8032
J . Phys. Chem. 1993,97, 8032-8038
Structure of Monolayers of Short Chain bAlkanoic Acids (CH~(CHZ)&OOH,n = 0-9) Spontaneously Adsorbed from the Gas Phase at Silver As Probed by Infrared Reflection Spectroscopy Earl L. Smith+and Marc D. Porter' Department of Chemistry and Ames Laboratory4J.S.Department of Energy, Iowa State University, Ames, Iowa 5001 1 Received: April 8, 1993; In Final Form: May 19, 1993
This paper presents the results of an infrared spectroscopic and wettability examination of monolayers formed by the spontaneous adsorption of short chain n-alkanoic acids ( C H ~ ( C H Z ) ~ C O OnH=, 0-9) from the gas phase onto Ag. The infrared spectral data, collected in an external reflection mode, indicate that the monolayers chemisorb as the corresponding salt and that the chains contain all-trans conformational sequences for n = 1-9. The chemisorption process is further exemplified by the formation of a carboxylate group that is symmetrically bound at Ag as either a bidentate or bridging ligand. The conformational insights are developed on the basis of an unusually prominent series of the methylene wagging progression. An in-depth analysis resulted in the complete assignment of the fundamental modes in the progression as well as a reassignment of the C C stretch in the region of this progression. Structural insights developed from the infrared spectral data and contact angle measurements are compared to analogous monolayers formed from dilute solution. This comparison points to the subtle, but important, role of solvent in influencing the structure of the monolayer.
1. Introduction The study of ordered monomolecular arrays of polar, amphiphilic molecules supported at metal surfaces has grown explosively in the past few Ordered arrays can be prepared by the transfer of a compressed layer of amphiphilic molecules using Langmuir-Blodgett (LB) and related transfer techniques8 and by the spontaneous adsorption of carboxylicacid,3.4 aroma ti^,^ and s~lfur-containing~~~ compounds from dilute solution. Both types of preparations are capable of producing densely packed structures from precursors containing long alkyl chains (Le.,polymethylene backbones longer than 14 methylene groups*). We present herein the results of infrared reflection spectroscopic (IRS) and contact angle characterizations of monolayers formed at evaporated Ag substrates by the chemisorptionfrom the gas phase (as opposed to dilute solution) of short chain n-alkanoic acids (CH3(CHz),COOH, n = 0-9). We were led to explore the utility of gas-phasedeposition based on our interests in unraveling the relationship between the length and packing density for various forms of the above-mentioned monolayers. Because packing density plays a central role in the observed macroscopic interfacial properties (e.g., ionic permeability,9-12wettability,13J4and corrosion contro1l5-l7),a better understanding of factors which affect the surface structure would be useful. Upon initiating such studies, we found that the preparation of monolayers from dilute solution led to an unexpected observation: corrosion of the metal surface. The corrosion effect was most notable for monolayers formed from ethanolic solutions of alkanoic acids at reactive surfaces such as Cu and, to a lesser extent, Ag. For both Cu and Ag, the surfaces were pitted when layers were prepared from ethanolic solution. We have observed this transformation for alkanoic acids of both long (e.g.,n = 18) and short (e.g.,n = 3) chain lengths, with the rate more rapid the shorter the chain length. The transformation can also be influenced by the polarity of the solvent. A nonpolar solvent such as hexadecane greatly impedes the corrosive process for the longer chain alkanoic acids, facilitating the preparation of densely packed alkyl chain arrays.18 However, substrate
-
* To whom correspondence should be addressed. ?Present address: ESCIL-E.R. CNRS 48, 43, Bd. 1 1 novembre 1918, 69100 Villeurbanne Cedex, France.
corrosion remained problematic using hexadecane for the formation of monolayers from the short chain analogs. Based on solubility considerations (i-e., alkyl chain length and solvent polarity) of the adsorbate precursors, we attribute these observations to an adsorbate-induced corrosion of the substrate. Importantly, similar though much less remarkable complications for layers formed for alkanethiols at Au and Ag have been found previously.19.20 The following sectionspresent the results of a beginning study of the formation and structure of monolayer films that are prepared from the gas-phase deposition of n-alkanoic acids at Ag and a comparison to analogs formed from hexadecane solution. We have also conducted studies at Cu which will appear elsewhere.21 The formation and structure of gas-phase deposited monolayers of alkanoic acids have to date been studied in ultrahigh vacuum (UHV).22q23However, few studies have correlated formation and the structural diagnostics from infrared spectroscopic characterizations for layers prepared at ambient pressures.*'z6 Thus, the first sections of this paper describe the findings of an IRS characterization. These data indicate that the deposition leads to the formation of a monomolecular film of the corresponding carboxylate salt and that the polymethylene chains contain alltrans conformation sequences for n = 1-9. The latter conclusion is based largely on the observationsand analysis of an unusually prominent progression of bands for the methylenewagging modes. A brief discussion of the correlations of the structures and wettabilities of these layers is given. An assessment of the differences in the structure and wettabilities of these layers with those formed from the solution phase as well as of the role of the solvent follows.
