J. Phys. Chem. 1981, 85, 2313-2316
visioned. One possibility is simply to use pulses with small flip angles. Another possibility is to use line narrowing sequences21which eliminate static second-rank tensor interactions. These sequences require phase-coherent pulse trains, and should be modified (for example, by addition of 180° pulses) to refocus the static inhomogeneity. Dephasing contributions from random fluctuations which occur rapidly compared to the pulse spacing will not be affected. Such experiments are currently in progress, and will be discussed in ref 13. (21) Haeberlen, U. “Advances in Magnetic Resonance”; 1976;Supplement 1 and references therein.
2313
Finally, the above findings are relevant to several recent experiments on impurity molecules in host crystals and excitons in molecular crystals. In this laboratory two systems were found to exhibit a low-temperature ( 1.5 K) absorption line width which appears to be homogeneously broadened. It is very possible that this effect is due to the multilevel nature of the exciton k states. More on this will be discussed elsewhere. N
Acknowledgment. We thank Wm. R. Lambert for many helpful discussions. The research described herein was supported by the National Science Foundation under grant No. DMR81-05034 and grant No. CHE79-05683.
Tricritical Phenomena in “Quasi-binary” Mixtures of Hydrocarbons. 2. Binary Ethane Systems Jiirgen Specovius, Mlguel A. Leiva,+ Robert L. Scott, and Charles M. Knobler” Department of Chemistry, UniversnY of California, Los Angeles, California 90024 (Received: May 8, 798 1)
Measurements are reported of the temperatures and pressures at the upper and lower critical end points of three-phase regions in the binary mixtures ethane + n-octadecane,n-nonadecane,and n-eicosane. The three-phase regions, which are found near the critical point of ethane, 32 “C, extend over temperature ranges of 0.16, 1.29, and 2.92 K, respectively. Concentrations of coexisting phases have been measured for the ethane + octadecane system. The relation of the studies on these binary mixtures to the existence of tricritical points in ternary mixtures containing ethane and two other n-alkanes is discussed and the coordinates of a tricritical point in such mixtures are predicted.
Introduction The existence of tricritical points in ternary mixtures of hydrocarbons is associated with the presence of threephase regions that are bounded by upper and lower critical end points.’p2 Such regions of limited miscibility are found in a number of binary mixtures of hydrocarbons and usually occur in the neighborhood of the critical point of the more volatile component. A knowledge of the phase diagrams of binary mixtures makes it possible to select, in a systematic way, ternary mixtures that are likely to exhibit such tricritical behavior. In essence, one looks for a transition region in the phase behavior of closely related binary mixtures in which there is a change from “complete miscibility” (i.e., no three-phase region) to “limited miscibility” (Le., a limited three-phase region). For example, the systems methane + n-pentane and methane + 2,2-dimethylbutane show complete miscibility at pressures and temperatures near the critical point of methane and are described as type I1 in the classification system devised by van K0nynenburg.l On the other hand, under similar conditions, methane + nhexane and methane + 2,3-dimethylbutane have limited three-phase regions, behavior typical of van Konynenburg’s type IV. Tricritical points occur at the border between type-I1 and type-IV behavior. It is highly unlikely that any binary mixture would fall precisely at this border, but ternary mixtures containing methane with a type-I1 and a type-I11 solute behave very much like binary mixtures and can be used to interpolate smoothly between the two types of behavior. This “quasi-binary” procedure has recently been described by Instituto Mexican0 del Petroleo, Mexico 14, D.F.
