SOAP BOILING on a LABORATORY SCALE WALTER C. PRESTON The Procter and Gamble Company, Ivorydale, Ohio
T
HE preparation of cold process soap on a small numerous articles on the subject4by J. W. McBain and scale has been desaibed in the literature1 and his students and by R. H. Ferguson, et al. Briefly summarized, the first step in soap making there have been numerous discussions2 of soap boiling as commonly carried out in industry. Reduc- consists in boiling oil or fat with slowly increasing tion in scale from the huge kettle of the factory to the amounts of sodium hydroxide solution, maintaining at one-pound kettle of the laboratory is not simple, how- all times the proper strength of alkali. The reaction ever, and descriptions of procedure and apparatus suit- then proceeds to practical completion, and further able for the latter have been lacking. The result is addition of alkali (or of NaCl) beyond this point throws that soap-boiling experiments are seldom attempted in the soap out of solution, or "grains" it. The soap rises school and college chemistry courses, although such to the top as a separate layer, below which lies an aqueexperiments would be both interesting and instructive ous layer which contains almost no soap, but most of additions to laboratory training. The following paper the glycerin, electrolyte, and water-soluble impurities. is an attempt to rectify this situation, for neither is the This aqueous layer is withdrawn, and the remaining requisite apparatus costly nor the technic difficult, and soap is boiled again either with strong alkali, to insure saponification of the last traces of oil, or with salt a laboratory product of high quality can be produced. The common saponification reaction, whereby soap brine, to wash out excess alkali, which would be oband glycerin are obtained from fat and alkali, is well jectionable in the finished soap. After each caustic or salt wash, the contents of the kettle are allowed to known : stand (hot) while stratification occurs, and the aqueous (CH.(CH2).C00)8 C& 3NaOH +3CHs (CH& COONa layer is removed and discarded. CsHs (0H)s The final purification step i3 variously known as Despite the simplicity of this equation, soap boiling pitching, fitting: or settling. When the overall contents demands more than mere mixing of the two ingredients. of a kettle of tallow soap consists of approximately 50% The first difficulty is that of initiating the reaction; soap, 1.5% salt, and 48.5% water, a separation into once started, i t becomes practically autocatalytic. two distinct and immiscible liquid phases occurs, or Emulsification of the oil and alkali is essential a t the even three phases when a little. more salt is added. beginning in order to multiply the area of contact be- These phases are: (a) Neet soap, an anisotropic phase constituting tween the two, since the initial react& takes place a t this interface. However, the soap formed plays a more 6040% of the total and consisting of some 6648y0 important role than that of a mere emulsifying agent; soap, 30-33% water, and 1-2% electrolyte, glycerin, it acts as a mutual solvent for both oil and alkali, and unsaponifiable organic matter, etc. This is the phase the chief reaction of the two takes place in this solvent which, on cooling and solidifying or drying, gives the medium.% The accomplishment of this reaction is the usual soaps of commerce. Nigre, an isotropic solution of soap in salt water, (b) first step in the soap-making process. Thereafter, the soap concentration varying widely but being usually purification is the object, and to this end changes in phase are brought about by varying the proportions of around 2MO% and the salt content some Myo. In soap, electrolyte, and water. For a full understanding industry, this nigre, which is high in impurities, is reof the physical chemistry involved in these phase worked with soap of a lower grade. (c) Soap lye, or pitchwater, a salt solution practichanges, the student is referred in particular to the cally free from soap. Pitching consists in adjusting concentrations in the EVANS, "Experimental soap making," J. CBM. EDUC., 14, kettle so that the first two, and preferably also the third. 53443 (1937). I For example, MCBAIN AND WALLS, "Colloid chemistry of of these phases shall separate into layers. The top soap. Part 11. The soap boiling process," Fourth report of colloid chemistry and its general and industrial applications, layer only, which is neat soap, is saved. "The modern soap indusLondon, 1927, pp. 244-63; PRESTON, It is possible to saponify the oil with such a small try," J. CHEM.EDUC., 2, 1035-44 (1925); and WEBB,"Soap and glycerine manufacture," Davis Bros., London, 1927, 224 pp. ' For example, MCBAINAND WALLS, b ~ Cit.. . and FEEDUSON. a SMITH,"The kinetics of soap making." J. Sor. C h m . Ind., 51, "Phase phenomena in commercial soap systems." Oil end Soap. 9,
+
33748T (1932).
