1028
Langmuir 1994,10, 1028-1033
Order and Disorder in n-Alkylcarboxylic Acid Monolayers. Chain-Length Dependence and Lateral Interaction Effects F. J. Touwslager* and A. H. M. Sondag Philips Research Laboratories, P.O. Box 80.000,5600J A Eindhoven, The Netherlands Received February 8, 1993. I n Final Form: February 23, 1994" The chain-length dependence of order and disorder phenomena in n-alkylcarboxylicacids, CH~(CHZ)~COOH, chemisorbed on oxidized aluminum, was studied by infrared reflection-absorption spectroscopy (IRRAS). The frequencies of the CH2 stretching modes decrease continuously with increasing chain length, which implies a gradual transition from conformationally disordered chains with many gauche conformation to all-trans ordered chains. The band position of the symmetric carboxylate etetching vibration in a fully packed monolayer exhibits a shift to higher frequencies for chain lengths between 10 and 14 methylene groups. This suggests a transition from a rather open structure for short chains to a densely packed structure for longer chains. The observed maximum integrated intensitiesfor the symmetric carboxylate stretching vibration, which have a lower value for the shorter chain molecules, support this suggestion. A comparison has been made between the coverage-dependent shifts of the symmetric and the asymmetric carboxylate vibration for methylene chain lengths of n = 0, 2, and 14. The data are comparedwith computationsbased on dipole-dipole coupling theory for modes perpendicular and parallel to the metal surface. A discussion is given about the possibility of an ordered adlayer structure for the n = 14 chain at low coverage. Introduction Self-assembled monolayers have been the subject of numerous investigations14 in recent years. They form a suitable substitute for Langmuir-Blodgett monolayers, because they are easily prepared and are chemically bonded to the surface. Different reactive groups have been used to attach the monolayers to the surface of interest. The most commonly used reactive groups are thiols,' carboxylic acids,26 and silanes.- Although much effort has been put into proving the formation of a chemical bond with the surface,9Jo the clearest evidence for a true chemical bond exists for carboxylic acid monolayers chemisorbed on oxidized metals. After formation of a chemical bond with an oxidized metal surface, the change from a carboxylicacid to a carboxylate group immediately follows from the infrared reflection-absorption spectrum. Moreover, the frequencies observed for the carboxylate vibrations are rather specific for the metal counterions involved.ll On the other hand, the thiol and silane groups do not exhibit pronounced bands in the infrared spectrum, from which information about a possible chemical reaction with the surface can be obtained. Therefore, if one intends to study the interaction between organic molecules on a surface in a way similar to the studies of the well-known example of CO on metal surfaces,1%14carboxylic acids are
* To whom all correspondence should be addressed. Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.Am. Chem. SOC.1987,109,3559. Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; 1989,111, Evall, J.; Whiteeides, G. M.; Nuzzo,R. G. J.Am. Chem. SOC. @
321. (2) Allara, D. L.; Nuzzo,R. G. Langmuir 1986,1,45. (3) Allara, D. L.; Nuzzo,R. G. Langmuir 1986, 1, 52. (4) Sondag, A. H. M.; Raas, M. C. J. Chem. Phys. 1989,91,4926. (5) Sondan, A. H. M.; Raas, M. C.; van Velzen, P. N. T. Chem. Phys. Lett. 1989, 135, 503. (6) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100,465. (7) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101, ma.
