SOME SOLVENT PROPERTIES OF SOAP SOLUTIONS. I1 In the

SOME SOLVENT PROPERTIES OF SOAP SOLUTIONS. I1

Downloaded by UNIV OF SUSSEX on September 7, 2015 | http://pubs.acs.org Publication Date: January 1, 1931 | doi: 10.1021/j150336a004

BY E. LESTER SMITH

In the previous paper of this series, attention was drawn to the extraordinary solvent powers of soap solutions for organic liquids insoluble or slightly soluble in water. The phenomenon was studied by determining the proportion of an organic liquid absorbed by a soap solution in equilibrium with excess of the liquid. In the present paper, the solvent properties of soap solutions will be studied from a different point of view. An organic substance almost insoluble in water-a sterol or an oil for example-may be dissolved to a limited extent in a soap solution; if this solution is then shaken with an organic solvent such as ether, the dissolved substance is by no means completely extracted by the solvent; on the contrary a definite and reproducible equilibrium is set up, the substance being partitioned between the two liquid phases. Until recently almost the only problems involving the extraction of soap solutions occurred in connection with the analysis of oils, fats and waxes, where determinations of the quantity and nature of the unsaponifiable fraction afford in some instances criteria of genuineness. Very little systematic work is to be found in the literature, dealing with the problems underlying the extraction of soap solutions. Published papers include descriptions of analytical methods for the determination of the unsaponifiable matter in oils, fats and waxes, often unaccompanied by strict evidence as to their accuracy, and some experiments which prove that solvents such as petroleum spirit are less efficient than ethyl ether for the extraction of fish liver oil soap solutions. This work has been reviewed by the author elsewhere.' With the discoveries that the fat-soluble vitamins A, D and E are present in the unsaponifiable fractions of the oils in which they occur, such studies assumed a new importance, particularly in connection with the preparation of concentrates of these vitamins for incorporation into foodstuffs and medicinal preparations. The work here reported was in fact undertaken initially from the point of view of these practical problems, but was continued on account of its own intrinsic interest. The unsaponifiable fraction of natural oils is a complex mixture of substances of ill-defined constitution, and is difficult to estimate with precision; the vitamins are even more difficult to estimate, and it was felt that more useful information might be obtained by studying instead the partition of some pure chemical substances. p-Dimethylaminoazobenzene and aniline were finally selected, largely on account of the ease with which these basic substances could be extracted with acid from their solutions in organic solvents and estimated colorimetrically or volumetrically. Analyst, 53, 632 (1928).

SOLVENT PROPERTIES O F SOAP SOLUTIONS

1673


Partition of p-Dimethylaminoazobenzenebetween Soap Solutions and Ether p-Dimethylaminoazobensene is a yellow dye, almost insoluble in water, but fairly readily soluble in soap solutions and in organic solvents. Dilute solutions of its hydrochloride in hydrochloric acid have a pink colour, very suitable for colorimetric estimation. I t was originally desired to study the partition of the natural unsaponifiable matter of cod-liver oil, and the idea was conceived of using this dye as an indicator, in the.hope that it would be partitioned in the same ratio as the unsaponifiable matter. This hope received some justification in the fact that it had not proved possible (with rather crude analytical methods) to demonstrate any fractionation between the sterol and liquid portions of the unsaponifiable matter on successive extractions of a soap solution; nevertheless it was not fulfilled, as the dye showed a partition coefficient in favour of the ether phase considerably higher than the corresponding value for the unsaponifiable matter. Previous experiments had indicated that the addition of methyl or ethyl alcohol increased the partition coefficient for unsaponifiable matter. Accordingly the effect of varying the methyl alcohol concentration on the partition of the dye was investigated, and also the effects of varying the soap concentration, the excess of alkali, and the concentration of the dye itself.

