Langmuir 1986,2, 412-417
412
Spontaneous Organization of Carboxylic Acid Monolayer Films in Ultrahigh Vacuum. Kinetic Constraints to Assembly via Gas-Phase Adsorption Lawrence H. Dubois,* Bernard R. Zegarski, and Ralph G. Nuzzo* AT&T Bell Laboratories, Murray Hill, New Jersey 07974 Received October 26, 1985. I n Final Form: March 14, 1986 The organization and bonding of vapor-phase-deposited,monomolecular C1-C4 and C6 n-carboxylic acid filmssupported on a Cu(100) surface are described. It is shown that, contrary to adsorption from solution, closest-packed monolayers of the n-carboxylic acids are not readily obtained due to the existence of a severe kinetic barrier (presumablylargely steric in origin) to their formation. For all the acids studied, at least two discrete bonding states have been identified: (1)a symmetrically bonded, low-converage state and (2) a high-coverage state showing significant tilting of the carboxyl head group with respect to the plane of the surface. The implications of these results as regards molecular self-assembly are discussed. Introduction The organization and bonding of monomolecular organic films at solid surfaces or condensed phase interfaces continues to be an area of immense interest. That this should be so follows naturally from the relevance of such structures in a broad range of important technologies. For example, the close relationship between molecular thin films and the processes and materials relevant to corrosion (and passivation), lubrication and wear (tribology), catalysis, wetting, colloid stabilization, and surfactancy, to name but a few, comprises the core of much of classic surface The recent speculation that such materials might also find application in such diverse areas as microelectronics: optics: advanced electrode and eletroanalytical materials,’ and molecular biologyS has only served to increase this attention. Of relevance to all of these areas is the preparation and physical characterization of organic thin films. As a result, there has arisen much interest in such techniques as molecular self-assemblyg and Langmuir-Blodgett transfer depositionlo as preparative routes to complex organic surface films. Both of these procedures, when suitable (1)See, for example: Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1976 and references cited therein. (2)Polymer Surfaces; Clark, D. T., Feast, W. J., Eds; Wiley: New York, 1978; and references cited therein. (3)Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell Universitv Press: Ithaca. New York. 1981:.and references cited therein. (4)Ziiman, W. A. In’Friction and Wear; Davies, R., Ed.; Elsevier: New York, 1959; pp 110-148. (5)See, for example: (a) Vincett, P. S.; Roberts, G. C. Thin Solid Films 1980,68,135-171.(b) Special issue on molecular electronics: Proc. IEEE 1983,130,197-263. (6)See, for example: Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D. J., Ed.; ACS Symposium Series 233; American Chemical Society: Washington, DC, 1983;and references cited therein. (7) Faulkner, L. R. Chem. Eng. News 1984,2845and references cited therein. (8)The literature here is too massive to cite in detail. Particularly elegant examples are provided by McConnel and co-workers. See, for example: Ta”, K. K.; McConnell, H. M. Biophys. J. 1985,47,105-113. Watts, T. H.; Brian, A. A.; Kappler, J. W.; Marrack, P.; McConnell, H. M. Proc. Natl. Acad. Sci. 1984,81,7564-7568.Brian, A. A.; McConnell, H. M. Proc. Natl. Acad. Sci. 1984,81, 6159-6163. Thompson, N.L.; Brian, A. A.; McConnell, H.M. Biochim. Biophys. Acta 1984,772,10-19 and selected references cited therein. (9)A review of classic references can be found in: Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Oxford University Press: London, 1968;Part 11, Chapter 19. (10)For a discussion and relevant references, see: Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966.
