SOME SOLVENT PROPERTIES OF SOAP SOLUTIONS. I
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B Y E. LESTE23 SMITH
I t is well known that phenol and cresol are more soluble in soap solutions than in water; advantage is taken of this phenomenon in the preparation of disinfecting solutions (e.g. Liquor Cresol Saponatus B.P., Liquor Cresol Glycerinatus B.P.C.) and it has formed the subject of several researches. Cyclohexanol and certain hydrocarbons have been incorporated with soaps to enhance their detergent action. I t does not seem to be realised, however, that the solvent power of soap solutions is by no means restricted to these substances but is perfectly general. Every organic liquid that has been investigated is many times more soluble in soap solution (e.g. 10% sodium oleate), than in water; these solvated soap solutions differ great,ly among themselves in t,heir viscosity, foaming power, and the stability of their emulsions with excess of the organic liquid. The mixture of sterols, alcohols, hydrocarbons, etc., constituting the unsaponifiable fraction of most natural oils, though quite insoluble in water must' obviously be soluble in soap solution, since soaps prepared from such oils yield clear aqueous solutions unless the unsaponifiable content is excessive. Moreover this unsaponifiable matter is retained by the soap with considerable tenacity against the competing solvent effect of extracting solvents, which also vary considerably among themselves in their effectiveness for extracting the unsaponifiable matter. Although some of these facts are mentioned without comment in papers dealing with the estimation of unsaponifiable matter, they have hitherto excited curiously little interest, despite t,heir evident bearing on a number of problems in t,heoretical and practical chemistry. The present series of communications may be regarded as an attempt to map out this field, to link up and extend the few investigations on record and to relate the whole to our knowledge of the nature of aqueous soap solutions. Review of Previous Publications Pickering' observed the highly interesting fact that soap solutions can not only emulszj'y oils, but can actually dissolre considerable proportions of oil under suitable conditions. It appears that a certain soap manufacturer noticed that oils (presumably glyceridic) were soluble in soap solutions, and communicated the observations to Pickering, who extended it, to mineral oils. Benzene and various paraffin oils mere mixed intimately with potassium stearate or palmitate (in the form of j o y o paste with water), the mixture was then diluted considerably with water, and the amounts of oil dissolved and emulsified, were determined after the emulsions had creamed. The soap would dissolve its own weight or more of oil, and retain it on dilution with water, but less oil was dissolved if the soap was diluted before mixing with the oil. J. Chem. SOC., 1917 111, 56.
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E. LESTER SMITH
Fischerl measured the “gelatin capacity” of a series of pure soaps for water, alcohol, and various organic liquids, i.e. the amount of liquid which could be held by I mol of the soap ~ 1 8a gel showing no syneresis. The entirely different physical states of gel and curd were however not clearly distinguished in this work.2 Bailey’ describes the following system: water, phenol, sodium oleate at 20°, 40’ and 60’; water, cresol, sodium oleate at 20’. The data are plotted in the manner usual for ternary systems, i.e. in equilateral triangles. The curves are of the binodal type which Bancroft has shown to be typical of systems of two partially miscible liquids and a third consolute liquid.‘ The phenolic substance is completely miscible with soap solutions above a certain concentration. JenEiE6 has studied the solubility of cresols in soap solutions of the acetic series, in order to determine the minimum of soap necessary to give a homogeneous solution with a mixture of equal parts of water and cresol at ordinary temperatures, and also (ibid.168)the minimum soap concentrations at which soap cresol mixtures yield gels. Weichherz‘ in a study of xylol-water emulsions stabilised by soap (sodium oleate) found that concentrated soap solutions yield water-in-oil emulsions provided the phase-volume ratio is within a certain range, whereas dilute soap solutions yield in general oil-in-water emulsions: in the former case soap is dissolved in the xylol. Thus on adding water to a solution of soap in xylol a water-inail emulsion is first produced, which on addition of water beyond a critical phase volume ratio inverts to an oil in water emulsion. On account of the low solubility of sodium oleate in xylol, the work was extended by the addition