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The Journal of Physical Chemistty, Vol. 83, No. 25, 1979
Atake and Angeli
conjunction with the similarities of the enthalpy values for reactions C-F imply a sixfold coordination for all cobalt complexes. In recent investigations, Dienstbach and Emmenegger have reported that NiInzC1818is -8 kcal/mol less stable than CoInzC18,17a finding that is indicative of a coordination change on going from the solid phase to the vapor complex for the nickel system (and presumably the cobalt system). The measured difference is close to that expected from the octahedral-site stabilization energy of nickel in an all-chloride e n v i r ~ n m e n t However, .~~ in evaluating the enthalpy of NiIn2C18the authors used quenching and optical absorption experiments18 at 823 K, where they claimed high indium chloride pressures (up to 5 atm for their experiment 5). In view of the recent vapor-pressure measurement of Kuniya,2Othe pressure of indium chloride at 823 K cannot exceed 0.7 atm. Much lower InC13(g) pressures are expected which yield an enthalpy for NiInzC18 closer to that of CoInzC18and imply a sixfold coordination for both complexes. It must be emphasized, however, that the structure of the vapor complexes is rather intriguing. Investigations by means of resonance Raman spectroscopy have been enlightening,lZ-l4 but in many cases (e.g., CoAlzC18, CoA.lzBrJ3),because of experimental difficulties,they have failed to reveal the structural particularities of the molecules. Other types of structural investigations (e.g., by electron diffraction) of the vapor phase also appear to be difficult due to the presence of large amounts of the carrier gas.
Sciences of the Department of Energy.
References and Notes E. W. Dewing, Metall. Trans., 1, 2169 (1970). H. Schafer, Angew. Chem., Int. Ed. Engl., 15, 713 (1976). M. Binnewies, 2. Anorg. Allg. Chem., 437, 25 (1977). G. N. Papatheodorou, J. Phys. Chem., 77, 472 (1973). G. N. Papatheodorou, Z . Anorg. Allg. Chem., 411, 153 (1975). A. Deii’Anna and F. P. Emmenegger, Helv. Chim. Acta, 56, 1145
(1975). A. Anundskas, A. E. Mahgoub, and H. A. aye, Acta Chem. Scand., Sect. A , 30, 193 (1976). F. P. Emmenegger, Inorg. Chem., 16, 343 (1977). G. N. Papatheodorou and G. H. Kucera, Inorg. Chem., 16, 1006
(1977). J. Hastie, “High Temperature Vapors”, Academic Press, New York, 1976, p 145. G. I. Novikov and E. S. Kotava, Zh. Flz. Khim., 47, 483 (1973). G. N. Papatheodorou and M. A. Capote, J. Chem. Phys., 69, 2067
(1978). G. N. Papatheodorou in “Characterization of Hm i Temperatwe Vapors and Gases”, J. Hastie, Ed., NBS Special Publication, 1979, in press. C. W. Schlpfer and C. Rohsbasser, Inorg. Chem., 17, 1623 (1978). M. A. Capote, G. H. Kucera, and G. N. Papathecdorou, “Proceedings of the Symposium on High Temperature Metal Halide Chemistry”, Vd. 781, D. Cublcblti and D. L. Hibnbrand, Ed., The Electrochemical Society, 1978, p 367. An early report of the present study is given in the Abstracts of Papers, National Meeting of the American Chemical Society, Chicago, IL, Aug. 29, 1977; American Chemical Society, Washington, D.C.;
INOR-194.
F. Dienstbach and F. P. Emmenegger, Helv. Chim. Acta, 60, 1966
(1977).
