1094%
J. Phys. Chem. 1992,96, 10948-10953
(3) The selectivity of hydroformylation increased from 20 to 50%.
(4) Selenium was selectively bound to Rh at the bond distance of 0.2434.244 nm by Se K-edge EXAFS. ( 5 ) The Rh6 cluster framework was maintained after the reaction with (CH&Se as suggested by the coordination number of Rh-Rh in the Rh K-edge EXAFS and the peak at 1800 cm-’ for the r 3 - C 0 ligands. (6)XPS showed that selenium on the Rh6 cluster was situated in an oxidation state of ca. -1, while no shift of Rh 3d binding energies from the metallic level was observed. (7) The structures of the partially-decarbonylatedRh, cluster and the Se-modified Rhs cluster on MgO were proposed on the basis of the number of CO ligands, its change by the treatments, FT-IR, and EXAFS. (8) The active sites for the catalytic hydroformylation reaction were suggested to be RhAor Rhs on the Rh6 framework. R@3W NO. Rh6(C0)16, 28407-51-4; MgO, 1309-48-4; Se, 778249-2; CO, 630-08-0.
Refereoces and Notes (1) (a) Gates, B. C., Guczi, L., Knbzinger, H., Eds. Metal Cluster in Catalysis; Elsevier: Amsterdam, 1986. (b) Iwasawa, Y., Ed. Tailored Metal
Catalysts; Reidel: Dordrecht, The Netherlands, 1986. (2) Kirlin, P. S.; KnBzinger, H.; Gates, B. C. J . Phys. Chem. 1990, 91, 8451. (3) Lee, G.; Ponec, V. Catal. Rev.-Sci. Eng. 1987, 29, 183. (4) Jackson, S. D.; Brandreth, B. .J.; Winstanley, D.J. Catal. 1991,129, 540. (5) Chuang, S. S.; Pien, S. I.; Sze, C. J . Catal. 1990, 126, 187. (6) Izumi, Y.; Asakura, K.; Iwasawa, Y. J. Catal. 1991, 127. 631. (7) Izumi, Y.; Asakura, K.; Iwasawa, Y. J . Catal. 1991, 132, 566. (8) MacLaren, J. M.; Pendry, J. 8.;Joyner, R. W. Surf. Sci. 1986, 178, 856. (9) Teo, B. K. EXAFS Basic Principles and Data Analysis; Springer: Berlin, 1986. (IO) Chini, P. J . Chem. Soc., Chem. Commun. 1967, 440. (1 1) Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S.P.; Shirley, D. A. Phys. Rev. B 1974, 9, 600. (12) Baltanas, M.A.; Onuferko, J. H.; McMillan, S.T.; Katzer, J. R. J . Phys. Chem. 1987, 91, 3772. (1 3) Galli, D.; Garlaschelli, L.; Ciani, G.; Fumagalli, A,; Martinengo, S.; Sironi, A. J . Chem. Soc., Dalton Trans. 1984, 55. (14) Sachtler, W. M. H. Faraday Discuss. Chem. Soc. 1981, 72, 7. (1 5) Siegel, S. J . Catal. 1973, 30, 139. (16) Konishi, Y.; Ichikawa, M.; Sachtler, W. M. H. J. Phys. Chem. 1987, 91, 6186. (17) Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 198, L329.
