Heat Capacities and Derived Thermodynamic Functions of n

Relaxations in the metastable rotator phase of n-eicosane. C. Di Giambattista , R. Sanctuary , E. Perigo , J. Baller. The Journal of Chemical Physics ...
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J. Chem. Eng. Data 1999, 44, 715-720

715

Heat Capacities and Derived Thermodynamic Functions of n-Nonadecane and n-Eicosane between 10 K and 390 K J. Cees. van Miltenburg,*,† Harry A. J. Oonk,† and Valerie Metivaud‡ Chemical Thermodynamics Group, Utrecht University, Padualaan 8, NL-3584 CH Utrecht, The Netherlands, and Centre de Physique Mole´culaire Optique et Herzienne, URA 283 au CNRS, Universite´ Bordeaux I, 351, Cours de la Libe´ration, F-33405 Talence Cedex, France

Heat capacity measurements were made on n-nonadecane and n-eicosane from 10 K to 390 K with an adiabatic calorimeter. These measurements were used to calculate the entropy and the enthalpy relative to 0 K. The sum of the enthalpy of the solid-solid transition and of melting of n-nonadecane is 62659 ( 20 J‚mol-1 at the triple point, 305.00 K. The enthalpy of melting of n-eicosane was calculated to be 69922 ( 100 J‚mol-1 at the triple point, 309.65 K. The entropies were extrapolated from the liquid phase just above the melting point to 298.15 K and found to be respectively 715.8 and 748.1 J‚K-1‚mol-1. The crystallization behavior of both compounds is discussed.

Introduction The low-temperature thermodynamic functions of the n-alkanes have been measured by several groups between the 1930s and the 1960s. The research on the hydrocarbons was encouraged by the oil industry. The complete set of n-alkanes, ranging from ethane to n-octadecane, was measured (Parks et al., 1930; Huffman et al., 1937; Parks et al., 1937; Stull, 1937; Witt and Kemp, 1937; Kemp and Egan, 1938; Aston and Messerly, 1940; Parks et al., 1949; Finke et al., 1954; Schaerer et al., 1954; Messerly et al., 1967). The availability of such a set of data on a homologous series of compounds offers the possibility to look for relations such as group contributions and dependence of thermodynamic properties on the number of carbon atoms in the linear molecule. One important property, the absolute entropy S°(T), is calculated by integrating the measured heat capacities divided by the temperature between 0 K and the stated temperature. Messerly et al. (1967) published a critical evaluation of the available entropy values at 298.15 K and proposed an improved quadratic fit for the n-alkanes ranging between n-pentane and n-octadecane. It is however always risky to extrapolate fitted data out of the measured range, and one of the objects of this study is to see if this extrapolation is warranted. More recently, there is renewed interest in the n-alkane series, especially for the higher members of the series, as shown by the literature (Barbillon et al., 1991; Mondieig et al., 1997; Oonk et al., 1998). Much work has been done on the study of phase diagrams formed by binary mixtures of neighbors and next-nearest neighbors of the n-alkanes. We work on these kinds of systems, together with other research groups united in the REALM (Re´seau Europe´en sur les Alliages Mole´culaires). The properties of the nalkanes seem to make them good candidates for the storage of thermal energy. In this application, the cooling behavior of the compounds is very important. One of the requirements is little or no subcooling in the bulk phase. That is * Corresponding author. Fax: +31 302533946. E-mail: miltenb@ chem.uu.nl. † Utrecht University. ‡ Universite ´ Bordeaux.

