Langmuir 1995,11, 1244-1251
1244
Reorganization and Collapse of a C 18-FerrocenecarboxamideLangmuir Monolayer Britta Lindholm-Sethson" and Svante Biberg Department of Analytical Chemistry, Umed University, S-901 87 Umed, Sweden Received March 31, 1994. In Final Form: December 30, 1994@ This paper describes details of a reorganization and collapse taking place when a ClSferrocenecarboxamide Langmuir monolayer is compressed in a Langmuir trough at the surface of a 1 M HClO4 solution. Surface pressurelarea isotherms were recorded and analyzed regarding hysteresis effects and compressibility. Other techniques employed include AVmeasurements,cyclic voltammetry,ellipsometry, and atomic force microscopy (AF'M). A dynamic transformation is initiated at the moment the surface pressure starts to rise. This comprises the simultaneous formation of two new phases from the liquid expanded phase that is initially present. One of the phases consists of nuclei of the three-dimensional phase that is formed at collapse and we propose that the other is a liquid condensed phase.
1. Introduction The common view of a monolayer that is compressed in the airlliquid interface is that it transforms from a gaseous phase through different types of liquid phases and finally approaches a solid phase. During all these transitions the head group is assumed to be solvated in the subphase until the film collapsesinto three-dimensional structures. Analogies with three-dimensional P-V isotherms are commonly drawn. Kato et al. recently pointed out that the picture might not be that simple.1,2They claim that so-called "solid films" of monolayers of long chain acids should not be described as two-dimensionalsolids. From their measurements of enthalpy release, they suggest a two-dimensional first-order transition region from twodimensional liquids to solids. As soon as the twodimensionalsolid is formed, the film collapses. Moreover, when stearic acid was spread and compressed on an aqueous phase containing polymeric counterions, Chi et al. found indications of small grains of LC phase inside what was earlier believed to be a pure LE phase.3 The collapse of a film usually starts in a highly compressed state and is often accompanied with a fall off in surface pressure and a compressibility which approaches in fin it^.^ It always involves the formation of a new phase5 and a criterion proposed for identification of collapse is an accelerating loss of monolayer area at constant surface pre~sure.~,'Bilayers or trilayers are suggested to be formed during collapse of fatty acid monolayers, mainly based on the observation of a plateau in surface pressure beyond the collapsepoint and a second sharp rise in surface pressure at half the molecular area or one-third the molecular area.s~gHowever, &to et al. recently demonstrated that a series of these long-chain
* Author to whom correspondence should be addressed. Abstract published in Advance A C S Abstracts, February 15, 1995. (1)Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1991,7 , 2208-2212. (2)Kato, T.;Akiyama, H.; Tanaka, T. Chem. Phys. Lett. 1991,184, @
- -- - - -
A M - A60 .
(3) Chi, L. F.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Thin Solid Films 1994,242,151-156. (4)Roberts, G. G.Langmuir-Blodgettfilms; Plenum press: New York, 1 win (5)Birdi, K.S.Lipid and biopolymer monolayers at liquid interfaces; Plenum Press: New York, 1989 325 pp. (6)Brooks, J. H.; Alexander, A. Retardation of Evaporation by Monolayers; Academic Press: New York, 1962;245. (7) Smith, R. D.; Berg, J. C. J . Colloid Interface Sei. 1980,74,273286. (8) Larsson, K.; Lundquist, M.; Stallbergstenhagen, S.;Stenhagen, E.J . Colloid Interface Sei. 1969,29,268-278. (9)McFate, C.; Ward, D.; Olmsted, J.Langmuir 1993,9,1036-1039. _"I".
acids are transformed into three-dimensional structures when compressed above the equilibrium spreading pressure, ESP.'O Similar results were earlier obtained by Vollhardt et al. who studied relaxation at constant surface pressure of a Langmuir monolayer of stearic acid with Brewster angle microscopy." Thus, a decreasing compressibility and a decelerating decrease in mean molecular area at constant surface pressure does not necessarilyrule out monolayercollapse. This is also demonstrated in the present paper where we have investigated a C18-ferrocenecarboxamidemonolayer that is compressed in a Langmuir trough. Several techniques have been utilized, such as analysis of the surface pressurelarea isotherms including hysteresis studies, AV measurements, cyclic voltammetry, ellipsometric determination of the mean film thickness, and atomic force microscopy of the film transferred to silicon wafers. We propose that reorganization of the monolayer begins at the moment the surface pressure starts to rise. This involves development of a two-phase coexistence region resembling a two-dimensional LE-LC transition, with a simultaneous nucleation of a three-dimensional phase. During this process, the surface pressure is continuously and smoothlyincreasing,and when it has reached a critical value, n,,,a sharp break in the surface pressurdarea isotherm is observed at 34-37 mNlm. The surface pressure continuesto rise, until the film eventually breaks at 50-70 mNlm. The AFM pictures indicate that the twodimensional transition dominates the reorganization before the surface pressure has reached JC,, whereas nucleation and growth of the three-dimensional phase is the dominating feature of the relaxation process at high surface pressures. The appearance of a sharp second oxidation wave in cyclic voltammetry when the highly compressedfilm is transferred t o gold electrodes is related to the three-dimensional phase. The reorganization has been mentioned earlier in the literature by Nakahara et a1.12 They examined the monolayer below and above the transition point with interferometry and report an exact doubling in film thickness. However, interferometry and also ellipsometry fail to resolve the fine structure of the collapsing monolayer (10)Kato, T.;Matsumoto, N.; Kawano, M.; Suzuki, N.; Araki, T.; Iriyama, K. Thin Solid Films 1994,242,223-228. (11)Siegel, S.; Honig, D.; Vollhardt, D.; Mobius, D. J . Phys. Chem. 1992,96,8157-8160. (12)Nakahara, H.; Fukuda, K.; Sato, M. Thin Solid Films 1986, 133, 1-11.