Ellipsometrlc Study of Surfactants Comprlslng Linear and Branched Hydrocarbon Chains at the Air-Water Interface L. Chicha, R. M. Leblanc, Centre de Recherche en Photobiophysique, UniversitP du QuPbec d Trois RiviPres, CP.500 Trois RiviPres, QuPbec G9A 5H7,Canada
and A. K . Chattopadhyay* ICI Explosives Group Technical Centre, McMasterville Site, 801 Richelieu Blvd., McMasterville, Quebec J3G 1 T9, Canada (Received: June 23, 1992; In Final Form: September 14, 1992) The ellipsometric isotherms of diethanolaminederivatives of n-alkylsuccinic anhydride (where the alkyl group comprised C12, C14, and C16 hydrocarbon chains) and polyisobutylene succinic anhydride (where the average backbone carbon numbers of polyisobutylene chains were 14, 22, and 34) at the air-water interface were compared with a view to the development of an understanding of the structural organisation of their monomolecular films. The ellipsometric angle, Ahs, has proven to be very sensitive to the nature of the physical state of the film that occurs during film compression. At 20 f 1 OC,the two series of surfactants exhibited very different behavior upon compression. A rodlike model was used to describe the different molecular behaviors exhibited by the surfactants comprising linear and branched hydrocarbon chains. Equations relating variations of refractive indexes, film thicknesses, and molecular areas were used for the determination of the molecular properties of the films during monolayer compression. Calculated refractive indexes and film thicknesses were obtained from experimental hsA values. The results indicated that the change in hsA values during film compression is more sensitive to the refractive indexes than to the film thicknesses. For the surfactantswith n-alkyl chains the refractive index component inmasad Linearly, whereas a reverse trend was observed with the surfactant molecules comprising polyisobutylene chains. Such an anomaly in refractive index can be explained by the effect of substituents in the hydrocarbon chains and subsequent differences in packing behavior of monolayers. The l6Al of n-alkyl derivatives increased upon compression and reached a steady state at collapse pressure, whereas a sharper rise in l6Al was observed with the derivatives comprising polyisobutylene chains. Such phenomena indicated multilayer formation by the molecules with polyisobutylene chains in the collapse region.
riItrodllction The role of molecular structure, polar head-group charge, and the influence of counterions on the polar head groups of surfactants has great signifcance in determining their interfacial pr0Perties.l” These phenomena have bcm studied in detail in the past with little attention having been paid to the nonpolar components of surfactants. However, more recently, attention has been turned toward the nonpolar group of the amphiphilic Biologists, for instance, arc intmsted in the componentsof surfactant molecules which possess unsaturation or substitution in their hydrophobic To whom correspondence should be addressed.
0022-3654/92/2096-10948$03.00/0
region. It has been found that in some membranes fatty acids possess unsaturation or substitution in their hydrophobic chaii? The properties of such fatty acids adsorbed at the interface in biological membranes are dependent partly on the lateral interactions between adjacent alkyl chains. This leads to greater fluidity and permeability of surface films in membranes compoead of such fatty acids. The behavior of certain fatty acids and lecithin has beem studid at the air-water interface by surface pmrsurearea “ents. It was reported that the unsaturated or branched hydrocarbon chains of such molecules lead to an expansion of their molecular area when compared to the corresponding saturated or unsub stituted ~ o ~ c c u ~ c s . * J ~ ~ ~ 0 1992 American Chemical Society
Surfactants at the Air-Water Interface
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10949
(a)
c- polyisobutylene
Figure 2. Rodlike model of the surfactant molecule: Do diameter of chain; D,, total length of the molecule; D2, length of the polar head group; XYZ, coordinate axis; 6, angle with respect to the vertical Z axis.
(b) Figure 1. Molecular structurs of surfactants comprising n-alkyl and polyisobutylene hydrocarbon chains.