2. Experimental Section A. Sample Preparation. The substrates were prepared by the resistive evaporation of 300 nm of Ag onto siliconusing an Edwards 306Acryopumpedevaporator. The silicon substrateswere primed with 10 nm of Cr prior to Ag deposition. The silicon substrates were cut into 1- X 3-in. plates from 4-in.-diameter Si( 100)wafers and were rinsed thoroughly with absolute ethanol before loading into the evaporator. The pressure during all depositionswas less than 1 X 10-6 Torr. The Ag and Cr deposition rates were 0.3 and 0.2 nm/s, respectively.
-
0022-365419312097-8032%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 8033
IRS of Alkanoate Monolayers/Ag
TABLE I: Mode Assignments and Peak Positions for Monolayers Formed at Silver by the Spontaneous Adsorption of Short Chain Length RAIkanoic Acids (CHJ(CH~),COOH,n = 0-9) from the Gas Phase peak position (cm-l) vibrational mode
-
n=O
2966" -2937" -2876" 141 1 1344 1375 (sh)
0
1 2 29700 2963 294 1 2936
3 2965 2938
4 2963 2937
5 2965 2938
2878
2876
2877
2877
2877
1405 1295 137 1
1401 1257
1402 1236
1400 1224
1402 1216
1380
1382
1382
1412 1382
1415 1382
1382
1371
1340
1311 1358
1287 1337
1268 1318 1356
1
1
2
2
3
-
6 2964 2937
7 8 2964 2965 2938 2937 -2918" -29200 2878 2878 2878 2850 -285 1" -285 1" 1398 1400 1402 1199 1209 1205
9 2966 2938 -2918" 2878 -2849" 1397 1198
1415 1382
1416
1416
1254 1300 1338
1245 1284 1321 1350
1237 1271 1304 1335
3
4
4
1230 1262 1293 1322 1345 5
CDdCJWKO2-IAg
1391 1206
0 These peak positions have higher uncertainty based on the low signal-to-noise ratio. These bands were occasionally observed as shoulders on the ul(COO-) which leads to only an estimate of their position. The number of o(CH2) is n/2 for even values of n and (n 1)/2 for odd values of n.37
+
Monolayers deposited from the gas phase were formed by placing the freshly evaporated Ag substrates into a covered glass Petri dish containing 1-2 drops of the acid (CH3(CH2),COOH, n = 1-9) (Aldrich). Vapor pressures of the gaseous acids are estimated to be 0.1-1 .O Torr. The longer chain acids (n = 8,9) were recrystallized once in methanol; the shorter chain acids (n = 0-7), which are liquids, were used as received. Samples were removed every few days and characterizedby IRS to monitor the course of formation. Formation in general appeared complete within in the first few hours of exposure, with layers exhibiting comparable spectroscopiccharacteristicsfor several samples. The acids were replenished as needed to replace that lost by evaporation. Samples were placed in flowing dry nitrogen in the sample chamber of the IR spectrometer for a few minutes before characterization to ensure the removal of any unreacted acid. There were no detectable changes in the spectra for samples handled under such conditions for up to 1 h; longer tests were not conducted. Monolayers formed by spontaneous adsorption from solution were prepared by the immersion of freshly evaporated Ag substrates into 1 mM hexadecane solutions of the n-alkanoic acid. Immersion times varied from a few minutes for the short chain acids to a few hours for the longer chain acids to optimize film formation and to minimizeeffects of corrosion. Upon removal from solution, the samples were rinsed thoroughly with hexane and dried on a spin mater. B. Instrumentation. ( i ) Infrared Spectroscopy. Infrared spectra were acquired with a Nicolet 740 FT-IR interferometer. Reflection spectra were obtained using ppolarized light incident at 82O with respect to the surface normal. These spectra are reported as -log(R/Ro), where R is the reflectance of the sample and Ro is the reflectance of a reference octadecanethiolate437 monolayer at Au. A home-built sample holder was used to position the substrates in the spe~trometer.~~ Both sample and reference reflectances are the average of 1024 scans. Reflection spectra were collected at 2-cm-I resolution (zero-filled) with HappGenzel apodization. Spectra of the bulk liquid n-alkanoic acids were collected at 4-cm-1 resolution using a ZnSe internal reflectance (ATR) cell (Harrick) and are the average of 64 scans. Liquid N2 cooled HgCdTe and InSb detectors were each used in the appropriate spectral region. The interferometer and sample chamber were both purged with boil-off from liquid N2. Further details of the spectroscopic methods as well as the preparation of the reference substrates have been given elsewhere.28 (ii) Contact Angle Measurements. Advancing (8,) and receding (e,) contact angles were measured in air with a Ram&
-
3)
n = even Ag n = odd Figure 1. Generalization of the structure for the alkanoate monolayers at Ag. The structuresare shown for chains with an even or odd number of methylene groups; 8 is the tilt from the surface normal.
