J. Phys. Chem. 1990, 94, 1550-1554
1550
Thermal Characterization of the Mode of Phase Transition In the Dioctadecykiimethylammonlum Bromide-Water System in Relation to the Stability of Its Gel Phase Michiko Kodama,* Department of Biological Chemistry, Faculty of Science, Okayama University of Science, 1 - 1 Ridai-cho, Okayama, 700 Japan
Toyoki Kunitake, Department of Organic Synthesis, Faculty of Engineering, Kyushu University, Fukuoka, 81 2 Japan
and SyQz6 Seki Department of Chemistry, Faculty of Science, Kwansei Gakuin University, Nishinomiya, 662 Japan (Received: May 2, 1989; In Final Form: August 24, 1989)
The binary system of water and dioctadecyldimethylammonium bromide (DODAB) having a double hydrocarbon chain was found to exhibit a complex and characteristic phase transition. That is, when the sample is cooled to a temperature that is not low enough to crystallize the coexisting water, the mode of phase transition of the present system was classified into three types according to water content: at a water content below 67 wt %, only the T, transition of coagel to liquid crystal reversibly appears on cooling and heating; at a water content of 67-93 wt %, the additional T,* transition of gel to liquid crystal, which also reversibly appears on thermal cycling, takes place and the fraction occupied by the T,* transition increases with increasing water content, in competition with the T, transition; and above 93 wt %water content, only the T,* transition reversibly appears. On the other hand, when the sample is cooled to a temperature as low as -20 OC (low enough to crystallize the coexisting water), the T,* transition is no longer realized in the heating direction and is replaced by the T, transition. The present mode of phase transition was investigated from the thermodynamic viewpoint, and it was revealed that the stability of the metastable, supercooled liquid crystal phase relative to the coagel phase is a factor to determine the mode of the T, and T,* transitions.
Introduction
In our previous thermal studied2 on the binary systems of water and a homologous series of octadecyltrimethylammonium halides of different counterions, it was found that the gel phase exists in the thermodynamic stable or metastable states and its thermodynamic stability depends on the negative counterion of the polar head groups. This seems to be greatly concerned with the interaction between the counterions and water molecules. In the present study, we deal with dioctadecyldimethylammonium bromide (DODAB), which has a double hydrocarbon chain, in comparison with dioctadecyldimethylammonium chloride (DODAC) in our previous work;*s these double-chain amphiphiles have been reportedb8 to form the bilayer lamellar liquid crystal in the presence of water at temperatures above the T, transition, in contrast with the micellar solution of homologous amphiphiles of a single chain. The mode of phase transition in the DODAB-water system, related to the stability of the gel phase, was thermodynamically characterized by focusing attention on the number of the hydrocarbon chain and also the kind of counterion associated with the polar head group. Experimental Section
DODAB was synthesized by Toyoki Kunitake9 and was purified ( 1 ) Kodama, M.; Seki, S.J . Colloid Interface Sei. 1987, 117, 485. (2) Kodama, M.; Tsujii, K.; Seki, S.J . Phys. Chem., in press.
(3) Kodama, M.; Kuwabara, M.; Seki, S. Thermochim. Acta 1981,50,81. (4) Kawai, T.; Umemura, J.; Takenaka, T.; Kodma, M.; Seki, S. J. Colloid Interface Sei. 1985, 103, 56. ( 5 ) Fujiwara, T.; Kobayashi, Y.; Kyogoku, Y.; Kuwabara, M.; Kodama, M.; Seki, S. J. Colloid Interface Sei. 1989, 127, 26. (6) Kajiyama, T.; Kumano, A.; Takayanagi, M.; Okahada, Y.; Kunitake, T. Chem. Lett. 1979, 645. (7) Nakashima, N.; Asakuma, S.;Kunitake, T.; Hotani, H. Chem. Leu. 1984, 227. ( 8 ) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710. (9) Kunitake, T.; Okahata. Y.; Tamaki, K.; Kumamaru, F.; Takayanagi, M . Chem. Lert. 1977, 387.
