English Method (Wet Method)
The acacia solution is place
in a mortar and a small quantity of the oil is added. (The amount of oil to be
added at one time depends on the quantity of emulsion being prepared). The
pestle is rapidly and lightly rotated, dashing the oil into globules, which are
prevented from coalescing by the intervention of the emulsifying agent. It is
advisably necessary to add an additional portion of the oil at this point, but
as emulsification progresses and the nucleus grows, it will also be necessary
to add water occasionally to maintain the proper consistency.
Example of Emulsion using English Method
To prepare 120 ml of a 50%
emulsion of cod liver oil the following shall be needed:
Formula:
Oil 60 ml
Acacia 15 g
Purified Water, q.s. __________
To make 120 ml
Preparation:
Add 15 ml of water to the
acacia to make the mucilage and being adding oil in approximately 2 ml portion,
increasing the quantity slightly as the emulsion develops.
The English Method is slower
and calls for considerable care and skill and recognition of various conditions
which should or may develop in the emulsification process. In this method the
emulsifying agent is always used in liquid form, as represented by mucilages of
acacia, tragacanth, or chondrus, agar solution, egg yolk, etc. This requires,
as in the case of acacia, which preferably used, that thick mucilage is
prepared. To make satisfactory mucilage of acacia, granular acacia should be
mixed in a mortar with an equal quantity of water added all at once, while
stirring with the pestle. This will eliminate lumps and produce smooth mucilage
in a very short time. It is advisable to dilute the original mucilage slightly
by the addition of a little more water. The amount of oil to emulsified,
although a smaller quantity may serve the purpose equally well.
When to add water and when to
add oil is a question which must be mastered by those who attempt to use the
English Method. It may resolve itself into a case of alternating one with the
other but the question depends entirely on how much of either is added at a
time. If too much water is added and the emulsion additional becomes too thin,
it will be impossible to emulsify additional oil. Like, if insufficient water
is added, the emulsion becomes too thick and “cracks” because there is not
enough aqueous mucilage to cover the globules of the oil present. The emulsion
must be kept of such consistency that the pestle makes a characteristic
crackling sound as it spins around the mortar. This crackling sound is due to
the thick emulsion, or mucilage stretching out between the sides of the mortar
and the pestle, and snapping back as it finally breaks away from one or the
other. At this stage the emulsion should be smooth and creamy in appearance and
show a high uniform gloss throughout. On the other hand when cracking develops,
the emulsion loses its uniform gloss and takes on a “pearly” appearance,
showing a definite sheen and evidence of free oil globules. Should this occur,
it is often possible to save the emulsion by setting it aside for a few
minutes, to allow progress of the separation. Much of the air which has likely
been beaten into the emulsion separates and the oil and aqueous phases become
more pronounced. If water is now added and the mixture triturated, the emulsion
will immediately turn normal. If too little water is found to have been added,
trituration should be discontinued, the emulsion set aside for a few minutes
then more water added and the mixture triturated. But if too much water has
been added the emulsion may become so thin that it will be impossible to
emulsify any additional oil. In this case a new mucilage may be prepared and
the broken emulsion reclaimed by adding it slowly to the new mucilage. When all
of the oily mixture has been incorporated, the emulsion may be brought to its
final volume by the addition of water, preferably added in small portions.
It is an accepted fact that
the smaller the oil globules, the slower the rater of creaming; thus according
to Stoke’s Law – if an oil globule is cut in half, the creaming rate is reduced
to one-fourth. Consequently, the longer is the trituration given to the
emulsion while in its original thick condition; the better is the finished
emulsion.
The Continental Method (4-2-1 Method) or (Dry Method)
When a hand homogenizer is not
available the Continental Method is preferred over the English Method because
it will produce an emulsion more quickly. The Continental Method has a real
merit of never failing to produce a good emulsion if the proportions are
maintained in forming the nucleus and if the directions are strictly followed.
For olive oil, cod liver oil, and castor oil the proportion must be “one-half
as much water as oil, and one-half as much acacia as water” or “4 oil, 2 water,
1 acacia”. This produces only the nucleus although all of the oil is
emulsified, so additional water should be finally added to dilute the emulsion
to the proper strength. The method consists in the following: - the acacia, in
fine powder, is added to the oil and mixed intimately (powdered acacia is the
only natural gum which produces a good emulsion by the Continental Method when
a mortar and pestle are used). The 2 parts of the water is added all at once,
while stirring vigorously with the water and the mucilage thus formed envelops
the oil globules which result from the active stirring of the mixture.
