69s
NOTES
Vol. 57
Summary
2. The reaction has value in analytical chemistry but, from the quantitative standpoint, its 1. Attention has been called to a non-elecapplication is limited. trolytic reaction between molten lithium salts 3. The nature of the phenomenon w;ts studied and glass wherein sodium ions of the glass are in detail. displaced by lithium ions. LEXINGTON, KENTUCKY RECEIVED M A R C H 1, 1935
NOTES Comparison of Viscosity Data BY FRANCIS T. MILES'
log 7 = A f B/TAb.. and this relation was used by Miss Waller to extrapolate viscosity data to the melting and boiling points. It follows from this equation that
In a recent paper2 Miss Mary D. Waller calculates for a number of compounds the ratios of log I = IOg r ) f - log r)b = B(I/Tt - I/Tb) the viscosities at the melting and boiling points Thus it is possible to separate log r into two factors and correlates the values of this ratio, ??I/%, of which one, B , represents the variation of with the molecular symmetry of the compounds. viscosity with temperature, and the other, l/Tf Molecules of high symmetry are found to have l/Tb, represents the extent of the liquid range and low values of r]f/qb, and v u e versa. In a preis independent of the viscosity of the liquid, exliminary paper3 the same author states that the cept in so far as this may be a factor in determelting and boiling points can be used as comparimining the melting and boiling points. Miss son temperatures. It is well known, however, Waller tabulates values of Tb - Tf and states that the melting point of a compound is influenced that changes in this do not explain the changes in by the symmetry of the molecule. Dietz and T. In view of the above equation, however, it is Andrews4 have pointed out that in the series, interesting to compare log T and l/Tf - l/Tb, benzene, dihydrobenzene, tetrahydrobenzene, and this is done in Tables I and 11. The first cyclohexane, the melting points of the two symfour columns in each table are taken directly metrical compounds, benzene and cyclohexane, from Miss Waller's paper. In the fifth column axe 100' higher than those of the unsymmetrical are values of log r calculated from these data. compounds, while the boiling points of all four Values of l/Tf - 1 / T b , calculated from the compounds lie in a range of only 2'. These melting and boiling points, are given in column authors have proposed an explanation for this six, and in the last column values of B are calcuinfluence of molecular symmetry on the melting lated from the relation point. If a pair of compounds had the same logy = B(l/Tf - 1/Tb) boiling points and equal viscosities at equal temTABLE I peratures, the less symmetrical of the two com(1/Tf - 1/Tb) B pounds, having the lower melting point, would log r X 10s X 10-8 TI nb r 0.0082 0.0034 2.2 0.38 0.76 5.0 have a higher viscosity at the melting point and Benzene .055 .0026 21 1.33 2.92 4.5 therefore a higher ratio, 7]f/??b. I t is interesting Toluene Ethylbenzene .059 .0025 24 1.37 3.13 4.4 .019 .0026 6 . 6 0.86 1.67 5.2 to calculate whether the changes of ??f/??b in a +Xylene . O M .0024 7.5 ,823 2.14 4.1 series of similar compounds are due primarily to m-Xylene $-Xylene .00703 ,0024 3 .47 1.04 4.6 the variation of viscosity with temperature or TABLE I1 merely to the lower melting points of the un(1/Tf - 1/Tb) B ?f vb I logy x 10' x 10-p symmetrical compounds. 0.038 0.0021 18 1.26 3.83 3.3 The variation of viscosity with temperature Pentsne Hexane .0194 .0021 9 0.96 2.67 3.6 ,0279 .0021 13 1.12 can be expressed approximately by an equation Heptane 2.77 4.05 Octane ,0189 ,0021 9 0.96 2.11 4.55 of the form Nonane .025 ,0021 12 1.08 2.14 5.05 (1) Exchange Fellow at the University of Basel from Princeton University under the auspices of the Institute of International Education. (2) Waller, Phil. Mug., 18, 579 (1934). (3) Waller, ibid., 18, 505 (1934). (4) Dietz and Andrews. J . Chcm. Phys., 1,62 (1933).