Experimental The extractions were carried out in 2 5 0 cc stoppered separating funnels, into which all solutions were measured by pipette or burette, taking precautions against evaporation of ether. Fatty acids from cod-liver oil stearin were kept as a 50% stock solution in ether, which also contained the pdimethylaminoazobenzene, in the proportion of 0.01 gm. to the fatty acids from I O O gm. of the oil. In most experiments the fatty acids from 1 5 gm. of oil were used, being mixed in the separator with the necessary amounts of water, z N sodium hydroxide, methyl alcohol and ether. After vigorous hand shaking for several minutes, the separators were allowed to stand at room temperature until complete separation into two layers occurred, noting the time required. The lower and upper layers were then run out separately into graduated flasks of capacity slightly greater than the volumes of the layers, afterwards filling to the mark with water-saturated ether measured from a burette. The volumes of the layers could thus be obtained by difference, with an error probably less than 0.5 cc. The ether layer was then transferred to a separating funnel, and extracted three times with 10% hydrochloric acid using 1 2 5 cc in all. The acid solution was warmed on a waterbath to remove ether, made up with water to 250 cc, and compared in a Hellige colorimeter with a standard containing 0 . 2 5 mg. of the dye in I O O cc of 5 % hydrochloric acid. This gave the weight of dye in the ether layer, and from a knowledge of the total amount present, and the volumes of the layers, the partition coefficient could be calculated. The volume of ether used for an extraction was adjusted after some preliminary experiments, so that approximately half


1674

E. LESTER SMITH

the dye present was extracted since experimental errors in the estimation cause the least error in the calculation of partition coefficients under these conditions. I n the first series of experiments the fatty acids from 15 gm. of cod-liver oil stearin were made up to yield in every case a volume of 1 5 0 ccs of alcoholic soap solution (referred to as IO$^^'' soap solution). On adding ether a very appreciable contraction occurred, and the final volume was always less than the sum of the volumes added by 5-10 cc, an amount considerably greater than could be accounted for by evaporation of ether during the measurements

Discussion of Results Table I records the data for the first series of experiments in which the methyl alcohol concentration was varied, the concentrations of soap, alkali and dye remaining constant. The influence of methyl alcohol on the time required for complete separation of the emulsion is striking; 69 hours were needed in absence of the alcohol, only a quarter of an hour with 15% of methyl alcohol in the soap solution. The use of alcohols for breaking emulsions is of course a familiar procedure in analytical chemistry, but nevertheless it does not seem to be explained by any of the theories of emulsification that have been advanced. I n absence of methyl alcohol, the volume of the soap solution increases on saturation with ether from 150 cc to 187.5 cc. Additions of methyl alcohol up to about 2 5 % cause only small increments in the ether absorption but higher concentrations of methyl alcohol greatly increase the absorption of ether by the alcoholic soap solution. The partition coefficients after a very slight rise, decrease steadily with increasing methyl alcohol concentration; this point will be discussed later. Slight irregularities in these variations are due partly to experimental error and partly to variations in the room temperature a t which the experiments were performed.

TABLE I Excess NaInitial Soap OH over Concentra- amount retion quired to neutraliae Fatty Acids 10%

21.5%

I1

' I

Methyl A& coholas ,o of initial volume of Soap Solution

Nil 2%

It

14

'I

I(

II

II

11

II

(I

'I

I1

'I

(I

I(

II

I(

Time Separate

69 hours ?

5%

7 hours

10%

45 mins.

15%

I5

' I

20%

II

"

25%

IO

('

27%

5 5

"

3.5

"

33% 40%

"

Volume of Ether-eaturated Lower Layer (Initially 150 cc) 187.5 cc

189.5 cc 188.3 cc 191.4 CC 190.7 CC 194.5 CC 199.9 CC 200.4 cc 219.0 cc 250.0 cc

Volume of

upper Layer

Partition Coeficient

37 cc 5.8 36.2 cc 5.8 6.0 35.8 cc 33.2 cc ' 6.1 33.2 cc 5.6 30.0 cc 5.3 23.8 cc 4.5 43.0 CC 4.2 29.2 cc 3 .o 2.15 57.5 cc

