Eric Hutchinson and Fumikatsu Tokiwa
Stanford University Stanford, California
Freezing Point Observations on Micellar Solutions
In the teaching of phase relationships in elementary chemistry a distinction is generally made between the cooling curve obtained for a pure substance above, at, and below its freezing point and that obtained for a solution from which pure solid separates. In the first case, of course, the temperature remains constant during the entire freezing period, as it must in a one-component, two phase system maintained a t constant pressure. I n the second case, dealing with a twocomponent, two-phase system, the system is univariant a t constant pressure.l Since the composition of the liquid solution is a function of the extent to which solid solvent has separated, the freezing point of such a binary solution is in general a function or time. I n the course of a recent research program we have come across an interesting case of a binary solution from which pure solid solvent separates but whose freezing point is nevertheless not time-dependent. An interesting contrast is provided by p-methylphenyl glucose and phutylphenyl glucose, two members of a series of compounds which we have synthesized by the method of Hurd and Bonner.2 The first of these two compounds does not form micelles in aqueous solutions in concentrations less than 10% by weight, whereas the second forms micelles readily. If one studies the cooling curves obtained for a variety of aqueous solutions of pmethylphenyl glucose one obtains the results shown in Figure 1. These are -
'DANIELS, F., AND ALBERTY,R. A., "Phpical Chemistry," John Wiley and Sons, Inc. (New York), 1961, p. 244. s H ~ C.~D.,~ AND , BONNER, W. A., J. Am. Chem. Soe., 67, 1972 (1945).
H ~ T C H I N ~E., ON INABA, , A,, A N D BAILEY, L. G.,Za'l. physik. Chem. (Frankfurt), 5,344 (1955).
typical of the time-dependent freezing point results for normal, approximately ideal solutions. Here the equilibrium is one involving pure ice and aqueons solutions of time-dependent concentration. I n contrast, the results obtained with p-butylphenyl glucose, shown in F -i r e 2, are time-independent beyond the ueriod of initial supe~cooling.
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Figure 2.
Freezing behavior of p-butylphenyl glucose rolutionr.
The author has shown, in a numher of earlier public a t i o n ~that , ~ it is convenient and physically sensible to regard the formation of micelles in solution as the formation of a distinct phase in the system, so that a t room temperature a solution of, say, sodium dodecyl sulfate above its critical micelle concentration is a twophase system consisting of a micellar phase bathed in a solution of single surfactant i0ns.l When such a solution begins to freeze, the system consists of a solution phase, pure solid solvent (ice) and the micellar phase, and a t constant pressure it is invariant. The fact that the micellar phase is in the form of small micelles which are probably liquid in structure distinguishes this case from the usual eutectic system in which two solid phases are present. Moreover, a micellar solution in which the solute is a non-electrolyte, as in the present case, is distinguishable from one in which the solute is an electrolyte, e.g., sodium octyl sulfate. I n this latter case the rnicelle bears a net electrical charge and the concentration of counterions does change somewhat with the micelle concentration, with the result that in sodium octyl sulfate solutions above the critical concentration the freezing point is timedependent,&though less markedly so than in the case of a simple solute such as sodium chloride.
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Figure 1.
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Freezing behavior of p-methylphenyl glucose rolution.
Journal of Chemical Education
B E A N K. , E., M. S. Dissertstiotion, Stanford University, 1951.