Another investigation of the interfacial properties of surfactants with branched hydrocarbon chains revealed some interesting differences compared to surfactants with linear alkyl chains. It was reported recently that the structure of aliphatic chains of surfactants as well as their polar head group are equally important in determining their properties and performanccs at different interfaces such as air-liquid, liquid-liquid, solid-liquid. le16 We report in this work the interfacial characteristics of six molecules having the same polar head group of which three of them have aliphatic alkyl chains (dodecyl, tetradecyl, hexadecyl) and the other three have branched hydrocarbon chains (polyisobutylene) of average backbone carbon numbers 14,22,and 34. The polar head groups of the surfactants are comprised of the diethanolaminederivative of succinic anhydride. The molecular structum of such molecules are shown in Figure 1. The molecules were spread at an air-water interface and were subjected to a lateral compression in a Langmuir trough. Along with the monomolecular film compression, optical ellipsometricangle measurements were carried out at different surface prtss~res.~’ The method of optical ellipsometry is suitable for the investigation of adsorbed monolayers at the air-water interface and is an appropriate and elegant tool with which to study the influence of hydrocarbon chains on monomolecular From this method valuable information concerning refractive index and film thickness can be obtained. The aim of this work was to understand the structure and organization of such diethanolamine derivatives at the air-water interface by obtaining information about the fundamental parameters affecting surfactants with branched and classical chains during film comp~e$sion.*~ To explain the optical parameters provided by ellipsometricdata in terms of molecular properties, we used a molecular dynamic simulation with a geometric model. The CPK model of molecules allows one to mimic the variation of the ellipsometric angle as a function of the molecular area during film compression.20 Geometric Model For a thin nonabsorbing film, the relevant ellipsometric parameter is 6A = A - A where A is obtained in the presence of a f h and A in the absence of a f h (water surface). The refractive indexes and film thickness are related to i3A by the following relation:2l.z2
X is the wavelength of light, 4Jo is the angle of incidence, n, and
n2 are respectively the refractive indexes of the air and the s u b strate, d is the film thickness, n, and n, are respectively the
perpendicular and the parallel f h refractive indexes with respect to the optical axis. From eq 1, the 6A measurement alone does not allow us to calculate the unknown parameters such as d, n,, and n,. To obtain relevant information on these parameters, a rodlike model for each kind of molecule was developed for rationalizing the experimental data. This allowed the determination of these optical constants as a function of the orientations of the surfactant molecules at the air-water interface. In the geometric model described elsewhereZo(in Figure 2) the acyl chain and the polar head group are included in a rectangular parallelepiped whose width and length are Do and D, respectively. According to this model it is assumed that the polar head groups are fully extended in the subphase and cannot be detected by the light beam because of their high degree of hydration. The approximation is that the hydrated moiety has a refractive index equal to that of the subphase. It is also considered that the conformation of the molecule is independent of the orientation regardless of its physical state. The orientation, as the molecules “lift off” from the water surface, is described by the angle 0 which exists between the normal and molecular axis. The variation of the molecular area, the film thickness, and the refractive indexes with respect to 0 may be obtained from the following equations: A = oo2/(c0se)
(2)
+ Do
(3)
sin2 e]
(4)
d = (Dl - D2 - Do) cos 0 n, = [
v2(1 + cos2 e)€, +
n, = (E, sin2
e + e,
t,
cos 8)1/2
(5)
where ex and e, are the dielectric constants (c = n2) obtained for 6 = 0. By fUing the geometric parameters Do, Dl, and D,, n, and n, can be determined using a computer analysis using the leastsquares method.
Materials and Methods The ellipsometric study was carried out with two classes of surfactants, viz., the diethanolamine derivative of (i) polyisobutylene succinic anhydide (PS)and (ii) n-alkyl succinic anhydride. Three different molecular weights of PS was used for this purpose, viz., 500,710, and 1050, comprising average backbone carbon numbers of the polyisobutylene chain of 14,22,and 34, respectively. In the n-alkyl derivatives, the alkyl group possessed dodecyl (C12), tetradecyl (C14). and hexadecyl (C16) chains, respectively. The surfactants with n-alkyl chains were dissolved in chloform, and the surfactants with polyisobutylene chains were dissolved in hexane for the purpo$c of spreading. Both the solvents (spectrophotometricgrade, Burdick and Jackson) were distilled before use. The surfactants were spread on the aqueous substrate containing ammonium nitrate of IC2M. The ammonium nitrate
PO950 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 TABLE I: Determination of the with Tetradecyl Chainsa 8, deg n, n, 65 1.472 1.469 60 1.471 1.470 55 1.471 1.471 50 1.470 1.472 45 1.470 1.473 40 1.469 1.474 35 1.469 1.475 30 1.468 1.476 25 1.468 1.477 20 1.468 1.478 15 1.467 1.478 10 1.467 1.479 5 1.467 1.479 0 1.467 1.479
A
g -1.2a
g
-.9-
t;
$
-.6-
B7
-3-
h
-E
Ghdcha et al.
