J. Phys. Chem. 1990, 94, 1619-1624 at flat band conditions is high. This feature does not occur in the experimental curves of Figure 3-5 and 7. The conclusion is that the value of sfbcannot be higher than sfb= 100. Together with the lower limit value, which was derived from the measurements in the anodic range, we can estimate the real rate constant of the surface recombination. By use of eq 2 our results for CdSe, 20 C sb C 100, yield 8.6 X lo5 cm/s < ko, C 4.3 X lo6 cm/s. Similar data have been reported for polycrystalline CdSe films.2s In the above discussion we have neglected the contribution to the photoluminescence from the recombination in the accumulation layer. This contribution has, however, been tacitly taken into account in our model. The holes generated in the space charge layer will either be driven into the bulk by the electric field, where (25) Storr, G.J.; Haneman, D. J . Appl. Phys. 1985, 58, 1677.
1619
they recombine, or recombine in the layer or contribute to the surface recombination. The near-band-gap fluorescence stems from a second-order recombination process governed by the product of free carriers np. This product remains constant in the accumulation layer or even decreases, if the onset of degeneracy is taken into account. By using eq 3 and 4 to calculate the photoluminescence intensity and neglecting the extension of the space charge layer, we have practically included the recombination in the space charge layer in our calculation. The exponential dependence of s on the potential drop A& in the semiconductor yields a satisfying representation of the photoluminescence behavior in the accumulation range for the CdSeand CdS-electrolyte systems. Registry No. CdSe, 1306-24-7; Gap, 12063-98-8; KOH, 1310-58-3; HC1,7647-01-0; Fe(CN)t+, 13408-63-4; Fe(CN)l+,13408-62-3; Na,S, 1313-82-2; KCI, 7447-40-7; cadmium sulfite, 13477-23-1.
Novel Size-Dependent Chemistry within Ionized van der Waals Clusters of 1,l-Difluoroethane M. Todd Coolbaugh, William R. Peifer, and James F. Garvey* Acheson Hall, Department of Chemistry, State University of New York at Buffalo, Buffalo, New York I4214 (Received: June 29, 1989; In Final Form: August 28, 1989)
We present in this paper evidence for size-dependentcluster chemistry occurring in van der Waals clusters of 1,l-difluoroethane. Clusters of C2H4F2are produced from a neat adiabatic expansion and are ionized via electron impact. In addition to the anticipated fragment ions, we observe ions with the general empirical formula of M,H+ (where n L 4). The reactive process that generates this species cannot be rationalized in terms of intramolecular analogues of known gas-phase bimolecular ion-molecular reactions. Hence, we feel the production of this product cluster ion represents an additional example of a brand new class of ion-molecule reactions that can only occur within the unique "solvated" environment of the cluster.
Introduction A subject of great interest in recent years has been the study of the physics of weakly bound van der Waals clusters. These species have been probed in a variety of ways to gain an understanding of their formation and to determine their various physical properties. However, the study of chemical reactions within clusters is especially intriguing to the physical chemist since the study of reactions within clusters may serve to join the disparate fields of bimolecular gas-phase reaction dynamics and solution chemistry. Most of the recent work in this area consists of utilizing the neutral cluster as one of the reagents for bimolecular reacs) created within the tion.'-* That is, an ion is rapidly (neutral cluster, via either electron impact or photoionization, which may then react with one (or more) of the solvating neutrals. The product cluster ion can then be directly detected via coqventional mass spectrometric techniques. Apart from the observation of protonated clusters, e.g., (H20),H+,9 (NH3)nH+,Io,II (1) Whitehead, J. C.; Grice, R. Faraday Discuss. Chem. SOC.1973, 55, 320. (2) King, D. L.; Dixon, D. A.; Herschbach, D. R. J . Am. Chem. Soc. 1974, 96, 3328. (3) Gonzalez Urena, A.; Bernstein, R. B.; Phillips, G. R. J . Chem. Phys. 1975,62, 1818. (4) Behrens, R. B., Jr.; Freedman, A.; Herm, R. R.; Parr, T. P. J . Chem. Phys. 1975, 63, 4622. ( 5 ) Wren, D. J.; Menzinger, M. Chem. Phys. 1982, 66, 85. (6) Nieman, J.; Na'aman, R. Chem. Phys. 1984, 90, 407. (7) Morse, M. D.; Smalley, R. E.Ber. Bunsen-Ges. Phys. Chem. 1984,88, 208. (8) Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Surf. Sci. 1985, 156, 8. (9) Hermann, V.;Kay, B. D.; Castleman, A. W., Jr. Chem. Phys. 1982, 72, 185.
