Emulsions are mixtures containing fats, oils, resins, or waxes reduced to a fine state of division, and uniformly diffused in aqueous liquids, subsequent aggregation of the oily particles being prevented by the intervention of a viscous or chemical substance, termed the emulsifying agent. Milk, yolk of egg, and the products of the trituration of certain gum-resins and seeds with water are typical natural emulsions. Artificially prepared emulsions should be as uniform in appearance as those produced naturally, the oily particles being diffused so that they become invisible to the naked eye. The suitability of the methods available for reducing emulsifiable substances to a sufficiently fine state of division, depends, upon the quantity to be manipulated, and the emulsifying agent employed. Brisk and light trituration in one direction with a pestle in an ample sized mortar, serves for the production of small quantities of emulsions containing a considerable proportion of fatty oil. Small quantities of volatile oils may be efficiently emulsified by shaking in a suitable container with a mucilaginous agent. The manufacture of large quantities of emulsions is effected in machines constructed for the purpose. The first object in the preparation of emulsions should be the production of a concentrated primary emulsion, and dilution should not be attempted before this is accomplished. The following substances are principally employed as emulsifying agents:—
Powdered Gum Acacia.—Fatty oils (almond oil, castor oil, codliver oil), oleoresins, liquid or soft paraffins and volatile oils may be readily emulsified by means of this agent, if the proportions of powdered gum and water used for producing the primary emulsion are strictly observed. The quantity of water should usually be double the amount of gum used. Four parts of fatty oil or oleoresin (or two parts of volatile oil) require one part of powdered gum and two parts of water. Place the powdered gum acacia in a dry mortar, incorporate the oil thoroughly, add the whole of the water at once, and triturate until complete emulsification ensues. For the emulsification of soft paraffin use a warm mortar, incorporate two parts of melted soft paraffin with one part of powdered gum and triturate with two parts of warm water. Emulsification of waxes and spermaceti may be effected in a similar manner, using an equal weight of powdered gum, and one and a half times as much warm water as gum. Liquid paraffin way be emulsified with the same proportions of powdered gum and water as used for soft paraffin without the use of heat. Copaiba -and balsam of Peru require half their weight of powdered gum for emulsification. When the substance emulsified forms a small proportion of the finished product (as is often the case when volatile oils are ordered in emulsion form) it is necessary to considerably increase the amount of gum, or to use decoction of Irish moss or mucilage of tragacanth as a diluent, in place of distilled water. As an alternative to the dry gum method the requisite quantity of gum acacia (not too finely powdered) may be triturated with double the quantity of water to form a mucilage, or an equivalent quantity of recently prepared mucilage of gum acacia may be employed, and the oil added gradually as emulsification ensues. Emulsions prepared with the best grades of gum acacia yield products of perfectly white appearance. Gum acacia emulsions do not require the addition of preservatives. Emulsions of resins, waxes and volatile oils prepared with gum acacia may often be improved by the introduction of almond oil before emulsifying, the quantity of emulsifying agent being increased accordingly. Such substances as bromoform, chloroform, creosote, croton oil, and oil of cubeb may be usefully incorporated with almond oil and exhibited in the form of an emulsion. Substances (such as salol, sulphonal, and terpin hydrate) which are insoluble or only slightly soluble in water, may in some cases be more uniformly diffused in aqueous vehicles if triturated with almond oil and emulsified, than by suspension with the usual mucilaginous agents. Salol should be melted before incorporating with the almond oil.
Decoction of Irish Mossmay be substituted for mucilage of gum acacia. The resulting emulsions are prone to fermentation and separation unless preventing substances are added. Decoction of Irish. moss is a more efficient emulsifying agent if allowed to stand for about eighteen hours before use. Six to eight parts of the decoction, set to the consistence of a thin jelly, suffice for the emulsification of eight parts of fatty oil. The oil should be added in portions, shaking or beating strongly, by means of any suitable machine, after each addition. For small quantities an egg whisk may be used. The inclusion of thirty grains of tragacanth diffused in alcohol (go per cent.) and mixed with two and a-half ounces of glycerin in each pint of finished product, and the addition of ten grains of benzoic acid prevent fermentation and separation. In consequence of its extreme viscosity decoction of Irish moss is a suitable suspending agent for resinous tinctures.