the reason that we report cooling curves made by adiabatic calorimetry. Experimental Section C19H40 was purchased from Fluka Chemica, with an estimated purity of better than 99%. C20H42 came from Aldrich with a stated purity of 99%. Measurements were performed on 4.20 and 3.65 g, respectively. The hydrocarbons were sealed in a gold-plated copper calorimeter vessel. After a short evacuation, about 1000 Pa of helium gas was added in order to improve heat conduction. The calorimeter used (laboratory-design indication CAL VII) was built as a copy of CALV, which has been described before (Miltenburg et al., 1987). The temperature calibration of the platinum-resistance thermometer used was corrected to the ITS-90 temperature scale (Preston-Thomas, 1990). The heat capacities are estimated from measurements on n-heptane and synthetic sapphire to deviate within (0.02% and have an accuracy of (0.2%. Often, the thermal history of the sample has an influence on the heat capacity measurements. With these compounds, however, no differences were found between the different series at the same temperatures. For each compound, two cooling curves, including the crystallization, were measured. In these cases an amount of about 5000 Pa of helium was admitted into the separate vacuum chamber of the calorimeter which surrounds the vessel and the shields. This resulted in a cooling rate, outside the crystallization region, of about 1 deg‚min-1. Results and Discussion Our experimental data series are given in chronological order in Tables 1 and 2. The derived thermodynamic properties H°(T) - H°(0) and S°(T) are given in Tables 3 and 4. The values at 10 K were calculated using the lowtemperature limit of the Debye equation Cp ) RT3. Our measurements do not extend to sufficient low temperatures for constancy of Cp/T3 to be reached. The fitted values of Cp at 10 K were used to estimate R. The values of R used for C19H40 and C20H42 are respectively 0.0033 and 0.0032 J‚K-4‚mol-1. An accuracy in these R values of about 10%

10.1021/je980231+ CCC: $18.00 © 1999 American Chemical Society Published on Web 05/08/1999

716 Journal of Chemical and Engineering Data, Vol. 44, No. 4, 1999 Table 1. Experimental Data Series of n-Nonadecane T/K Series 1 272.37 273.54 275.48 277.41 279.33 281.23 283.12 284.99 286.85 288.69 290.51 292.29 293.99 295.06 295.34 295.41 295.47 295.54 295.65 296.30 297.55 298.83 300.04 301.15 302.14 302.97 303.59 304.01 304.28 304.45 304.57 304.65 304.71 304.75 304.79 304.81 304.83 304.85 304.87 304.88 304.89 304.89 304.90 304.91 304.93 305.21 306.35 308.08 309.81 311.53 Series 2 241.17 241.60 242.80 244.78 246.76 248.74 250.72 252.70 254.67 256.65 258.63 260.61

Cp/J‚K-1‚mol-1 460.49 464.72 471.02 477.53 484.42 492.21 500.63 510.14 521.16 534.23 551.52 580.13 672.32 3939.21 31223 35430 34967 28741 15708 1232.76 1018.02 1109.81 1238.59 1434.40 1765.82 2401.28 3705.12 6210.22 10267 15886 23239 33064 46911 61774 78606 99267 119468 129501 179280 215038 193478 2208807 340488 179312 102804 3421 604.92 606.08 606.63 607.69 392.60 393.45 396.94 400.59 404.54 408.11 411.95 416.09 419.89 424.01 428.08 432.21

T/K 262.59 264.56 266.54 268.51 270.47 272.42 274.36 276.30 278.22 280.13 282.03 283.91 285.78 287.63 289.46 291.26 293.02 294.54 295.32 295.48 295.54 295.58 295.64 295.88 296.77 298.09 299.34 300.50 301.56 302.48 303.22 303.76 304.13 304.36 304.51 304.62 304.69 304.74 304.77 304.80 304.83 304.85 304.86 304.87 304.88 304.89 304.90 304.91 304.93 305.54 306.98 308.71 310.43 312.15 313.86 Series 3 7.30 8.65 9.61 11.26 13.06 14.78 16.56 18.54

Cp/J‚K-1‚mol-1 436.50 441.09 446.13 451.03 456.23 461.73 467.73 474.27 480.90 488.03 495.72 504.76 514.52 526.15 540.95 560.92 601.69 1032.76 8306 31139 49914 52253 34889 4605 978.98 1056.36 1162.64 1316.73 1562.29 1992.93 2830.67 4464.64 7304.18 11659 17936 26713 38298 52131 69863 92456 106879 130176 191213 222384 263585 302733 154872 180533 62277 1227.60 605.52 605.98 606.88 607.87 603.59 1.61 2.53 3.51 4.41 6.62 9.38 12.70 16.55

T/K 20.70 22.94 25.28 27.73 30.26 32.88 Series 4 8.30 9.91 11.48 13.38 15.17 17.00 19.06 21.26 23.54 25.92 28.39 30.94 Series 5 6.86 8.32 9.94 11.82 13.66 15.40 17.29 19.44 21.64 23.94 26.34 28.82 31.39 Series 6 34.07 36.73 39.06 41.33 43.70 46.11 48.55 51.04 53.57 56.13 58.72 61.34 63.99 66.66 69.36 72.08 77.63 80.38 83.15 85.95 88.76 91.60 94.43 97.27 100.08 Series 7 102.54 103.87 105.98 108.90

is to be expected. Figure 1 displays the R values that were estimated from the Cp(10 K)/1000 values as given by Messerly et al. (1967) for C5H12 to C17H36, by us for C19H40 and C20H42, and by Andon and Martin (1976) for C26H54. Despite a large error margin in some of these values, an even-odd effect is clearly visible. Melting Behavior and Purity Determination for n-Nonadecane. In Figure 2 molar heat capacity data of n-nonadecane are given around the solid-solid transition and the melting event. The molar heat capacity between

