7141
J. Phys. Chem. 1993,97, 7141-7143
Orientation of Surfactants Adsorbed on a Hydrophobic Surface R. N. Ward and P. B. Davies' University Chemical Laboratories, Lensfield Road, Cambridge CB2 1E W, England
C. D. Bain' Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, England Received: April 26, 1993
While it is generally accepted that surfactants adsorb at the interface between water and a hydrophobic surface with their nonpolar parts in contact with the hydrophobic surface and their polar regions oriented toward the water, this orientation has never been experimentally verified. In this paper, infrared-visible sum-frequency spectroscopy is used to demonstrate that dodecanol adsorbs from aqueous solution onto a hydrophobic surface (a monolayer of octadecanethiol on gold) with the terminal methyl group of the dodecanol oriented toward the hydrophobic surface.
Introduction Oriented films of surfactants at interfacesplay a fundamental role in a wide range of technological applications. The experimental determinationof the orientationof surfactantsat surfaces is not straightforward. For example, it has long been supposedl that both soluble and insoluble surfactants form oriented films at the air-water interface with the polar end of the molecule immersed in the aqueous phase and the nonpolar hydrophobic part directed toward the air. Although many measurements of macroscopic properties, such as surface tension and surface potential,2supported this hypothesis, it has only recently been confirmed unequivocally by X-ray3 and neutron ~cattering.~ According to conventional wisdom, surfactants adopt a similar orientation at the interface between water and a hydrophobic surface,namely, with the nonpolar part directed toward the surface and the polar head group immersed in the aqueous phase. Yet, to our knowledge, no direct experimental proof of this model exists. In this letter, we use sum-frequencyspectroscopy (SFS) to demonstratethat surfactant molecules adsorbed from aqueous solutionsonto hydrophobic surfaces do indeed have the expected orientation. Infrared-visible sum-frequency spectroscopy is a nonlinear optical technique in which a pulsed visible laser (frequency wviS) and a tunable pulsed infrared laser (COIR) are overlapped at a surface, and the light emitted at the sum frequency (asum = @vis COIR)is detected.5 The electric fields of the infrared and visible lasers interact with molecules at the surface and produce an oscillating polarization, P, given, within the electric dipole approximation, by6
+
where E(co~) and E(COIR) are the macroscopic electric fields at the interface,and ~ ( 2is) the second-order nonlinearsusceptibility of the surface. This oscillating polarization then emits light in a well-defined direction with an intensity proportional to lP12. The properties of $2) have two important consequences for the spectroscopy of molecules at surfaces. First, x(2),and hence the intensity of the sum-frequencysignal, is greatly enhanced when the frequencyof the infrared laser is in resonancewith avibrational mode that is both infrared and Raman active. Scanning the frequency of the infrared laser while monitoringthe sum-frequency emission yields a vibrational spectrum.' Second, ~ ( 2 ) is~ ak thirdrank polar tensor which changes sign upon inversion of the coordinate system and therefore vanishes in any medium that remains unchanged when the coordinates are inverted.6 There 0022-3654/93/2097-7 141$04.00/0
-
Reflection in plane parallel to surface
x'2' R
-
Figure 1. Schematic drawing of the methyl terminus of a surfactant adsorbed at a solid surface. 8 is the angle between the axis of the methyl group and the outward-pointing normal to the surface. Reflection of the molecule in a plane parallel to the surfacechanges the sign of the resonant second-order susceptibility, x(~)R.
is no sum-frequency emission from bulk liquids, gases, or amorphous and centrosymmetric solids-SFS has unique interfacial selectivity. Two alternative orientationsof a linear surfactant molecule at a hydrophobic surface are shown schematically in Figure 1. In the left-hand picture, the hydrocarbon chain is oriented with the terminal methyl group facing the aqueous solution; in the righthand picture, the terminal methyl group faces the solid substrate. Conventional vibrational spectroscopic techniques, including infrared absorption spectroscopy, surface-enhanced Raman spectroscopy, electron energy loss spectroscopy, and inelasticneutron scattering, do not distinguish between these two orientations. In SFS the polar orientationof the adsorbatescan be deduced from the sign of the nonlinear polarization, P(2)(see below). Unfortunately, the intensityof the SF emission dependsonly on 1P2)l"a phase-sensitivemethod of detection is required.8 Our solution to this problem is a variation of the approach used by Superfine et al. to determine the orientation of the terminal methyl groups in monolayers of pentadecanoic acid on water.9 In that study, the authorsdetermined the phaseof P(2)from the interferencebetween 0 1993 American Chemical Society
7142 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 sum-f requency signal
visible
infrared
\
laser aqueous dodecanol
\
Figure2. Schematicdrawingof the experimental geometry and reference coordinates. d-ODT= djT-octadecanethiol. 6 0 0 1 7
.:
.-
u) v)
E
1
2800
2900
3000
Infrared Wavenumber (cm-') Figure 3. Sum-frequencyspectra in the C-H stretching region with the electric fields of the visible and infrared lasers and the emitted sumfrequency light all p-polarized. (A) Monolayer of octadecanethiol on gold in contact with water. (B) Monolayer of d3roctadecanethiol on gold in contact with a saturated aqueous solution of dodecanol.
