J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1217-1221 Thermophysical Properties of Liquid m-Xylene at High Pressures Mercedes Taravillo, Susana Castro, Valenth Garcia Baonza," Mercedes Caceres and Javier Nuiiez Departamento de Quimica Fkica ,Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, 28040-Madrid, Spain Experimental ppT measurements of rn-xylene obtained with an expansion technique are reported from 230 to 298 K and up to 110 MPa, or freezing pressure when lower. The experimental results have been correlated in terms of a recently proposed equation of state which has been used to calculate the isothermal compressibility, ,K~ and the thermal expansion coefficient, cc,, as functions of pressure and temperature. Experimental density results for liquid toluene, which was used to calibrate our experimental device, are also reported for the iso- therms of 223 and 303 K.The ability of simple power functions recently proposed to correlate high-pressure results has been tested. The extrapolation capabilities of these functions have been confirmed using experimen- tal results from the literature for the two compounds studied. Very recently we have reported experimental pp T measure-ments and derived thermodynamic properties of mesitylene obtained with an expansion technique.' We also studied the ability of simple power functions to represent the experimen- tal measurements as well as the possibility of using those functions for extrapolation. In this work we present an analogous study for rn-xylene for temperatures between 230 and 298 K and pressures up to 110 MPa.Experimental measurements for toluene, a com- pound which was used to calibrate the experimental device, are also given for the temperatures 223 and 303 K. The experimental results have been correlated using a recently derived equation of state (EOS) which has been used ,to calculate the isothermal compressibility, K~ and the thermal expansion coefficient, a,, of rn-xylene as functions of pressure and temperature. These quantities have been com- pared with data taken from literature up to 200 MPa. Besides the importance of the experimental results them- selves, which cover a wide ppT region not previously studied, the present work is inteded to confirm the usefulness of the functions referred to above as well as to test the performance of the equation of state.Experimental The experimental device is based on an expansion principle.2 Both method and apparatus have been described previously in the literat~re.~.~ Temperatures were measured with an accuracy of 0.01 K, by using a Leeds and Northup calibrated platinum resistance thermometer, and were referred to the international tem- perature scale ITS-90. Temperatures in both the high-pressure cell and the expansion cell baths were controlled electronically to within kO.01 K. High pressures were mea- sured with a Heise bourdon gauge with an absolute accuracy of 0.01 MPa together with a Sensotec TJE1108-20 transducer with an accuracy of ca.0.02%. Both devices were calibrated against a Desgranges et Huot 5403 dead-weight gauge. The low pressures reached in the expansion system were mea- sured with a Maywood P-102 transducer with an accuracy of 0.07Yo. A reference density for m-xylene, p (298.15 K, 0.1 MPa) = 8.1011 mol dm-3 was taken from ref. 5. The second virial coefficient at 333.15 K of m-xylene in the vapour phase was calculated from data of Cox and Andon.6 Although the apparatus is essentially the same as that used for mesitylene,' a calibration procedure was necessary due to a rearrangement of the expansion system. Since accurate measurements have been reported in the literature for toluene by many this substance was chosen to calibrate our experimental device.Three isotherms, 248.15, 273.15 and 298.15 K, were mea- sured and compared to direct measurements reported by .