PM FTIRRAS Studies of Potential-Controlled Transformations of a

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PM FTIRRAS Studies of Potential-Controlled Transformations of a Monolayer and a Bilayer of 4-Pentadecylpyridine, a Model Surfactant, Adsorbed on a Au(111) Electrode Surface Vlad Zamlynny, Izabella Zawisza, and Jacek Lipkowski* Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, N1G 2W1 Canada Received August 28, 2002. In Final Form: October 23, 2002 Polarization modulation Fourier transform infrared reflection-absorption spectroscopy (PM FTIRRAS) has been combined with electrochemistry to monitor the potential-induced transformations of a monolayer and a bilayer formed by 4-pentadecylpyridine (C15-4Py), a model amphiphilic compound, at a Au(111) electrode surface. The optical constants for a solution of randomly oriented C15-4Py molecules have been determined and used to calculate the integrated band intensities for a monolayer and bilayer of randomly oriented molecules. The orientation of the hydrophobic and hydrophilic parts of the molecule with respect to the surface normal was determined from the ratio of the intensity of experimental to calculated bands. The CH stretch region of the spectra was used to determine the tilt angle between hydrocarbon chains and the surface normal. The tilt angle varied with potential. The minimum value of the tilt angle was 16° for the monolayer and 27° for the bilayer. In the monolayer, the headgroup of the C15-4Py molecule lies almost flat on the Au(111) surface. At negative potentials, the angle between the plane of the pyridine ring and the surface normal is 68°. As applied potential becomes more positive, the pyridine group stands up gradually and the tilt angle decreases to 63°. In the bilayer, the C2 axis of the pyridine group in the leaflet turned to the electrolyte solution is tilted at ∼70° with respect to the normal and this tilt angle changes little with potential. In the leaflet turned to the electrode, the angle between the C2 axis of the headgroup and the surface normal changes from 64° to 54° by moving from negative to positive potentials. The PM IRRAS data also show that in the bilayer, the plane of the pyridine group rotates with respect to the C2 axis when the electrode potential changes. In the monolayer, the tilt angle of the C2 axis changes but the plane of the pyridine moiety does not rotate with potential.

Introduction Preparation and characterization of well-ordered monolayers and multilayers of organic molecules on solid surfaces is a challenge for the design of biomimetic systems,1,2 sensors and biosensors,1,3 molecular devices such as switches and memories4-6 or transistors.7 Electrochemistry offers a possibility to control the assembly of amphiphilic molecules into organized films by applying a potential and to study potential-driven phase transitions or reorientation of molecules in these films.8-10 Electrochemical methods measure average macroscopic properties and have to be employed jointly with spectroscopic techniques that provide description of the film at the molecular level. Infrared reflection-absorption spectroscopy (IRRAS) is a particularly powerful tool to study orientation of molecules in thin organic films deposited at solid surfaces.11-16 (1) Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990. (2) Xu, J.; Li, H. L. J. Colloid Interface Sci. 1995, 176, 138. (3) Guidelli, R.; Aloisi, G.; Becucci, L.; Dolfi, A.; Moncelli, M. R.; Tadini Bounisegni, F. J. Electroanal. Chem. 2001, 504, 1. (4) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (5) Whitten, D. J.; Chen, L.; Geiger, H. C.; Perlstein, J.; Song, X. J. Phys. Chem. B 1998, 102, 10098. (6) Wong, E. W.; Collier, Ch. P.; Behlorodsky, M.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 5831. (7) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16, 241. (8) Adsorption of Molecules at metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1992. (9) Gierst, L.; Franck, C.; Quarin, G.; Buess-Herman, Cl. J. Electroanal. Chem. 1981, 129, 353. (10) Nelson, A.; Benton, A. J. Electroanal. Chem. 1986, 202, 253. (11) Greenler, R. G. J. Chem. Phys. 1966, 44, 310.

When IRRAS is used to investigate thin films adsorbed at electrode surfaces in situ, one has to employ either a potential or photon polarization modulation technique to subtract the signal due to the solvent and to differentiate between the molecules adsorbed at the electrode surface and those present in the solution.17,18 While the potential modulation technique (known as potential difference or subtractively normalized interfacial Fourier transform infrared sperctroscopy (SNIFTIRS)18) is presently more popular, the photon polarization modulation method is particularly suitable to study insoluble films at electrode surfaces.12,19-24 In polarization modulation Fourier trans(12) Golden, W. G.; Dunn, D. S.; Overend, J. J. Catal. 1981, 71, 395. (13) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (14) Neselli, C.; Rablot, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (15) Faguy, P. W.; Fawcett, W. R. Appl. Spectrosc. 1990, 44, 1309. (16) Chollet, P. A.; Messier, J.; Rosolio, Ch. J. Chem. Phys. 1976, 64, 1042. (17) Pons, S. J. Electroanal. Chem. 1983, 150, 495. (18) (a) Bewick, A.; Pons, S. Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden and Son: New York, 1985; Vol. 12, Chapter 1. (b) Liport, R. J.; Lamp, B. D.; Porter, M. D. Specular Reflection Spectroscopy. In Modern Techniques in Applied Molecular Spectroscopy; Mirabella, F. M., Ed.; Wiley and Sons: New York, 1998; Chapter 3, p 83. (c) Popenoe, D. D.; Stole, S. M.; Porter, M. D. Appl. Spectrosc. 1992, 46, 79. (19) Buffeteau, T.; Desbat, B.; Turlet, J. M. Mikrochim. Acta 1988, 2, 23. (20) (a) Green, M. J.; Barner, B. J.; Corn, R. M. Rev. Sci. Instrum. 1991, 62, 1426. (b) Barner, B. J.; Green, M. J.; Saez, E. I.; Corn, R. M. Anal. Chem. 1991, 63, 55. (21) Blander, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escatre, N.; Pezolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47, 869. (22) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45, 380.

10.1021/la026488u CCC: $25.00 © 2003 American Chemical Society Published on Web 12/10/2002

Potential-Controlled Transformations

Figure 1. Differential capacity of the Au(111) electrode: (dotted-dashed line) the film-free interface; the interface covered with a film of C15-4Py molecules deposited at E ) 0 V using the single horizontal touching (dashed line) and the double horizontal touching (solid line) methods. The electrolyte solution was 0.1 M NaF in water.