Hart Model 100-00 115 goniometer. Hexadecane, diiodomethane, glycerol, and water were used as probe liquids. Contact angles were measured with the needle in the 2-pL droplet formed on the substrate. The value of 8, was determined as the volume of the droplet was slowly increased; 8, was determined as the volume of the droplet was slowly decreased. Full details of these measurements have been given previously.28
3. Results and Discussion A. IR Spectrmopy. The infrared reflection spectra and related information for the monolayers formed by the chemisorption of n-alkanoic acids (n = 0-9) deposited from the gas phase onto Ag are shown in Figures 1-7. Mode assignments and peak positions are summarized in Table I. These spectra provide several important insightsinto the formation and structureof the resulting monolayer: (1) the carboxylic acid head group dissociatively chemisorbs at Ag to form a carboxylate species with a specific orientation, (2) the alkyl chains contain densely packed all-trans sequences,and ( 3 ) there are no detectablehydrocarbon impurities incorporated into the structure of the longer chain films. Discussions of each of these points, which are generalized in the structures in Figure 1, are given in the following sections. An appendix is given for discussion of the assignments of the vibrational modes in the monolayers. ( i ) Carboxylate Stretching Modes. Figure 2 shows the IR spectra between 1800 and 1100 cm-1 for the monolayers formed from n-nonanoic acid from both the gas phase and solution phase, and from gas-phase perdeuteriononanoic acid, and for the bulk liquid nonanoic acid. The band near 1705 cm-I for bulk nonanoic
8034 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993
A
Smith and Porter
n
T
T
n ~ 8 C
n=7 n=6
I
1800
I
1700
I
1600
I
1500
I
1400
I
1300
I
1200
1
n=5
1100
Wavenumber (an”)
n-4
Figure 2. Infrared reflection spectra (low-energy region) of (a) nonanoic acid deposited from the gas phase at Ag, (b) nonanoic acid spontaneously adsorbed from hexadecane solution at Ag, (c) pcrdeuteriononanoic acid, CDg(CDz)$OOH, deposited from the gas phase at Ag, and (d) bulk liquid nonanoic acid. s = 0.0015 AU for (a), (b), and (c) and 0.25 AU for (d).
acid is due to the v ( C 4 ) of the hydrogen-bonded (head-tohead dimer) carboxylic acid group. The spectrum of the bulk acid also exhibits localized vibrations that are associated with short all-trans sequences of methylene groups punctuated by gauche kinks,29 but none clearly attributable to a specific conformational arrangement (see below). For the three types of monolayers, the bands at -1400 cm-l are attributed to the v,(COO-) (- 1406 cm-1 for bulk silver carboxylate saltsg0)for a carboxylate group bound to Ag. The absence of the v ( C 4 ) together with the presence of the v,(COO-) indicates that the chemisorption process leads to the formation of the corresponding carboxylate salt for the films formed from both the gas phase and solution p h a ~ e . ~ ~ , ~ * Additionally, the lack of a detectable v,(COO-) (- 1515 cm-1 for bulk silver carboxylate saltsgo) argues that the carboxylate head group is effectively bound to the Ag surface in a symmetrical orientationas either a bridging or bidentate ligandgzasgeneralized in Figure 1. These conclusions are based on two interrelated factors: (1) the estimated directions of the transition dipole moments for the v,(COO-) (Le., perpendicular to the C-COObond and in the 04-0 plane) and v,(COO-) (i.e., parallel to the C-COz- bond) and (2) the infrared surface selection rule.33 Because the infrared surface selection rule results from the preferential excitation of vibrational modes which have a component of their transition dipole perpendicular to the surface of a highly reflective metal, the presence of only the v,(COO-) indicates that the carboxylate head group is bound as generalized in Figure 1. The spectra in this region also point to a subtle difference between the layers formed from the gas phase and solution. Both types of layers exhibit a similar carboxylate-substrate chemistry, as is evident from the comparable peak positions and shape of v,(COO-) bands for the nonanoate layersfrom the gas and solution phases. However, the absorbance of the nonanoate monolayer from the gas phase is -20% greater than that of the monolayer from the liquid phase. We have observed this trend consistently in comparisons for the layers of all chain lengths (n = 1-9).30.31 The differencebetween gas-phaseand solution-phasemonolayers could arise from three major possibilities: (1) lower surface coverage, (2) higher cant of the head group (Le.,tilt angle of the 04-0 plane with respect to the surface normal), or (3) a combination of both. A further discussion of these differences is presented after an examination of the spectra for the hydrocarbon chains. The IR spectra between 1500 and 11OOcm-l for the monolayers formed for the series of n-alkanoic acids ( n = 0-9) from the gas phase are presented in Figure 3. These spectra indicate that all of the layers form as the corresponding carboxylate salt.
n=3 n=2
n=l
I 1400
lis0
1100
I
I
1350 1300 1250 Wavenumber (an.’)
