J. Phys. Chem. 1994, 98, 13517-13523
13517
Reaction between O(3P) and Organized Organic Thin Films Y. Paz, S. Trakhtenberg, and R. Naaman* Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel Received: April 22, 1994; In Final Form: October 10, I994@ The reaction of an atomic beam of O(3P)with organized organic thin films (OOTFs) of amphiphilic molecules was investigated applying in-situ IR,X P S , and wettability measurements. Both kinetics and surface temperature effects were monitored. The reaction probability was much larger for the OOTF relative to the gas phase process. The reaction rate was found to depend on the organization of the thin films. Structural phase transition was observed by monitoring the changes in reactivity. A curve-crossing model explains the observations and rationalizes the lack of activation energy and the dependence of the reactivity on the penetrability in between the chains.
Introduction During the past years increasing attention has been drawn to organized organic thin films (OOTFs) due to their possible use in a variety of applications such as protective layers, biosensors, optical sensors, and nonlinear optic devices. Naturally, the chemical durability of these closed packed organized structures is of great importance. In addition, reactions of OOTFs may be governed by the ability of gas phase reactants to penetrate in between the chains. Nevertheless, only a few works were concentrated on studying the correlation between the structure of OOTFs and their reactivity with gas phase molecules. In a former work we presented qualitative evidence for the reactivity of OOTFs with oxygen atoms, and how it is affected by the structure of the OOTF.' In the present work our aim was to explore how gas phase reaction changes quantitatively, when one of the reactants is bound to the surface, to address the relation between the reactivity and structure of organized organic thin films, and to probe whether their reactivity can serve as a tool for exploring their structure. In order to answer these questions, it was important to choose a system whose gas phase analog was well studied. One of the reactions which were thoroughly investigated both in the gas phase and in the condensed phase and which can be studied with ultrathin organized organic films is the reaction between ground state atomic oxygen (O(3P)) and saturated hydrocarbons. In the gas phase these reactions proceed through an abstractionmechanism, generating a hydrocarbon radical and OH. The exothennicity of the process (in kcal/mol) is 2.3 for primary (RCH3) carbons, 7.0 for secondary (RzCHZ),and 10.3 for tertiary (R3CH) carbons. The energy barriers to reaction vary substantially in the series: 6.9, 4.5, and 3.3 kcavmol, respectively.2 Since the activation energies and the preexponential factors were found to be almost independent of the exact nature of the hydrocarbon,rates for large complex hydrocarbons can be approximately predicted as a sum of the rates of individual C-H bonds. The absence of rotational excitation in the OH results from the colinearity of the 0-H-C ~ o m p l e x . ~In. ~contrast, electronically excited oxygen, in its singlet state (O('Dz)), has no energy barriers for these reactions which proceed via in~ertion.~ Therefore the reactions on the singlet potential energy surfaces are faster. For example, the rate constant for the reaction of singlet oxygen with methane was found to be as large as 1.4 x 10-lo cm3 molecules-' s-l compared with 1 x cm3 molecules-' s-l for the triplet state.' 'Abstract published in Advance ACS Abstracts, November 15, 1994.
0022-365419412098-13517$04.50/0
Structural information in general and phase transitions in particular have been observed in OOTFs by IR8,9and optoacoustic1° spectroscopy, X-ray diffraction," atomic force microscopy,'*J3 and helium scattering. l4 Phase transition of LB films was studied also by observing the frequency of a quartz crystal microbalance deposited with LB f h s . 1 5 Recently, X-ray grazing angle incidence diffraction studies16 on LB films of cadmium arachidate (Cd.4; (CH3(CHz)1&O0-)zCdZ+) on silicon indicated that, for a number of layers greater than 3, the structure at room temperature is orthorhombic with lattice dimensions of a = 4.85 8, and b = 7.50 A. Increasing the temperature above 60 "C results in gradual expansion of the lattice, mainly in the b dimension and the formation of a hexagonal lattice. The oxidation reaction of unsaturated OOTFs by KMn04 in the liquid phase was found to depend on the density of molecules in the reacting layer and on the type of bonding to the surface (covalent or i ~ n i c ) . ~ ~ItJwas * claimed that this oxidative attack proceeds through a two-stage mechanism. The first step involves the reaction at structural defects (domain boundaries, kinks) accessible to the reagent upon its perpendicular penetration. The second step is via lateral propagation from initially attacked sites. Since reactions of OOTFs must involve penetraton of the gas phase reactant in between the chains, the study of the permeation of gas molecules through OOTFs is of great relevance. It was established that well-packed Langmuir-Blodgett (LB) layers adsorbed on water reduce the penetration of small gas phase molecules into the ~ a t e r ' and ~ , ~that ~ the penetration may be modeled assuming an energy barrier linearly proportional to the OOTF chain length.21