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by recrystallization three times from acetone after it was completely dehydrated according to the procedure previously employed for at least six amphiphile~.l-~~'~*" Forty samples of the DODAB-water mixture, containing from 0 to about 99 wt % water, were prepared by using a microsyringe to add increasing amounts of water to the completely dehydrated compound. All samples were annealed by repeated thermal cycling at temperatures above and below the T,transition a t a scanning rate of 0.1 OC mi& for about 12 h to ensure homogeneous mixing and to attain the equilibrium state. DSC measurements were carried out with a Mettler DSC TA-2000 and a Seiko-Denshi DSC ssc/560 by placing the sample in a high-pressure crucible and heating it from -20 OC to temperature above the T, transition at a heating rate of 0.2 OC m i d . Results
Figure 1 shows sets of typical DSC curves in cooling (+) and heating (+) directions describing the so-called T, transition of the DODAB-water system at three different water contents. As shown in this figure, the thermal behavior of the T, transition in the present system can be classified into representative three types (a, b, and c) according to water content. Furthermore, two sets of DSC curves (runs I and 11) of the same water content are compared on the basis of the difference in cooling procedure; whether the sample is cooled to a temperature to crystallize the coexisting water or not. On the basis of the thermal behavior shown in Figure 1, the mode of phase transition in the present system is summarized as follows: (i) In a low-water-content region below 67 wt % shown in Figure fa,when the sample is cooled to temperatures below the T, transition and then heated, exothermic and endothermic peaks reversibly appear at about 47 and 53 "C, respectively, regardless of the temperature to which the sample is cooled, as (10) Kodama, M.; Kuwabara, M.; Seki, S.Biochim. Biophys. Acta 1982, 689,561. (11) Kodama, M.; Seki, S . Prog. Colloid Polym. Sei. 1983, 68, 158.
0 1990 American Chemical Society
Phase Transition in the DODAB-Water System
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1551
80
J l e liquid crystal
60 -
40 -
20 \
‘\
0 65
I
70
80
90
100
Figure 2. Variation with increase in water content above 65 wt 96 of apparent enthalpy changes (AH) associated with the T, (-0-) and T,* (-0-) transitions shown in runs I of Figure lb,c. Both transitions re-30-20 -10 0
40 50 60 70
t / “c Figure 1. Typical DSC curves describing the so-called T, transition of the DODAB-water system in the low-(a), intermediate-(b), and high(c)-water content regions. At each water content region, two sets of DSC curves in the cooling (e) and heating (+) directionsshown in runs I and I1 are compared at the same water content according to the temperature to which the sample is cooled. The dotted peaks represent the T,* transition of metastable gel to metastable supercooled liquid crystal.
shown in runs I and 11. The size of the exothermic peak is nearly the same as that of the endothermic peak. This fact indicates an Occurrence of the reversible phase transition which is independent of the limit of cooling temperature. (ii) In a intermediatewater-content region shown in Figure Ib, on cooling, the exothermic peak at the same transition point as that of the exothermic peak of Figure l a first appears and is followed by a new type of exothermic peak at about 40 OC, as shown by the cooling DSC curves of runs I and 11. As the cooling temperature is not low enough to crystallize the coexisting water, two endothermic peaks corresponding to two exothermic peaks reversibly appear at 44 and 53 OC, respectively, as shown by the heating DSC curve of run I. The peak size is nearly the same between the reversible exothermic and endothermic peaks, respectively. When water content is changed, however, the size of the reversible peaks at the low-temperature side (shown by dotted peaks) becomes larger with an increase in water content, contrary to a gradual diminution of that of the reversible peaks at the high-temperature side. This behavior continues to the limiting water content of approximately 93 wt %, at which the reversible peaks at the low-temperature side become the largest and those at the high-temperature side completely disappear. On the other hand, when the sample is cooled to as low as -20 “C, only the endothermic peak at the high-temperature side is observed in the heating direction and that at the low-temperature side completely disappears, as shown in run 11. Furthermore, the size of the endothermic peak observed in run I1 is always larger than that of the endothermic peak at the same transition point in run I and is not affected by water content in this region. (iii) In a high-water-contentregion above 93 wt % shown in Figure IC, only the exothermic and endothermic peaks at the low-temperature side reversibly appear on cooling to the noncrystallization temperature of the coexisting water and then on heating, as shown in run I. The peak size is nearly the same between them and is not affected by water content. Undergoing enough cooling to the crystallization temperature of the coexisting water, the endothermic peak in run I is no longer observed in the heating direction and is replaced by the endothermic peak at the high-temperature side, as shown in run 11, similar to the thermal behavior in the intermediate-water-content region.