an obvious choice. Lateral interactions between the molecules may influence the band position of the carboxylate vibrations in a quitespecificway, therebyenabling a determination of the packing density. In the preceding paperls we attempted to interpret the band shifts of the v,-COz- observed for the adsorption of CH&H2)1&OOH on oxidized aluminum as a function of surface coverage by using the dipole-dipole coupling formalism. In the present paper the study is extended to chains of different lengths: CH~(CHZ),COOH. Firstly, the effects of the chain length on the CHz vibrations in densely packed monolayerswill be investigated. Secondly, the changes in the v,-COz- vibration are considered. Finally, the results for both the v,-COz- vibrations will be given for chain lengths of n = 0, 2, and 14 with varying surface coverage. They are discussed by comparing them to calculated line shifts. A comparison of the data obtained for long- and short-chain moleculesmay provide additional information with respect to the interaction mechanism and the blocking of surface sites by the methylene ~ h a i n . ~ J ~ In addition to the presented data on band shifts, some attention will be given to the bandwidth of the vibrations of the carboxylate group. Theoretical Considerations In the present paper, both the symmetric and asymmetric carboxylate vibrations are considered. The surface selection rule of infrared reflection-absorption spectroscopy16predicts a high sensitivity for modes perpendicular to the metal substrate and a low sensitivity for modes parallel to the surface. The relatively high intensity of the v,-COz- vibration compared to the va-COz- vibration in the spectra of the carboxylic acids bonded to an oxidized aluminum surface suggests an almost perpendicular orientation of the C-C bond in the C-COz- group with
ZUI.
(8)Tillman, N.;Ulman,A.; Penner, T. L. Langmuir 1989,5,101. (9) Miller, J. D.; Ishida, H. J. Chem. Phys. 1987, 86, 1593.
(10)Nuzzo,R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. SOC.
1987,109,733. (11) Kagarise, R. E. J.Phys. Chem. 1966,59,271. Tackett, J. E. Appl. Spectrosc. 1989,43, 483. (12)Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1986, 19, 275.
(13) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107. (14) Willis, R. F.; Lucas, A. A.; Mahan, G. D. The Chemical Physics
of Solid Surfaces and Heterogeneow Catalysis; King, D. A., Woodruff, D. P., Ede.; Elsevier: Amsterdam, 1982; Vol. 2, p 159. (15) Sondag, A. H. M.; Touwslager,F. J. Precedingpaperin thisjournal. (16) Greenler, R. G. J. Chem. Phys. 1966,44,310.
0743-7463/94/2410-1028$04.50/00 1994 American Chemical Society
Order and Disorder in Carboxylic Acid Monolayers
respect to the surface. To a first approximation?J7 the v,-C02- and the v,-C02- can therefore be regarded as perpendicular and parallel to the surface, respectively. The theory presented in our preceding paper16 has to be slightly extended to cope with both dipoles parallel and perpendicular to the surface. An expression for the dipole sum for parallel modes has been given previously by Hayden et al.18 For convenience the results for both situations will be reconsidered here starting from the general formula for the energy of two interacting dipoles. The interaction energy between two dipoles i and j is proportional to12
Langmuir, Vol. 10, No. 4, 1994 1029
2
1
(xi;
3xij
+ + yi; + 4d2)3/2 (xi; + yi; + 4d2)'l2
)
(3)
The transition dipole moment, or the dipole derivative with respect to the normal coordinate of the vibration, is introduced into the equation describing the line shift by defining the vibrational polarizability (4)
Finally, the equation for the dipole-dipole copuling shift is given by12 w=wo
l + -aL u)
(
1/2
where pi and pj are the dipoles or the dipole derivatives as in the present case. The distance between the dipoles is denoted by rij = eij = rij/lrilj is the unit vector along the straight line that connects the center of the two dipoles. The simplest situation exists for perpendicular dipoles on a rectangular lattice, i.e., rij = ( x i j , yij) and lrijl = (xi? + yi?)'/'. For 8n array of identical dipoles on this lattice, interacting with each other and with the images induced in the metal, it can be derived that12J8
l+aeu where w and 00 are the frequency perturbed by dipole coupling and the unperturbed frequency,respectively. The denominator expresses the effect of dielectric screening on the shift and a. is the electronic polarizability. The computations were performed assuming values of d = 1 A, a, = 0.2 A3, and ae= 2.5 A3,which are the same as used before.l6 Frequencies of the uncoupled oscillators wo = 1470 cm-l and wo = 1600 cm-I have been used to simulate the band shifts for the v,-C02- and the va-C02- vibration, respectively. The dipole coupling shift is calculated by inserting U from eq 2 or eq 3 into eq 5. Results for different coverages are obtained by multiplying U in eq 5 by the fraction @ of the surface sites covered. We did not attempt to fit line shifts in this study but merely aimed at a better qualitative understanding of the observed shifts.
