Emulsion

EMULSION, Emulsio [literally "milking" (of milk), from Latin mulgere-to milk], the name of a dispersed system in which both phases are liquid. Systems are particularly distinguished in which the external (continuous) phase is liquid, while the dispersed phase is dense; these systems are usually called suspensions or suspensions. Suspensions generally follow the same laws as emulsions, and often both types pass into each other, for example depending on temperature; thus, an emulsion prepared from melted lanolin with hot water turns into a suspension upon cooling; the same happens with emulsions of salol and others. In the vast majority of cases in medicine, only systems are discussed in which one of the phases is water (or aqueous solution), and the other is fat; much less frequently, fat is replaced by some other substance insoluble in water, for example waxes, essential oils, paraffins, resins, etc. (Fig. 1). Formation and stability of emulsions. Emulsions formed by simple shaking of water with fat are very unstable and separate upon standing. The formation of emulsions occurs more easily the lower the surface tension at the interface of both phases (the so-called interfacial tension); for example, water with cresol (σ = 4.28) easily forms an emulsion, while emulsions with olive oil (σ = 30) or toluene (σ = 34.6) separate almost immediately. On the other hand, the rate of separation of emulsions is inversely proportional to the viscosity of the medium (Stokes' law).-Emulsifiers. To form stable emulsions, emulsifiers are used-substances that reduce surface tension or increase the viscosity of the medium. Other methods for increasing the stability of emulsions are used less frequently; thus, for emulsifying heavy substances, water with glycerin or glycerin (containing about 12% water) is rarely used; for light substances (such as essential oils, fats, paraffins)-mixtures of water with alcohol. Reducing the radius of dispersible particles to 100 μ on practice is achieved; as is known, dispersions below 0.2 μ, or, according to other authors, below 0.1 μ, already fall under the category of colloidal (ultramicroscopic) and can only be obtained by using special methods of dispersion (see Colloids). Depending on the effect produced, the following emulsifiers are used: 1) reducing surface tension: soaps, saponins (decoctions of soap root or quillaia bark, licorice extract), bile and its preparations, lecithins and sterols; 2) increasing the viscosity of the medium (water): mucilages and gums (gum arabic, tragacanth), dextrins, starch paste, pectins, agar-agar, gelatin, gelatose (see below), egg or blood protein, albuminates and caseinates. As can be seen, the same substances are used as emulsifiers that are used as protective colloids in colloidal preparations. There is still no generally satisfactory theory explaining the formation of emulsions. According to the generalization of Behhold, Zsigmondy, Clayton and others, emulsifiers more soluble in water than in fats (or hydrophilic colloids) give predominantly fat-in-water emulsions. These include: egg or blood protein, gelatin, gums (gum arabic, tragacanth), mucilages, pectins, dextrins, starch paste, agar-agar, saponins, bile, soaps (true and proteinaceous, for example sodium caseinate), alkalis. Emulsifiers less soluble in water than in fats give predominantly water-in-fat emulsions. Such emulsifiers are lecithin (and the egg yolk containing it), sterols (cholesterol, phytosterols, as well as lanolin, which consists largely of cholesterol esters), calcium, magnesium and ammonium soaps, complex esters. However, with a significant predominance of water, emulsifiers of the second group can give fat-in-water type emulsions, and with a significant predominance of oil, emulsifiers of the first group can give water-in-oil emulsions. Pharmaceutical emulsions and techniques for their preparation. In natural raw materials, emulsifiers may be contained in the material itself: thus, in gum-resins there are gums, in oilseeds-mucilages or proteins. Therefore, emulsions are obtained from these materials by simple fine grinding and mixing with water-these are so-called true emulsions (Emulsiones verae). Most often such emulsions are prescribed from almond seeds cleaned from the shell, sometimes from hemp seeds and others. These emulsions from seeds are used as vehicles, having a gentle coating effect or even as easily digestible food substances. To obtain emulsions from substances that do not contain emulsifiers (false emulsions, Emulsiones spuriae), an emulsifier is added additionally; as emulsifiers for these emulsions, gum arabic and egg white are allowed; imported gum arabic is replaced by gelatose (emulgin). The technique for preparing emulsions from oil ("continental method"). The emulsifiable substance is mixed with half its weight of fine powder of gum arabic or gelatose; during this process, the dispersible phase is mechanically broken down. Then water is added in an amount equal to the arithmetic mean of the weight of the emulsifier and the emulsifiable substance, and everything is vigorously rubbed until a white homogeneous mass is obtained, which emits a characteristic crackle when rubbed. This crackle indicates that when air enters, sufficiently strong films are formed that envelop air bubbles and burst with a crackle. The resulting base is gradually diluted with water to obtain the required total weight. If the prescription does not specify from which oil to prepare the emulsion, it is prepared in the pharmacy from almond oil or from another non-drying vegetable oil with low acidity (apricot, pine nut). Of oils with therapeutic action, castor oil is most often used in emulsions (30 g of castor oil per 150-200 g of emulsion). For emulsifying oil with egg yolk, "Vitellum ovi" is indicated in the prescription, and it is taken into account that one yolk can emulsify 15 g of oil (for more viscous oils, such as castor oil-up to 30 g). If no emulsifier is specified in the prescription, the pharmacist always takes gelatose or gum arabic; if it is not specified how much of the main emulsifiable material (oil, seeds, almond or gum-resin) to take, the pharmacist takes it at 10% of the weight of the entire emulsion. If substances soluble in oil or water are prescribed in the emulsion, they are first dissolved in the corresponding phase; substances soluble in water can also be added at the end to the ready emulsion. Electrolytes (salts and acids) easily destroy emulsions, so they cannot be added to emulsions in concentrated form (acids should not be prescribed in emulsions at all). All the described emulsions are opaque, usually white (if no colored ingredients are added) liquids; they are prescribed and used in the same way as mixtures. Ready emulsions are strained through cloth, gauze or rare fabric stretched over a jar or funnel, but by no means filtered, since in the latter case the fat or resin (in general the emulsified substance) would be retained on the filter pores. For preparing emulsions in the pharmacy, mortars or special (double) cups are used; in the latter case, weighing of ingredients directly into the cup is facilitated (Fig. 2). In large quantities, emulsions are prepared in vessels with rapidly rotating mechanical stirrers (machines for preparing emulsions, emulsors of various systems, "cream machines"); in such devices, ingredients can be loaded together into the machine. The method in which oil (10 parts), water (7.5 parts) and gelatose or gum arabic (5 parts) are loaded together and mixed at once is called, in contrast to the "continental" method, the "English" method; the continental method more reliably leads to the goal. Emulsions and suspensions are used mainly for internal, less frequently for external use. In the case of prescribing emulsions for subcutaneous, intravenous or intramuscular injections, or for application to open wounds or skin damage, the admixture of emulsifying substances not provided for in the prescription is not allowed. In order for an emulsion (suspension) to have greater strength, in these cases, media more viscous than water are usually used: emulsions are prepared with glycerin, liquid paraffin, oil, sometimes thickened by dissolving lanolin or cocoa butter or other denser substances in it. In the indicated cases, all ingredients should be pre-sterilized and mixed aseptically. Liquids are thoroughly filtered before sterilization, solid particles are crushed to the greatest possible extent. To prepare mercury emulsions, sometimes used in the form of mercury oil for injections, ready-made mercury emulsion in anhydrous lanolin is used, which is rubbed with warm oil or paraffin to the required concentration of mercury. For external application, in addition to liniments (see Ointments), emulsions are sometimes prepared with such emulsifiers as saponins (decoction of soap root), soaps, etc. Tar emulsions, used for obtaining experimental epitheliomas, are prepared with these emulsifiers, with solutions of gelatin or protein;