of phenol to the system. The investigation of the quaternary system was limited to a study of the phase volume ratios and emulsion types produced by addition of water to an arbitrarily chosen mixture of xylol 79.94%, phenol 12.917~, and sodium oleate 7.1570. The mixture remained homogeneous up to a certain small water concentration, then further addition caused separation into two phases, the aqueous phase being small in volume, and forming unstable water-in-oil emulsions with the hydrocarbon phase; the aqueous phase decreased in volume as water was added, then disappeared, so that a second narrow homogeneous region appears on the diagram; still further additions of water again caused separation into two phases which formed relatively stable oil in water emulsions. Three of the four ternary systems possible with the four components xylol, phenol, sodium oleate, water, were investigated, and also the variation of the viscosity of the quaternary system with increase of water concentra“Soaps and Proteins” (1921). See Laing and McBain: Kolloid-Z., 35, 18 (1924). 3 J. Chem. Soc., 123, 2579 (1923). 1 Bancroft: Phys. Rev., 3, 21 (1895)‘ J. Phys. Chem., 1, 3 1896); 1, 760 (1897); 3, 217 (1899); Proc. Am. Acad., 30, 324 (1894); Taylor: J.Phys. &;em., 1,461, 542 (1897). 5 Kolloid-Z., 42, 69 (1927). 6 Kolloid-Z., 47, 133; 49, 158 (1929). 1 2
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SOME SOLVENT PROPERTIES OF SOAP SOLUTIONS
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tion, and its behaviour at different temperatures. The findings could all be explained satisfactorily on the basis of the micelle theory of the structure of soap solutions developed by McBain. The author points out that it is not certain whether the solvents are adsorbed on the surfaces of the micelles, or whether any significant penetration or permeation occurs, but observes that the latter possibility is not unlikely. More recently, Angelescu and Popescul have studied in considerable detail the systems ortho-cresol, water, and the sodium potassium, ammonium, and lithium soaps of stearic, palmitic, and oleic acids, paying particular attention to the surface tension and viscosity of the solutions. Addition of o-cresol to sodium or potassium oleate solutions stronger than 0.1N, causes a t 20' first an increase in the viscosity to a maximum a t about 2 % cresol, then a decrease to a minimum at about 4% cresol, followed by a slow increase. With the palmitates, in general only a slow increase in viscosity occurs, similar to that which occurs with sodium hydroxide solution. The stearates are solid a t 20' but a t 40' to 50' behave similarly to the palmitates. The surface tension in most cases falls to a minimum for a small phenol concentration, then rises slowly. The solubility of 0.-cresol in solutions of the three sodium soaps and in sodium hydroxide solutions was also measured. I n the latter case a chemical reaction occurs, whereas in the case of the soap this possibility is excluded. Nevertheless, the solubility in the soap solutions is much greater than in the sodium hydroxide, particularly with the oleate and palmitate. These results are explained on the micellar theory of the constitution of soap solutions. The maximum in the viscosity curve can probably be explained by Ostwald's hypothesis that, as the dispersion of a colloidal solution is increased, the viscosity passes through a maximum. Addition of cresol increases the dispersion of the colloidal particles until they almost reach the molecular state, corresponding to the minimum of the viscosity curve, whereupon further addition causes a slow increase as with sodium hydroxide or dilute soap solutions. This conclusion is substantiated by measurement of the specific conductivity of 0 . 2 K sodium oleate-cresol solutions, which passes through a maximum corresponding to the minimum viscosity, indicating that the soap is most fully dispersed a t this stage. The authors conclude that the whole of the observations can be explained in a highly remarkable and satisfactory manner if it is assumed that the 0.-cresol effects a reduction in the size of the colloidal soap particles, and that according to the theory of W. Ostwald a viscosity maximum occurs a t a certain degree of dispersion. I t will be shown later that the results obtained in the present research (much of which was completed before the papers of Weichherz and of Angelescu and Popescu became available to the author) may be explained on very similar lines. The author's interest in this subject was aroused by a technical research on which he was engaged, concerning the extraction of the unsaponifiable fraction from saponified fish liver oils, as a stage in the preparation of a conKolloid-Z., 51. 247, 536 (1930).