because the first term in the numerator of eq 1 is zero for the direct dipole-dipole interaction and nonzero for the dipole-dipole image interaction. For convenience we omitted the magnitude of the dipole vectors in the definition of U,i.e. eq 2 should be multiplied by p2 for identical dipoles to obtain the interaction energy. The distance 2d is defined as the distance between the position of the real (point) dipoles and the position of the image dipoles. In other words, d is the distance between dipoles and the image plane. The last two terms in eq 2 originate from the dipole-dipole image interaction. The last term originates from the fact that the dipole, i, and the image of the dipole, j , subtend an angle different from 90° with the line that connects them. The cosine of this angle equals
Details of the experimental setup and the sample preparation procedure have been given elsewhere.*$ Briefly summarized, infrared reflection-absorption spectroscopyhas been applied to study n-alkylcarboxylic acids on oxidized aluminum substrates. The experiments were performed with a Mattson Cygnus 100 FT-IR spectrometer purged with liquid nitrogen boil-off. A multiple specular reflectance setup was used. Superficially oxidized aluminumsubstrateswere freshly prepared in a vacuum coater by depositing approximately 2000 A of aluminum on a silicon wafer and venting with pure oxygen. Samples of the n-alkylcarboxylic acids were prepared by spin-coating from methanol solutions. Various concentrations(0.01-2 mg/mL) of these acids in methanol were used to obtain different coverages on the surfaces. The samples were evacuated prior to the FT-IR measurements. The reaction of freshly prepared oxidized aluminum surfaces with carboxylic acids is instantaneous as opposed to reaction from hexadecane solutions.23 Sample preparation by immersion in solution instead of spin-coatingdoes not result in significantly different spectra. The chance of contamination, however, is considerably higher, especially for submonolayer quantities of carboxylic acids. For this reason the spin-coatingtechnique has been chosen as a general method for sample preparation. No significant infrared absorptions are observed for oxidized aluminum films treated with pure methanol.
Experimental Section
+
2 d / ( ~ i f yif
+ 4d2)1/2.
A somewhat more involved situation arises for dipoles parallel to the surface, because the line connecting the two dipoles in general subtends an angle different from 90°, in contrast to dipoles perpendicular to the surface. The cosine of this angle can be expressed as X i j / ( x i f + yif + 4d2)1/2. In that case, both the direct dipole-dipole and the dipole-image interaction consist of two nonzero terms in eq 1 and the dipole s u m follows from18 (17) The microroughness of the oxidized aluminum surface may be the cause for the slighly off-perpendicular alignment of the carboxylate groups. From F' e 2 of ref 6 it is immediately clear that this effect is much less for w%&c acids chemisorbed on oxidized silver surfaces. (18) Hayden,B. E.;Prince, K.; Woodruff,D. P.;Bradehaw, A. M. Surf. Sci. 198S, 133,689. Hayden,B.E.;Prince,K.;Woodruff,D.P.;Bradshaw, A. M. Phye. Reo. Lett. 1983,61,476.
Results and Discussion Chain-Length Dependence. Figure 1 gives examples of spectra, relevant for the present discussion, which have already been presented in previous publications.4*6J@The present paper dealswith the band positions and line widths ~~
(19) Sondag, A. H. M.; Raas, M. C.; Touwslager,F. J. Appl. Surf. Sci. 1991, 47, 206.