Cp/J‚K-1‚mol-1 21.33 26.55 32.30 38.94 46.00 53.20 2.18 3.30 4.27 7.02 9.92 13.50 17.51 22.39 27.83 33.80 40.56 47.31 1.20 1.82 3.33 4.44 7.66 10.56 13.95 18.37 23.26 28.76 34.99 41.72 48.58 56.71 64.52 71.12 77.59 84.42 91.40 98.19 104.82 111.57 118.22 124.95 131.48 137.79 144.05 150.23 156.08 167.21 172.60 177.87 183.01 188.03 192.96 197.70 202.30 206.72 210.57 212.59 215.80 220.02

T/K 111.79 114.68 117.58 120.48 123.38 126.29 129.20 132.12 135.04 137.96 140.89 143.82 146.75 149.69 152.63 155.57 158.51 161.46 164.40 167.34 170.29 173.24 176.20 179.16 182.11 185.06 188.01 190.97 193.93 196.88 199.84 202.80 205.76 208.72 211.68 214.64 217.59 220.51 223.42 226.31 229.18 232.04 234.88 237.70 240.50 243.29 246.06 248.81 251.55 254.27 256.98 259.67 262.34 265.00 267.64 270.26 272.86 275.44 278.00 280.54 283.06 285.56 288.02 290.45

Cp/J‚K-1‚mol-1 224.10 228.10 232.06 236.00 239.82 243.60 247.32 250.99 254.54 258.13 261.67 265.20 268.67 272.13 275.62 279.12 282.59 286.13 289.86 293.58 297.18 300.64 304.19 307.77 311.47 315.02 318.69 322.47 326.28 330.13 334.06 337.58 341.54 345.64 349.76 353.86 358.17 362.41 366.71 371.07 375.36 379.92 384.42 389.11 393.85 398.62 403.54 408.67 413.91 419.20 424.61 430.31 436.13 442.33 448.87 455.85 463.33 471.23 479.88 489.54 500.73 514.07 529.70 551.51

T/K 292.81 294.67 295.44 295.54 295.59 295.71 296.65 298.34 299.96 301.41 302.63 303.52 304.08 304.39 304.57 304.68 304.75 304.80 304.83 304.86 304.88 304.89 304.91 304.92 304.92 305.02 306.23 308.50 310.79 313.08 315.37 317.65 319.93 321.31 322.30 324.15 326.14 328.13 330.12 332.10 334.09 336.08 338.07 340.06 342.05 344.04 346.04 348.03 350.02 352.01 354.00 356.00 358.00 359.99 361.99 363.99 365.99 367.98 369.98 371.97 373.97 375.96 377.96 379.96

Cp/J‚K-1‚mol-1 594.49 1470 19482 54110 56664 15519 1091 1077.10 1239.71 1523.20 2118.56 3559.78 6831.39 12638 21564 35268 52972 77663 107255 149733 183709 179657 240230 428085 398384 16704 631.94 605.32 606.70 608.10 609.44 611.12 612.82 614.51 615.21 616.67 618.26 619.75 621.46 623.17 624.46 626.56 628.20 629.48 630.86 632.71 634.75 636.51 637.99 639.84 641.78 643.60 645.49 647.38 649.46 651.16 652.92 654.71 656.69 658.52 660.68 662.55 664.85 666.53

the transition and the melt temperatures is exceptionally high and is completely reproducible. It does not depend on the thermal history of the sample or the mean heating rate of the interrupted measurements. The effect is much too large to be attributed to a premelting effect. We consider this effect to be a true aspect of this compound. Previously, similar observations have been made for other odd members of the n-paraffin’s (Messerly et al., 1967). The effect was attributed to the “rotary state” in the solid phase, in which the linear molecules can rotate along the long axes.