form infrared reflection-absorption spectroscopy (PM FTIRRAS), one measures a ratio of the difference between reflectivities for s- and p- polarized light |Rs - Rp| divided by the average reflectivity signal (Rs + Rp)/2.12,19,22 The electric field strength of the s-polarized light is close to zero, while the field of the p-polarized light is enhanced at a metal surface. Consequently, Rs is used as the background to subtract absorbance by the solvent and solution species from Rp that carries the spectrum of molecules adsorbed at the electrode surface. In contrast to SNIFTIRS that measures differential spectra, PM IRRAS measures directly the absorbance by molecules at the surface. PM IRRAS has been successfully employed to measure spectra at the solid-air and solution-air interfaces,19,21-24 while only a few papers describe its application for in situ studies of adsorption at electrode surfaces.25 The purpose of the present work was to apply PM FTIR spectroscopy to study the orientation of a monolayer and a bilayer formed by 4-pentadecylpyridine (C15-4Py), a model amphiphilic compound, at a Au(111) electrode surface. Electrochemical, elastic light scattering, and neutron reflectivity techniques have been used earlier to study spreading of C15-4Py molecules from the gassolution (GS) interface onto the metal-solution (MS) interface of a gold electrode.26-29 The knowledge gained from these studies may be summarized with the help of Figure 1, adopted from our earlier work.29 It shows the potential-driven transformations of the monolayer and bilayer of C15-4Py observed by recording differential capacity curves. The capacity decreases when surfactant molecules spread onto the electrode surface. At potentials higher than -0.2 V (saturated calomel electrode (SCE)), a condensed film is formed and the capacity attains a minimum value of ∼7 µF cm-2 when the electrode is covered by a monolayer and ∼0.8 µF cm-2 when it is covered by a bilayer. Neutron reflectivity experiments have (23) Dicko, A.; Bourgue, H.; Pezolet, M. Chem. Phys. Lipids 1998, 96, 125. (24) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pe´zolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47, 869. (25) (a) Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437. (b) Faguy, P. W.; Richmond, W. N. J. Electroanal. Chem. 1996, 410, 109. (c) Faguy, P. W.; Richmond, W. N.; Jackson, R. S.; Weibel, S. C.; Ball, G.; Payer, J. Appl. Spectrosc. 1998, 52, 557. (26) Bizzotto, D.; McAlees, A.; Lipkowski, J.; McCrindle, R. Langmuir 1995, 11, 3243. (27) Sagara, T.; Zamlynny, V.; Bizzotto, D.; McAlees, A.; McCrindle, R.; Lipkowski, J. Isr. J. Chem. 1997, 37, 197. (28) Bizzotto, D.; Zamlynny, V.; Burgess, I.; Jeffrey, C. A.; Li, H.-Q.; Rubinstein, J.; Galus, Z.; Nelson, A.; Pettinger, B.; Merrill, A. R.; Lipkowski, J. Amphiphilic and ionic surfactants at electrode surfaces. In Interfacial Electrochemistry, theory, experiment and applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (29) Zamlynny, V.; Burges, I.; Szymanski, G.; Lipkowski, J.; Majewski, J.; Smith, G.; Satija, S.; Ivkov, R. Langmuir 2000, 16, 9861.

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shown that the monolayer is 2 nm and the bilayer 3.3 nm thick and that the coverage of the electrode surface by the bilayer is 100% while it is only 80% in the case of the monolayer.29,30 In the potential range between -0.2 and -0.5 V (SCE), the capacity increases to a value of ∼10 µF cm-2 for the monolayer and ∼2 µF cm-2 for the bilayer. The elastic light scattering and fluorescence quenching experiments demonstrated that a hemimicellar film is formed in this region. At E < -0.6 V, the films are desorbed and the capacity for the electrode that was initially covered by the film becomes equal to the capacity of the film-free electrode surface. However, the desorbed surfactant molecules remain close to the electrode surface in the form of aggregates (micelles, flakes, or bilayers) and when direction of the voltage sweep is reversed they spread back onto the metal surface. The hysteresis on the differencial capacity curves shows that kinetics of desorption and respreading of the films are slow and that the corresponding reaction paths involve different rate-determining steps. We will demonstrate here that PM FTIR spectroscopy is an invaluable tool to determine changes in the orientation of the hydrocarbon tail and the polar headgroup that accompany the potential-controlled transformations of the monolayer and the bilayer of this model surfactant at the electrode surface. Experimental Section Solutions and Experimental Procedures. All glass, a syringe type IR cell, similar to that described in ref 18, with a BaF2 prism IR window was used in these studies. The working electrode (WE) was a Au(111) single crystal grown, cut, and polished in our laboratory.31 The counter electrode (CE) was a cylindrical Pt foil. Before each experiment, the Au electrode was cleaned by flame-annealing and quenching with MilliQ water (tandem MilliQ and MilliQ plus UV (18.2 MΩ) systems), as described previously.26 The Pt electrode was sealed into the IR cell and cleaned along with the cell. The reference electrode (RE) used in the infrared cell was a Ag/AgCl electrode in a solution of saturated KCl containing some AgCl (to prolong the life of the electrode) in a KelF compartment, connected to the cell via a frit and a salt bridge. However, the potentials were always converted to the SCE scale, and all potentials in this paper will be quoted with respect to the SCE. Prior to experiments, all glassware was cleaned by immersion in a hot mixture of concentrated nitric and sulfuric acids (ratio ≈ 1:3) for at least 1 h. After cooling, the glassware was rinsed with copious amounts of MilliQ water. The cell was, in addition, soaked in MilliQ water for several hours and rinsed again. The supporting electrolyte for infrared experiments was 0.1 M NaF (Merck, Suprapur) in D2O. Solutions were made using deuterated water (D2O, Cambridge Isotope Laboratories, Cambridge, MA), and all glassware was thoroughly dried before use. Solutions were purged of oxygen by bubbling with argon (BOC Gases) for at least 45 min, and an argon blanket was maintained throughout the experiment. The surfactant, C15-4Py, was dissolved in chloroform and spread at the electrolyte-air interface in a beaker at the equilibrium spreading pressure (ESP) of 33 mN m-1. The electrode was flame-annealed, cooled in air, and assembled into a dry IR cell without the IR window. To transfer the surfactant spread at the GS interface to the MS interface, the dry working electrode was slowly approached to the electrolyte surface and brought in contact with the surfactant film. The surface of the electrode was preadjusted to be parallel to the surface of the electrolyte, and the transfer was performed at an open circuit potential. Next, the electrode was detached from the film-covered electrolyte surface. The cell was sealed with the IR window and filled with the deaerated electrolyte solution to ∼80% of its total (30) Zamlynny, V. Ph.D. Thesis, University of Guelph, Guelph, Ontario, Canada, 2002. (31) Richer, J.; Lipkowski, J. J. Electrochem. Soc. 1986, 133, 121.

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volume. The film-covered surface of the working electrode was kept out of contact with the electrolyte during the filling stage. Finally, the working electrode was brought in contact with the electrolyte solution in the IR cell to form a hanging meniscus configuration, and the electrochemical characterization of the surfactant-covered Au(111) surface was carried out. The deposition procedure was considered as successful if the differential capacity curve recorded for the film-covered electrode agreed with the curve presented in Figure 1. When the quality of the film of C15-4Py was good, the electrode was pressed against the BaF2 window to form a thin layer configuration and the spectra were recorded. We will refer to this procedure as the singletouching procedure. In addition, a bilayer of C15-4Py molecules was deposited at the gold electrode surface using the doubletouching method. In this case, the electrode covered by a monolayer of C15-4Py was detached from the solution surface at the open circuit potential and brought in contact with the film spread at the air-solution interface for the second time. Collection and Processing of Spectra. The Nicolet Nexus 870 spectrometer, equipped with an external optical bench, MCT-A detector (TRS 50 MHz, Nicolet, Madison,WI), photoelastic modulator (PEM) (Hinds Instruments PM-90 with II/ZS50 ZnSe 50 kHz optical head, Hillsboro,OR), and demodulator (GWC Instruments Synchronous Sampling demodulator, Madison, WI), was used to perform PM FTIRRAS experiments. The electrode potential was controlled via the potentiostat (HEKA PG285, Lambrecht/Pfalz, Germany) using in-house software, Omnic macro, and a digital-to-analog converter (Omega, Stamford, CT). In addition, Omnic macro was used to collect and save spectra. The details of the PM FTIR cell and the optical setup were described elsewhere.30,32 The IR window was a 1 in. equilateral prism of BaF2 (Janos Technology, Townshend, VT); prior to experiment, it was washed in water and methanol and then cleaned for 10 min in an ozone chamber (UVO-cleaner, Jelight, Irvine, CA). The WE was set at a starting potential E ) -0.89 V, and spectra were collected at a series of potentials, which were programmed as a cyclic sequence of steps whose amplitude was progressively increasing using a 0.050 or 0.2 V increment. In total, 20 cycles of 400 scans each were performed to give 8000 scans at every applied potential. The instrument resolution was 2 cm-1. At the end of the measurement, the blocks of scans were individually checked using software for anomalies before averaging. Measurements of IR spectra were carried out with the PEM set for half-wave retardation at either 2900 cm-1 for the CH stretching region or at 1600 cm-1 for pyridine ring vibrations and the CH bending region. At the PEM setting of 2900 cm-1, the 2400-3200 cm-1 band-pass filter was employed. In the CH stretch region, the angle of incidence of the infrared beam was set to 50° and the thickness of the electrolyte layer between the electrode and the prism was ca. 2 µm. These parameters guaranteed the maximum mean square electric field strength of p-polarized radiation at the electrode surface. In the pyridine ring vibration region, the angle of incidence was set to 60° and the electrolyte thickness was ∼3.5 µm. The thickness of the thin layer was determined by comparing the experimental reflectivity spectrum of the thin layer cell, attenuated due to the layer of solvent between the electrode and the IR window, to the reflectivity curve calculated from the optical constants of the cell constituents, as described in ref 33. The demodulation technique developed by Corn and coworkers20a was used in this work. After demodulation, two signals were measured: (i) the intensity average (IA(ω)) and (ii) the intensity difference (ID(ω)) described by the following equations:20b