0022-3654/90/2094- 1619$02.50/0
(CH3COCH3),H+,I2(CH30H),,H+,I3(CH30CH3),H+,I3and a few heterocluster ions such as [(ROH),.(H,O),]H+ I43ls and [(CH30H)n(H20),]H+,16there are few reported cases of chemical reactions taking place within the cluster ion itself,'2,1b20 even though it is known that electron impact ionization of clusters leads to ions that closely resemble many of the intermediates found in bimolecular ion-molecule reactions.21*22 When ionized, a polyatomic m o l e c ~ l (such e ~ ~ as ~ ~halohydro~ (10) Stephan, K.; Futrell, J. H.; Peterson, K. I.; Castleman, A. W., Jr.; Wagner, H. E.; Djuric, N.; Mark, T. D. Int. J . Mass Spectrom. Ion Phys. 1962, 44, 167. (11) Echt, 0.;Morgan, S.; Dao, P. D.; Stanley, R. J.; Castleman, A. W., Jr. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 217. (12) Stace, A. J.; Shukla, A. K. J . Phys. Chem. 1982, 86, 865. (13) Grimsrud, E. P.; Kebarle, P. J . Am. Chem. SOC.1973, 95, 7939. Morgan, S.; Castleman, A. W., Jr. J . Phys. Chem. 1989, 93, 4544; J . Am. Chem. SOC.1987, 109, 2867. (14) Stace, A. J.; Shukla, A. K. J . Am. Chem. SOC.1982, 104, 5314. (15) Stace, A. J.; Moore, C. J . Am. Chem. SOC.1983, 105, 1814. (16) Kenny, J. E.; Brumbaugh, D. V.; Levy, D. H. J . Chem. Phys. 1979, 71, 4757. (17) (a) Klots, C. E.;Compton, R. N. J . Chem. Phys. 1978,69, 1644. (b) Klots, C. E. Radiat. Phys. Chem. 1982, 20, 51. (c) Klots, C. E.Kinetics of Ion-Molecule Reactions; Ausloos, P., Ed.; Plenum: New York, 1979; p 69. (18) Ono, Y.; Ng, C. Y. J . Am. Chem. SOC.1982, 104, 4752. (19) Nishi, N.; Yamamoto, K.; Shinohara, H.; Nagashima, U.; Okuyama, T.Chem. Phys. Lett. 1985, 122, 599. (20) Stace, A. J. J . Am. Chem. SOC.1985, 107,755. (21) Milne, T. A.; Beachey, J. E.;Greene, F. T. J . Chem. Phys. 1972,56, 3007. (22) Ceyer, S. T.; Tiedemann, P. W.; Ng, C. Y.; Mahan, B. H.; Lee, Y. T. J. Chem. Phys. 1979, 70, 2138. (23) Mansell, P. I.; Danby, C. J.; Powis, I. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1449. (24) Baer. T.; DePristo, A. E.;Hermans, J. J. J . Chem. Phys. 1981, 76, 1449.
0 1990 American Chemical Society
1620 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Coolbaugh et al.
