M. DELLA MONICA,L. JANNELLI,AND U. LAMANNA
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Physicochemical Properties of Sulfolane by Mario Della Monica, Liliana Jannelli, and Ugo Lamanna Institute of Physical Chemistry, University of Bari, Bari, Italy Accepted and Transmitted by The Faraday Societu
(June 16,1967)
Cyroscopic behavior of solutions of n-heptane, carbon tetrachloride, naphthalene, diphenylmethane, benzene, pyridine, and cyclohexane in sulfolane was investigated in the molality range 0-1. Data supply evidence that sulfolane solidifies at 28.45' in plastic crystals, which undergo a transition in a new solid phase at 15.45", owing to the LLglobular" structure of the molecule. Negative conclusions are drawn concerning the use of sulfolane as a reliable solvent in cryoscopy, in the domain of mesomorphic phase, because molar depressions are indicative of a partition of solute between solid and liquid phases. Below the transition temperature, dissolved substances lose their individuality, in the limits of their different activity coefficients.
Introduction During the past 15 years, a considerable number of papers have been written on the subject of molecular crysta1s.l These crystals resemble a liquid in some aspects and they may be regarded as a mesomorphicstate intermediate between typical solids and liquids.2 The mesomorphic phase is characterized by a high dielectric constant value. This implys that the molecules which have quasi-spherical (globular) repulsion envelopes when close-packed in the crystals possess sufficient energy to rotate frequently over the restricting potential energy barriers. Therefore, the entropy of fusion does not exceed 3-5 e q 3 because changes of vibrational entropy are negligible and only positional disordering in melting is important. Some degrees below the melting point, the mesomorphic phase undergoes a transition in a nonrotational crystalline phase; enthalpy change on the transition considerably exceeds the enthalpy of fusion. As globular substances are very often polymorphic, more than one transition point may be encountered with decreasing temperature. Therefore, in the proximity of a transition point, nucleation of two forms may occur within the crystal matrix, and hybrid single crystals may be formed. I n this case, the transition is called continuous, in the sense of U b b e l ~ h d e . ~ ~ ~ Evidence of globular properties of sulfolane and of some molecular rotational freedom between the freezing point (28.45') and a transition point (15.43') is given by our previous studies on this substance.6 I n the present paper, thermal behavior of a highly purified sample of sulfolane in the temperature range 10-30" is reported. Also, cryoscopic data are collected on several binary mixtures in which the solventis the sulfolane and the other component may be: n-heptane, cyclohexane, carbon tetrachloride, benzene, benzoic acid, pyridine, diphenylmethane, or naphtalene. Experimental Section Muteriuls. Sulfolane (tetramethylene sulfone), furThe Journal of Phgsical Chemietry
nished by Shell Industrial Chemical Division, was carefully purified and dehydrated on PzOj by repeated distillation in vucuo torr) through a 1.5-m Podbielniack column packed with glass helices. Benzene, n-heptane, and cyclohexane were boiled under reflux over phosphorous pentoxide and finally distilled in a stream of dry air. The purified material was stored under dry air. Carbon tetrachloride, free of carbon disulfide, was dried over fused calcium chloride and distilled. Benzoic acid was twice recrystallized from hot water and dried in a vacuum desiccator. Naphthalene was kept overnight at 140' under a stream of nitrogen, in order to remove water. The product was twice recrystallized from anhydrous methanol and distilled. Diphenylmethane, obtained from the Eastman Kodak Co., was subjected to three fractional crystallizations. The resulting product melted at 25.1'. Reagent grade pyridine was purified by repeated fractional freezing, refluxing over freshly heated barium oxide, and distilling through a column, 1 m in length, packed with glass helices. Cryscopic Apparatus and Procedure. The measuring cell (300 cm3) was constructed in order to prevent errors due to intrusion of water vapor and, generally, to secure a satisfactory standard of accuracy. It was cleaned with chromic acid and carefully washed with distilled water. It was then evacuated with a mercury diffusion pump and subsequently heated to 150' for a period of 24 hr.
(1) A. R . Ubbelohde, Quart. Rev. (London), 11, 246 (1957); Angew. Chem., 4,587 (1965); G. €3. G u t h i e and J. p. McCullough, J . PhW. Chem. Solids, 18, 53 (1961); L. A. K. Staveley, Ann. Rev. Phys. Chem., 13, 351 (1962).
(2) A. R.Ubbelohde, Quart. Rev. (London), 11, 246 (1957). (3) B. Timmermans, J. Phys. Chem. Solids, 18, 1 (1961). (4) G. B. Guthrie and J. P. McCullough, ibid., 18, 53 (1961). (5) J. Jannelli, M.Della Monica, and A. Della Monica, Gazz. Chim. Ital., 94, 552 (1964); U . Lamanna, 0.Sciacovelli, and L. Jannelli, ibid., 96, 114 (1966).