IA(ω) )

(Is(ω) + Ip(ω)) |Is(ω) - Ip(ω)| + J0(φ0) 2 2

(1)

ID(ω) ) |Is(ω) - Ip(ω)|J2(φ0)

(2)

and

where Is and Ip are the intensities of s- and p- polarized light, φ0 is the maximum retardation of the incident radiation imposed by the PEM, and J0(φ0) and J2(φ0) are the PEM response functions.

The maximum retardation was set to π for either 1600 or 2900 cm-1 and deviated from π for other frequencies, introducing a slowly varying background to the measured spectrum. For quantitative analysis, one needs to calculate ∆S(ω) described by

∆S )

2(Is - Ip) ) 2.3A ) 2.3Γ Is + Ip

(3)

where is the decimal molar absorption coefficient and Γ is the surface concentration of the adsorbed species. ∆S(ω) measures the absorbance of the film of adsorbed molecules. The following procedure, that is, a modified version of the method applied earlier by Buffeteau et al.,34 has been used to calculate ∆S(ω) from the measured signals IA(ω) and ID(ω). The experimental setup was assembled with a dielectric mirror (CaF2 prism) in place of the spectroelectrochemical cell and a second static polarizer inserted in the path of the beam just after the PEM. The two static polarizers were set to transmit only the p-polarized light. The PEM was turned off and the first reference spectrum was recorded. This spectrum represents the intensity of the p-polarized light that traverses the whole optical bench Ip(ω)cal. Then, the PEM was turned on and after demodulation two calibration signals IA(ω)cal and ID(ω)cal were recorded. Since only p-polarized light was passed through the second static polarizer, Is(ω)cal ) 0 for the calibration signal, and according to eqs 1 and 2, one can write

IA(ω)cal )

Ip(ω)cal Ip(ω)cal + J0(φ0) 2 2

(4)

and

ID(ω)cal ) Ip(ω)calJ2(φ0)

(5)

These equations show that the response of PEM, represented by J2(φ0)and J0(φ0), can be determined experimentally from the measured calibration signals and the following formulas:

J2(φ0) )

ID(ω)cal

IA(ω)cal

J0(φ0) ) 2

(6)

Ip(ω)cal

Ip(ω)cal

-1

(7)

The experimental values of functions J0(φ0) and J2(φ0) are plotted in Figure 2. Knowing the PEM response functions, the measured signals IA(ω) and ID(ω) can be corrected and the corrected average 〈I(ω)〉 and difference ∆I(ω) signals may be expressed as equal to

〈I(ω)〉 )

(

)

Is(ω) + Ip(ω) ID(ω) Ip(ω)cal ) IA(ω) 22 2 IA(ω)cal

(8)

and

Ip(ω)cal ∆I(ω) ) Is(ω) - Ip(ω) ) ID(ω) ID(ω)cal

(9)

Buffeteau and co-workers22 demonstrated that signals 〈I(ω)〉 and ∆I(ω) have to be corrected further to take into account that the ratio of the optical throughputs of the experimental setup for p- and s-polarized light γ is not equal to unity. When γ * 1, the corrected PM FTIR spectrum can be calculated with the help of the formula30

S(ω) )

|Is - Ip| (Is + Ip)/2

(γ + 1)∆I(ω) + 2(γ - 1)〈I(ω)〉 )2 (γ - 1)∆I(ω) + 2(γ + 1)〈I(ω)〉

For the experimental setup used in this work, γ ) 1.06.

(10)


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Figure 3. The PM FTIRRA spectrum (thick line) in the CH stretching region of the 4-pentadecylpyridine bilayer on a Au(111) surface in 0.1 M NaF in D2O, E ) -0.89 V, determined with the help of eq 10. The thin line is the spline. Circles mark points used to build the spline. The inset shows the IR spectrum of the C-H stretch region after background subtraction.

Figure 2. The experimental PEM response functions: (a) J0(φ0) and (b) J2(φ0) determined for the PEM set for half-wave retardation at (solid line) 2900 cm-1 and (dashed line) 1600 cm-1. Finally, when in situ experiments are performed in a thin layer cell that contains electrolyte the measured spectrum has a background due to absorption of the IR beam by the aqueous solution in the thin layer. To correct the spectra for the background, a baseline was built for each spectrum using the spline interpolation technique.20 The same data points were used to build the spline for all spectra. This procedure is illustrated in Figure 3, which shows the spectrum with the background. The circles show points used to define the spline, and the inset shows the spectrum determined after background subtraction. The background-corrected spectrum plots ∆S, which is proportional to the absorbance A of adsorbed molecules (see eq 3).

Results and Discussion Optical Constants of 4-Pentadecylpyridine and Band Assignment. For linearly polarized light and a molecule adsorbed at or in front of a reflecting metal surface, the integrated intensity of the absorption band is proportional to the square of the dot product of the transition dipole µ and the electric field of the photon E:18b,35

∫A dν ∝ |µ‚E|2 ∝ cos2 θ|µ|2〈E2〉

(11)

where θ is the angle between directions of the electric (32) Horswell, S. L.; Zamlynny, V.; Li, H.-Q.; Merrill, A. R.; Lipkowski, J. Faraday Discuss. 2002, 121, 405. (33) Li, N.; Zamlynny, V.; Lipkowski, J.; Henglein, F.; Pettinger, B. J. Electroanal. Chem. 2002, 524-525, 43.

field of the photon and the transition dipole in the molecule, |µ| is the absolute value of the transition dipole moment, and 〈E2〉 is the mean square electric field strength of the photon. At the gold electrode, the electric field of ppolarized IR radiation is normal to the surface. Hence, the orientation of molecules at the metal surface can be found if the direction of the transition dipole can be related to the molecular geometry. For randomly oriented molecules, cos2 θ ) 1/3. The orientation of molecules adsorbed at the interface can therefore be determined by calculating the spectrum for randomly oriented molecules from isotropic optical constants, with the help of the formula18b,36,37

cos2 θ )

