doi:10.1006/jcht.2000.0778Available online at http://www.idealibrary.com on

J. Chem. Thermodynamics2001, 33, 775–786

Excess molar enthalpies of (carbon dioxide+ethene) in the liquid and near-critical regions

C. J. Wormalda and J. M. EyearsSchool of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.

Flow calorimetric measurements of the excess molar enthalpyHEm for {xCO2+ (1− x)

C2H4} are described. The measurements cover the rangeT = 260.95 K to T = 315.75 Kat pressures from 3.5 MPa to 12.6 MPa. Liquid phase measurements were made at thepressure 11.0 MPa over the rangeT = 260.95 toT = 300.45 K, and at pressures slightlygreater than the saturation pressure of ethene or the critical pressure locus for the mixture.At x = 0.5 and at the four temperatures (285.85, 300.45, 306.25, and 315.75) K, excessenthalpy measurements were made over a range of pressures from 3.0 MPa to 12.6 MPa.The measurements at (285.85 and 300.45) K traverse the critical locus for the mixture,and those at (306.25 and 315.75) K are in the supercritical region. The measurements arecompared with curves generated from the Patel–Teja equation of state, which does notquite fit the measurements but reproduces the main features of the observed behaviour.c© 2001 Academic Press

KEYWORDS: excess enthalpy; critical region; carbon dioxide; ethene; flow calorimetry

1. Introduction

A flow mixing calorimeter capable of operation at temperatures fromT = 220 K toT = 330 K and pressures up to≈12 MPa was previously described.(1) The calorimeter wastested by making excess enthalpy measurements on (carbon dioxide+ toluene), and theresults were found to be in good agreement with other work.(2) The calorimeter was thenused to make extensive measurements(3–5) of HE

m for {xCO2+ (1− x)C2H6} in the liquid,gas, two-phase, and near-critical regions. Similar measurements on{xCO2+ (1− x)C2H4}

are now reported.The (p, T) phase diagram for{xCO2+ (1− x)C2H4} is illustrated in figure1, which

shows the critical locus and the vapour pressure curves for carbon dioxide (lower curve)and that for ethene (upper curve). The critical temperature and pressure of carbon dioxideare(6) Tc = 304.13 K, and pc = 7.375 MPa, and the values for ethene are(7) Tc =

282.34 K, and pc = 5.040 MPa, respectively. The critical locus has been studied byRowlinson and Sutton(8) and the critical points for the mixture at 4 compositions are listedin the compilation of the critical properties of binary mixtures by Hickset al.(9) Unlike

aTo whom correspondence should be addressed (E-mail: chris.wormald@bristol.ac.uk).

0021–9614/01/070775 + 12 $35.00/0 c© 2001 Academic Press

776 C. J. Wormald and J. M. Eyears

14

10

6

2

260 280T/K

p/M

Pa

300 320

FIGURE 1. The(p, T) phase digram for (carbon dioxide+ ethene). The critical locus lies betweenthe pure component critical points which are indicated by circles. The upper vapour pressure curveis for ethene and the lower is for carbon dioxide. Squares indicate thermodynamic states at whichmeasurements have been made over a range of composition. Vertical lines indicate ranges of pressureover which measurements atx = 0.5 have been made.

{xCO2+ (1− x)C2H6}, for which the critical temperatures of the pure components arealmost the same but for which the critical temperature of the mixture atx = 0.5 exhibits aminimum 14 K below the mean of the pure component values, the criticalTc(x) andpc(x)loci of {xCO2+ (1− x)C2H4} are both smooth curves without minima as shown by thecritical locus in figure1. Dew and bubble point curves have been measured by Haseldenet al.(10,11) Mollerup(12) has reported isothermal (vapour+ liquid) equilibrium (v.l.e.) dataand has compared the dependence of the azeotropic composition and Gibbs energy onpressure and temperature with that for (carbon dioxide+ ethane). Although the mixture hasa minimum boiling azeotrope, as has (carbon dioxide+ ethane), the minima in the(x, T)v.l.e. loops are not as far below the mean of the two pure component vapour pressures asthey are for (carbon dioxide+ ethane). AtT = 243 K the carbon dioxide mole fractioncomposition of the azeotrope is 0.25 and atT = 284 K, where it joins the critical locus ofthe mixture, the mole fraction is 0.43.

