STUDlES OF THE COKSTLTUTlON OF SOAP IN SOLUTION. 191

XXII1.-Studies of the Constitution o f Soap in Soh- t i on : the Electrical Conductiiuity of Sodium Xt ear at e Solutions.

By RICHARD CHARLES BOWDEN.

THE study of the constitution of soap solutions is of the greatest importance in a large number of directions, both practical and theoretical. One of the most obvious problems still awaiting explanation arises out of the fact that there is no reason for suspect- ing the existence of any colloid whatsoever in solutions of sodium acetate, whilst it is equally impossible to deny the existence of

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192 BOWDEN: STUDIES OF THE

colloids in thesolution of the salts of the higher homologues of this series. The question as to whether, say, sodium palmitate, C,,H33,0,Na, itself is a colloid or an electrolyte is a t present still entirely open, although it seems more probable that it is an electro- lyte (McBain and Taylor, Zei t sch . physikal. Chern., 1911, 70, 1; Rer., 1910, 43, 321; compare Ubbelohde and Goldschmidt, Hand- h ~ c h der Chemie und Technologie der Oele und Fette, Vol. 3).

Since the classical work of Chevreul, there are very few papers bearing directly on this problem. The boiling-point investigations of Krafft and his collaborators have become classical, but unfor- tunately the method has been shown to be quite untrustworthy (McBain and Taylor, loc. c i t . ) on account of the impossibility of removing dissolved and “ sorbed ” (McBain, Phil Mag., 1909, [vi], 18, 916; Zeitsch. physikal. Chem., 1909, 68, 471) air from the easily frothing, viscous solutions. h i t s ’ vapour-tension work suffers from the same defect. The conductivity measurements of Kahlen- berg and Schreiner (Zeitsch. physikal. Chew., 1898, 27, 552) in dilute solution unfortunately did not go far enough to give information about the concentrated solutions under discussion.

The investigations of McBain and Taylor have revealed some unexpectedly interesting results, in that, for example, they have shown that the equivalent conductivity of sodium palmitate passes through a pronounced maximum at a concentration of T / 2 , and a well-defined minimum at a concentration of N / 5 . Such a phenomenon has never before been met with in aqueous solution, although exactly similar cases occur in non-aqueous solutions (com- pare Franklin and Gibbs, J . Amer. Chem. SOC., 1907, 29, 1389).

The present paper contains measurements of the conductivity of solutions of sodium stearate, C18H3502Na, carried out in order to observe the effect of the addition of two CH, groups on the form and position of the conductivity curve. The result has been that, although, like sodium palmitate, sodium stearate is an excellent conductor at all concentrations, yet the values of its conductivity differ markedly from those of the palmitates, and the maximum and minimum of the conductivity curve have been considerably displaced.

The experimental method has been fully described in the paper of McBain and Taylor. The method is that of Kohlrausch, using telephone and induction coil. The solutions were made up and shaken until homogeneous in pure silver tubes, and they did not come into contact with the Jena-glass of the dipping electrodes until a few minutes before the actual reading. The stearic acid was recrystallised twice from absolute alcohol, and melted at 69’45O.

The “ resistance capacity ” of the dipping electrode was, on

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CONSTITUTION OF SOAP IN SOLUTION. 193

account of the conductivity of the outer silver tube, a function of the observed resistance. It was measured in solutions of specially purified potassium chloride of concentration N , N / 10, N / 5 0 , and N / 100, and also in a saturated solution of specially purified sodium chloride.

The values obtained were within one per cent. of those determined a year previously by Miss Taylor. All vessels and instruments; were carefully calibrated.

The work of McBain and Taylor has shown that in the case of sodium palmitate, the conductivity of a given solution is the same within the limits of experimental measurement, whether the solution is prepared by dissolving pure sodium palmitate, or by mixing sodium hydroxide solution with an equivalent amount of palmitic acid. Hence it seemed safe to make the same assumption in the case of the stearate, and to prepare the solutions by mixing pure sodium hydroxide (from sodium and conductivity water) with stearic acid. The solutions were shaken in each case for about the same length of time as the corresponding palmitates. In the follow- ing table the first column gives the weight-normality, the second the number of grams of sodium @earate in 100 grams of water, the third the capacity factor of the electrode, the fourth the specific conductivity in mhos at 90*OOo, the fifth the density (due to E. C. V. Cornish, Zeitsch. physikal. Cliem., 1911, 76, 32), the sixth the molar or equivalent conductivity a t 90.00°, and the last the final results for hhe equivalent conductivity.