Experimental Section In the present study we have performed two types of experiments. In the first, we investigated the dependence of the reaction probability on the surface temperature whereas the second used in-situ measurements of reaction kinetics. For both experiments, atomic oxygen in its electronically ground state was produced by a 2450 MHz microwave discharge and was introduced into the vacuum through a 30 cm long quartz tube, to insure the quenching of electronic excited species like O(1D).z2 The absence of excited oxygen atoms was verified by reacting the oxygen atoms with saturated hydrocarbons (methane and cyclohexane) and monitoring the OH formed. While the reaction with O(3P) produces rotationally cold OH, the reaction with O('D) produces rotationally hot molecules. This method for O(lD) detection was verified experimentally, 0 1994 American Chemical Society
13518 J. Phys. Chem., Vol. 98, No. 51, 1994 and a sensitivity of 1:1@ was obtained. It means that one atom of O(lD) in lo4 atoms of O(3P) can be detected. The tube was pretreated with phosphoric acid to inhibit heterogeneous recombination of the O(3P) atoms. Two classes of thin films were investigated: LangmuirBlodgett (LB) films and self-assembled (SA) films. Three types of LB films were studied: cadmium arachidate (CdA) films on glass slides, cadmium behenate (CdB; Cd2+(CH3(CH:!):!0COO-)z), and deuterated cadmium behenate (CdBD) on glass and silicon wafers. For CdA, the temperature and surface pressure during the transfer were 20 "C and 30 mN/m, respectively. For CdB and CdBD, the pressure and temperature during transfer were 32 mN/m and 25 "C, respectively. The pH of the subphase was balanced at 8.5 by adding a minute amount of ammonia. An isotherm was measured each time a new solution was prepared. An indication of the quality of the films was obtained by measuring the area during the transfer from the water surface onto the solid substrate. In addition, direct absorption FTIR measurements were performed. The IR absorption was measured also for one, three, five, and seven layers of CdA. A linear correlation between the absorption and the number of CdA layers was observed. Self-assembled (SA) monolayers of octadecyltrichlorosilane (OTS) or methyltrichlorosilane (MTS), deposited on glass or on silicon slides, were studied. The OTS monolayer consists of 18-carbon alkyl chains, while the MTS monolayer consists of a single methyl group attached to the surface through siloxyl groups and cross linked through Si-0-Si bonds. For the temperature dependence studies each slide (either with a LB or SA film) was heated in the vacuum (background pressure 1 x low6Torr) to the required temperature and then exposed to a beam of ground state oxygen atoms for 40 s at an estimated flux of 7 x 1014atoms cmP2s-l. This process was repeated for the other side of the slide. FTIR direct absorption was then used for monitoring the reduction in the CH:! and CH3 groups due to the reaction. In order to investigate the reaction kinetics, FTIR measurements were performed on slides that were positioned inside a high-vacuum chamber, which enabled us to perform the reactions and probe the surface "in-situ". The apparatus consists of a stainless steel chamber having two 1.75 in. BaF2 windows, implanted inside the sample compartment of a BRUKER IFS66 FTIR spectrometer. The chamber was pumped by a turbomolecular pump (Balzers TPU-050) backed by a small rotary pump. Oxygen atoms were produced by a microwave discharge as described before, external to the measurement compartment. Complementary XPS (Kratos AXISHS) measurements were performed on an OTS monolayer adsorbed on a 500 8, aluminum film evaporated on glass. The angle between the X-ray source, the scattering point at the surface, and the detector was kept at 60" whereas the surface itself could be rotated. Measurements were taken at angles of 0" and 70" between surface normal and detector. The monolayer was measured prior to exposure to the oxygen beam and after 90 and 300 s of exposure.
Results This section is divided into two parts: the frrst concentrates on results related to the measuring of the kinetics of the reaction of OOTFs due to exposure to O(3P), while the second part focuses on the interplay between reactivity and structure in organized organic thin films. (a) Reaction Rate Measurements. Figure 1 presents changes in a direct absorption (DA) spectrum of cadmium behenate adsorbed on silicon as a function of reaction time,
Paz et al.
