COMMUNICATIONS TO THE EDITOR
165
I n the photolysis of formaldazine, methyleneimine and hydrogen cyanide were obtained. Therefore,othe excitation of formaldazine by the light of 2537 A is followed by rapid intramolecular transfer and then bond scission before the molecule returns to the ground state. CH~ZN--N-@X&
hu
7 (CH2eN-N=CHz) .amA
(CH2=N--N=CH2)
*
--j
2CRz=-N* -* CHz=NH
2CHz=N
Careful measurements on these spectra do show changes which may be helpful in elucidating the interactions of hydrophobic groups with water and with each other. For example, the procedure of Stone, et aZ.,4for getting the tumbling correlation time 7 from the nitroxide spectra leads to the results in Figure 1.
*
*
+HCzN
The details of the nature and the reactivity of methylene imino radical will be reported at a later date. Acknowledgment. The authors are greatly indebted to Professor Koeo Hirota for his encouragement.
40 -
( 5 ) Author to whom correspondence should be addressed at the Department of Chemistry, The University of Iowa, Iowa City, Iowa 52240.
20 ,//
MIKIHARU KAMACHI* KEIJI KUWATA SHUNSUKE MURAHASHI
FACULTY OF SCIENCE OSAKAUNIVEELSIT’Y TOYONAKA, OSAKA,JAPAM
RECEIVED JUNE 10, 1970
Electron Paramagnetic Resonance Probes for Hydrophobic Interaction in Aqueous Solutions Publication costs borne completely by the Journal of Physical ChemiBtry
8ir: Aqueous solu.lions of hydrophobic solutes or solutes with substantial hydrophobic groups exhibit a number of remarkable properties’ among which is a relatively large dy/dm, increase in viscosity q with molality m, even a t low m. While one expects to be able to trace this hack to the local influence of the hydrophobic solute particle upon neighboring water molecules (its “cospherc”j, nothing is known in detail either of the nature of the influence nor of the size of the cosphere. Such solutions also show a large sound absorption, in one case arising from a process as slow as lo8Hz at Oo,2 which might be due to a slow relaxation process in the cospheres or t o a slowly relaxing interaction of two or more hydrophobic particles. This suggests that an epr probe might he useful for the study of the dynamics of thew solutions since it might just enable one to get away from the extreme narrowing limit characteristic of the widely investigated nmr probes in this problem, in which one sees only a global response not so different from the viscosity.3 We have studied the epr spectra of relevant solutions of several hydrophobic epr probes, typically 2,2,6,6Y
L
I
Y
icolinatOvanadyl, Only small changes in epr spectra are found, with no new lines, even at 0”.
IIA, W. P. Mason, Ed., Academic Press, New York, N. Y., 1965, p 281; G. E. McDuffie, R. G. Quinn, and T. A. Litovitz, J. Chem. Phys., 37, 239 (1962). The Journal of Physical Chemistry, Val. 75, No. 1 , 1971
COMMUNICATIONS TO THE EDITOR
166 consistent with this view. These results are all negative in the aense that getting microscopic information about hydrophobic interactions from them depends more on the power of the theory which may be applied than anything else. A more promising result is shown by line C in Figure 1, which gives evidence of a probe, BPh4- interaction markedly at variance with Walden's rule behavior. This can be discussed in terms of the expression dTldm
-
Jgp,(r)epA(r)dar
where gpH is the equilibrium pair correlation function for the probe P and the other hydrophobic species H and ~ P His a measure of the effect of H upon T of P at the separation (p. It remains to learn in this case whether the extra interaction compared to line B is more due to hydrophobic binding (gpH >> 1 at some r ) or to a slowly relaxing cosphere of the hydrophobic species [large epIr(y) at some r ] . It is hoped that this can be elucidated by more detailed epr studies. A most surprising feature of the present results is the marked difference i11 behavior of BPh4- and such similar species as AsPh4-+which are found near line B. TJsing VOPiz as the probe, aqueous glycerine and tert-butyl alcohol are again indistinguishable at the same viscosity white Ru4NBr solutions deviate markedly from the Walden's rule behavior. (6) Fellow of the Nat,ional Research Council of Canada, 1969-1970. (7) The support of this research by the National Institutes of Health is gratefully acknowledged.
DEPARTMENT OF CHEMISTRY STATEUNIVERSITY OF 'NEWYORK AT STONY BROOK STONY BROOK, NEWYORK 11790
cally symmetrical molecules, w > Oe2 Any thermodynamic property of a large number of fluids (with the exception of associated fluids, or fluids containing molecules of high dipole moment) can be characterized as a function of the three parameters T r j p,, and a. Pitzer3n4has shown that a group of molecules will obey the principle of corresponding states if they obey Boltemann statistics, are approximately spherically symmetrical (i.e., the rotational, vibrational, and translational partition functions are separable), and have intermolecular potential of the same general form, i.e.
where E and u are energy and distance parameters, and f is any general function. For a molecular species obeying the above assumptions, the entropy of vaporization, As, at a constant ratio of saturated gas to liquid molar volume should be constant. This has indeed been verified by examining the inert gases Ar, Kr, and Xe, where As at a gas to liquid molar volume ratio of 335.0 has been found to be 18.67,18.60,and 18.66, respectivelyS5 Therefore, a measure of the nonideality of a given molecular species may be obtained if As is examined at a constant gas to liquid molar volume ratio. Choosing a ratio of 335.0, the inherent problem which remains is finding that point in p-V-T space where this ratio is satisfied. This can be accomplished by solving the following set of equations.
CARMEL JOLICOEUR~ HAROLD L. FRIEDMAN* 7
RlEClnIVED AUGUST10, 1970
As = 0.024218V1
The Relationship between the Acentric Factor and the Entropy of Vaporization Publication costa borne completely by The Journal of Physical Chemistry
Sir: The acentric factor, w, was introduced by Pitzer*v2 as an imperical fa,ctor to characterize the deviation of molecules from spherical symmetry. The factor is defined as OJ
=
log pr"(0.7) - '1.00
(1)
where pr"(T,) is the reduced vapor pressure at the reduced temperature, Tr. The particular method of defining w , results from the experimentally observed fact that for sphericdly symmetrical molecules such as Xe, Ar, and Kr, u is essentially zero. For nonspheriThe Joztrnal of Plq&al
Chemistry, Vol. 76, No. 1, 1971
g(2
-1
(5)
In the above, p" is the vapor pressure of the pure component, Z is the compressibility factor, M is the molecular weight, and T is the temperature. For a given ratio of V,/V,, there exists only one unique solution for T, which can be found by solving eq 2, 3, and 4. This was done using the Newton-Rapbson (1) K. 5 . Pitzer, J. Arne?. Chem. SOC.,7 7 , 3427 (1955). (2) K. 8. Pitzer, D. Z. Lippmann, R. F. Curl, C. M. Huggins, and D. E. Peterson, {bid., 77, 3433 (1955). (3) K. S. Pitzer, 1.Chem. Phys., 7 , 583 (1939). (4) G. N. Lewis and M. Randall, "Thermodynamics," 2nd ed, revised by K. S. Pitzer and L. Brewer, McGraw-Hill Book Co., New York, N. Y., 1961, p 605. (5) J. H. Hildeband and R. L. Scott, "Regular Solutions," PrenticeHall, Englewood Cliffs, N. J., 1962, p 75,