I
I
I
1200
1150
1100
Figure 3. Infrared reflection spectra (low-energy region) of n-alkanoic acidsdepositedfromthegasphaseatAg(8-0.0010AU). Thespectrum for n = 0 is plotted at two-thirds its intensity (s = 0.0015).
Interestingly, the absorbances of the v,(COO-) for n = 1-9 (i.e., all methylene-containing structures) arecomparable. It is unlikely that a change in cant would exactly offset a change in coverage. This argues, assuming the cant of the head group remains unchanged, that the coverage of these layers is effectively independent of chain length; i.e., for all chain lengths n = 1-9, the strength of interaction between the head group and substrate is sufficient to lead to the formation of a layer which is stable upon handling in the laboratory ambient. (ii) Polymethylene Stretching Modes. Infrared reflection spectra in the C-H stretching region for the n-alkanoic acids ( n = 0-9) deposited from the gas phase at Ag are shown in Figure 4. For n 1 7, five bands are observed and are ascribed9Jc36 in descending energy to va(CHg,ip), v8(CH3,FR1),v,(CHz), v,(CHg,FRz), and v,(CHz). The components of the Fermi resonance (FR) couplet, v,(CHg,FR1) and v,(CHg,FRz), are designated by the subscripts 1 and 2, which refer to the higher- and lowerenergy components, respectively. Based on comparisonsto many other types of monolayers,30J1 the spectra of the gas-phase formed monolayers are remarkable. For n 2 7, the u,(CHz) appears at 2917-2920 cm-l and u,(CHz) at 2849-2851 cm-l, indicating that the packing and orientation of alkyl chains are similar to those found in a crystalline-like e n ~ i r o n m e n t .The ~ ~ v,(CHz) and v,(CHz) for the solution-phase monolayersg0are slightly higher in energy and absorb about a factor of 2 more strongly. The low absorbances of both the v.(CH2) and v,(CHz) for the gas-phase monolayers indicate a near normal orientation of the polymethylene chain with respect to the Ag surface. This conclusion follows from the IR surface selection rule, which indicates that an all-trans polymethylene chain with its molecular axis exactly normal to a smooth metallic surface has surfaceforbidden (zero absorbance) methylene C-H stretching modes. Unfortunately, the low signal-to-noise ratio for these modes hinders a more in-depth description using existing orientational analysis computation^.^^^^^^ Nevertheless, a comparison to the
IRS of Alkanoate Monolayers/Ag
The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 8035
9
- a
-1.6
2
5
1.2 -
i0.8 10.4
-
4
L
0
2
4
6
CH3(CHJnCOOH
1
8
10
n=6
0.8
3
'0.6
5
4
T^
j0.2
< 0
I 3000
I
I
2950
I
I
2900
I
I
2850
I
1 2800
Wavenumber (an.')
Figure 5. Absorbance of (a) v.(CH,,ip) and (b) u8(CH3,FR2) versus numberof methylenegroups for monolayersof n-alkanoic acidsdeposited from the gas phase at Ag.
Figure 4. Infrared reflection spectra (high-energy region) of n-alkanoic acids deposited from the gas phase at Ag (s = 0.0010 AU).
l
L
findings for other types of monolayers (e.g., alkanethiolates at Au and Ag) is instructive. Alkanethiolates at Au have a chain tilt of 30° from the surface normal, whereas the tilts at Ag are -13O.9~2~ In both cases, the absorbances for the methylene stretches are 3-5 times greater than those in Figure 4. This analysis, which is based on a comparison of data obtained under the same experimental conditions used in Figures 2-7, qualitatively supports our orientation assessment. Together, these findings from the CH2 modes argue that the gas-phase layers are more densely packed and contain fewer defects than those prepared from the solution phase. (iii) Methyl Stretching Modes. Insight concerning the orientation of these monolayers near the chain terminus is obtained from examining the C-H stretching modes of the methyl groups. As discussed, the band at 2963 cm-l is ascribed to the in-plane asymmetric stretch of the methyl group, v,(CH,ip), and those at -2940 and -2877 cm-1 are ascribed to the respective high- and low-energy components of the vS(CH3)Fermi resonance couplet. The data in Figure 4 and Table I show that the positions of the bands are largely independent of the length of the alkyl chain. For n L 2, however, the absorbances of these bands exhibit a clear dependence on whether n is an odd or even number. The data for u,(CH3) and v8(CH3,FR2) are summarized in Figure 5 . The 'odd-even" effect differs notably for the two modes. The higher absorbance values occur for the odd-length chains with v,(CHo,ip) and with v,(CHs,FRz) for the even-length chains. The basis for both observations can be developed from considerations of the spatial orientation of the methyl groups at odd and even length chains in an all-trans conformational arrangement. As revealed by the structural projections in Figure 1, the spatial orientation of the dipole moment for the methyl stretching modes is controlled by whether n is odd or even. When n is even, the dipole moment for v,(CH3), which is oriented along the CH2CH3 bond, is aligned closer to the surface normal in comparison to an odd value of n. It follows that the absorbance for u,(CH3)
-
o
1350,-
0
h
'E
0
,13@)! E 0
1250-
a 0
1200-
:: A A a
1150-
0
k= 1 k=3 k-5
k-7
k-9
c-c 2
4 6 8 1 CH,(CH,),,COOH
0
Figure 6. Array of methylenewagging modes and CHAOO-stretching mode for monolayers of n-alkanoic acids deposited from the gas phase at Ag.