Journal of Chemical and Engineering Data, Vol. 44, No. 4, 1999 717 Table 2. Thermodynamic Properties at Selected Temperatures for Nonadecane T T (C19H40) (M ) 268.52 g‚mol-1; Φ°m def ) ∆0 S° m - ∆0 H° m/T) T/K

C°p,m/ J‚K-1‚mol-1

∆S°m/ J‚K-1‚mol-1

∆H°m/ J‚mol-1

Φ°m/ J‚K-1‚mol-1

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 305b 305c 310 320 330 340 350 360 370 380

3.378 9.894 19.60 31.46 44.84 59.47 73.79 88.20 102.04 115.29 128.20 140.18 151.67 161.98 171.86 181.29 190.20 198.62 206.60 214.31 221.58 235.36 248.33 260.60 272.50 284.35 296.83 308.82 321.23 334.27 347.42 361.66 376.64 392.99 410.93 431.01 455.13 487.38 547.32 545.4a 602.12 606.76 613.31 621.22 629.52 638.20 647.28 657.77 666.68

1.087 3.415 7.522 13.13 20.05 28.03 36.92 46.44 56.46 66.81 77.40 88.14 98.95 109.77 120.54 131.25 141.86 152.38 162.77 173.04 183.18 203.05 222.41 241.26 259.65 277.61 295.23 312.53 329.56 346.36 362.98 379.47 395.87 412.24 428.64 445.14 461.84 478.69 496.68 524.3a 729.18 739.33 758.70 777.71 796.38 814.74 832.84 850.79 868.26

8.155 38.00 110.52 237.30 427.98 687.90 1021 1426 1902 2446 3055 3726 4456 5240 6075 6958 7887 8859 9873 10925 12015 14300 16719 19264 21930 24714 27620 30648 33798 37075 40481 44026 47717 51564 55581 59789 64215 68919 73933 82281a 144276 147290 153392 159568 165821 172158 178584 185104 191725

0.2718 0.8818 1.997 3.640 5.783 8.379 11.38 14.74 18.41 22.34 26.49 30.81 35.29 39.90 44.60 49.39 54.23 59.12 64.04 68.99 73.95 83.88 93.80 103.66 113.45 123.15 132.75 142.26 151.67 160.99 170.21 179.35 188.41 197.39 206.31 215.18 224.01 232.55 241.73 252.77 253.07 264.20 279.35 294.17 308.67 322.86 336.77 350.51 363.72

a

c

Extrapolated from the solid before the transition. b Solid. Liquid

It does however differ from the well-known solid-solid transition into a plastic crystalline phase as in the case of carbon tetrachloride. In those kinds of substances, the heat capacity between the rotation transition and the melting point does not reach such high values, nor does it increase that much with temperature. For n-nonadecane, the anomalously high values of the heat capacity between the transition and the melting point make it impossible to separate the two enthalpic effects. We therefore give in Table 5 the total heat effect, including the transition and the melting. For the calculation the heat capacity of the solid was fitted between 200 K and 260 K to the quadratic function

Figure 1. Even-odd effect at 10 K. The molar heat capacity was assumed to fit Cp ) RT3 between 0 K and 10 K.

Figure 2. Molar heat capacities of n-nonadecane; different measuring series are given: (b) series 1; ([) series 2; (0) series 7 and 8.

Equation 1 was used as the baseline, and no iterative calculation was applied. The end-point of the melting process was calculated in the usual way, that is, by plotting the melted fraction against the equilibrium temperature. For this calculation the heat capacity data of the rotary phase were fitted between 297 K and 301 K. The plot of the reciprocal of the melted fraction against temperature was straight, indicating that the choice of baseline was justified. The resulting triple point was 305.00 K. The purity calculated was 99.54%. The molar heat capacity data of the liquid phase were fitted to a second-order polynomial equation:

Cp(liquid 305 K to 390 K) ) {638.14 - 0.88814T + 0.002537T2} (2) where Cp is in J‚K-1‚mol-1 and T is temperature in K. The maximum deviation of this fit from the experimental data is 0.15%. Melting Behavior of n-Eicosane. n-Eicosane does not show a solid-solid-phase transition during heating. The results of three melting experiments are given in Table 6. Schaerer and Busso (1954) reported an enthalpy of fusion of 69875 J‚mol-1 and a melting temperature of 309.7 K. Claudy and Le´toffe (1991) found 66935 J‚mol-1 and a melting temperature of 310.0 K. The data are all within the combined error margins. The heat capacity data of the liquid were fitted to the following polynomial function:

Cp(liquid 310 K to 390 K) )

Cp(solid 200 K to 260 K) ) {458.46 - 2.32824T + 0.008564T } (1) 2

where Cp is in J‚K-1‚mol-1 and T is temperature in K.