I
I
translated alo
blower package turbo-pump turbo-pump 1OOOm3 -h 360 1-s valve 360 1-s Figure 1. Schematic side view of differentially pumped cluster beam apparatus and quadrupole mass spectrometer. The temperature of the nozzle in the stagnation region is regulated by a circulating chiller.
carbon^^^-^') is internally excited, and this randomized excess energy can initiate bond fission, with the ion fragment distribution (mass spectrum) being well accounted for by the quasi-equilibrium theory. Indeed, the ion generated within a cluster does exhibit unimolecular fragmentation in a similar fashion. In addition to this simple bond cleavage, there can also occur extmsive bond re-formation within these ionized clusters. This suggests that reactions similar to gas-phase ion-molecule chemistry are occurring within a given cluster ion (here a solvated cation). Many of the major reactive pathways 0 b s e r v e d ~ ~can 9 ~ be ~ understood in terms of bimolecular ion-molecule reactions, observed previously in conventional ICR experiment^^^^^ and via high-pressure mass ~pectrometry.’~ In addition to the unimolecular fragmentation and bimolecular gas-phase chemistry occurring within the cluster, we have recently observed the generation of new cluster ions that cannot be explained by either of these two processes. That is, we observe product ion formation that has absolutely no counterpart in the gas-phase bimolecular reaction and that only occurs within a van der Waals cluster. These particular examples may be considered as truly “cluster” chemistry (i.e., chemistry that can only occur within a cluster). These new processes include the generation of (NH3),N2H8+ions from ammonia clusters,35the generation of (CH30CH3),H30+ions from dimethyl ether clusters,36the generation of (CH3F),CH3CH3+ ions from methyl fluoride clust e r ~ , and ~ ~ most * ~ ~recently the photogeneration of MOO+ and Moo2+ ions from van der Waals clusters of molybdenum hexa~arbonyl.~’Indeed, all of these systems, which fall outside of bimolecular chemistry, might best be described as “termolecular” ion-molecule reactions occurring within the confined environs in a cluster. We feel this body of work has revealed a new class of chemical reactions, and we hope to employ these systems to study in detail the role played by ion solvation in the reaction dynamics within (25) Simm, I. G.; Danby, C. J. J . Chem. Soc.,Faraday Trans.2 1976,72, 861. (26) Powis, I.; Danby, C. J. Chem. Phys. Lett. 1979, 65, 390. (27) Powis, I. Mol. Phys. 1980, 34, 31 1. (28) Garvey, J. F.; Bernstein, R. B. Chem. Phys. Lett. 1986, 126, 394. (29) Garvey, J. F.; Bernstein, R. B. J . Phys. Chem. 1986, 90, 3577. (30) McAskill, N. A. Aust. J . Chem. 1970, 23, 2301. (31) Herd, A. A.; Harrison, A. G.; McAskill, N. A. Can. J. Chem. 1971, 49, 2217. (32) Beauchamp, J. L.; Holtz, D.; Woodgate, S. D.; Patt, S. L. J. Am. Chem. SOC.1972, 94, 2798. (33) McMahon, T. B.; Blint, R. J.; Ridge, D. P.; Beauchamp, J. L. J. Am. Chem. SOC.1972, 94, 2798. (34) Blint, R. J.; McMahon, T. B.; Beauchamp, J. L. J . Am. Chem. SOC. 1974, 96, 1269. (35) Garvey, J. F.; Bernstein, R. B. Chem. Phys. Lett. 1988, 143, 13. Coolbaugh, M. T.; Peifer, W. R.; Garvey, J. F. Chem. Phys. Lett. 1989,156, 19. (36) Garvey, J. F.; Bernstein, R. B. J . Am. Chem. SOC.1987,109, 1921. (37) Peifer, W. R.; Garvey, J. F. J . Phys. Chem. 1989, 93, 5906.