~Mopsik7 and Kashiwagi et ~1 at the same temperatures. Absolute differences in density were always less than 0.004 mol dm-3 with the data of Mopsik and 0.0025 with those of Kashiwagi. The second virial coefficient at 333.15 K of toluene in the vapour phase was calculated from data of Scott et a/.'' Two additional isotherms, those of 223.16 and 303.14 K, were measured in order to check the quality and consistency of the calibration procedure. The 48 experimental ppT points are recorded in Table 1. All of the data are plotted in Fig. 1. Differences in our density results along the 223 K isotherm compared with those reported by Mopsik7 and Muringer et aL9 at the same temperature are ca.0.002 mol dm-3 on average if the data are referred to the same density at 0.1 MPa. Toluene, with a purity greater than 99.5%, was sup- plied by Carlo Erba. The accuracy of the densities reported here for both toluene and rn-xylene is always greater than 0.003 mol dm-3. The uncertainty in xT is ca. 0.02-0.03 GPa-'. a, for m-xylene is accurate within 0.03 kK-' at high pressures and within 0.02 K-at effectively zero pressure. m-Xylene, with a purity greater than 99%, was supplied by Carlo Erba. 10.6 10.4 10.2 I E 0 10.0---.9.8P 9.6 9.4 I' 0 10 20 30 40 50 60 70 80 90 100 110 120 PIM Pa Fig. 1 Comparison of densities of toluene. (0)This work, Table 1 ; (0)this work, isotherms measured for calibration ;(0)ref.7; (0)ref. 8; (A) ref. 9. T = (a) 223.16, (b) 248.15, (c) 273.15, (d) 298.15 and (e) 303.14 K. Table 1 Experimental values of density, plmol dm-3, of liquid toluene for pressures, plMPa, and temperatures, T/K P P P P P P P P T = 223.16 102.82 10.606 74.0 1 10.482 46.63 10.362 16.78 10.218 99.31 10.591 70.56 10.467 43.40 10.347 14.05 10.204 95.68 10.575 67.05 10.452 40.25 10.333 11.37 10.190 92.01 10.560 63.58 10.437 37.14 10.319 8.67 10.175 88.40 10.545 60.15 10.422 33.99 10.304 6.02 10.160 84.81 10.529 56.72 10.407 30.9 1 10.289 3.43 10.145 81.22 10.513 53.34 10.392 27.49 10.273 1.08 10.132 77.65 10.498 50.00 10.378 19.59 10.233 0.13 10.126 T = 303.14 105.50 9.987 62.12 9.756 36.52 9.596 14.32 9.434 99.10 9.954 55.36 9.716 30.65 9.556 9.29 9.393 91.22 9.913 48.86 9.676 25.02 9.5 15 4.47 9.351 69.05 9.796 42.57 9.636 19.57 9.475 0.39 9.314 Results and Discussion Freezing Pressures The melting curve of m-xylene was measured in the range 8-101 MPa following the procedure described in ref.1 and 11. The results, taken along two different series, are recorded in Table 2. Coexistence pressures are accurate to within 0.6 MPa. The melting point obtained by extrapolation of the measurements at 0.1 MPa is (224.9 0.3) K in good agree- ment with values found in the literature: 225.27 K,12 225.28 K.13 ppT Results and Mechanical Coefficients The 182 experimental ppT points, 30 along an isobar and 152 along eight isotherms, are recorded in Tables 3 and 4.These results have been fitted to an EOS (hereafter referred to as EOS1) recently proposed by our group to represent experi- mental ppT data of 1iq~ids.l~ This EOSl is an extension of the expression derived by Alba et a1.” The derivation of both EOS starts by representing the iso- therms of a, as a function of the pressure, p, by a simple power function’ a,@) = a*@ -pJ-1/2 (1) where ps is the divergence pressure along the so-called spino- dal curve16 and a* is a proportionality constant. This expression together with the widely known experi- mental observation of intersections occurring at high pres- sures for the isotherms of aP,l7-l9 led Alba and co-workers to derive a simple expression for representing the general Table 2 Experimental freezing pressures for m-xylene 249.43 101.2 250.43 105.3 247.76 94.1 248.97 100.1 246.68 89.6 247.66 94.7 245.34 83.9 245.66 86.6 243.34 77.1 243.70 78.2 242.75 73.0 241.77 69.7 241.55 68.0 239.76 61.4 240.10 61.4 237.13 50.5 238.94 56.7 234.92 41.4 237.69 51.6 232.99 33.2 235.91 44.4 230.83 24.9 233.98 36.5 229.44 19.1 232.31 29.9 227.42 11.2 230.38 22.2 226.52 7.9 228.40 14.3 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Experimental values of density, p/mol dm-3, of liquid m-xylene at temperatures, T/K,along the isobar of 0.37 MPa T P T P 226.15 8.704 263.69 8.397 229.80 8.676 266.37 8.375 231.73 8.661 268.96 8.353 234.07 8.641 27 1.27 8.334 236.51 8.622 