Sodium Stearate at 90*OOo.

I. 11. 111. IV. 0,06326

0,03194 0.4942 15-14 g:i;4 0.03106

0 -031 15

0'01395 0.1988

0.007105

0 -003680

0-979 29'98 3'77 0.06374 I

6.088 3-72 0.01398

0.0997 3'054 3'70 0.007086

0-0499 1.529 3.675 0.003716

0.306 3.60 o*oo1208 0.001213 0.00999

V. v I. VII. 88'33 87.98

77'52 0.9600 75.37 76.1

75*60*

0.9632 7777:;: i7 .44

0.9642 ;:::: 76.05

0.9648 ;i::i 77.98

0.9652 :;::! 125'9

:0'9547) 88-65

* A perfectly independent measurement made at another time by Miss Taylor.

Both the molar and the specific conductivities contain a correction of 0.1 per cent. (subtracted) on account of the thermal expansion of the electrodes.

The conductivity remains nearly constant at the value of 77 mhos between the concentrations

The results itre sufficiently remarkable.

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194 STUDIES OF THE CONSTITUTION OF SOAP IN SOLUTION

N / 2 and N / 2 0 , the values then rising by 12 per cent. to 88.3 for the normal solution, and to 125.9 for the N/lOO-solution. The figure below shows their conductivity values plotted against the volume of water in litres containing 1 gram-molecule of sodium stearate, and the values for sodium palmitate are also plotted for comparison. In the first place, these high values obtained for the conductivity

even of the most concentrated solutions prove that much electrolyte is present, and that therefore soaps cannot be simple colloids (com- pare Krafft and his co-workers, Ber., 1894, 27, 1747,1755; 1895, 28, 2566, 2573; 1896, 29, 1328; 1899, 32, 1584; Smifs, Zeitsch. physikal. Chem., 1903, 45, 608). On the other hand, the presence of a colloid in these solutions cannot well be denied; if

seems therefore that the colloids mesent are the acid sodium

28

salts, C&~~O~,ClGH3102Na and C18E~0,,~18H3,0zNa. The peculiar shape of the conductivity curve might be explained as due t o the change in the degree of dispersion of the colloid. Acid sodium salt (containing an excess of sodium depending on the concentration of the free alkali in the solution) is found in dilute alkaline solution, such as the dilute soap solutions, in the form of a coagulated pre- cipitate; it is also precipitated quantitatively (salted out) by excess of free alkali.* Intermediate solutions, such as the concentrated sodium stearate and sodium palmitate solutions, are opalescent or

* McBain and Taylor, loc. eit. ; thus the coagulated precipitate in equilibrium with a 1*6N-solution of sodium hydroxide has the empirical composition of 59 per cent. C,,H,,O,Na 4- 41 per cent. C,,H,,O,,C,,H,,O,Na, as calculated from the in- crease in concentration of the sodium hydroxide solution employed for the salting out.

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Hicrophotograph, No 9. 25 per cent. silver ; 75 per cent. tin, cooled in ten minictes.

Magni$cution = 50 diameters; etched by HCl. Two elemmts : dark: granular eutectic and light

crystalline Ag,Sn.

[Face p. 194.

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Microphotograph, No. 57. 50 per cent. silver ; 50 per cent. tin, quenched after one hotcr

at 400". Magnification =600 diameters ; etched by KCN.

Two elements : dark granular eutectic and light crystalline Ag,Sn.

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JOYNER : AMALGAMS CONTAINISG SILVER AND TIN. 195

even quite transparent, depending on the concentration, Thus, the conductivity results might be explained by the sorptive powers of the colloidal acid saltl decreasing so much as its degree of dispersion increased that the relative amount of free hydroxide in the solution increases for a time with increase of concentration. In still stronger solutions one might expect more and more of normal salt to be formed, and this, together with lessening dissociation, would explain a second decrease in the conductivity in the strongest solutions.

However, as this problem is being attacked from many different points of view in this laboratory, it would be premature as yet to regard this as being more than a plausible explanation which is in agreement with all the facts.

My sincere thanks are due to Dr. James W. McBain, a t whose suggestion this work was undertaken, for his continued interest and advice .

CHEMICAL DEPARTMENT, UNIVERSITY, BRISTOL.

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