TABLE I: Electron Impact Mass Spectrum for the CHjCHFz Monomep ion CHF2+ CH3CF2’ CHqCHF’ C2H2F+
m/z 51 65 47 45
C2H3+
27
C2H2F2+ C2H3F+ C2HF+ C2H2+ CF+
64 46 44 26 31
method of generation loss of CH3 loss of H loss of F loss of H2F loss of HF2 loss of H2 loss of HF loss of H3F loss of 2HF loss of CH4F
intensity 1000 492 129
117 54 41 39 30
27 24
the cluster ions. That is, observations of novel reactive pathways and unexpected dynamics within cluster ions can demonstrate the profound effect that only a few “solvent” molecules have on the course of a reaction and hopefully answer questions such as the following: (a) To what degree can current knowledge about gas-phase ion-molecule reactions be applied to explain such reactions occurring within the cluster ions, and under what circumstances is it inadequate? (b) How are these elementary reactions influenced by stepwise solvation (i.e., increasing cluster size)? (c) How do the rates of reactions occurring “inside” the clusters compare with those for the bimolecular analogues? Typically the dominant reactive process that occurs within cluster ions consists of a bimolecular reaction between the monomer ion and one of the neutral solvent molecules to generate a protonated ion and a radical. However, many molecules do not generate a stable parent ion; hence in these systems one might then expect the absence of any protonated cluster ion peaks in the mass spectrum. That is, the mass spectrum of such a cluster beam would be composed of only solvated fragment ions. The mass spectrum of the molecule 1,I-difluoroethane (DFE), shown in Table I, represents such a case (Le., the ground-state parent ion is thermodynamically unstable with respect to fragmentation). We report in this paper that for van der Waals clusters of DFE we observe the formation, for n > 4, of M,H+ (where M = CH3CHF2throughout this paper). We attempt to rationalize this unusual behavior by postulating two distinct mechanisms that can account for the generation of protonated DFE clusters, in the absence of a stable precursor parent ion.
Experimental Section The experimental apparatus has been described in detail elsewhere35and is illustrated schematically in Figure 1. Briefly, the apparatus consists of a differentially pumped Campargue continuous molecular beam source38coupled to an Extrel C-50 quadrupole mass spectrometer. Neat DFE (Linde, >98%) is (38) Campargue, R.; Lebehot, A. Rarefied Cas Dyn. 1974, 9, 1. Campargue, R. J . Phys. Chem. 1984,88,4466.
The Journal of Physical Chemistry, Vol. 94, No. 4 , 1990 1621
I,l-Difluoroethane van der Waals Clusters
m=528
n=8
m=462
0
m
d
A,
m&
,Mi6
m
k n=3
9-t-H-tI'''I ' L ' m-2
m
m+2 mL
mii
m/z Figure 2. (a) Raw 100-eV mass spectra showing generation of M,(CH,CHF)+ (Le., loss of F), as a function of cluster size (50 scans/
spectrum). (b) (a) expanded by a factor of 10 (300 scans/spectrum) showing generation of M,(C2H4F)+ (i.e., loss of F and H). Peak observed at m + 1 is due to I3C contribution of the adjacent M,(CH3CHF)+ peak. In both (a) and (b) the number over the empirical formula at the top of the figure indicates the appropriate reactions referred to in the text. introduced into the stagnation volume of the apparatus in order to maintain a stagnation pressure of 3 atm. Neutral clusters form in the free-jet expansion created as the high-pressure gas expands adiabatically through a 250-pm-diameter nozzle into the source chamber. In these experiments, the nozzle temperature was maintained (within 0.1 K) at 278 K by a recirculating chiller. Pressure in the source chamber is maintained at about 50 mTorr during operation of the cluster beam by a lo00 m3/h roots blower. The cluster beam passes from the source chamber, through a 0.5-mm-diameter skimmer (about 40 nozzle diameters downstream from the start of the expansion), and into a collimation chamber. This chamber is pumped by a 360 L/s turbopump and is maintained at Torr during beam operation. The beam passes out of the collimation chamber through a second 0.5-mm-diameter skimmer and into the mass spectrometer chamber. This chamber is also pumped by a 360 L/s turbopump and is maintained at 5 X IO-' Torr during beam operation. The cluster beam enters the ion source and quadrupole mass filter in an axial configuration. The mass filter is typically operated at better than 0.3 amu mass resolution up to about 1200 am^,^^ although we can select the best compromise between resolution and sensitivity, depending on the demands of a particular experiment. Emission current in the source is maintained at 1 mA by current regulation at fixed electron energy. In these experiments, mass spectra were collected at various electron kinetic energies ranging from about 20 to 100 eV. Cluster ions and fragments formed in the ion source are filtered by the quadrupole and imaged onto an off-axis channeltron. The amplified signal from the channeltron is then averaged by a digital storage oscilloscope (Le Croy 9400) for all the small mass range scans (Figures 2-4). In order to assure the validity of the mass spectral data, we regularly calibrated the mass scale and the sensitivity of the quadrupole mass spectrometer, through the use of a Teknivent (39) Peifer, W. R.; Coolbaugh, M. T.; Garvey, J. F.J . Phys. Chem. 1989, 93, 4700.
