SOLUBILITY THEORY
Consider Sodium Stearate. (C17H35
COONa). The non-polar hydrocarbon chain
is lipophilic while carboxylic (COONa) part is hydrophilic. The balance of hydrophilic-lipophilic properties
determines whether o/w or w/o emulsion results.
Generally, HLB 9 – 12 form o/w and HLB 3 – 6 form w/o emulsion. A blend of tween 20 and span 20 form o/w. Span 60 alone forms w/o emulsion.
Bancroft’s Rule:
The type of emulsion is a function of the relative solubility of the
SAA, the phase in which it is more soluble, being the continuous phase. Emulsifiers with high HLB are soluble in
water and form oil/water emulsion. The
contrary is true for low HLB.
RATE OF COALESCENCE THEORY AND EMULSION TYPE
Another theory, according to Davies, is based on coalescence
kinetics. When a mixture of oil and
water is shaken together with an emulsifying agent, multiple dispersions are
initially produced which contain oil dispersed in water and water dispersed in
oil.
Therefore we have o/w and w/o initially. On shaking,
interface is disturbed so that fingers or threads of one liquid pass into the
second liquid and vice versa. The
threads are unstable, become varicosed or beaded, the beads separate and become
spherical. Depending on agitation or
shear rate, larger droplets are deformed to give small threads with produce
smaller droplets etc. When the number of
drops increases, there occurs collision and coalescence ensue.
The type of final emulsion depends on whether the water
droplets or the oil droplets coalesce more rapidly. If the coalescence rate of oil globules
dispersed in water is turned rate 1. and that of water dispersed in oil is rate
2, if rate 2 (w/o coalescence rate) is greater than rate 1 (o/w coalescence
rate) an o/w emulsion is formed. This is
a more stable type.
Coalescence
rate = Ce –w/RT
C = collision factor (and is
proportional to phase volume of oil relative to water, also inversely
proportional to viscosity of continuous phase)
W = Energy barrier resisting coalescence (that must
be overcome before coalescence takes place).
It depends on the fraction of interface covered by emulgent, and
electrical potential of dispersed droplets).
For HLB > 7, rate 2 > Rate 1 and o/w forms.
2. MULTIMOLECULAR
ABSORPTION
Hydrated lyophillic colloid, emulsify by acting like SAA by
appearing at o/w interface.
BUT: (a) They do not cause any appreciable
lowering of interfacial tension
(b)
They form multi- rather than monolayer film at
interface. This contributes more to the
emulsification power, because the film formed is strong and resists
coalescence. Moreover they increase
viscosity of the dispersion medium.
Because of hydrophilicity they promote o/w emulsions.
3. SOLID PARTICLE
ADSORPTION:
Finely divided solid particles that are wetted to some
degree by both oil and water can act as emulsifying agent, due to being
concentrated at the interface, where they produce a particulate film around the
dispersed droplets so as to prevent coalescence. The powder easily wetted by water form o/w
emulsions and these wetted preferentially by oil form w/o emulsions, eg Veegum,
Bentonite form o/w, carbon black form w/o.
SELECTION OF EMULSIFIER
The choice of emulsifying agent is very important or crucial
to the successful formulation of an emulsion.
The material should:
-
have emulsifying properties (should be soluble in both
phases but not too soluble in either phase).
-
Be non-toxic, chemically stable
-
Have good taste, odor,
-
Be compatible with other ingredients
The choice also depends on the
type of emulsion to be prepared, ie o/w or w/o.
-
use of the prep external/internal/penetrated
The selection is based on the HLB
system:
-
o/w 8 – 18, w/o 3 – 16
-
polarity of the material?
-
commonly used ingredients have HLB regard values of o/w
or w/o emulsions eg
w/o o/w
Beeswax 4 12
Cetylalcohol - 15
Liquid
paraffin 5 12
Soft
paraffin 5 12
Wool
fat 8 10
Cotton
seed oil 4 11
CLASSIFICATION OF
EMULSIFYING AGENTS (PROPERTIES, USES)
TYPE
|
TYPE
OF FILM
|
EXAMPLES
|
NOTES
|
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Natural
|
Multimolecular
|
Hydrophillic
colloids
(i) Polysardrarides: Acacia, agar tragacanth, Na Alginate,
chondrus, pectin
|
Acacia: Best emulsifying properties
(for extemporaneous preps) for oral administration. CH2O, promote o/w, stable for wide range of
pH, precipitated by alcohol, high concentration of electrolytes.