will be greater when n is even relative to when n is odd. Since the dipole moment of u,(CHs,ip) is oriented perpendicular to the molecular axis but in the plane of the C-C-C all-trans chain, the pattern for the odd-even dependence is reversed. The data for the methyl stretching modes clearly exhibit an odd-even dependence.28 This dependence is consistent with the trend expected from the structural orientation developed for the carboxylate head group. Qualitatively, a symmetrically bound carboxylate leads to a projection of the C H A O O - bond along the surface normal. Based on a conventional bond angle for the CHrCH2-COO- of 109O,39an all-trans polymethylene chain then leads to the patterns in Figure 6. We also note that the patterns for the odd-even effect are offset by one methylene group in comparison to those for n-alkanethiolate monolayers chemisorbed at Au.28 In the latter case, the absorbances of the -.Y (CH3,ip) are larger for the even-length as opposed the odd-length chains. The trend is reversed for u,(CH3,FR2). A typical divalent
-
Smith and Porter
8036 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993
a
3000
I
I
2950
I
I
I
2900 Wavenumber (mi')
I
2850
I
1
2800
Figure 7. Infrared reflection spectra (high-energyregion) of deuteriumlabeled alkanoic acids deposited from the gas phase at Ag (s = 0.000 75 AU for both spectra): (a) nonanoic acid-dl7(CD,(CD&COOH) and (b) 6,6,6-trideuteriohexanoicacid (CDs(CHz)&OOH).
bond angle for sulfur (- 110°a) readily accounts for the offset of the patterns. Thus, the spatial orientation of the binding between the head group and substrate dictates the orientation at the chain terminus of these types (i-e.,methyl terminal groups) of monolayers. The data in Figures 4 and 5 also exhibit a gradual increase in the magnitude of the absorbance values as n increases. Since the data for the v,(COO-) argue that the coverage for the monolayers for n = 1-9 is effectively constant, the gradual increase in the absorbances of both v,(CH3,ip) and v,(CHJ,FR~)points to an increase in the order of the layer near the chain terminus. Such ordering, which would reflect the increase in cohesive attractive forces between neighboring chains as n increases, leads to an increase in the overlap between the surface electric field of the IR beam and the transition dipoles of both modes. (iu) Vibrations of the Polymethylene Chain between 1420 and 1200 cm-].In bulk solid alkanoic acids, the series of bands observed between 1345 and 1220 cm-1 is characteristic of an alkyl chain in an all-transconformation. Thus, the bands between 1382 and 1200 cm-1 in Figure 3, which are assigned to the progression of methylene wagging modes (w(CH2)), indicate that the gas-phase deposited monolayers exhibit this extended conformation. This spectral region also contains diagnostics that reveal the presence and type of conformational disorder in the alkyl chains.29,41g42For example, disorder in terms of an endgauche conformation exhibits a o(CH2) at 1341cm-'. Signatures of other types of structural disorder (e.g., kink (-1367 cm-1) and double gauche (- 1354 cm-l) conformers) also have spectral signatures in this region. These types of modes are not apparent in the spectra in Figure 3, suggesting that any conformational disorder is below that detectable by IRS. The prominence of the v,(COO-) and the high-energy wagging modes from the progression further hinder the assessment. A technique such as He diffraction,43 which probes the order of the outermost portion of an interface, may prove valuable in further resolving this issue. Our attempts to assign the bands in this region indicate that a v(C-C) stretch has been generally misassigned as part of the wagging progression. This is a likely consequence of both its comparable magnitude and its proximity to the wagging modes. The reassignment, which facilitated the complete assignment of the wagging modes, is supported by several pieces of structural information. First, a weakly absorbing band is reproducibly found a t 1205 cm-' in the spectra in Figure 2 for both the nonanoate and perdeuteriononanoate monolayers. The bands attributed to the wagging modes for the nonanoate monolayer (1245, 1284, 1321, and 1350 cm-l) are notably absent in the spectrum for the perdeuteriononanoate monolayer. Second, as Table I and Figure 7 indicate, the position of this mode undergoes a large (- 150 cm-l) decrease in energy as n increases. The trend toward decreasing energy with increasing chain length is also observed for the highest-energy v(C-C) mode of n-alkyl paraffins.4 Third, the monotonic array of bands in Figure 7 (see below) enables the prediction of the position of the a-C-COz- mode for the acetate
-
monolayer. Such an extrapolation yields a peak position that agrees well with the observed position. As further proof, this band reassigned to the v(C-C02-) is not observed in monolayers that are devoid of a carboxylate head group, e.g.,alkanethiolates at Au9 and Ag.28 The assignment of the v(C-COz-) mode aided a full assignment of the wagging progression. The latter modes are affected by both the type of end group and the length of the alkyl chain. Polar end groups such as a carboxylate cause increases in both absorbance cross section and vibrational energy with respect to nonpolar (e.g.,methyl) end groups. The absorbance cross section and the vibrational energies of these modes also increase as chain length decrea~es.3~94Additionally, the wagging modes in the bulk solid can be described by an oscillator function3' where the integer k characterizes an individual normal mode. Both odd and even values of k are IR-active for chains when n is odd; only