NOTES
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Vol. 60
of the solution; AV is the volume change per mole mole. The results obtained for potassium laurate of metal above that to be expected from the sums in water indicate a break in the concentration of the metal and ammonia volumes; N is the nor- range 0.019 t o 0.020 mole soap per liter, and we mality; V is the observed volume of ammonia per estimate the heat of micelle formation to be -0.088 gram atom of the metal; V' is the volume of the i 0.002 kcal. per mole. solution per gram atom of the metal; 1 is the specific conductance, and A is the equivalent or atomic conductance. The values of V' in Table I1 were oboi tained from a graph of V vs. V', constructed from 8 6.8 the data of Table I. \ From the graph it may be seen that the atomic 8 Y conductance of sodium is about 20% greater than $6.6 that of potassium when the two solutions are at a .concentration of 4.96 moles per liter. I n absolute s + 0 magnitude this amounts t o about 160,000 ohm-' ," 6.4 cm.2, scarcely a negligible amount. It is evident that this variation is well outside the range of experimental error and we must conclude that the 6.2 conductivities are not identical. I I I It seems evident that a successful theory regard0.01 0.015 0.02 0.025 ing these solutions must be capable of offering a Molarity. reasonable explanation for the observed variation Fig. 2.-Plot of the heat of solution (integral) in kcal. in their atomic conductivities. per mole of potassium laurate in water as a function of the
42
' .c)
concentration of eoap.
HEATS O F MICELLE FORMATION BYERICHUTCHINSON A N D LORRAINE WINSLOW Department of Chemistry, Stanjord University, Stanjord, California Received July 11, 1866
I n a recent publication' we reported some observations on the calorimetrically determined heat effect associated with the formation of micelles in solutions of sodium decyl sulfate in water. This work has been extended t o cover the case of sodium decyl sulfate in aqueous solutions of sodium chloride and the cas,eof potassium laurate in pure water. The results which we have obtained are shown in Figs. 1 and 2, in which the heat of solution (integral) per mole is plotted as a function of concen-
I
I
I
I
J
0.005
0.01 0.015 0.02 Molarity. Fig. 1.-Plot of the heat of solution (inte al) in kcal. per mole of sodium decyl sulfate in 0.1 M sogum chloride solution as a function of the concentration of sodium decyl sulfate.
tration. The results for sodium decyl sulfate in 0.1 M aqueous sodium chloride solutions indicate a break in the curve in the concentration range 0.010 to 0.011 mole detergent per liter. Associated with this break is the heat of micelle formation, which we estimate to be -0.288 f 0.047 kcal. per mole. This may be compared with the value obtained in pure water,l namely, -0.21 kcal. per (1) E. Hutchinson, K. Msnchestef and L. Winslow, T i m JOURNAL, 68, 1125 (1954).
The effect of added sodium chloride on the heat of micelle formation in sodium decyl sulfate solutions appears t o be quite small, despite the fact that it is known from light scattering measurements2 that added salts have a considerable effect on the charge of the micelle. In the presence of added sodium chloride at concentrations as high as 0.1 M the micelle has a charge of only some 5 or 6 electronic charges per 50 or so units, and even so, according t o our results, the process of micelle formation occurs with relatively small entropy and heat changes. This would seem t o add further evidence to support the view that in the concentration ranges discussed here the principal type of micelle has a loose structure such as was suggested by H a r t l e ~even , ~ when carrying only a small charge. The value for the heat of micelle formation in potassium laurate solutions is appreciably less than that for sodium decyl sulfate. This result is t o some extent misleading, however, in that the micelle of potassium laurate contains some lauric acid formed by hydrolysis, and, as will be discussed in a forthcoming paper, the heats of solubilization of a variety of solubilizates are slightly positive. The true heat of micelle formation in this case, then, may be expected t o be somewhat closer t o that of sodium decyl sulfate when allowance is made for this fact. The hydrolysis which occurs in the case of potassium laurate does not interfere with the ease of making the necessary experiments, but we have failed, after many experiments, t o obtain corresponding heats of micelle formation for laurylamine hydrochloride. Hydrolysis, in this case. seems to lead to very erratic and irreproducible results. This work was supported by the Ofice of Naval Research. (2) E. Hutchinson and J. C . Melrose, 2. physik. Chem. (Nciie Folge), 2, 363 (1954).
( 3 ) G . 6. Hartley, ".4q11eous Solutions of Porafin.Cliain Salts," Hermann e t Cie, Paris, 1936.
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