CXCVI. - Emulsions

Abstract

When paraffin oil is churned up with a solution of soft soap, an emulsion is formed which rises to the surface and generally contains 65 to 82 per cent. by volume of oil. Emulsions containing as much as 99 per cent. can, however, be obtained, and these are so stiff as to be almost solid. The character of the oil does not much affect the results, neither does the extent of the churning nor the proportion of soap, so long as this proportion is within certain limits. The amount of oil in an emulsion which has arrived at a state of equilibrium decreases somewhat from the top downwards, but there is often a tendency to arrange itself in layers of approximately uniform composition. Solutions of other organic substances, such as glue, flour, milk, starch, albumen, saponin, &c., act as emulsifiers for paraffin oil. All these emulsions will oft en spontaneously become de-emulsified, and this occurs at once if the emulsifier is destroyed; for example, if an acid or any sodium salt is added to an emulsion with soap. Electrolytes, as such, seem to have no de-emulsifying action. The addition of unemulsified paraffin to an emulsion will gradually de-emulsify the whole. The oil globules in an emulsion are probably prevented from coalescing by being enveloped in a pellicle consisting of particles of solid much more minute than the globules themselves. The solid particles would be derived from the solution, which in all cases contains a substance with but little affinity for water, and insoluble in paraffin, it being, therefore, precipitated in the neighhourhood of the paraffin globules. Apparently, a precipitate consisting of any insoluble substance which is wetted more easily by water than by oil, if in a sufficiently fine state of division, will equally act as an emulsifier, and in some cases it is possible under a microscope to see the coating of solid particles which envelop the oil globules. Emulsions made with an insoluble emulsifier are in every respect similar to those made with soap, &c., except that they never seem to de-emulsify spontaneously; spontaneous de-emulsification being, no doubt, due to the fact that the solid particles, as in the case of soap, are soluble in water, and are continually being redissolved and reprecipitated, thus affording opportunities for the coalescence of the oil globules. The basic sulphates of iron and copper are amongst those substances which give excellent emulsions. They may be formed by adding lime, or lime-water, to the normal sulphates, and then the paraffin, when the slightest churning, or even shaking of the mixture, produces emulsification. Besides the ease of manufacture, and the absence of spontaneous de-emulsification, these emulsions possess the advantage of not being decompose,d by the addition' of caustic soda, as are emulsions with soap, and caustic soda is required when the emulsion is used as a winter wash for trees. When the copper salt is used, the emulsion possesses all the fungicidal properties of Bordeaux mixture. Many other precipitated substances act as emulsifiers, but this property is destroyed as soon as they have been dried, or have by any other means been deprived of their fine-grained structure. Solids which are not sufficiently fine-grained to emulsify will, in many cases, when present in considerable proportions, form quasiemulsions. In these, the particles of oil are merely entangled with the particles of solid, and may be separated from them by such simple means as dilution with water. Lime is an instance of a substance which forms a quasi-emulsion which can be used for spraying purposes. Other substances, including many recentlyformed precipitates, and, probably, all crystalline solids, seem to be incapable of forming even quasi-emulsions.