60 Of-------
Optical Parameters of Surfactant
d, A 9.8 10.8 11.7 12.6 13.4 14.1 14.8 15.3 15.8 16.3 16.6 16.8 17.0 17.0
- 6 4 deg A , A2/moiccule 0.67 0.73 0.78 0.83 0.87 0.91 0.94 0.96 0.98 1.o 1.01 1.01 1.02 1.02
50 42 37 33 30 28 26 24 23 23 22 21 21 21
“Do= 4.6 A, D,= 29 A, D2 = 12 A.
molecule
MOLECULAR AREA, ( ) Figure 3. 11-A and 6A-A isotherms of the diethanolamine derivatives of n-alkyl = C12, C14, and C16, shown as 1, 2, and 3, respectively.
a w^ -3.5-
TABLE 11: Determination of Optical Parameters of Surfactant with Polyisobutylene Chains of Average Backhone Carbon Number 14” 4 deg n, n, d, A - 6 4 dcg A , A2/molecuic 65 1.556 1.559 10.3 1.24 70 60 1.556 1.558 11.2 1.35 59 55 1.557 1.557 12.1 1.47 51 50 1.557 1.556 12.9 1.58 46 45 1.558 1.554 13.6 1.68 42 40 1.559 1.553 14.3 1.78 38 35 1.559 1.552 14.9 1.87 36 30 1.560 1.551 15.4 1.96 34 25 1.560 1.550 15.9 2.03 32 20 1.561 1.549 16.3 2.09 31 15 1.561 1.548 16.6 2.14 31 10 1.562 1.547 16.8 2.18 30 5 1.562 1.547 17.0 2.20 30 0 1.562 1.547 17.0 2.20 29
‘Do= 5.43 A, D,= 29 A, D2 = 12 A.
-I
with an ellipsometer which has been described elsewhere.”J5
-2.84
Results and Mscussion
g -2.1w 5 -1.4-
-i
. E 60Of-------
MOLECULAR AREA, ( i22/molecule )
Figure 4. 11-A and 6A-A isotherms of the diethanolamine derivatives of polyisobutylene succinic anhydride (average backbone carbon numbers of the polyisobutylene chains are 14, 22, and 34, respectively).
was obtained as pure recrystallized material from ACP chemicals. The water used for the subphase was first purified with a Milli-Q filtering system (Millipore Co. Canada) and then distilled twice in an all quartz distilling unit (Brinkman Instruments Ltd. Canada). All experiments were done at 20 f 0.1 OC. Surface pressure isotherms and ellipsometric angle measurements were simultaneously carried out using a Langmuir trough combined
The experimental surface pressure and ellipmetric angle versus area (II-A and 6A-A) isotherms of surfactants with n-alkyl chains and the surfactants with polyisobutylene chains are presented in Figures 3 and 4. Surface pressure (II) and ellipsometric angles (sa)were simultaneously recorded from the onset of detectable surface pressure. At a very large area 6A is either zero or shows an unstable signal. From the Figures 3 and 4 representing II and 6A against A, several points are apparent: (i) Irrespective of the nature of hydrocarbon chains, the molecular area increases with increasing chain length. (ii) On following the shape of the E A isotherms, it can be seen that the monolayers of the surfactants comprising n-alkyl chains are more condensed and collapsed at higher pressure than those of the surfactants comprising polyisobutylene chains. (iii) The change in ellipsometric angle (6A)upon compression of the monolayers was found to be linearly dependant on the hydrocarbon chain length. (iv) The values of IsA( of the molecules with polyisobutylene chains are higher compared to the molecules with n-alkyl chains. (v) The ellipsometric angle lA6l of the molecules with n-alkyl chains increased upon compression and reached a steady state at the collapse pressure. The l6Al of the molecules with polyisobutylene chains increased steadily upon compression and a sharper rise in IbAl was noticed beyond the collapse pressure observed in 11-A isotherms. Such differences between the two series of surfactants are further delineated by comparing molecules with similar hydrocarbon chain length. In Figure 5 , II and l6Al versus A profiles of the diethanolamine derivative of tetradecylsuccinic anhydride and PS of molecular weight 500 are shown. Despite the differences in the nature of the hydrocarbon chains, both the surfactants showed similar Ii-A isotherms with the exception of higher
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10951
Surfactants at the Air-Water Interface
TABLE III: Optical Purwten of Surfrctrnts with Linear and Branched Hydrocarbon Chains straight chains, Do = 4.6 A av backbone C no. of C no. of n-alkyl chain n, n, d, A polvisobutvlene chain 14 12 1.4625 1.4665 15.5 14 16
e
1.4668
1.4696
1.4792
17.0
1.4849
20.5
-3-
1.5618
1.5580 1.5165
-2-
8 -1.5I
MOLECULAR AREA, ( i2/molecule
Figure 5. Comparative II-A and 6A-A isotherms of the surfactants comprising C14 alkyl chains and polyisobutylene chains of average backbone carbon numbers 14, shown as 1 and 2, respectively.