0743-7463/86/2402-0412$01.50/0
adsorbates and substrates are used, can yield closestpacked, highly oriented (and perhaps ordered) organic monolayer films. The Langmuir-Blodgett technique rigorously requires the assembly of the monolayer at a liquid-gas interface (e.g., air-water) by mechanical compression. This feature has led some to question whether many of the structures thus obtained are in fact equilibrium as opposed to metastable structures. In this regard, the study of materials formed by spontaneous adsorption are of importance; in the absence of kinetic constraints to assembly, the structure of these materials should closely reflect the equilibrium condition. Because much of the interest in the above centers on the properties of surface phases formed by large molecules, most of the self-assembly data that is available in the literature is for adsorption from solution. In principal, however, comparable structures should be accessible via gas-phase chemisorption. With this in mind, we have undertaken an exploratory ultrahigh vacuum (UHV) study using a series of simple adsorbates, the n-carboxylic acids.” Further, to provide a point of comparison to other UHV data,I2 a high quality, Cu(100) single-crystal surface was employed as a substrate. The results we present below suggest that severe kinetic constraints of steric origin can limit the preparative utility of gas-phase adsorption under these reaction conditions. The data also suggest a possible origin of the minimum chain length dependence observed in several solution adsorption ~ t u d i e s . ~ J l The J ~ complex structural and coordination chemistry of the materials we obtain is also described. Experimental Section Experiments were performed in two UHV chambers. One was a diffusion and titanium sublimation pumped system equipped with four-grid low-energy electron diffraction optics (Varian), a single-pass cylindrical mirror analyzer (Phi) for Auger electron spectroscopy (AES),a quadrupole residual gas analyzer (Inficon) for thermal desorption studies (TDS), and a high-resolution electron energy loss spectrometer. For the EELS experiments, the angle of the incident electron beam (60’ to the surface normal) (11)The n-carboxylic acids represent, perhaps, the most extensively studied adsorbate in classical surface chemistry. The interested reader is directed to: A h a , D. L.; Nuzzo, R. G. Langmuir 1985,1,45-52,5246 and references cited therein. (12)(a) Sexton, B. A. Surf. Sci. 1979,88,319-330. (b) Dubois, L.H.; Ellis, T. H.; Zegarski, B. R.; Kevan, S. D., submitted for publication in Surf. Sci. (c) Sexton, B. A. Chem. Phys. Lett. 1979,65,469-471. (13)Timmons, C. 0.; Zisman, W. A. J.Phys. Chem. 1965,69,984-990 and references cited therein.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 4, 1986 413
Organization of Carboxylic Acid Monolayer Films and ita impact energy (-4.5 eV) were held constant, and electrons were collected only in the specular direction. Typical incident beam currents were (1-2)X A, and the elastic scattering peak from a clean copper surface was (1-6) X 106 counts s-l, with a full width at half-maximum (fwhm) between 40 and 70 cm-'. The X P S experiments were run in a second ion-pumpedUHV chamber containing a modified Kratos X-SAM 800 X-ray photoelectron spectrometer. The hemispherical electron energy analyzer was operated in a fixed analyzer transmission mode with an instrumental resolution of -1.1 eV. A Mg Ka X-ray source was used throughout. All core levels have been referenced to the copper 2p3 core level (binding energy = 932.8 eV). AES could also be pdormed in this system. Both UHV chamberscontained ion sputtering guns for sample cleaning and simple effusive molecular beam sources for gas dosing. Two Cu(100) single crystals (>99.999% pure) were oriented (fl/*O), cut, and polished by standard metallographictechniques. The samples were cleaned of trace carbon, oxygen, and sulfur impurities by repeated cycles of either argon or neon ion bombardment (1000eV, 12 PA) at both 25 and 700-750 "C followed by annealing in vacuum at 700-750 "C to restore surface order. Sample cleanliness was carefully monitored by using both AES and XPS. The sample could be cooled to -150 OC with liquid Nzfor low-temperature adsorption studies. The carboxylic acids used were purified by distillation under an argon atmosphere; a center, constant boiling fraction was collected for each. The acids were carefully degassed via repeated freeze-pump-thaw cycles prior to dosing the sample. All dosing was performed by using a simple effusive doser located directly in front of the sample in order to minimize any wall reactions. In the discussion that follows, all of the acids used are referred to in terms of the number of carbon atoms they contain (Le., HC02H = C1, CH8CH2C02H= CB,etc.).