versibly appear on thermal cycling under the limited cooling condition that the coexisting water is not crystallized. To clarify the nature of the two kinds of reversible peaks at. the high- and low-temperature sides shown in runs I of Figure 1, three samples of different water contents corresponding to the three water-content regions of this figure were observed by the naked eye at -20 “C, to which these samples were cooled from -60 OC of the liquid crystal states: (i) in the low-water-content region, only the coagel phase of a crystalline sediment exists; (ii) in the intermediate-water-cntent region, two phase coexists, that is, the cage1 phase dispersed into the gel phase of semitransparent homogeneous state; and (iii) in the high-water-content region, only the gel phase exists. Therefore, the reversible peaks at the high-temperature side in runs I of Figure la,b are considered to be due to the phase transition of coagel to liquid crystal, while those at the low-temperature side in runs I of Figure 1b,c are due to the phase transition of gel to liquid crystal. Furthermore, the appearance of only the endothermic peak at the high-temperature side in the intermediate- and high-water-content regions shown by the heating DSC curves of runs I1 of Figure 1b,c indicates that only the phase transition from the coagel to liquid crystal phases takes place; that is, the gel phase which appears at temperatures below the gel to liquid crystal transition is transformed successively into the coagel phase by a further cooling to the crystallization temperature of the coexisting water. Thus, it is suggested that the gel phase exists in the thermodynamic metastable state at all temperatures below the gel to liquid crystal transition and only the coagel phase exists in the stable state in this temperature region. Therefore, the phase transition of gel to liquid crystal corresponds to the “Tc* transition” which we named the phase transition of metastable to metastable states in our previous work2*12on the OTAB-water system, in contrast to the so-called transition” of stable coagel to stable liquid crystal. On the basis of these facts, the simultaneous appearance of two kinds of reversible peaks in the intermediate-water-content region (see run I of Figure lb) suggests that on cooling a part of the liquid crystal is transformed directly into the stable coagel phase (T,) and the remainder into the metastable gel phase (T,*). This behavior is clearly distinguished from the transformation of all of the liquid crystal phase into either of the stable coagel and metastable gel phases in the low- and high-water-content regions. To make clear this phenomenon, Figure 2 shows the variation of apparent enthalpy changes (AH) per 1 mol of DODAB associated with the T, and T,* transitions with water content above 65 wt %. In this figure, the enthalpy changes were obtained from the corresponding endothermic peaks shown in runs I of Figure 1b,c. The T,* enthalpy curve rises with increasing water content up (12) Kodama, M.;Seki, S. Adu. Colloid Interface Sei., in press.
1552 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1001
I
I
I
"
"
I
"
1
TABLE I: Fractions of the Enantiotropic-like and Monotropic-like T , Transitions in the Intermediate- and High-Water-Content Regions water T, transition fraction/%
80 0
\ %
4ol
,
content/wt %
enantiotropic
monotropic'
69.4 78.3 85.2 89.7 95.0 98.2 99.0
96.1 72.7 42.0 36.3 0 0 0
3.9 27.3 58.0 63.1 100 100 100
-=I
60
I
Kodama et al.