collapse pressure for the surfactant with the tetradecyl chain. In Tables I and I1 typical data for the film thickness and refractive indexes of the monolayers of the surfactants are given. The unknown optical parameters for the molecules comprising two different types of hydrocarbon chains were determined by simulation at various molecular areas for which the film shows a unique physical state. As can be Seen from the isotherms, none of these surfactants shows a phase transition. For the surfactant with tetradccyl chains, the optical parameters were calculated with Do = 4.6, while Do = 5.43 was used for the molecules with polyisobutylene chains. At a detectable surface pressure both the surfactants have an inclination of 6 5 O with respect to the normal. From the molecular model the optical parameters for the surfactant with tetradecyl chains were calculated at molecular areas ranging between 50 and 21 AZ/molecule, and the optical parameters for the surfactant with polyisobutylene chains were calculated at molecular areas ranging between 70 and 29 A 2 / m o l d e . The rodlike surfactant molecules at an inclination greater than 6 5 O float almost horizontally in a random configuration. Both the surfactant films behave as isotropic media at an orientation of 55" with mpect to the normal which correspond to the molecular area of 37 and 5 1 Az/molecule for the surfactants with tetradecyl and polyisobutylene chains, respectively. Upon compression of the monolayer, the average area occupied by each surfactant decreases, and as a consequence the acyl chains are lifted from the aqueous surface and stand parallel in an orientation perpendicular to the plane of the subphase. During such changes in molecular orientation the film thickness increases. For the surfactant system comprising tetradecyl chains, the optical parameters calculated from the molecular model are given in Table I. The difference between the refractive index components, viz., n, (parallel to the optical axis) and n, (perpendicular to the optical axis) increased with decreasing inclination of the molecules from 65" to 55". At the inclination of 5 5 O (corre-
1S469 1.5140 1.4776
17.0
28.0 41.0
sponding molecular area 37 A2/molecule)the optical birefringence is null (n, = n,) and becomes positive (n, < n,) for smaller molecular areas. Such changes in optical anisotropy upon film compression (n, < n, n, = n, n, > n,) strongly suggest a shift from bent to predominatly upright oirientation of the surfactant molecules. The optical parameters for the surfactant molecules with polyisobutylene chains with average backbone carbon number 14 are given in Table 11. The optical parameters are calculated at the same angles of orientation as the molecules with tetradecyl chains. The null birefringence was observed at the inclination of 5S0 (corresponding molecular area 5 1 A2/molecule). Contrary to the surfactants with tetradecyl chains, the optical anisotropy of the surfactant film comprising polyisobutylene chains showed a reverse trend. Upon compression of the monolayer (Le., from 51 A2/molecule and onward) the film shows negative anisotropy (Le., n, < n,). According to the ellipsometry data, the l6Al values obtained for the molecules with tetradecyl and polyisobutylene (backbone carbon number 14) chains differed greatly. This strongly suggests that the films of the two kinds of molecules behave in a different manner. The change in the ellipsometric angle upon compression of the film is very sensitive to the nature of the hydrophobic environment of the surfactant molecules which seem to be of great importance in determining the molecular structure of the monolayers and their properties. This indicates that the change in [SA1 from the onset of detectable surface pressure to the collapse pressure of the monolayer cannot be explained solely by a change in the film thickness. The change in refractive indexes has also a major role to play in the 6A changes. In Table 111, refractive indexes and film thickness values for the vertically oriented (i.e., at 6 = 0) surfactant molecules with n-alkyl and polyisobutylene chains of different carbon numbers are given. For all the surfactant systems of the present study the experimental 16A(-A isotherms are fitted by the least-squares fit method based on the geometric model. The absence of phase transitions of n-A and 16AFA isotherms were best fitted by taking into account the change in film density that may occur due to the change from the liquid expanded state to the liquid condensed state through the variation of molecular area. The use of a model gives a valuable approximation of the optical parameters. In all the calculations, the general assumptions are as follows: (a) Irrespective of the hydrocarbon chain length the hydrocarbon chains are vertically oriented near collapse pressure. (b) Regardless of the nature of the hydrocarbon chains (n-alkyl or polyisobutylene type) the thickness should be similar for similar hydrocarbon chain length. The refractive index components obtained for the surfactants with n-alkyl chains showed positive anisotropy, whereas those obtained for molecules with polyisobutylene chains exhibited negative anisotropy. According to eq 1, which relates 6A to the thickness and refractive indexes, the difference in the experimental data found between the molecules with n-alkyl and polyisobutylene chains shows clearly that 6A is more sensitive to the refractive indexes than to the monomolecular film thickness. While comparing the calculated optical parameter (n,and n,) versus l6Al for the surfactant molecules of similar hydrocarbon chain length (Tables I and 11), it can be seen that the refractive indexes of the molecules with branched hydrocarbon chains are higher than those of the molecules with linear chains. Despite such differences in optical parameters (n, and n,) and the calculated thickness of the monolayer films of both surfactants appear to be similar. This reveals that the differences in l13Al values observed in these two types of molecules arise mainly from the refractive indexes instead
- -
w. -2.5a
pa
22 34
substituted chains, Do = 5.43 A n, n. d. A
10952 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
0
Ghakha et al.