Decane Undecane
.0232 ,0021 11 .0295 ,0020 16
1.04 1.17
1.90 1.92
5.5 6.1
Table I gives the results for benzene and some of its homologs. Examination of the last two
April, 1935
NOTES
699
columns of this table shows that while B is ap- should be used as a standard temperature for the proximately constant (within 30%) and shows no comparison of viscosities. regular dependence on molecular symmetry, The writer is indebted to Professor Hans Erlenl/Tf 1/Tb varies over a four-fold range and meyer for advice and help on this note. shows a definite increase with decreasing sym- ANSTALTPUR ANORGANISCHE CHEMIE JANUARY 9, 1935 metry. In other words the increase of r with de- BASEL,SWITZERLAND RECEIVED creasing molecular symmetry can be largely attributed to changes in 1/Tf - 1/Tb. In the An Explanation of Hysteresis in the Hydration case of the parafEins (Table 11) B increases reguand Dehydration of Gels larly with the increase in the number of carbon BY JAMES W. MCBAIN atoms, as has been pointed out by DunnJ6while l/Tf - l/Tb decreases on changing from a It is well known that hysteresis in the hydration hydrocarbon containing an odd number of carbon and dehydration of porous gels at moderately high atoms to one containing an even number of relative humidities is connected with the filling carbon atoms and increases on proceeding to the and emptying of pores with liquid.’ next (odd) hydrocarbon. This is evident from Such hysteresis sometimes persists even after 1/Tb are rigorous evacuation in the attempts to eliminate Fig. 1 in which log r, B and 1/Tf plotted against the number of carbon atoms in other impurities.2 An explanation is usually the molecule. The alternation in r, pointed out sought in terms of “friction in the contact angle” by Miss Waller, can thus be traced to an alterna- or the familiar “Jamin effect.” tion in l/Tf - 1/Tb which is due to the well It is the purpose of this note to present an known alternation in the melting points. alternative mechanism, suggested in discussion with the class on Sorption at Stanford University. Figure l a gives in well-known fashion an illustration of the fact that capillary condensation occurs (4 in wettable pores of sufticiently small radius and relatively high humidities in accordance with the formula of Lord Kelvin3 as used by Anderson’ In p / p , = -2m/rRT - 6.0 X 10’ where p is the pressure a t concave surface, ps is the pressure of saturated vapors of liquid in bulk a t that temperature, Q is the surface tension, v is the volume of 1 gram mole of condensed liquid, r the - 5.0X 10* radius of the capillary, R the gas constant, T the absolute temperature, and In represents the B natural logarithm to the base e. I The applicability of the formula depends upon 4.0 X lo* the liquid and is generally restricted to pores of radius from about 20 A. up to visible dimensions. 2 Figure l b gives a diagram of a lecture experi/ I I 1 ment we have used to show two possible positions 5 6 7 8 9 1 0 1 1 of true stable reversible equilibrium, where a Number of carbon atoms. stoppered bell jar has a narrow capillary passing Fig. 1.-Logr. 0 ; ( l / T - l / T ) , A; B, 0. through the stopper. Either the capillary is From these calculations it appears that the filled to the same height as in Fig. la, the bell jar (1) McBain, “The Sorption of Gases and Vapours by Solids,” correlation between values of r and molecular George Routledge and Sons, Ltd., London, 1932; Zsigmondy, 2. symmetry is due, in a large part a t least, to the anorg. Chcm., 71,356 (1911); “Kolloidchernie,” 0.Spamer, Leipzig, Aufl., Bd. 11, p. 76,1927; Zsigmondy, Bachmann and Stevenson, dependence of the melting point on molecular 26$*anurg. Chcm. 75, 189(1012); Anderson 2. physik. Chem., 88, 212 symmetry, which does not involve viscosity. It (1914). (2) Lambert and Clark, PYOC.Roy. SOC.(London), 8132, 507 is doubtful, therefore, whether the melting point
-
-
I
I
( 5 ) Dunn, Truns. Furaduy Soc.. 22,401 (1926).
(1929); Foster, ibid., Al47, 128 (1934). (3) Thomaon, Phil. Mag., [4]49,448(1871).