∫Aexp dν 3∫Acal dν

(12)

where Acal has to be calculated using the matrix method for reflection from an interface consisting of four phases: Au/C15-4Py/D2O/BaF2.33,38,39 This method was employed in the present work. To calculate Acal, the C15-4Py monolayer was represented as being 2.0 nm and the bilayer as a 3.3 nm thick film of randomly oriented C15-4Py molecules on the metal surface. The thickness of these films was taken from earlier neutron reflectivity experiments.29 The optical constants for Au and BaF2 were taken from ref 40a, while optical constants for D2O were taken from ref 40b. Similar calculations of the absorption spectra for a monolayer of long-chain alkenethiolates at a gold electrode surface in a thin layer spectroelectrochemical cell were performed previously by Popenoe et al.18c At room temperature, C15-4Py is a liquid. The isotropic optical constants for this compound were obtained using (34) Buffeteau, T.; Desbat, B.; Blaudez D.; Turlet, J. M. Appl. Spectrosc. 2000, 54, 1646. (35) Moskovits, M. J. Chem. Phys. 1982, 77, 4408. (36) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (37) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 11, 1215. (38) Hoon-Khosha, M.; Fawcett, W. R.; Chen, A.; Lipkowki, J.; Pettinger, B. Electrochim. Acta 1999, 45, 611. (39) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (40) (a)Handbook of Optical Constants of Solids II; Palik, E., Ed.; Academic Press: San Diego, 1998. (b) Bertie, J. E.; Ahmed, M. K.; Eysel, H. H. J. Phys. Chem. 1989, 93, 2210.

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Figure 4. Isotropic optical constants of 4-pentadecylpyridine, calculated from the transmittance of its 0.5485% (w/w) solution in a mixture of 12.98% (w/w) of (0.1 M NaF/D2O) in CD3OD: (a) CH stretch region and (b) HCH bending and pyridine ring vibration region of the spectrum; n is the refractive index (solid line), and k is the attenuation coefficient (dashed line).

the procedure recently developed to determine optical constants for pyridine.33 The IR bands of the pyridine moiety in the film of C15-4Py at a gold electrode surface bear a closer resemblance to the bands of hydrated pyridine than to the bands in the spectrum of a neat pyridine. Hence, instead of using optical constants for neat C154Py, the optical constants were determined for C15-4Py in a binary solvent consisting of perdeuterated methanol and deuterium oxide. Two BaF2 disks were used as windows of the thin layer cell, and 0.1 M NaF was added to D2O in order to suppress the solubility of the window material. The C15-4Py is insoluble in D2O but is soluble in CD3OD. It was experimentally found that addition of approximately 10% (w/w) of D2O to CD3OD was sufficient to ensure the hydration of the pyridine moiety in a 0.5485% (w/w) solution of this surfactant. Figure 4 plots the isotropic optical constants of C154Py determined using this procedure. Panel a shows the region of the spectrum where stretching vibrations of the hydrocarbon chain are observed, while panel b shows the region where deformation bands of the pyridine moiety and bending vibrations of the aliphatic chain are observed. The attenuation coefficient k was determined from the transmission spectra, and the refractive index n was then calculated from k using the Kramers-Kroenig transformation as described in refs 18c, 33, and 36. For the C-H stretch region, the value of the refractive index at infinite

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frequency n∞ was taken from ref 41 as equal to 1.41. For the pyridine moiety vibrations, we took n∞ ) 1.49 determined for neat pyridine in ref 33. The iterative procedure described in refs 33 and 36 has been employed in these calculations. The plot of k may be used for band assignment. In panel a of Figure 4, the four bands observed at 2858, 2874, 2933, and 2962 cm-1 can be assigned to symmetric νs(CH2), symmetric νs(CH3), asymmetric νas(CH2), and asymmetric νas(CH3) stretching vibrations of alkyl chains, respectively.42-50 The spectrum of k in panel b of Figure 3 displays four bands at 1610 (a1), 1559 (a1), 1499 (b1), and 1414 cm-1 (b1) that can be assigned to the deformational vibrations of the pyridine ring of C15-4Py having either a1 or b1 symmetry.33,51-54 In addition, two bands at 1467 and 1458 cm-1 can be assigned to δ(CH2) bending (scissoring) and asymmetric δas(CH3) bending vibrations, respectively.42-50 Figure 5 shows a space filling model of the C15-4Py molecule and directions of transition dipoles corresponding to major bands in the IR spectrum of this molecule. The transition dipoles of methylene stretching and bending bands are located in the plane defined by the positions of C and the two H atoms, and for a fully stretched, all-trans hydrocarbon chain they are perpendicular to the chain axis. The transition dipole of the symmetric CH3 stretch is directed along the terminal C-C bond of the hydrocarbon skeleton, and that of the asymmetric CH3 stretch is located in the plane defined by the position of the three H atoms of the methyl group. Clearly, the intensity of these bands can provide useful information concerning changes of the chain tilt angle. The direction of the transition dipole of a1 bands of the pyridine moiety is parallel to the C2 axis of this group.33 Transition dipoles of bands with b1 symmetry are located in the plane of the moiety. Their direction is perpendicular to the C2 axis.33 Intensities of these bands can be used to determine the rotation of the pyridine moiety of the C154Py molecule around the C2 axis. Below, we will discuss IR bands corresponding to the tail and the head regions of the C15-4Py molecule in separate sections. Tail Region. Figure 6a,b shows the absorption spectra recorded in the CH stretch region for the mono- and bilayer of C15-4Py deposited onto the Au(111) surface, for selected electrode potentials. In each panel, the top curve, drawn with a thick line, is the spectrum calculated either for a 2.0 nm thick (monolayer) or 3.3 nm (bilayer) film of (41) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. 1997, 101, 58. (42) Casal, H. L.; Mantsch, H. H. Biochim. Biophys. Acta 1984, 779, 381. (43) Snyder, R. G.; Liang, G. L.; Strauss, H. L.; Mendelsohn, R. Biophys. J. 1996, 71, 3186. (44) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (45) Dicko, A.; Bourque, H.; Pe´zolet, M. Chem. Phys. Lipids 1998, 96, 125. (46) Le Bihan, T.; Pe´zolet, M. Chem. Phys. Lipids 1998, 94, 13. (47) Cameron, D. G.; Casal, H. L.; Mantsch, H. H. Biochemistry 1980, 19, 3665. (48) Casal, H. L.; Mantsch, H. H. Biochim. Biophys. Acta 1983, 735, 387. (49) Okamura, E.; Umemura, J.; Takenaka, T. Biochim. Biophys. Acta 1985, 812, 139. (50) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochem. Acta 1978, 34A, 395. (51) Corrsin, L.; Faxand, B. J.; Lord, R. C. J. Chem. Phys. 1953, 21, 1170. (52) Long, D. A.; Thomas, E. L. Trans. Faraday Soc. 1963, 59, 783. (53) Hebert, P.; La Rille, A.; Zheng, W. Q.; Tadjeddine, A. J. Electroanal. Chem. 1998, 447, 5. (54) Pettinger, B.; Lipkowski, J.; Hoon-Koshla, M. J. Electroanal. Chem. 2001, 500, 471.