SOLVENT PROPERTIES OF SOAP SOLUTIONS

1675

TABLE I1 Initial Excess Na- Methyl AlOH over cohol as % Soap Concentra- amount of initial required to volume of tion neutralim Soap Fatty Acids Solution 10% 11


11

1% 21.5%

42%

1I 11

1% 21.5%

20%

11 11

I8

33% 'I

Volume of Ethersaturated Lower Layer (Initially 1 5 0 cc)

Volume of

30.3 CC 30.0 cc

'(

193.0 CC 194.5 cc 198.5 CC 212.5 cc

li

219.0 cc

39.7 cc 29.2 cc

Time to Separate

? mins.

5 5

2g

25.0

cc

Partition Coefficient

5.4 5.3 4.55

3.55 3.0

TABLE 111 Initial Excess Na- Methyl AlSoap OH over cohol a % Concentra- amount of initial tion required to volume of neutralise Soap Fatty Acids Solution

8%

7 mins.

Volume of Ethersaturated Lower La er (Initidy 150 cc)

Volume of

184.2cc 194.5 cc

36.0 cc 30.0 cc 41.5 cc 34.8 cc 48.0 cc

21.5%

20%

11

1'

11

11

6

205.5

'1

8

221.5

11

7

10% 12%

16%

Time t o Separate

II

cc

cc 236.0 cc

2;:;

Partition Coefficient

7.85 5.3 3.95 3.55 2.9

The second table indicates that for a fixed soap concentration of IO% and methyl alcohol concentration of zoyo and 3370, respectively, increasing t h e free alkali in the soap solution slightly increases the ether absorption and decreases the partition coefficient. The third table illustrates the effect of increasing soap concentration for a fixed methyl alcohol concentration of 2 0 7 ~ . The ether absorption is observed to increase rapidly, while the partition coefficient falls from 7.85 to 2.9 on doubling the soap concentration.

Interpretation of Results We have seen in the first paper of this series, that the solvent powers of soap solutions can only be accounted for by postulating adsorption of the organic solute on the colloidal soap particles. The resistance which such solutions show towards extraction of the solute by organic solvents favours the same hypothesis. Fortunately also, some of the partition data are amenable to mathematical treatment which enables the hypothesis to be tested. It is convenient to consider the solute as being distributed between three phases, the organic solvent, the water and the colloidally dispersed soap. The portions of solute in the solvent and in the water, respectively, are in equilibrium according to the law of partition, while the portions of solute in the water and in the soap micelles are in equilibrium according to the adsorption law. It is true that the concentration of solute in the water may be vanishingly


1676

E. LESTER SMITH

small in some cases, and also that solute may pass direct from soap to solvent, but these considerations do not render inapplicable the above equilibrium relationships. Thus we have:Let U = Total weight of solute in system. X = Weight of solute adsorbed by colloidal soap. Y = Weight of solute in true solution in aqueous phase (excluding that adsorbed by the soap). W = Weight of solute in organic solvent phase. kl = Adsorption coefficient. kz = True partition coefficient for solute between organic phase and aqueous phase (without colloidal soap). K = Measured partition coefficient for solute between organic phase and aqueous plus colloidal soap phases. v = Volume of organic solvent phase. V = Volume of aqueous phase. S = Concentration of colloidal soap in aqueous phase. Then according to the usual adsorption formula :X = ki S YI'" (I) (2)

or Y =

X" -

kt" S"

According to the partition formula:-

Substituting for Y,

(5)

U=X+V+R

But since Y is usually extremely small, it can be neglected in comparison with U giying:(6)

X = U - W

Substituting in (4), W k2v (7) ___ = (u- W)" kl"S"F' The ordinary partition formula used for calculating K (in which aqueous and colloidal phases are treated as one phase) is:W Kv (8) -- -