Creek et ala3(paper 1of this series), who investigated the ternary mixtures methane + (n-pentane + 2,3-dimethylbutane) and methane + (2,2-dimethylbutane + 2,3-dimethylbutane) and were able to come very close to tricritical points. Mixtures of ethane with hydrocarbons in the C16-C20 range are also likely candidates for quasi-binary tricritical studies. Kohn and co-workers4have reported that limited three-phase regions are present in the binary mixtures ethane + n-eicosane and ethane + n-nonadecane and that there is complete miscibility for ethane n-octadecane and ethane + n-heptadecane. They have also shown that limited miscibility exists for ethane + n-nonadecane + n-eicosane5and what is now recognized as tricritical behavior for ethane n-hexadecane + n-eicosane.6 The three-phase regions in the ethane systems lie at temperatures close to 40 OC and are therefore more convenient to study than those in the methane systems, which are found near -70 OC. As a preliminary to measurements of tricritical phenomena in ternary hydrocarbon mixtures containing ethane, we have investigated the phase behavior of three binary mixtures, ethane + n-eicosane, n-nonadecane, and n-octadecane, in the vicinity of the ethane critical point. A description of the apparatus, which differs sig-
+
+
(1)P. H.van Konynenburg and R. L. Scott, Phil. T r a m . R . SOC. London, Ser. A , 298,495 (1980). (2)P. H.van Konynenburg, Dissertation, UCLA, Dec 1968. (3)J. L. Creek, C. M. Knobler, and R. L. Scott, J. Chem. Phys., 74, 3489 (1981). (4)J. P.Kohn, Y. J. Kim, and Y. C. Pan, J . Chem. Eng. Data, 11,333 (1966). ( 5 ) Y. J. Kim, J. A. Carfagno, D. S. McCaffery, Jr., and J. P. Kohn, J. Chem. Eng. Data, 12,289 (1967). (6)J. R. Wagner, D. S. McCaffery, Jr., and J. P. Kohn, J.Chem. Eng. Data, 13,22 (1968).
0022-3654/81/2085-2313$01.25/00 1981 American Chemical Society
2314
The Journal of Physical Chemistty, Vol. 85, No. 16, 198 1 VACUUM
A
I
I I
I I I
HIGH PRESSURE
N2
A
I
I I I
I
I I I I I I
I I
I I I
I
I I I
I I
I
I II
I I
Flgure 1. Schematlc diagram of the apparatus. VI to V, are valves and S is the stirrer. Other parts and their functions are described in the text.
nificantly from that used in paper 1, is presented in the next section. The experimental results are discussed in the following section and the relation of the results to ternary diagrams and tricritical behavior is considered in the last section. Experimental Section Apparatus. A schematic diagram of the apparatus is shown in Figure 1. The central portion of the cylindrical Pyrex cell C is 10 cm high and has an internal diameter of 11 mm and a 2.5-mm wall thickness. Sections of heavy-walled 2-mm capilllary tubing 40 mm long are sealed to the ends of the cell. The upper capillary terminates in a high-pressure connector of the type described by Davis et al.' The bottom half of the lower capillary is inserted into a close-fitting steel tube and sealed with epoxy cement. The outer surface of the capillary was etched to enhance bonding between the glass and metal. A reservoir R, essentially identical with the cell, is connected to the cell by 1/16-in.0.d. stainless steel capillary. The volume of the cell can be changed by allowing mercury to flow between the reservoir and cell through the stainless steel needle valve VI. Calibrations performed by weighing mercury allow the volume corresponding to any mercury level to be determined with a precision of 0.01 cm3; the volume change produced by pressurizing the cell to 6 MPa is less than this value. To minimize dead space, the pressure in the cell is measured by a null method. A variable reluctance differential pressure transducer D (Validyne, DP 15) with a sensitivity of 0.14 kPa is brought to a null by adjusting the pressure of N2gas above the mercury in the reservoir. This pressure can be read to *2 kPa with a Bourdon gage G (Heise). The parts of the apparatus within the dashed line in Figure 1are immersed in a well-stirred 65-liter water bath. The cell and the reservoir are also enclosed in an openended jacket made from a 3-in. diameter copper pipe, which serves to reduce vertical temperature gradients and affords protection against explosion. The cell can be (7) P. C. Davis, T. L. Gore, and F. Kurata, Ind. Eng. Chem., 43,1826 (1951).
