1965 Volume 38 Issue 10 Pages 1700-1704
The sound velocity and the density of aqueous solutions of sodium hydrogen (SHS), methyl (SMS), octyl (SOS), dodecyl (SDS) and tetradecyl (STS) sulfates have been measured by using, respectively, a differential ultrasonic interferometer and a float balance, and the compressibility (β) has then been calculated from these results. Each of these three quantities, when plotted against the concentration (C), has shown a nearly straight line for SHS and SMS, while it consists of two nearly straight lines, with a break point between them at CMC, in the cases of SOS, SDS and STS. The values of dβ/dC below and above CMC, when plotted against the number of carbon atoms in the alkyl chain of the solute molecule, again showed two straight lines which intersected with each other at the zero number of carbon atoms. This value of dβdC is equal to that of SHS at a moderate concentration. These results have been explained by taking account of the dissolved state of these solutes. The compressibility of the micelle (βm), the molar volume of the CH2 radical in the micellar state (Vm), and the ionic hydration number of solute have been calculated; the values of βm and Vm show evidence for the view that the hydrocarbon interior of the ionic micelle is liquid-like rather than solid-like.
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