=vi'
30
"The fraction of the monotropic-like T, transition corresponds to
0
20
40
60
80
100
WH20 / 9 % Figure 3. Phase diagram of the DODAB-water system. T, and T,* represent the temperatures of phase transitions of coagel to liquid crystal
and gel to supercooled liquid crystal, respectively. Phase aspect at temperature below the T, transition is classified into three types at boundary water contents of about 67 and 93 wt % indicated by the two hatched lines.
herg E]
that of the T,* transition.
TABLE II: Enthalpy Changes (AHe AH,*) Associated with the T , and T.* Transitions at Different Water Contents
water content/wt % 30.5 49.8 62.5
i+r cooling
I
coagel phase
1
Figure 4. Schematic diagram showing the mode of phase transition in the DODAB-water system. The solid lines (a) represent the T, transition of stable coagel to stable liquid crystal which reversibly appears on thermal cycling. The solid lines (b) represent the T,* transition of metastable gel to metastable supercooled liquid crystal which also reversibly appears on thermal cycling under the limited cooling condition that the coexisting water is not crystallized. With enough cooling, the T,* transition in the cooling direction is replaced by the T, transition in the heating direction, which is represented by the dotted lines (c). Therefore, the T, transitions of stable coagel to stable liquid crystal shown by the solid line a and dotted line c are called the enantiotropic-likeand monotropic-like transitions, respectively.
to about 93 wt %, at which the T, transition enthalpy reaches zero value. Figure 3 shows the phase diagram of the DODAB-water system obtained from all the DSC curves for the samples of different water contents. The phase diagram is revealed to be composed of two transition curves of the T, and T,* which depend on the limit of cooling temperature; the diagram exhibits only the T, transition curve over all water contents by enough cooling to around -20 O C , but it is lacking in the T, transition curve in the high-water-content region under the limited cooling condition that the coexisting water is not crystallized. Furthermore, the T, transition curve exhibits a stepwise decreasing behavior, particularly in the low-water-content region, which we commonly found in our previous work^'-^*"'-'^ on amphiphile-water systems. Discussion
The present system was found to exhibit the characteristic mode of phase transition according to water content and thermal history of the sample. To make this point clearer, Figure 4 presents a schematic picture showing an outline of the mode of phase transition in the present system. In the low- and intermediatewater content regions, the T, transition of stable coagel to stable liquid crystal phases represented by solid lines (a) reversibly (13) Kodama, M. Thermochim. Acra 1986, 109, 81.
AH,*/kJ mol-'
101.0 105.2 105.6
Intermediate Water Content
liquid crystal phase
i;
AH//kJ mol-' Low Water Content
69.4 78.3 85.2 89.7
91.9 92.6 92.5 91.5
28.0b 29.0 30.8 31.4
High Water Content 95.0 98.2 99.0
82.1 82.1 82.1
32.6 32.9 31.8
'The nature of AH, depends on water content as follows: in the low-water-content region the enantiotropic-like T, transition; in the intermediate-water-content region a combination of the enantiotropiclike and monotropic-like T, transitions; and in the high-water-content region monotropic-like T, transition. AHc* in the intermediatewater-content region was calculated on the basis of T,* transition enthalpy shown in Figure 2 and the fraction of the monotropic-like T, transition shown in Table I. appears on the thermal cycling. In the intermediate- and high-water-content regions, the T,* transition of metastable gel to metastable liquid crystal represented by solid lines (b) also reversibly appears under the limited cooling condition of the noncrystallization temperature of the coexisting water. With enough cooling to as low as -20 O C , however, the T,* transition is no longer realized in the heating direction and is replaced by the T, transition. This phenomenon is expressed by a combination of the T,* transition in the cooling direction and the T, transition in the heating direction represented by dotted lines (c). Therefore, there are two types of the coagel phase depending on whether or not it is attained by way of the metastable gel phase. To distinguish between both coagel phases, the T, transition in the heating direction shown by dotted line (c) is called the "monotropic-like one", in contrast to the "enantiotropic-like T, transition" shown by solid line (a). According to this definition, the T, transition in the intermediate-water-contentregion observed in