2800
2850
2900
3000
2950
cm-' Figure 1. Direct absorption spectra of a CdB/Si monolayer exposed to atomic oxygen at several exposure times. 0.0
-0.5 h
9 \
-
v I
c
-1 .o
-1.5
0
100
I
I
200
300
400
Time (sec) Figure 2. Logarithmic scale plot for the absorption signals of the CH2 (filled squares) and CH3 (open squares) asymmetric stretching bands as a function of reaction time for a monolayer of CdB. obtained by the "in-situ" FTIR apparatus. Three absorption peaks are shown corresponding to the symmetric and asymmetric stretch modes of the CH:! group and to the CH3 asymmetric stretch mode. The decrease in both methyl and methylene absorption signals is clearly observed. In order to extract the reaction kinetics, the changes in the area under each peak were measured. The normalized logarithmic scale plots of the CH:!(a) and CH3(a) signals as a function of reaction time are presented in Figure 2. This data was obtained in a continuous flow reactor where the flux and velocity of the oxygen atoms remained constant during the reaction. Therefore, the linearity of the logarithmic scale plot indicates a pseudofirst-order reaction kinetics:
I = Io exp(-k[O]t) where I is the amount of functional groups at time t and [O]is the concentration of oxygen. The slope for the CH2(s) signal was found to be the same as for the CHZ(a) signal within *lo%. The same first-order kinetics was observed also for CdBD, mixed monolayers of CdBD and CdB, and OTS adsorbed on both glass and silicon substrates. After the consumption of ca. 60% of a monolayer, the reaction rate was found to level off. In order to compare with the identical gas phase reactions, the reaction probabilities (Rp) for both phases have been
J. Phys. Chem., Vol. 98, No. 51, 1994 13519
Reaction between O(3P) and Organic Thin Films calculated. Rp represents the ratio of the rate of “productive” collisions between the oxygen and the hydrocarbon (either attached to the surface or in the gas) to the total number of collisions per second. In the gas, the “rate of productive collisions per functional group” is the kinetic rate constant, taken from the literature; multiplied by the concentration of oxygen atoms during the reaction. For the monolayer, this number is extracted from the slope in Figure 2. The total number of collisions per second per one functional group was calculated for both phases, assuming a hard sphere model:
Z = n d 2F -
TABLE 1: Bond Energies and Atomic Concentrationfor an OTS/Al Monolayer Exposed to O(3P) for Various Times atomic concentration normal to surface (%)
grazing angle (%) peak
0 (1s) C1 (1s) C2(1s) All (2p) A12 (2p) Si(2p) Cl(2p)
Eb(eV) t = O 532.9 285.5 288.0 75.7 73.0 103.3 200.1
t=90
t=300
t=O
t=90 t=300
23.4 31.84 51.07 34.3 11.29 16.34 14.96 4.34 3.41 3.94 3.64 0.91 0.56
36.35 24.13 15.76 16.00 3.82 3.65 0.29
30.21 43.73
38.58 27.36 7.69 16.32 6.66 2.82 0.57
15.45 6.77 2.90 0.94
42.07 21.01 11.56 15.91 6.36 2.73 0.36
A
where d is the radius of collision (taken as 2 A), F is the total flow of atomic oxygen, and A is the cross section of the oxygen beam at the surface. For the surface reactions, the concentration of oxygen atoms was estimated by measuring the flow of 0 2 during the reaction ((2 i~0.5) x 10l6 molecules s-l). Massspectrometry measurements indicated that the concentration of O(3P) in the oxygen beam is about 10%. A beam divergence of 25” was assumed. It means that the radius of the beam at the surface was 14 mm. The mean velocity of the oxygen atoms is 6 x 104 cm s-l. On the basis of the above values, the reaction probability at 300 K for the methyl group of the monolayer was found to be 3 x about 3 orders of magnitude larger than the gas phase value (7 x As for the methylne, the difference in the reaction probability between the gas and the monolayer is less pronounced ( 2 x and 5 x respectively). The errors in the calculations for the reaction probabilities are less than 1 order of magnitude and result from some uncertainty in the angular distribution of the beam and in the percentage of atomic oxygen in the beam. The high reaction probability is another indication that no contamination of electronically excited oxygen is responsible for the observations, since it requires a few percent contamination of O(’D). This amount could be easily detected by the methods used by us and by others (as described in ref 22). Direct comparison between the reactivity of the methyl groups in MTS and OTS was possible by direct absorption FTIR spectroscopy. A decrease of no more than 7 f 4% of the MTS CH3 signal was observed at a time corresponding to a loss of 45 f 7% of the OTS methyl signal. External reflection IR spectroscopy measurements on the same monolayers adsorbed on aluminum and exposed simultaneously to the oxygen beam revealed a decrease of 14% in the methyl signal for the MTS surface and 43% for the OTS surface. The products could be detected either by FTIR or by XPS measurements. In the IR spectrum a broad peak appeared between 3100 and 3450 cm-’, indicating the formation of an OH bond. The increase in the peaks was proportional to the exposure time of the sample to the oxygen beam. The results of the XPS measurements, taken from four different sites on the surface and averaged, are summarized in Table 1. The table reflects changes in the atomic concentration of oxygen, carbon, aluminum, silicon, and chlorine. Two peaks are attributed to aluminum: The peak at 73.0 eV (A12) corresponds to aluminum in its free metal form while the 75.7 eV peak (All) corresponds to aluminum in the oxidized form. Note the increase in the ratio of the oxidized aluminum to the “metallic” aluminum due to the reaction (from 2.28 to 2.50, for the measurements taken normal to the surface-the normal configuration). In principle, this increase in the ratio may reflect either an oxidation of the metallic aluminum by the atomic oxygen or an artifact resulting from measurement with a decreased penetration depth. The fact that the total concentration of aluminum does not decrease
Binding Energy (eV)
Figure 3. Carbon (1s) signal in an X P S measurement of OTS/Al exposed to triplet oxygen: (A) prior to reaction; (B) after 90 s of reaction; (C) after 300 s of exposure.