Results and Discussion The chemisorption of C1-CI and C6n-carboxylic acids on Cu(100) was studied with high-resolution electron energy loss spectroscopy and thermal desorption mass spectrometry. The vibrational spectra of adsorbed formic and acetic acid can be understood completely by comparing the observed data to that obtained from suitable model compounds (i-e., inorganic formates and acetates;14J6 see below). The vibrational spectra of the larger acids are far too complex to analyze fully as a result of the limited resolution of EELS. Consequently, only two modes of diagnostic value have been identified unambiguously; these are the symmetric and asymmetric 0-C-0 stretching vibrations in the 1300-1700-~m-~region.16 Modes assignable to CH2and CH, bending, stretching, and rocking, as well as to the 0-C-0 deformation, were observed but were found to be of little utility in determining the gross structural features of the adsorbed layer. High-resolution EELS spectra of acetic acid chemisorbed on Cu(100) are shown in Figure 1. Results for formic acid chemisorption are discussed in detail elseTwo types of surface structures can be formed readily. In the lower trace of this figure we present clear evidence for the formation of a stable, symmetrically bound acetate.120 This species can be made by dosing a clean Cu(100) surface with a multilayer (-5 langmuirs) of acetic acid at -150 O C and then slowly warming the sample to room temperature. Desorption of the multilayer occurs near -60 OC while the residual hydrogen atoms (resulting from the dissociative chemisorption of the acid) desorb as H2molecules above - 5 O O C. Flashing the sample to 120 O C tends to sharpen both the elastic peak and the loss peaks slightly. No evidence for an ordered overlayer (14)Ito, K.;Berstein, H. J. Can. J. Chem. 1956,34, 170-178.
(15) See for example: Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd Ed.: Wiley: New York, 1978;and references cited therein.
cU(l00)
+ CHjCOOH 1410
1400
I
650 I
NORMAL
l
0
400
,
l
,
l
800 1200 ENERGY LOSS
,
l
1600
,
I
2000
)
Figure 1. High-resolution EELS data showing the vibrational
spectra which originate from acetic acid monolayers of differing structure on a Cu(100)substrate. In the lower trace, the spectrum of a low-coverage, symmetrically bonded carboxylate phase is presented. The upper trace shows the spectrum that develops when additional acetic acid is introduced into the layer described above. This latter monolayer corresponds to a mixed carboxylate/carbosylic acid phase exhibiting significant intramolecular hydrogen bonding and asymmetricbonding of the carboxyl head group to the substrate. Preparative methodologies and mode assignments are described in the text and in Table I, respectively. Table I. Vibrational Mode Assignments of Adsorbed Acetic Acida mode stable solutionb (C2" symmetry) (CH,COO) canted (CH,COO) 81
u1 u2 v3 u, u5
C-H stretch CH3 deform
u,
C-H stretch
bi
C
d
1350 (1080)ef 0-C-0 stretch 1400 (1395) 1410 (1420) C-C stretch (900) 1020 (880) COO deform 650 (635) 670
ug 0-C-0 stretch vg CHS deform
ul0 CH3 rock ull COO rock
d
1630 (1660) g (1060)h 1040 (840) 480 (460) 290 (270)
2935 (2111) 1344 (1985) 1413 (1406) 926 (883) 650 (619) 3010 (22264) 1556 (1545) 1429 (1047) 1020 (832) 471 (419)
Cu-0 stretch 285 (275) "All frequencies in cm-'. *Reference 14. CBroad,very weak mode centered at -3000 cm-'. dOverlapped by large hydrogen bonded OH band stretching from 2000 to 3200 cm-'. A similar broad band occurs for the deuterated species beginning at about 2000 cm-'. e Deuterated species are shown in parentheses. /Shoulder. 8Blocked by intense 0-C-0 stretch (4at 1410 cm-'. Tentative assignment, could also be the symmetric deformation (UZ).
was observed by LEED, though the acetate species was found to be sensitive to the electron beam (similar ob-
414 Langmuir, Vol. 2, No. 4, 1986
Dubois et al.