273.73 8.314 238.69 8.602 276.35 8.292 241.37 8.580 278.93 8.272 243.46 8.562 281.75 8.248 245.88 8.543 284.33 8.227 248.83 8.519 286.94 8.205 251.20 8.499 289.28 8.180 253.59 8.480 291.84 8.157 256.32 8.457 294.57 8.133 258.84 8.440 296.65 8.115 261.41 8.419 299.23 8.092 behaviour of ap on the following basis: (a) the intersection of the ap isotherms occurs at a single point, (b) the spinodal originates at the critical point and (c) p,(T) can be repre-sented by a [1/1] Pade approximant which expresses ps as a function of a reduced temperature, t = (1 -T/T,),where T,is the critical temperature.The final expression for ap as a func- tion of t and the pressure, p, is: with p, given by Pm = pc(a1t + w2 t + 1) (3) where a1 and a2 are characteristic parameters for each sub- stance. The EOS can be directly obtained by integration of the standard thermodynamic relation In V = apdT. Since the complete derivation of the EOS is fully described in ref. 14, we shall give here a brief description of the more relevant remarks. Performing the integration between a reference tem-perature T,and the temperature T, the whole ppT surface of the liquid can be obtained from any reference isotherm in molar volume V (or molar density p). The final expression for the EOS is the following In [ = iz 7(a + b)F::: i: +{[-I -[-] F} (4) where R = (1 -o)aoT,/a,, o= (a1/a2),LI = (a-po/pc), b = (P/Pc -4and x = -[(Po -PJ(P -P31l’’.The function F depends only on x, a and b and takes dif- ferent forms for positive and negative values of the product (ab) and, if we call c = I ab Ill2, it can be written: For (ab) > 0 1F(a, b, x) = -[arctan(xc/a) -arctan(x, c/a)] (5)C For (ab) < 0 1 [“+ xc)(a -x.31F(a,b, x) = -In (6)2~ (a -XCXU + X,C) Although eqn. (4) is relatively complex and a non-linear numerical procedure to determine the characteristic param- eters is required, it depends only on four parameters, all of them with a clear physical meaning. In our opinion, these J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Experimental values of density, p/mol dm-', of liquid m-xylene at pressures, p/MPa, and temperatures, T/K P P P P P P P P T = 229.95 13.67 8.741 7.01 8.7 10 3.55 8.693 0.14 8.677 10.80 8.728 T = 233.19 27.12 8.768 17.35 8.727 7.14 8.683 0.15 8.649 22.13 8.747 12.87 8.707 2.50 8.661 T = 238.15 55.33 8.842 39.77 8.783 24.35 8.720 7.50 8.646 50.24 8.823 34.55 8.762 17.72 8.691 3.61 8.626 44.98 8.803 29.60 8.742 12.85 8.669 0.32 8.608 T = 248.11 94.08 8.913 67.70 8.818 41.52 8.7 15 15.95 8.604 88.61 8.894 62.59 8.799 36.47 8.695 11.00 8.579 83.39 8.875 57.35 8.779 3 1.05 8.673 5.48 8.552 77.79 8.855 51.88 8.757 25.95 8.649 0.20 8.525 72.83 8.837 46.91 8.737 21.07 8.628 T = 263.15 109.31 8.871 78.05 8.755 47.10 8.629 15.59 8.483 104.27 8.852 72.93 8.736 41.12 8.606 10.54 8.456 99.08 8.834 67.60 8.7 15 36.69 8.583 5.22 8.428 93.90 8.815 62.49 8.694 3 1.38 8.559 0.17 8.400 88.60 8.796 57.33 8.674 26.03 8.534 83.22 8.775 52.24 8.652 20.92 8.509 T = 273.15 109.16 8.803 78.18 8.683 46.44 8.551 15.79 8.402 104.26 8.784 73.26 8.663 41.60 8.529 10.50 8.374 98.90 8.764 67.60 8.640 36.43 8.505 5.22 8.345 92.98 8.742 62.31 8.619 31.22 8.480 0.16 8.316 88.01 8.723 56.82 8.596 25.66 8.454 83.38 8.703 51.71 8.574 20.84 8.428 T = 283.17 109.28 8.737 78.08 8.615 46.80 8.480 15.33 8.321 104.32 8.717 72.51 8.593 41.38 8.455 10.22 8.292 98.65 8.696 67.93 8.572 36.25 8.429 5.14 8.263 93.49 8.676 62.09 8.548 31.09 8.403 0.14 8.233 88.30 8.656 56.89 8.527 25.88 8.376 83.01 8.635 51.81 8.504 20.60 8.349 T = 298.15 109.63 8.653 74.78 8.5 11 47.96 8.386 21.36 8.243 105.91 8.639 72.25 8.500 45.56 8.374 18.89 8.228 101.92 8.623 70.06 8.489 43.16 8.362 16.45 8.214 98.18 8.609 67.56 8.478 40.73 8.349 14.00 8.199 94.50 8.593 65.09 8.467 38.24 8.336 11.56 8.183 88.90 8.571 62.76 8.456 35.84 8.324 9.25 8.168 86.22 8.560 60.17 8.444 33.38 8.311 6.82 8.153 83.59 8.549 57.72 8.432 30.97 8.298 4.34 8.136 81.80 8.542 55.27 8.421 28.54 8.284 1.97 8.119 79.50 8.532 52.79 8.409 26.22 8.271 0.40 8.108 77.21 8.522 50.36 8.397 23.79 8.257 features justify its extensive use