HEm for (carbon dioxide+ ethene) 777

Gas phase measurements ofHEm for {xCO2+ (1− x)C2H4} have been made by Ba

et al.(13) at temperatures from 298.15 K to 338.15 K and at pressures up to 4.835 MPa.Comparison with the Benedict–Webb–Rubin (BWR) and the Redlich–Kwong (RK)equations of state was made and it was shown that the RK equation gave a better fit thanthe BWR equation. TheHE

m(x) curves were found to be almost parabolic at all pressures.

2. Experimental

The apparatus was the same as that used to makeHEm measurements on{xCO2+ (1− x)

C6H5CH3}.(1) The flow-mixing calorimeter was fitted with both a Peltier cooling deviceand a heater. The carbon dioxide used was(0.999 CO2+ 0.001 N2) and the ethene usedwas (0.999 C2H4+ 0.001 C2H6). The materials were used without further purification.The pure components were stored as liquids in cylindrical vessels of 0.5 dm3 capacitymaintained atT = (253.2± 0.1) K. They were displaced from these vessels by mercurypumped from similar cylinders, and mercury in turn was displaced by water from meteringpumps operating under computer control. After passing through the mixing calorimeter thefluid mixture entered a reservoir of 2 dm3 volume which was back pressured with nitrogengas. The reservoir was fitted with a motorized needle valve operated under computercontrol. As the fluid mixture was pumped into the reservoir the pressure tended to rise. Thisincrease in pressure was sensed by a pressure transducer linked to the control computer,and when it had risen by 2 kPa the needle valve was opened to release gas and restorethe pressure to the required value. The molar flow rate and the composition of the mixturewere calculated from the flow rate of water entering the metering pumps, the densitiesof the liquified gases atT = 253.2 K, and the pressure chosen for the experiment. Thedensities of carbon dioxide were taken from reference (14) and the densities of ethenefrom reference (15).

The composition dependence ofHEm was investigated by scanning the mole fractionx

at chosen temperature and pressure under computer control. Scans were begun by passingpure carbon dioxide through the apparatus, and were completed by passing pure ethene.This procedure checked that the zero heat-flux baseline obtained at the outset by passinga pure fluid though the calorimeter had not drifted during the course of the experiment.Runs on the pure components were also used to check the calibration of the Peltier device.The computer was programmed to make measurements at mole fraction increments of 0.05from x = 0.05 to x = 0.95, though because of the complicated nature of the apparatusthese compositions were not always achieved.

The pressure dependence ofHEm(x = 0.5) was investigated by beginning with measure-

ments at high pressure and incrementally reducing the pressure under computer control.Chosen pressures could be achieved to within±2 kPa, and the temperature of the bathcontaining the calorimeter was controlled to±10 mK. Small fluctuations in temperature,pressure, and flow rate caused corresponding fluctuations in the temperature of the mixtureleaving the calorimeter. These were reduced by using the computer to sample the output at10 s intervals to average the signal. The uncertainty on the excess enthalpies is±2 per cent.Occasionally, excessive noise on the signal resulted in the standard deviation of the calcu-latedHE

m values being unacceptably large, and these runs were rejected by the computer.