“uldkno
I
/
1.I
-
1.a
3
4
e
I
7
8
0
10
cut#nFkmbr
10
Figure 6. Refractive index as a function of carbon number of n-alkanes
20
25
30
35
Carbon number Figure 7. Refractive index components as a function of hydrocarbon chain length in the diethanolamine derivatives of n-alkylsuccinic anhydride and polyisobutylene succinic anhydride.
and substituted butanes.
TABLE W. Optical Parameters of n-Alkanes and Substituted Butanesa R M P “D 58.12 0.6012 1.3543 21.03 butane 72.15 0.6201 1.3537 25.27 butane, 2-methyl 86.18 0.6485 1.3688 29.86 butane, 2,2-dimethyl 100.21 0.6901 1.3864 34.13 butane, 2,3-trimethyl butane, 2,2,3,3-tetramethyl 114.23 0.8242 1.4695 38.63 58.12 0.6012 1.3543 2 1.03 butane 72.15 0.6262 1.3575 25.26 pentane 86.18 0.6603 1.3751 29.88 hexane 100.21 0.6837 1.3878 34.57 heptane 114.23 0.7025 1.3974 39.19 Octane
15
1024~ 8.33 10.01 11.83 13.52 15.31 8.33 10.01 11.84 13.70 15.53
M,molar mass; p, density; nD, refractive index; R, molar refraction; a, polarizability. of the monolayer thicknesses. Such discrepancies in refractive indexes observed with the surfactants with two different kinds of hydrocarbon chains can only originate from the substituent methyl groups present in the polyisobutylene chains. Refractive index data of some normal alkanes and substituted butane24are compared (Table IV and Figure 6) in this regard. It shows that despite having similar molecular length, the refractive index of butane increases exponentially with increased number of substitutions by methyl groups. This reveals that the large differences in 6A observed in these surfactant systems have a strong dependence on the structure of the hydrocarbon chains. The differences in refractive indexes obtained for the linear and substituted hydrocarbon chains arise from the differencesin molecular polarizability which depends on the total number of atoms present in the molec~le.~~,~~ The refractive indexes calculated from the molecular model show (Figure 7) that the refractive index increases linearly with an increase in alkyl chaii length, whereas for the polyisobutylene chains the refractive index decreases with increased chain length. The explanation for such an anomaly is not very clear. However, it seems that the difference in the phase behavior of monolayers with regard to the packing of the surfactant molecules has an important role to play. The phase behavior of monolayers at relatively high surface density is strongly influenced by the packing of the hydrocarbon chains.27 The differences in the molecular area and the collapse pressure observed with the molecules of two different kinds of hydrocarbon chains indicate that branched chains do not pack in a similar way to that of n-alkyl ones. For all surfactant systems investigated, it was observed that branching of the hydrocarbon chains causes a greater expansion of the monolayer films compared to the molecules with straight chains. The branching in alkyl chains also caused a lowering in collapse pressure. These features are probably due to a packing perturbation of the branched molecules at the monolayer level. The perturbation resulting from the weaker attractive forces in the hydrophobic region prevents the packing of the hydrocarbon chains in a close-packed manner. It is well-known that branched chains have weaker intermolecular cohesive forces than straight-chain o n e ~ . ~ JThe ~ , *negative ~ optical birefringence ob-