the odd values of k correspond to IR-active modes when n is even. Odd-numbered chains have ( n 1)/2 wagging modes with dipole moments that are parallel to the molecular axis of an all-trans chain. The integer k is odd-valued for those modes. Oddnumbered chains also have ( n - 1)/2 wagging modes with dipole moments that are parallel to the molecular axis of an all-trans chain. The integer k is odd-valued for those modes. Oddnumbered chains also have ( n - 1)/2 wagging modes with dipole moments that are perpendicular to the molecular axis of the chains. For these latter modes, k is even-valued. They have a cross section lower than those parallel to the chain axis and are not observed because of the surface selection rule. Even-numbered all-trans chains, on the other hand, have n/2 IR-active wagging modes. All of these modes have dipole moments that are parallel to the molecular axis of the chain. The result is that only the wagging modes with odd k values are observable for all-trans chains oriented normal to a metal surface. For example, a monolayer formed from octanoic acid ( n = 6) with the above structural characteristics would have three wagging modes in the progression, whereas that from nonanoic acid ( n = 7) will have four surface active modes in the progression. The results of these assignments are collected in Figure 7. This figure plots the position of each progression of the wagging modes as a function of n. Following the above discussion, each of the progressions exhibits the correct number of normal modes, with the position of each mode shifting monotonically to lower energy as n increases. A comparison of the wagging modes for the gas-phase and solution-phase monolayers (Figure 2a,b) serves as a means for assessing the structural differences that arise from the two preparative routes. The absorbances are greater and the widths are narrower for the gas-phase monolayers. This suggests that the gas-phase monolayers may be aligned more toward the surface normal than the chains for the solution-phase monolayers. The difference in widths argues that the chains for the solution-phase monolayers are more disordered than those for the gas-phase monolayers. This conclusion is also supported by the larger coverages of the gas-phase monolayers as suggested by the earlier comparison of the absorbances of the v,(COO-). That is, a larger coverage for the gas-phase monolayers is consistent with a more densely packed array of alkyl chains than for the solution-phase monolayers. (u) Contaminant Incorporation. The possible adsorption of hydrocarbon-containing impurities was tested using deuteriumlabeled alkanoic acids of intermediate (CDS(CD~),COOHand CD3(CH2)4COOH) and short length chains (CD3COOH). Earlier work on self-assembly from solution demonstrated that no hydrocarbon impurities were incorporated into a monolayer film of arachidic acid self-assembled from dilute hexadecane solution at Ag.31 The C-H stretching regions for the layers deposited for the labeled nonanoic and hexanoic acids are shown in Figure 7. The spectra indicate a trace level of hydrocarbon
+
The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 8037
IRS of Alkanoate Monolayers/Ag
. X
O
0
2
4
n
6
Hexadccane Methylene Iodide Glyc~l Watu
8
r 10
Figure 8. Advancing contact angles for n-alkanoic acids deposited from the gas phase at Ag. The probe liquids are hexadecane (e), di-
iodomethane (m), glycerol (X), and water
(0).
contaminant, based on the presence of the broad shoulder at -2965 cm-I for v(CH3) for the films from the labeled hexanoic acid and acetic acid (not shown). In contrast, the absence of C-H stretches in the film from labeled nonanoic acid indicates no detectable hydrocarbon impurity. Thus, the longer chain structures (e.g., n = 7) are effectively free of contamination, whereas the shorter length structures are slightly susceptible to contaminantadsorption. This findingargues that the longer chain acids competitively displace potential contaminants. B. Contact Angle Measurements. The advancing contact angles (e,) for the layers formed from n-alkanoic acids (n = 1-9) deposited at Ag from the gas phase are shown in Figure 8.45 Hexadecane, diiodomethane, glycerol, and water were used as probe liquids. The uncertainty in these data is f2’. For all probe liquids, 8, increases with n, reaching limiting values at n 6. The limiting 0, values are comparable to those for other types of monolayers that are composed of ordered long alkyl ~hains.l~-28.~6 The data for hexadecane and, to a lesser extent, for diiodomethane iodide also exhibit a dependence on whether n is even-valued or odd-valued for n 1 4. That is, the s0: of the monolayers with an odd value of n are slightly less than those of the monolayers with one fewer methylene group. As substantiated by our earlier findings for alkanethiolates at AgZ8and A u and ~ ~ by the correlation with the IRS data in Figures 4 and 5 , we ascribe this dependence to a change in the screening of the dispersion forces of the underlying interfacial structure by the permanent dipole moment of the methyl end Additionally, the data in Figure 8 are collectively similar to the values found for the solution-phase layers. The receding contact angles (e,) were much less reproducible than the e, but not notably different from the data for the solution-phaseanalogs. For both types of monolayers, the reproducibilityin 8,was poorest with water as the probe liquid, possibly attributable to dissolution of the monolayer formed in the probe liquid. Together, these data argue that there is little difference between the two types of monolayers as judged by wettability measurements.
