J. Phys. Chem. 1981, 85, 759-761
759
Effect of Water upon the Critical Points of Carbon Dioxide and Ethane G. Morrison Thermophysics Division, National Bureau of Standards, Washlngton, D.C. 20234 (Received: February 4, 198 1)
The effect of water upon the critical points of carbon dioxide and ethane has been measured. A t the end of the critical locus for COz + HzO, x H z O = 1.1 x the critical temperature of COz is raised 0.372 K. For CzHB + H 2 0 ,the critical temperature of C2H6 is lowered 0.022 K at the end of the critical locus, X H ~ O= 5.5 X lo-*. T A B L E I: Critical Temperature f o r CO, + H,O Introduction The phase diagrams and critical lines of the mixtures sample xH.0/10-4 TC/" C COz HzO and CzH6 + HzO have been studied in detail, 1 0 31.052a especially in the intermediate composition and water-rich 2 31.11 1.8 ranges.Q Interest in these mixtures arises from their 3 3.6 31. 195a 4 5.4 31.23 importance both in geochemistry and in the chemical and 5 7.2 31.30 petrochemical industries. Both mixtures have discontin6 31.41 satd uous liquid-vapor critical lines and both systems exhibit 7 satd 31.42 "gas-gas" immiscibility above the critical point of Hz0.3 satd 3 1.424a 8 There is, however, little quantitative information about end p o i n t 1.1 31.424 either mixture near the COPor CzH6 critical p ~ i n t .In~ ~ ~ a Except f o r these samples, t e m p e r a t u r e s were m e a s u r e d both mixtures, the low-temperature branch of the critical only to k0.005 K. line (Le., the part near the COz or CzH6critical point) is very short. The development of this branch of the critical TABLE 11: Critioal Temperature f o r C,H, + H,O line for the two mixtures is shown schematically in Figures sample xH20/10-4 TCI0C 1 and 2 and explained in detail in the legend for Figure 1. 1 0 32.229
+
Experimental Section Samples for this study were prepared by distilling either C 0 2 (Matheson research grade, 99.995%16 or CzH6 (Matheson research grade, 99.96%)6from a gas buret into an ampule made from heavy walled Pyrex capillary (i.d. 2.0 mm, 0.d. 7.7 mm); no effort was made to purify the materials further. The amount of COPor CzH6 in the ampules was known to &0.4%. Trace amounts of water were introduced by distilling known volumes of the vapor in equilibrium with liquid water at 0 "C into the ampule. The amounts of water were known to *5%. Uncertainty in the amounts was determined primarily by uncertainty in the pressure in the buret; uncertainty in the buret calibration was important only for determining the uncertainty in the amount of water. Samples at the critical density were prepared by shortening the length of the ampule until the meniscus disappeared near the center of the ampule; the critical behavior of the meniscus was accompanied by the expected strong opalescence. The temperature a t which the meniscus appeared or disappeared was determined to &0.001 "C with a quartz crystal thermometer which had been calibrated against an NBS-calibrated platinum resistance thermometer. The bath tem-
-
-
(1) (a) S.Takenouchi and G. C. Kennedy, Am. J. Sci., 262,1055 (1964); (b) K. Todheide and E. U. Franck, 2. Phys. Chem. (Frankfurt am Main), 37,387 (1963); (c) R. Wiebe and V. L. Gaddy, J. Am. Chem. Soc., 61,315 (1939); 62, 815 (1940). (2) (a) A. H. Wehe and J. J. McKetta, J. Chem. Eng. Data, 6, 167 (1961); (b) R. G. Anthony and J. J. McKetta, ibid., 12, 17 (1967); (c) A. Danneil, K. Todheide, and E. U. Franck, Chem. Ing. Tech., 39,816 (1967); (d) H. H. Reamer, R. H. Olds, B. H. Sage, and W. N. Lacey, Ind. Eng. Chem., 35, 790 (1944). (3) J. S.Rowlinson, "Liquids and Liquid Mixtures", 2nd ed, Plenum Press, New York, 1969, section 6.5. (4) R. Wiebe and V. L. Gaddy, J . Am. Chem. SOC.,63, 475 (1941). (5) J. P. Kuenen and W. G. Robson, Phil.Mag., 48, 180 (1899). (6) Because no effort was made to purify materials, manufacturers and sources are provided for scientific completeness. Mention of such names does not represent an endorsement by the National Bureau of Standards or the United States Government.