0743-7463/95/2411-1244$09.00l00 1995 American Chemical Society
Reorganization of a Langmuir Monolayer 0
FC
Figure 1. C18-ferrocenecarboxamide (C18FcA).
and one might be misled to believe that the film that is formed at higher surface pressures constitutes one single phase. Results from atomic force microscopy and AV measurements reveal that this is not the case. 2. Experimental Section 2.1. Materials and Equipment. Chemicals. C18-ferrocenecarboxamide (Figure 1)was synthesized as described in the 1 i t e r a t ~ r e . HPLC l~ grade chloroform and HC104 were obtained from Aldrich; octadecyltrichorosilane (OTS)was purchased from Petrarch and was used as received. Ultrapure water (R> 18.4 MQ cm) was obtained by running house deionized water through a Milli-Q Plus water purification system (Millipore) and was used for preparation of the subphase and in all cleaning procedures. Electrodes and Silicon Wafers. Thin gold film electrodes were prepared with an evaporation technique on glass slides using (3-mercaptopropyl)trimethoxysilaneas a molecular glue between glass and g0id.1~The electrode pattern was defined by metal masks consisting of two rectangular pads of 0.64 cm2 area linked by a 1mm wide and 16 mm long strip. The microscope slides accommodate a total of five electrodes where one of the rectangular pads is intended to serve as electrode surface and the other as a contact pad. They were scratched with a diamond pen and split into smaller pieces with one electrode on every piece. Single crystalline silicon p-doped wafers were obtained from Aurel GmbH, Germany, and were cut with a diamond pen in 10 mm wide and 35 mm long strips. Langmuir-Blodgett Equipment. Two home-built Teflon troughs both equipped with a Teflon barrier were employed in the monolayer experiments. The LB transfers were carried out in the smaller trough (30 x 15 cm2)whereas isotherms and AV measurements were performed in a longer and more narrow trough (75 x 12 cm2). The surface pressure was monitored with a KSV film pressure measuring system and a 20 mm wide Wilhelmy platinum plate and AVwas recorded with a vibratingcapacitor apparatus (KSV SP-meter). AKSV 5000 film deposition system was used for controlling LB transfers. The equipment was supervised by a KSV multitasking software package. Ellipsometry. The average thickness of transferred films on siliconwafers was estimated with ellipsometricmeasurements. The ellipsometer was a Rudolf, Model 43603, Research Instrument with a 633-nm laser and an incident angle of 70". A FORTRAN program originally developed by McCrackin and further modified by Sollander and Olofsson was exploited to calculate film thicknesses andor refractive indices for thin monoand multilayer films from the ellipsometric data.16.16 Electrochemistry. Cyclic voltammetry was performed with a Princeton Applied Research Potentiostat, Model 174 A, connected to a Watanabe xy-recorder,WX4421. The experiments were performed in the trough, with the doublejunction reference electrode (Ag/AgCVAl2S043-) and the Pt-wire counter electrode positioned at the far side of the barrier. Atomic Force Microscopy. A Nanoscope I11 (Digital Instruments, Santa Barbara, CA) was used to image a few samples ofthe monolayer transferred to silicon wafers at ambient air conditions. Commercial Si3N4 tips integrated on Si3N4 cantilever, Le., Nanoprobes, were also obtained from Digital Instruments. The cantilevers were 200 pm long triangularly (13)Charych, D. H.; Landau, E. M.; MJda, M. J.Am. Chem. SOC. 1991,113,3340- 3346. (14)Goss, C. A.;Charych, D. H.; Majda, M. Anal. Chem. 1991,63,
-- --. FIE-%?
(15)McCrackin, F.National Bureau of Standards, Washington, DC, 1969. (16)Sollander, S.; Olofsson, G. Fortranprogram fdr Ellipsometrimatning, the Defence Research Institute, 1981.
Langmuir, Vol. 11, No. 4, 1995 1245 shaped and had a reported spring constant of 0.06 N/m. Some of the experiments were carried out in TappingMode, an oscillating technique, and some in normal contact mode.