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E. LESTER SMITH
centrate of vitamins A and D. Besides the tendency of the soap solutions to emulsify almost all the solvents with which extraction was attempted, some quite unexpected problems arose. Among these were the high solubility of the solvents in soap solutions, the occurrence in some cases of systems of three liquid phases, and principally the surprisingly low partition coefficients for the unsaponifiable matter, between some solvents and soap solutions (saturated with the solvent.) The observation that the presence of alcohol in the system, besides rendering emulsions less stable, often increased the partition coefficient, did not render the problem less perplexing. The small value of the partition coefficient with even the best solvents, such as ether, renders difficult the complete extraction of the unsaponifiable fraction. Tests of the published methods for estimating the total unsaponifiable matter of oils and fats showed that scarcely any of them give accurate results for this reason. I n some cases, however, it was found that this source of error is partially compensated by failure to remove or estimate soap and fatty acid present in the extract. The details of this work have been published elsewhere.’ A few measurements of partition coefficients for vitamin A have also been published in a paper relating to a technique for the colorimetric estimation of this vitamin.* Apart from the purely technical side, the present research has followed three main lines, which will form the subjects of this and subsequent papers. I . Measurement of the solubility of a range of organic liquids in sodium oleate solutions, and observations on the salting-out of soap in the presence of organic liquids. 2 . Measurements of partition coefficients for unsaponifiable matter, an azo dye and aniline between certain solvents and soap solutions saturated with the solvent. 3 . Study of the phase equilibria in the quaternary system, sodium oleate, water, ethyl acetate, and sodium chloride. Solubility of Organic Liquids in Sodium Oleate Solution Experimental. Oleic acid was prepared from olive oil by the method described by Lawrence.* Sodium oleate was prepared in solution only, by heating the oleic acid with the calculated volume of standard sodium hydroxide (prepared from washed sticks and containing not more than 0.2% carbonate). Slight measured additions of oleic acid or soda were then made until a sample of the solution was just neutral to phenolphthalein on mixing with an equal volume of neutral ethyl alcohol. Laing and McBain4 have shown that certain sodium oleate solutions can be obtained at room temperatures as sol, transparent jelly or opaque white curd. The preliminary experiments were
3
Analyst, 53, 632 (1928). Biochem. J., 24, 1942 (1930). “Soap Films.” J. Chem. SOC., 117, 1507 (1920).
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SOME SOLVENT PROPERTIES O F SOAP SOLUTIONS
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made on a 0.4 N,’ solution, in the sol form. To a weighed quantity in a narrow-necked flask the organic liquid was added in small portions until a slight excess was present. Small additions of the soap solution were then made until the solution was just saturated with the organic liquid at zoo, becoming cloudy from separation of the latter at lower temperatures. No attempt was made to purify the solvents rigorously; in most cases they were taken from laboratory stock and redistilled, the fraction of correct boiling point being used. Accuracy was limited by the high viscosity of some of the solutions, the slowness with which some of the liquids dissolve, and the difficulty of detecting visually an excess of such liquids as ethyl ether, hexane, ethyl and methyl acetates, which yield almost transparent emulsions with the soap solution. The solubility of the liquids in water was also determined when no value could be found in the literature. In addition an emulsion of equal parts of the organic solvent and the saturated soap solution was prepared as follows, and its stability noted: to j ml of the soap solution in a test-tube was added 5 ml of the solvent in I ml portions, giving 20 vigorous shakes by hand after each addition, and an extra 20 shakes after the last. Some of the emulsions “broke” or “separated” in a short time, i.e. separated completely into two clear layers; others were “permanent” or “stable,” i.e. the oil phase remained dispersed in globules for months, although in most cases “creaming” occurred, i.e. partial separation into aqueous phase and emulsion richer in the disperse phase. The mechanism of these phenomena is discussed in previous papers.* Most of the organic liquids investigated show a perfectly definite solubility in the soap solution. At the saturation point the solution becomes cloudy on cooling and clears at the same temperature on warming slowly. This is not the case, however, with some solvents practically insoluble in water, such as anisol and the hydrocarbons. A certain proportion of such liquids can be dissolved by shaking and warming the 0.4 N, soap solution with the liquid. A larger amount can however be “coaxed” into solution by other methods, for example by mixing the solvent into a stronger soap solution, which may be a curd initially, and then gradually adding water, or by mixing the solvent with oleic acid, stirring with the requisite amount of strong alkali (e.g. z N) and then diluting. The apparent solubility of these organic liquids in the soap solution varies with the manner in which the solution is prepared. For example if a mixture of equal weights of oleic acid and benzene is stirred with sufficient 2 X sodium hydroxide to neutralise the oleic acid, and the resulting gel of middle soap then diluted with water, a clear solution will result; neutralisation with N/z alkali will on the other hand yield a cloudy solution. This is in line with the findings of P i ~ k e r i n g . ~ A value for the solubility in 0.4 N, sodium oleate of a substance which behaved in this way was obtained as follows: oleic acid and the organic 0.4 weight normal, Le. 0.4 mols per kilo of water; roo gm. contains 10.8 gm. sodium oleate. * Quart. J. Pharm., 3, 354,362 (1930). Loc. cit. J
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E. LESTER SMITH
liquid were weighed into a tube and the amount of 2 N sodium hydroxide necessary to neutralise the oleic acid stirred thoroughly into the mixture by means of a perforated plunger. The resulting gel was then diluted gradually with sufficient water to make the soap 0.4 N,. If the solution was clear, the experiment was repeated with increasing proportions of the organic liquid, until the solubility limit was reached, and the final solution appeared cloudy with excess of the liquid. The value obtained by this trial and error method does not necessarily represent the maximum solubility of the organic liquid in 0.4 N, sodium oleate. In the case of benzene, however, which was studied in some detail, no greater amount could be got into solution, whether by the use of stronger or weaker sodium hydroxide solution to neutralise the mixture of benzene and oleic acid, by mixing the benzene with strong sodium oleate solution, or by using an excess of benzene and estimating the amount dissolved by methods similar to those used by Pickering. Where two figures for solubility are recorded in the tables, the lower refers to the amount which can be dissolved directly by the 0.4N, solution, the higher to the value obtained by the above procedure.