778 C. J. Wormald and J. M. Eyears

TABLE 1. Measurements of the excess molar enthalpyHEm of {xCO2 + (1− x)C2H4} at

11.0 MPa made over a range of compositionsx at temperatures from 260.95 K to 306.62 K

xHE

m

J ·mol−1x

HEm

J ·mol−1x

HEm

J ·mol−1x

HEm

J ·mol−1x

HEm

J ·mol−1

T = 260.95 K

0.396 714 0.473 756 0.396 772 0.636 750

0.433 741 0.515 791 0.595 759

T = 276.65 K

0.149 338 0.339 700 0.548 734 0.699 631 0.849 377

0.297 602 0.447 720 0.598 722 0.750 584 0.899 246

0.349 645 0.497 734 0.648 688 0.799 499 0.948 126

T = 285.5 K

0.148 361 0.310 620 0.449 730 0.697 682 0.847 399

0.198 448 0.362 660 0.499 741 0.749 600 0.900 295

0.260 555 0.405 703 0.598 750 0.799 503 0.951 98

T = 300.45 K

0.049 103 0.247 592 0.447 830 0.648 812 0.849 452

0.098 229 0.297 665 0.497 831 0.699 726 0.899 300

0.149 373 0.349 730 0.549 830 0.750 659 0.948 139

0.199 473 0.399 792 0.598 847 0.799 561

T = 306.62 K

0.099 305 0.298 739 0.499 981 0.749 799 0.950 155

0.148 442 0.348 824 0.599 989 0.799 701

0.199 544 0.399 880 0.650 968 0.847 599

0.249 641 0.447 940 0.699 898 0.899 337

3. Results and discussion

The temperatures and pressures at which measurements were made are marked on figure1.Squares indicate the temperatures and pressures at whichHE

m measurements were madeover a range of composition, and vertical lines indicate the pressure ranges over whichmeasurements atx = 0.5 were made. Three sets of measurements have been made. Table1lists measurements at five temperatures made over a range of compositions at the pressure11.0 MPa, and these are plotted in figure2(a to e). Table2 lists measurements made overa range of compositions at pressures close to the saturation pressure of carbon dioxide andclose to the critical locus of the mixture, and these are plotted in figures3 to 5. Table3lists measurements made atx = 0.5 and over a range of pressures, and these are plotted infigures6 to 8.

The measurements on (carbon dioxide+ ethane) reported previously(3–5) were com-pared with the Patel–Teja(16) equation of state which, although not exactly fitting any ofthe sets of measurements, reproduced all the main features of theHE

m(x) curves. The mea-

HEm for (carbon dioxide+ ethene) 779

TABLE 2. Excess molar enthalpiesHEm for {xCO2 + (1 − x)C2H4} measured over a range

of compositionsx at temperatures from 260.95 K to 305.75 K and at pressures form 3.50 to8.05 MPa