-
4. Conclusions This paper has demonstrated the utility of gas-phasedeposition for preparing short chain (n = 0-9) n-alkanoate monolayers at Ag. The monolayer forms as the corresponding carboxylatewhich is symmetrically bound to the Ag surface. The chains appear strongly oriented toward the surface normal. Formation yields the same type of head groupsubstrate bond as determined in earlier characterizations from the solution pha~e.30.3~.~~ Differences in the IRS data, however, for monolayers from the two preparative routes point to a subtle but important role of solvent
in influencing structure. Though effectively indistinguishable using wettabilitymeasurements,the gas-phasemonolayers exhibit a slightly higher surface coverage which leads to a more densely packed array of alkyl chains. These insights developed from the observation of an unusually prominent array for the progression of the methylene wagging modes. We attribute the improvement in packing to solvent either impeding the formation processes or activating the gradual corrosion of the surface. In closing, we point out potential advantages of the gas-phase versus solution-phase preparation processes. The principal advantage is that, unlike the preparation from solution of monolayers at a reactive surface such as Ag, where we have found that the solution-based processing can lead to the corrosive dissolution and subsequent roughening of the metal surface, no corrosion of the Ag surface occurs. Preliminary tests reveal that monolayers can be prepared by gas-phase deposition of n-alkanethiolates at Ag and Au. (See ref 24 for a recent study of the latter using microbalance and ellipsometric methods.) This type of processing is also applicable to the preparation of monolayers of fluorinated thiols and carboxylic acids. At Cu, however, studies in our laboratory have shown that the gas-phase deposition of n-alkanoic acids results in multilayer formation. We presently attribute this finding to the high reactivity of Cu in comparison to Au and Ag, which facilitates multilayer formation. These and other studies are underway, along with considerations for constructing a sample preparation chamber that will afford improved control of experimentalconditions(e.g., atmosphere and temperature) of preparation.
Note Added in hoof. A recent article (Tao, Y.-T. J . Am. Chem. SOC.1993, 115, 4350-8) concerning the structure of n-alkanoic acids at silver supports the infrared and wettability findings described in this work. Acknowledgment. The Endicott Technology Laboratory of the IBM Corporation sponsored this research. M.D.P. expresses appreciation for a Dow Corning Assistant Professorship and an Alcoa Foundation Faculty Development Grant. This work was also supported in part by the Chemical Sciences Division of the Office of Basic Energy Sciences of the U S . Department of Energy. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract W-7405-Eng-82. Appendix. Band Assignments of CH Stretching and Bending Modes The assignment of alkyl chain modes other than the CH stretching and wagging modes is briefly presented in this section. These assignments are based on the extensive literature of the IR spectroscopyof bulk hydro~arbons3~3~J9~@ as well as the results of spectroscopy of monolayer research.3~~*~~ The spectral feature at 1467cm-I in Figure 3 is clearly observed only in a few of the spectra and is assigned to the CH2 “scissors” mode(6(CH2)). Thelow absorbanceofthismodeisaconsequence of the surface selection rule; Le., the transition dipole is parallel to the surface for an all-transchain orientednormal to the surface. The spectral feature at 1380-1382 cm-1, present as a shoulder on the v,(COO-) for n = 1-7, is assigned to the CH2 wagging fundamental.@ Overlap with the strong v,(COO-) obscures the 1382-cm-lshoulderforn = 8,9. Thebandat 1412cm-1,observed only as a shoulder on the v,(COO-) for n > 4, is assigned to the a-CH2 deformation, b(a-CH2).39 In this case, overlap with the strong v,(COO-) hinders detection for the short chain layers. Upon completing the above assignments and those for the methylene wagging progression, the feature at 1370 cm-l could then be assigned to the methyl umbrella mode (6(CH3)).3’
References and Notes (1) Bigelow, W.C.; Rckett, D.L.; Zisman, W.A. J . Colloid Inrerjuce Sci. 1946, I , 513-38.