(u-w)- v

Whence:-



I677

If this expression is correct we should expect that the measured part'ition coefficient K would not remain constant if the amount of solute in the aqueous layer varied over any considerable limits. I t would only remain constant if n = I , when the expression (V-w)"-* reduces to I . This would be the case if the solute distributed itself between t,he colloidal soap and the aqueous phases according to the simple law of partition and not according to the usual adsorption law. In a series of experiments, however, it may be taken that small variations in the concentration of solute in the aqueous phase will not affect the value of K, the permissible limits of variation becoming smaller the furt,her the value of n departs from I. We should also expect, that K would be increased by any factor which would increase the true partition coefficient kz and decreased by any factor tending to increase either the adsorption coefficient or the concentration of colloidal soap (not total soap concentration) in the aqueous layer. Tables 1-111 provide data which enable some of these conclusions t o be tested. Table 111shows the pronounced influence of soap concentration of the partition coefficient K ; the value of the latter is reduced to much less than half its former value by doubling the soap concentration. This is in agreement with expectations, since according to formula (9) K should be proportional to I/S"; S, the concentration of colloidal soap, would more than double on doubling the total soap concentration, since a relatively greater proportion would be in molecular solution in the weaker solution; also n is greater than I ; both these factors therefore tend to make K decrease more rapidly than the soap concentration increases. Similarly the effect of excess alkali in the alcoholic soap solution would probably be to increase the proportion of soap in the colloidal state, without much affecting other factors. This is reflected (see Table 11) in an increase in the ether absorbed by the soap solution, and a decrease in the partition coefficient K. Variation in the methyl alcohol concentration has a composite effect. On the one hand, the solubility of the solute in the aqueous phase is likely to increase with increasing alcohol concentration, and since its solubility in the organic phase is not much affected, k2 the ratio of these solubilities, decreases, so that K tends to decrease. On the other hand, the soap passes more completely into molecular solution as the alcohol concentration is increased, so that S decreases; also the increased solubility of the solute in the aqueous phase is liable to decrease the adsorption coefficient kl. Both these factors tend to increase K so that the net effect on K cannot be predicted. Table I shows in the main a decrease in K for increasing methyl alcohoi concentration, indicating, if formula (9) is correct, that k:! decreases more rapidly than kl"S". Neither of these factors can be measured with any degree of accuracy under the conditions of the experiments; it was possible, however, to get a rough measure of k:! by making the assumption that the solubility of the solute in the aqueous phase (containing methyl alcohol, dissolved ether, and soap in molecular solution) is the same as its solubility in aqueous methyl alcohol of the same strength.


1678

E. LESTER SMITH

The solubility of the dye in a series of methyl alcohol-water mixtures was therefore determined, by adding the requisite volume of a methyl alcohol solution of the dye to a measured volume of water, allowing the excess dye to crystallise out overnight, and estimating colorimetrically the dye concentration in the acidified filtrate. The solubility of the dye in ether a t 15.5' was found to be 2.99 gm. per 100 cc of solution. The ratio of the solubilities in ether and in aqueous methyl alcohol gives kz approximately. Since (U- W)"-* was approximately constant for the experiments recorded in Table I, kl"S" is proportional to K/kz. The results obtained are shown in Table IV, a few of the intermediate values for kz being read from curves. I t will be observed that the solubility of the dye in water increases rapidly with increasing methyl alcohol concentration, so that kz decreases in like fashion. The values of k z / B in the last column, which should be proportional of kl"S", also decrease, though less rapidly than does kz, thus confirming the previous conclusion. I n other words, the addition of methyl alcohol has a greater effect in increasing the solubility of the dye in the aqueous phase than it has in decreasing the proportion of soap in the colloidal state and the value of kl; hence the measured partition coefficient K falls.