xHE

m

J ·mol−1x

HEm

J ·mol−1x

HEm

J ·mol−1x

HEm

J ·mol−1x

HEm

J ·mol−1

T = 260.95, p = 3.5 MPa

0.322 643 0.405 783 0.483 805 0.604 779 0.763 562

0.362 716 0.443 800 0.525 793 0.684 685

T = 276.65 K, p = 4.75 MPa

0.158 507 0.360 851 0.523 858 0.682 720 0.841 429

0.199 604 0.403 853 0.563 855 0.723 646 0.880 332

0.239 691 0.447 885 0.602 811 0.762 590 0.920 217

0.320 822 0.484 886 0.643 780 0.802 500 0.959 102

T = 285.85 K, p = 6.25 MPa

0.157 490 0.318 936 0.478 1059 0.641 892 0.801 547

0.198 595 0.358 1024 0.521 1053 0.680 829 0.839 410

0.238 703 0.401 1055 0.561 1007 0.721 737

0.281 820 0.439 1074 0.600 957 0.760 648

T = 300.45 K, p = 7.50 MPa

0.036 131 0.237 1242 0.437 2043 0.639 2249 0.839 954

0.075 328 0.279 1422 0.477 2131 0.679 2148 0.878 675

0.119 600 0.317 1586 0.519 2200 0.719 1925 0.919 415

0.156 768 0.357 1762 0.559 2286 0.759 1639 0.958 210

0.197 1021 0.399 1895 0.598 2309 0.799 1295

T = 300.45 K, p = 6.57 MPa

0.348 585 0.501 630 0.652 582 0.750 471 0.850 340

0.451 631 0.602 610 0.669 539 0.799 418 0.899 247

T = 305.75 K, p = 8.05 MPa

0.165 613 0.332 1360 0.500 1943 0.666 2204 0.833 2028

0.205 904 0.375 1580 0.542 2076 0.707 2340 0.875 1764

0.249 991 0.417 1681 0.585 2157 0.750 2223 0.916 1254

0.291 1304 0.458 1833 0.626 2194 0.791 2176 0.928 887

surements listed in tables1 and2 have also been compared with the Patel–Teja equation.It was shown previously(17) that the Patel–Teja equation is a better fit to saturated liquidand vapour densities than many similar cubic equations of state. The fit to residual volumesor enthalpies is not perfect, and one would not expect it to fit values of excess volumes orexcess enthalpies over anything but a small range of temperatures or pressures. Patel(18)

has recently suggested an improved expression for the temperature dependence of theα

function of the equation. This more complex form gives a better fit to vapour pressures,

780 C. J. Wormald and J. M. EyearsH

E m/(

kJ. m

ol−

1 )

1.0

0.8

0.6

0.4

0.2

00.0 0.5

a b c d e

x (CO2) x (CO2) x (CO2) x (CO2) x (CO2)

1.0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0

FIGURE 2. Excess enthalpiesHEm for {xCO2 + (1− x)C2H4} at the pressurep = 11.0 MPa made

at: ◦, Table 1. Curves were calculated from the Patel–Teja equation of state with values of theinteraction parameterki j listed in table4. a, T = 260.95 K; b, T = 276.65 K; c, T = 285.85 K;d, T = 300.45 K; e,T = 306.25 K.

HE m

/(kJ

. mol

−1 )

a1.0

0.8

0.6

0.4

0.2

0.00.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6

xx

0.8 1.0

1.0

0.8

0.6

0.4

0.2

0.0

b

FIGURE 3. Excess enthalpiesHEm for {xCO2+ (1− x)C2H4} at: a,T = 260.95 K, p = 3.50 MPa;

and b,T = 276.65 K, p = 4.75 MPa in the liquid region.◦, Table2. Curves were calculated fromthe Patel–Teja equation of state with the values ofki j listed in table4.

liquid densities and liquid heat capacities of both polar and nonpolar compounds, and in afuture publication, when excess volumes for (carbon dioxide+ ethene) are reported, com-parison with our data will be made. For the present purposes the original form of theα

function is adequate. The method for the calculation of the residual enthalpy was givenpreviously.(17)

To obtain the mixture parametersa, b, andc for the Patel–Teja equation of state,(16) the1-fluid combining rules were used:

a = x2· a1 · α1+ x(1− x)(1− k12)(a1 · α1+ a2 · α2)

1/2+ (1− x)2a2 · α2, (1)

HEm for (carbon dioxide+ ethene) 781

HE m

/(kJ

. mol

−1 )

a b

1.0

0.8

0.6

0.4

0.2

0.0

2.0

1.5

1.0

0.5

0.00.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

x x

FIGURE 4. Excess enthalpiesHEm for {xCO2+ (1− x)C2H4} at: a,T = 285.85 K, p = 6.25 MPa;

and b,T = 300.45 K, p = 7.50 MPa at thermodynamic states just above the critical locus.◦, table2.Curves were calculated from the Patel–Teja equation of state with the values ofki j listed in table4.

