NOTEB
1158
that the kinetics of desorption for SOS was diffusion controlled.+l' In the mixed film, the kinetics of desorption for SOS should be unaffected by the presence of COL if no interactions between the two film components occur. However, the AT-t relation for the mixed film system was unlike that for the 100% SOS monolayers in two major respects. First, all the mixed films were characterized by an initial relatively rapid decrease in area. To compare the initial rate at which the area decreases for the pure and mixed SOS films, we used the function15 d In AT --- ki
(2)
dtl'2
where ki is the rate constant in the initial stage of diffusion limited monolayer desorption. Estimates for ki were obtained from the slopes of In AT-W plots for the first 5 min of each of the curves in Figure 1. For example, ki at T = 10 dyn/cm for 100% SOS was 0.012 min--ll2,while the value for the mixed film was 0.036 min-1'2. It should be noted that the latter value must be corrected for the initial 50% molar dilution of SOS in the surface.16 At t 7 5 min the correction factor will be about 2, and hence the apparent value of ki for SOS in the mixed film becomes 0.072 niin-l'z, about sixfold greater than the value for the 100% SOS film. Approximately the same relative increase in ki was observed for each mixed film at the other surface pressures. The second major difference in the mixed film behavior was that the initial rapid decrease in area was followed by a period in which A T was constant with time. Where SOS desorption should have been in the steady state (Figure 1) the mixed film was stable. An estimate of the composition of the stable mixed film may be obtained by calculating the surface concentration of COL (assuming no loss of COL from the surface) from the final value of AT and the amount of COL added initially, These values, at four surface pressures, are shown in Table I. column 3. For comparison the surface concentration of 100% COL films a t the same values of ir are shown in column 2 of Table I. Table I: Surface Concentrations of Cholesterol, and Surface Potential AV, at Various Surface Pressures, T (1) T,
dyn/ cm
5 10 15 20
(2)
(3)
A-1, mol/ ---ems X 1010--COL COL SOS~J (100%)
+
4.42 4.48 4.54 4.60
2.78 2.90 3.00 3.08
(4)
(5)
(6)
mVCOL COL~ (100%) sos
--AV,
390 400 400 400
+
210 180 180 180
A(AV),
mV (col 4
- ~015) 180 220 220 220
Obtained from the horizontal portions of the curves ;n Figure 1. Spreading solution contains an equimolar mixture of SOS and COL. a
The JOUTnal of Physical Chemistry
These values were obtained from the n-A isotherm of the pure COL film. The surface concentration of COL in the nondesorbing mixed film is about 35% lower than for the pure films of COL. We must conclude, therefore, that along with COL an additional component must still be in the film, namely SOX. In support of this conclusion were the results of the AV measurements also given in Table I. AV values for 100% films of SOS are between -80 and - 100 mV. Column 4 gives the surface potential for pure cholesterol films, while column 5 gives the values of AV obtained for the mixed film from which no desorption occurred. The decrease in AV of -200 mV from the value for the pure cholesterol film (column 6) is consistent with the conclusion that a significant amount of SOS is present along with COL in the s ~ r f a c e . ~ The AT-t results for the mixed film appear to be paradoxical. I n the initial stages of the mixed film experiment SOS appears to desorb much more rapidly than from pure SOS films, while in the latter stages of the experiment SOS desorption is prevented. Perhaps the initial decrease in area for the SOS-COL system is due, in addition to desorption, to other mechanisms which can lead to contraction of the film such as a decrease in partial molecular areas or a rearrangement of the molecular packing. KO quantitative assessment of these factors is possible without an independent measure of the mole fractions of each component in the surface. However, whatever the stoichiometry of the SOS COL system may be, the results indicate that adhesive forces do exist between COL and SOS, which are of sufficient magnitude to prevent desorption of SOS. This precludes consideration of this system as an ideal surface ~ o l u t i o n . ~Whether this constitutes an example of molecular complex formation cannot he decided until additional evidence of the colligative properties of the system can be obtained.
+
(16) The actual correction factor is 1 i(ncoL u c o ~ ) / n ~usoa) os where 7t refers to the number of moles (or molecules) per unit area of each film species at time t , and u to the corresponding partial molar (or molecular) area.l*
Kinetics of Hot Hydrogen Atoms from H,S Photodissociation at 1850 A l a by L. E. Compton, J. L. Gole,lb and R. M. Martin Department of Chemistry, University of California, Santa Barbara, California (Receiued August 1 , 1 9 6 8 )
The H2S 2000-A vapor band shown in Figure 1 consists of a diffuse vibrational progression. This transi(1) (a) This work was supported in part by the Sulphur Institute and in part by the National Science Foundation under Grant GP5834. (b) NSF Undergraduate Research Participant, Summer, 1967.
