Soaps can occur in several physical modifications. In the con dition of "neat soap" as found in the pan (containing 3o% water) soda soaps display the phenomenon of anisotropy or "liquid crystals." As the neat soap cools the characteristic long thin "curd fibres," which exhibit many of the properties of true crystals, grow from and through the mass of fluid crystals. Trans parent solid soaps (q.v. below) are supercooled solid solutions in which crystallisation has been retarded ; they are optically homo geneous (isotropic) and may be true colloidal gels. In old speci mens crystallisation of fibres may occur, and the formation of these can be induced by "seeding" with fibre crystals. The "figging" (growth of irregular opaque masses) in potash soft soap is due to the crystallising out of harder constituent soaps of the mixture, the bulk consisting of fluid crystals and isotropic jelly. From alcoholic solutions of pure soaps, such as sodium palmitate, true crystals can be obtained.
In aqueous solution soap was originally regarded as an or dinary colloid; the solution, however, possesses a high electrical conductivity, which is normally associated with electrolytes.
These apparently conflicting facts are reconciled by McBain (to whom much of the recent physico-chemical investigation of soap solutions is due) by the theory of "colloidal electrolytes," pos tulating the occurrence of the "ionic micelle," or multiply-charged and heavily hydrated colloidal aggregate of ionised molecules.
In alcoholic solution, soap behaves as a simple unhydrolysed non electrolyte. When dissolved in water, soap suffers hydrolysis to an extent dependent on the dilution, resulting in the precipitation of acid soaps (e.g., sodium hydrogen palmitate), and the libera tion of a small amount of free alkali and infinitesimal quantities of free fatty acid.
The detergent power of soaps was orig inally attributed to the alkali formed by hydrolysis (Berzelius), but recent work has proved (McBain, Hillyer and others) that the amount of such alkali is far too minute to account for the cleansing properties ; moreover, hydrolysis is greater in cold dilute solution, whereas the detergent power of hot relatively concen trated solutions is far superior. The detergent action is now considered to be due to the soap itself, and principally a result of its physical characters. Spring considers that soap forms a "colloidal absorption compound" with the dirt, whereby the latter is hindered from re-deposition on the fabric whence its removal was facilitated by the low surface tension between soap solution and grease. In support of the theory, he cites the ob servation that lamp-black suspended in soap solution is not re moved by filtration. It has also been suggested that the soap "lubricates" the dirt particles, rendering them less adherent and more easily removable by rubbing, and that soap may exert a solvent or emulsifying action on oils and grease in the dirty fabric.
The concentration of a soap solution has considerable in fluence on the cleansing power; as a rule, the best results are obtained with 0.25-0.5% solutions (laundry practice).
Almost any fat can be utilised in the man ufacture of soap, the choice being determined by the price of the oil or fat and the quality and type of soap required. The most important of the animal oils used are tallow and grease (toilet soap), and of the vegetable oils, cottonseed, coconut (cold process, marine soaps) palm, castor (transparent soaps) and olive (textile, toilet soaps) oils. Sulphur extracted olive oil is very
for the manufacture of potash soft soaps for the tex tile industries. Linseed oil is the principal ingredient of soft soaps for other purposes. This is infrequently used in America, soya bean or comoile being more commonly substituted. Rosin (the residue from the distillation of crude oil of turpentine) is an important ingredient of yellow ("primrose") household and washer soaps. Lower grade soaps (brown) are made from bone fat, kitchen grease and low grade tallows. As a result of the increased demand for first quality oils for edible purposes, there has been considerable employment recently of hardened oils e.g., whale, soya-bean, etc. For alkalis, caustic soda is usually em ployed ; sodium carbonate may be substituted if the soap is to be made from fatty acids. Caustic potash is used for the pro duction of soft soaps.
The processes for the manufac ture of soap fall into three classes: formation of soaps by (I) Neutralization of fatty acids with alkali (caustic or carbonate). This is not strictly "saponification" but a simple combination similar to the formation of rosin soap. The method enables the manufacturer to employ fats from which the glycerin has been removed by other methods of hydrolysis, and such ma terials as the "oleine" (liquid fatty acids) which is a by-product of the candle-stearine industry, and "soap-stock fatty acids" recovered from the refining of fatty oils. The soap is made by pouring the melted fatty acids slowly into the preheated al kali solution in the soap-pan, the mass being kept boiling by steam to avoid the formation of lumps ("bunching"). The boiling is continued and the soap "finished" as in the boiling process (see below). The production of soft soap by this method has increased latterly, especially on the Continent, as thereby loss of glycerin by inclusion in the soap is avoided. A slight advantage gained by using the cheaper sodium carbonate instead of caustic alkali is counterbalanced by the additional care needed to avoid losses by boiling-over due to the evolution of carbon dioxide. The various methods Of saponification or fat-splitting, by which fatty acids may be prepared from oils may be briefly mentioned.