( a i SYMMETRIC
7
( b ) CANTED
777777 Cl
CZ
277777 c3
///’//’/’
7-77-
c4
c6
Figure 2. Schematicdiagram showing the hypothetical structures of extended confirmations of the n-carboxylic acids in the (a) low-coverage, symmetrically bonded phase and (b) the highcoverage, canted head group phase. Note, that in the latter form, distortion of the originally symmetrical bonding of the carboxyl group to the substrate tends to align the alkyl chain closer to the surface normal.
servations were made for all of the adsorbed acids). The vibrational mode assignments for this form of adsorbed acetic acid are summarized in Table I (see also ref 124. Inspection of these data leads to the conclusion that the adsorbate exhibits a preferred orientation of its C-C axis along the surface normal. Such an assignment is based on a straightforward application of the dipole selection rule; only those modes whose dipole derivative contains a component perpendicular to the surface can be excited.16 Consistent with this, only modes of a1 and no modes of either bl or b2 symmetry are observed. The asymmetric carboxylate stretch is completely absent (reflecting a symmetric disposition of the carboxylate oxygens toward the surface) as are such modes as the CH, rocking deformation (suggesting alignment of the molecular plane along the surface normal). By analogy with recent NEXAFS data on a similarly prepared surface formate on Cu(lOo), we believe that the carboxylate head group of this acetate species is of C2”symmetry with the chemically identical oxygen atoms asymmetrically disposed in two adjacent, 4-fold-hollow sites.17 As was found with chemisorbed formate species on Cu(100),12bthe surface acetate can be made to cant (Figure 2) by chemisorbing additional acetic acid at low temperature. When an acetate-covered surface, prepared as described above, is cooled to -150 O C , dosed with 1-5 L of CH,COOH, warmed to -50 “C (to remove any multilayer adsorption), and cooled once again (to enhance the stability of the new overlayer), the spectrum in the upper trace of Figure 1 is observed. The mode assignments for this structure are listed in Table I. As in the above, a simple application of the dipole selection rule now reveals that the major species on this surface is canted (see Figure 2); we observe three modes of bl symmetry (the asymmetric (16) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. ( 1 7 ) Stahr, J.; Outka, D. A.; Madix, R. J.; Diibler Phys. Rev. Lett. 1985, 54, 1256-1259.
I
545
535
1
52
BINDING ENERGY ( e V )
Figure 3. XPS data showing the 0 1s core levels in the highcoverage (upper trace) and low-coverage(lower trace) forms of acetic acid adsorbed on Cu(100). These spectra have been normalized to the spectral intensity observed for the high-coverage
state. Clean surface background subtractions have been made in each case. Preparative methodologies are comparable to those in Figure 1. The middle trace shows a computer fit to the data for the high-coverage phase using two Gaussian peaks at BE N 533.9 and 532.2 eV and fwhm = 2.1 and 1.9 eV, respectively. carboxylate stretch, 1630 cm-l; the CH, rocking deformation, 1040 cm-’, and the carboxylate rocking deformation, 480 cm-’) which become active only if the carboxyl group is asymmetrically tilted away from the plane of the surface. Of particular importance in this latter regard is the fact that such an orientation results in an inequivalent bonding environment about the oxygens of the head group. The general conclusions reached above are also strongly supported by the XPS data shown in Figures 3 and 4. In the former figure, two normalized 0 Is core level spectra are shown which correspond to the low-coverage (lower trace) and high-coverage (upper trace) states of adsorbed acetic acid (a clean-surface background subtraction has been made in each case). Two features of the data merit specific attention. First, increasing coverage does result, as expected, in an increase in total integrated intensity (here by a factor of -2). Second, the high-coverage state is characterized by a complex 0 Is line shape. With the exception of the pronounced tailing to higher binding energy, this latter spectrum (as shown in the upper trace of Figure 3) is reasonably fit by two overlapping Gaussians (BE 532.2 and 533.9 eV with fwhm = 1.9 and 2.1 eV, respectively) of comparable integrated intensity. As to their origin, it seems intuitive that these two peaks reflect core level emission from the two chemically distinct oxygen atoms present in the high-coverage, canted form. The lower binding energy peak (BE N 531.9 eV), as a result of increased screening of the core hole by the electrons of the metal, is presumed to be that oxygen which is closest to the surface. This assignment is further supported by the close similarity of this latter transition to the single, discrete peak seen for the low-coverage phase at BE N 532.1 eV.