1030 Langmuir, Vol. 10, No. 4,1994
I
Touwslager and Sondag
1 1
I
0.001
1480
0.005
I;&
.-
ui
0 W
E 3
E W
e
a
0
5
10
15
nFigure 3. Band position and bandwidth (fwhm) of the eymmetrical carboxylate stretching vibration u,-COr for CHs(CHg).COOH molecules chemisorbed on oxidized aluminum at
maximum obtainable surface coverage. 2801 1800 1400 wavenumber (cm-')
3100
1O(
Figure 1. FTIR-MSRspectra of CHs(CH2)nCOOH molecules chemisorbed on oxidized aluminum at maximum obtainable surface coverage (refs 4,5, and 18).
Us-CH2 2860
(cm-1)
2855 2850
I 0
5
10
15
nFigure 2. Shift of the symmetric (v.-CH2) and asymmetric methylene (v,-CHz) stretching vibrations of CHa(CH2),COOH
molecules on oxidized aluminum. Only molecules with chain length n 2 4 are included (see text) (ref 4).
deduced from these spectra. Figure 2 shows the band positions for fully covered monolayers of CH&Hz),COOH chemisorbed on oxidized aluminum. Clearly, the band positions for both the v,-CH2 and the v,-CH2 decrease with increasing number, n, of CH2 groups. The accuracy of these band-position determinations is fl cm-1. Data for n < 4 have not been included in the plot, because the band positions of the CH2 groups become dominated by the CH3 and COz- end groups.20 The changes observed in Figure 2 may be interpreted to be the result of an overall diminished level of gauche interactions between neighboring CH2 groups. The range of the shifts, -10 cm-', complies exactly with the shifts observed4for hexadecanoic (20) Hill, I. R.;Levin, I.
W.J. Chem. Phys. 1979, 70,842.
acid, CH&Hz)14COOH (HDA), when going from a disordered state a t low coverage to an ordered state at high coverage. However, instead of the abrupt change observed in ref 4 as a function of surface coverage, a more continuous change occurs in Figure 2. Therefore, the relative number ofgauche conformations seems to decrease gradually as the chain length increases. Nevertheless, since these changes relate to vibrations in the hydrocarbon tails of the monlayer, a discontinuous vibration effect within the monolayer parallel to the oxide surface cannot be excluded, as will be shown. A similar effect on the CH2 vibrations as presented in Figure 2 has been reported in ref 1for n-alkanethiol monolayers on gold, although the authors did not show it explicitly. Figure 3 illustrates the changes in v,-C02- brought about by the variation in the methylene chain length. The band positions are accurate within fl cm-l. The band shift for chain lengths n < 10is less than for the longer chain lengths. It should be realized that the v8-C0z-band positions of the short-chain molecules in Figure 3 are shifted with respect to their position at low coverage or, similarly, with respect to the position in their aluminum salts by about 7 cm-1 (vide infra). The chain-length dependent shift of the usCOz- band indicates an enhanced lateral interaction between the molecules.12 Consequently, the shorter chains must be less densely packed than the larger chains. Whereas a rather abrupt transition is observed for v8-C02-, the CH2 stretch vibration changes only gradually (see Figure 2). Possibly, a minimum number of CH2 groups is required for the attractive forces between the chains to become sufficiently effective for inducing a disorder-order transition. In other words, the loss in the entropic term of the free energy has to be overcome by an energy gain due to dispersive interactions, which depends on the number of CH2 groups. Notice the difference between the order in the chains, which increases gradually with increasing chain length and the order in the arrangement of molecules a t the surface, which appears suddenly above a certain threshold in the chain length. Presumably, the latteral diffusion on the surface during the adsorption process is facilitated by the attractiveinteractions between the long tails. The observed phase transition corresponds with previously obtained data. In studying the chemisorption of n-alkylcarboxylic acids, Allara and Nuzzo2 showed, that
Order and Disorder in Carboxylic Acid Monolayers
1
1480
Langmuir, Vol. 10, No. 4, 1994 1031
1810
u.-co; (em-1)
1470
1
II
CH3COi 1480
u.-coi (Cm.1)
1410
f
1480
1610
u*-co; (Cm-1)
(Cm-1)
1470
t
..