Emulsions with these emulsifiers are obtained by simple vigorous shaking of all the mixture ingredients together. Suspensions are prepared technically in the same way as Emulsions. Low-melting substances (wax, salol, etc.) are emulsified with hot water in a mortar heated above the corresponding melting temperature. When preparing a suspension from solid substances whose melting is undesirable or impossible (bismuth salts, tannin preparations, benzonaphthol, etc.), the finest possible powder of the latter is triturated with half the amount of gum arabic or gelatin, and then proceed as in the preparation of false Emulsions. If oil and substances soluble in it (salol, camphor, etc.) are prescribed simultaneously, first a solution of the substance in oil is prepared, and then emulsification is begun; but if these substances are prescribed in amounts exceeding their solubility in the given oil, they are mixed with the total amount of emulsifier, then the oil is added and further proceed as indicated; in this case it is usually necessary to take somewhat more emulsifier than calculated. All Emulsions and suspensions are unstable preparations: the enormous surface of the dispersed phase significantly facilitates and accelerates the oxidation of emulsified substances, their hydrolysis and other chemical and enzymatic processes; at the same time, the composition of Emulsions usually makes them favorable media for the development of microorganisms. Therefore, the preparation of Emulsions in advance is not recommended. Emulsions prepared industrially are released either in sterile form or with the addition of suitable antiseptics, most often harmless essential oils, thymol, chloroform, or nipagin. To increase the physical stability of Emulsions by reducing the diameter of the particles of the dispersed phase, homogenizing apparatus are used; in them, the Emulsion is forced through a narrow slit between (rotating) well-ground surfaces; good apparatus give, after passage, particle sizes within narrow limits up to 10 μ. Bacterial and organ-preparation Emulsions ('opsonic Emulsions') are usually suspensions obtained by triturating bacterial cultures (attenuated or killed) in the first case or animal organs in the second case with water, often with an admixture of glycerin or antiseptics, to obtain a stable, apparently uniform suspension, which is strained from large fragments; preparation in both cases is aseptic. The use of bacterial Emulsions - see Vaccination, vaccines; bacterial Emulsions and suspensions of cultures of non-pathogenic bacteria - see Protein therapy; Emulsions from organs - see Organ preparations.-Emulsions in the food industry. The prototype of all artificial Emulsions is the natural Emulsion-milk, which is an Emulsion of fat in water; the opposite type-water in fat-we have in oil and margarine. Artificial food Emulsions-see also Vegetable milk. Emulsions are widely used in technology.

N. Obergard.