Letters
viewed through a slit milled in the pipe. A proportional controller (Tronac) maintains the bath temperature constant to h 3 mK and the temperature is measured with a quartz thermometer. Procedure. The less volatile hydrocarbon can be added to the cell through a metal-gasket fitting F (Cajon VCR) that is welded into the stainless steel cross that forms the upper half of the high-pressure connector at the top of the cell. A syringe is used to withdraw a sample from a container heated above the melting temperature. After the syringe is cooled it is weighed on an analytical balance and then warmed prior to injection of the sample. The amount of hydrocarbon remaining in the syringe is determined after cooling by another weighing. The number of moles of ethane added to the cell is also determined gravimetrically. A weighed 10-cm3stainless steel sample cylinder E (empty weight 144 g) filled with ethane is connected to the filling line by means of a metal-gasket fitting. The cell is cooled below room temperature and pumped out and then ethane is admitted. After the cell has been filled, any ethane left in the lines is recondensed by immersing the sample cylinder in liquid nitrogen. The cylinder is then reweighed to obtain the amount of ethane added. After the cell has been filled, the contents are stirred with a steel ball activated by a magnet that can be moved vertically in the bath. The level of mercury and the position of any meniscus can be measured to f0.02 mm with a cathetometer. It is necessary to know the volume of the lines and fittings at the top of the cell to determine the volume of the upper phase of the sample (and the total volume). Gas expansion measurements after the cell was assembled give 1.272 f 0.010 cm3 for this dead-space volume. In a typical series of measurements, the positions of the menisci and the pressure are measured as a function of temperature for a fixed cell volume. The mercury level can then be altered to change the overall density and height and pressure measurements can again be performed. Materials. Ethane (Phillips Petroleum Research Grade 99.99 mol 5%) was used without further purification. Measurements in the cell of the critical temperature and pressure give t , = 32.19 "C and p , = 4.88 MPa, in good agreement with recent literature values.8 n-Octadecane (99.8%), n-nonadecane (99%), and n-eicosane (99.9%) were obtained from Chemical Samples Co. When the nonadecane was melted and placed in contact with mercury, a black precipitate formed and a straw color developed when the nonadecane was shaken with concentrated sulfuric acid. The liquid nonadecane was therefore washed with acid until no coloration appeared, neutralized by repeated washings with sodium bicarbonate solution, rinsed in distilled water, and dried under vacuum. When treated in this way the nonadecane no longer reacted with mercury even on long standing at elevated temperatures. None of the other hydrocarbons showed any tendency to react with mercury or acid and, except for degassing, they were used without purification. Results Three-phase behavior was observed for mixtures with eicosane, nonadecane, and octadecane. Values of the temperatures and pressures at the upper critical end point (UCEP) and lower critical end point (LCEP) that bound the three-phase region are given in Table I. The LCEP (8)D.R. Douslin and R. H. Harrison, J. Chem. Thermodyn., 5 , 491 (1973).
The Journal of Physical Chemistry, Vol, 85, No. 16, 1981 2315
Letters
-'r
TABLE I: Critical End Points for the Systems C,H, + n-C,H,,+,
n,
tl/"C
t,,/"C -
18 19 20
39.141 36.341 33.540
39.298 37.633 36.467
AT/K
0.157 5.511 1.292 5.211 2.927 4.927
I
3930
Pll MPa
I
40
Pul
MPa
APl MPa
5.531 5.363 5.26
0.020 0.152 0.33
'
I
-
,/%371 36
-
351
-4-
39.25
\
-
34
t /OC 3920 -
39.15
e
9
c 2 /mol
IO
lite?'
I
I
17
18
19
20
"C
Figure 3. Variation of upper and lower critical-end-point temperatures with carbon number. The curves in (a) have been drawn to be consistent with that in (b).
In preparation for a detailed study of the approach to the tricritical point in ethane + (n-octadecane + n-heptadecane) mixtures, we have also determined the compositions of the coexisting phases throughout the three-phase region for ethane n-octadecane. The method by which the concentrations of the components can be determined from measurements of the volumes of the coexisting phases has recently been de~cribed.~Since it allows the concentrations of each of the components to be determined independently, two temperature-composition diagrams can be constructed. A plot of the coexisting molar concentrations of ethane derived from our data is given in Figure 2a and the corresponding plot for octadecane is given in Figure 2b. The error bars represent the standard deviations obtained from the least-squares analysis. Where no error bars are shown the standard deviation is no greater than the size of the symbol.
+
I
I
0.I
0.0
I
0.2
I
I
0.3
c ~ B/mol lite?'
Figure 2. Temperature-concentration diagrams for the three-phase region of ethane n-octadecane: (a) ethane concentrations, (b) octadecane concentrations. The curves have simply been drawn through the points.
+
and UCEP temperatures reported by Kohn, Kim, and Pan4 for the eicosane and nonadecane systems are roughly 0.2 K systematically higher than our values, and the pressures are also correspondingly higher. Such discrepancies are probably caused by small differences in the purity of the hydrocarbons. (We have found variations between samples of as much as 0.04 K.) The differences between the properties at the end points, AT and Ap, which are less sensitive to impurities, agree very well. Kohn et al. found no evidence for limited miscibility in ethane + n-octadecane. They made no measurements above 38 "C, however, and the narrow three-phase region that we discovered lies at 39 "C. Complete miscibility was also reported4 for ethane + n-heptadecane and in the following section we will show that it is highly unlikely that any region of immiscibility will be found at higher temperatures.