indicates that the observed increase in the ratio is due to oxidation of the metallic aluminum. The ratio of the atomic concentration of the oxygen atoms to that of the oxidized aluminum before exposure is 1.9 for the normal configuration. If one subtracts the amount of oxygen attached to the silane atoms, a value of 1.6 is obtained. This value is very close to 1.5, the stoichiometric ratio in aluminum oxide. Exposure to O(3P)for 90 and 300 s leads to an increase in the ratio to 2.36 and 2.64, respectively. This evidently shows that the increase in the atomic concentration of oxygen reflects an increase in the absolute amount of oxygen due to the reaction. As will be shown below, part of this oxygen is attached to the chains, while a small amount oxidizes the aluminum surface. Residual amounts of chlorine atoms were observed prior to the reaction. These atoms are attributed to some deficiency in the hydrolization stage during the production of the monolayers. The decrease in the relative concentration of the chlorine atoms due to reaction (from 0.94% to 0.36%) suggests that oxygen atoms replace the chlorine atoms. When calculating the atomic concentration of the chlorine atoms, while disregarding the signal from the oxygen atoms in the summation over all atoms, the atomic concentration of the chlorine atoms decreased from 1.3% to 0.6%. This result indicates that the oxygen atoms penetrate to the bottom of the chains and replace the chlorine atoms. This conclusion also suggests that imperfections, often related to chains that are not fully cross linked, are the first to be attacked. As for the carbon atoms, apart from an aliphatic carbon at 285.5 eV (Figure 3A), another broad, shifted peak at 289 eV was found for the exposed monolayers (Figure 3B and C). This shift is known to correspond to carbon atoms which are attached to oxygen atoms. By fitting the peak of the carbon to two Gaussians, it is possible to obtain the ratio between the
Paz et al.
13520 J. Phys. Chem., Vol. 98, No. 51, 1994 I
'
'
0.0 0
-
A
-0.5
:"I
A
0
-0.10
0
-
h
-0.15
0
A
\
AAAA
=tI
-0.20
C
A
-0.25
A
-1.5
I 0
0
A
-1.0
-2.0
0
I
A
1
-0.30
I 2000
4000
6000
280
300
320
340
360
380
400
420
Time (sec) Figure 4. The methylene signal as a function of time for a CdB multilayer constructed from two protonated layers (filled triangles) on top of a deuterated one (open squares).
Surface Temperature (K) Figure 5. CHZ(a) DA signal of OTS slides exposed to atomic oxygen at various temperatures. The solid line represents a polynomial fit of the data points.
methylenic groups that reacted to form carbon-oxygen bonds and the remaining methylenic groups (Table 1). For an exposure time of 90 s, as much as 21 f 5% of the total carbon concentration was attributed to carbon atoms attached to an oxygen. For 300 s of exposure, about 35 f 5% of the carbon atoms reacted to form C-0 bonds. The fact that these values are consistent with the loss of CH2 groups measured by FTIR suggests that the amount of cross links via C-C bonds is minor and that the main channel for CH2 reaction is the formation of C - 0 bonds. It is well-known that probing the X P S signal at a grazing angle emphasizes the outer atoms.23 As shown in Table 1, the ratio of the atomic concentration of the carbon atoms attached to oxygen to that of the CHI groups is higher in the grazing angle measurement than when monitoring at the normal configuration. This indicates that the chain tail (the part of the chain toward the vacuum) reacts first. The formation of the C-0 bonds observed by the X P S measurements relates most probably to the formation of C-OH bonds, as observed by the FTIR measurements. Figure 4 shows the decrease in the methylene signal as a function of time for a three-layered OOTF of Cdhehenate in which two protonated layers were deposited on top of a deuterated one. First-order kinetics is clearly observed for the protonated layers, as in the case of the single layer. With regard to the deuterated underlayer, no reduction in its methylene signal could be observed even when the decrease in the protonated methylene signal is as large as 42%, i.e. 84% of a monolayer. This fact clearly indicates that the two outer layers protect the inner deuterated layer from reacting. It demonstrates the close packing of the multilayer and suggests that imperfections formed in the layers are not constructed epitaxially, namely the pinholes do not go through the three