J . Phys. Chem. 1987, 91, 4653-4655
4653
Removal Rate Constant Measurements for CH,O by O2 over the 298-973 K Range Paul J. Wantuck,* Richard C. Oldenborg, Steven L. Baughcum, and Kenneth R. Winn Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: April 6, 1987; In Final Form: July 16, 1987)
Removal rate constants for CH30 by O2were measured over the temperature range 298-973 K by using a laser photoly&/laser-induced fluorescence technique. The removal rate constant shows a distinct non-Arrhenius behavior suggesting that other removal processes, in addition to reaction, are important at higher temperatures.
Introduction
The methoxy radical (CH30) plays a role in chemical reactions important in both atmospheric and combustion environments. Rate constant measurements for oxidation of the methoxy radical by oxygen via the reaction CH30
+ O2
-
CH20
+ HOz
are of considerable interest. Several inve~tigatorsl-~ obtained rate constants for reaction 1 relative to a reaction for which the rate constant is well established (Le., CH30 NOz). These rate constant measurements, spanning a temperature range of approximately 300-400 K, exhibit a considerable amount of scatter. Sanders, Butler, Pasternack, and McDonald: using a combined laser photolysis/laser-induced fluorescence technique to measure removal rate constants for C H 3 0 by various reactants at room temperature, obtained by an upper limit value for reaction 1 of cm3 molecule-' s-I. Using a similar laser-induced 99.99%) and oxygen (Matheson UHP, 99.99%) are mixed in a high-vacuum, gas handling system, enter the cell through a inlet at the base of the HTC, and flow vertically through the cell. The photolytic precursors C H 3 0 H and C H 3 0 N 0 are premixed with argon (1% C H 3 0 H or C H 3 0 N 0 in Ar) and are introduced through a water-cooled inlet at the base of the cell. Gas flow through the cell is sufficient to assure that a fresh sample of C H 3 0 H or C H 3 0 N 0 is present in the observation zone for each successive laser shot yet essentially static on the time scale of the reaction (C1 ms). The bulk flow velocity is on the order of 5 cm SKI. Cell pressure is monitored by various MKS Baratron pressure sensors. Flow rates for the argon diluent, oxygen reactant, and the precursor mix are measured and regulated by calibrated Tylan mass flowmeters. (7) Inoue, G.; Akimoto, H.; Okuda, M. J. Chem. Phys. 1980, 72, 1769. (8) Wantuck, P. J.; Oldenborg, R. C.; Baughcum, S. L.; Winn, K.R. J . Phys. Chem. 1987, 91, 3253. (9) Felder, W.; Fontijn A.; Volltrauer, H. N.; Voorhees, D. R. Rev.Sci. Instrum. 1980, 51, 195. (10) Baughcum, S. L.; Oldenborg, R. C. In The Chemistry of Combusrion
Processes; Sloane, T. M., Ed.; American Chemical Society: Washington, DC, 1984; ACS Symp. Ser. No. 259, pp 257-266.
0 1987 American Chemical Society
rhe Journal of Physical Chemistry, Vol. 91, No. 18, 1987
4654
I
I
1
I
I
Letters 0.025
Po, = 50 torr T = 673 K
- =
BEST FIT SINGLE EXPONENTIAL
ccn
0.020
1
r
z W
0.015
v)
. -
I
v
I-
I-
z
0.010
cc
J
0.005
0
-i 0
f 20
40
60
0
80
0
20
10
Figure 1. C H 3 0 decay curve together with its logarithm and best-fit single exponential (PO,= 50 Torr, PAr= 25 Torr, T = 673 K).
The excimer and probe laser beams are introduced collinearly into the HTC. The size of the excimer beam is approximately 3 X 3 mm at the cell's center while the probe beam is focused to approximately 0.5 mm at the same location. The large relative size of the photolysis beam relative to the probe beam minimizes diffusional mixing effects at the edges of the probe beam. The excimer and dye laser fluences are typicaly 0.15 and 0.25 J cm-2, respectively. For these experiments, the methanol and methyl nitrite pressures are held at approximately 30 and 2 mTorr, respectively. The use of methanol as a methoxy precursor allowed the investigation of the CH30 O2reaction to be conducted at higher temperatures. Methyl nitrite has been utilized by other investigators (Le., ref 4-6) as a source of C H 3 0 from laser-induced photolysis; however, decomposition limits the use of C H 3 0 N 0 in our apparatus to temperatures below -573 K. Methanol, although not as efficient a photolysis source of C H 3 0as methyl nitrite (at 193 nm), is found to be stable for temperatures up to and exceeding 1000 K. Good agreement (at the same temperature, T = 423 and 573 K) is noted when either methanol or methyl nitrite is utilized as a methoxy precursor. The total methoxy L I F signal is imaged through a 305-nm long-pass filter onto the photocathode of a RCA 7265 gated photomultiplier tube. The photomultiplier signal is amplified, processed by a PAR Model 162 boxcar signal averager, and then recorded and analyzed with an interfaced Data General Nova/ Eclipse computer system.