Langmuir, Vol. 2, No. 4, 1986 415
Organization of Carboxylic Acid Monolayer Films
A
cis
I
Table II. 0-C-0 Stretching Vibrations of n -Carboxylic Acids on Cu(100)”
C1 C2
Cs C4 Ce
1325 1400 1410 1400 1400
1350 1410 1410 1410 1440
1650 1630 1620 1640 1700
300 220 210 230 260
1.7 0.7 0.5 0.2 0.1
OAll frequencies in cm-’. b N o asymmetric OCO stretching vibration is observed for the low-coverage, stable species. c A = voco,, - %co.,.
m
P
a
d \I t
300
290
2 80
BINDING ENERGY ( e V )
Figure 4. XPS data showing the C 1s core levels in the lowcoverage (lower trace),high-coverage (middletrace), and multilayer forms (upper trace) of acetic acid adsorbed on a Cu(100) surface. The spectral intensities have been normalized to that in the high (saturation)-coveragemonolayer. Preparative methodologiesare comparable to those in Figure 1. The carbon 1s core level data presented in Figure 4 also shows comparable trends to that described above, though the line shapes observed are much more complicated (we do not currently understand why they appear as such). As in the above, transition from the low-coverage (lower trace) to the high-coverage (middle trace) state results in an increase in the total integrated intensity in the C 1s core level spectra. The peak(s) centered in the carboxylate region (BE = 289 eV) broadens in such a way as to suggest the presence of more than one discrete species (note the broad and complex line shape as compared to the methyl carbon at 285.1 eV). For comparison purposes, the spectrum of the multilayer precurser to this latter phase is shown in the upper trace of Figure 4. These data, together with the 0 1s data described above, suggest that the high-coverage phase is characterized by considerable structural and/or chemical heterogeneity as regards the bonding (surface and intermolecular) of the two oxygens in the carboxylic acid/carboxylate head group (see below). Two questions raised by previous studies of the chemisorption of formic acid on Cu(100)’2bcan be asked in the case of acetic acid as well. These are the following: (1) are the new species on the surface in the high-coverage phase completely deprotonated and (2) is there only a single type of canted species (as regards the head group) rather than a mixture of canted and symmetrically bound molecules. In the case of formic acid chemisorption, the conclusions were that some of the acidic hydrogens were still present and that at saturation coverage all of the molecules were tilted comparably; this latter phase was believed to be held together in a tight hydrogen bonded array.’% These same conclusions are appropriate for acetic acid chemisorption as well. Our reasoning for each is as follows. First, in EELS spectra of the high-coverage state, one observes that there is a broad, weak mode that stretches from below 2800 to above 3200 cm-’. Both the frequency and the width of this mode are characteristic
of intermolecular hydrogen bonding (the CH3 stretching modes tend to be sharply peaked below 3000 cm-’). An equally broad mode is seen for the chemisorption of CD3C02D under comparable conditions. Second, the high-coverage state reverts to the low-coverage phase at a temperature (T ‘v -40 O C ) well below that where a copper surface carboxylate is expected to decompose.lZbJ8 Thus, like the chemisorption of formic acid, the surface most likely contains large (the exact size of these domains is unknown),hydrogen bonded arrays of mixed carboxylate and acid molecule^.'^ The answer to the second question-dealing with whether a mixture of species exists on the surface or if, in fact, there is a single, canted species-is relatively easy to answer by looking at Figure 1in the area of the symmetric CH3 deformation mode (1350 cm-’). When the molecule is oriented perpendicular to the surface (lower trace), this mode is dipole-active; as the molecule tilts, it must and does become weaker. Since this mode disppears (or possibly becomes a very weak shoulder) in the upper trace of Figure 1, it is clear that the initial, symmetrically bound molecules must develop significant cant in the head group as the surface coverage increases. Again, this is consistent with the conclusion that large, hydrogen bonded arrays of “acetic acid” and acetate are present on the surface. It is also clear from these data that the bonding of the residual protons in the acid, as judged by the position of the carbonyl bands,must be significantly different as regards their chemical environment relative to the bulk state (note that there are no modes seen in the 1700-1750 cm-’ region).lg The exact nature of the bonding involved is unknown at present. Even though we cannot completely characterize the vibrational spectra of the other acids studied, the results of Table I1 clearly show that the type of bonding observed above for acetic and formic acids must be similar for all. In all cases a stable species can be formed by first condensing a multilayer of the acid on the surface at low temperatures and then warming the sample to room temperature to remove excess physisorbed acid and hydrogen gas. In all instances, only a single 0-C-0 stretching vibration, the symmetric carboxylate stretch near 1400 cm-’, is observed (Table 11),indicating that a symmetrically bound carboxylate species is formed (C2uhead group). When the surface is cooled once again and additional carboxylic acid added, a canted species is formed as eviden-by the presence of both symmetric and asymmetric 0-C-0 stretching vibrations. With respect to the structures of these latter canted species, examination of the column labeled A in Table I1 (18) Iglesia, E.;Boudart, M. J. Catal. 1983,81, 214-223. (19) It should be noted that mixed carboxylic acid-metal carboxylates, so called ‘half-acids”, are well-known compounds which poaseea extremely strong hydrogen bonds. See, for example: Hadzi, D.; Orel, B.; Novak, A. Spectrochim. Acta 1973,29,1745-1753 and references cited therein.