{581.23 - 0.38942T+ 0.001852T2} (3) where Cp is in J‚K-1‚mol-1 and T is temperature in K.

718 Journal of Chemical and Engineering Data, Vol. 44, No. 4, 1999 Table 3. Experimental Data Series for n-Eicosane T/K Series 1 297.46 298.64 300.60 302.53 304.44 306.29 307.95 309.00 309.39 309.51 309.56 309.58 309.60 309.61 309.62 309.62 309.62 309.63 309.63 309.63 309.63 309.63 309.64 309.64 309.81 310.89 312.70 314.51 316.32 318.12 319.92 321.72 323.51 325.31 Series 2 275.01 275.44 276.65 278.64 280.63 282.61 284.59 286.58 288.56 290.54 292.52 294.50 296.48 298.46 300.43 302.37 304.29 306.16 307.86 308.97 309.39 309.51

Cp/J‚K-1‚mol-1 529.43 532.41 542.76 556.77 580.52 638.03 955.50 3654.50 15184 38814 77460 135209 225510 389388 606765 860726 935781 1145634 1159154 1381878 1977671 2835136 1125440 230966 7344 638.50 640.80 642.12 643.55 644.70 646.05 647.37 648.62 650.06 458.46 459.33 464.12 470.01 475.40 480.97 486.57 492.28 497.93 503.27 509.67 516.45 523.55 531.45 541.41 555.51 579.44 636.23 911.56 3214 15113 38232

T/K 309.56 309.59 309.63 309.64 309.95 311.16 312.99 314.82 316.64 318.47 320.29 Series 3 7.12 8.37 9.42 10.76 12.24 14.03 15.74 17.66 19.76 21.92 24.19 26.57 29.04 Series 4 7.54 9.38 10.88 12.68 14.52 16.32 18.29 20.44 22.66 24.98 27.41 29.92 32.52 Series 5 6.69 8.98 11.12 12.69 14.53 16.37 18.35 20.49 22.72 25.05 27.46 29.97 32.57 Series 6 32.43 33.76 35.59 37.93

Cp/J‚K-1‚mol-1 84757 192528 4046399 380825 3754 640.07 641.39 642.48 644.08 645.36 646.72 1.21 2.26 2.87 4.20 5.58 8.80 11.39 14.97 19.68 24.70 30.66 37.12 44.28 1.65 2.91 4.09 5.98 8.84 12.16 16.20 21.10 26.65 32.58 39.28 46.87 54.17 0.88 2.73 4.22 5.89 8.76 12.27 16.20 21.01 26.46 32.45 39.30 46.62 54.04 54.02 57.86 63.49 70.54

T/K 40.22 42.54 44.92 47.34 49.81 52.31 54.85 57.42 60.02 62.65 65.30 67.99 70.69 73.41 78.98 81.73 84.42 87.05 89.60 92.08 94.51 96.89 99.22 Series 7 102.20 104.94 107.73 110.61 113.49 116.38 119.27 122.18 125.09 128.00 130.91 133.80 136.66 139.50 142.30 145.07 147.82 150.54 153.24 155.92 158.58 161.22 163.84 166.44 169.02 171.58 174.13 176.67 179.18 181.69 184.18 186.65 189.11 191.56

The maximum deviation of the experimental data from this fit is 0.2%. Entropy at 298.15 K. Finke et al. (1954) fitted the available experimental entropy data at 298.15 K for the 14 alkanes ranging from C5H12 to C18H38. They used a linear fit of the entropy against the number of carbon atoms. Messerly et al. (1967) applied a quadratic fit, which gave a smaller standard deviation. Although the compounds we measured have melting points higher than 298.15 K, the entropy of the liquid phase can be extrapolated to 298.15 K using a fit of the heat capacity data of the liquid. In Table 7, we compare the measured entropy values and the calculated with these fits. Both fits correspond well with our data. The quadratic fit differs by only 0.1% for C19H40 and 0.05% for C20H42. The larger difference