TABLEI V Methyl Alcohol Solubility of as % of initial volume Dye (parts of soap solution per 1 0 6 ) 0% 0.32 5% 0.41

K2

K

k?/K a k,nSn

5'8

16,000

15%

6 .o 6.1 5.6

I2,OOO

I .oo

93Jooo 73,000 43,000 30,000

20%

1.95

I57000

5.3

11J900 10,300

4.5

10%

25%

27%

30% 33% 40%

-

4.2

-

-

-

6,400

3.0

2,100

10.2

2,900

2.15

Ii3Oo

3.33'

Determination of c W A further series of experiments was planned to determine partition coefficients for widely differing concentrations of the dye. This provides data for the calculation of a series of values for the index 'k," the constancy of which afford a further test of the validity of formula (9). If all factors except solute concentrations are kept the same in two experiments (for which the subscripts a and b are used) we have: K, - (U-W)t-' (IO) Kb (U-W)g-'


I679

Partition coefficients were measured for t’he dye p-dimethylarninoazobenzene using a 20% concentration of methyl alcohol, and other concentrations as in the first series of experiments, except that the fatty acids were prepared from cod-liver oil stearin soap solution exhaustively extracted with ether to remove unsaponifiable matter, in case this might interfere. The temperature at which the separations occurred was maintained a t 19’ IO.

*


TABLEV Initial Weight of Dye

K

(U-W)

n

1.0

gm.

6.55

0.574

gm.

0.10

”

,

5.72

0.0663

”

0.010

”

4.86

0.00563

”

0.0010

”

4.18

0.00010 ”

3.60

!

,063

\I

0.00063

I

,065

I

,069

,, \ / I ”

T

0.00007 2

The results, recorded in Table V, show that the value of n remains constant within the limits of experimental error. Owing to the fact that n is only slightly greater than I , the partition coefficient is barely doubled for a 10,ooo fold increase in initial dye concentration. This means that it would almost be correct to say that the dye is dissolved, rat’her than adsorbed, by the colloidal soap, a conclusion which is of interest in connection with the further experiments to be described. Partition of Aniline between Sodium Oleate Solutions and Ethyl Acetate The experiments described in this section were carried out three years later than those in the preceding section, and in connection with an investigation of the system sodium oleate, water, sodium chloride, ethyl acetate, which will form the subject of a later communication. Sodium oleate was chosen as a pure soap, of which isotropic solutions could be prepared over a considerable concentration range a t room temperature; ethyl acetate because it was one of the few organic solvents known at the date of these experiments which did not form a permanent emulsion when shaken with a soap solution; aniline because it could easily be estimated volumetrically and because it was more soluble in water than other solutes investigated and promised to yield interesting results on this account. Experimental. Sodium oleate was prepared as described previously. Ethyl acetate was purified by washing repeatedly with calcium chloride solution or brine to remove alcohol, drying over calcium chloride and fractionating. Aniline of C.P. quality was freshly redistilled before use. To determine a partition coefficient, sodium oleate solution and a solution of aniline in ethyl acetate were weighed into a IOO cc separating funnel, which was stoppered both a t the top, and a t the bottom of the stem, and then


1680

E. LESTER EMITH

immersed to the neck in a thermostat a t 2s'. After a period suffic.ent to ensure temperature equilibrium, the separator was shaken vigorously by hand, and replaced in the thermostat until separation was complete. It was then carefully removed and samples comprising almost the whole of each layer transferred to weighed separating funnels. The aniline was estimated in both samples, so that the concentration of aniline per gram could be calculated for both layers, the ratio of these concentrations being taken as the partition coefficient. This method of calculating partition coefficients needs no apology, for as Hand remarks' Nernst's original statement of the partition law was in terms of weight in weight concentrations. Aniline was extracted from the ethyl acetate layer by shaking out thrice, each time with a quantity of 10% sulphuric acid approximately equal in volume to the sample. The bulked acid extracts were boiled down to small bulk to remove ethyl acetate and alcohol resulting from its hydrolysis (acetic acid was found not to affect the estimation) cooled and transferred to a stoppered bottle with enough water to bring the acidity back to about IO% sulphuric acid. Aniline was then estimated volumetrically in this solution by the method of Pamfilov and Kisseleva*. About I gm. of potassium bromide was added, then a measured excess of N/IO sodium hypobromite solution (usually 50 cc); after standing about 3 minutes, I gm. of potassium iodide was added in solution, and the mixture back-titrated with N / I O sodium thiosulphate using starch as indicator. Aniline was extracted similarly from the soap solution after adding a little more ethyl acetate, but it was necessary to filter the acid solution before titrating, in order to remove the trace of oleic acid usually present, which was found to interfere with the aniline estimation. Tested against pure aniline this volumetric method gave results I-Z% too high. 30corrections were applied however since the same errors affected the estimations of aniline in both phases, and cancelled out in the calculation of the partition coefficient. Partition coefficients for aniline were measured over the largest possible range of sodium oleate concentrations, namely from zero to 1.1 N,. Slightly above the latter concentration a third liquid phase appeared, which was characterised as neat soap on account of its properties, and from a knowledge of the phase diagram which will be described in the next paper in this series. In some experiments the aniline concentration was varied over as wide a range as was practicable, and in others the effect of sodium chloride on the partition coefficient was studied. The initial sodium oleate concentrations are expressed in weight normalities (mols of soap associated with I kilo of water) and also as weight in weight percentages. The weight normality is not affected by saturation with ethyl acetate, and shaking with excess ethyl acetate alters it only very slightly on account of the water abstracted from the soap solution by the upper layer.