layers. The same results were obtained for two deuterated layers on top of a protonated one. By studying several three layers samples, we found that at room temperature, even for a long exposure time, only a single layer out of three reacted. (b) Effect of Surface Temperature on Reactivity. Structural changes in the OTS monolayer due to heating were studied by grazing angle external reflection infrared spectroscopy' (EXRIR), since this method is very sensitive to the alignment of the amphiphilic chains. The EXRIR signal of vibrational modes having a dipole moment perpendicular to the surface is enhanced while that of those parallel to the surface is reduced. For well aligned alkylsilane chains, this quality is expressed by enhancing the signal of the methyl groups while reducing that of the methylene. The loss of perpendicular alignment at
surface temperatures above 360 K is manifested by the sharp increase in the CHz(a)/CH3(a) ratio. The reversibility of this change is demonstrated by cooling down from 388 K to 308 K. Heating to 450 K causes an irreversible loss of the monolayer alignment. The reversibility of the 360 K transition and the irreversibility at 450 K were observed also by wettability measurements; for both OTS on glass and on aluminum, heating the monolayer to 375 K resulted in a chnage of no more than 1" in the contact angle, which is approximately the error in these measurements. In contrast, monolayers heated to 452 K showed a well observed decrease in their contact angles (typically from 112" to 103" for water and from 51" to 40" for bicyclohexane) with some hysteresis. In Figure 5, the changes in the methylene direct absorption signal of OTS due to reaction at various surface temperatures are presented. The depletion here is a monotonic function of the surface temperature (TJ with an enhanced slope above 360 K and below 320 K. Unlike the case of methylene, the depletion in the methyl signal was found to be almost independent of the surface temperature. The depletion of the methyl group is faster than that of the methylene at all temperatures, in agreement with the data obtained from the reaction kinetics measurements. An additional indication for the reversibility of the structural changes observed in the FTIR measurements and probed by the variations in reactivity was obtained by reacting simultaneously two OTS monolayers, at room temperature. One of these monolayers was preheated to 390 K. Here, not only did the monolayers on the two slides show the same contact angles before and after preheating but also no significant change between their contact angles could be observed after the reaction. The effect of structural changes on the reactivity is not limited to self-assembled monolayers. Figure 6 presents the change in the CH2(a) absorption signal of a cadmium arachidate pentalayer due to reaction at various surface temperatures. Each point in the figure was obtained with a different slide. An apparent linear dependence of the CH2 depletion on the temperature was observed with an increased slope at 333 10 K. Heating the CdA multilayer to 383 K followed by cooling and exposure at 298 K revealed no significant change in reactivity in comparison to a room-temperature exposure without heating. In Figure 7 the depletion of the R absorption, due to the reaction of a CdA monolayer, is shown as a function of the surface temperature. Here, again, each point corresponds to a different slide. The existence of a correlation between the quality of a monolayer and its reactivity is clearly observed in Figure 7A, where the initial absorption is also presented in units of OD. The more packed the monolayer is, the more
J. Phys. Chem., Vol. 98, No. 51, 1994 13521
Reaction between O(3P) and Organic Thin Films I
I
’
I
’
I
I
’
reaction rate is almost insensitive to the temperature. Although the data in Figure 7B only vaguely support the existence of the low-temperature phase below 273 K and one may claim that a single line could be fitted to the data up to 333 K, the observation of the same three phases in OTS (Figure 5) supports the conclusion of three regions.
‘
Discussion
I
-0.25
-0.30
1
-0.35
\A
-0.40
240
280 320 360 Surface temperature (K)
400
Figure 6. Dependence of the depletion of the CH2(a) absorption peak on the surface temperature during the reaction, for five layers of CdAr. The dashed line represents a depletion corresponding to one-fifth of the initial signal.