416 Langmuir, Vol. 2, No. 4, 1986 (the frequency difference between the asymmetric and the symmetric 0-C-0 stretching vibrations) is most useful. In analogy with the vibrational spectra of a wide variety of transition-metal carboxylates, it is likely that a value of A above -180 cm-' corresponds to unidentate bonding. Bidentate bonding is expeded to yield values of A less than 80 cm-' while bridging species exhibit values intermediate between the two.16 In all instances, the observed values of A are, in fact, quite large; this finding further supports the dipole selection rule arguments that phases containing asymetrically bonded head groups (i.e., canted head group structures as formally depicted in Figure 2) are formed at high coverage. The orientation of both the hydrocarbon tail and the canted head group on the surface cannot be precisely determined from the data currently available to w. In Figure 2 we present one possibility for postulated trans zig-zag structures which is consistent with monolayer adsorption studies in solution using longer chain n-carboxylic acids." In (a), where we have a symmetrically bound carboxylate moiety, the angle of the carbon chain is -35' to the surface normal. In (b), where we show a canted species (as defined by the head group), we see the chain is forced to align closer to the surface normal; indeed, in several systems, structures with canted head groups and nearly normal chains have been definitively characterized.20 An additional point can be raised as regards the relative intensities of the symmetric and asymmetric 0-C-0 stretching vibrations of the canted species (see Table 11). It is expected that equal tilting of the carboxylate group should produce spectra with nearly the same relative intensities in these modes across the series. This is clearly not the case. These variations imply one of two possibilities: either (a) the tilt angle is intrinsically different for the different chain lengths studied or (b) there is a larger kinetic and/or steric barrier to the adsorption of the longer chain acids (i.e., redwing does not increase coverage uniformly for all acids). CO titration studies on a number of the symmetricallybonded carboxylate species described in Figure 2 show that there are, as expected, large open areas present on the surface. These experiments were performed by first forming the stable carboxylate species, cooling the sample, and dosing with a saturation exposure of carbon monoxide (5-10 L). The ratio of the CO stretching intensity (ca. 2100 cm-') to that determined from CO chemisorption on the clean surface, provides an approximate measure of the open surface area. Although this ratio varied somewhat from run to run, -'/* of the surface was found to be free to chemisorb CO for all of the acids studied. When the canted species was present, however, very little, if any, CO adsorption was observed for both formic and acetic acids while for the higher acids of the series signiicant CO coadsorptionwas observed even after repeated exposures or the sample to the carboxylic acid. The vibrational spectra show that, a t saturation coverage, one type of species predominates for both acetic and formic acids (at lower coverages there may be a mixture of islands containing canted and symmetricallybound adsorbates). For the longer chain acids, no such definitive conclusions can be reached in large part due to the width of the observed 04-0stretching vibrations (several weak CH2 and CH, scissor and deformation modes cannot be spectroscopically resolved from the symmetric 0-C-0 stretching vibration and add to ita width). In fact, for the longer chain materials there may be persistent islands of (20) There is evidence that the coverage of the stable, symmetrically
bonded formate may be increased by predosing the Cu(100) surface with oxygen.'*J'
Dubois et al. both types of species present. Such a circumstance would account for the low relative intensity in the asymmetric 0-C-0 stretch and the persistent patchiness, as judged by CO coadsorption, seen for these higher molecular weight adsorbates (see Table 11). The thermal stability of both the symmetrically bound and the canted head groups species was studied but with only limited success. The heat of adsorption of both species on copper, as determined by thermal disorption mass spectroscopy, appears to be a very sensitive function of surface coverage, so much so that the desorption spectra were not particularly reproducible. The only exception to this limitation was formic acid, the least hindered adsorbate examined (see ref 12b). Speaking qualitatively, however, we can make two general inferences. First, for all of the acids studied herein, the low coverage state is the only phase found to be persistent in UHV at ambient temperature. Second, we find that as the carbon chain increases in length, the canted structure (normal chain) increases in thermal stability. This trend is consistent with the increasing boiling points of the higher molecular weight acids but cannot be quantified rigorously on the basis of the data presented here.