instead of other polynomial EOS, depending on a large number of parameters lacking in physical meaning, described in the literature.l4 In order to be consistent with the spinodal concept it is convenient to represent the reference isotherm by the follow- ing function (7) where p, is the divergence density along the liquid branch of the spinodal, K* is a proportionality constant related to the isothermal compressibility through the K&) = K*(P -ps)-0'85 (8) In summary, if eqn. (7) is used to represent the reference isotherm, the number of parameters required to represent the equation of state of a given liquid by means of EOSl is, except for the critical parameters, six: ao, po, u1 and a, for a,@, T), and p, and K* for the reference isotherm, since p, at the reference temperature is given by eqn. (3). The character- istic parameters of EOSl for m-xylene, obtained with a weighted least-squares procedure, are recorded in Table 5.1219 Table 5 Coefficients of EOSl determined by fitting the experimen- tal densities recorded in Tables 3 and 4 a,/kK-' po/MPa a, u2 p,/MPa" TJK" 0.904 24.14 -23.40 -1.067 3.56 619.0 ~~ " Ref. 22. These results have been obtained by using the isotherm of 298.15 K as the reference with parameters recorded in Table 6. Eqn. (7) reproduces the experimental densities of the refer- ence isotherm within 0.001 mol dm-3. The average deviation between experimental densities and those calculated from EOSl is 0.002 mol dm-', in agreement with our estimated uncertainty. From parameters a, and u2 we have calculated the diver- gence pressures at the experimental temperatures and each isotherm has been fitted to eqn.(7). The parameters p, and K* are also recorded in Table 6. Experimental densities are always reproduced within 0.001 mol dm-3. An interesting issue that deserves analysis is the influence of the reference isotherm on the values of the characteristic parameters of EOS1. Our experience reveals that this influ- ence is only significant when, as in the present case, a rela- tively small range of temperature is considered. In general, while variations in po are typically about 10-20 MPa, the values of a. do not change too much.14 Nevertheless, the global performance of the EOS, i.e. standard deviation in the density, derived properties, divergence pressures, etc. remains almost the same.Eqn. (2) takes into account the occurrence of a crossover of the a, isotherms at the point (ao,po) where (aolplaT),= 0. This assumption, as in the case of mesitylene,' is compatible with the estimated uncertainties of our a, results and the rela- tively narrow range of temperatures covered by our measure- ment~.~~This observation is in agreement with the relatively large uncertainty achieved in po . For other substances a clear displacement of the intersec- tions of the a, isotherms can be observed.'* However, it usually requires a temperature interval of 200 K or greater and an accuracy on a, of CQ. l-2%.'83'9 In such a case, one can follow an analogous scheme to that used to derive eqn. (4) by using the expression suggested by Ter Minassian et al.l8 for representing their a, measurements of toluene.Selected values of a, for m-xylene calculated from eqn. (2) at round values of pressures and temperatures are recorded in Table 7, and indicate, through the following standard ther- modynamic relation, (aC,/aP)T = -V/P)c@: + (aap/aT),l (9) that a minimum in the isobaric heat capacity C, is expected at ca. 130 MPa in the range of temperatures covered in this Table 6 Coefficients of eqn, (7) for the isotherms recorded in Tables 1 and 4 m-xy lene 298.15" 4.8758 38.909 -88.62 283.17 4.9764 37.881 -98.87 273.15 4.9994 37.869 -106.44 263.15 5.0 129 38.003 -114.66 248.11 5.0817 37.455 -128.52 238.15 5.1208 37.178 -138.84 233.19 5.1322 37.134 -144.38 toluene 303.14 5.4968 40.840 -81.84 223.16 5.2458 45.237 -181.02 * Data represent the reference isotherm which must be included in eqn.