-05
.I 0 46 4P
.I j
. h
0
e
I
-
-2c
58
v
E
t -0 j
In what follows the results will be discussed in terms of the reaction mechanism and the effect of the structures of the OOTFs on their reactivity. (a) Reaction Mechanism. The most striking result of the current work is the fast disappearance of the CH3 groups. The reaction probability calculated for the methyl group (3 x is 3 orders of magnitude larger than the reaction probability in the gas phase (7 x As for the methylene groups, the reaction probability observed for the monolayer (5 x is approximately 25 times larger than that for the gas phase homolog. This finding is surprising, since one would expect a smaller rate constant for the process in the monolayer due to a steric effect resulting from the close packing of the chains. Assuming a Boltzmann distribution for the translational energy of the oxygen atoms, the fraction of atoms impinging on the surface with sufficient translational energy to overcome the activation energy barrier for the reaction between O(3P) and a methyl is 1.2 x Taking into account the spatial divergence of the atomic beam in the reaction chamber, the number of oxygen atoms impinging on 1 cm2 of the sample in a minute and having sufficient energy is ca. 4 x 10”. At that exposure time, our measurements show a depletion of 50% of the methyl signal, that is 2.5 x 1014methyl groups per square centimeter. Actually, the above consideration leads to the conclusion that almost all oxygen atoms react, independent of their kinetic energy. This is the first hint that the gas phase mechanism is not applicable to the reactions of the OOTFs. The pseudo-first-order rate constant for the reaction is expressed by
.I c
-I 5
-2 c
-2 5
L
240
280
320
360
40
Surface temperature (K) Figure 7. (A) Dependence of the depletion of the CH2(a) absorption peak on the temperature for a CdA monolayer. The absorption of the monolayer prior to the reaction is presented for each experimental point. (B) Data presented with a weighted average for each surface temperature. material per area is placed on the slide and therefore the higher the absorption. While, for a single monolayer, the density of molecules per area is strongly dependent on the preparation, this type of sensitivity to the preparation was not observed for three and five layers. In order to average data obtained at each temperature with different slides, a weighted average was taken. This was done by a linear interpolation between points having different initial absorptions. In Figure 7B the averaged data is presented. Three distinct regions are observed in Figure 7B. Up to 273 f 10 K and above 333 f 10 K a linear temperature dependence of the reactivity exists, as is observed also for the five-layer case. For surface temperatures between 273 and 333 K, the
If one assumes that C(T) depends only weakly on the temperature, it is possible to extract an apparent activation energy from the temperature dependent studies. For a monolayer of CdA, apparent activation energies of 4.5 f 1.5, 0.75 f 0.2, and 4.5 f 1 kcdmol are obtained for the low-, medium-, and high-temperature regions, respectively. As for monolayers of OTS, the obtained values are 5 f 1, 0.6 f 0.3, and 4 f 1 kcal/mol. Although the “activation energy” in the hightemperature region is significantly higher than that in the middletemperature range, the reaction probability is much larger in the high-temperature range. The probability in this range is 2 x lo-*, 30 times larger than that in the gas phase. Therefore, the assumption that the temperature dependence of the rate constant is controlled by the exponential term in (3) must be excluded. These results, combined with the very low “apparent activation energies” at the middle-temperature range, indicate that the temperature dependence of the rate constant is probably controlled by the steric factor C(T). This conclusion is further supported by the fact that the reactivity of the methyl group, where steric hindrance is less important, does not depend on the surface temperature. Therefore, we conclude that the real activation energy for the reaction with OOTFs is much smaller than the one obtained from the gas phase. The above conclusion can be rationalized on the basis of the formation of a long-lived collision complex which enables curve
Paz et al.
13522 J. Phys. Chem., Vol. 98, No. 51, 1994 crossing of the atomic oxygen to a singlet potential energy surface. A similar mechanism was proposed before for explaining the efficient quenching of O(lD) by collisions with various molecules.24 Such a mechanism was observed in the gas phase, for the reaction between cyclohexane clusters and O(3P),22where the low-frequency van der Waals modes of the cluster provided the density of states required to stabilize the collision complex. Stabilization of the collision complex is essential for curve crossing, since the curve crossing occurs at a bent configuration of the 0-H-C complex. The higher reactivity observed for the OTS methyl group in comparison with that of a MTS monolayer supports this mechanism (see also results in ref 1); the “floppiness” of the long chain amphiphile serves to increase the lifetime of the collision complex, while the residence time on the more rigid MTS is shorter. Both FTIR and the XPS studies indicate that C-0 bonds are formed in the reaction. An abstraction mechanism, common for O(3P) reactions, would have created mainly C-C cross-linking instead of the observed alcohols and carbonyls, which indicate an insertion process typical for the reaction on the singlet potential energy surface. (b) Temperature Dependence of Reactivity. As mentioned above we propose that the temperature dependence of the reactivity of the methylenes results from changes in the steric preexponential factor of the rate constant. Hence, the ratedetermining step is the penetration of the oxygen in between the chains and not the reaction itself. In the limited temperature range that could be investigated, the dependence of reactivity on temperature was fitted to a linear function. The dependence of the reation rate constant of the methylenes on the surface temperature can be explained by considering the above-mentioned thin films as assemblies of close packed, long chain amphiphiles anchored to the substrates and free to vibrate laterally. Therefore, C(T) is proportional to the cross section for penetration (a),which can be represented by u = n3 = n(req Ar)2, where r is the radius of the channels formed between chains and res and Ar are the radius at equilibrium and the change in the radius due to the vibrational motion, respectively. Obviously, this equation holds only if r is greater than the minimal distance needed for the penetration of the oxygen atoms. If the interchain vibrations can be described by an “effective” two-dimensional harmonic potential, then the energy of the two-dimensional oscillator is given by