2.2. Monolayer and Self-AssemblyTechnique. Monolayer Technique. The Langmuir trough was thoroughly cleaned on an everyday basis, which comprised soaking trough and barrier in 96% ethanol, wiping with Techni-Cloth texwipes, and rinsing for at least 20 min in ultrapure water. Hereafter the trough was placed in a laminar flow hood and filled with 1 M HC104. The cleanliness of the surface of the subphase was established by repeatedly sweeping the barrier back and forth. When the surface area was at a minimum, surface active contaminants were aspirated away with a capillary. The cleaning procedure was not terminated until the change in surface pressure was less than 0.1 mN/m upon compression from maximum to minimum area. This procedure was repeated every time a new monolayer was to be spread. A monolayer spreading solution (ca 2 pM) was prepared fresh every day by weighing and dissolving the surfactant C18 ferrocenecarboxamidein chloroform. The molecules were initially spread to amean molecular area of ca. 0.8- 1nm2by micropipeting a precise aliquot of the spreading solution onto the surface of the subphase with a Hamilton gas-tight microliter syringe equipped with a Teflon plunger. A delay of 20-30 min was allowed before starting the compression. The reason for the choice of subphase is that the perchlorate ion is known to stabilize the ferrocenyl group.l7 An attempt to determine the equilibrium surface pressure, ESP, was performed by placing a few crystals of C18-ferrocenecarboxamide on the surface of the subphase. No rise in surface pressure was observed within 1 h of observation. All experiments were performed at ambient room temperature, i.e. 23 3~ 2 "C, since the features of the isotherm did not show any significant variations because of the small variation in temperature. Consequently, any temperature dependence ofthe reorganization is beyond the scope of this report. Self-Assembly of Octadecyltrichlorosilane. The glass/ gold electrodes were covered with a monolayer of OTS with a self-assembly technique developed by Sagiv.1*-20Initially, the electrodes were immersed into a boiling solution of EtOH + CHC13, (l:l), for 20 min, followed by exposure to an argon plasma for 10 min at 80-90 Torr. It has been pointed out in the literature that a crucial requirement for the successful self-assembly of OTS onto a solid surface is the presence of trace amounts of water in the reaction vessel.21 Hence, the electrodes were rinsed with ultrapure water and dried in an argon stream thus leaving diminutive quantities of water at the surface. They were then immediately immersed in a silanization solution (2 mM OTS in 80% hexadecane, 12% cc14, 8% CHCls), where they were kept for 2 x 7 min. The silanization solution was stirred with a magnetic bar during the self-assembly. In between and after the immersion, the substrates were rinsed thoroughly with CHC13 and dried in an argon stream. 2.3 Preparation of LB-films. In many cases the reduction of area because ofthe continuous reorganization was larger than the area of the transferred monolayer itself. In order to obtain an estimate of the transfer ratio, the mean value of the intrinsic barrier motion was subtracted. The transfer rate was always 5 mdmin. A hydrophilic silicon wafer was lowered 20 mm below the clean airAiquid interface before the monolayer was spread onto the subphase. The monolayer was compressed to a number of surface pressures both below and above ncrand then transferred to the substrate in an upstroke. The transfer ratio was always close to 1. These LB layers were used in ellipsometric measurements and AFM studies. The compressed monolayer was transferred to hydrophobic gold electrodestreated with OTS in a down stroke. The transfer ratio was constant during transfer, but in most cases was (17)Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992,8,2521-2530. (18)Sagiv, J. J.Am. Chem. SOC.1980,102,92. (19)Netzer, L.;Iscovici, R.; Sagiv, J. Thin Solid Films 1983,100, 67-76. (20)Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984,100,465-496. (21)Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 1120-1126.
1246 Langmuir, Vol. 11, No. 4, 1995
Lindholm-Sethson and Aberg
n/(mN/m)
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isotherm. At nc,there is a break in the isotherm and the second part of the isotherm is characterized by another smooth increase in surface pressure. The compressibility, c, of a Langmuir film allows detection of transformations of higher order and is defined by the equation: c = -1fA (dAfdrt),,n
0
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(mean molecular area)-’/nm Figure 2. Results from measurements on a Langmuir monolayer of C18FcA on 1M HC104 compressed with a rate of ca. 0.005 nm2/min*molecule:(a, top) surface pressurdarea and compressibility/area isotherms; (b, middle) surface pressure/ area and surface potentiallarea isotherms; (c, bottom) surface pressurdsurface concentration and surface potentiallsurface concentration isotherms.