F. Dlenstbachand F. P. Emmenegger, Inwg. Chem., 16,2957 (1977). F. Dienstbach and F. P. Emmenegger, J. Inorg. Nucl. Chem., 40,
129 (1978). Y. Kuniya, S. Hosada, and M. Hosaka, DenklKagaku, 42, 20 (1974). H. Schafer, Z . Anorg. Allg. Chem., 278, 299 (1955). C. W. DeKock and D. Gruen, J. Chem. Phys., 44, 4387 (1966). R. Gale, R. E. Godfrey, and S. F. Mason, Chem. Phys. Lett., 38,
Acknowledgment. Work performed under the auspices of Materials Science Offices of the Division of Basic Energy
441 (1976). G. N. Papatheodorou, J. Inorg. Nucl. Chem., 35, 465 (1973).
Pressure Dependence of the Glass Transition Temperature in Molecular Liquids and Plastic Crystals 1.Atake and C. A. Angell” Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received July 5, 1979) Publication costs assisted by the National Science Foundation
The effect of pressure on the glass transition temperature of a variety of molecular liquids both hydrogen bonded and non-hydrogen bonded, and their solutions has been investigated. It is found generally that the more extensive the hydrogen bonding in the liquid the smaller is the effect of pressure on Tg. This obserfation applies also to a binary mixture, in which dT,/dP is found to change linearly between the end member values. The effect of pressure on the glasslike transition in the cyclohexanol plastic crystal phase is as expected for a liquid alcohol. For the lubricant 5P4E, the data are in accord with predictions based on high-pressure light scattering data. The results are interpreted by using the second Davies-Jones relation, dT,/dP = VT(Aa/AC,). Some evidence suggesting that the excess entropy frozen in at the glass transition is not quite independent of pressure as often supposed is presented.
Introduction In recent papers from this laboratory the effect of pressure on the glass transition temperature of a variety of ionic liquids and solutions has been investigated.lP2 It was found that in aqueous solutions dTg/dP tended to be small, and in certain water-rich systems could assume small 0022-3654/79/2083-3218$01 .OO/O
negative values, a result not anticipated from simple free-volume considerations. In the present paper we extend this type of investigation to molecular liquids in which the intermolecular interactions vary from pure van der Waals, to multilateral hydrogen bonding. Included in the study is the much-in0 1979
American Chemical Society
The Journal of Physical Chernistty, Vol. 83, No. 25, f979 3219
Pressure Dependence of the Glass Transition Temperature
vestigated polyphenyl hydrocarbon ~ - t e r p h e n y l ~and -~ some of its more crystallization-resistant mixtures with rn-terphenyl, Also included is the polyphenyl ether 5P4E (an isomeric mixture of five phenyl rings connected by four ether links) currently under the investigation as a high traction fluids6 Theories of traction link high traction properties with large pressure coefficients of vis~osity.~ This implies, through the VTF equation
P / kbar 0.5
I .o l
-
o terphenyl
27(
-10
rl
= A exp[Bl(T-
7'011
(1)
a large value of dTg/dP. Since the possibility of empirically testing for high traction potential through simpleto-measure dTg/dP coefficients has not, to our knowledge, been explored, it seemed reasonable to include one such case in this project.