Figure 3. Raw 100-eV mass spectra showing generation of M,(C2H3FZ)+ (Le., loss of H) and M,H+ (cluster reaction) as a function of cluster size (100 scans/spectrum). Peaks observed at m and m 2 at large cluster size are due to I3C contribution of the m - 1 and m 1 ion peaks,
+
+
respectively. The number over the empirical formulas at the top of the figure indicates the appropriate reactions referred to in the text.
0
/L
60eV
CI
m o Y 50eV
A
Y
.cI
$ 0 Y
30eV
0
U
t o
A
U
0
I
20eV
m +I '
262
264
'
266
m/z Figure 4. Mass spectra of the generation of M4(C2H3F2)+(i.e., loss of H) and M4H+ (cluster reaction) as a function of electron energy (100 scans/spectrum). Each spectrum is normalized with respect to the M4H+ (m 1 ) peak. The number over the empirical formulas at the top of the figure indicates the appropriate reactions referred to in the text.
+
data acquisition system. This data system was also used to acquire the large mass range scans (Figure 6 ) . In the range below 500 amu, we routinely calibrated the instrument against the mass
1622 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Coolbaugh et al.
spectrum of perfluorotributylamine, while beyond 500 amu, we used an argon cluster beam as a calibration standard. All mass spectra are reported with the use of a mass scale of nominal masses.
Results and Discussion In the following discussion we will suggest mechanisms for the generation of the four types of cluster ions which we observe (channel 1, M,CH3CFH+; channel 2, M,C2H,F+; channel 3, M,C2H3F2+;and channel 4; M,H+). All ions listed in italics in the following reactions are ions that are directly observed by our mass spectrometer. Clearly the largest cluster ion signal observed is for the sequence of peaks corresponding to the empirical formula M,CH3CFH+, as shown in Figure 2a. This cluster ion is generated, following electron impact, by the unimolecular fragmentation of the monomer ion through fission of the C-F bond
-
which is the most intense ion peak.40 However, in terms of clusters, we are unable to discern any presence of cluster ions with the formula M,CHF2+. This difference between the monomer and the cluster can be understood from ICR work by Ridge," in that the CHF2+ion can rapidly react with the monomer to form M
+ CHF2+
-
+
CH3CFH+ CHF,
(14-2)
Clearly then any cluster ions of the formula M,CHF2+ can internally react within the cluster, via channel 1 4 - 2 , to produce M,,CH3CFH+ and fluorofom. This is an example of a sequential bimolecular reaction (1 4 - 2 ) following a unimolecular fragmentation (14-1) and explains the prominence of M,CH3CFH+ peaks as well as the absence of M,CHF2+ peaks. In addition to the generation of M,CH,CFH+ there are two minor fragmentation pathways. The first is shown in Figure 2b, where following the formation of M,CH3CFH+ (via channel l ) , an additional fission of a C-H bond occurs. CH3CHF'
---+
C2H3FC
+H
(2)
This channel is highly unlikely and is an order of magnitude less in intensity compared to channel 1. The third channel consists of a single unimolecular fragmentation, which results in just the fission of a C-H bond as follows. M+
-
CzH3Fz'
+H
(3) This generates the cluster ion with a formula of M,C2H3F2+,as shown in Figure 3. We see in Figure 3 another sequence of peaks which have the empirical formula of M,H+. These peaks particularly stand out in the mass spectrum in that they only appear at n > 4 and continue to become more prominent with increasing cluster size. This is in direct contrast to the preceding three channels discussed, which monotonically decrease with increasing cluster size. Figure 4 shows how M,H+ and M,C2H,F2+ vary as a function of electron energy. As we can see, channel 3 drops much more rapidly in intensity, showing that its mechanism of generation is much more energy dependent than the generation of M,H+ (Le., a higher activation barrier). Typically the appearance of cluster ions with the empirical formula M,H+ would not be particularly surprising since they are readily generated by the bimolecular ion-molecule reaction of the tY Pe M + M+ MH' + C2H3F2 (4-i) -+
However, if that were the case, this reaction would then be ex(40) Ridge, D. P.Ph.D. Thesis, California Institute of Technology, 1972.