+
.58 and 25 (1932).
excess of alkali that one can "drop a nigre" a t once, without intermediate treatments with lye or salt brine, but it is customary to grain with salt a t the end of the saponification and to "wash a t least once each with strong caustic and with salt brine before proceeding to the $itch. More detailed instructions for carrying out these steps will be given below. APPARATUS REQUIRED FOR LABORATORY SOAP BOILING
The essential equipment consists of a steam line and a vessel which permits stratification and separation of hot, and often viscous, liquid layers. No mechanical or manual stirring is required, since this can be accomplished by steam alone. Figure 1 shows a simple assembly suitable for work upon a one-pound scale. The soap kettle can be made from a 3-liter pyrex Erlenmeyer flask, the neck of which has been sealed as shown, while a hole three or four inches in diameter is cut in its bottom. The need for sturdier and larger kettles than those of this type led this laboratory 14 or 15 years ago to have 12-liter kettles of thick pyrex glass molded to order, hut more recently, conical pyrex "percolators" have become commercially available,' and these make satisfactory kettles, whether in the 2-, 4- or 8-liter size. The kettle can be fitted with a collar and hung from a support above. In the case of the larger "percolators," a rope and pulley will be found convenient for lifting from and lowering into the water bath. Temperature control of the hot water bath may be as desired, but it would he hard to find a more foolproof system than that pictured, namely, steam from the laboratory line bubbling through water a t such a rate as to keep the latter boiling. Excess steam escapes into the room, while the comb'matiou of condensation and overflow to the drain maintains constant water level. Steam for the kettle itself may be drawn from the laboratory line, or it may be generated as shown. The former method is advised for an.8-liter "percolator," the latter for a one-pound kettle. When a lower liquid layer is to be dra& off from the kettle, this may be done by suction from the sealed inverted Erlenmeyer, or by gravity from the percolator if rnbber tubing, closed by means of a screw clamp, is fitted over the outlet nipple of the latter. Refinements will occur to the user, but the apparatus just described should be capable of assembly in any laboratory a t small expense. SOAP STOCKS
Kitchen grease, lard, vegetable shortening, olive oil, and salad oils are everywhere available. Beef tallow, coconut oil, and "red oil" (technical oleic acid) can be bought with slightly more difficulty. A 3:l mixture of unrefined tallow and coconut oil is an ideal stock for a beginner to use. SOAP-BOILING PROCEDURE
In the laboratory, no attempt need be made to re-
See laboratory supply house catalogs.
cover the glycerin formed as a by-product, and one can use a large excess of NaOH without qualms as to cost, such as beset the manufacturer. Each type of oil has its own kettle characteristics which can best be learned in the school of experience. No universal law can be laid down as to the proper strength of lye to use or the rate a t which i t should be added. Each new oil entails a change in technic. Coconut oil soap, for example, is so soluble that unusually strong lyes and brines must be used in handling it, and its rate of saponification (after the initial induction period) is so rapid that only alertness and skill can prevent a boil-over.
Tallow, on the other hand, saponifies slowly, and relatively low concentrations of electrolyte throw its soap out of solution. A refined tallo,w,'free from fatty acids, is always hard td boil, and if such a soap stock must be used, the additiou of a small amount of fatty acid or of powdered soap a t the beginning is recommended, to facilitate initial emulsification. In the description that follows, it will be assumed that a 4-liter pyrex percolator is used as;a kettle, and that the soap stock is tallow.
(1) Safionificatwn Charge the kettle with 500 g. of fat and 100 cc. of water. Boil with steam while adding slowly 200 cc. of 15 per cent NaOH solution. This may require half an hour or more. If the lye is added more slowly than necessary, no harm will result, but if it is added too fast, emulsification is interfered with and no saponification occurs. When saponification does begin, thickening takes place, and if a spatula be inserted and withdrawn, a smear of soapy, greasy emulsion remains on it. This state being reached, lye should be added as fast as possible without breaking the emulsion, and a more concentrated solution (28 per cent) should be used. While too rapid addition may still break the emulsion, yet too slow additiou will produce middle sou?, the stiff, gummy soap phase which is the bane of the beginner. The remedy for the former situation is to add water; that for middle soap is prolonged boiling with strong NaOH or NaCl solution.
Granted that an emulsion is maintained and middle soap avoided, the soap mass becomes thicker until finally all of the fat bas been saponified. There will then be present soap, water, glycerin, and excess NaOH, all forming an apparently homogeneous phase through which steam is being continuously blown. If a spatula be dipped in, a smooth layer of soap should adhere to it; if the hot soap be squeezed'between thumb and forefinger, it should solidify to a thin, dry, non-greasy flake; if tasted, there should be an alkali bite, and this bite should not disappear upon further boiling. If these criteria are met, the saponification is complete and the soap is next "grained out," "salted out," or "opened" by adding more NaOH, or, alternatively, by adding NaC1. Curds of soap, floating on clear lye, result, the sudden heterogeneity being clearly recognizable by eye. Steam is now turned off and the kettle is allowed to stand for 10 minutes. The lower, aqueous layer, called seat, is drawn off and discarded (unless glycerin is to be recovered), and the upper, soap layer is ready for the second step.