where A is the mean molecular area at the liquidair interface and JC is the surface pressure. Note that the transformation at J C is~ manifested as a sharp and almost 10-fold increase in the compressibility. A rapid increase in AV from 0 to +240 mV occurs at mean molecular areas between 0.76 and 0.64 nm2. The latter is the mean molecular area where the surface pressure starts to deviate from zero. Thereafter the surface pressure increases slowly as an almost linear function of the decreasing mean molecular area up to nCr where AV attains an almost constant value of 310 mV. Note that for faster compression rates, Le., -0.09 nm2/ minmolecule, a bigger increase in AV, i.e., 110-120 mV was observed, when the surface pressure increases between zero and ncr. A series of hysteresis experiments was also conducted, with a compression rate of either ca. 0.006 or 0.08 nm2/ min-molecule and a reversal of the barrier at various surface pressures (Figure 3). The hysteresis effects increase, the further the film is compressedand the slower the compression rate. It is only when the barrier is reversed at 5 mN/m and a compression rate of 0.08 nm2/ min is employed that no hysteresis is observed. We have also noticed that the faster the compressionrate, the larger is the mean molecular area at Jtcr and also at the take-off from zero surfacepressure. Moreover, a rapid compression induces a decrease in ncr. Relaxationat constant surface pressure and at constant surface area was conducted and some of the relaxation models proposed in the literature were None was found appropriate and the results are therefore not reported. 3.2. Ellipsometric measurements. By determination of the ellipsometric parameters A and Y at five different locations or more, we intended to estimate the thicknesses of transferred LB films on silicon wafers. However, the ellipsometric measurements fail to resolve the microstructure of the investigated film. What one really obtainsis an estimate ofthe totalamount of material that is residing on the surface of the silicon wafer within the area of the laser spot. This can be translated into mean film thickness when the refractive index of the film is known. However, Y was almost invariant in these measurements and therefore it was not possible to obtain an unambiguous value of the refractive index of the film, nF. Ulman recommends nF = 1.5 to be used for film thickness determinations of aliphatic monolayers with ellip~ometry.~~ We have chosen that value in our calculations, since ferrocene does not adsorb at the frequency of the laser. (22) Bois, A. G.; Panaiotov, I. I.; Baret,
265-277. _1984.34. ___ , - - I - - -
significantly smaller than unity, which indicates a change in molecular orientation during tran~fer.~ Transfers were performed at surface pressures between 20 and 50 mN/m. 3. Results 3.1. Surface pressure and AVcurves. The surface pressurdarea isotherm, lateral compressibility, and AV are shown in Figure 2a-c for a compression rate of ca 0.005 nm2/min,molecule. A slow and continuous increase in surface pressure is perceived in the first part of the
(1)
J. F. Chem. Phys. Lipids
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(23) Vollhardt, D.; Retter, U. Langmuir 1992, 8, 309-312. (24) Vollhardt, D.; Retter, U. J. Phys. Chem. 1991,95,3723-3727. (25) Vollhardt, D.; Retter, U.; Siegel, S. Thin Solid Films 1991,199, 189-199. (26) Bois, A. G. J. Colloid Interface Sci. 1986, 105, 124-128. (27) Gabrielli, G.; Guarini, G. G. T.; Ferroni, E. J . Colloid Interface Sci. 1976, 54, 424-429. (28)Keyser, P. D.; Joos, P. J . Phys. Chem. 1984,88,274-280. (29) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G.; Kim,M. W. Macromolecules 1989,22, 2332-2337. (30) Ulman, A. An introduction to. “Ultrathin Organic Films” from
Langmuir-Blodgett to Self-Assembly;Academic Press, Inc.: San Diego, CA, 1991.
Reorganization of a Langmuir Monolayer
Langmuir, Vol. 11, No. 4, 1995 1247 50
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Figure 3. A series of hysteresis experiments for a C18FcA monolayer on 1M HClOl with a reversal of the barrier at (a, b) 30mN/m, (c, d) 5mN/m, and (e) 42 mN/m. The compression rates are indicated in the figures. Highly compressedfilms are very impermeableto water and drying of cotransferred subphase takes several hours. Therefore, it was not possible to obtain reliable ellipsometric measurements for monolayers of mean molecular areas smaller than 0.17 nm2(Figure 4). Two conclusions can be drawn from the observationthat the mean thickness is directly proportional to the surface concentration. Firstly, no material is lost from the airfliquid interface since that would give a negative deviation in the mean thicknesdsurface concentration plot. Secondly, the transferred film is completely dried out since residues of the subphase beneath the film would yield a positive deviation. This latter point is of great importance when the AFM pictures are to be interpreted. 3.3. Electrochemistry of the Transferred LangmuirFilm. In a recent work a C18-ferrocenecarboxamide monolayer at surface pressures between 0 and 29 mNlcm was transferred to either planar gold macroelectrodes or to microarray gold electrodesboth treated with As in the present study the cyclic voltammograms revealed single symmetric peaks with almost no peak splitting and
a standard potential of ca. 440 mV vs Ag/AgCl. No resting time was allowed between spreading of the monolayer in the air11 M HClOI interface and the commencement of the rather rapid compression of -0.1 nm2/min.molecule. This is the reason for the discrepancy between the isotherms in the present and the former work. In this work the monolayer was transferred to planar gold macroelectrodes treated with OTS and cyclicvoltammetry was performed in situ in the trough (Figure 5). A second oxidation wave is emerging slightly positive of the original oxidation wave for films transferred at surface pressures greater than irm,The wave is sharp and becomes more and more significantthe smaller the mean molecular area at transfer. Subsequent voltammetric experiments show a decreasing electroactivity as a function of time. Extrapolation to zero time reveals that the initial amount of electroactive material on the surface of the gold electrodein most cases is smaller than that expected from the transfer ratio and the surface concentration of the ferrocene amphiphile at the airfliquid interface before transfer. The voltammetric response seems to be sensitive to the compression rate of the monolayer and therelaxation time at the target surface pressure before transfer. Thus, for monolayerstransferred at surface pressures immediately prior to xC,,only around 50%was electroactive when a relaxation time of 10 min and a slow compression rate was employed. However, an electroactivity of 80% was observed for a monolayer transferred at the same surfacepressure, with a relaxation period of only 2 min after a rapid compression. In the earlier work as short a relaxation time as possible was used as well as the rapid compression as described above. In that case almost 100%of the transferred material was found to be electroactive after extrapolation back to zero time. The reason for using such a rapid transfer technique was that in an earlier study of the lateral diffusion of the ferrocene amphiphile in the airfliquid interface it was observed that a significant loss of electroactivity was obtained ca. 30 min after spreading.I3 When a slow compression rate was employed before transfer at a mean molecular area of 0.12 nm2,only 15 to 20% of the transferred ferrocene was electroactive. However,with a faster compressionrate the electroactive ratio at the electrode surface was almost 50%for the same mean molecular area at transfer. 3.4. Atomic Force Microscopy. Films were transferred on an upstroke to hydrophilic silicon wafers and investigatedwith an atomicforce microscope. No attempts (31)Lindholm-Sethson,B. D.;Om,J. T.; Majda, M. M. Langmuir 1993,9,2161-2167.