1590
30 0
01
02
A(abs un x cm 11
03
04
0
01
02
Alabs un x cm 1)
03
04
t 0
Bandwidth, 0.1
0.2
A(abs.un.xcm.11
0.3
0.4
0
0.1
0.2
A1abs.un.x cm.1)
/
0.3
0.4
Figure 4. Band shifta of the symmetric (v,-COs-) and asymmetric (v.-COz-) carboxylak stretching vibrationsversus the integrated absorption of the v . 4 0 2 - band.
Figure 5. Av(fwhm), of the symmetric (v.-COs-) and asymmetric(u,-C02-) carboxylatestretchingvibrations versus the integrated absorption of the v,-CO2- band.
the advancing contact angle of n-hexadecane on a monolayer of methylene chains below n = 10 is considerably lower than for longer-chainmolecules. The work of Porter et al.' on n-alkanethiols should also be mentioned here. They observed a discontinuity in the intensity of the vaCH2 as a function of the chain length and suggest this is due to a lower packing density for shorter chains. Our results provide additional information, allowing a more complete picture. An assumption of a linear dependence of the shift of the v,-C0~-vibration on the packing density would result in about a 7/10lower packing density for the chains below n = 10 as compared to the chains in the ordered state. The relative number of gauche conformations in the chains below n = 10 decreases with an increasing number of CH2 groups, but this does not result in a higher coverage for these chains. Hence it appears that the cross-sectional area of these compressed disordered chains must be of similar magnitude independent of the chain length. The bandwidths fwhm (full width at half maximum) of the v,-COz- vibrations at the maximum obtainable surface coveage for various chain lengths are depicted in the lower half of Figure 3. There is not much correlation between the width and shift measurements. The width is approximately constant except for the region of low n. Especially the width of the v,-COz-vibration for the acetate moiety, CHaC02-, is considerably lower. Perhaps this is related to the various possible conformationsof CH2 groups (all CH2 groups except the a-CH2) near the carboxylate group for longer-chain molecules. However, it does not imply that the width is independent of the surfacecoverage. Coverage Dependence. The dependence of the line shift and line width on coverage for chain lengths of n = 0,2,and 14 are given in Figures 4 and 5, respectively. The coverage is represented by the integrated absorption of the v,-COz- band. It appearsto be impossibleto chemisorb a fixed amount of molecules for the n = 0 and 2 chains with the spin coating technique. The previously developed method in which a Langmuir-type adsorption behavior was assumed6 could not be applied for these small
molecules. However, it has been shown in ref 18 that the integrated absorption provides a good estimate for the surface coverage, except sometimes at very high coverages.21 Therefore, possible effects of dipolar interaction on the infrared absorption intensity of the v&Oz- band are neglected. The results for the highest packing density of n = 14 are included in parentheses in Figures 4 and 5, because the intensity at this coverage differs substantially from the intensity as predicted with the Langmuir i~otherm.~ It is evident from Figures 4 and 5 that for the two smaller chain lengths, the magnitude of the integrated absorption A is limited to A = 0.35 AU cm-1 (the integration interval used is 1365-1525 cm-9. On the other hand, the data for CH~(CHZ)~&O~approach a value A = 0.40 AU cm-l. Assuming equal dipole strengths for the v,-CO2- vibrations of the different molecules, it can be inferred from these limiting values that the packing density is higher for the longer-chain molecule. A comparison of the v,-COz- shifts in Figure 4 reveals that a relatively abrupt shift is only observed for the n = 14molecule. Smaller molecules show a continuous increase of the band position as a function of their packing density. Thisjustifies our suggestionmade in the preceding articlel6 that the long methylene chains initially prevent chemisorption in the proximity of an already occupied site. Interpretation of the Coverage-Dependent Shift. The shifts of asymmetriccarboxylate stretching vibration, va-C0z-, are displayed in the right-hand side of Figure 4. The band positions presented are accurate within f2 cm-'. The shifts are positive, i.e. to higher frequency, for CH3(CH2),C02- with n = 0 or 14 but negative for n = 2. The latter behavior seems anomalous, but calculations according to eq 3 inserted in eq 5 show that the anomaly isjust the other way around. Figure 6presents calculations for both the symmetric (eq 2 inserted in eq 5) and (21)H o b s , P.; Pritchard, J. Vibrational spectroscopies for adsorbed species; Bell,A. T.,Hair,M. L., Eds.;ACSSymp.Ser., American Chemical Society: Washington,DC, 1980; Vol. 137, p 51.
