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- 94 - MICRCWAVE ABSORPTION W POLAR LIQUIDS * W.M. Heston, Jr.^ E.J. Hennelly, A.D. Franklin and CP. Symth Frick Chemical Laboratory, Princeton University Princeton, N. J. The d i e l e c t r i c constants and loss factors of twenty-seven l i q u i d organic halides have been measured at 1.27 and 3.22 cm. vJavelength and used, together with c e r t a i n additional measvuremdnts at 10 cm. dhd with measureihents of v i s c o s i t i e s , densities and r e f r a c t i v e indices, to examine the accuracy of the equations f o r d i - e l e c t r i c behavior and the relations between d i e l e c t r i c relaxation and molecular structure. The method of measurement employs the v a r i a t i o n i n the r e f l e c t i o n c o e f f i - cient of a uniform d i e l e c t r i c layer as the depth of the layer i s varied. A short- c i r c u i t i n g plunger terminates the d i e l e c t r i c layer and a d i r e c t i o n a l coupler i s used t o measure the power reflected from t h i s plunger. The calculations Involve only the r a t i o of the amplitudes of the reflected waves f o r two d i f f e r e n t lengths of the d i e l e c t r i c - f i l l e d guide, the free-space wavelength, the wavelength i n the a i r - f i l l e d guide, and the wavelength i n the d i e l e c t r i c medivim. /# 4 The loss factors, £ , have been plotted against the d i e l e c t r i c constants, ^ , on c i r c u l a r arc plots by the method of Cole and Cole. The value of the em- p i r i c a l constant vC obtained from these arc plots gives an in d i c a t i o n of the d i s t r i b u t i o n of relaxation times f o r a molecule. The curves f o r the dependence of oC f o r the straight-chain a l k y l bromides upon the number of carbon atoms show a rapid r i s e with increasing chain length from a small value f o r the eth y l bromide molecule with a f l a t t e n i n g out at 1° i n the v i c i n i t y of a twelve-carbon atom cliain and a f l a t t e n i n g out at 55° i n the v i c i n i t y of a nine-carbon atom chain. The i n - crease i n oC with increase i n chain length may be a t t r i b u t e d to an increase i n the number of shapes which the molecules can acquire by t w i s t i n g around the carbon- carbon bonds or to the increase i n the number of carbon-carbon bonds about which r o t a t i o n can occur to give dipole orientation i n the applied f i e l d . Both types of or i e n t a t i o n may occiir to a greater or lesser extent. The f l a t t e n i n g out of the curve f o r the longer molecules may be due to the r e l a t i v e l y smaller e f f e c t upon molecular shape produced by the addition of each successive carbon atom or to tlie p o s s i b i l i t y that not more than about ten bonds may be involved i n dipole orienta- t i o n by i n t e r n a l t w i s t i n g . The d i e l e c t r i c behavior of four of the twenty-seven polar l i q u i d s which have been investigated conforms closely to that required by the Debye theory of anomalous d i e l e c t r i c dispersion with a single relaxation time. The values of are very nearly zero f o r these four molecules, i-propyl, i - b u t y l , s-butyl, and t - b u t y l bromide, which are not f a r from the sperical form postiolated i n the deriva- t i o n of the Debye equations. The c y c l i c molecules have lower values of i^C since they possess less variety of modes of orientation than the corresponding s t r a i g h t - chain molecules having the same number of carbon atoms. I t would appear that while or i e n t a t i o n of the molecule as a whole i s occurring f o r the six-carbon atom mole- cules, such i s not the predominant mode of orientation f o r the normal ten cprbon atom molecule as compared to the r i g i d «C-bromo-naphthalene molecule. * This research was carried out with the support of the Office of Naval Research, Navy Department.
- 95 - Table 1 Di s t r i b u t i o n Constants and C r i t i c a l Wavelengths at 25°C. Compoimd ^m C^Hi3Br . 0,17 2,96 cm. .10 3.09 C^KiiBr .11 3o67 CioH2iBr .25 6.32 C^oHyBr .16 16.2 n - The c r i t i c a l wavelengths increase i n the series chloride, bromide, iodide as do the vi s c o s i t i e s (See Table 2) Table 2 C r i t i c a l Wavelengths and Viscosities at 25°C. Compound X m ^ X 103 CICH2CH2CI 1.09 cm. 7.90 poises BrCHgCHjBr 2.18 I6.O n-CgHi^Cl 3.22 11.5 n-CgH27Br A. 09 14-.9 n-CgHj_7l' (7.24) 19.5 C^H^Cl 1.9A 7.56 C^H^Br . 3.09 ⢠10.8 0C-C10H7CI . 9.24 . 2% A cy;-CioH7Br 16.2 4-5.2 The close parallelism between c r i t i c a l wavelength and viscosity i n de- pendence upon molecular size and shape and temperature gives fiu-ther evidence of the previously observed s i m i l a r i t y between the processes of d i e l e c t r i c relaxation and viscous flow.