PHYSICOCHEMICAL PROPERTIES OF SULFOLANE
1069
An opening in the cell head provided a way to introduce the solvent and solutes with a suitable delivery apparatus. In order to prevent intrusion of water vapor during measurements, it was essential to grease all ground joints with Apiezon 1\1 grease. The stirrer consisted of a glass spiral. Temperatures were measured by a platinum resistance thermometer calibrated by XBS. The resistance was measured by a Rlueller G2 bridge (Leeds & Northrup Co.). Heating curves were preferred to cooling curves, in the case of solutions, in order to avoid undercooling and related errors.6 Before starting the measurement of a heating curve, the solid-liquid mixture was kept at a temperature slightly below the initial melting point for at least 1 hr. The molality of the solutions investigated ranged between 0 and 1.
Results and Discussion Data on Pure Substance. The mesomorphic-phase domain may be detected by the cooling curve in Figure 1. It reveals a transition at 15.43' whose latent heat considera,bly exceeds that of fusion at 28.45'. This melting point was first determined by us on a highly purified sample of the material; it is in good agreement with the value (28.85') predicted by Birch and Mac Allen' using Rossini's extrapolation method. We report here, for the first time, the value of the transition temperature, directly determined on the pure substance. It checks well wit11 the cryoscopic preliminary data of Burwell and Langford,* which are consistent with the existence of a new solid phase below 15'. Dielectric behavior indeedg evidences that sulfolane molecules still possess rotational freedom below the melting point, until the transition temperature is reached, at which point a nonrotational crystalline phase is formed. Data on Solutions. Several binary systems were studied by thermal analysis, as previously described, up to m = 1. I n the case of the n-heptane and cyclohexane solutioizs, measurements are restricted to dilute solutions, on account of their low solubility. Initial freezing points (liquid-solid equilibrium temperatures) of these solutions are plotted os. molality in Figure 2. The values lie on straight lines, but the slope of the lines appears to depend upon the solute in the first composition range (0-0.2 m). The steeper the slopes are, the more the dissolved substance is different, in size and shape, from the solvent. Afterward, the plots exhibit a sharp break, which, in most cases, checks well with the transition temperature (15.43') of sulfolane itself. The initial freezing points of the solutions, having a composition higher than the molality at the transition point, lie on a Eitraight line, which is practically common
\
t m- 28.45OC
\ 0
10
20
30
t ' d
Figure 1. Cooling curve of pure substance.
to all dissolved substance, up to in = 1. Because of their low solubility, solutions of n-heptane and cyclohexane may not gain transition points.
Discussion Cryoscopic behavior may be better described by Figure 3, where molal depressions, 8/m, calculated from the fusion point, 28,45', of the pure substance, are plotted vs. molality. I n the composition range 0-0.2 m, these values lie on straight lines and they appear to be influenced slightly by temperature. The straight lines, for several solutes-napht halene, diphenylmethane, n-heptane, and benzoic acid-extrapolate to a value of 65, which may be indicated, reasonably, as the cryoscopic constant of sulfolane in the stability region of the plastic phase. Practically all acids are not dissociated in sulfolane.1° The values of 8 / m for carbon tetrachloride, pyridine, and cyclohexane solutions lie on straight lines, but these do not extrapolate to normal values of 8 / m when fn + 0. Therefore, they are suspected of forming plastic mixed crystals with the solvent. From the limiting values of 8/m, distribution coefficients of (6) W. M. Smit, "Purity Control by Thermal Analysis," Vol. 7, Elsevier Publishing Co., Amsterdam, The Netherlands, 1957, pp 2335. (7) A. R. Birch and J. Mac Allen, J. Chem. Soc., 2556 (1951). (8) R. L. Burwell and C. Langford, J. Amer. Chem. Soc., 81, 3799 (1959). (9) U.Lamanna, 0. Sciacovelli, and L. Jannelli, Gam. Chem. Ital., 94, 552 (1964). (10) E. M.Arnett and C. F. Douty, J. Amer. Chem. Soc., 86, 409 (1964). Volume 78. Number S March 1968
M. DELLAMONICA,L. JANNELLI, AND U. LAMANNA
1070
30
t
0 0
I
.2
I
.6
.4
1.0
.8
m
c
Figure 2. Initial freezing points us. molality: 0, diphenylmethane; 0, naphthalene; A, benzoic acid; 0, n-heptane; XI benzene; A, pyridine; . , carbon tetrachloride; 0, cyclohexane.
75
9. m 50
25
(
I
I
I
.2
.4
.6
Figure 3. Molal depression vs. molality.