1
t
F
I I
-
CH,CFH+ + F (14) It is interesting to note that in the mass spectrum of the DFE monomer, reaction 14 is the second most prominent path (Table I). The most intense peak corresponds to fission of the C-C bond generating the CHF2+ ion M+ CHF2++ CH, (1-ii-1) M+
k ,Mn., [ M + C,H,F+ (4,
M,.,H+
cluster reaction n24
I
+ C,H,F
Figure 5. Schematic flowchart representing the four main channels
available to the 1,l-difluoroethane cluster ion: either fragmentation (channels 1-3) or cluster reaction (channel 4). Empirical formulas in boldface represent ions detected via the mass spectrometer. pected to occur for all n, not just n 2 4. In addition, for D E ,the monomer cation is relatively unstable41 (i.e., heat of formation is 0.65 eV above the sum of the heats of formation of H F and CH2CHF+). As a result, the M+ ion (parent ion) is absent in the mass spectrum of the monomer (Table I). If the C2H4F2+ion cannot be generated, how then are we forming the protonated cluster ions? As Jungen and co-workers point out,"1 the parent ion might be observed as a metastable ion if formed in the Franck-Condon region of the neutral molecule. Thus, if the parent ion is generated within the cluster, the presence of solvating molecules may stabilize it long enough, such that it may react with one of the neutral monomers by channel 44. Therefore, the fact that the protonated cluster ions only appear for n 1 4 suggests that M5+is the critical size for stabilizing the monomer ion.42 However, since we do not observe the presence of the M,+ ion, for any n, that must mean channel 4-i is 100% efficient and consumes all the parent ion generated. A different possible explanation for the formation of the protonated cluster ion may come from one of the newly created fragment ions (channels 1-3), reacting with a solvating monomer in a sequential step. Of the three possible fragments, CH3CFH+ seems to be the best candidate since the reaction M
+ CH3CFH+
-
MH+ + CH2CHF
(44)
is only 1.5 eV more endoergic than reaction 4-i.43 Should this reaction occur one would expect an increase in the ion intensity of M,H+ at the expense of M,CH3CFH+ ion intensity. To see if this was the case, we wished to observe the change in the ion intensity over as large a size range as possible, for the M a + and M,CH,CFH+ cluster ions. Figure 5 shows a schematic diagram displaying the three known fragmentation channels (channels 1-3) and the proposed "cluster" reaction (channel 4). While we do indeed observe more product channels, other than the four already mentioned, their intensity is over an order of magnitude smaller than the smallest channel (channel 3) and will be discussed in a future publication. Figure 6 shows the product yield as a function of parent cluster size as well as a function of process channel, for two different electron energies. In calculating the percent yield we assumed (41) Heinis, T.; Bar, R.;Bijrlin, K.; Jungen, M. Chem. Phys. 1985,94,235. (42) In the case that evaporation is taking place in conjunction with the
chemical reaction, the actual reaction may be taking place in a much larger cluster than MS+. (43) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.;Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref Data 19%8, 17, Suppl. I . (44) EPA-NIH Mass Spectral Data Base: National Bureau of Standards: Gaithersburg, MD, 1978.