2 3 4 5 6 7 8 9 10 11 end p o i n t
0 2.8 2.8 2.8 5.7 5.7 8.5 8.6 satd satd 5.5
32.228 32.218 32.219 32.217 32.205 32.208 32.207 32.207 32.206 32.208 32.207
perature was controlled to f0.0003 "C by using a proportional controller. Results A t low temperatures, there is the possibility of forming three phases. To use the COz + HzO system as an example, the phases are nearly pure COz vapor, nearly pure COz liquid, and nearly pure liquid water. In the absence of the nearly pure water phase, the critical point of COz-rich phases depends upon the concentration of water. With the formation of the third, nearly pure water phase, the critical point of the C0,-rich phases becomes fixed.' The maximum effect of water upon the critical points of C02 and C2H6 can be determined by measuring the appropriate critical conditions in the presence of an excess of water. The compositions at which these systems become saturated with HzO are determined by measuring the critical temperature as a function of overall water composition. The composition, at which the critical temperature no longer changes, marks the end of the critical line. This point was located by finding the intersection of straight-line segments (7) The fixed nature of the end of the critical locus can be shown by using the Gibbs phase rule, f = c - p + 2 - n, where c is the number of components, p , the number of phases, and n, any additional material or mechanical constraints on the system. In a binary mixture, there are two constraints at a critical point, ( 8 2 G , / 8 r 2 ) ~ ,=p 0 and (d3G,/8xS)T,, = 0 where G, is the molar Gibbs function. Thus, where c = 2 and p = 1 or 2 and n = 2, f = 1 or 0.
This article not subject to US. Copyright. Published 1981 by the American Chemical Society
760
The Journal of Physical Chemistry, Vol. 85,
No. 7, 7987
Letters
32,230 I
0
X
32 220 I
--
u t\
0
X
32,210
I
32,200 I
0
I
I
0
X
0,O
X
I
Flgure 1. A schematic representation of the phase behavior of COP H20 (Tvs. xHfl at constant p). (i) At p C pcco,. Point A is the nearly critical temperature of COP;point B, the vaporization temperature of H20. Vapor phases are above line ADB. Line DEF represents a three-phase equilibrium, a C0,-rich vapor (D), a C02-rich liquid (E), and a H,O-rich liquid (F). Area BDEF encloses a two-phase region, the vapor along BD and the liquid along BF. Area ADE is a two-phase region. The area below line segment EF is a liquid two-phase region. (ii) At p > pCm , the phase behavlor is simllar to that shown In (I) except that the C02-&h two-phase region (ADE) has pulled away from the pure COz axis. The critical point for this equilibrium is at point A. (iii) At p > pii the C0,-rich two-phase region has contracted to a point (ADE) where the locus of CO,-rich critical points ends. (iv) At p > plii, there is only a two-phase region. The C0,-rich two-phase region is remembered only in the "elbow" on the left side of the two-phase region. At a much higher presure the two-phase region pulls away from the pure-H,O axis (not shown); even so, the two-phase region persists-the so-called "gas-gas" immiscibility.
+
2,O
SAT
4,O
X
0
X
31 30 I
\
31.20 31 10 I
5,0
10.9
I
I
X
I
Flgure 2. A schematic representation of the phase behavior of C2H, H,O (Tvs. xH,o at constant p ) . The sequence of figures is similar to those shown in Figure 1. The major difference between C02 H20 and C,H, -k HO , is that PcA , , > , ,T whereas PCo2 . , ,< , ,T
+
15,O SAT'D
~ / 2 10-4 ~
Flgure 4. The critical temperature of C2HB4- H20 vs. xw, The cross (+) marks the end of the critical locus.