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Figure 5. Directions of the transition dipoles of major IR bands in 4-pentadecylpyridine.

randomly oriented C15-4Py molecules, using the optical constants presented in Figure 4. At the most negative potential, the shape of the spectra determined for the two films resembles the shape of the spectra calculated from the optical constants, indicating that no major errors were made in the IR signal processing and background correction. The νas(CH3), νas(CH2), and νs(CH2) peaks at 2960, 2926, and 2853 cm-1, respectively, are well resolved. The νs(CH3) peak at wavenumber 2872 cm-1 has low intensity. It is better resolved in the monolayer than in the bilayer film. The frequencies of these peaks are somewhat shifted with respect to the peak positions in the spectra calculated from the optical constants. This can be explained as the effect of the environment. The optical constants were determined for a homogeneous solution of C15-4Py in CD3OD/D2O solvent where the surfactant molecules are surrounded by the solvent. In contrast, experimental spectra correspond to C15-4Py molecules assembled into a film.

CH2 Stretching Modes. A brief inspection of the two panels of Figure 6 reveals that while for the bilayer the shape of spectra changes little with potential, for the monolayer the changes are quite pronounced. One notes that at positive potentials the CH2 stretch bands split and shift toward lower frequencies. The band at 2926 cm-1 has a complex structure because it contains a Fermi resonance band.50 Therefore, we will use the symmetric CH2 band at ∼2850 cm-1 for quantitative data analysis. The frequencies of νs(CH2) stretch in the spectra for the monolayer and bilayer films are plotted as a function of the electrode potential in Figure 7. In the monolayer, the symmetric CH2 stretch frequency moves from 2853 cm-1 at -0.89 V to 2850 cm-1 at 0.26 V. For the symmetric CH2 stretch, frequency values below 2850 cm-1 are characteristic of conformationally ordered polymethylene chains in the solid state of hydrocarbons55 or in the gel phase of (55) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.

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Figure 6. The PM IRRA spectra in the C-H stretch region: (a) monolayer and (b) bilayer of C15-4Py at the Au(111) electrode surface; E versus SCE is marked for each spectrum in the figure. In each panel, the top spectrum (thick line) is the spectrum calculated from the isotropic optical constants for (a) 2.0 nm thick and (b) 3.3 nm thick films of C15-4Py in the Au//D2O/BaF2 thin layer cell.

Figure 7. The symmetric methylene stretch νs(CH2) peak position vs the electrode potential plots for (O,b) the C15-4Py bilayer and (0,9) monolayer; the potential changed in the (b,9) positive and (O,0) negative directions.

lipids.42 Therefore, this shift may be interpreted as a liquid-solid or liquid crystal-gel phase transition. At E < -0.2 V, where the C15-4Py film has either hemimicellar structure or the molecules are desorbed and form ag-

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gregates in the vicinity of the electrode surface, the band frequency is ∼2853 cm-1, characteristic for a liquid (liquid crystalline) state. The monolayer becomes solidified (transformed to the gel state) at E > -0.2 V, where it forms a condensed film. The splitting of this band indicates that initially the liquid and solidified domains coexist and that the transformation progresses gradually with potential. However, even at the most positive potentials, the 2850 cm-1 peak has a shoulder at 2853 cm-1, indicating that the solidification has not been fully completed. In addition to the change of frequency, the band becomes narrower. The full width at half-maximum (fwhm) of the band changes from 12 cm-1 at -0.89 V to 7 cm-1 at 0.26 V (SCE). Similar changes have been observed for the liquid-solid phase transition in long-chain hydrocarbons.55 While the frequency of the C-H stretching bands depends on conformational changes, that is, changes in the gauche/trans ratio of the alkyl chain, the bandwidth depends on the mobility of the film.56 Therefore, the narrowing of the symmetric C-H stretch band suggests that a decrease of the film mobility accompanies the potential-controlled solidification of the monolayer. Clearly, the spectroscopic data are in accord with the differential capacity curve shown earlier in Figure 1. They demonstrate that the capacity peak seen at ca. -0.2 V (SCE) corresponds to a potential-controlled two-dimensional liquid to solid phase transition. In addition, the hysteresis seen on the band frequency versus potential plots resembles the hysteresis on the differential capacity curves. In contrast to the properties of the monolayer, the frequency of the νs(CH2) stretch in the bilayer is ∼2853 cm-1 and is independent of the applied potential. Apparently, this film is liquid crystalline even in the condensed state. This behavior shows that packing of C15-4Py molecules in the film is influenced by the solvent (water). Pyridine has a strong affinity to gold.38 Therefore, in the monolayer, pyridine moieties are turned to the metal and hydrocarbon chains are turned to the solution. The hydrophobic interaction between the tails and the solvent apparently assists ordering of chains into the more organized gel state. In the bilayer, the pyridine moieties in the first leaflet are oriented toward the metal and in the second leaflet toward the solution. Consequently, the tails of the first leaflet face tails of the second leaflet. In this environment, the less ordered liquid crystalline state is formed. The dependence of the integrated νs(CH2) stretch intensity on the electrode potential is shown in Figure 8a for the monolayer and in Figure 9a for the bilayer. In each film, the band intensity decreases by moving from the desorbed to the adsorbed state. Qualitatively, these changes correlate quite well with the change of the differential capacity of the interface (Figure 1). Moving in the positive direction, the band intensity drops down when C15-4Py spreads onto the electrode surface. It attains a plateau between -0.5 and -0.2 V (SCE) where the hemimicellar film is formed and decreases further at E > -0.2 V in the region of the condensed film. Apparently, orientation of alkyl chains changes during the potentialdriven transformations of these films. These changes may be quantified by calculating the angle between the direction of the transition dipole of the νs(CH2) stretch and the surface normal, using eq 12. The angles are plotted against potential in Figures 8b and 9b for the monolayer and the bilayer, respectively. It is useful to recall that the direction of this transition dipole is (56) Mantsch, H. H.; Madec, C.; Ruthven, R.; Lewis, N. A. H.; McElhaney, R. N. Biochemistry 1985, 24, 2440.


Figure 8. For the C15-Py monolayer on the Au(111) electrode surface: (a) integrated intensity and (b) angle Θ between the direction of the transition dipole of the νs(CH2) band and the surface normal vs potential plots; (9) the potential changed in the (9) positive and (0) negative directions.

perpendicular to the hydrocarbon chain. In the gel state, the chains are almost all-trans fully extended.36 Hence, in the monolayer at E > -0.2 V (SCE), where the transition dipole is oriented at 74° with respect to the surface normal, the hydrocarbon chains are tilted at the angle of 16° with respect to the normal. In the liquid crystalline state, the chains are not fully extended due to the presence of gauche conformations.42 Here, the chain direction is poorly defined. Nevertheless, the angle between the transition dipole of the νs(CH2) stretch may be taken as an approximate measure of the tilt angle of the chain and the data in Figures 8b and 9b show clearly that the tilt angle of the chain progressively decreases when the potential moves in the positive direction. In addition, the data show that the chains are more tilted in the bilayer (larger angle with respect to the normal) than in the monolayer. CH3 Stretching Modes. The asymmetric νas(CH3) and the asymmetric νas(CH2) stretch bands overlap. Likewise, the symmetric νs(CH3) and νs(CH2) bands overlap as well. However, Figure 10a shows that these bands can be deconvoluted by fitting Gaussian functions. The deconvoluted bands may then be integrated, and the angle between the direction of the transition dipole and the surface normal can be determined from the integrated band intensities using eq 12. For the monolayer, the angle between the directions of transition dipoles of the symmetric and the asymmetric ν(CH3) stretches is plotted against potential in Figure 10b. The direction of the transition dipole for the symmetric νs(CH3) stretch is aligned with the direction of the terminal C-C bond in