the heating direction is found to be composed of two components of the monotropic-like and enantiotropic-like transitions, while the T, transitions in the low- and high-water-content regions consist of one component of either transition, respectively. Furthermore, as shown in Table I, the fraction occupied by the enantiotropic-like T, transition in the intermediate-water-contentregion decreases with increasing water content, contrary to the behavior of the monotropic-like T, transition. The fraction of the enantiotropic-like T, transition is given by a ratio of the endothermic peak at the high-temperature side in run I of Figure 1b (corresponding to the T, transition enthalpy of Figure 2) to the enthalpy change of the endothermic peak at the same transition point in run I1 of the same figure (corresponding to the T, transition enthalpy in the intermediate-water-content region shown in Table 11), and the remainder can be taken as the fraction of the monotropic-like
Phase Transition in the DODAB-Water System
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1553
0
10 20
30 40 50 60 70 80 90 100
Figure 6. Variation with increasing water content of entropy changes (AS,,ASc*) associated with Tc and T,* transitions. The nature of ASc in the three different water content regions corresponds to that of AHc in Table 11.
cr, 20
30
40
t /'C
50
60
Figure 5. Relationship between schematic diagram of G-T (above) and transition entropy (below) curves of the DODAB-water system at a water content of about 80 wt %. O1 and O2 represent angles made of G T curves of liquid crystal and coagel and of those of supercooled liquid crystal and metastable gel, respectively. AS,* and AS, represent entropy changes associated with the Tc* and Tc transitions.
T, transition. Under the limited cooling condition mentioned above, the fraction of the monotropic-like T, transition can be looked upon as that of the Tc* transition. In summary of the phase transition phenomenon in the present system, it may be concluded that the T,* transition is facilitated with an increase in water content, and hence the monotropic-like T, transition is also enhanced, contrary to an increased suppression of the enantiotropic-like T, transition. To discuss the present mode of phase transition, in more detail, from the thermodynamic viewpoint, the upper half of Figure 5 shows the schematic diagram of the Gibbs energy (G)versus temperature (7') curve for the sample at -80 wt % water content as a representative example. The diagram was constructed on the basis of the entropy changes (AS,, AS,*) and the transition temperatures (T,, T,*) shown in the lower half of the same figure. Following the G-T curve of the liquid crystal in the cooling direction by reference to the diagram of Figure 5 , it is suggested that the selection of either supercooled liquid crystal or coagel phases at the T, transition point is associated with stability of the former relative to the latter. On the other hand, the stability can be measured by the difference (AG) in the Gibbs energy between the liquid crystal (or the supercooled liquid crystal) and coagel phases in the neighborhood of the T, transition, and the AG is quantitatively proportional to the angle 8, which is made of the G-T curves of both phases. According to the Gibbs energy definition of G = H - TS and the relation (BGIBT), = -S, the angle 8, corresponds to the T, transition entropy (AS,), similar to the correlation of the angle O2 (made of the G T curves of the supercooled liquid crystal and gel phases) and T,* transition entropy (ASc*). Consequently, the T, transition entropy can be schematically looked upon as the AG between the liquid crystal and coagel phases. From this viewpoint, the entropy changes associated with the T, and T,* transitions were calculated on the basis of the enthalpy changes shown in Table I1 and the results are plotted against water content in Figure 6 . In this figure, the T, transition entropy decreases stepwise at the boundary water contents of 67 and 93 wt % as water content increases, reflecting the difference in the mode of the T, transition, enantiotropic and/or monotropic, in the three water-content regions. Based on this result, the relation of AG in these water content regions is
Figure 7. Relationship between schematic diagram of G T (above) and transition entropy (below) curves of the DODAC-water system at a water content of about 80 wt %. T cl and Tc represent the temperatures of phase transitions of stable coage! to stable gel and stable gel to stable liquid crystal, respectively. S8,and AS, represent entropy changes associated with the TBcland T, transitions.