Touwelager and Sondag
1032 Langmuir, Vol. 10, No. 4,1994
t
u,-co;
t
v,-co;
I
d
1599
0.00
I
0.04
0.08
theta
Figure 6. Band shifts calculated according to eq 5: top view, a mode perpendicular to the surface representative of the usCOz- vibration; bottom view, a mode parallel to the surface representative of the u,-COyvibration. The dashed line has been calculated for a square lattice adlayer structure with a lattice constant of 6 A. The solid lines have been calculated for a rectangular lattice of 4.8 x 20 Az.
asymmetriccarboxylate vibrations. The surface coverage
fl is defined as the ratio between the number of sites
occupied and the total number of sites. The dashed lines have been calculated by assuming a molecular crosssectional area of 20A2 which, at maximum packing density, results in a (2 X 2) adlayer structure on a square lattice of 3 X 3 A2. The latter is supposed to represent the aluminum hydroxide surface with one site per 9 A2.22 According to dipole-dipole coupling theory,18the shift of a mode parallel to the surface is clearly negative in this case as indicated by the dashed line in the lower half of Figure 6. The only possibility to obtain a positive value for the dipole sum for parallel modes in eq 3 is through a pronounced anisotropic adsorption mechanism.18 For instance, if it is assumed in the computations that the CHs(CH2)&02- molecules adsorb parallel to the surface along one of the lattice vectors, the results indicated by the solid lines in Figure 6 are obtained. At fl = 0.09 a fully covered substrate is already obtained, with no space left between the molecules. Therefore, the results for this "anisotropic adsorption" are slightly over estimated as compared to the "chemisorbed" (2 X 2) case, where only 20136 of the totalsurface area is covered. Ordered surface structures of n-alkyl chains parallel to the surface have previously been observedm and the existence of such a structure for hexadecanoic acid at low coverage can therefore not be totally excluded. It might be worthwhile testing this suggestion by low-energy electron diffraction or scanning tunneling microscopy. However, this "anisotropic adsorption" provides no explanation for the small positive shift of the ua-C0z- vibration observed for the CNsCO2- groups, because an "anisotropic adsorption" behavior is very unlikely for this molecule. The other (22) Peri, J. B.J.Phys. Chem. 1966,69,221. w a n ,J. D.; Hansma, P.K.Surf. Sci. 1978,52, 211.
(23)Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1977,66, 2901.