Relation to Tricritical Phenomena In paper 1 we showed that calculations with simple equations of state suggest that the critical-end-point temperatures and pressures should be smooth functions of any parameter that is a measure of the strength of the interaction between solute and solvent. In addition, for a binary mixture in which a single interaction parameter l can be varied continuously, the analytic (mean-field) thermodynamic theory of tricritical points leads to predictions of the dependence of the width of the three-phase region, either in terms of AT or A p , on A t = l - lt,where ltis the value of the parameter at the tricritical point. For mixtures of a single substance with members of an homologous series, the number of carbon atoms n, in the solute molecule is a convenient choice for an interaction (9) C. M. Knobler and
R.L. Scott, J. Chem. Phys., 73, 5390 (1980).
2318
J. Phys. Chem. 1981, 85, 2316-2319
parameter. If varies linearly with n, and if the individual binary mixtures are thought of as differing only in this parameter, then the analytic theory predicts that AT and Ap are both proportional to Figure 3a shows the upper- and lower-critical-end-point temperatures tu and tl as a function of n, and Figure 3b is a plot of (At)2/3 against n,. The cusplike variation of the end-point temperatures is in accord with the analytic theory and the linearity of (At)2/3with n, demonstrates that, certainly to a first approximation, n, is proportional to 5: The tricritical point is located at the intersection of the LCEP and UCEP curves, which from Figure 3b corresponds to n, = 17.6, and experiments currently under way with the quasi-binary system ethane (n-heptadecane +
+
n-octadecane) confirm this. Thus, limited miscibility should not be found in binary mixtures of ethane with normal alkanes below octadecane. Although the phase rule restricts tricritical points to systems of at least three components, occasionally one can find a true binary mixture that lies close to a tricritical point. This appears to be the case in ethane n-octadecane: all three coexisting phases appear opalescent throughout the 160-mK wide three-phase region. Lightscattering investigations of this fascinating system are being pursued.
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Acknowledgment. This work was supported by the National Science Foundation.
ARTICLES Hydrated Electrons in Surfactant Solubilized Water Pools in Heptane Victor Calvo-Perez, Godfrey S. Beddard, and Janos H. Fendler' Department of Chemistty, Texas ABM University, College Station, Texas 77843 (Receive& Januaty 28, 1981; In Flnal Form: April 21, 198 I)
Hydrated electron (eaq-)formation has been observed in the laser photoionization of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and dodecylammonium propionate reversed micelle solubilized phenothiazine, 8hydroxy-1,3,6-pyrenetrisulfonate, and potassium ferrocyanide in heptane. Changes in the H20/AOT ratios, w0 values, were found to affect substantially eaq-lifetimes and spectra. Decreasing wo values or the temperature resulted in increasing lifetimes and in narrowing the absorption spectra of ea;. These results are rationalized in terms of increasing viscosities with decreasing wo values.
Introduction Surfactant solubilized water pools in apolar solvents, referred to as reversed micelles, provide unique microenvironments for reactions and intera~tions.l-~A variety of microscopic polarities can be attained by altering the water-to-surfactant ratios, Le., the size of the water The ability of reversed micelles to organize molecules in compartments of controlled hydrophobicities has been exploited in a variety of processes including photochemical energy conversion.s14 Employing photochemical and (1)Fendler, J. H. Acc. Chem. Res. 1976,9,153. (2)Eicke, Ha-F.Top. Curr. Chem. 1980,87,85. (3)Kitahara, A. Adu. Colloid Interface Sci. 1980,12, 109. (4)Fendler, J. H.; Nome, F.; Van Woert, H. C. J. Am. Chem. SOC. 1974,96,6745. (5) Correll, G. D.; Cheser, R. N.; Nome, F.; Fendler, J. H. J.Am. Chem. SOC.1978,200, 1254. (6) Wells, M. A. Biochemistry 1974,13,4937. (7)Wong, M.;Thomas, J. K.; Gratzel, M. J.Am. Chem. SOC.1976,98, 2391. (8)Zulauf, M.; Eicke, H.-F. J.Phys. Chem. 1979,83,480. (9)Gratzel, M.Micellization, Solubilization, Microemukiions, [ h o c . Znt. Symp.], 1976 1977,531. (10)Calvin, M.Acc. Chem. Res. 1978,10, 369. (11)Gratzel, M.; Thomas, J. K. In
