858
L. B E K J S M I N AND G. c. BENSON
Vol. 67
A DEUTERIUM ISOTOPE EFFECT ON THE EXCESS ENTHALPY OF bIETHAKOLWATER SOLUTIONS1 BY L. BEN JAM IN^
AND
G. C. BESSON
Division of Pure Chemistry, National Research Council, Ottawa, Canada Received October 11, 1968 Heats of mixing of methanol and water and of methanol-d and heavy water were determined a t 25’ in an adiabatic calorimeter. Deviations between the two systems are most pronounced for solutions dilute in alcohol and a Tian-Calvet microcalorimeter was used for additional meaciurements in this region. The results are consistent with the concept of the strengthening of hydrogen bonding by deuterium substitution.
Introduction Differences in the physicochemical properties of a system due to isotopic substitution are of interest since in general they reflect variations in the degree of interaction within the system as distinct from changes in the fundamental nature of the interaction. Thus the substitution of hydrogen by deuterium has been used to study hydrogen bonding in a number of casesn3 In one recent investigation by Rabinovich and coworkers4 the mutual solubility behavior of a number of organic liquids in water was compared with that of their suitably deuterated analogs in heavy water and the changes in miscibility attributed to differences between hydrogen bonds involving normal and heavy hydrogen. The present study of heats of mixing was undertaken to investigate further the isotope effect in hydrogen bonding. The methanol-water system was chosen because of the relative simplicity of the two species and the availability of deuterated analogs, vix., methanol-d and heavy water. Calorimetric studies of the mixing of methanol and mater were carried out by Bose6 over 50 years ago and more recently by Oc6n and T a b ~ a d a . ~ JHowever, since only a small isotope effect was expected it seemed advisable to repeat the methanol-water determinations and thus to ensure that the same experimental techniques were used for this system as for the methanol-d-heavy water system. Experimental In the study of aqueous methanol solutions, Matheson Coleman and Bell “Spectroquality Reagent” methanol and conductivity water were used. The latter was obtained by distillation of alkaline permanganate solutions in a Pyrex still. Heavy water (DzO content 99.82%) purchased from Atomic Energy of Canada Ltd. and Merck methanol-d having an isotopic purity greater than 98co were used for measurements on the correspondipg deuterated system. In both systems heats of mixing for values of 2 2 , the mole traction of alcohol in the final solution, in the range 0.03 to 0.95 were measured in the adiabatic calorimeter described by Benson and Benson.8 The onlv modification of this equipment was the replacement of the guillotine arrangement inside the stainless steel vessel by the Pyrex mixing cell shown in Fig. 1. The liquids to be mived in the cell are separated by mercury; filling (1) Issued as N . R C. No. 7314. Paper presented in part a t the 17th Annual Calorimetry Conference, Unii ersity of California, Berkeley, August 23, 1962. (2) National Research Council of Canada Postdoctorate Fellow, 19581960 (3) G. C. Pimentel and A. L. hIcClellan, “The Hydrogen Bond,” W. H. Freeman and Co.. San Francisco, Calif., 1960. (4) I B. Rabinovich, V. D. Fedorov, N. P. Pashkin, &I. A. Avdesnyak, and N. Ya. Pimenov, Dokl. Alcad. ,Vauk SSSR, 105, 108 (1955) (English translation N R C TT-875). (-5) E. Bose, Z. phgszk. Chem., 58, 585 (1907). (6) J. Oc6n and C. Taboada, Anales 8 s . y quim. (Madrid), SSB, 243 (1959). (7) J. Oc6n a n d C. Taboada, abid., 66B, 263 (1959). (8) G. C. Benson a n d G. W. Benson, Rev. Sci. Inatr., 26, 477 (1955).
is accomplished by displacing mercury from the outer chamber with one of the liquids and removing mercury from the central tube to accommodate the second liquid. The amounts of the two liquids are determined from an appropriate series of weighings. In general the total weight of the mixture was 10 to 20 g. and the vapor volume above the liquid in the central tube m a s about 2 cc. The ratio of the volumes of the tn-o liquids to be mixed in the cell was 5 to 1 or greater. I n order to cover the whole concentration range it was necessary to work from both ends and to reach cuncentrations with mole fractions 0.3 t o 0.7 by multiple dilutions; the overlap of results around xz = 0.5 was quite satisfactory and indicated the self-consistency of tho technique. The mixing cell is supported by a metal frame inside the steel calorimeter vessel and the intervening space filled with water to improve the thermal contact. Mixing is initiated by tipping the calorimeter assembly through 180”. The experimental procedure followed during a calorimetric determination has been described previously.8 Quantities of heat evolved by the mixing process ranged from 10 to 100 cal. All calorimetric measurements were carried out at temperatures close to 25” (generally within i 0 . l ’ ) and heat capacity values were used to correct the results t o 25.00’. Vapor volumes in each determination were noted and a correction applied for heat effects arising from the change in vapor composition which accompanied the mixing. Data for heat capacities and vapor pressures needed for these corrections are available in the literature for aqueous methanol solutions .6, QJO Similar information is lacking for the deuterated system and corrections were estimated on the basis of the values for the ordinary methanol solutions. I n all cases the total correction amounted t o less than 1% of the measured heat and usually was less than 0.5%. In addition to the work in the adiabatic calorimeter, a number of solutions with mole fractions of alcohol less than 0.1 were investigated in a Tian-Calvet microcalorimeter.l*~12 The present form of this apparatus, which is similar in many respects to the calorimeter used by Attree, et aZ.,13 has been described briefly by Benjamin and Benson.14 I n it the transfer of thermal energy from a silver reaction tube to a large Dural block surrounding the tube is measured by a calibrated thermopile; a flux of less than 1 X 10-6 cal. sec.-l can be detected. The dilution studies were carried out in a metal cell immersed in silicone oil in the reaction h b e . A drawing of the mixing cell is shown in Fig. 2. A thin (0.0005 in.) platinum foil separates the two liquids before mixing. The position of this divider is adjustable in the metal cell. After a steady state has been reached the foil is pierced by a pointed cutter and the e.m.f. of the thermopile measured continuously until the steady state is re-established. The area under the e.m.f.-time curve is proportional to the heat effect taking place in the reaction tube. A manganin heater wound on the outside of the metal cell is used for electrical calibration. The heat associated with rupture of the foil is determined in a blank experiment and a correction applied. In general the heat effects studied in the microcalorimeter were in the range 0.05 to 2.0 cal. The accuracy with which these could (9) G. Bredig and R . Bayer. Z. physik. Chem., 130, 1 (1927). (10) J. B. Ferguson and W. S.Funnell, J . Phys. Chem., 33, 1 (1929). (11) E. Calvet a n d H. Prat, “Microcalorimetrie,” Masson et Cie, Paris, 1QB6. ( I 2) F. D. Rossini. “Experimental Thermochemistry,” Interscience Publishers, New York, N. Y., 1956, Chapter 12. (13) R. IT. Attree, R. L. Cushing, J. A. Ladd, and J. J. Pieroni, Rev. S