Yolk of Egg possesses approximately double the emulsifying power of powdered gum acacia, volume for weight. The yolk of an egg of average size measures from four to five fluid drachms, and suffices for the emulsification of at least four fluid ounces of fatty oil or two fluid ounces of volatile oil. Oils should be added gradually to the requisite quantity of yolk of egg, freed from enveloping albumen and previously triturated in a mortar to a perfectly smooth consistence, distilled water being added in small portions if the emulsion thickens inconveniently. Emulsions prepared with yolk of egg are not so liable to separate upon additions of alcoholic preparations, acid salts, diluted acids, glycerin, syrups, or large quantities of soluble salts as are those produced by means of gum acacia. Yolk of egg is therefore a suitable agent for the preparation of turpentine liniments containing acetic acid. Emulsions prepared with yolk of egg may be preserved by the presence of five per cent. of alcohol (90 per cent.), or if desired an equivalent quantity of brandy or rum will serve the purpose of a flavouring agent in addition to that of a preservative. Yolk of egg may be effectively preserved if mixed with an equal volume of glycerin, and by this means can be kept in a suitable condition for immediate use. The emulsifying power of the mixture is approximately equivalent to that of powdered gum acacia. If desired, flavouring agents such as gluside or aromatic oils may be incorporated with the yolk of egg before admixture with the glycerin.
Mucilage of Tragacanthproduces coarse emulsions, and is therefore rarely used except in conjunction with other emulsifying agents.
Tincture of Quillaia, Liquid Extract of Quillaia, Tincture of Senega, and Saponin may be advantageously used as emulsifying agents in certain cases. In consequence of their therapeutic activity the use of these agents is generally limited to the. preparation of emulsions intended for external application, from which it is desirable to exclude mucilaginous matter. For the emulsification of fatty oils or small quantities of volatile oils in such preparations, or for preventing the complete separation of oil when adding alcoholic solutions of oils to aqueous liquids, these agents may be used with advantage. They are unsatisfactory for preparations containing volatile oil in appreciable quantity.
One part of Tincture of Quillaia will emulsify eight parts of fatty oil or one part of volatile oil. The requisite quantity of tincture of quillaia should be diluted with two parts of water and thoroughly shaken in an ample sized bottle with the oil until emulsification ensues.
Liquid Extract of Quillaia is more suitable as an agent for the emulsification of fatty oils in lotions than for volatile oils; about ten minims of liquid extract of quillaia diluted to 120 minims with distilled water will emulsify one fluid ounce of almond oil and yield a product of milky appearance.
Saponin may be used with advantage in place of tincture of quillaia for the emulsification of fatty oils in creams or lotions. Four grains of saponin dissolved in 120 minims of distilled water, and shaken with one fluid ounce of fatty oil, produces a satisfactory emulsion. Emulsions made with quillaia or saponin are not affected by the addition of substances liable to "break" gum acacia emulsions.
Soap (usually in the form of soft soap) is also used for the emulsification of oils in creams, liniments, or lotions, and owes the rapidity of its action to the presence of free alkali. Volatile oils may be emulsified with soft soap by the method used in the preparation of Liniment of Turpentine, B.P. One part of soft soap heated in a tared pan with five parts of distilled water and subsequently adjusted so that six parts by weight of the final product contains one part of soft soap, forms a convenient stock solution, which may be used for the emulsification of fatty oils in preparations intended for external use. Eight parts of almond oil are rapidly emulsified if shaken in an ample sized bottle with one part of such a solution, previously diluted with an equal quantity of distilled water.
Wool Fat may be used in aiding the incorporation of aqueous liquids and fatty oils ordered in the form of creams intended for external application. One part of melted wool fat is capable of emulsifying at least sixteen parts of almond oil in the presence of from twenty to thirty parts of water. Emulsions of more fluid consistence may be produced by using suitable proportions of wool fat and powdered Castile soap or powdered curd soap.
Alkaline Solutions (such as Lime Water and Solution of Potash) are used as emulsifying agents.
For the emulsification of substances intended for internal administration, preference should be given to methods which do not involve hydrolysis of the oils contained. Emulsions intended to be stored should be poured immediately after manufacture into wide-necked bottles (preferably dry) and securely closed with corks, the lower portions of which have been coated with hard paraffin.