Lindholm-Sethson and Aberg
1248 Langmuir, Vol. 11, No. 4, 1995 I I
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but their number has increased immensely and they grow predominantly in the LE phase. When the monolayer is compressed more rapidly and transferred at 35 mN/m, a different pattern is evolving where the “flowers” are lacking. Now the growth of the three-dimensional phase seemsto dominatethe relaxation process,with a maximum height and width of 0.21 and 0.5 pm, respectively. Fragments of collapsed film with a height of ca. 3 nm and a maximum width of 1 pm are also observed (Figure 61). When an increased contact force was applied to the microscope tip, we noticed that the “flowers” were unaffected whereas the surrounding phase was distorted to a great extent. The result from the “forced i s s e ~ t i o nis ”~~ shown in Figure 6J and indicates a difference in firmness in the two phases. 4. Discussion
\ vI Figure 5. Typical voltammograms for C18FcA monolayers immediately after transfer to gold electrodes. The potential sweep rate was 20 mV/s. (a, top) Effect of relaxation time at target surface pressure, nt, = 30 mN/m: compression rate ca. 0.08 nmVminmolecule; resting time at target surfacepressure (I) 2 min and (11) 10 min; mean molecular area at transfer (I) 0.334 nm2 and (11) 0.293 rima percentage C18FcA that is electroactive (I) 77% and (11) 46%. (b, bottom) Effect of compression rate, nt, = 50 mN/m: compression rates (I) ca. 0.08 nm2/min.molecule up to 30 mN/m, then 0.025 nm2/ min-moleculeand (11)0.025 nm2/min.molecule;resting time at target surface pressure(I)4 min and (II)10min; mean molecular area at transfer (I)0.0109nm2and (11)0.0101nm2;percentage C18FcA that is electroactive (I) 50% and (11) 16%. were made to find structural differences in the film topology due to different compression rates prior to transfer. Therefore almost all films were prepared in a similar manner. The compression rate was always around 0.015 nm2/min-molecule,the rise in surface pressure limited to 5 mN/mmin, and a relaxation period of 10 min a t the target surface pressure was allowed before transfer unless otherwise stated. For a film transferred at 10 mN/m, small grains of 4070 nm width and 2 to 7.5 nm height are visible on an almost flat surface (Figure 6A). Flower-likeplateaus with diameters up to 6-10pm and an apparent height of 1.01.5 nm are observed for films transferred at 20,25, and 32 mN/m, respectively, Figure 6B-D. The maximum diameter of the “flowers” seems to be invariant with increasing surface pressure, whereas their total amount appears to be increasing. In this region, both the number of the 3D grains and their size seem to be constant with a maximum height and width of 10-20 and 150-300 nm, respectively. For films transferred close to nc,(Figure 6E,F)the basic pattern of “flowers”is unaffected, whereas the number of grains has increased significantly. Their size, however, has increased only slightly as compared to the samples transferred a t surface pressures between 20 and 32 mNI m. Figure 6G,H, finally, display films transferred a t the surface pressures 35 and 35.5 mN/m, respectively. The grains are of comparable size as at lower surfacepressures
In order to shed some light on the dynamics that govern the rise in surface pressure during the first part of the C18FcA isotherm, a discussion of the physical shape of the molecule is necessary. However,there does not seem to be any consensus in the literature about how large an area the ferrocene head group really occupies at the air/ water interface. The mean molecular area for the ferrocene head group from crystallographicdata is around 0.34 nm2, which would yield a maximum surface coverage of 4.5 x 10-lo m o l / ~ m ~ This . ~ ~surface concentration is in fair agreement with that obtained by Chidsey et al. who obtained -5.5 x 10-lo mol/cm2when a ferrocene-terminated alkanethiol was self-assembled on gold. The discrepancy was attributed to folding of some ferrocene groups among the poorly packed polymethylene chains.34 Hickman et al. obtained a surface coverage of 4 f 1 mol/cm2 for self-assembly of a bis[(ferrocenylcarbonyl)decyll disulfide on gold electrode^.^^^^^ However, Charych and Facci seem to agree on a significantlylarger molecular area, i.e., 0.47-0.5 nmZ,l3v3’by referring to molecular models. Lenhard et al. also reported lower surface coverage, i.e. 3.3 x 10-lo mol/cm2 when a ferrocenyl carboxylic acid was self-assembledon PtO surfaces.38In the latter case, however, it is arguedthat the alkyl aminesilane chemistry used in the self-assembly sets the limit and not the size of the ferrocene head group. We prefer to trust crystallographic data and, therefore, we assume that the size of the ferrocene head group is 0.34 nm2. Thus, the diameter of the ferrocene head group is significantlylarger than the diameter of the carbon chain. This will obviously influence the dynamics of the monolayer at compression, since a compact packing cannot be obtained with all head groups solvated in the aqueous phase and the carbon chains closelypacked. We also want to stress that the mean molecular area at na is significantly smaller than the cross-sectionalarea of the ferrocene head group. This underlines the possible instability of the monolayer, as does the fact that the head group contains hydrophobicelements. The fnritless attempt to determine the equilibrium spreading pressure (ESP)indicates that the amphiphile does not spontaneouslyform an organized, (32)Hoh, J. H.; Lal, R.; John, S. A.; Revel, J.