+
E =f((Ar)2)= k,T
(4)
where T is the temperature, f is the effective harmonic force constant, and kb is the Boltzman constant. Hence nkbT
+
+
B = - 2nreq(kb~”).5 const
f
For the limited temperature range measured in the present study, it is impossible to distinguish between the linear and the square root dependence on temperature as predicted by eq 5. The change in the reactivity slope observed at ca. 330 K for multilayers of cadmium arachidate (Figure 6) can now be rationalized as due to reduction in the effective force constant for the interchain vibrations. This reduction is in accordance with the X-ray diffraction results, which suggest a less compact structure following the phase transition from the orthorhombic structure to hexagonal. This phase transition is characterized by “pseudo free rotation” of the chains and by an expansion in the b dimension of the unit cell. The increase in the reactivity due to the expansion of the unit cell is in line with the claim that the reaction proceeds through vertical penetration. The area needed for the expansion reduces the area in between the
domains. Therefore, if the penetration had occurred through the domain boundaries, one could have expected the reactivity to decrease at temperatures higher than the phase transition temperature. Indeed, an observed decrease in the permeation of water molecules through CdA multilayers at temperatures above 325 K was attributed to the fact that the molecules penetrate through domain b o ~ n d a r i e s . ~ ~ In contrast to the multilayer behavior, the temperature dependence of the reactivity of CdA monolayers reveals three distinct regions as observed in Figure 7. In each of the regions the dependence is linear, with a very weak dependence on the temperature between 293 and 333 K. The proposed structure for the five-layer system is valid also for the monolayer at temperatures below 293 K (where the monolayer is solid like) and above 333 K (where the monolayer is in a hexagonal phase). The remaining question is the nature of the phase between 293 and 333 K, where the increase in reactivity as a function of temperature is less pronounced. On the basis of the simple model presented above, it is possible to speculate on the nature of this phase. The weak linear dependence on temperature results (see 5) from a larger force constant, suggesting a more compact structure. When the temperature increases, the chains near the imperfections may bend fist, leaving the head groups intact. Due to the existence of kinks and bends, the chains fill the free volume and there is a stronger repulsion force between them. In other words, the force constant characterizing the “effective” two-dimensional harmonic potential is higher in this phase. The observed pseudo-first-order kinetics (Figure 2) is consistent with the above model. This type of kinetics means that at a given temperature several methylenic groups, in each chain, are equally accessible to the oxygen atom. The reaction does not affect the accessibility of either methylene. However, with increasing temperature the number of accessible methylenes increases. The hexagonal structure observed by X-ray diffraction for a CdA monolayer at a temperature lower than 333 K results from structural isotropy. This apparent isotropy may have two origins. One possibility is that all the chains are fixed laterally yet are rotating freely. The second case is a quasi random structure in which each chain is oriented on this rotational axis differently but does not rotate freely. As suggested above, the hexagonal phase between 293 and 333 K results from disorder and not from free rotation. Since the five-layer system is more ordered, this “disordered” phase does not exist there. Indeed, kinks and jogs were observed in multilayers only above 363 K.26 This interpretation of the data also rationalizes the sensitivity observed, in the case of the monolayer, to the quality of the monolayer, as indicated by its IR absorption. Well packed monolayers have less holes between the chains, and therefore the oxygen atoms can not penetrate easily between them. Hence, less reactivity was observed for those samples. The effect of structural changes on the reactivity is not limited only to LB films. The reversible structural changes observed by EXRIR for OTS at 360 K are manifested by an increase in the reactivity at that surface temperature. For OTS, the 360 K phase transition, observed also by the change in its reactivity (Figure 3,does not represent a change in the lateral spacing between the head groups, since the head groups are immobilized by covalent bonds. Therefore, the increased penetrability at elevated temperatures is attributed to the free rotation of the chains around their axis, while keeping the head groups intact. The role of the static head groups in restricting the penetration of oxygen atoms is demonstrated by the use of multilayers, where an extinction of the reaction after the loss of ca. one layer
Reaction between O(3P) and Organic Thin Films was observed at room temperature. Above the phase transition temperature, the head groups can no longer prevent the penetration of the oxygen; thus, we observed a depletion corresponding to more than one layer. Our results demonstrate, therefore, the possibility and limitations in using multilayers as protective layers. To complete this discussion we would like to discuss the validity of another possible mechanism. This mechanism was pr~posed'~J* for the liquid phase reaction between KMn04 and unsaturated OOTF,which was claimed to have two steps. The first step involves the reaction at imperfections such as dislocations or domain boundaries whereas during the second, slower step, the reaction proceeds through lateral diffusion of the reactants. Such a mechanism is in line with the evidence for oxidation of the residual amounts of chlorine atoms attached to the silicon in OTS,which was observed by X P S . Moreover, since the lateral penetration length is supposed to increase as the reaction proceeds, one would expect the reaction rate to decrease with time. We believe that although it is reasonable that the domain boundaries are the first sites to react, lateral penetration can not be the dominant mechanism for the proceeding of the reaction. If this was the mechanism, one would expect a fast depletion of ca. 5-10% of the signal, followed by very slow kinetics. This was not observed. Besides, the fact that we were able to observe a total loss of the signal further supports our claim that reaction kinetics is a majority phenomena. Moreover, our XPS measurements indicate that most of the oxygen which is attached to carbon atoms is located preferentially at the outer part of the chains.