served with the monolayers of the surfactants comprising polyisobutylene chains can be attributed to a conformationaldefect which occurs at the level of chains. This defect can alter the molecular polarizability of substituted chains to a large extent. The positive anisotropy observed with the surfactants comprising n-alkyl chains is due to the fact that the polarizability acting along the molecular axis of the surfactant is greater than that acting perpendicularly to the molecular axis.2°,28Therefore, it is possible that molecules with substituted hydrocarbon chains may have a bent conformation where the upper portion of their alkyl chains tilt leaving the remainder of the chains vertical to the surface. The presence of a large number of methyl substituents on the polyisobutylene chains restricts the rotational degree of freedom around the C-C bond which cause the chains to tilt and prevent them from packing in a close manner.”JO The degree of bending increases with increasing chain length, and as a result it promotes a tendency toward the enlargement of the expanded state and the lowering of the collapse m u r e of the monolayer^.'^ The decrease in the collapse pressure with increasing chain length confers poorer stability to the monomolecular films.8J3 The data on refractive indexes obtained from simulation also reflects the pacling behavior of the surfactant molecules. In contrast to the molecults with n-alkyl chains, the refractive indexes of the molecules with polyisobutylene chains decrease with increasing chain length. It was noticed while the measurements of ellipsometric angle were being made that the molecules with polyisobutylene chains of 34 backbone carbon number generated unstable signals which made the measurements Micult. Such fluctuations in the signal seem to result from conformational disorder. In our earlier studies, infrared spectroscopy of such long chain molecules on Langmuir-Blodgett films indicated the presence of gauche cordormations in the hydrocarbon chains.5 The conformational defects that occur mainly at the end of the hydrocarbon chains3Z3’v y in a random manner in the monolayer. This may cause the oham to push apart and result in the formation of domains. The size of the domains depends on the length of the alkyl chains of the surfactant molecules spread at the air-water interface.34 Due to the loss of film organization, the molecules with longer branched hydrocarbon chains form liquidlike chains. The structure of such films can be described as transient. The continuous up-and-down trend of the ellipsometric signal reveals instability of the film which is the consequnce of formation, disappearance, and re-formation of small domains. Another striking feature observed with the surfactant molecldes with polyisobutylene chains was the sudden rise in (6A(at the collapse pressure (Figures 3 and 4). The dramatic change in slope of the ellipmetric isothermsabove the collapse preswe contrasts with the surfactant molecules with n-alkyl chains. This indicates the differences in the behavior between the surfactants with branched and linear alkyl chains. Such sharp transitions in slopes for the surfactants with polyisobutylene chains signify a new arrangement of the surfactant molecules.36 The collapse region is poorly understood but is often regarded as a region where a multilayer is f ~ r m e d . ~The ~ J aforesaid ~ phenomenon reveals that
J. Phys. Chem. 1992, 96, 10953-10959 the surfactant molecules with polyisobutylene chains form multilayers in the collapse region and that this property is inherent to such surfactant systems.
Acknowkdgment. The authors are deeply grateful to Dr.Daniel Ducharme for many valuable discussions. L.G.is thankful to IC1 Explosives Group Technical Centre for supporting the study.
