1. The stress conditions used for speeding up the instability
of emulsions include:
• Centrifugal force,
Agitational force, Aging, and temperature.
2. Physical parameters are evaluated to assess the
effect of any of the above stress conditions:
• Phase separation
• Viscosity
• Electrophoretic
properties
• Particle size and
particle count
1. Determination of particle size and
particle count: Determination of changes in the average
particle size or the size distribution of droplets is an important parameter
used for the evaluation of emulsions. It is performed by optical microscopy, and
sedimentation by using Andreasen apparatus and Coulter counter apparatus.
2. Determination of viscosity: Determination of viscosity is done to assess the changes that might take place during aging. Emulsions exhibit non-newtonian type of flow characteristics. The viscometers which should be used include cone and plate viscometers.
3. Determination of phase separation:
This is another parameter used for assessing the stability of the
formulation. Phase separation may be
observed visually or by measuring the volume of the separated phases.
4. Determination of electrophoretic
properties: Determination of electrophoretic
properties like zeta potential is useful for assessing flocculation since
electrical charges on particles influence the rate of flocculation.
Types of Surfactants
• Anionic
• Cationic
• Non-ionic
• Ampholytic
(a) Anionic:
Monovalent, polyvalent, and inorganic soaps sulfate and sulfates, alkali soaps include sodium, potassium, and
ammonium salts are lauric, myristric, palmitic,
stearic, and oleic acid are water soluble which forms o/w emulsion.
Metallic soaps like calcium or magnesium salts are
fatty acids that are water-insoluble and tend to form w/o emulsion. Organic
soaps form o/w emulsion. Sulfated alcohols such as sodium lauryl sulfate form
o/w emulsion.
Sodium dioctyl sulphosuccinate is frequently used
sulphonates di-(2-ethyl hexyl) sodium sulphosuccinate called Aerosol OT.
(b) Cationic emulgent:
They are commonly used in lotion and cream due to their remarkable bactericidal
property. For example: domiphen bromide, cetyl pyridinium bromide, benzalkonium chloride, cetyl trimethyl
ammonium bromide.
(c) Non-ionic emulgents:
For example: Glyceryl ester, fatty acid esters of sorbitol, and their poly
oxyethylene derivative, polyoxyethylene glycol esters and Sorbitan fatty acid
esters e.g. Sorbitan mono palmitate (span-40) are non-ionic and oil soluble
promoting w/o emulsion.
Polyoxyethylene sorbitan monopalmitate (tween 40) are
hydrophilic, water-soluble derivative that promotes o/w emulsion.
(d) Amphoteric:
N-dodecyl, N, N- dimethyl betaine, lecithin.
PEG esters such as polyoxyethylene glycol 400 monostearate are widely
used to prepare emulsified lotion and creams.
Natural emulgents: The
most commonly used is acacia. Others are Gelatin which is amphoteric in nature,
Lecithin (phospholipid), and Cholesterol.
Acacia and gelatin:
Form interfacial monolayer.
Lecithin and cholesterol:
Form interfacial mono-molecular layer.
Lecithin is a phospholipid and forms o/w emulsion, Darken on
storage.
Cholesterol form:
w/o emulsion.
Finely dispersed solids:
Colloidal clays such as bentonite, veegum, oxide, silica gel, aluminum
hydroxide, and Magnesium oxide. Magnesium hydroxides are most commonly used
finely dispersed solids which act by forming particular particulate film around
dispersed globules.
• Bentonite produce
both o/w and w/o type of emulsion depending on the order of mixing.
• Veegum is also as an
emulgent for o/w emulsion but it is chiefly used as a stabilizer in cosmetic
creams and lotion.
• Auxiliary emulgents
are incapable of forming stable emulsions. Have thickening properties. Thus,
consistency of an o/w emulsion prepared by using acacia. It can be increased by
tragacanth or agar which act as auxiliary emulsion.
List of antioxidants (0.001-0.1%)
• Gallic acid
• Propyl Gallate
• Ascorbic acid
• Sulphites
• alpha-Tocopherol
• Butylated hydroxyl
toluene
• Butylated hydroxyl
anisole
• Ascorbyl palmitate