Conclusion This study was aimed at addressing the relation between the structure and reactivity of organized organic thin films. It reveals fiist-order kinetics, with rate constants which are much higher than those obtained for the gas phase. The surface effect is attributed to formation of a long-lived collision complex, which facilitates a curve crossing into the more reactive singlet potential energy surface. This catalytic effect is important, since it is a surface nonspecific effect of adsorbed hydrocarbons. This work also demonstrates that, for a case where reactivity depends on the penetration of the reactant into the monolayer, reactivity may become a sensitive tool for obtaining structural
J. Phys. Chem., Vol. 98,No. 51, 1994 13523 information. Reactivity may provide new insight on the nature of each phase, being complementary to the well established X-ray diffraction methods.
Acknowledgment. This work was partially supported by the Israel Science Foundation. We thank Professors L. Leiserowitz and M. Lahav for the use of their LB trough. References and Notes (1) Pa,Y.; Trakhtenberg, S.; Naaman, R. J . Phys. Chem. 1992,96, 10964; 1993,97,9075. (2) Herron, J. T.; Huie, R. E. J . Phys. Chem. 1969,73,3327. (3) Kleinermannes, K.; Luntz, A. C. J . Chem. Phys. 1982,77,3533. (4) Andersen, P.; Luntz, A. C. J. Chem. Phys. 1980,72,5842. (5) Luntz, A. C. J . Chem. Phys. 1980, 73, 1143. (6) Davidson, J. A.; Schiff, H. I.; Streit, G. E.; McAfee, J. R.; Schmeltekopf, A. L.; Howard, C. J. J. Chem. Phys. 1973,67,5021. (7) Hampson, R. F. Chemical Kinetic and Photochemical Data Sheets for Atmospheric Reactions; U.S. Department of Transportation: 1980. (8) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J . Chem. Phys. 1985,82, 2136. (9) Cohen, S.R.; Naaman, R.; Sagiv, J. J . Phys. Chem. 1986,90,3054. (10) Rothberg, L.; Higashi, G.S.; Allara, D. L.; Garoff, S . Chem. Phys. Lett. 1987,133,67. (11) Jacquemain, D.; Leveiller, F.; Weinbach, S. P.; Lahav, M.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. J . Am. Chem. SOC. 1991,113, 7684. (12) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Science 1993, 261 449. (13) Bourdieu, L.; Silberzan, P.; Chatenay, D. Phys. Rev. Left. 1991, 67,2029. (14) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J . Chem. Phys. 1991,94,8493. (15) Okahata, Y.; Kimura, K.; Ariga, K. J . Am. Chem. SOC. 1989,111, 9190. (16) Tippmann-Krayer, P.; Kenn, R. M.; Mohwald, H. Thin Solid Films 1992,210,577. (17) Maoz, R.; Sagiv, J. Thin Solid Films 1985,132, 135. (18) Maoz, R.; Sagiv, J. Langmuir 1987,3, 1034. (19) Rose, G. D.; Quinn, J. A. J . Colloid Interface Sci. 1968,27, 193. (20) Blank, M. J. Phys. Chem. 1962,66,1911. (21) Hawke, J. G.; White, I. J . Phys. Chem. 1970,74,2788. (22) Rudich, Y.; Hurwitz, Y.; Lifson, S.; Naaman, R. J . Chem. Phys. 1993,98,2936. (23) Woodruff, D. P.; Delchar, T. A. Modern techniques of surface science; Cambridge University Press: Cambridge, U.K. 1990 p 112. (24) (a) Tully, J. C. J . Chem. Phys. 1974,61,61. (b) Zahr, G. E.; Preston, R. K.; Miller, W. H. J. Chem. Phvs. 1975,62, 1127. (25) Osiander, R.; Korpiun, P.; Duschl,C.; Knoll, W. Thin Solid Films 1988,160,501. (26) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J . Chem. Phys. 1987,86, 1601. ~