TABLEI Solubility of Aliphatic Compounds in 0.4 N, Sodium Oleate Solution a t Substance
Hexane Chloroform Carbon tetrachloride N butyl alcohol
Formula
100gm.
0.4 N,
sodium oleate dimlvea:
100 gm.water
dissolves:
CeHir CHC13
2.4 - 7.3
CCla CaHsOH
6.65 gm 0 . 0 8 ~gm Miscible 0.2 N, 8.48bgm NaOH dissolves 59 gm Miscible 0.2 N, 5.2 gm NaOH dissolves 58 gm 71 gm 32.0~ gm
Amyl alcohol” Methyl acetate
gm 20.6 gm
Trace
Permanent
0 . 8 ~ gm
Ethyl acetate
18.7 gm
8.6b gm
Ethyl ether
20.6
gm
7.3b gm
Paraldehyde
Not determined
Furfuraldehyde Cd-IaOCHO
14.75 gm.
11.6~
gm
9.05~gm
”
”
Separated‘ in 4 mins. Separated’ in 4 mins. Separated in I$ mins. Separated in I O mins. Separated in 3$ hrs. Separated in IO mins. Separated in
zo ._
Sharples Cor oration’s “Pentaaol.” b Inter. Crit. qablea. 0 Using 0.2 N, sodium oleate. a
20’
Stability of Emulsion
mins.
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SOME SOLVENT PROPERTIES O F SOAP SOLUTIONS
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Discussion of Results The results obtained for aliphatic compounds or various types are shown in Table I, and for a series of mono-substitution products of benzene (including a few hydrogenated and di- and tri-substitution products) in Table 11. At first sight there appears to be no rhyme or reason in the widely different solubilities and emulsifiabilities of these substances. For example, the extremes of complete miscibility and low solubility are exhibited by such closely related compounds as phenol and its methylation product, anisol. Substances of the same class behave quite differently. Among the esters for example, ethyl benzoate and amyl acetate yield permanent emulsions, while methyl and ethyl acetates yield emulsions which break in a few minutes. TABLEI1 Solubility of Aromatic Compounds in 0.4 N, Sodium Oleate at gm. 0.4 N, sodium oleate dissolves : 100
Substance
Formula
Benzene Toluene Nitrobenzene Aniline
CsHs C8H5CH3 C6H5No2 CsHsNHz
p-Toluidine Phenol
C&OH
o-Cresol Thymol
4.0
3.5
4.32 gm 11.5
gm
Benzyl alcohol Cyclohexanol
CsHiiOH
2.0
Ethyl benzoate C s H ~ c O O C ~j H C6HsOCH3 2.1 Anisol Acetophenone CsHsCOCH3 C7H120 CsHsCHO
- 6 . 3 gma 19.0gm j9
gm
5.4 gm
- 7.2
gmb Trace 0.19 gm' 3.62 gm' 0.1j
gmg
3.8 gm 6
gm
0.08 gm Trace Trace
gm
2.2
7.7
22.5
0.08
gm
5.8 gm
At 45". b Hill: J. Am. Cbem. SOC.,45, 1143 (1923) c Using 0.2 N, sodium oleate. d Angelescu & Popescu: LOC.cit. e Bailev: Lor. rit. .~ f Inter: Crit;