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1623
1,I-Difluoroethane van der Waals Clusters
a) 100eV
b) 30eV
I
Channel 1 loss of F Channel 2 loss of F and H Channel 3 loss o f H Channel 4 loss of F and generate C2H3F
-
-
Figure 6. (a) Bar graph representing percent yield of daughter cluster ion as a function of parent cluster ion size (n) and process channel (as given in Figure 5 ) at 100-eV electron impact energy. Note how all three of the fragmentation channels drop as a function of the cluster size (channel 1, 87% 43%;channel 2, 8% =$ 3%; channel 3, 5% 2%) while the cluster reaction channel (channel 4) increases with n (0% =+ 52%). (b) Bar graph representing percent yield of daughter cluster ion as a function of parent cluster ion size (n) and the process channel (as given in Figure 5 ) at 30-eV electron impact energy. Note how all three of the fragmentation channels drop as a function of the cluster size (channel 1, 90%=+ 29%; channel 2, 7% 2%; channel 3, 4% =$ 3%) while the cluster reaction channel (channel 4) increases with n (0% 66%).
the parent-daughter relationship between the ions as indicated in Figure 5 and calculated a percent yield for each parent cluster size (n = 3-15), assuming it had to appear as an ion from one of the four channels. In making this analysis we also assumed that no evaporation occurs. Typically, for very small clusters, the barrier to reaction is smaller than the barrier for e ~ a p o r a t i o n . ~ ~ However, should any evaporation occur, it will only shift rows relative to each other and not affect the qualitative conclusions which we draw from the analysis.
-
In Figure 5, at small cluster size, all that occurs is primarily channel 1, unimolecular fragmentation. However, after n = 4 we see a rise in channel 4 and a subsequent drop in channel 1. This effect continues all the way up to the limit of our mass spectrometer and is apparently independent of electron energy. We feel that this change of branching ratio as a function of cluster size is consistent with the suggested reaction 4-ii, in that the fragment ion CH3CFH+ is consumed to form the protonated cluster ion and fluoroethylene. We do not know why this reaction
1624
J . Phys. Chem. 1990, 94, 1624-1626
only occurs in clusters above a critical size, but it may suggest a steric barrier to reaction 4-ii at small cluster sizes that is absent above a certain critical size. That is, there may exist an orientational effect, which is only overcome by enhancing the probability of a properly positioned collision encounter (e.g., by surrounding the reactant ion by a full shell of solvent molecules). We hope to continue to study other fluorinated hydrocarbons in order to better understand the systematics of this cluster chemistry.
Conclusion For van der Waals clusters of 1,l-difluoroethane, we have observed a reaction that forms a protonated cluster ion only above a certain cluster size. The generation of this product appears to also exhibit larger reaction efficiency with increasing cluster size. We have attempted to rationalize this result in terms of two possible reactive mechanisms: a solvated metastable parent ion
directly reacting with a neutral monomer (reaction 44) or a fragment ion, CH3CFH+, reacting with a neutral monomer to generate fluoroethylene (reaction 4 4 ) . Whatever the true nature of the reactive mechanism, it is clearly a process that has not been observed in bimolecular experiments, suggesting unique ionmolecule chemistry that can only occur in the "solvated" environs of a cluster. Such intramolecular processes are providing an important bridge between bimolecular gas-phase reaction dynamics and condensed-phase chemistry.
Acknowledgment. This research was supported by the Office of Naval Research, which is hereby gratefully acknowledged. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. Registry No. C2H4F2,75-37-6.
Mechanbm of Solute Transfer across Water/Oil Interfaces in Biphasic Microemulsion Systems C. Tondre* and A. Derouiche Laboratoire d'Etude des Solutions Organiques et Colloidales (LESOC), UA CNRS No. 406, UniuersitC de Nancy I, B.P. No. 239, 54506 Vandoeuvre-les-Nancy Cedex, France (Received: July 7 , 1989)
The microdroplets constituting the dispersed phase of a microemulsion have numerous potential applications as transfer agents. The mechanism of loading/unloading of these droplets with substances to be transferred through liquid/liquid interfaces is not easy to ascertain. We compare in this work the transport of Ni2+ ions by two microemulsion systems: CI2EO4/1hexanol/ndecane/water (system I) and AOT/n-decanelwater (0.25 M salt) (system 11). From the effect of the anion associated with Ni2+on the measured flux, we can conclude that a 'direct" interfacial transfer prevails in the case of system I1 and an "indirect" transfer in the case of system I.