Experimental Section Materials. Commercial high-purity samples were used without purification; o-terphenyl and rn-terphenyl were from Eastman Kodak Co., d-sorbitol, cyclohexanol, and benzyl alcohol from Fisher Scientific Co. The sample of the lubricant 5P4E, which was studied because of the availability of high-pressure viscosity data: was provided by ONRS8 Gas-liquid chromatography showed that the purity of o-terphenyl, cyclohexanol, and benzyl alcohol was higher than 99.9 mol %. Chemical analysis of d-sorbitol certified the purity should be at least 99 mol %. Apparatus. The apparatus and procedures for the high-pressure differential thermal analysis (DTA) were essentially the same as described in earlier papers.lV2 The main problem to be solved in the present work was one of finding a pressure-transmitting medium which would not interact with the molecular liquids under study and which would remain liquid at their vitrification temperatures. A solution was found in the earlier observation that concentrated LiCl-H20 solutions, which are quite insoluble in organics, are very resistant to crystallization and have low vitrification temperatures which are only weakly dependent on pres~ure.~ A tendency of these solutions to give erratic warmup behavior at high pressures (perhaps due to liquid-liquid phase ~ e p a r a t i o n )was ~ eliminated by adding a small amount of ZnClz to the medium thereby converting the strongly hydrogen-bonding chloride ions into very weakly bonding ZnC1:- ions.1° The glass transition temperature of this solution is 145 K and its pressure dependence is only 0.030 K MPa-I (3 deg/kbar). As further protection, the samples were separated from the denser salt solution by a silicone rubber plug though this was not 100% effective, and some difficulty was encountered with gravitational displacement of sample material during a sequence of measurements. This tendency, which was minimized by maintaining a horizontal posture for the pressure tube and sample, lead frequently to less welldefined thermograms at higher pressures. The sample was contained in a thin-walled Pyrex glass tube (2 mm in i.d., 30-35 mm in length), into which a thin stainless steel sheathed, MgO-insulated Cr/Al thermocouple was inserted. The output of this thermocouple was opposed by that of a colinear reference thermocouple which terminated in the pressure-transmitting fluid. The glass transition of the latter was always sharply registered during warmup. The accuracy of the temperature measurement was within 0.5 K. Procedures. In the case of o-terphenyl, the effects of three different methods of pressure application were checked. (1) The pressure change was carried out while the sample was in the liquid state (above T,) and the latter then
26(
Tg/K
Tg/"C
- 20 25C
-30 24(
50
100
3
P/ M Pa Figure 1. Values of Tg,and shapes of DTA thermograms, for different pressure change-temperature change sequences, T, always being determined during heating run. Details in text.
vitrified. The glass transition temperature was measured during heating at the same pressure. These points are shown as open circles in Figure 1. (2) The sample was first vitrified under atmospheric pressure. The pressure was then increased to some chosen value, and the transition temperature measured during heating at the same pressure. Such data are represented by triangles in Figure 1. (3) The sample was pressurized while in the liquid state (T always > T,). It was then vitrified and the pressure released to some lower value, usually 1 atm, making sure that always T < Tr T, was then measured during heating under the final pressure. Such a point is indicated by the square in Figure 1,the pressure of the initial vitrification being indicated by the dotted line connection. Whether or not the data obtained from these different procedures are judged to differ from one another significantly depends on the way the resulting thermograms are analyzed. There are clear, and mostly expected, differences in the shapes of DTA traces observed in the different procedures. Examples of these are included in Figure 1. Because the sensitivity and baseline reproducibility of our measurements is not high, a quantitative analysis of these effects has not been feasible. The shape differences are real, however, and their significance needs to be borne in mind in interpreting the origin of the pressure dependence of T9. This will be discussed further below. With all other samples studied, including the extended (to 250 MPa) series on the o-terphenyl, procedure 1 described above was used exclusively.
Results The T,-pressure relations obtained in this study are shown in Figures 2-5. The DTA traces of glass transition of these samples were not generally as well defined or reproducible as were those for the ionic substances1v2 studied earlier. The reasons were not properly clarified though probably relate to sample-thermocouple configuration problems (including sample displacement) and to differences in thermal conductivity of the samples. The data above 100 MPa (1 kbar) were generally of poorer quality, and more weight should be placed on lower pressure results.
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The Journal of Physical Chemistry, Vol. 83, No. 25, 1979 p/kbar
300F-
290
05
10
Atake and Angel1 250~
I 5
2 0
'
i
/
/ 0
I
75
0-terphenyl ' + benzyl 1 3 0 alcohol solutions
0
1
0
1
?.40r0\
12"
2301
00 0 .
I
220
0,
\
1-40
o
I \
1-50 -60
280t
lo
o\
Tg/T
o - terphenyl
Tg/K 270
-
p/k bar
c x
190-
-
1"
32
z 180-
~-~ - -..2
'
150
100
--.
ic
