reduction in surface free
energy can help to maintain the surface area generated during the dispersion system By ii.
preventing the coalescence of droplets Coalescence of droplets can be prevented by two
methods - (a) By formation of rigid film ,(b) By formation of electrical double layer.
a. By formation of rigid interfacial film – mechanical barrier to coalescence.
Coalescence of droplets can be prevented by formation of films around each droplet of
dispersed material. This film forms a barrier that prevents the coalescence of droplets.
This film should possess some degree of surface elasticity, so that it does not break when
compressed between two droplets. If broken it should form again rapidly. These films are
of three types :
i. Monomolecular films :
The surface active agents form a monolayer at the oil water interface. This monolayer
serves two purposes :
Reduces the surface free energy.
Forms a barrier between droplets so that they can not coalesce.
Multimolecular films :
Hydrated lyophilic colloids and finely divided solids form multimolecular films around
droplets of dispersed oil. They do not reduce the interfacial tension but form a coating
around droplets and prevent coalescing. The hydrocolloid which is not absorbed on the
surface of droplet, increase the viscosity of continuous phase hence stabilizes the
emulsion.
solid particle films
Small solid particles which are wetted to some extent by both oily and aqueous phase,
can act as emulsifying agent. If the particles are too hydrophilic, they get dispersed in
aqueous phase. If the are too hydrophobic, they get dispersed in oily phase. Other
requirement is that the particles should be smaller than the droplet size.
By forming electrical double layer
Presence of a well developed charge on the droplet surface increases stability by causing
repulsion between approaching drops. This charge is likely to be greater if ionized
emulsifying agent is employed. i/v fat emulsions are stabilized with lecithin due to the
electrical repulsion.
In an o/w emulsion stabilized by sodium soap, the hydrocarbon tail is dissolved in the
oily phase and ionic heads are facing the continuous aqueous phase. As a result the
surface of the droplet is studded with –vely charged carboxylic group. This produces a
surface charge on the droplet.. The cations of opposite charge are oriented near the
surface , producing a double layer of charge. The potential produced by double layer
creates a repulsive effect between the oil droplets and thus hinder coalescence.