1159
NOTES
14
%
IO
w6
2
I800 1850 1900 1950 2000 2050
2100 2150
WAVELENGTH (8)
Figure 1. Absorption spectrum of H,S a t 25'. Units for E : 1. mol-' cm-'.
tion has been qualitatively described as due to 49, + 3ps orbital excitation, and the vibrational intervals appear to be due to predominant excitation of the bending vibration.2a Gann and DubrinZb have studied the photodissociation of HzS a t 2138A, which is in the continuous region before the first vibrational peak a t 2000A, and by a comparison of hot hydrogen atom reaction yields they have shown that most of the excess energy of photochemical dissociation a t 2138 A appears as kinetic energy of separation of H and SH rather than internal energy of the SH radical. We have used the kinetic method to determine the average hydrogen atom energy from the photodissociation of H2S a t 1850A, which is between the fourth and fifth vibrational peaks of the 2000-11 band. HBr-C4Dlo and H2S-C4Dlomixtures were irradiated with a low-pressure mercury lamp over a range of relative concentrations, and the resulting H2/HD product ratio was plotted as a function of the HBr/C4DlO or H2S/CdDlo reactant ratio, as shown in Figure 2. As has been observed previously for HI, HBr, and DBr photochemical hot atom ~ y s t e m s the , ~ ~product ~ ratio increases linearly with reactant ratio and positive intercepts are obtained when the line is extrapolated to a reactant ratio of zero. Reactants were prepared on a mercury-free vacuum line and pressures measured
with a Wallace and Tiernan FA 141 diaphragm gauge. Products were analyzed with a CEC 620 mass spectrometer calibrated with standard mixtures of the same composition range as the product mixtures. The HZS spectrum in Figure 1 was taken on a Cary 15 spectrophotometer purged with nitrogen gas. The indicated extinction coefficients are the average of three determinations, which gave an average deviation ranging from 3.3% a t l S O O h , to a minimum of 0.7% a t 2000 A. The results of the 1850-A photolyses are compared with those of Gann and Dubrin in Figure 3. The intercept I is interpreted as the HZ/HD ratio under conditions where C4D10 is the only significant reactant for hot atoms above the H C4D10 abstraction threshold energy, and the HBr or H2S function as hot atom sources and as scavengers for atoms which are C4D10 abstraction threshold moderated below the H energy. Therefore ( I 1)-l = HD/(Hz HD) indicates the fraction of the atoms which react to form H D before they are moderated below the threshold energy for abstraction with C4Dlo.6 The ordinate of Figure 3 should go to zero at a threshold energy of
+
+ +
+
+
Figure 3. Abstraction probability ( I l)-I for reaction with C4D10 as a function of initial H atom energy. The values a t 1.6 and 2.0 eV are those of ref 2b. The parallelogram indicates the range of average initial H atom energy from the photolysis of HzS a t 1850 A, bracketed by the uncertainty in ( I l)-l for HzS and for HBr photolyzed a t various wavelengths. (I f 1)-l will go to zero at the threshold energy ET which is expected to be within the indicated range.
+
16
12
(2) (a) 9. D. Thompson, D. G. Carroll, F. Watson, M. O'Donnell, and 9. P. McGlynn, J. Chem. Phys., 45, 1367 (1966); (b) R. G. Gann and J. Dubrin, i b i d . , 47, 1867 (1967). (3) R. L. Carter, W. H. Hamill, and R. R. Williams, J. Amer.
d
S8 4
Chem. Soc., 7 7 , 6457 (1955). (4) R. M. Martin and J. E. Willard, J.Chem. Phys., 4 0 , 3007 (1964). (6) Gann and Dubrin found that within experimental error ( l a 1) - 1 for H,S was unaffected by substitution of HBr as the scavenger. However, a t our higher energies some substitution, H * C4D10 4 04D9H D, may occur; see 0.0. Chou and B. 9. Rowland, J. Amer. Chem. Soc., 8 8 , 2612 (1966). The occurence of substitution rather than abstraction by the D atom with the scavenger could lead to error in the calibration curve a t 2.9 eV. Based upon private communications with I?. S. Rowland and D. R . Davis on abstraction/ substitution ratios of hot and thermal H atoms, we estimate the possible error due to these effects to be less than 3 %.
+
01
0
I
0.5
I
1.0 HX/RD.
I
1.5
I
2.0
Figure 2. Product ratios as a function of reactant ratios for the 1849-Aphotolysis of HX-RD mixtures a t 25".