(4) to fit the ppT surface of m-xylene. Table 7 Thermal expansion coefficient, a$K -of liquid m-xylene computed from eqn. (2) at round values of pressure, p, and tem- perature, T p/MPa 233.15 243.15 253.15 263.15 273.15 283.15 293.15 0.0 0.96 0.97 0.98 0.99 1.00 1.02 1.03 10.0 0.94 0.94 0.95 0.95 0.96 0.97 0.97 20.0 0.92 0.92 0.92 0.92 0.92 0.92 0.92 50.0 -0.86 0.85 0.84 0.83 0.82 0.81 100.0 --0.75 0.74 0.72 0.70 0.68 work. Unfortunately, we have not found experimental data of C,at high pressures for rn-xylene but, in any case, owing to the appearance of the solid phase, a minimum in C, should not be observed experimentally for temperatures below 260 K.Selected values of IC~ for rn-xylene calculated from eqn. (4) at round values of pressures and temperatures are recorded in Table 8. Fig. 2 shows the IC~results computed by finite differences from the densities recorded in Table 4 at the tem- peratures 298.15 and 265.13 K. IC~values calculated from eqn. (4) are represented by continuous lines in order to show the reliability of derived thermodynamic properties obtained by using EOS1. Our results at effectively zero pressure agree well with values found in the literature for this substance (see Table 9). IC~values at high pressures obtained from experimental den- sities found in the literature are compared in the next section.Comparison with Other Measurements at Higher Pressures Eqn. (l), (7) and (8) described above have been used recently to correlate experimental measurements of mesitylene.' We 1 & 0.7 c I t 0-0.41 1 ULI0.30L, 40 60 PIMPa Fig. 2 Isothermal compressibility of m-xylene. Symbols : calculated by finite differences of densities recorded in Table 4. Lines: calculated from eqn. (4). T = (0)263.15 and (0)298.15 K. Table 8 Isothermal compressibility, K,/GPa- ', of liquid m-xylene computed from EOSl at round values of pressure, pand tem-perature, T p/MPa 233.15 243.15 253.15 263.15 273.15 283.15 293.15 0.1 0.58 0.60 0.63 0.67 0.71 0.77 0.83 10.0 0.54 0.56 0.59 0.62 0.66 0.71 0.76 20.0 0.50 0.53 0.55 0.58 0.62 0.66 0.70 50.0 -0.44 0.47 0.49 0.51 0.54 0.57 100.0 --0.37 0.39 0.41 0.42 0.44 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 9 Isothermal compressibility of m-xylene at 0.1 MPa from different sources T/K ref. 283.15 293.15 298.15 this work 0.77 0.83 0.86 21 0.788 -0.869 23 -0.846 0.874 24 -0.852 0.889 also studied the possibility of using the functions for extrapo- lation with satisfactory results. Since experimental measure- ments at high pressures are available in the literature for the two compounds studied here, at temperatures covered by our measurements, we present here a similar study to that of mesitylene. .~~Sun et~2 reported values of the thermophysical proper- ties of toluene from speed of sound data up to 260 MPa.Isothermal compressibilities are compared in Table 10 with those calculated from eqn. (8) with the parameters recorded in Table 6. Our results for pressures greater than 100 MPa are extrapolated values. Both sets of data agree within their estimated uncertainties. Yokohama etd2'reported high-pressure density results for rn-xylene at 283.15 and 298.15 K. Table 11 records the comparison between our isothermal compressibilities, calcu- lated from eqn. (8) with the parameters recorded in Table 6 Table 10 Comparison of isothermal compressibilities obtained for toluene in this work with those given in ref. 20 T = 223.15 K T = 303.15 K p/MPa this work ref. 20 this work ref. 20 0.1 0.55 0.545 0.95 0.965 20.0 0.50 0.499 0.80 0.802 40.0 0.45 0.460 0.69 0.689 60.0 0.42 0.427 0.6 1 0.605 80.0 0.39 0.399 0.55 0.541 100.0 0.36 0.375 0.50 0.490 120.0 0.34 0.354 0.46 0.449 140.0 0.32 0.335 0.44 0.414 160.0 0.3 1 0.318 0.40 0.385 180.0 0.29 0.303 0.37 0.360 200.0 0.28 0.290 0.35 0.338 220.0 0.27 0.277 0.33 0.319 240.0 0.26 0.266 0.32 0.302 260.0 0.25 0.256 0.30 0.287 Values of this work for pressures greater than 100 MPa are extrapo- lated values calculated from eqn.