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the hydrocarbon tail of the C15-4Py molecule (see Figure 5). This bond forms an angle of 35° with the fully extended all-trans hydrocarbon chain. When the potential changes in the negative direction, the angle increases. Such a change indicates that the angle between the direction of the chain and the surface normal increases. This result is consistent with the analysis of the symmetric νs(CH2) band, presented earlier. At positive potentials, where the monolayer is in the gel state, the analysis of the νs(CH2) band (see Figure 8b) suggested that the chains are tilted at 16° with respect to the normal. Consequently, the angle between the transition dipole of the νs(CH3) stretch and the surface normal should be 51°. Open points in Figure 10b plot tilt angles for the transition dipole of the νs(CH3) stretch. At positive E, the angle is equal to 55°, in reasonable agreement with the expected value. Black points in Figure 10b show changes of the tilt angle for the asymmetric νas(CH3) stretch plotted against E. The transition dipole of this vibration forms an angle of 90° with respect to the direction of the transition dipole of the νs(CH3) stretch (see Figure 5). Consistently, with this geometrical relation, the tilt angle of the νas(CH3) stretch changes in the opposite direction to the change of the tilt angle of the νs(CH3) stretch. In addition, when the two angles are added the result is 90 ( 10° as predicted by simple geometric considerations. Looking back to Figure 5, one may recall that the transition dipole of the νas(CH3) stretch forms a 35° angle with the transition dipole of the νs(CH2) stretch. Indeed, tilt angles of these two vibrations, plotted in Figures 8b and 10b, display quite similar dependence on potential. At positive E, the tilt angle for the νas(CH3) stretch attains a maximum value of 35° (see Figure 10b). The maximum value of the tilt angle for the νs(CH2) stretch equals 74°. The difference between the maximum tilt angles of the νs(CH2) and the νas(CH3) stretches is 39° to be compared to 35° expected from geometrical considerations. The experimental and the expected values are in accord within the limits of the experimental error. One should note that methyl stretch bands are much smaller than methylene stretch bands and hence the tilt angles calculated for CH3 vibrations are affected by a larger error. Figure 10c shows that in the bilayer, the tilt angles of the two CH3 stretches are almost identical and approximately equal to ∼60°. They change little with potential. This number is close to the average angle of randomly oriented dipoles equal to 55°. We can therefore conclude that in the bilayer the methyl groups have no preferential orientation. CH2 Scissoring Mode. The C-H scissoring vibrations are located in the frequency range of 1480-1430 cm-1 in the spectra for the headgroup region shown in Figure 11a,b. The broad band seen in this region is a combination band that consists of the δ(CH2) scissoring band, located at 1468 cm-1, and the δas(CH3) asymmetric bend at 1456 cm-1.13,36 In addition, another mode, particularly strong in the monolayer film, is observed around 1445 cm-1. This band has been described in the literature as the feature characteristic to the liquid crystalline-gel transition.47 For the monolayer film, due to low peak intensities, overlapping of a few bands, and poorer signal-to-noise ratio, the quantitative analysis of the band intensity was not performed in this spectral region. However, even a qualitative analysis provides useful information concerning the structure of the film, because the number and the frequency of the CH scissoring bands depend on the alkyl chain packing in the film. Clearly, the shape of the CH scissoring band is distinctly different in the monolayer than in the bilayer film. While

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Figure 9. For the C15-4Py bilayer on the Au(111) electrode surface: (a) integrated intensity of the νs(CH2) band, (b) angle (Θ) between the direction of the transition dipole of the νs(CH2) band and the surface normal, (c) integrated intensity of the δ(CH2) scissoring band, and (d) angle (Θ) between the direction of the transition dipole of the δ(CH2) bend and the surface normal, plotted vs potential; the potential changed in the (b) positive and (O) negative directions.

in the bilayer the CH scissoring bands resemble the band for the dissolved C15-4Py, in the monolayer, the bands are much broader, the amplitude of the 1468 cm-1 band is much smaller, and hence the 1445 cm-1 band appears to be stronger. This behavior resembles the properties of alkyl chains in the so-called pretransition region where liquid expanded and liquid condensed phases coexist.47 The presence of the band at 1468 cm-1 is characteristic of a hexagonal packing of chains.57,58 Therefore, the spectra in Figure 11a suggest that alkyl chains are hexagonally packed in the monolayer in the gel state. Due to the liquid crystalline state of the bilayer, in this film, the packing of alkyl chains is not well-defined. The shape of the band changes little with potential in the bilayer film. In contrast, in the monolayer film the band becomes narrower when potential moves in the positive direction. This behavior suggests that the packing of chains improves with potential. The intensity of the scissoring δ(CH2) vibration changes with the electrode potential. This trend is seen well for the bilayer film. In the desorbed state of the bilayer, the δ(CH2) peak is strong and its amplitude is comparable with the amplitude of the same peak in the calculated spectrum. The peak intensity decreases when potential (57) Snyder, R. G. J. Mol. Spectrosc. 1963, 10, 5145. (58) Mendelsohn, R.; Mantsch, H. H. In Progress in Protein-Lipid Interactions; Watt, A., De Pont, J. J., Eds.; Elsevier: New York, 1986; p 103.

becomes more positive and the film spreads onto the Au(111) surface. For the bilayer film, the peaks present in this spectral region may be deconvoluted. This deconvolution will be described later in the section on the pyridine head region. The integrated intensities of the deconvoluted δ(CH2) peak were determined and used to calculate the angle between the transition dipole and the surface normal. Since the change of the δ(CH2) scissoring peak intensity and the tilt angle is expected to be similar to the change seen for the νs(CH2) stretch, for the benefit of comparison, these quantities are plotted in Figure 9c,d. The data for δ(CH2) display a similar dependence on potential to the data determined earlier for the νs(CH2) stretch. Clearly, consistent information concerning the conformational changes of the alkyl chains can be extracted from different regions of the IR spectra. IR bands corresponding to vibrations of the alkyl chain of the C15-4Py molecule provided rich information concerning the chain conformation and chain packing. The results show that the monolayer film is more ordered than the bilayer. Both the hemimicellar and condensed states of the bilayer have a liquid crystalline structure. In the monolayer, the hemimicellar state is liquid crystalline; however the condensed film is solid (gel state). When the potential is moved in the positive direction, a potentialdriven liquid crystal-gel phase transition takes place. In the gel state, the mobility of the molecules is restricted and the alkyl chains are hexagonally packed assuming a


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Figure 11. PM IRRA spectra of the pyridine ring vibrations and the CH bending region: (a) the monolayer and (b) the bilayer film of C15-4Py at the Au(111) electrode. The potential E vs SCE is marked for each spectrum in the figure. In each panel, the top spectrum (thick line) is the spectrum calculated from the isotropic optical constants for (a) 2.0 nm thick and (b) 3.3 nm thick films of C15-4Py in the Au/D2O/BaF2 thin layer cell.