Cp/J‚K-1‚mol-1 77.60 84.66 91.60 98.62 105.84 112.79 119.73 126.76 133.68 140.50 146.96 153.43 159.70 165.86 177.10 182.67 187.94 192.91 197.59 201.98 206.27 210.30 213.98 218.84 223.23 227.49 231.73 235.71 239.83 243.78 247.71 251.61 255.60 259.45 262.99 266.70 270.30 273.71 277.12 280.46 283.75 286.98 290.13 293.35 296.43 299.61 302.77 305.90 308.98 312.15 315.23 318.34 321.40 324.45 327.55 330.69 333.79

T/K 193.99 196.41 198.82 201.22 203.61 205.98 208.34 210.70 213.04 215.37 217.69 220.00 222.29 224.58 226.86 229.13 231.39 233.64 235.88 238.11 240.33 242.54 244.74 246.94 249.12 251.30 253.47 255.63 257.78 259.92 262.05 264.18 266.29 268.40 270.50 272.59 274.68 276.76 278.83 280.89 282.94 284.99 287.02 289.04 291.06 293.06 295.05 297.03 299.00 300.96 302.89 304.80 306.64 308.23 309.15 309.44 309.54 309.58

Cp/J‚K-1‚mol-1 336.92 339.99 343.04 346.41 349.39 352.53 355.75 358.90 362.11 365.48 368.79 372.16 375.49 378.96 382.31 385.54 389.13 392.62 396.22 399.73 403.02 406.33 409.80 413.41 416.95 420.72 424.21 428.04 431.78 435.60 439.33 443.27 447.25 451.35 455.48 459.78 464.00 468.72 473.14 477.84 482.87 488.11 493.07 498.80 504.52 510.68 517.77 525.07 533.27 543.97 559.16 585.33 658.99 1118.89 5001 20532 47936 116969

T/K 309.59 309.60 309.61 309.62 309.62 309.63 309.63 309.63 309.64 309.65 310.06 311.36 313.17 314.99 316.80 318.61 320.41 322.22 324.02 325.82 Series 8 327.31 328.75 330.47 332.45 334.43 336.42 338.40 340.38 342.37 344.35 346.34 348.32 350.31 352.30 354.29 356.27 358.26 360.25 362.25 364.24 366.23 368.22 370.21 372.21 374.20 376.19 378.19 380.18 382.18 384.17 386.17 388.16 390.16 392.16 394.15 396.15 398.15

Cp/J‚K-1‚mol-1 229893 365175 405579 442195 554664 694079 1192110 2235015 2709567 241313 2586 640.15 641.34 642.52 643.71 645.03 646.24 647.71 649.15 650.70 652.12 653.38 654.40 655.96 657.79 659.90 661.28 663.18 664.66 666.56 668.31 669.95 671.97 673.88 675.79 677.73 679.42 681.79 683.73 685.39 687.09 689.04 691.34 693.07 695.38 697.52 699.11 700.99 703.08 705.00 707.62 709.42 711.43 713.64 713.94 717.40 719.24

of the quadratic fit from the data of Andon and Martin (1976) indicates that the quadratic fit cannot be extrapolated to C26H54. Cooling Behavior. In Figures 3 and 4 a cooling curve for each compound is given. In the case of nonadecane, crystallization started at 304.80 K with no visible subcooling effect. This is remarkable, since the end melting point, which is the equilibrium temperature at a melted fraction (F) of 1, is 304.90 K. This is not exactly the same temperature as the triple point (305.00 K), which was determined by extrapolation to 1/F ) 0. During a cooling experiment, the outer part of the calorimeter vessel will be colder than the contents, since it is not an equilibrium process. The thermometer will show the temperature of the wall of the vessel, so the crystallization process actually

Journal of Chemical and Engineering Data, Vol. 44, No. 4, 1999 719 Table 4. Thermodynamic Properties at Selected Temperatures for n-Eicosane (M ) 282.55 g.mol-1, Φ°m def ) ∆T0 S°m - ∆T0 H°m/T) T/K

C°p,m/ J‚K-1‚mol-1

∆S°m/ J‚K-1‚mol-1

∆H°m/ J‚mol-1

Φ°m/ J‚K-1‚mol-1

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300a 309.65a,b 309.65c 310 320 330 340 350 360 370 380 390

3.20 9.69 20.22 32.57 46.68 61.69 76.92 91.83 106.39 120.15 133.62 146.22 158.12 169.09 179.17 189.04 198.31 207.12 215.24 223.33 230.86 244.77 258.27 270.92 283.09 295.00 307.07 319.34 331.82 344.68 357.97 372.16 386.90 402.53 418.46 435.74 454.49 475.77 501.52 528.24 556.19 638.78 639.02 645.19 654.11 662.67 671.74 681.23 691.03 701.02 711.10