1.0
m
(Symbols have the same meanings as they have in Figure 2.)
solutes between solid and liquid sulfolane could be calculated, which are consistent with the hypothesis that the greater the disturbing effect of a solute on the mesomorphic phase, the more the second component departs from criteria of mesomorphism. The Journal of Physical Chemistry
.8
I n the phase-I1 domain, experimental points lie practically on one curve which crosses the straight lines, previously described, corresponding to the transition temperature. This value, in the case of carbon tetrachloride, pyridine, and cyclohexane solutions,
NOTES varies from substance to substance, still affording support to hypothesis that plastic sulfolane can dissolve solutes. Since an ideal solution is defined as one in which the activities of the components are equal to their mole fractions, their freezing points can be calculated by replacing the activity of the solvent by its mole fraction in Lewis and Randall’s equation.” In the case of solutions in sulfolane, thermal data on molal heat of solidification and on heat capacities of the solid and liquid phases are lacking. Anequation8 = 65m - 17.9m2,valid in .the range 0-0.2 m, can be obtained if we put (8/m)m=o= 65 and we disregard the small difference between the heat capacities of liquid and plastic sulfolane. As can be seen from Figure 3, the experimental points, in the case of solutions of diphenglmethane and naphthalene in sulfolane, lie close to the dotted line calculated by this equation; n-heptane and benzoic acid are indeed some examples of nonideal solutes, as might be expected. The initial freezing points of solutions, having
1071 molality higher than the transition value, may be represented well by the expression 17.07 - 7.57m (dotted line in Figure 2), where 17.07 is the virtual fusion temperature of the crystalline nonrotational phase I1 and 7.57 may be reasonably indicated as the cryoscopic constant of sulfolane in equilibrium with the phase 11. Starting from these values, an entropy change associated with the transition phase I1 + liquid can be calculated as 9.1 eu, whereas the corresponding value for the fusion of phase I does not exceed 1.1eu. Therefore, the entropy that characterizes the transition I1 -t I amounts to 8 eu. Both transition and fusion entropies are in good agreement with the figures referred to by Staveley12 and Guthrie and McCullough4 for some substances forming plastic crystals. (11) S. N. Lewis and M. Randall, “Thermodynamics and Free Energy of Chemical Substances,” MoGraw-Hill Book Co., Ino., New York, N. Y., 1923, p 283, eq 17. (12) L . A . K. Staveley, Ann. Rev. Phys. Chem., 13, 351 (1962).
NOTES
On the Reaction of D2+ with Cyclohexane by Jean H. Futrell,la Fred P. Abramson,lb and Thomas 0. Tiernan Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 46433 (Received September 22, 1967)
In a recent communication, Wolfgang and Menzinger2 presented a preliminary account of an ion-impact experiment in which cyclohexane depositied on a surface was bombarded with Tz+and T + ions as a function of ion kinetic energy in order to evaluate the threshold energy and energy dependence of a typical hot-atom reaction. The initial stage of the reaction could involve charge exchange and/or various ion-atom interchange processes--?‘+ transfer, H atom abstraction, and T stripping. To aid in the interpretation of this initial stage of the reaction, we have performed the related gas-phase experiment of impacting D2+on cyclohexane a t various ion energies and examining the ionic products of this reaction. The experiments which we report here were carried out using the ARL tandem mass spectrometer described in detail el~ewhere.~In the present experiments, Dz+
ions were formed by electron impact on D2 in the ion source of the first-stage mass spectrometer, accelerated, both energy and mass selected, decelerated to terminal energy, and impacted on cyclohexane vapor in a differentially pumped collision chamber. The energy spread in the ion beam, determined by the defining slit of the electrostatic analyzer, was 0.5 eV in these experiments. The lowest energy investigated approached thermal velocities (ion energy 0-0.5 eV) and is reported in Table I. Ions monochromatic in energy to this degree were then impacted on cyclohexane a t various energies up to 20-eV nominal ion kinetic energy. Cyclohexane pressure in the collision chamber was maintained at 0.005 torr, and the chamber temperature was ca. 230’ ; under these conditions secondary reactions of the ions initially produced are negligible. The distribution of cyclohexane ions from the reaction of 04.5-eV Dz+ ions is reported in Table I. At this energy no signal was detected a t m / e 2 (D+) or a t (1) (a) Department of Chemistry, University of Utah, Salt Lake City, Utah 84112. (b) Consolidated Electrodynamics Corp., Monrovia, Calif. 91017. (2) R. Wolfgang and M. Menzinger, J. Am. Chem. SOC.,89, 5992 (1967). (3) J. H. Futrell and C. D. Miller, Rev. Sci. Instr., 37, 1521 (1966).
Volume 79, Number 3 March 1968