Figure 10. (a) Deconvolution of the asymmetric stretch region of the PM IRRA spectrum for a bilayer of C15-4Py on the Au(111) electrode surface. Solid line, experimental spectrum; dotted line, the methyl and methylene group vibrations after deconvolution. (b) For the monolayer film, the angle (Θ) between the transition dipole of (0) νs(CH3) and (9) νas(CH3) bands and the surface normal plotted vs the electrode potential. (c) For the bilayer film, the angle (Θ) between the transition dipole of (O) νs(CH3) and (b) νas(CH3) bands and the surface normal plotted vs the electrode potential. The potential changed in the positive direction.

small angle (16°) with the surface normal. The potential also affects the structure of the bilayer. While the film remains in the liquid crystalline state, when E increases, the alignment of chains with the surface normal progres-

sively improves. However, due to the presence of gauche deformations the chains are never fully extended. At the most negative potentials, in the desorbed state, the chains in the bilayer are disordered. In contrast, a residual order is seen at the negative limit of potentials for the monolayer film. Pyridine Moiety Vibrations. The spectra in Figure 11a,b show four bands due to pyridine moiety vibrations. They correspond to the in-plane stretches of the pyridine ring with a1 symmetry at 1610 and 1560 cm-1 and with b1 symmetry at 1421 and 1501 cm-1. The a1 mode at 1610 cm-1 and the b1 mode at 1421 cm-1 are the strongest bands, and they will be used in further discussion of the headgroup region behavior. First, we will turn our attention to the a1 band. Apparently, this is a combination band that can be deconvoluted into two bands at 1603 and 1610 cm-1. For the monolayer, the band at 1610 cm-1 dominates and the band at 1603 cm-1 appears as a shoulder at E < -0.6 V (SCE). In the bilayer, the band at 1603 cm-1 dominates at negative potentials while the band at 1610 cm-1 becomes stronger when potential moves in the positive direction. Figure 12a plots the frequency of the combination band as a function of the electrode potential. For the monolayer film, the band maximum is potential independent. In contrast, for the bilayer, the band frequency is equal to

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Figure 13. For the monolayer of C15-4Py at the Au(111) electrode surface, the angle (Θ) between the direction of the transition dipole of the a1 band at ∼1610 cm-1 and the vector of the electric field of the photon plotted vs the electrode potential; E changed in the (9) positive and (0) negative directions.

Figure 12. Position of (a) the a1 band at ∼1600 cm-1 and (b) the b1 band at ∼1421 cm-1 of the pyridine moiety of the C154Py molecule plotted vs the electrode potential: (0,9) monolayer and (O,b) bilayer; the potential changed in the (9,b) positive and (0,O) negative directions.

1610 cm-1 when the film is spread at the electrode surface and to 1603 cm-1 when it is desorbed. The hysteresis seen on the frequency plots recorded when the potential is moved in either positive or negative directions resembles the hysteresis observed earlier on the capacity curves in Figure 1. We will now discuss behavior of the b1 band. In the monolayer, the band is so small that it is essentially lost in the background. In the bilayer, it is also a combination band that consists of two overlapping peaks at 1414 and 1421 cm-1. The peak at 1414 cm-1 dominates at negative potentials, and the peak at 1421 cm-1 at positive potentials. Panel b in Figure 12 plots the frequency of the global maximum of the b1 band as a function of the electrode potential. Clearly, these changes follow the trends observed in Figure 12a for the a1 band. It is important to note that both the a1 and b1 bands of the pyridine moiety undergo splitting and the new splitoff bands shift toward higher wavenumbers when the film spreads onto the electrode surface. The split-off bands are shifted by the same amount of ∼7 cm-1 for both a1 and b1 modes. This behavior suggests that the splitting and the shift are due to the environmental effects. Our previous studies of pyridine adsorption at a Au(111) electrode surface demonstrated that pyridine ring deformations are sensitive to the change of the environment. The frequency and the shape of these bands are somewhat different for a neat pyridine, pyridine dissolved in an aqueous solution, and pyridine co-coordinated to a gold electrode surface.54

Since the 1610 cm-1 band predominates in the monolayer, we assign the a1 peak at 1610 cm-1 and the b1 peak at 1421 cm-1 to C15-4Py molecules co-coordinated to the Au(111) electrode surface through the pyridine moiety. The a1 peak at 1603 cm-1 and the b1 peak at 1414 cm-1 are assigned to C15-4Py molecules either in the desorbed state or residing in the second leaflet of the bilayer, where the pyridine moiety is turned toward the solution. At this point, we can perform quantitative analysis of the band intensities in order to determine the orientation of the pyridine moiety. We discuss the orientation of the polar head in the monolayer first. The bands at 1610 cm-1 have been integrated. The angles between the directions of the transition dipole of the a1 mode and the normal to the surface were then determined, from the ratio of the experimental to the calculated integrated band intensities, using eq 12. The direction of the transition dipole of the a1 mode is parallel to the C2 axis of the pyridine moiety; therefore that angle shows how the C2 axis is tilted with respect to the surface normal. Figure 13 plots the tilt angle as a function of the electrode potential. The angle is equal to ∼68° at E < -0.2 V where the film is in the liquid crystalline state and decreases to a value of ∼63° at E > -0.2 V where monolayer is in the gel state. At the most positive potential, the tilt angle for the hydrocarbon chain of the molecule is equal to 16°. The values of the tilt angle of the chain and the tilt of the C2 axis of the pyridine moiety can be used to calculate the angle between the chain and the pyridine group. The experimental value is ∼133°. The geometry of the molecule indicates that this angle should be equal to 144°. Taking into account that even at the most positive potential the transformation into the gel state is not fully completed and hence that certain chains are not fully trans stretched, the agreement between the experimentally measured and expected values is quite good. Figure 11a shows that in the monolayer film, the b1 band is very weak and almost lost in the background. It essentially disappears from the spectrum recorded at E ) 0.26 V (SCE). The transition dipole of this band is located in the plane of the pyridine moiety, and its direction is normal to the C2 axis (see Figure 5). This behavior suggests that the angle between the transition dipole and the surface normal is either close or equal to 90° and is almost parallel or parallel to the electrode surface. Apparently,


Figure 14. For the bilayer of C15-4Py at the Au(111) electrode surface: (a) deconvolution of an experimental spectrum (solid line) into two components with maxima at 1610 and 1603 cm-1 (dotted line); (b) influence of the electrode potential on the angle between the direction of the transition dipole of a1 bands and the surface normal, determined from the integrated intensity of (b) the total a1 band and deconvoluted bands at (4) ∼1610 cm-1 and (2) ∼1603 cm-1.

during the potential-driven transition from the liquid crystalline state at E < -0.2 V to the gel state at E > -0.2 V, the tilt of the C2 axis changes without any significant rotation of the plane of the pyridine moiety. We will now discuss the orientation of the pyridine moiety in the bilayer. For that purpose, the composite bands were deconvoluted by fitting a Gaussian and the intensities of the peaks assigned to the first (turned to the metal) and the second (turned to the solution) leaflets of the bilayer were analyzed independently. Figure 14a shows the deconvolution of the a1 band. The intensities of the a1 band and the intensities of its two constituent peaks were integrated. We assumed that an equal number of C15-4Py molecules are present in each leaflet of the bilayer and hence half of the intensity of the spectrum calculated from the optical constants was used to determine the angle between the C2 axis of the pyridine moiety and the surface normal. The tilt angles are plotted in Figure 14b. The full circles plot a change of the average tilt angle calculated by integrating the whole a1 band with its two peaks. Open triangles mark the angle between the C2 axis of the pyridine moiety and the surface normal calculated from the integrated intensity of the 1610 cm-1 peak. It shows the orientation of the pyridine group in the first leaflet of the bilayer. Full triangles mark tilt angles of the C2 axis of the pyridine moiety in the second leaflet

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Figure 15. For the bilayer of C15-4Py at the Au(111) electrode surface: (a) deconvolution of an experimental spectrum in the 1400-1500 cm-1 range (solid line) into b1 modes at ∼1414 and 1421 cm-1 and overlapping bending CH modes (dotted lines); (b) influence of the electrode potential on the angle between the direction of the transition dipole of b1 bands and the surface normal, determined from the integrated intensity of (b) the total b1 band and deconvoluted bands at (4) ∼1421 cm-1 and (2) ∼1414 cm-1.

that is turned toward the solution. We note that the pyridine group forms a smaller angle with the surface normal in the first than in the second leaflet. This behavior illustrates the influence of the electric field. The bilayercovered Au(111) electrode surface becomes positively charged when E > -0.2 V (SCE). The pyridine moiety has a permanent dipole moment of ∼2 D whose negative pole is on the nitrogen atom and direction is parallel to the C2 axis. Hence, the dipole-field interaction assists in turning the pyridine moiety toward the surface normal at positive potentials. The deconvolution of the b1 band is more complex. The 1400-1480 cm-1 region is rich in overlapping bands, and as Figure 15a shows, one has to deconvolute all bands in this region to determine intensities of the 1414 and 1421 cm-1 peaks. Figure 15b plots angles between the transition dipole of the b1 band and the surface normal determined from integrated band intensities. Full circles plot the average angle determined from the total intensity of the b1 band (sum of the intensities of the two peaks). Open triangles mark the angle for the pyridine moiety in the first leaflet and full triangles denote the angle for the moiety in the second leaflet of the bilayer calculated from intensities of individual deconvoluted peaks.

