R. N. SELBY AND J. G. ST ON
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CRYOGENIC LABORATORY OF THE COLLEGE OF CHEMISTRY AND PHYSICS,THE PENNSYLVANIA STATE UNIVERSITY J
The Entropy of the Compound Formed between Cyclopentane and 2,2-Dimethylbutane at 0OK.l BY R. N. SELBY AND J. G. ASTON RECEIVEDJUNE 7, 1958 The heat capacities from 14 to 29S°K., melting points and heats of fusion of two c~-clopentane-2,2-dimethylbutane solutions (66.6 and 69.3 mole 70 cyclopentane) have been measured. Values for the temperatures and entropies of the rotational transitions which are of second or higher order have been determined. The heat of vaporization of the 66.6 mole 70 cyclopentane solution at 298.16"K. has been determined experimentally and that of the 69.3 mole Yo cyclopentane solution has been estimated. From these data the entropy of the solutions at the absolute zero has been determined. The zero point entropies have been interpreted t o indicate that a compound or complex consisting of two units of cyclopentane and one of 2,Z-dimethylbutane exists in five isomeric forms randomly arranged at the absolute zero.
Introduction A maximum in the solid-liquid phase diagramzaof cyclopentane-2,2-dimethylbutanesolutions at 66.7 mole 7 0 cyclopentane indicates compound formation a t this composition. X maximum in the liquid-vapor diagramzb at the same composition shows that a constant boiling solution exists. Nuclear magnetic resonance measurements3 have shown that a rotational transition occurs a t 65'K. for the 66.7 mole 7 0 cyclopentane solution. The present work was undertaken to determine whether this cyclopentane--2,2-dimethylbutane solution behaves as a compound or complex as it is cooled to O'K. or as a solid solution of the individual components. This paper presents the results of a calorimetric study of the 66.7 mole Tu cyclopentane solution leading to a value of the entropy of the system with this composition a t O O K . In addition, the results of a calorimetric study of a 70 mole % cyclopentane solution, which are useful in determining the effects of excess cyclopentane on the entropy a t O'K., are presented and discussed. Experimental Apparatus.-Platinum calorimeter G of this Laboratory was used in this work. Temperatures were measured with platinum resistance thermometer Pt-G 10. This calorimeter together with its cryostat and temperature scale were previously described .* The Cyclopentane and 2,Z-Dimethylbutane Samples.The cvcloDentane and 2,2-dimethvlbutane samdes were "reagent grade" specimens obtacned from the Phillips Petroleum Company. The purity of a sample of each was determined calorimetrically by observing equilibrium temperatures of solid and liquid with increasing fractions of the sample melted.5 By this procedure it was found that the cyclopentane was 99.99 mole % pure and the 2,2-dimethylbutane 99.98 mole yo pure, assuming no solid solution. The observed melting points were 179.62 and 174.28"K., respectively, showing excellent agreement with previously reported v a l ~ t e s . ~ - ~ (1) Aided in part by contract N6onr-269, T.O. 111 of the Office of Xa\-:il Research. Keproduction in whole or in p a r t is permitted for an>- p u r p o s e i o f the United States Government. ( 2 ) (a) 11. I, F i n k . XI R. Cines, Q. E. Frey and J. G . Aston. THIS J O L R N A I . ,69, 1301 ( 1 9 4 7 ) : (11) A . S. Czrrney, Ph.D. Disiertation, T h e Pennsylvania S t a t e Uni, v r s i t y , 1955. (3) J. C. Aston. B. Rolger, R . Trarnburulo and H. Srgall, J , Ciienz.
( 5 ) J. G. Anton J I \I'cwd and T 'I Zolkl, T H I S J o r r R N A L , 7 6 , 6202 ( 1 0 5 3 ) . ( 5 ) J C;. A \ t o i i , k1. 1.. l , i : i h , J. \\.. '1 t l i i h e .mtl 31. I