References and Notes (1) Gorwyn, D.; Barnes, G.T. Lungmuir 1990,6, 222. (2) Johnston, D. S.;Coppard. E.; Parera, G.P.; Chapman, D. Biochemistry 1984, 23, 912. (3) Maqio, 8.; Lucy, J. A. FEES Lett. 1978, 94, 301. (4) Ghaicha, L.; Chattopadhyay, A. K.; Tajmir-Riahi, H. A. Lmgmuir 1991, 7, 2007. ( 5 ) Ghahha, L.; Chattopadhyay, A. K.;Leblanc, R. M. Lmgmuir, submitted. (6) Tomoaia-Cotiscl, M.; Chifu, E. J . Colloid Interface Sci. 1987, 95, 355. (7) Rakshit, A. K.; Zografi, G.; Jallal, I. M.; Gunstone, F. D. J . Colloid Interface Sci. 1981, BO, 466. (8) Menger, F. M.; Wood, M.G.; Richarson, Jr., S.;Zhou, Q.; Elington, A. R.; Sherrod, M. J. J. Am. Chem. Soc. 1988, 110, 6797. (9) (a) Kaneda, T. Bacreriol. Rev. 1977,41,391. (b) Rey, P.; McConnell, H. M. J. Am. Chem. Soc. l!W7,99,1637. (c) Oldfield, E.; Chapman, D. Fed. Eur. Biochem. Soc. Lett. 1977,23,285. (d) Block, M. C.; Van Deenen, L. L. M.; de Gier, J. Biochem. Biophys. Acta 1977, 464, 506. (10) Kellner, 8. M. J.; Cadenhead, D. A. J . Colloid Interface Sci. 1978, 63, 452. (1 1) Welles, H. L.; Zografi, G.; Scimgeour, C. M.; Gunstone, F. D. Adv. Chem. Ser. 1975,144, 135. (12) Nagarajan, M. K.; Shah, J. P. J . Colloid Interface Sci 1981,80, 7. (13) Rice, D. K.; Cadenhead, D. A,; Lewis, R. N. A. H.; McElhaney, R. N. Biochemistry 1987, 26, 3205. (14) Wormuth, K. R.; Zushma, S.Lungmuir 1991, 7, 2048.
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Partial Reduction of a Vanadia/Slllca-Titania Catalyst with NH3 by Electrlcal Conductance and ESR Measurements
+ NO at 473 K Studied
Robert B. Bjorklund,*pt Lars A. H. Andemson,* C. U. Ingemar Odenbrand,* Lars Sjtiqvist,s and Anders Lurid* Laboratory of Applied Physics and Chemical Physics Group, Department of Physics, Linkdping University, S-581 83 Linkdping, Sweden, and Department of Chemical Technology, Lund University, Box 124, S-221 00 Lund, Sweden (Received: June 30, 1992; In Final Form: September 17, 1992)
Partial reduction of vanadia supported on silica-titania by NH3 + NO has been studied by electrical conductance and ESR measurements. The relationship between electrical conductance and degree of reduction was determined by oxidative and reductive titrations of V(IV) and V(V) species leached from catalyst samples which had been reduced to different levels. In situ monitoring of the steady-state V(1V) ion concentration by electrical conductance during reduction with NH3 + NO in the concentration range M O O ppm for each reactant in a 2 vol 7% 02/Ar carrier gas was performed. In a large excess of NH,, the V(IV) ion concentration increased sharply with even small additions of NO. In a large excess of NO, the NH, NO mixture exhibited first an oxidizing character, and the V(IV) ion concentration increased when PNHl> The reaction order with respect to the NO concentration, determined from both the NO conversion and the initial rate of catalyst reduction in excess NH3, was found to be less than unity. Determination of the stoichiometry of the reaction with respect to O2indicated that the gas-phase O2concentration required to balance the reducing character of NH3 + NO mixtures on the surface was significantly higher than predicted by the balanced equation describing the reaction. ESR measurements on the catalyst detected V(IV) ions as vanadyl groups having two different coordination spheres. Reduction of the catalyst with NH, + NO caused an increase in the V(IV) signal and a decrease of the hyperfine structure.
+
latroduction
Selective catalytic reduction of nitrogen oxides by ammonia (SCR) is an established technology for reducing emissions from sb,tatiow power stations, A thoroUa description of the catalysts *To whom correspondence should be addressed. Laboratory of Applied Physics, Linkbping University. Lund University. f Chemical Physics Group, Linkbping University.
*
and details of the reaction have recently been published.' Vanadia-based catalysts, supported on Ti02, exhibit high activity for the SCR reaction in the Presence of 1-6 VOl % oxygen. Commercial catalysts are promoted with tungstate to reduce the activity for oxidation of SO,? and work has recently been reported on optimizing the catalyst pore structure to increase the resistance against SO2and As203 p~isoning.~ Commercial reactors operate in the temperature range 620-670 K with water vapor concentrations of 6-12 vol %.
0022-3654/92/2096-10953$03.00/0Q 1992 American Chemical Society