-P.; Amsdorf, M. F. Science 1991,253, 1405-1408. (33) Seiler, P.; Dunitz, J. D. Acta Cryatallogr. 1979, B35, 10681074. (34) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mqjsce, A. M. J. Am. Chem. SOC.19W,112,4301-4306. (35) Hickman,J. J.; Laibinis, P. E.;Auerbach,D. I.; Zou,C.;Gardner, T.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357-359. (36) Kondo, T.;Yanagisawa, M.; Fujihira,M. Electmhim. Acta 1991, 36, 1793-1798. (37) Facci, J.;P. A. Falcigno;J. M. Goldhngmuir 1986,2,732-738. (38) Lenhard, J. R.; Murray, R. W. J.Am. Chem. SOC.1978, 100, 7870-7875.
Langmuir, Vol. 11, No. 4, 1995 1249
Reorganization of a Langmuir Monolayer
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7:s
10.; I
Figure 6. AJ?M pictures of C18FcA monolayers transferred to silicon wafers at various s d a c e pressures (mean molecular areas a t transfer are indicated in parentheses: (a)10mN/m (0.501nm2);(b)20 mN/m (0.378nm2);(c) 25 mN/m (0.338nm2);(d) 32 mN/m (0.297nm2); (e) 34 mN/m (0.283 nm2);(f) 34.5 mN/m (0.269nm2);(g) 35 mN/m (0.246nm2);(h) 35.5 mN/m (0.239nm2);(i) 35 mN/m (0.226 nm2); (j) 25mNlm (0.338 nm2). Details on compression rates prior to transfer are given in the text. close-packed monolayer in the aidliquid interface. Remember that amphiphiles, such as certain detergents and membrane destabilizing agents like lysophospholipids
with bulky head groups and one carbon chain, are not likely to form stable bilayers but are subject to micellar formation!39
Lindholm-Sethson and h e r g
1250 Langmuir, Vol. 11, No. 4, 1995 MacDonald and Simon derived an equation of state for pure lipid films in the LE-state based on the assumption that the rise in surface pressure is dominated by the entropy term.40 They obtained the following simple expression showing a linear n-VA dependency
n = nkm
(2)
where n equals the number of carbon atoms in the chain,
K is Boltzmann's constant, Tis the absolute temperature,
and r is the surface concentration of the amphiphile at the airAiquid interface. Good experimental agreement was demonstrated for a few lipid molecules. Equation 2 predicts a straight line with slope 18KTwhen n is plotted as a function of 1/Afor the Cl8-FcA amphiphile. As can be seen in Figure 2c the n versus 1/Aplot is indeed linear during the first part of the isotherm, but the slopeis 4.5-5 kT, where the higher value was obtained at higher compression rates. Obviously the monolayers in the present work do not obey the simple relation above. This indicates that the monolayer in this region is not in a simple LE state, as might be assumed from the shape of the isotherm. AVmeasurements. A Galvani potential, which cannot be measured, is established across a gasfliquid interface. See for instance ref 41. The presence of an oriented monomolecular film in the interface o h n leads to a shift in the Galvani potential, which can be measured. This is due to the surface dipole moments, p 1, introduced by the molecules residing in the interface. In the monolayer field this change in Galvani potential is frequently called the surface potential, which should not be mixed up with the classical definition of surface potential, x, which is related to accumulated charges and oriented dipoles in an interface within a single phase.42 In order to avoid confusion we therefore use the term AV throughout this paper. In the analysis of AV data, an airAiquid interface with a spread monolayer is frequently regarded as a parallel plate capacitor where the film is thought of as an array of dipoles. The interactions between monolayer constituents in a LE phase are assumed to be unchanging as the available area is varied, and thus the orientation and size of the dipoles should not be affected at compression. In this simple model the following equation which is attributed to Helmholtz is assumed to be valid, 43
where r is the sufface concentration of dipoles. Several authors have pointed out that the Helmholtz formula is appropriate only in special cases, e.g., for isolated dipoles at a metal-vacuum or air interface. See for instance ref 44. Furthermore, in AV-surface pressure-area isotherm measurements for 38 lipid species, Smaby and Brockmann showed that AV was indeed a linear function of reciprocal area but with a nonzero intercept (39) Cullis, P. R.; Kruijff, B. d.; Hope, M. J.; Verkleij, A. J.; Nayar, R.; Farren, S. B.; Tilcock, C.;Madden, T. D.; Bally, M. B. In Membrane fluidity in biology;Aloia, R. C., Ed.;Academic Press: New York, 1983; Vol. 1. Concepts of membrane structure, pp 40-83. (40) MacDonald, R. C.; Simon, S. A. Proc. Natl. h a d . Sci. U.S.A. 1987,84, 4089-4093. (41)Antropov, L. I. Theoretical Electrochemistry; Mir Publishers: Moscow, 1972. (42) Schottky,W.; Rothe, H. Handb. Exp.-Physik Bd. lS28,13,145ff. (43) Gaines, G. L.Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley and Sons: New York, 1966. (44) Schuhmann, D. J. Colloid Interface Sci. 1990, 134, 152-160.