HE m

/(kJ

. mol

−1 )

a b0.6

2.0

1.5

1.0

0.5

0.0

0.4

0.2

0.00.0 0.2 0.4 0.6 0.8 1.0

x

0.0 0.2 0.4 0.6 0.8 1.0

x

FIGURE 5. Excess enthalpiesHEm for {xCO2 + (1− x)C2H4} at: a,T = 300.45 K, p = 6.57 MPa

in the gaseous region close to the saturation pressure of carbon dioxide; and b,T = 306.25 K,p = 8.05 MPa in the supercritical region.◦, Table2. Curves were calculated from the Patel–Tejaequation of state with the values ofki j listed in table4.

782 C. J. Wormald and J. M. Eyears

TABLE 3. Measurements of the excess molar enthalpyHEm of (0.5CO2 + 0.5C2H4) made over the

ranges of pressurep shown by the vertical lines in figure1 and of temperatureT

T

K

p

MPa

HEm

J ·mol−1

p

MPa

HEm

J ·mol−1

p

MPa

HEm

J ·mol−1

p

MPa

HEm

J ·mol−1

p

MPa

HEm

J ·mol−1

285.85 3.49 63 4.21 144 5.03 4195 5.60 4109 8.51 736

3.79 95 4.62 849 5.18 4262 5.81 1168 10.0 575

4.00 123 4.84 4284 5.39 3944 7.01 873 11.5 662

300.45 2.98 63 6.23 491 7.02 284 7.79 1922 10.1 940

4.00 100 6.45 639 7.23 2579 8.30 1540 11.1 854

5.03 202 6.61 1390 7.44 2327 8.61 1300 12.1 716

6.02 397 6.82 3056 7.64 2050 9.03 1121

306.25 3.48 109 5.48 202 6.95 486 8.44 1741 10.0 1208

5.08 181 5.69 226 7.45 862 8.94 1591 11.0 956

5.28 184 5.89 242 7.94 2139 9.50 1340 12.5 818

315.75 3.00 75 6.05 208 8.30 516 10.1 1099 12.0 846

4.00 109 7.08 313 8.91 722 10.7 1016 12.6 765

5.01 149 7.68 408 9.52 1053 11.3 896

HE m

/(kJ

. mol

−1 ) 4.0

2.0

0.04 6 8

p/MPa

10 12 14

FIGURE 6. Excess enthalpiesHEm for (0.5CO2+0.5C2H4) at T = 285.85 K measured over a range

of pressures.◦, Table3. The continuous curve was drawn with a flexicurve. The broken curve wascalculated from the Patel–Teja equation of state withki j = 0.06.

HEm for (carbon dioxide+ ethene) 783

HE m

/(kJ

. mol

−1 )

3.0

2.0

1.0

0.04 6 8

p/MPa

10 12 14

FIGURE 7. Excess enthalpiesHEm for (0.5CO2+0.5C2H4) at T = 300.45 K measured over a range

of pressures.◦, Table3. The continuous curve was drawn with a flexicurve. The broken curve wascalculated from the Patel–Teja equation of state withki j = 0.06.

where

α = {1+ F(ω)}(1− T1/20 )2, (2)

and

F(ω) = 0.480+ 1.574ω − 0.176ω2, (3)

whereω is the acentric factor. Parameterb was obtained from

b = x · b1+ (1− x)b2, (4)

and a similar expression was used to obtainc. The parameterk12 in equation (1) isadjustable. Measurements ofHE

m andVEm for {xCO2+ (1− x)C2H6} reported previously

were compared with curves calculated usingk12 = 0.1325 which gave the best overall fit.This value did not fit the excess function measurements individually, and to achieve this itwould have been necessary to allowk12 to vary by as much as 20 per cent. In this paper wehave chosen to compare our measurements ofHE

m(x) with curves calculated with values ofk12 which best fit each individual set, and these are listed in table4.