1.07 3.59 7.76 13.56 20.75 29.06 38.29 48.22 58.65 69.44 80.48 91.68 102.96 114.25 125.48 136.65 147.72 158.68 169.51 180.21 190.77 211.46 231.59 251.19 270.29 288.95 307.19 325.09 342.69 360.03 377.17 394.14 411.01 427.81 444.56 461.30 478.09 495.00 512.13 529.58 546.73 772.34 773.08 793.44 813.45 833.11 852.45 871.51 890.30 908.85 927.19

8.0 39.8 113.4 244.4 442.7 713.4 1060 1482 1978 2544 3179 3879 4640 5459 6330 7251 8219 9233 10289 11385 12521 14900 17415 20061 22831 25722 28732 31864 35119 38501 42014 45663 49459 53407 57510 61780 66229 70878 75760 80907 86137 155996 156219 162645 169135 175720 182398 189170 196035 202994 210047

0.270 0.939 2.089 3.778 5.990 8.681 11.80 15.29 19.10 23.18 27.50 32.00 36.67 41.46 46.36 51.35 56.39 61.49 66.62 71.77 76.94 87.30 97.62 107.90 118.09 128.19 138.18 148.07 157.85 167.53 177.10 186.58 195.97 205.28 214.52 223.69 232.80 241.86 250.89 259.89 268.56 268.56 269.15 285.17 300.92 316.29 331.31 346.04 360.48 374.65 388.61

a Extrapolation from the solid; temperature range used 230295 K. b Solid c Liquid

Table 5. Measured Enthalpies of Transition and Melting of n-Nonadecane series

∆Htrans + ∆Hfus/J‚mol-1

1 2 7 mean value and estimated error

62672 62652 62653 62659 ( 20

starts at a temperature even higher than 304.80 K. We conclude that this compound crystallizes at or very near its melting point. To determine if the material of the vessel influenced the start of the crystallization, we did some measurements in a Hart calorimeter. This is a commercial microcalorimeter with three measuring vessels having volumes of about 1 cm3. A Teflon holder was placed in one of the stainless steel vessels. Crystallization started at

Figure 3. Cooling curve of n-nonadecane; crystallization starts within 0.1 K of the final melting point.

Figure 4. Cooling curve of n-eicosane. The compound crystallizes in a metastable state; when about 40% is crystallized, it transforms to the stable crystal form. Table 6. Measured Enthalpies of Fusion of n-Eicosane series

triple point/K enthalpy of fusion/J‚mol-1 purity/%

1 2 7 mean value

309.65 309.65 309.65 309.65

69876 70013 69877 69922 ( 100

99.91 99.90 99.91 99.91

Table 7. Measured and Calculated Entropy Values at 298.15 K ∆S(C19H40)/ J‚K-1‚mol-1

∆S(C20H42)/ J‚K-1‚mol-1

∆S(C26H54)/ J‚K-1‚mol-1

this work

715.9

748.1

Finke (linear fit) Messerly (quadratic fit)

716.7 716.4

749.0 748.5

945 (Andon and Martin, 1976) 943 940.5

exactly the same temperature both with and without the Teflon holder at a cooling rate of 1 K‚min-1. This behavior makes this alkane an ideal temperature calibration compound for cooling experiments in a differential scanning calorimeter. Eicosane crystallizes first in a metastable phase. When about 40% is crystallized, a sudden transition to the stable phase takes place. Crystallization of the metastable phase started at 309.17 K; no subcooling was observed. Experiments in the Teflon holder mentioned before also showed no influence of the material of the wall of the vessel. Literature Cited Andon, R. J. L.; Martin, J. F. Thermodynamic Properties of Hexacosane. J.Chem. Thermodyn. 1976, 8, 1159-1166. Aston, J. G.; Messerly, G. H. The Heat Capacity and Entropy, Heats of Fusion and Vaporization and the Vapor Pressures of n-Butane. J. Am. Chem. Soc. 1940, 62, 1917-1923. Atkinson, C. M. L.; Larkin, J. A.; Richardson, M. J. Enthalpy Changes in Molten n-Alkanes and Polyethylene. J. Chem. Thermodyn. 1969, 1, 435-440.

