Anal. Chem. 1992, 64, 2197-2199
2197
External Reflectance Cell for Infrared Spectroscopy of Fluids at Elevated Pressure and Temperature Paul A. Flowers’ and B. Gilley Boaz, I11 Department of Physical Science, Pembroke State University, Pembroke, North Carolina 28372
INTRODUCTION Infrared spectroscopic cells capable of maintaining samples a t elevated pressure and temperature are useful in a variety of both fundamental and applied investigations, examples of which include studies of catalytic systems, supercritical fluids, and in situ process monitoring. Perhaps most relevant to the field of analytical chemistry has been the development of infrared-based detectors for supercritical fluid chromatography; consequently, a number of reports have described appropriate flow cells for this purpose.’-4 Other publications have described ‘static” cells suitable for infrared studies of solids5y6and fluids7,*a t high pressures and temperatures. Although design details of these cells vary, all share the common trait of transmittance sampling geometry, and many involve a somewhat complex window-to-cell sealing arrangement of spacers, gaskets, and O-rings. Described herein is an external reflectance cell of relatively simple design which may be used to obtain infrared spectra of fluids a t high pressure and temperature. The cell permits sampling of a thin layer of fluid (ca. 4 pm) isolated between a CaFz window and the reflective bottom of a stainless steel cup. This sampling geometry facilitates studies of two-phase phenomena, e.g., solubility and partition equilibria, as only the more dense phase occupying the lower portion of the cell is probed by the infrared beam. Further in this regard, the reflecting cup may serve to contain the more dense phase (fluid or solid) of a two-phase system, thus allowing the less dense layer to be sampled. The cell’s short path length, approximately 10 pm at 30’ incidence, is useful for the study of strongly absorbing features such as those of solvent or highly concentrated solute species and enables accurate spectral subtraction of such features. Applications to supercritical fluids, molten salts, and fluid mixtures are demonstrated, and potential applications to spectroelectrochemical studies are discussed.
EXPERIMENTAL SECTION Reagents. Tetrapentylammonium bromide (Aldrich, 98 % ) was dried by repeated melt/freeze cycles in Pyrex tubes under dynamic vacuumQprior to use. Subsequent manipulations of this hygroscopicsolid were performed in a nitrogen-purged glovebag to minimize rehydration. Supercritical-grade carbon dioxide (Air Products) and HPLC-grade methanol (Fisher Scientific) were used as received. Apparatus. An illustration of the spectroscopic cell is given in Figure 1. The cell body, top and bottom caps,and the reflecting cup were machined from a corrosion-resistant316 stainlesssteel rod. The reflecting cup’s bottom was polished to a mirror finish with successivelyfiner grades of aluminapowder. This cup serves (1) Shafer, K. H.; Griffiths, P. R. Awl. Chem. 1983,55, 1939-1942. (2) Olesik, S. V.; French, S. B.; Novotny, M. Chromatographia 1984, 18,489-495. (3) Hughes, M. E.; Fasching, J. L. J . Chromatogr. Sci. 1985,23,535540. (4) Jordan, J. W.; Taylor, L. T. J. Chromatogr. Sci. 1986,24, 82-88. (5) Gallei, E.; Schadow, E. Reu. Sci. Zmtrum. 1974,45,1504-1506. (6) Tagawa, T.; Amenomiya, Y. Appl. Spectrosc. 1985, 39, 358-360. (7) Tinker,H. B.;Morris,D. E.Reu.Sci.Zmtrum. 1972,43,1024-1026. (8) Blitz, J. P.; Fulton, J. L.; Smith, R. D. Appl. Spectrosc. 1989,43, 812-816. (9) Tissot, P. In Molten Salt Techniques; Lovering, D. G., Gale, R. J., Eds.; Plenum Press: New York, 1983; Chapter 6, pp 140-141. 0003-2700/92/0364-2197$03.00/0
fluid
/ inlet
11 c m ‘9
Pb gas1
‘\
IR window
Figure 1. Cross-sectional diagram of the high-pressure cell. The
dlstance between the reflecting cup and the CaF2 window Is greatly exaggerated. both to isolate a thin layer of the fluid being sampled between its reflective bottom and a CaF2 window (25 X 4 mm, Buck Scientific) and, in certain experiments, to contain a second fluid or solid phase. The top cap contains four porta accepting Vd-in. Swagelok pipethread fittings to accommodate fluid inlet/outlet lines, thermocouples, etc., and is sealed to the cell body via a Viton O-ring. Lead gaskets cut from 1-mm-thick foil (Aesar, 99.9%)were used to seal the window to the recessed bottom of the cell body. The total cell volume is roughly 20 mL, though the external reflectance geometry is such that less than 1pL of fluid is actually sampled by the infrared beam. The internal volume of the reflecting cup is ca. 0.5 mL. An Isco Model 260D syringe pump and controller were employed to control and monitor C02 pressure within the cell. Heating of the cell was accomplished with heavy insulated heat tape and an Omega Model CN370 temperature controller with a 1/16-in.stainless steel-sheathedtype J thermocouple. Infrared spectra were acquired with a Biorad Model FTS-40 Fourier transform infrared spectrometer equipped with a high-temperature ceramicsource, Ge/KBr beamsplitter,and DTGS detector. A Biorad specular reflectance accessory was used to introduce the infrared beam to the cell at 30’ incidence. Procedure. The experimentalarrangement employed in this work is depicted in Figure 2. The cell (D),charged with powdered salt, methanol, or water in appropriate experiments, was connected to the outlet of the syringe pump (B)with l/16-in.stainless steel tubing and positioned above the reflectance attachment (F) in the spectrometer’s sample chamber using a platform constructed in-house. After adjustment of the cell temperature and COz pressure to desired values, spectra were acquired by averaging typically 50 scans at 2 cm-l nominal resolution. At the end of each experiment, the system pressure was slowly reduced to ambient by venting through valve V3. Safety Precaution. Because of the possibility of window rupture (see discussion of pressure limit below), use of this cell should entail careful observation of standard safety precautions. For most applications,it is assumed that the cell will be housed within the sample compartment of a spectrometer, in which case the major concern is for instrumentation, not personnel.