as follows: AG(H) < AG(1) < AG(L), where G = GI - G,;G , and G, represent the Gibbs energies of the supercooled liquid crystal and coagel phases, respectively; and AG(H), AG(I), and AG(L) correspond to the AG in the high-, intermediate-, and low-water-content regions, respectively. This relation indicates that the thermodynamic stability of the supercooled liquid crystal phase relative to the coagel phase in the three water-content regions increases in the order of low < intermediate < high. Consequently, a part of the liquid crystal in the intermediate-water-content region is allowed to p r d along the G T curve of the supercooled liquid crystal phase at the Tc transition point, after which the resultant supercooled liquid crystal is forced to select the G T curve of the gel phase at the Tc* transition point. This tendency is much more enhanced for the sample in the high-water-content region, so that all of the liquid crystals go into the supercooled liquid crystal at the T, transition point. This causes the appearances of the metastable gel phase and also the T,* transition in both watercontent regions. In this respect, in the intermediate-water-content region, the stable coagel phase coexists with either of the metastable gel and metastable supercooled liquid crystal phases a t temperatures below the T, transition. To investigate the possibility of transformation of these metastable phases into the stable coagel
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J . Phys. Chem. 1990, 94, 1554-1564
phase, the following procedures were adopted: (i) the sample in this water-content region is heated at a scanning rate as low as 0.1 OC/min; (ii) the sample is annealed at temperatures just below the T, and T,* transition points for at least 6 h, respectively. On the basis of these procedures, any exothermic peak characteristic of transformation of the metastable phases into a more stable one was not observed in the heating direction and two endothermic peaks shown in run I of Figure 1b were not at all affected. This fact indicates that nuclear growth of the coagel phase requires a cooling procedure to crystallize the coexisting water, as is evident in run I1 of Figure 1b. Finally, we want to discuss the present mode of phase transition in comparison with homologous systems of different counterions and different chain numbers previously investigated by us. The octadecyltrimethylammonium bromide (OTAB) system,2.'2 which has a single chain, shows the same type of G-T curves as those of the present system shown in Figure 5. On the other hand, the dioctadecyldimethylammonium chloride (DODAC),3 which also has a double chain, exhibits a different type of the GTcurves shown in Figure 7 , similar to the octadecyltrimethylammonium chloride (OTAC) system.' The remarkable difference in two types of the G T curves shown in Figures 5 and 7 is the thermodynamic stability of the gel phase; the gel phase of chloride counterion exists in the stable state in the specified temperature region between the T, and Tgeltransitions, while the gel phase of bromide counterion exists in the metastable state over all temperatures below the T, transition. However, the simultaneous appearance of the T, and T,* transitions observed in the intermediatewater-content region of the present system is not observed in the OTAB system,2 although the mode of the GTcurves is the same for both systems. Presumably, this may be concerned with a
smaller T, transition entropy (AS,= 205 J/(K-mol)) of the OTAB system over all water contents and, consequently, a higher stability of the gel phase, compared with that of the gel phase of the DODAB system. Furthermore, when the T, transition entropy of the DODAB system is compared with a total entropy change associated with the transformation from the coagel to liquid crystal phases of the DODAC system, these double-chain systems exhibit nearly the same value as shown in Figures 5 and 7 . The corresponding entropy change is also nearly the same between the single-chain systems of OTAB and OTAC. However, the T, transition entropy of the double-chain systems is only 1.4 times larger than that of the single-chain systems and is smaller than that expected. As is well-known, a drastic change at the T, transition is attributed to a conformational change of the hydrocarbon chain. The small value of the T, transition entropy of the double-chain systems indicates that the molecular motion of the hydrocarbon chain of these systems is fairly restricted, even at temperatures above the T, transition, which is away from the generally accepted liquidlike state. This is connected with the formation of the liquid crystal phase of the double-chain systems above the T, transition,6-8 in contrast with that of the micellar solution phase of the single-chain systems. Summarizing the above discussion of the homologous systems of different counterions and different chain numbers, it may be concluded that the halide counterion determines whether the gel phase exists in the thermodynamics stable or metastable states at temperatures below the T, transition, which is intimately concerned with the mode of the phase transition. The number of hydrocarbon chains determines the aggregation state of amphiphile molecules, micelle and/or liquid crystal, at temperatures aboue the T, transition.