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Results and Discussion Experiments were performed to measure the C H 3 0 + O2 removal rate constants as a function of temperature. The present work provides what we believe are the first measurements of removal rate constants for the reaction of methoxy and oxygen at and above 673 K. Figure 1 shows a typical decay curve for ground-state C H 3 0 for 50 Torr of O2 (in 25 Torr of argon) recorded at a temperature of 673 K by LIF. Also shown in the figure is the logarithm of the decay curve together with the best-fit single exponential which establishes a decay time, r , for C H 3 0 at the conditions specified. At 973 K, the decay curves for C H 3 0 begin to deviate from single exponential at long decay times, indicating the presence of a fluorescing contaminant. This contaminant is believed to be OH produced, presumably, by the reaction of photolytic H fragments with 02.To investigate temperatures greater than 973 K, it should be possible to excite only the C H 3 0 radical by probing on another band. This experiment, however, was not pursued. A Stern-Volmer type plot of r-I vs. O2 pressure (in Torr) is shown in Figure 2. The slope of the line fitted through the data is used to calculate the bimolecular removal rate constant, k,, for C H 3 0 in the presence of O2(in this case at 673 K). Similar procedures are followed to establish removal rate constant values
30
40
50
0, PRESSURE (torr)
TIME (IS)
Figure 2. Stern-Volmer plot of the inverse first-order decay constant, T - ' , vs. O2 pressure ( T = 673 K, PAr= 25 Torr). 10
1
I
x
lo.'o
I exp (-6028n) + 3.6 x
I
lo.''
-1 3
10
+
@.
Y
CURRENT LORENZ, el al OUTMAN, et 81 -1 5
10
I
1 .o
I
I
I
I
1.5
2.0
2.5
3.0
Y
/ 3.5
1O O O n (K)
Figure 3. Arrhenius plot of C H 3 0 + O2removal rate constants. Error
bars, where not shown, are smaller than the symbol.
TABLE I: Measured Removal Rate Constants. k , for CH,O with 0, k,, cm3 molecule-' s-' k,, cm3 molecule-' s-I T, K T, K 298 (2.3 f 0.2) X 573 (1.4 0.1) x 1 0 4 4 (1.9 f 0.2) x 10-15 (1.2 f 0.3) x 1 0 4 4 348 (4.0 f 0.2) X (1.4 f 0.1) x 10-14 423 (4.7 f 0.3) X 673 (3.0 f 0.1) X (4.4 0.1) x 10-15 773 (6.7 f 0.4) X (4.6 f 0.2) x 10-15 873 (1.4 f 0.1) X lo-" 498 (7.2 f 0.6) X 973 (3.6 f 1.1) x 10-13
for each temperature investigated. Table I lists the results obtained for these experiments. The removal rate constant errors are (with exception of the 973 K data) the standard deviations of the slope of the line best representing the T-I vs. O2pTessure data. The error in the 973 K removal rate constant is an estimate of the uncertainty based on the observed onset of non-single-exponential C H 3 0decay. Removal rate constants for the C H 3 0 O2reaction measured in the present study are shown together with the reported rate constants of Gutman and co-workers5and Lorenz and co-workers6 in Figure 3. Clearly, the removal rate constant shows a distinctive non-Arrhenius behavior. The rate constant data obtained by G u t m a d and Lorenz6 are in good agreement with results established in this investigation. Our C H 3 0 O2 removal rate constant values were fit, over the 298-973 K temperature range, utilizing an expression of the form k,(T) = A(1000/T)-" exp(-E/RT). For this relation, A is equal to (2.3 0.6) X cm3 molecule-1 s-* K", n has a value 9.5 f 0.7, and E / R is -2768 335 K. These parameters appear to be physically unrealistic which suggests that a single Arrhenius expression is an incorrect representation of the data. This relation
+
+
*
*
J. Phys. Chem. 1987, 91, 4655-4657 can, at best, be. considered an empirical means of interpreting the data. This result also suggests the onset, at increasing temperatures, of another mechanism for methoxy radical removal by 0,. Both collision-induced isomerization by the reaction
-
C H 3 0 + 0, CHzOH + 0,
(2)
and decomposition of C H 3 0 to produce formaldehyde via
CH30 + 0, CH20 + H -+
+ 0,
(3)
'
are possible methoxy removal channels at higher temperatures.' Saeba, Radom, and Schaefer" predict that both the decomposition and isomerization channels for CH30 have high activation barriers (34.4 and 36 kcal mol-', respectively). Thus, contributions to methoxy radical removal by reactions 2 and/or 3 may be increasingly important at high temperatures. The solid line shown through the removal rate constant data in Figure 3 represents a fit to our measured C H 3 0 + O2 removal rate constants, along with the removal rate constant data measured by Gutman et aL5 and Lorenz et a1.,6 using a double-exponential expression of the A , exp(-E,/RT). For this form k,(T) = A I exp(-El/RT) double Arrhenius relation, where one expression can describe decomposition and/or isomerization and the other reaction, A I = 1.5 X cm3 molecule-' s-I, E I / R = 6028 K, A , = 3.6 X cm3 molecule-' s-', and E2/R = 880 K. Although the fit to the data is well represented by this form, we do not feel that it is unique and that all the fit parameters are well established.