RESULTS AND DISCUSSION Cell Characteristics. The most troublesome aspects of high-pressure spectroscopic cell designs are without doubt those concerning the optical window and the window-to-cell (P 1992 Amerlcan Chemlcal Society
2108
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
v2
vent
I I I
VI
I
4
I
2.0
I
,
/
,
!
,
,
,
,
,
,
4000 3800 3600 3400 3 2 0 0 3 0 0 0 2800 2 6 0 0 2400 2200 2 0 0 0 I800 1600 1400 1200 1000
Uavenumbers
Flguro 2. Schematic diagram of the experimental arrangement: (A) C 0 2 tank; (e) syringe pump; (C) pump controller; (D) cell; (E) temperature controller;(F)reflectance attachment;(V 1-V3) high-pressure needle valves. Solid lines represent tubing connections, and dashed
Flgwo 3. Infrared spectra of 21, 76, and 136 atm of Con(bottom to top, respectively) at 46 OC. ! 27
lines represent electrical connections.
seal. A window material must be chosen which is suitable in terms of ita optical, chemical, and mechanical properties. For mid-infrared work, the more commonly employed materials have been diamond, sapphire, zinc sulfide, zinc selenide, and calcium fluoride.'-*JO Optical-quality diamond is quite expensive, and sapphire exhibits a relatively high frequency cutoff of ca. 2000 cm-'. Zinc sulfide and zinc selenide are attractive candidates in most respects but are somewhat more expensivethan calcium fluoride;subsequently,CaFz has been widely used and was chosen for this work. Various arrangements for sealing the CaFz window to the steel cell were attempted with limited success prior to the use of Pb gaskets. In addition to ita favorable mechanical properties and low cost, lead is relatively inert and should be suitable for many chemical systemsother than those described here. The pressure limit of the cell is dictated by the maximum pressure the window may withstand, P,, which may be estimated according to the equation'
P,, = Fa(T/1.06D)2 where Fais the apparent elastic limit of the window material, Tis the window thickness, and D is the unsupported diameter of the window. Such a calculation for this cell (Fa= 360 atm, T = 4 mm, D = 10 mm) yields a pressure limit of ca. 50 atm. A 'safety factor" of 4was used in the derivationof the equation above; omitting this factor, the pressure limit of the cell is roughly 200 atm. If desired, an increase in P,, could easily be effected by use of a thicker window, a smaller opening in the cell's bottom cap (which determines the value of D), or a different window material with a higher elastic limit. The absorbance at 1960 cm-' of neat benzene was used to estimate the cell's optical path length,ll yielding a value of approximately 10 Fm. This is a sufficiently short path to permit observation of strongly absorbing spectral features, e.g., those of solvent or highly concentrated solute species, and consequently to permit accurate spectral subtraction of such features as is demonstrated below. Applications. Figure 3 shows infrared spectra of gaseous and supercritical carbon dioxide contained in the highpressure cell. Spectral features consistent with previous (10)Sherman, W. F.;Stadtmuller, A. A. Experimental Techniques in High-Pressure Research; Wiley: New York, 1987. (11)Willard, H.H.;Merritt, L: L.; Dean, J. A,; Settle, F. A., Jr. Instrumental Methods of Analysrs, 6th ed.; Wadsworth Publishing Company: New York, 1981;p 206.