720 Journal of Chemical and Engineering Data, Vol. 44, No. 4, 1999 Barbillon, P.; Schuffenecker, L.; Dellacherie, J.; Balesdent, D.; Dirant, M. Variation d’Enthalpie Subie de 260 K a` 340 K par les nParaffines, Comprises entre l’Octode´cane (n-C18) et l’Hexocosane (nC26). J. Chim. Phys. 1991, 88, 91-113. Claudy. P.; Le´toffe, J. M. Transition de Phase dans les n-Alcanes Pairs CnH2n+2 n)16-28. 22-e` mes Journe´ es de Calorime´ trie et d’Analyse Thermique 1991, 281-290. Finke, H. L.; Gross, M. E.; Waddington, G.; Huffman, H. M. LowTemperature Thermal Data for the Nine Normal Paraffin Hydrocarbons from Octane to Hexadecane. J. Am. Chem. Soc. 1954, 76, 333-341. Huffman, H. M.; Parks, G. S.; Barmore, M. Thermal Data on Organic Compounds. X. Further Studies on the Heat Capacities, Entropies and Free Energies of Hydrocarbons. J. Am. Chem. Soc. 1937, 53, 3876-3888. Kemp, J. D.; Egan, C. J. Hindered Rotation of the Methyl Groups in Propane. The Heat Capacity, Vapor Pressures, Heats of Fusion and Vaporization of Propane. Entropy and Density of the Gas. J. Am. Chem. Soc. 1938, 60, 1521-1525. Messerly, J. F.; Guthrie, G. B.; Todd, S.; Finke, H. L. Low-Temperature Thermal Data for n-Pentane, n-Heptadecane, and n-Octadecane. J. Chem. Eng. Data 1967, 12, 338-346. Mondieig, D.; Espeau, P.; Robles, L.; Haget, Y.; Oonk, H. A. J.; CuevasDiarte, M. A. Mixed Crystals of n-Alkane Pairs. A Global View of the Thermodynamic Melting Properties. J. Chem. Soc., Faraday Trans. 1997, 93, 3343-3346. Oonk, H. A. J.; Mondieig, D.; Haget, Y.; Cuevas-Diarte, M. A. Perfect Families of Mixed Crystals: The Rotator I N-Alkane Case. J. Chem. Phys. 1998, 108, 715-722. Parks, G. S.; Huffman, H. M.; Thomas, S. B. Thermal Data on Organic Compounds. VI. The Heat Capacities, Entropies and Free Energies

of Some Saturated, non-Benzenoid Hydrocarbons. J. Am. Chem. Soc. 1930, 52, 1032-1041. Parks, G. S.; Shomate, C. H.; Kennedy, W. D.; Crawford, B. L. The Entropies of n-Butane and Isobutane, with Some Heat Capacity Data for Isobutane. J. Chem. Phys. 1937, 5, 359-363. Parks, G. S.; Moore, G. E.; Renquist, M. L.; Naylor, B. F.; McLaine, L. A.; Fujii, P. S.; Hatton, J. A. Thermal Data on Organic Compounds. XXV. Some Heat Capacity, Entropy and Free Energy Data for Nine Hydrocarbons of High Molecular Weight. J. Am. Chem. Soc. 1949, 71, 3386-3389. Preston-Thomas, H. The International Temperature Scale of 1990 (ITS90). Metrologia 1990, 27, 3-10. Schaerer, A. A.; Busso, C. J.; Smith, A. E.; Skinner, L. B. Properties of Pure Normal Alkanes in the C17 to C36 Range. J. Am. Chem. Soc. 1955, 77, 2017-2019. Stull, D. R. A Semi-Micro Calorimeter for Measuring Heat Capacities at Low Temperatures. J. Am. Chem. Soc. 1937, 59, 2726-2733. van Miltenburg, J. C.; van der Berg, G. J. K.; van Bommel, M. J. Construction of an Adiabatic Calorimeter. Measurement of the Molar Heat Capacity of Synthetic Sapphire and of n-Heptane. J. Chem. Thermodyn. 1987, 19, 1129-1137. Witt, R. K.; Kemp, J. D. The Heat Capacity of Ethane from 15 K to the Boiling Point. The Heat of Fusion and the Heat of Vaporisation. J. Am. Chem. Soc. 1937, 59, 273-279.

Received for review September 16, 1998. Accepted March 22, 1999.

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