The table1 measurements, which are all liquid phase measurements made at thepressure 11.0 MPa, are plotted against mole fraction in figures2(a to e). All the curvesare slightly skewed parabolas with maxima at carbon dioxide mole fraction compositionsclose to 0.55. The maxima increase with increasing temperature. The broken curves shownin the figure were calculated from the Patel–Teja equation of state with values of theinteraction parameterki j listed in table4. Except for the measurements atT = 306.25 Kthe values ofki j were found to diminish with increase in temperature. The measurementsat T = 306.25 K were made on a mixture of two dense supercritical fluids, rather than twoliquids, and the value ofki j = 0.0325 does not quite follow the trend in the temperaturedependence of the values ofki j found at lower temperatures. Experimental difficultiesat T = 260.95 K resulted in successful measurements only in the central region of the

784 C. J. Wormald and J. M. EyearsH

E m/(

kJ. m

ol−

1 )

3.0

2.0

1.0

0.04 6 8

T = 306.25 K

T = 315.75 K

p/MPa

10 12 14

FIGURE 8. Excess enthalpiesHEm for (0.5CO2+0.5C2H4)measured in the supercritical region over

a range of pressures atT = 306.25 K (upper curve), andT = 315.75 K (lower curve). For clarity theupper curve has been displaced upwards by adding 500 J·mol−1 to the table3 values.◦, Table3.The continuous curves were drawn with a flexicurve. The broken curves were calculated from thePatel–Teja equation of state withki j = 0.06.

composition range, but these are sufficient. In a previous paper(3) measurements ofHEm for

(carbon dioxide+ ethane) at the thermodynamic states (T = 248.1 K, p = 10.9 MPa),(T = 272.1 K, p = 11.0 MPa), and (T = 308.4 K, p = 11.0 MPa) were reported. Thevalues ofHE

m(x = 0.5) for this mixture are almost 50 per cent larger than those for (carbondioxide+ ethene). This difference may be due to the different quadrupole forces betweenthe molecules. For ethene, ethane and carbon dioxide the quadrupole moments are(19)

5.3 · 10−40 C ·m2,−3.34 · 10−40 C ·m2 and−13.4 · 10−40 C ·m2, respectively. To a firstorder approximation, the quadrupole force between two molecules is proportional to theproduct of the square of their quadrupole moments, and it is evident that the quadrupoleforces in carbon dioxide are much stronger than in the other two fluids. The major dif-ference between the two mixtures is in the sign of the quadrupole moments of ethene andethane. To attempt an interpretation of the heats of mixing without information about thestructure would be naive. Adya and Wormald(19) have performed inelastic neutron scat-tering measurements on liquid phase CO2, C2D4, C2D6, and on(0.5CO2+ 0.5C2D4) and(0.5CO2+ 0.5C2D6) at T = 220 K. The measurements on(0.5CO2+ 0.5C2D4) are con-sistent with a nearly parallel orientation of CO2 and C2D4 molecules, as might be expectedfor quadrupoles of opposite sign. For(0.5CO2+ 0.5C2D6) the evidence suggests that the

HEm for (carbon dioxide+ ethene) 785

carbon–carbon axis of the ethane molecules is preferentially aligned perpendicular to theaxis of the carbon dioxide molecules. Only machine simulation can relate the structuralinformation to the excess enthalpy and volume, and this has not yet been attempted.

Table2 lists six composition scans. The first two, at the thermodynamic states (T =260.95 K, p = 3.5 MPa), and (T = 276.65 K, p = 4.75 MPa) are at pressures whichare about 0.3 MPa greater than the saturation pressure of carbon dioxide. The second two,at the thermodynamic states (T = 285.85 K, p = 6.25 MPa), and (T = 300.45 K,p = 7.5 MPa), are at pressures of about 0.3 MPa above the critical locus of the mixture.The measurements at (T = 300.45 K, p = 6.57 MPa) are in the gaseous region ata pressure of about 0.2 MPa less than the saturation temperature of ethene. Finally themeasurements at (T = 305.75 K, p = 8.05 MPa) are in the supercritical region at atemperature 1.62 K above the critical temperature of carbon dioxide. The measurementsare shown in figures3, 4 and5. Again the shapes of the curves are almost reproduced bythe Patel–Teja equation of state with values ofki j chosen to fit the maximum values ofHE

m,and again the calculated curves are displaced towards the carbon dioxide-rich side of thecomposition range.