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We recall that the transition dipole of the b1 mode is located in the plane of the pyridine moiety and forms a 90° angle with the C2 axis. Therefore, this angle shows rotation of the moiety plane around the C2 axis. Clearly, the rotation is different in the two leaflets. In the second leaflet, the plane of the moiety forms a large angle with the surface normal and is almost parallel to the electrode surface at all potentials. In the first leaflet, the plane of the pyridine moiety forms a smaller angle with the surface normal and this angle changes with potential. The results show that there are significant differences between the orientations of the pyridine moiety in the monolayer and the bilayer and in the bilayer between the first and the second leaflet. In the monolayer film, the moiety is tilted with respect to the surface normal with the tilt angle changing with potential from ∼68° at negative potentials (hemimicellar film) to ∼63° at positive potentials (condensed film). In the first leaflet of the bilayer (facing the metal), the tilt angle changes from 58° to 54° across the phase transition. The tilt angle is larger by about 10° than in the monolayer film. In addition, the plane of the moiety is rotated and the rotation angle measured with respect to the plane of incidence changes from ∼68° at negative potentials (hemimicellar state) to ∼65° at positive E (condensed state). In the second leaflet, the tilt and rotation angles are larger, ∼70° and ∼80°, respectively, and are essentially independent of the electrode potential. To discuss the potential-dependent change in the orientation of the headgroups of C15-4Py, it is useful to compare the behavior of the pyridine surfactant to the behavior of the pyridine molecule at the Au(111) electrode surface. Electrochemical and IRRAS studies38,54,59 demonstrated that when the potential is moved from negative to positive, the surface concentration of pyridine molecules increases from 1.4 × 10-10 to 6.7 × 10-10 mol cm-2, corresponding to the change in the mean molecular area (MMA) from 119 to 25 Å2. The orientation of pyridine molecules changes rapidly from nearly horizontal to nearly vertical to the surface (with respect to the surface normal, the tilt angle changes from 65° to 20°), and the plane of the pyridine molecule is rotated from 85° to 71° with respect to the plane of incidence.54 In contrast, the pyridine moiety in the film of C15-4Py displays a much smaller change in the tilt angle. Apparently, the alkyl chains, attached to the pyridine group, hinder the reorientation of the headgroups and impose a restriction on the value of the tilt angle. At the equilibrium spreading pressure, the MMA of the C15-4Py molecule is equal to 30 Å2.27 This value is larger than the MMA for a densely packed film of pyridine, suggesting that the packing of the pyridine surfactant is determined by the area needed to accommodate its hydrocarbon tails. Somewhat higher values of the tilt angle in the first leaflet of the monolayer than in the bilayer may be understood in terms of poorer order of the tail region of the film. The chains are less ordered in the bilayer than in the monolayer, and hence they impose less restriction on the orientation of the pyridine moiety. It is worth commenting on the large tilt and rotation angles for the pyridine moiety in the second leaflet of the bilayer. Here, the influence of the electric field of the electrode is weak and polar heads have a tendency to assume an orientation almost parallel to the bilayer. Similar orientation of the polar region is observed in bilayers formed by phospholipids.60 That orientation (59) Stolberg, L.; Morin, S.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 307, 241. (60) Hauser, H.; Pascher, I.; Pearson, R. H.; Sundell, S. Biochim. Biophys. Acta 1981, 650, 21.

Zamlynny et al.

ensures a small contribution of the permanent dipole of the polar head to the dipole potential in the direction normal to the surface of the bilayer. Finally, we note that dipoles of the pyridine moiety do not form an angle of ∼55°, expected for a random orientation, at the most negative potentials, where the capacity curves show that the film is desorbed from the electrode surface. This behavior suggests either that aggregates formed at these potentials remain partially ordered and are not spherical in shape or that the desorption is incomplete. Summary and Conclusions We have provided a molecular level description of potential-controlled transformations of a monolayer and a bilayer of C15-4Py, a model surfactant molecule, deposited at the Au(111) electrode surface. The monolayer and the bilayer form a condensed film at a positively charged surface (positive potentials). When the metal is negatively charged (negative potentials), the film is transformed into a hemimicellar state. At even more negative potentials, the film is desorbed and the molecules form aggregates (micelles) in the vicinity of the electrode surface. The C15-4Py molecule is shaped like a hockey stick in which the hydrocarbon chain is the “handle” and the pyridine moiety is the “blade” that can rotate. We have employed PM IRRAS to monitor changes in the tilt angle of the handle and the blade, bending of the handle, and rotation of the blade that accompany the potentialcontrolled phase change of the film. Our data show that in the desorbed or hemimicellar states, the hydrocarbon tail is bent due to the presence of multiple gauche conformations. When the potential is moved in the positive direction, the chains become more extended. For the monolayer, the transition from the hemimicellar to the condensed film coincides with the transformation from the liquid crystalline to the gel state of the film, with the majority of chains becoming fully extended (all trans), at the most positive potentials. For the bilayer, the film remains liquid crystalline in the condensed state and only a progressive stretching out and/ or change in orientation of the chains with potential is observed. The pyridine moiety has a relatively large permanent dipole moment. In the monolayer and in the first leaflet of the bilayer, the pyridine headgroup faces the metal. At positive potentials, the dipole-field interaction pulls the pyridine moiety to assume a vertical orientation with respect to the surface. This force is opposed by the restricted movement of the chain. These two forces twist the molecule causing a rotation of the pyridine moiety around the C2 axis. The field acting on the pyridine moiety in the second leaflet of the bilayer is weak. In the absence of a strong electric field, this pyridine group is oriented nearly parallel to the electrode surface. We have demonstrated the tremendous power of combined electrochemical and IRRAS studies of thin organic films at solid-liquid interfaces. Electrochemistry provides a unique means to apply an electric field to the interface. The field may then be used to manipulate amphiphilic molecules within the film. By turning a knob on the control instrument, one can force phase transitions in the film or force organic molecules to disperse or to aggregate at the surface. The IRRAS shows how the surfactant molecules rotate, change orientation, and undergo conformational changes during these field-driven phase transformations.


Acknowledgment. This work has been supported by a grant from the Natural Sciences and Engineering Research Council of Canada. J.L. acknowledges the Canada Foundation for Innovation for the Canada Re-

Langmuir, Vol. 19, No. 1, 2003 145

search Chair Award. V.Z. acknowledges an OGS scholarship from the Ontario Government. LA026488U

PM FTIRRAS Studies of Potential-Controlled Transformations of a

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