Considerable skill is required in handling the mortar and pestle at this point.
Trituration must be fast and must cover the entire surface of the mortar to
maintain until the mixture becomes very thick and a “perfect nucleus” is
formed. Then additional water may be added as required.
The powdered acacia should not
be triturated with the oils longer than just enough to mix the two intimately.
If the oil and acacia are allowed to remain contact any appreciable time, the
acacia imbibes the oil. This causes the
oil to dissolve poorly when water is added and interferes with emulsification.
Using the Continental Method, it is essential that the acacia dissolves very
rapidly when water is added to the oil and acacia mixture. The following factors are very important in producing a satisfactory
emulsion using Continental Method:
1. Use of a dry mortar
2. Correct proportion of
ingredient
3. Measuring oil in a dry
graduate
4. Adding all at once two
parts of water
5. Rapid and continued
trituration
The pestle should be lightly
held between the thumb and the first two fingers and it should be rotated
entirely by a finger and wrist motion to obtain the speed which is highly essential.
The head of the pestle should be forced against the curve of the mortar and not
rotated with a purely circular motion in order to produce a shearing action on
the oil globules.
In making Continental
emulsions of some oils, other than standard cod liver oil or castor oil type,
it is a necessity to change the proportions of the ingredients. In case of oil
of turpentine the proportion is “3 oil, 2 water and 1 acacia”. For liquid
petrolatum the proportion is “2, 2 and 1”.
Addition of Salt, Syrup and other ingredients:
It is a general rule that when
salts, syrups (as flavoring) and alcohol (preservative) are to be added to
emulsions, they should be dissolved in or diluted with water before adding to
the emulsion. Salts ionize in solution and if carelessly added to the emulsion,
they will neutralize the charge on the dispersed phase and cause the oil
globules to coalesce. If salts are added in concentrated solution, they may
break the emulsion by a dehydration process, taking water away from the
interface. Syrup added in a concentrated form may cause emulsion to break due
to dehydration at the interface.
If an ingredient in an
emulsion prescription of formula is soluble in a substance which is to become
the inner or dispersed phase, it should be dissolved in that substance first
before any attempt is made to make emulsion.
Hand Homogenizer
Hand Homogenizer in small
scale production may be used for preparing either O/W or W/O emulsions. In both
instances, the technique is the same. The water-soluble ingredient and the
oil-soluble ingredients are combined separately, using appropriate technique to
obtain a solution in each case. These two phases are shaken together in a
bottle or otherwise thoroughly mixed and the rough emulsion which forms is
pumped through the homogenizer several times. The hand homogenizer eliminates
all failures due to faulty techniques. If unsatisfactory emulsion results the
fault usually will be found in the formula employed.
Special Emulsions
1. Transparent Emulsions – known also as Micro-emulsions, are clear,
colloidal dispersions of one liquid dispersed in another. The resultant
emulsion is clear because the particle size of the dispersed phase is probably
in the range of 100 – 600A. Some uncertainty still exists as to the
demarcation line between micro-emulsion and clear dispersion brought about by
micelle formation (solubilization). Unfortunately, no exact methods are
presently available to distinguish solubilized oil from one which can be called
micro-emulsions.
According to Osipow there are 3 important aspects of micro-emulsion
theory:
(a) The combination of
emulsifying agents and their concentration must be such as to produce a
metastable negative interfacial tension.
(b) The emulsifier interphase
must not be too highly condensed.
(c) The molecules of the oil
phase must be able to interpenetrate or associate with the mixed interfacial
film constituting the interphase.
One of the chief advantages of
micro-emulsions is that they are thermodynamically stable; no emulsifying equipment
is needed to produce the emulsion. It is very important that the emulsion
formula be designed in such a manner that the micro-emulsion will be formed.
2. Gum-Resin Emulsions – these are prepared from asafetida,
ammoniac, etc. which are usually misnamed since they are not true emulsions but
suspensions. To prepare them, the materials are crushed and only a small
portion of water added at first, just sufficient to make a heavy paste. When
this becomes uniform, additional water is gradually added with constant
stirring.