Table3 lists four sets of measurements at constant temperature and atx = 0.5 madeover a range of pressure as shown by the vertical lines in figure1. These measurements areplotted against pressure in figures6 to 8, where the continuous curves through the pointswere drawn with a flexicurve and the broken curves were calculated from the Patel–Tejaequation of state withki j = 0.06. The calculated curves give a good fit to the low pressuremeasurements below 5 MPa, and reproduce the main features of the experimental curves,but are a poor fit to the high pressure measurements. Figure6 shows measurements atthe temperature 285.85 K which cover the pressure range from 3.49 MPa to 11.5 MPa.As shown in figure1 the measurements at 3.49 MPa, and pressures up to the saturationpressure (4.81 MPa) of carbon dioxide, are in the gaseous region. Above the saturationpressure the carbon dioxide enters the mixing calorimeter as a liquid which mixes withgaseous supercritical ethene to form a gaseous mixture. The excess enthalpy therefore has acontribution arising from the enthalpy of vaporization of carbon dioxide, and is large. Thisbehaviour continues until the pressure reaches the critical locus atp = 5.83 MPa. Abovethis pressure the liquid carbon dioxide entering the calorimeter mixes with supercriticalethene to form a liquid phase, and the excess enthalpy is the enthalpy of formation of aliquid mixture. As expected, the value ofHE

m decreases with increase in pressure.The measurements atT = 300.45 K shown in figure7 exhibit similar behaviour. The

saturation pressure of carbon dioxide at this temperature is 6.75 MPa and the critical locusof the mixture is at 7.20 MPa. As the enthalpy of vaporization of carbon dioxide at thistemperature is less than that atT = 285.85 K this contribution toHE

m is smaller and thepeak height is about 3 kJ·mol−1 rather than 4.3 kJ·mol−1. As the critical point of carbondioxide is approached the peak becomes narrower and smaller in amplitude.

Figure8 shows measurements in the supercritical region at the temperatures 306.25 K(upper curve), and 315.75 K (lower curve). To accommodate both curves in the same figure500 J·mol−1 have been added to theT = 306.25 K measurements. The figure shows thatabove the critical point of both fluids, at the temperature 306.25 K, there is still a maximumin the excess enthalpy. A similar behaviour was observed for(0.5CO2+ 0.5C2H6)

786 C. J. Wormald and J. M. Eyears

TABLE 4. Values ofk12 which best fit the measurementslisted in tables1 and2 at pressuresp and temperaturesT

T/K p/MPa k12 T/K p/MPa k12

260.95 11.0 0.037 260.95 3.50 0.040

276.65 11.0 0.034 276.65 4.75 0.038

285.85 11.0 0.031 285.85 6.25 0.034

300.45 11.0 0.027 300.45 6.57 0.065

306.25 11.0 0.0325 300.45 7.50 0.026

306.25 8.05 0.045

reported previously,(4) where the origin of the maximum was shown to be a consequenceof the way the residual enthalpy curves for the pure components and that of the mixturecombine. The maximum does not imply any special clustering of the molecules.

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dynamics1983, 15, 747–755.3. Wormald, C. J.; Eyears, J. M.J. Chem. Thermodynamics1988, 20, 323–331.4. Wormald, C. J.; Eyears, J. M.J. Chem. Soc. Faraday Trans. 11988, 84, 1437–1445.5. Wormald, C. J.; Hodgetts, R. W.J. Chem. Thermodynamics1997, 29, 75–85;

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(Received 24 March 2000; in final form 6 October 2000)

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