+
+
Volume 73,Number 4 April 1969
1160
NOTES
0.25-0.35 e v e 6 It is believed that only some 2% or less of the HBr photodissociations in the ultraviolet continuum yield 2Pl,zBr atoms.7 Such transitions will result in a lowering of the H atom recoil energy by about 0.45 eV. The predominant transition to groundstate Br atoms should give the H atom energies indicated in Figure 3 for the various wavelengths used. The three hot atom abstraction yields a t 1.6, 2.0, and 2.9 eV using HBr as the source are consistent and indicate that the present data in this region are best represented as a linear increase of yield with initial energy. The observed HzS-C4Dlo system ordinate of 0.18 then corresponds to an H atom energy of 2.3 f 0.1 eV. The range of reported values of the H-SH bond dissociation energy is 4.1-3.7 eV,8 giving 2.6-3.0 eV as the range of excess energy in the photochemical dissociation a t 1850 hi. Therefore an average of at least 7501, of the excess energy appears as kinetic energy of separation a t 1850 hi. This indicates that the energy partitioning is similar a t 1850 and 2138 hi despite the difference in initial vibrational state indicated spectroscopically, as Gann and Dubrin found that 80% or more of the excess energy goes to the H atom at the latter wavelength. A further conclusion of these results is that even at the high energy end of the 2000-k band the main photodissociation process is H2S + H HS rather than H2S+ H z S. The upper limit on the occurrence of the latter process which is consistent with our results is a quantum yield of about 0.25, corresponding to a quantum yield of 0.75 for hot atom production with all of the excess energy going to kinetic energy of separation. We have determined the quantum yield for Hz from HzS at 1850 as 1.0 4 0.1, but this does not distinguish between hydrogen atom and hydrogen molecule production in the primary process, since the fates of H and HS are expected to be H H2S + H z HS and HS HS +H2S S.8 We have also studied the photolysis of HzS-D~ mixtures a t 1850& and a comparison of the present H2S and HBr data with previous HBr data is presented in Table I. All of the systems in Table I have been found to follow the linear behavior Hz/HD =
+
+
A
+
+
+
+
Table I: H o t Hydrogen Atom Kinetic D a t a for D Abstraction from Deuterated Reactants System4
HBr-D2brc HBr-C4Dlo HBr-C2DGb HBr-CDf H2S-DzC HzS-C~DIO
(I
+ I)-’
0.61 0.23 0.14 0.062 0.43 0.18
w.
A-1
0.49 0.18 0.085 0.012 0.20 0.10
Data for photolysis a t 1850 * Data from ref 4. Intercepts and slopes have been multiplied by 2 to correct for the H D assumed to be produced thermally from reaction with HBr or HzSfollowing the hot atom abstraction producing D. The Journal of Physical Chemistru
A (HX/RD)
+
I, where HX is either HBr or HzS, over a wide range of (HX/RD) . The second column of the table gives the reciprocal of the slope. With the reservations cited previously14JA-l may be taken as an approximate measure of the average relative abstraction cross section ratio ( S R D / S H X ) over the relative energy distribution present in the system, Taking the ratio of the A-I values for the HBr-D2/HBrsystems and the H2S-D2/H2S-C4Dlo systems gives D ~and ~ ) 2.0 where the initial values of ( S D ~ / S Cof~ 2.7 hot atom energies are approximately 2.9 and 2.3 eV, respectively. It is interesting that the complex dynamics of monoenergetic atoms introduced into a thermal gas leads to the linear product dependence on reactant ratio which has now been observed with a variety of systems and initial energies. Further interpretation of such hot atom kinetics and the extraction of more definitive information on reaction cross section behavior above threshold requires more kinetic data than are presently available and the application of nonreactive cross section functions and statistical theory to describe the observed kinetics. (6) B. A. Thrush, Progr. Reaction Kinetics, 3 , 89 (1965). (7) R. 5. Mulliken, J. Chem. Phys., 8 , 382 (1940). (8) T. L. Cottrell, “The Strengths of Chemical Bonds,” Academic Press, London, 1958. (9) D. deB. Darnent. R . L. Wadlinger. and M. J. Allard, J. Phys. Chem., 71, 2346 (1967).
Activity Coefficients for Ionic Melts by R. Haase Instdtut f a r Physikalische Chemic, Rheinisch- Westfiilische Technische Hochschule, Aaehen, Germany (Received August 26, 1968)
While the definitions of activity coefficients for both nonelectrolyte solutions and electrolyte solutions are straightforward and well known, the situation with ionic melts is less clear and less well understood. It is true that most authors1 proceed in the same way as they do with nonelectrolyte solutions, but it is by no means evident whether this is the adequate scheme of description. As a matter of fact, it will be shown that it is expedient to introduce activity coefficients for ionic melts that are different from those in nonelectrolyte and electrolyte solutions. For simplicity’s sake, we shall restrict the discussion to binary ionic melts such as the systems NaC1 KC1 or PbClz PbBr2. Reciprocal salt pairs of the type NaCl+ KBr may also be treated as binary systems
+
+
(1) See, for example, H.Bloom, “The Chemistry of Molten Salts,” W. A. Benjamin, Inc., New York, N. Y.,1967.
