Communications to the Editor
TABLE I: Dielectric Constants of 1-Pentanol-Water Mixtures at 25"
wt Pdbhcalton costs assrsted by the Offtce of Sahfle Water
Sir: Additivity rules predict that the addition of' a substance of higher dielectric constant to one of lower permittivity wouid give a mixture with a higher dielectric constant than that of the second component. Herewith is reported a striking exception to' expectation: initial addition of water ( D = 78.35) to h-amyl alcohol (1-pentanol, D = 15.14) l o w ~ r sthe dielectric constant. A minimum appears in the D us. wt % curve a t about 3.5% water. The D us. wt % curve for methanol-water mixtures is nearly linear over the whole range of compositions; lor ethanol-water, it is slightly concave upward. The curvature increases on going to propanol (normal and iso) and still more with ierf-butyl alcohoLP These results, together with our obsdrvation on 1-pentand-water mixtures, suggest that the branch points which appear when water enters the hydrogen-bonded alcohol chains have a lower net dipole moment than that corresponding to the vector sum of the components previous to mixing, and that the decrease becomes greater as the number of carbon atoms in the alkyl group increases. Work on other mixtures is in progress; we hope to find a correlation between the structure of the components and the magnitude and sign of (3D12/8x&, where x 2 i s a measure of the water concentration, arid thereby to learn something about short-range interactions between polar compounds. Below is an account of the behavior of thc pentanol-water mixtures. As part of a study of the conductance of a series of quaternary ammonium salts in mixed solvents, l-pentanol was chosen as one of the components. Our ~ a l u e sfor the physical constants at 25" (dielectric constant, D = 15.14; viscosity, 0.0353 P; density, 0.8117 g/ml) did not agree with the literature vaiues. V i s c o s i t i e ~ ~of, ~0.03476 and 0,03947 arid densities234 of 0.81096 and 0.8095 have been repor Led. The greatest discrepancies appear in the dielectiic constants, for which the values 15.04,2 14.k5 13.77,4 and 13.186 are given. Evans and Gardam2 dried their amyl alcohol by refluxing over calcium oxide; Larson and Hunt4 used metaliic calcium; no informarion on the purification of the other materials is available. The method of measurement used by Evans and Cardam is completely reliable; the methods used by che earlier workers are suscep-
The Journal of Physical Chemistry, Val. 78,
No. 6 , 1974
% H*0
D
Wt 76 H*O D
0.00 15.14 3.13 14.63
0.75 14.97 3.76 14.65
1.16 14.91 4.14 14.68
1.80 14.SO 5.15 14.74
2.47 14.71
tible to errors if the conductance of the test liquid is not negligible. Our 1-pentanol (Fisher's Certified Grade) was dried by refluxing over calcium hydride; water content, as determined by a Perkin-Elmer 880 gas chromatograph, was 0.010 f 0.001%. The chromatograph showed only the pentanol peak and the water blip. Dielectric constants were measured at 1 MHz, using the General Radio 716-CS1 bridge, and a Lind-Fuoss7 cell. The results are given in Table I for 1-pentanol and for several mixtures with water. (The solubility of water, 2.19%, as reported by Gimmings and Baum,* is too low; our highest water concentration is just short of saturation.) The presence of a minimum near 3.5% water is distinctly visible. We conclude that the lower values of the dielectric constant previously reported were due to the presence of water in the samples measured. Acknowledgment. This work was supported by the Office of Saline Water, U. S. Department of the Interior, under Contract No. 14-01-0001-1308. References a n d Nates (1) (2) (3) (4) (5) (6) (7) (8)
H. S. Warned and B. B. Owen. "The Physical Chemistry of Electrolytic Solutions," 3rd ed, Reinhold, New York, N. Y., 1958, Table 5-1-4, p 161. D. F. Evans and P. Gardam, J. Phys. Chem., 73,158 (1969). W. J. Jones, S. T. Bowden, W. W. Yarnold, and W. H . Jones, J. Phys. Colloid. Chem., 52, 753 (1948). R. G. Larscn and H . Hunt, J. Phys. Chem., 43,417 (1939). P. Girard, Trans. Faraday SOC., 30, 763 (1934). H. R. Sarna and P. N. Trenhan, Curr. Sci. (India), 21, 306 (1952); Chem. Ab.?.fr., 47, 4153a (1953). J. E. Lind, Jr., and R. M. Fuoss, J. Phys. Chem., 65,999 (1961). P. M. Gimmings and R. Baurn, J. Amer. Chem. Soc., 59, 1111 (1937).
Sterling Chemistry Laboratory Yale University New Haven, Connecticut 06520
Alessandro D'Aprano
Received January 27, 1974