3. Seed – Emulsion – This is made by triturating oil seeds such as
almond seeds in water. They are examples of emulsions prepared without added
emulsifying agents, since all the ingredients necessary for emulsification are
in the seeds themselves. Preparation is the same as the gum – resin emulsions.
4. Emulsions of Phenyl Salicylate (Salol) – this type of emulsion may
be compounded by dissolving the salol in the oil with the aid of a little heat
and then proceeding in the usual manner to make an emulsion.
5. Emulsions of Volatile Oils – an example of emulsions of volatile
oils is the Turpentine Oil Emulsion which is made by the Forbes’ Method. This
is suitable for oils particularly volatile oils which are capable of being
dispersed as minute globules by shaking in a bottle. In another common method
of emulsification the volatile oil is mixed with a bland fixed oil, the latter
imparting sufficient viscosity to the mixture to allow use of the regular
continental method.
Emulsions may be intended for: (1) oral use (2) parenteral use and (3)
external use.
Emulsions for Oral Use:
This kind of emulsions should
contain ingredients which have been proved by both animal and clinical
experience to be safe. There should be adequate testing for the safety of the
final product. Legal requirements with respect to stability of the active
ingredients and of the final product may require further testing. Most oral
emulsions will require one or more flavoring agents to improve the palatability
of the product. Flavors should be considered from the standpoint of possible
long-term use of the product. In general, the flavor or blend of flavors should
be dissolved in the aqueous phase, but there are instances where it might be
advantageous to have the flavor in the oil phase, as for instance with very
distasteful oils. All oral emulsion products should be in such manner that the
product will remain free of the effects of bacteria, molds or yeasts. This is
particularly important for emulsions which contain gums and similar organic
substances. An extremely efficient preservative in emulsion products is alcohol
in 10 – 15% concentration. Other chemical agents useful as preservatives are:
benzoic acid (0.2%), parahydroxybenzoates (0.1 -0.2%), sorbic acid (0.2%) and
benzalkonium chloride (0.1%).
Antioxidant and buffers may be
incorporated into the emulsion product for a specific purpose and should be
selected as judiciously as preservative agents.
Emulsions for Parenteral Use:
Oil – in – water (O/W)
emulsions develop suitable intravenous preparations for a number of specific
clinical uses. By far the most important use made of emulsions is the
incorporation of fats and oils with high caloric contents in preparations to be
administered intravenously to patients who cannot ingest food and by the other
routes. The emulsion must be non-toxic, sterile and pyrogen-free. Some parenteral
emulsions are used for diagnostic purposes – example, contrast medium.
Emulsions for External Use:
All external emulsion
products, whether for therapeutic or cosmetic use, must be esthetically
acceptable. It is very important to consider the texture, aroma, staining characteristics
and color, since these may mean the success of failure of the emulsion.
Packaging of Emulsion
There are three considerations
for choosing packaging for emulsions.
1. Esthetic quality of the
final package
2. Protective effect of the
package on the product
3. Utility of the package to
the user
It is necessary that packaging
materials should be one which will not release a constituent into the emulsion
system nor sorb one or more of the ingredients.
Example of Emulsion
Liquid Petrolatum Emulsion, NF
Other Names: Emulsum Petrolati Liquidi; Mineral Oil Emulsion and
Liquid Paraffin Emulsion
Formula:
Mineral Oil 500 ml
Acacia, v.f.p. 125 g
Syrup 100 ml
Vanillin 40 mg
Alcohol 60 ml
Purified Water, q.s. _________
To make 1000 ml
Preparation:
Mix the mineral oil with the
powdered acacia in a dry mortar, add 250 ml of purified water all at once, and
emulsify the mixture. Add in divided portions, triturating after each addition,
a mixture of syrup, 50 ml of purified water and the vanillin dissolved in the
alcohol. Then add sufficient quantity of purified water to make the product
measure 1000 ml and mix well.
Uses: Intestinal lubricant and cathartic.
Dose: Usual is 30 ml.
Note: Other methods of emulsification may be used and the quantity
of acacia may be reduced or replaced by agar, gelatin, tragacanth or mixtures
of any of these provided the resulting emulsion is similar in viscosity and
appearance to the emulsion made by the given formula. Vanillin may be replaced
by not more than 1% of any other official flavoring substance or mixture of official
flavoring substances. 60 ml of sweet orange peel tincture or 2 g of benzoic
acid may be used as preservative in place of the alcohol.