Concluding Remarks Several features of the work presented abo-ve correlate closely with the data available in the literature for carboxylic acid monolayers prepared by adsorption from dilute solution. This close analogy is all the more remarkable when one considers both the physical differences in these contrasting experiments and the enormous chemical diversity of the substrate surfaces used.l0 In the paragraphs that follow, we will address these points of similarity in order to further clarify the nature of selfassembled carboxylic acid monolayers. The first point of interest to note is that stable adsorption states differing both in coverage and structure can be prepared in UHV. The low-coverage adsorption state we observe has little precedent in the condensed phase (solution and melt) literature. It, in fact, may arise uniquely from the physical nature of our experiments.for the following reasons. First, the substrate employed does not bear an oxide.20 Second, it is not aggressively active in the reduction of acidic protons. Third, the absolute flux of adsorbate molecules to the surface was low. Taken together, these features eliminate the importance of the assembly being driven exclusively by the strong acid-base reactions which characterize chemisorption from solution on most oxide-bearing substrates. Further, they make it technically simple to prepare a low-coverage surface phase independent of the competitive adsorption of soluentderived impurities. The high-coverage state has ample precedent in the literature. Indeed, many structural characterizations of self-assembled carboxylic acid (chain lengths >C,J monolayers on oxide-bearing substrates have deduced similar structures. Thus, for example, on All' and Ag21substrates the alkyl chain has been observed to densely pack and orient nearly along the surface normal (see Figure 2). The canted head group geometry described above has also been documented in the case of an aluminum substrate.l' Thus, despite the differing conditions (temperature, phase, etc.) and substrates (clean metal single crystals, polycrystalline metals bearing an oxide, etc.) which pertain to each, it is (21) Allara, D. L.; Schlotter, N. E.; Porter, M. A.; Bright, T. E., private communication. A similar structure on silver has also been deduced from electron diffraction data. See, for example, ref 2 and: Menter, J. W.; Tabor, D. Proc. R. Soc. London, Ser. A 1957,242, 96-107.
Langmuir 1986,2,417-423 possible to obtain qualitatively similar structures. Examination of the data presented above, albeit even if only for a small number of carboxylic acid adsorbates, demonstrates another important, though poorly recognized, characteristic of the self-assembly of closest-packed layers; these systems are strongly limited by kinetic constraints.ll Further, the simplest interpretation as to the nature of this barrier is that it is largely steric in origin. In solution adsorption experiments, this significant feature is less apparent (temperature and concentration being easily manipulated). In UHV,22 even for the simple adsorbates addressed herein, this limitation is pronounced and restricts the general preparative utility of gas-phase adsorption. The final point of interest we wish to note has to do with the thermal stability of the high-coverage adsorption state. There is a qualitative suggestion in the data we have obtained that the stability of this phase is enhanced-that (22) We have documented kinetic barriers to closest-packed aaeembly in solution experimenta in several system, most notably carboxylic acids on All1 and Ag2I substrates and dialkyl disulfides on A u ~ .All these systems form closest-packed assemblies under appropriate conditions, thus the conclusion that the problem is not one of thermodynamic instability but, rather, the kinetics which characterize the approach to equilibrium. In UHV, the problem is maintaining a high flux of reactanta at high surface temperatures. Clearly, absorbing a multilayer is a highflux experiment. The problem, however, is that multilayer8 are stable in UHV only at extremely low temperatures. (23)Nuzzo, R.G.;Allara, D. L., unpublished results. Whitesides, G. M., personal communication.