Gelatin: Protein, as two types:
Type A:
isoelectric pt pH 7/9, best pH 3, +ve
Type
B: isoelectric pt pH 5/9 best pH
8, -ve
Therefore: use type B with Acacia, Tragacanth, Agar
|
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Monomolecular
|
(ii) Lecithin
|
Phospholipid: Promote o/w need preservative
|
|||
(iii) Cholesterol
|
Wool
Alcohol (from wool fat, o/w
|
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Semi-Synthetic
|
Multimolecular
|
Methylcellulose
|
Can
be used as emulsifier and/or stabilizer for o/w emulsions for internal use as
well as external
-
For mineral and vegetable oil, (not Cod-liver oil)
-
Stable over wide pH, but coagulated by high concentration of electrolytes
|
||
Sodiumcarboxy
Methylcellulose
|
-
used as stabilizer
|
||||
Synthetic
surface active agents
|
Monomolecular
|
Anionic
surfactants ionize into large anions (for emulsification) and small cations.
|
Alkali
metal + NH4 soaps (Na, K, NH4 laurate
+ oleate)
-
Are good o/w emulsifiers, but disagreeable odour, irritate GIT,
therefore FEU
-
Not stable below pH 10 (form fatty acids)
-
Incompartible with polyvalent cation, sensitive to acids and high
concentration electrolytes.
-
Do not apply to broken skin
Soaps
of divalent and trivalent metals:
Can
soaps mainly for w/o, for Ext. use also Mg, Ac, soaps
Amine
Soaps: Triethanolamine N(CH2 CH2 OH)3
-
Act like NH3 (combine with FA to form
soaps)
-
Produce fine grained almost neutral o/w, so can be applied on broken
skin but not for internal use
-
Resist appreciably polyvalent
cation, but are affected by acids, electrolyte – stable at pH 8
|
||
SULFATED
ALCOHOLS:- Esters of fatty alcohols
and sulfuric acid,
Na Laurylsulfate,
CH3 (CH2)10
Na cetostearylsulfate
CH2 OSO3 Na+
-
Mainly wetting agents, also emulsifier with auxillary agents.
-
Compartible with Ca++, Mg++ (so can be used in hard water,
Alkylsulfate of these salts are water soluble.
-
Neutral, tolerate big changes of pH.
-
Incompartible with cationic emulsifiers e.g cetrimide and cationic
drugs, eg crystal violet, also I2, HgO, SH+
ALKYL
PHOSPHATE:
-
Similar to sulfates, for o/w creams
SULFONATES: eg Dioctylsodiumsulfocuccinate
CH3(CH2)12CH2SO3-Na+
- These do not hydrolyses as readily as
sulfates.
|
|||||
Cationic
surfactants:
Ionise
into big cation and small anion
|
Most
important are 4o NH cpds. eg
cetrimide, cetyltrimethyl NH4 Br CH3 (CH2)14 CH2 N+) Br-4 – Mainly used for
their disinfectant and preservative effects but also as o/w emulsifiers.
-
Most stable at pH 3 – 7, so suitable for dermatological preps, skin
pH is 4.5
-
No need of preservative, but need stabilizers for efficiency.
-
Compatible with cationic S and Ca++, but incompatible with anionic
surfactants.
|
||||
Non-ionic
surafactants:
Sorbitan
fatty acid esters eg sobitan monopalmitate
SPAN
40, ATLAS
Polyoxyethylene
sobitan
Monoleate
Tween 40, Atlas
|
Can
form o/w or w/o depending on which group is predominant hydrophobic or
hydrophilic.
Promote
w/o
When
blended properly, SPANS and TWEENS produce well textured emulsions
|
||||
FINELY
DIVIDED PARTICLES
|
SOLID
PARTICLE
|
Colloidal
clays
(montmorillonite
minerals: Bentonite veegum, Al (OH)3,
SIO2 Al Mg. Silicate
|
-
Form particulate film wound droplets course grained but stable
emulsion.
-
Can form o/w or w/o according to mode of mixing. If Bentonite is dispersed in water, allowed
to hydrate + form a magma. Add oil
slowly – o/w. If dispersed in oil
first __ w/o
|
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