(8) using the parameters recorded in Table 5. Table 11 Comparison of isothermal compressibilities obtained for m-xylene in this work with those given in ref. 21. T = 283.15 K T = 298.15 K p/MPa this work ref. 21 this work ref. 21 lG.0 0.7 1 0.728 0.78 0.798 50.0 0.54 0.562 0.59 0.604 100.0 0.42 0.442 0.45 0.469 125.0 0.38 0.400 0.4 1 0.423 150.0 0.35 0.366 0.37 0.385 175.0 0.32 0.338 0.34 0.355 200.0 0.30 0.314 0.32 0.329 Values of this work for pressures greater than 100 MPa are extrapo- lated values calculated from eqn.(8) using parameters recorded in Table 5. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1221 and extrapolated up to 200 MPa, and those derived from the results of Yokohama. As in the case of toluene, differences are within the estimated uncertainty of this property. Differences in molar densities are always less than 0.2%. 2 3 4 W. B. Streett and L. A. K. Staveley, J. Chem. Phys., 1971, 55, 2495. J. C. G. Calado, P. Clancy, A. Heintz and W. B. Streett, J. Chem. Eng. Data, 1982,27, 376.V. G. Baonza, J. Nunez and M. Caceres, J. Chem. Thermodyn., 1989,21, 231. Summary and Conclusion 5 6 J. L. Hales and R. Townsend, J. Chem. Thermodyn., 1972,4,763. J. D. Cox and R. J. L. Andon, Trans. Faraday SOC., 1958, 54, Accurate density measurements obtained with an expansion technique have been reported for rn-xylene from 230 to 298 K and pressures up to 110 MPa. Toluene was used to calibrate our experimental device and density measurements along the isotherms of 223 and 303 K have also been reported for this liquid. The experimental results have been correlated in terms of an EOS which has been used to evaluate the mechanical coef- ficients a, and icT of this substance as functions of pressure and temperature. The most important feature of this EOS is that it depends on six adjustable parameters only, all of them 7 8 9 10 11 12 13 1622.F. I. Mopsik, J. Chem. Phys., 1969,50,2559. H. Kashiwagi, T. Hashimoto, Y. Tanaka, H. Kubota and T. Makita, Znt. J. Thermophys., 1982,3,201. M. J. P. Muringer, N. J. Trappeniers and S. N. Biswas, Phys. Chem. Liq., 1985, 14, 273. D. W. Scott, G. B. Guthrie, J. F. Messerly, S. S. Todd, W. T. Berg, 1. A. Hossenlop and G. J. Waddington, J. Phys. Chem., 1962,66,911. V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1992, %, 1932. K. S. Pitzer and D. W. Scott, J. Am. Chem. SOC., 1943,65,803. R. De Goede, G. M. Van Rosmalen and G. Hakvoort, Thermo- with a clear physical meaning. The ability of simple power functions recently proposed to correlate high-pressure results of density, isothermal com-pressibility and thermal expansion coefficient has been tested.In addition, the extrapolation capabilities of these functions have been confirmed with experimental results at higher pres- 14 15 16 chim. Acta, 1989,156,299. V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1993, 97,10813. C. Alba, L. Ter Minassian, A. Denis and A. Soulard, J. Chem. Phys., 1985, 82, 384. V. P. Skripov, in Metastable Liquids, Wiley, New York, 1974, p. 226. sures found in the literature for the two compounds studied in this work. Since the complete equation of state (EOS1) is directly related to these power functions, their extrapolation capabilities can be directly transferred to the EOS. The results of this work together with those of ref. 1 17 18 19 20 V. G. Baonza, M. Ciceres and J. Nuiiez, J. Phys. Chem., 1993, 97,2002. L. Ter Minassian, K. Bouzar and C. Alba, J. Phys. Chem., 1988, 92, 487. Ph. Pruzan, J. Phys. Lett., 1984,45, L-273. T. F. Sun, S. A. R. C. Bominaar, C. A. ten Seldam and S. N. suggest the general application of EOSl to represent high- pressure thermophysical properties of normal liquids over wide ranges of temperature and pressure. 21 22 Biswas, Ber. Bunsenges. Phys. Chem., 1991,95,696. C. Yokohama, S. Moriya and S. Takakashi, Fluid Phase Equilib., 1990,60,295. K. H. Simmrock, R. Janowsky and A. Ohnsorge, in Critical Data This work was supported by CICYT (M.E.C., Spain), Project NO.: PB92-0553. 23 for Pure Substances, Chemistry Data Series 11, DECHEMA, Frankfurt, 1986. D. Tyrer, J. Chem. SOC., 1914,105,2534. 24 A. J. Richard and P. B. Fleming, J. Chem. Thermodyn., 1981,13, References 863. 1 V. G. Baonza, M. Caceres and J. Nuiiez, J. Chem. SOC., Faraday Trans., 1994,90, 553. Paper 4/001256; Received 10th January, 1994