X
I
0
The cross
U
X
0
+ H20vs. xnp.
- "Qp
0,0
0
10-4
XH~O
Figure 3. The critical temperature of C02 (+) marks the end of the critical locus.
D
8,O
6,O
+
passing through the water-saturated and -unsaturated parts of the critical line determination as shown in Figures 3 and 4. Data for these points are listed in Tables I and 11. The addition of small amounts of water to C02 causes its critical temperature to rise. The locus of critical points ends at Tendpoint - F ureCOz = 0.372 f 0.004 K and X H ~ O= (1.1f 0.1) X T i e measured critical temperature for pure COz was 31.052 "C. The water composition is comparable to those of Wiebe and Gaddy at 25 "C, but in sharp disagreement with the value predicted in a phase diagram fit by Heidemann, xHzO = 0.065,8 who also predicted a (8) R. A. Heidemann, ACS. Symp. Ser., No. 133, "Critical Points in Reacting Mixtures" (1980).
temperature rise of about 30 K. In contrast to COz, water causes the critical temperature of CzHs to drop. The critical line ends at Tend - Tcpurecz = -0.022 K (rt0.003 K) and xHpO = (5.5 f 0.7) X %he measured mole fraction is in a range consistent with a dew point composition for the equilibrium of C2H6 with H 2 0 under somewhat different conditions (T = 37.8 "C, p = 63.03 Pa, xHaO = 1.33 X reported by Reamer et a1.2d The measured critical temperature of pure C2H6was 32.229 "C, which compared well with an independent determination for the material from the same cylinder, 32.231 0C.9 Within the precision of the experiment, no effect of HzO upon the critical density of either C02or C2H6 could be determined; however, the effect would have to have been greater than - 5 % in the density to be noticed.
Discussion Because the three-phase line for COz + HzO lies a t a higher temperature than the critical point of pure COz,and d p / d T for the three-phase lines is positive, the topology of the phase diagram demands that T" rise with the addition of water (see Figure 1). Concomitant with this topology is a COz-liquid phase that is richer in water than the COz-vapor phase. A qualitative physical explanation of the behavior of COz + H 2 0 is the chemical interaction between COPand H20. Thus both the solubility of HzO is enhanced in liquid COz and the increased average at(9) T. D. Doiron, private communication.
J. Phys. Chem. 1981, 85,761-762
traction in the vapor phase due to the presence of HzO causes the critical point to rise. In CzH6 + HzO, the three-phase line lies at a temperature lower than Tc for pure CzH6 (see Figure 2). The phase diagrams for short-chained hydrocarbons (C, to C8, at least) fall into this class. The topology of such a diagram does not preclude either a rise or fall in F upon the addition of water; however, other alkane + H 2 0 mixtures appear to behave much like ethane. The temperature at the end of the critical locus reported for n-C4Hio + HzO by Reamer et al. is the same as the critical temperature of pure n-C4Hlo,within the sensitivity of their experiment.1° Water at these very small concentrations affects
76 1
the alkanes not as a highly polar hydrogen-bonding material, but as a small relatively unpolarizable molecule; indeed, its effect is the same that Nz would be expected to have on the critical point. For example, low concentrations of Nz in n-butane raise p c but leave Tc effectively unchanged.ll Acknowledgment. This work was supported in part by the Department of Energy (DOE Contract No. P7901196). (10) H. H. Reamer, R. H. Olds, B. H. Sage, and W. N. Lacey, Ind. Eng. Chem., 36, 381 (1944). (11) W. W. Akers, L. L. Attwell, and J. J. McKetta, Ind. Eng. Chern., 46, 2539 (1954).