8038 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 (2) Sagiv, J. J. Am. Chem. Soc. 1980,102,92-98. (3) Allara, D. L.; Nuzzo, R. G. Lungmuir 1985,1, 52-66. (4) Allara, D. L.; Nuzzo, R. G. Lungmuir 1985,I,45-52 and references therein. ( 5 ) Stem, D. A,; Laguren-Davidson, L.; Frank, D. G.; Gui, J. Y.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Walton, N.; Zapien, D. C.; Hubbard, A. T.J. Am. Chem. Soc. 1989,111,877-891. (6) Whitesides, G. M.; Laibinis, P. E. Lungmuir 1990,6,87-96 and references therein. (7) Ulman, A. An Inrroducrion io Ultra-Thin Organic Films From Langmuir-Blodgerr lo Self-Assembly; Academic: San Diego, 1991. (8) Gaines, G. L. J. Insoluble Monolayers ai Liquid-Gas Interfaces; Wiley: New York, 1966;p 386. (9) Porter, M. D.; Bright, T.B.; Allara, D. L.; Chidwy, C. E. D. J. Am. Chem. Soc. 1987,109,3559-68. Avery, S.;Lynch, M.; Furtsch, T.Lungmuir 1987, (10) Finklea, H. 0.; 3,409-13. (11) Sabatini, E.; Rubinstein, I. J. Phys. Chem. 1987,91,6663-9. (12) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T.M.; Mujsce, A. M. J . Am. Chem. Soc. 1990,112,4301-6. (13) Dubois, L. H.; Zeganki, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1*, 112,570-9. (14) Bain, E. D.; Whitesides,G. M.J. Am. ChemSoc. 1989,111,71647s. (15) Laibinis, P. E.; Whitesides, G. M.J . Am. Chem. Soc. 1992,114, 9022-9028. (16) Blackman, L. C. F.; Dewar, M. J. S.J. Chem. Soc. 1957,7,171-6. (17) Blackman, L. C. F.; Dewar, M.J. S.;Hampon, H. J. J. Appl. Chem. 1957,7, 160-71. (18) Smith,E. L.; Franek, J. E.;Porter, M. D.;Siperko, L. M. Manuscript in preparation. (19) Widrig, C. A.;Alves, C. A.; Porter, M. D. Unpublished results. (20) Edinger, K.; GalzhHuser, A.; Wall, C.; Grunze, M. Lungmuir 1993, 9,4-8. (21) Smith, E.L.; Porter, M. D. Manuscript in preparation. (22) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. Lungmuir 1986,2, 412-7. (23) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987,109,2358-2368. (24) Thomas,R. C.; Sun, L.; Crooks, R. M.; R i m , A. J. Lungmuir 1991, 7,620-622.
Smith and Porter (25) Schneider, T.;Buttry, D. A. Submitted for publication. (26) Cook, H. D.; R i a , H. E. J. J. Phys. Chem. 1959,63,226-230. (27) Stole, S.M.;Porter, M. D. Appl. Spectrosc. 1990,49, 1418-20. (28) Walczak, M. M.; Chung, C.; Stole, S.M.; Widrig, C. A,; Porter, M. D. J. Am. Chem. Soc. 1991,113,2370-8. (29) Snyder, R. 0. J . Chem. Phys. 1%7,47, 1316. (30) Smith, E.L.;Chau, L.-K.; Wolff, K. M.;Porter, M.D. Manuscript in preparation. (31) Schlotter, N.E.;Porter, M.D.; Bright, T.B.; Allara, D. L. Chem. Phvs. Len. 1986.132.93-98. i32) Canning, N. D.S.;Madix, R. J. J . Phys. Chem. 1984,88,2437-46. (33) Greenler, R. G. J . Chem. Phys. 1966,44, 310-15. (34) Nuzzo, R. G.;Allara, D. L. J. Am. Chem. Soc. 1983,105,4481-83. (35) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86,5145-50. (36) Snyder, R. G.; Hsu, S. L.; Krimm, S . Soecrrochim. Acra, Parr A i9i8,34~,-395-406. (37) Snyder, R. G. J. Mol. Spectrosc. 1960,4,411-434. (38) Porter, M.D. Anal. Chem. 1988,60,1143A-50A. (39) Umemura, J. J. Chem. Phys. 1978,68,4248. (40)Handbook of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.; Chemical Rubber Co.: Bcca Raton, FL, 1991-1992. (41) Sander, L.C.; Callis, J. B.;Field, L. R. Anal. Chem. 1983,55,10681075. (42) Maroncelli, M.; Qi, S.P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982,104,6237-6247. (43) Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G.; Wang, J. Lungmuir 1990, 6, 1804-1806. (44)Snyder, R. G.; Schachtschneider. J. H. SDectrochim. Acta 1963.19, 85-116. (45) We have also attempted to determine the thickness of the gas phase monolayenusing optical ellipsometry. Although the thicknweregenerally of the correct order of magnitude, the scatter precluded the development of any structural insights. We do not understand the cause of the poor reproducibility but suspect that a small change in the optical properties of the uncoated substrate during the film formation proccss may be an important factor. (46) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112,558-69. (47) Chau, L.-K.; Smith, E. L.; Porter, M. D. Manuscript in preparation. (48) Chau, L.-K.; Porter, M. D. Chem. Phys. Lett. 1990,167,198-204.