417
is, it persists to higher temperatures-as the length of the carbon chain increases. If this is correct, it is possible to imagine a critical chain length for a high-coverage phase which would be persistent at ambient temperature. Does such a process as this account for the minimum chain length dependences (typically CI0-Cl2,T 25 "C) which have been extensively reported in the solution literature for closest-packed adsorption of carboxylic acid monol a y e r ~ ? ~ * 'We ~ * 'simply ~ do not know on the basis of the data available, but such a suggestion remains attractive to us.24 The final answer to this question may not reside in additional UHV studies of the type reported herein. Rather, an examination of the influence of temperature on the stability and nature of surface phase formation by adsorption from dilute solution might help address fundamental issues such as these.
Acknowledgment. We thank C. Chidsey for a critical reading of this manuscript prior to publication. Jh&StV NO. C1,64-18-6;Ct, 64-19-7;CB, 79-09-4; Cq, 107-92-6; Ce, 142-62-1; CU, 7440-50-8. (24) We note for clarity that, in the case of solution experimenta, it is important to differentiate between the nature of the monolayer while immersed in an adsorbate-containing solution and that which is found to persist after ita removal. It is this latter state that is most easily compared to OUT UHV experimenta. Any desorption processes in either environment are irreuer8ibk. The solution environment on the other hand, presents the true equilibrium condition where desorption is reversible.
Structure of Microemulsions in the Brine/Aerosol OT/Isooctane System at the Hydrophile-Lipophile Balance Temperature Studied by the Self-Diffusion Technique J. 0. Carnali,*t A. Ceglie,i B. Lindman, and K. Shinodas Physical Chemistry 1, Chemical Center, Lund University, S-221 00 Lund, Sweden Received October 8, 1985. In Final Form: February 13, 1986 At the hydrophile-lipophile balance (HLB) temperature, the brine/isooctane/aerosol OT (AOT) system displays a large L2 phase which extends to very low AOT concentrations. At 5 wt %/system AOT, the self-diffusion of isooctane and water has been measured at isooctane weight fractions ranging down to 0.25. The system is oil-continuous and brine-discontinuous at low brine contents but becomes bicontinuous as the amounts of oil and brine become comparable. In this region, the self-diffusion of isooctane and water is high, and it is reasoned that each component can form temporary, continuous layers. A highly flexible oil/brine interface must exist under these conditions. At isooctane weight fractions under 0.25, it is necessary to go above or below the HLB temperature to obtain a microemulsion. The system is brine-continuous above the HLB temperature, but the isooctane self-diffusion is fairly large, indicating that the interfaces remain flexible. However, as the temperature is increased further, this flexibility is lost. The conclusion is made that the oil/brine interface possesses a large degree of flexibility at the HLB temperature.
Introduction Microemulsions are mixtures of oil, water, and surfactant
characterized by being a single, isotropic, and thermodyof namically stable The microscopic Permanent address: Department of Chemistry, Lehigh University, Bethlehem, PA 18015. Permanent address: Dipartimento di Chimica, Universitl degli Studi di Bari, Via Amendola 173, 701 26 Bari, Italy. *Permanent address: Department of Chemistry; Faculty of Engineering, Yokohama University, Tokiwadai, Hodogayaku, Yokohama 240,Japan.
*
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phase has been the subject of considerable interest and several models have been Originally, Hoar and fM"n2described these systems as an extension of the familiar emulsion systems to very small droplet sizes. For oil-rich systems, they pictured spherical droplets of water surrounded by a monolayer of surfactant. The droplets were dispersed in an oil-continuous phase. The roles of oil and water were interchanged if the systems were water rich. This model has withstood experimental studies by (1)Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3,391. (2) Hoar,T. P.; Schulman, J. H. Nature (London) 1943, 152, 102.
0 1986 American Chemical Society