+
AV = AVO 37.70pL/A
(4)
where AVOis attributed to a reorganization of water structure over the entire interface.45Neverthelesswe will use the Helmholtz formula as a quantitative tool to determine whether and how the average molecular dipole varies with mean molecular area. Since ferrocene is known to be protonated in strong an electrostatic term, YO,has to be included in the Helmholtz formula. Vertical dipoles arise from ion pairing between perchlorate ions and the protonated ferrocene head AV= 4 p p I P*,t = P I
+ Yo
+ Po
(5) (6)
wherepo is the dipole contribution of the ion pair in debye. As can be seen in Figure 2c, AVis not linearly dependent on 11A even in the first part of the isotherm, Le., up to nc,. On the contrary, the slope of the plot is continuously decreasing until the onset of the reorganization where AV attains an almost constant value of 310 mV. Thus, the mean value of the dipole moment for the film forming constituents is not constant during compression at surface pressures between zero and nm,which indicates that the moleculea do not behave as independent dipoles. The nonlinearity in the l/A vs AVplot might partly depend on loss of ion pairs in the air/liquid interface. This suggested squeezing out of counterions is probably not due to steric reasons since the perchlorate ion is significantly smaller than the ferrocenecarboxamide head Electrochemistry. When several consecutivevoltammograms were recorded immediately after transfer, a decrease in anodic peak height was observed (Figure 5). This is in good accordance with the earlier work 31 and might be explained by desorption from the electrode surface. However, a shoulder appears in the positive part of the sweep asthe anodic oxidationwave decreases. Smith et al. recently pointed out that when a redox center is located inside a thin monomolecular film on an electrode surface, the voltammetricwave is broadened and shifted.51 The effect is dramatic and explained by the influence the presence of a thin molecular film of low dielectric constant exerts on the interfacial potential distribution. When the redox center resides only 0.3 nm from the fildelectrolyte interface, the potential of the oxidation wave is shifted from the formal potential by 600 mV. One plausible explanation for the decreasing voltammetric response is therefore that an increasing fraction of the ferrocene head groups are slowly becoming embedded in the hydrophobic part of the monolayer. Furthermore, we have found that a decreasing fraction of the transferred ferrocene amphiphile is electroactive as the compressionrate is slowed down and the relaxation time is prolonged prior to transfer. We suggest that the observedtwo-dhellisional reorganization in the monolayer that is observed in the AFM is a transition from a LE to a LC phase. Hereby, some of the ferrocene head groups in the LC phase become buried in the aliphatic region. The transformation probably proceeds both at the air/ (45) Smaby, J. M.; Brockman, H. L. Biophys. J. 1990,58,195-204. (46) Curphey, T.J.;Santer, J. 0.;Rosanblum, M.; Richards, J. H. J. Am. Chem. Soc. 1980,82,5249-5250. (47) Ballhausen,C. J.;Dahl,J. P.Acta Chem. Scand. 1961,15,13331336. (48) Cerichelli, G.; Illuminati, G.; Ortaggi, G.; Giuliani, A. M. J. Organomet. Chem. 1977,127,357-370. (49) Davies, J. T.;Rideal, E. Can. J . Chem. 1955,33,947-963. (50) Ivarsson, G. Acta Chem. Scand. 1973,27,3523-3530. (51) Smith, C. P.;White, H. S . Anal. Chem. 1992, 64, 2398-2405.