(2) Strong Change, of First Lye Wash This step may be omitted if the soap boiler is sure that all of the fat was saponified in the 6rst step, but to get good results consistently, it is advised that it be included. Water is added slowly and the soap is "closed," i , e., it is boiled with steam until the curd redissolves to give again a smooth, homogeneous phase. Four hundred cc. of 28 per cent lye are then added, which again grains the soap. After boiling 10 minutes longer, the kettle is allowed to stand for the same length of time, and the seat is drawn off and discarded.
(3) First Salt Wash This step may be omitted'if the presence of some free alkali in the finished soap is not o~ectiouable,but unless the oil used is of high grade, one or more lye or salt washes are needed to wash out dirt and colored impurities. Four hundred cc. of saturated salt solution are boiled through the soap. Then, after a 10-minute stand, the seat is drawn off and discarded. ( 4 ) First Pitch
Add water slowly, boiling constantly, until soap curds begin to soften and the soap becomes almost, but not quite, "closed." If too much water is added and the soap does close, it should be "opened" to a soft grain by adding salt. After a 10-minute stand, the seat is drawn off and discarded. It seems to be legitimate to call this step either a pitch, because water only is added, or a salt wash, because only seat, and no nigre, separates as the second phase. It is not an essential part of the process. Its object is to reduce the electrolyte content so as to
avoid the need for an excessive amount of water in the next step.
(5) Second Pitch Add water slowly, while boiling very vigorously, until the soap is somewhat nearer to being "closed" than in the first pitch. When the proportions of soap, water, and electrolyte are correct, small steam bubbles entrapped in the viscous soap cause it to swell to the top of the kettle, if the boiling is sufficiently vigorous. (A lid may be necessary for protection against excessive splashing.) The soap will now slide from a spatula in coherent sheets or flakes, leaving the surface of the spatula clean6 If too little water has been added, the curd soap will not adhere to the spatula; if too much, the soap adheres in a smooth layer. In the latter case, more salt brine should be added. Steam is now cut off and the kettle allowed to stand. A characteristic folding over of the surface of the soap will be observed as the level falls, and dark streaks of nigre can be seen settling in the kettle to form a distinct layer a t the bottom. After an hour's stand, the nigre should be from 204070 of the total volume, but for complete settling a t least 16 hours should be allowed for a kettle of the size specified herein.' The nigre is next drawn off and discarded, leaving fmished soap in the kettle. This is usually covered with a dry crust, and to avoid mixing this with the neat soap, the latter is drawn off from the bottom into a suitable container in which i t is allowed to cool and solidify. Since the resulting cake of "kettle soap" will probably be streaked or mottled in appearance, it is usually ':crutched," or stirred thoroughly, while still hot and molten. A mecbauical crutcher, with a hot water jacket, is preferable, but not essential. It is here that perfume, coloring matter, sodium silicate, etc., may be mixed in, if desired, or air beaten in if a floating soap is the aim. Soap boiling is an art, to beinastered only by practice. The maestro then varies his technic to suit the particular requirements of the oil in hand. For example, a fatty acid such as "red oil" combines so rapidly with the added alkali that i t is usually advisable to reverse procedure, and add the oil to the alkali. In this way a large excess of electrolyte is assured, which will prevent the formation of middle soap. Lastly, both beginner and maestro have this consolation: So long as they do not boil the soap over the top of the kettle, there is little that they can do that will ruin it, since mistakes are not irrevocable. It is always possible to "open" the soap, drop a "seat" and discard it, and start over again.
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Wrem, loc c i l , gives photographs of the appearance of soap upon a trowel at various degrees of "openness." I n addition to near and nipre nhasrs. ~nirchine . . often dues and should result also in a third pha'se, a clear 1ye";hich ""d&li& the nigre. Study of the phase diagram of the system (for example, VOLDAND FEROUSON, "A phase study of the system sodium palmitate-sodium chloride-water at 9O0C.,"I. Am. Chem. Soc., 60. 206676 (1938). Finure 6 esoeciallv) will elucidate this a n d o t h k phase change'bwhkh occur-during'map boiling, ~~
