Some ingredients of emulsions promote growth of
micro-organisms by providing nutrients, e.g. bacteria feed on non-ionic and
ionic surfactants, glycerin, and Emusifying agents (natural polysaccharides)
cause deterioration of emulsions. . Oil e.g. arachis oil promote growth of aspergilus, Rhizopus, while liquid
paraffin promote growth of some spp.
Brucitisses. For these reasons, emulsions should be formulated with a
preservative.
Contamination of emulsions could be due to:
·
Contaminated Emulgent
·
Deionized water (Bacteria grow in resin beds)
·
Equipment
·
Poorly closed container
Preparation of emulsions therefore requires use of high
quality ingredients and clean equipment. Also water must be boiled and cooled
before use. Immediately after use of emulsion, the container must be closed
using a close fitting closures.
Emulsions for Parenteral use must be absolutely sterile.
Although it is not always necessary to achieve sterile conditions in an
emulsion for oral or topical use, it is essential that emulsions are formulated
to resist microbial attack for the reasons given above.
The growth of micro-organisms brings about certain
undesirable changes in the properties of emulsions. Instability include Phase separation, Discoloration, Gas – odor
formation and Changes in
rheological properties
There is one major problem
encountered in adding a preservative to an emulsion, and this is to get
adequate concentration in aqueous phase due to partition. It should be
remembered that Emulsions are heterogeneous systems, and therefore a
preservative tends to partition between oil and aqueous phase. But bacteria grow in aqueous phase. Hence if a preservative is partitioned
strongly in the oil phase, it is useless at normal concentration because a low
concentration remains in water. Here
phase Volume ratio is important. When a
preservative is more soluble in water than in oil, if you increase oil, the
proportion of the preservative in water is increased. The contrary is
true. Therefore allow for this during
formulation.
The preservative must be unionized (so as to penetrate the
bacteria membrane). Activity of weak
acid preservative decreases as the pH of aqueous phase raises.
The preservative must be free, not bound to other components
of preservative – e.g. phenols hydrogen based with oxyethylene group of
non-ionic agents e.g. polysorbate 20
The agent should be non-toxic (non-irritating, bactericidal
(rather than bacteriostatic), rapid in action, broad-spectrum, not attached by
microorganisms (e.g. Pseudomonas
aeruginos a attack phenols), act
even in large of bacteria (some bacteria uptake agents, therefore low
concentration).
The efficacy of a particular preservative is also influenced
by emulsion type, nutritive value of product, degree of aeration, and type of
container used. Examples of preservatives include Sodium benzoate, PABA, Sorbic acid, Chlorcresol, chlorphenol, Mercurial soaps and salicylic acid.
EMULSION STABILITY
The stability of emulsions is the most important aspect of
in the preparation of emulsions. This is
characterized by the absence of coalescence of the internal phase, creaming and
maintenance of elegance as regards appearance, odour, colour and other physical
properties.
(a) Breaking or
cracking:
This is coalescence of the dispersed globules and separation
of the dispersed phase, which is not reversed by shaking. Any chemical of physical or biological effect
that changes the nature of emulsifying agent or making it less stable causes
cracking.
(a)
(i) Addition of emulsifying agent of opposite
type. Monovalent metal soaps form
o/w, while Divalent metal soaps form w/o emulsions. Adding one to the other leads to
instabilities. Also anionic and cationic
surfactants are mutually incompatible. An o/w with sodium stearate can be
converted to w/o by adding calcium salt to form Calcium stearate (therefore
addition of electerlyte).
(ii) Decomposition or
precipitation of emulsifying agent:
Alkali soaps are decomposed by acids ( Free fatty acids– Alkali
salts). The products are not
emulsifiers. E.g. Add acid to Turpentine
Liniment BP
Na soaps are salted out by NaCl – cracking
Anionic and Cationic Emulgents are incompatible with large cautions and
anions respectively.
Non – ionic disinfectants are
incompatible with phenol.
Gums, gelatin, casein, are
insoluble in alcohol leading to cracking.
(iii) Addition of common
solvents: Both dispersion and
dispersed phase dissolve forming solution e.g. castor-oil, soft soap, water,
are soluble in Ethanol
(e) Microbial Action: Molds and Bacterial might destroy emulsifier leading to Cracking,
therefore emulsions are recommended to have a preservative if are to be stored
longer.
-
(f) Excess disperse phase: Alteration in
phase volume ratio also causes inversion, eg mixing an o/w emulsifier with oil
and adding a small amount of water (water is now dispersed in oil. Addition of more water gradually reach the
inversion point and oil globule are enveloped by water to form o/w. This is employed in the continental method.
Thus Emulsions with dispersed phase
concentration (phase vol. ratio) exceeding 74% have a marked tendency to
crack. (If a given space is filled with
closely packed small spheres of uniform diameters, they will occupy 74% of vol.
irrespective of their size).
(g) High Temperatures: Temperature
increases the number of molecule collisions cause cracking. Also it causes
coagulation of monomolecular emulsifiers (protein). Freezing on the other hand produces crystals
in aqueous phase, the salt concentrate in the remaining unfrozen part.
An emulsion prepared by heating
and mixing the two phase can invert on cooling probably due to temperature
dependent …. On solubilities
(b). CREAMING
This is concentration of dispersed globules in either upper
layer or lower layers of the emulsion, e.g. milk. It is caused by gravity and the upward or
downward creaming depends on the density of dispersed and continuous
phase. O/w emulsions cream upwards while
w/o cream downwards (oil less dense than H2O)
The dispersed globules retain their identity do not coalesce
(on in cracking) shaking re-obtains a uniform dispersion. Thus this is not as serious as breaking, but
it causes inelegance, inaccurate dosage, and likelihood of coalescence –
cracking.
The factors leading to creaming are similar to those
involved in the sedimentation rates of suspension and therefore stake’s law
holds.
V = d2
(P1-P2)g
18η
Thus to control creaming, we can utilize these factors:-
-
Reduce mean size of globules (use efficient
homogenizer).
-
Increase viscosity of the continuous phase, by use of a
thickening agent (ttragacanth,
methylcellulose for w/o, soft
paraffin for w/o).
-
Store in a cool place ( decreased temperature reduces
to viscosity, and the number of
collisions between globules) BUT avoid freezing.
(c) Flocculation or Coagulation:
Globules aggregate into looses masses within the emulsion,
but do not coalesce and may be re-dispersed on shaking. But if left uncontrolled, may lead to cracking.
(d) Phase inversion:
This involves the change of emulsion type from o/w to w/o or
vice versa. If this happens after
preparation it is considered as instability.
An o/w with Na Stearate can be converted to w/o by adding Calcium
chloride to form Ca Stearate (addition of electrolyte).
Alteration in phase volume ratio also causes inversion, e.g.
mixing an o/w emulsifiers with oil and adding a small amount of water (water is
now dispersed in oil). Additions of more
water gradually reach the inversion point and oil globule are enveloped by
water to form o/w. This is employed in
the continental method.