Reorganization of a Langmuir Monolayer liquid interface before transfer and at the electrodesurface after transfer. Zasadzinski et al. have recently shown that mono- and multilayers of LB films of cadmium arachidate can be subject to rapid reorganization on the substrate surface after transfer, when immersed in an aqueous ~ u b p h a s e . ~ ~ The electrochemical experiment demands transportation of charge to the electrode surface. This can be accomplished either by direct electron transfer across the monolayer and the underlying OTS layer or by lateral transport of charge by successive oxidation and reduction of the ferrocenehead groups until the charge has diffised to a defect where it can be transferred to the electrode surface. The first mechanism is ruled out since it requires electron tunneling over a distance of at least 5 nm, which would not yield the reversible voltammograms that are in fact observed. Therefore, we propose that the electron transfer occurs at defects in the transferred film after lateral charge diffusion which is in good accordance with the work of Chidsey et al. They conclude that the major contribution to the measured electron-transfer current in densely packed self-assembled monolayers of ferroceneterminated thiols on gold surfaces relies on rapid electron transfer at a few defect sites.34 A fast compression rate is likely to introduce more defects in a monolayer than a slow one, which is probably one of the reasons for the increasingfractionof nonelectroactiveferrocenesat slower compression rates. In the cyclic voltammetry for films transferred after the break point in the isotherm, a second sharp peak evolves which probably is related to the outgrowth of a three-dimensional phase. It becomes more pronounced the smaller the mean molecular area and the spiky shape of the oxidation wave suggests that the electroactive centers are not subjectto thermal fluctuations. The shape of the voltammogram is similar to that observed by Bilewics and Kublic when studying the electrodeposition of cuprous thiocyanate on a copper amalgam electrode. The mechanism involves a complex adsorption-nucleation and growth mechanism.53 Ferrocene is one of the molecules with JC systems that are known to align their molecular planes in stacks, thus forminglow-dimensional or quasi-one-dimensional lattices.54 We speculate that the spiky shape of the second oxidation peak stems from a one-dimensional metallic conduction in stacked ferrocenes in the bulk phase. 5. Conclusions
We propose a dynamic transformation within the filni that is initiated at the moment the surface pressure starts to rise. The absence of a discontinuity in the compressindicates that no transformation of ibility except at xCr second order is initiated in the first rise of isotherm. We conclude that two new phases are formed simultaneously, which is probably the reason for the failure to fit relaxation data to any of the theories presented in the literature. The longer the monolayer is kept at nonzero surface pressures and the higher the surface pressure, the further the reorganization is enforced and thus the larger is the hysteresis. Hence, we conclude that the monolayer is not in an equilibrium state at elevated surface pressures. The importance of time of observation as introduced by Kat0 (52) Schwartz, D. K.;Viswanathan, R.; Zasadzinski, J. A. N. J . Phys. Chem. 1992,96, 10444-10447. (53) Bilewics, R.;Kublik, Z. J.Electroanal. Chem 1985,195, 137149. (54) Ward, M. D. Electrochemical aspects of low-dimensional molecular solids; Marcel Dekker, Inc.: New York, 1989; Vol. 16, pp 181312.
Langmuir, Vol. 11, No. 4, 1995 1251 for studies of Langmuir films at isothermal compression is therefore emphasized.' AV measurements from the first part of the isotherm indicate that the total number of dipoles in the Langmuir film is decreasing upon compression and/or that there is a continuous decrease in the individual dipole moment for the film-forming constituent, ptot.The formation of a three-dimensional phase can partly explain the observed negative deviation from the Helmholtz formula, since it takes place at all nonzero surface pressures and probably involves loss of both the ion pair and the net orientation of the amphiphile. However, we have observed in the first rise in the isotherm that a fast compression rate induces a bigger increase in AV than a slow one. This indicates that a significant part of the loss of dipoles upon compression is linked to the formation of the two-dimensionalLC phase. For steric reasons, we propose that this phase transition involves a squeezingout of some the bulky ferrocene head groups from the aqueous phase and into a hydrophobic environment. In this process,which can be compared with an ordinary extraction, these ferrocene head groups are deprotonated and the ion-pairing ceases. Thus the total amount of ion pairs in the air-liquid interface is expected to decrease, because of the formation of the LC phase. It cannot be disregarded that the whole amphiphile might lose its orientation during this process and thus not only the electrostatic contribution to ptotis lost but also the contribution from the amide function and the carbon tail. AFM measurements show that the LC phase is more firm than the surrounding original LE phase and also suggest that it is 1.0-1.5 nm thicker. The proposed staggering of the ferrocenecarboxamide amphiphile would explain not only the increased thickness and rigidity of the LC film but also the decrease in electroactivity since the electroactive head groups become embedded inside the aliphatic chains. Only a minor increment in AV is observed during compression above ncrwhere the reorganization mainly proceeds as a collapse into a three-dimensional phase. This suggests that the number of dipoles contributing to AV is almost constant as a function of area. The threedimensional solid phase does not possess any net dipole moment and is probably located on top of the LE phase in the two-dimensional monolayer. AV is determined by oriented dipoles in the remaining LE-LC phase coexistence region. The small increase in AV is probably attributed to an increasing fraction of LC phase which is more compact than a LE phase. It is shown that nuclei of the three-dimensional phase are present in the film at considerably lower surface pressures than nCr,where the collapse of the monolayer commences. This is in agreement with the findings of Chi et al. who report the presence of small grains of LC phase inside what was believed to be a pure LE phase.3 We propose that the early observation of nuclei of a new phase might be of general importance for phase transformation.
Acknowledgment. We thank the Swedish Natural Research Council for financial support and P e r - h e Ohlsson at the National Defence Research Establishment, Department of NBC Defence in Umel, for invaluable support with the atomic force microscopy. We also want to express our appreciation to Professor Michael Sharp for useful discussions and linguistic revision of the text. LA940282E