Coadsorptlon of Carbon Monoxide and Hydrogen on the Ni(100) Surface: A Theoretical Investigation of Site Preferences and Surface Bonding Jing Li, Birgit S c h i ~ t t Roald ,~ Hoffmann,* and Davide M. Proserpio' Department of Chemistry and Materials Science Center, Cornel1 University, Ithaca, New York 14853-1 301 (Received: May 16, 1989; I n Final Form: August 18, 1989)
The CO/H coadsorption on the Ni( 100) surface is discussed in this study. Relative stabilities of various possible surface structures are compared for the initial state (lower temperature form), as well as the final state (higher temperature form) of the coadsorption system. The surfaceadsorbate bonding in the Ni( 100)/H(4)/CO(t) structure (H(4) stands for hydrogen atoms adsorbed in a 4-fold hollow site and CO(t) stands for carbon monoxide in the on-top position), the most favorable choice of the lower temperature state, resembles that of the singly adsorbed systems. The adsorbate-adsorbate interaction does not lead to any chemical bonds but does affect the surface-CO A bonding. Destabilization of the CO 2a orbitals due to the 2~-1s(H)interaction results in a depopulation of the 2a states and a strengthening of the C-O bond. For the observed higher temperature c ( 2 ~ ' 2 X d 2 ) R 4 5geometry ~ of CO, possible H and H, (adsorbed hydrogen molecule) arrangements in the final surface state are compared. Energy and crystal overlap population analyses show that the 4-fold adsorption site for both H and H2gives good agreement with the experimental observations. A new adsorbate-adsorbate coupling between CO(t) and H2(4) (hydrogen molecule adsorbed in a 4-fold hollow site) is seen, but this coupling does not significantly affect the surface-adsorbate bonding. Again, no C-H or 0-H bonds are formed. Besides the favorable CO(t)/CO(4) (half of the CO's adsorbed terminally and the other half in the 4-fold hollow site) configuration, our calculations also reveal the possibility of a CO(t)/CO(b) combination (half of the C O Sadsorbed terminally and the other half in a bridging manner) in the final state.
There have been many studies of the coadsorption of carbon monoxide and hydrogen on the various transition-metal surfaces during the past 10 years. Examples among these include many close-packed faces such as Ni( 11 1),1-3Pt( 1 1 1),4 Rh( 1 1 1),5 Pd'Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark. Istituto di Stereochimica ed Energetica dei Composti di Coordinazione, C. N. R., via J. Nardi 39, 50132-Florence, Italy.
*
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(1 1 I),6v7 and Ru(001)8 and more open faces such as W( Rh( 100),133'4 Fe( and Ni( The great activity in ( 1 ) Conrad, H.; Ertl, G.; Kiippers, J.; Latla, E. E. Proc. Inr. Congr. Curd., 6rh 1977, 427. (2) Bertolini, J . C.; Imelik, B. Surf. Sci. 1979, 80, 586. (3) Peebles, D. E.; Creighton, J. R.; Belton, D. N.; White, J. M. J . Cural. 1983, 80, 482. (4) Baldwin, Jr., V. H.; Hudson, J. B. J. Vue. Sci. Technol. 1971, 8, 49.
0 1990 American Chemical Society