Discussion of Results The results are combined in Table VI. Owing to the moderate solubility of aniline in water, the partition coefficient for this substance between ethyl J. Phys Chem., 34, 1961 (1930). 2

Z. anal. Chem., 75, 87 (1928); Analyst, 54, 60 (1929).

1681

SOLVENT PROPERTIES OF SOAP SOLUTIOSS

TABLEVI Initial Sodium Oleate ~ ~ i tAniline. i ~ l Gm. per 100gm. Solution NW Gm. per IOO gm. Sodium (.4)1 Upper (A)* Lower solution Chloride Layer Layer N, o ,0146 0 0 0.442 t,

t


,> Jl

0.05 0.1 0.1 0.15

0.2

0.3 0.4 0.4 0.4 0.4 0.4

0.4

t,

,, ,, ,, 1.50

2.95 2.95 4.37 5.73 8.36 10.83 10.83 10.83 I O .83 10.83 10.83

0.5 0.7 0.9 0.9

17.55 21 . 5

1.1

26.7

13.2

21.5

-

0.5 0.5 -

-

0.550 o.oj61 4.62

0.00182 0.157

0.0146

0.542

0.0242

0.539 0,399 0,436 0.316 0.396

0.0320

0.0256 0.0360 0.0303 0.0502

-

0.528 3.82 16.90

0.5

0.422

0.5 -

0.439

0,534 2.239 0.0610 0.064%

0,501

0.0844

0.509 0.649 0.620 0.469

0.1106 0.1584

-

-

K=(A)I (A),

0.0182

0.481

0,517

Partition coefficient

30.7 29.5 33 . o 22.4 16.8 15.6 12.1 10.5

7.9

0.0728 0.07jz

0 . I j07

0,1229

7.15

%!\Mean

6.85

5.93 4.61

44.11 . 1 ~ ) ~ e a4.10 n 3.82

acetate and water is not abnormally high, like the corresponding value for p-dimethylaminoazobenzene between ether and water. It will be observed however that 1% of sodium oleate is sufficient to cause a marked decrease in the partition coefficient for aniline, and that the value decreases steadily as the soap concentration increases. Comparison with Table I11 will show that this decrease in K is of lesser magnitude than in the earlier experiments using the dye as solute. This is largely because the amount of aniline dissolved in the water phase is comparable with the amount dissolved in the colloidal soap phase, a factor which operates in the direction of making K independent of soap concentration. It was hoped that these statements might be expressed mathematically, but although a formula can be derived along the lines of the one used in connection with the earlier experiments, the fact that Y cannot be neglected in comparison with U and X renders the final expression so complicated that it cannot be made to yield any useful information. I t will be observed that K is independent of aniline concentration, except when this is so high that the weight of aniline adsorbed by the soap is comparable with that of the ethyl acetate adsorbed. This means that the value of n in the adsorption formula is unity, so that in this case it is hardly correct to speak of adsorption of solute by the soap. Rather