+
(1 1) Saebci, S.;Radom, L.; Schaefer 111, H. F. J . Chem. Phys. 1983, 78, 845.
4655
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As can be noted in Figure 3, more C H 3 0 0,removal rate constant data have been recorded at lower temperatures than in the higher temperature regime, and g o d agreement is observed between the measured removal rate constants at these lower temperatures. Thus, the fit parameters established for the Arrhenius relation representing the methoxy removal rate constants at the lower temperatures are probably more reliable. For the higher temperatures, the fit parameters are clearly not as well defined, and the disagreement between our measured activation barrier and those predicted by S a e b et al." may not be significant. Efforts to better define these high-temperature fit parameters for the methoxy removal rate constant by determining, as a function of temperature, the relative contributions of reaction (q l ) , isomerization (eq 2), and decomposition (eq 3) are currently in progress. Evaluating the removal of CH30 by collisions with unreactive species such as argon, nitrogen, and CF4 will help independently ascertain the importance of reactions 2 and 3. Preliminary measurements of methoxy removal show that processes like (2) and/or (3) can be quite important and may well explain the observed non-Arrhenius temperature dependence observed in the CH,O O2removal rate constant data presented here. Further experiments and analysis are in progress to fully deconvolute the contributions of these various processes in the C H 3 0 0,system.
+
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Acknowledgment. This work was supported by the Morgantown Energy Technology Center (DOE) and was performed under the auspices of the Department of Energy. P.J.W. gratefully acknowledges the support provided by the LANL Photochemistry and Photophysics Group (CLS-4) during his postdoctoral appointment.
Dynamic Light Scattering in Semidllute Solutions of Nonionic Surfactants. Entanglement of Elongated Micelles Tadashi Kato,* Shin-ichi Anzai, and Tsutomu Seimiya Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Fukasawa. Setagaya-ku, Tokyo 158, Japan (Received: April 27, 1987; In Final Form: June 15, 1987)
Dynamic light scattering (DLS) has been observed for D 2 0 solutions of penta(oxyethy1ene glycol) n-dodecyl ether (Cl2E5) in the range 3-160 kg n i 3 and 5-25 O C . The mutual diffusion coefficient ( D ) obtained by DLS measurements takes a minimum which is shifted to lower concentrations when the temperature is increased. The log D-log c (c is concentration) plot becomes linear in the semidilute region, and the slope of the plot is close to the theoretical value for entangled polymers in good solvent. Self-diffusion coefficients of CI2E5in the lower concentration range have also been measured by means of pulsed-gradient IT-NMR to examine the effects of the critical concentration fluctuations.
Introduction Properties of entangled solutions of flexible polymers have been described successfully by the scaling theories.'" Recently, this approach has been extended to semidilute aqueous solutions of ionic surfactants. Candau et al.7-9 have investigated micellar (1) D e Gennes, P. G.Scaling Concepts in Polymer Physics; Cornel1 University Press: Ithaca, NY, 1979. (2) Doi, M.; Edwards, S . F. The Theory of Polymer Dynamics; Oxford Science Publications: Oxford, 1986. ( 3 ) Des Cloizeaux, J. J. Phys. (Les Ulis, Fr.) 1975, 36, 281. (4) Adam, M.; Delsanti, M. Macromolecules 1977, 10, 1229. (5) Schaefer, D. W.; Joanny, J. F.;Pincus, P. Macromolecules 1980, 13, 1280. (6) Schaefer, D. W.; Han, C. C. In Dynamic Light Scattering, Pecora, R., Ed.; Plenum: New York, 1985; Chapter 5 .
solutions of cetyltrimethylammonium bromide (CTAB) in H200.1 M KBr and H20-0.25 M KBr up to high concentrations by means of static and dynamic light scattering. They have shown that the concentration dependences of the scattered intensity and the mutual (collective) diffusion coefficient follow the power laws derived for semidilute polymer solutions in good solvent. Imae and Ikeda'O have discussed their static light scattering data for semidilute aqueous NaCl and NaBr solutions of tetradecyltri(7) Candau, S. J.; Hirsch, E.; Zana, R. J . Colloid Interface Sci. 1985, 105, 521. (8) Candau, S. J.; Hirsch, E.; Zana, R. J. Phys. (Les Ulis, Fr.) 1984, 45, 1263. (9) Candau, S. J. In Surfactant Solutions; Zana, R., Ed.; Marcel Dekker: New York, 1987. (10) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216.
0022-3654/87/2091-4655$01 .50/0 0 1987 American Chemical Society