Introduction Microemulsion droplets have been shown to behave as possible carriers for the transport of substances through a medium where This property has potential they are not (or poorly) applications in many biological as well as technological fields, among which the most promising are the use of microemulsions as drug delivery systems: blood substitute^,^ and transfer agents in separation and hydrometallurgical processes? Our contribution to the development of such applications has been concerned with the characterization of the ability of the microdroplets to play the part of mobile carriers. We have applied for this purpose liquid membrane techniques to biphasic microemulsion systems consisting of a microemulsion phase in thermodynamic equilibrium with an external phase. This has allowed us to measure in different situations the flux of a solute transported from a source phase to a receiving phase across a microemulsion phase used as liquid membrane."-3 The data were found to be consistent with a classical facilitated diffusion model which implies the transfer of the solute across the first liquid-liquid interface, solubilization in a micro( I ) (a) Tondre, C.; Xenakis, A. Colloid Polymn. Sci. 1982, 260, 232. (b) Tondre, C.; Xenakis, A. In SurJucranrs in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, pp 1881-1896. (c) Tondre, C.; Xenakis, A. Faraday Discuss. Chem. SOC.1984, 77, 115-1 26. (2) (a) Xenakis, A.; Tondre, C. J . Phys. Chem. 1983, 87, 4737. (b) Xenakis, A.; Tondre, C. J. Colloid Inreface Sei. 1987, 117, 442. (3) Xenakis, A.; Selve, C.: Tondre, C. Tulunra 1987, 34, 509. (4) Fubini, 8.;G a m , M. R.;Gallarate, M. In?. J. Phorm. 1988,42, 19-26. Halbert, G. W.;Stuart, J. F. B.; Florence, A. T. Ibid. 1984, 21, 219-232. (5) Mathis, G.; Leempoel, P.; Ravey, J. C.; Selve, C.; Delpuech, J. J. J . Am. Chem. SOC.1984, 106, 6162-6171. Cecutti, M. C. Thesis, Toulouse, France, 1987. (6) Fourre, P.; Bauer, D. C.R. Acad. Sci., Ser. 2 1981, 292, 1077. Bauer, D.; Komornicki, J. Ini. Soluen? Extr. Conf. [Proc.] 1983, 315. Wu, C.-K.; et al. Scienria Sinica (Engl. Trunsl.) 1980, 23, 1533; Inr. Solvent Exrr. Conf., [Proc.] 1980,80-23. Osseo-Asare, K.; Keeney, M. E. Sep. Sei. Techno/. 1980, IS, 999. Kim, H. S.: Tondre, C. Sep. Sei. Technol. 1989, 24, 485.
0022-3654/90/2094- 1624$02.50/0
droplet, diffusion of the droplet through the stagnant layers, and the reverse operations at the second liquid/liquid interface.lcv2a It is very important to know the mechanism of interfacial transfer precisely. This is particularly true if we are interested in the optimization of the transport process. It may also considerably influence the effectiveness of liquid membrane separation based on the use of microemulsions. Previous experiments had suggested to us that there may be no general mechanism valid for any microemulsion system.' Instead, different transfer modes could be operative depending mainly on the nature of the surfactant. We were missing so far a clear and convincing demonstration. We think that the results reported below provide a definitive evidence.
Experimental Section The surfactants had the following origins: Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) was purchased from Sigma and tetraethylene glycol dodecyl ether (C12E04)from Nikko Chemicals (Tokyo, Japan). They were both used without further purification. The liquid membrane transport experiments are relative to two different quaternary systems, both leading after phase separation to a water-in-oil microemulsion phase in equilibrium with an external phase mainly constituted of water. The first system (I) consisted of CI2EO4/1-hexanol/n-deanelwater with weight ratios 7.5/2.5/40/50. The composition of the second system (11) was AOT/n-decane/H,O, 0.25 M KBr (7/40/53). The microemulsion phases obtained after phase separation included 6.5% water in the first case and 8.5% in the second one. Following the previously described procedure,lCq3we have used the reversed micelles to carry Ni2+ions. The nickel salt was introduced in the source aqueous (7) Derouiche, A.; Tondre, C. J . Chem. Soc., Faraday Trans. I 1989,85, 3301.
0 1990 American Chemical Society