E. LESTER SMITH


1682

must the aniline be regarded as being distributed according to the partition law between the three phases; ( I ) wet ethyl acetate layer; (2) water (plus dissolved ethyl acetate and soap in molecular solution); and (3) colloidal soap (plus adsorbed ethyl acetate and water of hydration). The effect of sodium chloride is noteworthy. It increases, by the familiar salting-out effect, the partition coefficient for aniline between ethyl acetate and water. When soap is present, however, salt decreases the partition coefficient below the correspondingvalue in theabsenceof salt. This isattributed to the effect of salt in increasing the proportion of the soap in the colloidal state, thus increasing the size of this phase in relation to the others. I t may be noted in passing that the temporary emulsion produced by shaking 0.9 N, sodium oleate solution with ethyl acetate is perfectly transparent a t 2 j". I n general, this series of experiments using aniline as solute confirm the conclusion suggested by the earlier experiments. Both series demonstrate clearly the remarkable solvent powers of colloidal soap. Consideration of the magnitudes of the partition coefficients in conjunction with those of the corresponding soap concentrations indicates that the solubilities of the dye or the aniline in the colloidal soap phase must be comparable with their solubilities in organic solvents. This solvent power of soap is even more in evidence when the solute is the natural unsaponifiable matter of oils, for the available data indicate partition coefficient of M to % the corresponding values for the dye or the aniline.

Partition between Water and Colloidal Soap When the organic solvent phase is absent, and a small proportion of a solute such as aniline is added to a soap solution, the solute must be distributed between the remaining two phases, water and colIoidal soap. Such partition or adsorption phenomena can only be studied indirectly, but two examples of their effects may be cited from the literature. McBain and Bolaml and Beedle and Bolam2 measured the hydrolysis alkalinity of soap solutions by the catalytic effect of the hydroxyl ion on the decomposition of nitrosotriacetonamine. They obtained abnormally low results for the more concentrated soap solutions and ascribed the discrepancy between determinations by this and by electrical methods to adsorption of the amine on the soap micelles. Hampi13 found that soap had a marked inhibiting effect on the germicidal activity of phenolic compounds towards B . Typhosus and other of sodium oleate destroyed the activity of hexyl organisms; thus 0.57~ resorcinol a t a dilution of I : 1000 which is a t least five times the strength necessary to kill the organisms in the same time in aqueous solution. Hampil considered the distribution of the phenolic substance between the water and the colloidal soap, as the most probable explanation of these phenomena. J. Chern. SOC., 113, 825 (1918).