more likely reason for a positive shift is the not exactly planar orientation of the asymmetric carboxylate vibration. Naturally, this argument could also apply to CH3(CH2)1&02-, but it is not obvious why the orientation of the carboxylate group of CH3(CH2)2C02- on the same surface should be different from the other two examples. More experimental work would be needed to clarify this matter, but it is beyond the scope of the present investigation. Interpretation of the Coverage-Dependent Line Width. Figure 5 presents the line-width measurements. The determination of the width is not always straightforward, e.g. because the v,-C02- band exhibits a shoulder at low wavenumber (see Figure l).24 We have not tried to deconvolute or to fit the spectra. Although there is a large scatter in the data, the bandwidth generally increases with increasing surface coverage as measured in integrated intensity units. The broadening of adsorbate vibrational bands is less well understood than the shift. Inhomogeneous broadening is expected to dominate the bandwidth for disordered structures.13 The inhomogeneity may be caused by differences between adsorption sites (heterogeneity) or by a distribution of intermolecular distances. The latter would only be sharp for an ordered array of molecules. In any case, one should expect a band narrowing at high coverage, at least for the ordered longchain molecules. In fact, the observed behavior is just the opposite; the bands broaden at high coverage. The order within the chains and the order of the adsorbates in the plane of the surface do not necessarily coincide. However, according to the explanation of the shift dependence on the chain length in Figure 3, it seems unlikely that there would be a broader distribution of intramoleculardistances for densely packed long-chain molecules than for less densely packed structures. Attractive forces between the chains tend to decrease the width of the distribution. A similar steep increase of the bandwidth at high coverage has been observed by Hayden et al.18for HCOOH chemisorbed on Cu(ll0). They observed a narrowing of the v,-C02- at low coverage, followed by an increase at intermediate coverage and finally a steep rise at high packing densities. It is difficult to estimate the effects of differences between adsorption sites on the bandwidth as a function of surface coverage, but the effects cannot be completely ruled out on an ill-defiied substrate such as oxidized aluminum. The similarity between the curves for the observed widths and shifts (albeit negative) leads us to the conclusion that the mechanism causing the broadening of these lines must have a similar basis as the mechanism causing the shifts. As discussed in the preceding paper,15 it is most likely that a short-range instead of the long-range dipole coupling interaction is responsible for the observed shifts. It may even be of chemical rather than of physical nature. Hayden et al.la suggest the removal of electrons of an antibonding orbital of the 0-C-0 bond due to depolarization effects with increasing coverage. Unfortunately, the lack of quantitative models for chemical shifts urged us to confine ourselves to dipolar coupling models at present. (24) Data pinta for the low coverages for C&(CH&COOH have been omitted from the bandwidth measurements in Figure 5. The intensity of the low-frequency shoulder (1420 cm-1) of the v.-COs- band (1469 cm-1) is higher for the n = 2 than for the n = 0 and 14 molecules. It makes a reliable measurement of the width without fitting impossible. Comequently, we omitted the same data p i n t a from the bandwidth measurementa of the v.-COz- vibration. T h e width of the v.-COs- band is approximately 90 cm-1 at 0.16 AU cm-1.
Order and Disorder in Carboxylic Acid Monolayers
Conclusions Monolayers of n-alkylcarboxylic acids on oxidized aluminum have been examined using infrared reflectionabsorption spectroscopy. A slow continuous decrease in the band position of the CH2 stretching vibrations with increasing chain length is found, suggesting a gradual reduction of the relative number of gauche conformations in the methylene chain. On the other hand, the band position of the symmetric carboxylateStretchingvibration, however, exhibits an abrupt transition to a higher frequency between 10 and 14 methylene groups. An approximately constant band position is only observed below 10 CH2 groups. Assuming these shifts to be caused by lateral interactions, it can be inferred that the packing density is highest for the longest-chain molecules and that a phase transition occurs between 10 and 14 methylene groups. Possibly, the attractive energy between the long chains facilitates the formation of a densely packed structure at longer chain lengths. The constant frequency observed for the symmetric carboxylate stretching for the
Langmuir, Vol. 10, No. 4,1994 1033 shorter chains at full coverage reveals that the surface area occupied per molecule remains constant, although the ratio of gauche and trans conformations decreases with increasing chain length. It is estimated that the packing density for the smaller chains is 70% less than for the longer chains. The band positionsof the symmetricand the asymmetric stretching vibration of the carboxylate group with varying surface coverage were also exained in some cases. Dipoledipole coupling predicts a negative shift for a mode parallel to the surface, except for very asymmetric adsorption behavior. Our results show a negative shift for the asymmetric carboxylate stretching for 2, but a positive shift for 0 and 14 CH2 groups. An asymmetric adsorption behavior for chains containing 14 CH2 groups seems acceptable, if the chains do adsorb in rows parallel to the plane of the surface. However, this cannot explain the positive shift for the molecule without CH2 groups. More research is needed to clarify this interesting matter.