I I
04
0 Q P
0.6
a n
:0
1 4]
0 0 1 I , I , , , I I 4000 3000 3600 3400 3200 3000 2800 2600 240C 2200 2000 ilavenumbers
1
1
1800 160C .400
1
1
.ZOO
,000
Infrared spectra of molten TPAB (bottom)andCOzdlssolved In molten TPAB (top, P = 136 atm, solvent bands subtracted) at 110 Flguro 4. OC.
reports are exhibited,2J2but with the notable difference that the most intense band, the asymmetric stretch around 2350 cm-', remains well "on-scale" for even the most dense fluid phase. In addition to the expected broadening, this band undergoes apronounced splitting as a result of increased COz density; closer scrutiny of this phenomenon is presently underway. An infrared spectrum of molten tetrapentylammonium bromide (TPAB)is shown in the lower part of Figure 4.Raising the carbon dioxide pressure above the melt results in the appearance of a strong, sharp feature at 2334cm-l and a much weaker band at 3700 cm-1 due to dissolved COZ (see upper part of Figure 4). These spectra exemplify use of the cell in high-pressure and -temperature studies of a solute dissolved in a strongly absorbing solvent. The cell's short optical path results in a maximum absorbance by the molten TPAB of ca. 0.5 in the C-H stretching region around 2900 cm-l, a value below the recommended absorbance limit for accuratespectral subtraction.l3 The success of spectral subtraction in this case is clearly demonstrated by the difference spectrum of Figure 4,which displays an excellentbaseline even in regions of most intense solvent absorption. Results of equal quality were obtained using perhaps the most difficult of all solvents for infrared studies, water (spectra not shown). Shown in Figure 5 are infrared spectra for neat methanol and methanol dissolved in supercritical carbon dioxide (solventbands subtracted). The latter spectrum was obtained by filling the cell's reflecting cup with methanol prior to (12)Johnson, C.C.;Jordan, J. W.; Taylor, L. T.; Vidrine, P. W. Chromatographia 1985,20, 717. (13)Anderson, R.J.; Griffiths, P. R. Anal. Chem. 1976,47, 2339.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1002
,
I
,
/
I
I
I
,
I
I
I
,
I
I
,
I
4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 Uavenumbers
Figure 5. Infrared spectra of neat methanol (bottom) and methanol dissolved in supercrftlcal COn (top, P = 110 atm, solvent bands subtracted)at 46 OC. The top spectrum's ordinate Is expanded by a factor of 20.
introduction of the less dense supercritical COz. Observed spectral differences reflect the different molecular environment experiencedby the methanol in the fluid solution relative to the pure, condensed phase, e.g., the appearance of the sharp feature around 3650 cm-' characteristic of "free" (nonhydrogen bonded) 0-H ~tretching.1~ Again, the short path length of this cell is proven beneficial, as this feature would be completely obscured by C02 absorption in any of the transmittance cells previously Finally, it is worth noting that the cell as designed may be easily adapted for use in spectroelectrochemical studies. In (14) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic Limited London, Chemistry, 3rd ed.; McGraw-HillBook Company (UK) 1980; pp 49-50. (15) Widrig, C. A.; Porter, M. D.; Ryan, M. D.; Strein, T. G.; Ewing, A. G. Anal. Chem. 1990,62,1R-20R. (16) Flowers, P. A.; Wightman, R. M., University of North Carolina at Chapel Hill,1990, unpublished work.
2199
such use, a reflective disk working electrode would be used in lieu of the reflecting sample cup. Spectral changes accompanying the electrolysis of a thin layer of solution isolated between this electrode and the cell window could then be followed via external reflectance sampling in the fashion which has become most popular for the infrared region.16 Our laboratory is presently pursuing such a modification of the cell, with the first study planned being that of CO2 reduction in molten quaternary ammonium salts. Previously obtained results16 suggestthat carbon dioxide may be reduced at copper, glassy carbon, and platinum electrodes in molten tetrahexylammonium nitrate. An infrared study of the electrode processes involved in these systems should be quite valuable in gaining an understanding of this potentially important phenomenon. In summary, the cell described herein has been shown to poesess severalattractive traits, includingits design simplicity, use of relatively inexpensive materials, short optical path, and sampling geometry permitting the study of two-phase fluid systems. The cited applications demonstrate the utility of this cell in a variety of situations where previously reported cella would be unsuitable.
ACKNOWLEDGMENT Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Partial support by a CottrellCollege Science Grant from Research Corporation is likewise gratefully acknowledged. The FTIR spectrometer was purchased with funds from the Instrumentation and Laboratory Improvement Program of the National Science Foundation, grant no. USE-9052349. We thank R. Mark Wightman of the University of North Carolina at Chapel H ill for helpful discussions and technical assistance. RECEIVED for review March 9, 1992. Accepted June 15, 1992.