* J. SOC.Chern. Ind.,40, 27T (1921). 3

J. Bact., 16,287 (1928).

SOLVENT PROPERTIES O F SOAP SOLUTIONS

1683


The Extraction of Soap Solutions We may now consider the bearing of these conclusions on the original problem of how best to extract organic substances dissolved in soap solutions. The first desideratum is evidently that the soap should be as far as possible in true or molecular solution, with the minimum proportion present as colloid. This may be achieved by the use of solvents which tend to render the soap “less colloidal,” such solvents being in general those which are appreciably soluble in water, as shown in the first paper of this series.’ The same result may be secured by the addition of alcohols. The measurements with pdimethylaminoaeobeneene show, however, that this beneficial effect of alcohol may be counterbalanced by its effect in increasing the solubility of the solute in the aqueous phase. The choice of a solvent of the type suggested, or the use of alcohol, has a second very practical advantage in that the emulsions produced by shaking are relatively impermanent. The value of the adsorption coefficient for the solute by the colloidal soap is usually outside our control. The partition coefficient on the other hand, is evidently proportional to the solubility of the solute in the extracting solvent, if other factors remain unchanged. In connection with the extraction of unsaponifiable matter from fish liver oil soap, it was sought to utilise this principle by determining the solubility of cholesterol, which constitutes a considerable proportion of the unsaponifiable matter, in a series of solvents. The solubilities decreased in the order ether, benzene, petroleum spirit; the partition coefficients for the same soap concentration decreased in the same order, but not proportionately, because other factors did not remain constant. In practice, the choice of solvents is limited chiefly by the tendency of most solvents to emulsify in the soap solution, and the large proportion of alcohol necessary to break the emulsions so produced. Such solvents as benzene and its homologues, chloroform and chlorinated hydrocarbons generally are unsuitable on this account except in special cases.* When mixed with sufficient alcohol to render emulsions unstable, the proportion of solvent absorbed by the soap solution becomes unduly high. Also in some cases (noted particularly in the extraction of unsaponifiable matter with xylene from alcoholic soap solution), the value of n (see formula 9) is large, so that the efficiency of extraction decreases rapidly as the weight of solute remaining behind in the soap solution is reduced. Petroleum spirit does not suffer from these defects, even when the proportion of alcohol in the soap solution is over jO%, but the partition coefficients for unsaponifiable matter are too low to render this solvent serviceable in this connectionP Ether is the most generally useful solvent for extracting soap solutions, requiring relatively little alcohol to break its emulsions. Ethyl acetate, however, deserves attention as it does not emulsify even in absence of alcohol and is a good solvent for many organic substances. Its disadvantages are that the soap solution must be neutral in order to avoid 1

*

J. Phys. Chem., 36, 1401(1932). See for example Smith and Hazley, Biochem. J., 24, 1942 (1930). See Smith: Analyst, 53, 632 (1928).


1684

E. LESTER SMITH

hydrolysis of the ester, and that more fatty acid is extracted from a neutral than from a slightly alkaline soap solution. The extraction of soap solutions is no exception to the general rule that the efficiency of extraction increases as a given total volume of extracting solvent is subdivided into smaller portions. In practice however, both in laboratory extractions for the determination of unsaponifiable matter, and in large-scale extractions for the preparation of fat soluble vitamin concentrates, it is found best to give only 3 or 4 extractions. For a given degree of extraction, the solvent saved by giving more extractions with a smaller total volume is more than counterbalanced by the extra time consumed in the operations. The author has shown elsewhere that in general the greatest efficiency is secured by subdivision of the extracting solvent into equal portions.‘ The rule applies to the extraction of soap solutions when the value of “n” approximates to unity. When “n” is greater than I , it can be shown mathematically that it is more efficient to extract with decreasing volumes of solvent.

summary

Partition coefficients have been measured for the dye pdimethylaminoazobenaene between ether and soap solution. The partition coefficients decrease considerably with increasing soap concentration, and slightly with increasing methyl alcohol concentration, and with increasing excess alkali, in the soap solution. The results are explained by postulating adsorption of the dye on the colloidal soap. A formula has been derived on the assumptions that the solute is distributed between the colloidal soap and the aqueous phases according to the adsorption law, and between the organic solvent phase and the water according to the partition law. The formula receives quantitative confirmation in the decrease of the partition coefficients with decrease in the dye concentration. The addition of methyl alcohol greatly increases the solubility of the dye in the aqueous phase and so decreases the partition coefficients, despite the counterbalancing effect of the alcohol in diminishing the proportion of soap in the colloidal form. Partition coefficients have also been measured for aniline between ethyl acetate and sodium oleate solutions. The partition coefficients decrease considerably with increasing soap concentration and decrease slightly on addition of sodium chloride, but are almost independent of aniline concentration. Thesystem behaves as though the aniline were distributed according to the partitionlaw between the three phases, ethyl acetate, water, and colloidal soap. The bearing of these experiments on the problem of how best to extract organic solutes, particularly the natural unsaponifiable matter of oils and the attendant fat-soluble vitamins, from soap solutions, is discussed. GEazo Research Laboratory, London, N . W . 1. Research Laboratory, Chelsea Polytechnzc, London, S. W . 9. January if‘, 13.52.

J. Soc. Chem. Ind.,

47, 159T(1928).

SOME SOLVENT PROPERTIES OF SOAP SOLUTIONS. I1 In the

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