Selective Multiphoton Ionization of Geometric Isomers: cis- and trans-I ,2-Dichloroethene Jeffrey W. Hudgens,” Mark Seaver,+ and J. J. DeCorpo Laser Chemistry Group, Code 61 10, Naval Research Laboratoty, Washington, D.C. 20375 (Received: February 2, 1981)
Mass spectral resolution of the cis and trans isomers of 1,2-dichloroethene is demonstrated by the use of multiphoton ionization (MPI). These isomers are resolved by using three photon resonances predicted from vacuum UV spectroscopy of each isomer. A fourth photon is absorbed to form the ions. A plot of isomer selectivity vs. wavelength shows that discriminationof one isomer from the other of >lo1 can be realized. The MPI mass spectra of ions formed from both isomers reveal identical fragmentation patterns with wavelength (440-465 nm). The MPI mass spectrum shows greater elimination of C1 than observed under electron impact ionization.
Introduction Several authors, including ourselves, have suggested that geometric isomers which produce identical mass spectra with electron impact ionization can be identified by their electronic absorption fingerprint using multiphoton ionization (MPI) in a mass ~pectrometer.l-~One study along these lines observed the CloHs isomers azulene and naphthalene and found the fragmentation patterns dependent on ~ a v e l e n g t h .However, ~ these two CloHs isomers are distinguishable under conventional electron impact ioni~ation.~?~ Currently, mass spectral identification of geometric isomers in mixtures requires prior separation by gas chromatography. Reference tabulations5 of the fragmentation patterns of cis- and trans-1,2-dichloroethene, as well as measurements in our mass spectrometer (Table I), show the isomer pair of this study to have identical fragmentation patterns. Experimental Section The principal characteristics of the experimental apparatus are described e1sewhere.l Briefly, it consists of a nozzle beam, a Nd:YAG pumped dye laser beam focused with a 50-mm lens, and a quadrupole mass spectrometer. The molecular beam and laser beam crossed at right angles within the ionizer. A diffusion pump and 77 K beam traps maintained the pressure of the quadrupole region of the mass spectrometer at less than 5 x torr. A filament below the beam plane provided electron impact data. Gas chromatography showed the samples of cis- and trans1,2-dichloroethene to contain about 5% isomer cross NR,L-NRC Postdoctoral Associate, 1978-1980. Sterling Chemistry Laboratory, Yale University, New Haven, CT 06511.
contamination, -10% CHC13, and 5% CC4.
Results and Discussion The total ion signal spectra of trans and cis isomers between 440-465 nm at 0.5-cm-l resolution are shown in Figure 1. In both molecules the MPI spectrum arises from a three-photon resonance with a Rydberg state followed by absorption of a fourth photon into the ionization continuum. These resonance states were studied previously by Walsh and Warsop using vacuum UV spectroscopy and were assigned as either the (a,4s) or (a,3d) transition^.^^' These one-photon assignments are indicated on the spectra. The strong transition in cis-1,2-dichloroethene which Walsh labeled “A”, but could not assign, appears in these results at 460.2 nm. Considerable structure which cannot be assigned by comparison with the vacuum UV spectra is also seen. These features are currently under further investigation. Toward shorter wavelength the trans isomer shows a continuum of increasing intensity. Figure 2 shows the isomeric selectivity vs. wavelength. Selectivity is represented as the log of the ratio of trans to cis isomer total ion signals. Greater discrimination of (1) M. Seaver, J. W. Hudgens, and J. J. DeCorpo, Int. J.Mass Spectrom. Ion Phys., 34, 159 (1980). ( 2 ) J. H. Brophy and C . T. Rettner, Chem. Phys. Lett., 67,351 (1979). (3) U. Boesl, H. J. Neusser, and E. W. Schlag, J. Chem. Phys., 72,4327 (1980). (4) D. M. Lubman, R. Naaman, and R. N. Zare, J. Chem. Phys., 72, 3034 (1980). (5) A. Cornu and R. Massot, “Compilation of Mass Spectral Data”, Vol. I, 2nd ed, Heyden, London, 1975. (6) A. D. Walsh and P. A. Warsop, Trans. Faraday Soc., 63, 524 (1967). (7) A. D. Walsh and P. A. Warsop, Trans. Faraday SOC.,64, 1418 (1968).
This article not subject to US. Copyright. Published 1981 by the American Chemical Society