the sum-frequency signal from the sample and from a reference quartz plate. We use the surface of an evaporated metal film as an internal phase reference. Results and Discussion The experimentalarrangementis shown schematically in Figure 2. Two thousand angstroms of gold are first evaporated onto a chromium-primed silicon wafer. This film is immersed in a dilute solution of octadecanethiol (ODT) or d3roctadecanethiol (dODT) in ethanol to form a densely packed, self-assembled monolayer bound to the gold through the sulfur.1° The outer surface of the monolayer is composed predominantly of methyl groups and is thereforehydrophobic. A thin layer (- 1 pm thick) of pure water or saturated aqueous dodecanol is then sandwiched between the substrate and a fused silica prism, which is used to couple the laser beams into and away from the monolayer-solution interface.' Details of the Cambridge sum-frequency spectrometer have been reported el~ewhere.~J2 Figure 3A shows a sum-frequency spectrum in the C-H stretching region of a monolayer of ODT on gold in contact with pure water. Figure 3B shows a SF spectrum in the same region for a monolayer of d-ODT in contact with a saturated aqueous solution of dodecanol. CH~(CH~)IIOH. The sum-frequency, visible, and infrared electricfields were p-polarized. The constant background signal in both spectraoriginatesfrom the gold surface. In Figure 3A, the three main peaks all arise from the terminal methyl groupof the ODT: the methylene modes are weak because
Letters an all-trans hydrocarbon chain is sum-frequency inactive? In Figure 3B, all the features in the spectrum arise from molecules of dodecanol adsorbed at the monolayer-water interface: the fully deuterated self-assembled monolayer (d-ODT) has no resonances between 2800 and 3000 cm-1; the bulk solution is isotropic and hence sum-frequencyinactive;and the spectrum is independentof the thicknessof the layer of solution and therefore cannot arise from molecules adsorbed onto the silica prism.13 We notice the same three principal features from the adsorbed dodecanol molecules as from the ODT molecules in Figure 3A, but now as "dips" rather than peaks. To understand why the resonances in these two spectra have different shapes, we have to consider the consequences of the symmetry of x ( ~on ) the nonlinear polarization of the organic moleculesadsorbedat the gold surface. In the absenceof external fields,these molecules will be distributedisotropically in the plane of the interface (the xy-plane, see Figure 1). As a consequence of this in-plane symmetry,only 7 of the 27 Cartesian components of x ( ~are ) nonzero,14and of these only x(2)zzz, x(2)uz,and x(2)yyz play a role in the experiments reported here. Since ~ ( 2 is) a polar tensor, the sign of each of these components of x ( 2 ) changes ~ if the molecular coordinatesare reflected (2- -2) in a plane parallel to the surface, as shown schematically in Figure 1. SFS is a coherent technique, and thus the total nonlinear polarization at the interfacearisesfrom a linear superpositionof the nonresonant susceptibility of the gold substrate, x(~)NR,and the resonant susceptibility of the organic molecules at the interface, x(~)R. If only one Cartesian component of ~ ( 2 )makes a significant contribution to the SF signal, the tensor product in eq 1 can be written as The magnitude, Ix(~)NRI, and phase, e, of the nonresonant susceptibility change little with infrared frequency. The phase of the resonant susceptibility,~(oIR), increasesby .rr as WIR sweeps through the resonant frequency, a",of a vibrational mode that is sum-frequency active. The intensityof the emitted SF radiation depends on IP12
The third term in eq 3 represents the interference between the resonant and nonresonant polarizations. It is this term that carries the phase of x ( ~ ) Rand the information on the polar orientation of the surfactant molecules. When X(~)NRis larger than x(*)R, as is the case in these spectra, the first term in eq 3 provides a constant background signal and the resonant features arise principally from the interference term. For molecules such as ODT and dodecanol, which have no electronictransitionsin the visible region, x ( ~ ) Ris purely imaginary when OIR = a,:the phase 6 = &n/2. If we assign a positive sign to 6 for a monolayer of ODT on gold, then the SF spectrum in Figure 3A can be modeled with the phase of x(~)NR, e = n/2. On resonance (OIR = a,),cos(€ - 6) = 1, so there is constructive interferencebetween the resonant and nonresonantpolarizations which give rise to "peaks" in the SF spectrum. For the dodecanol adsorbed at a hydrophobic surface(Figure 3B) there is destructive interferencebetween the resonant and nonresonantpolarizations when WIR = a,. Since the phase e is unchanged, we infer that 6 is now -7r/2 on resonance: x ( ~ ) Rhas changed sign. It is well-established that long-chain alkanethiols bind to gold through the sulfur atom and, hence, that the terminal methyl groups are oriented away from the metal surface (Figure 1, lefthand side).IS We may thus deduce from the change in sign of x ( ~ ) Rthat the methyl groupsof dodecanol are oriented toward the metal surface, as in the right-hand diagram in Figure 1. We should make it clear exactly what we mean by "oriented toward
Letters the surface". x ( ~ ) Rdepends on an orientational average over all the molecules a t the interface (indicated by angle brackets). For example, x ( ~ ) R , ~for ~ , the symmetric methyl stretch (r+) depends on (cos 0) - 0.7(cos3 0 ) (Figure 1). Our data therefore do not rule out the presence of a minority of molecules with the opposite orientation. The magnitude of x ( ~ ) Ris, however, comparable for the monolayer of ODT and for the adsorbed dodecanol, suggesting a very high degree of orientation in the surfactant film. These data provide strong evidence for the proposed (and intuitively correct) orientation of the dodecanol molecules at the monolayer-water interface. The use of p-polarized laser beams does, however, introduce one complication into the interpretation of the data. Gold is an excellent reflector in the infrared. When the I R laser is p-polarized, almost complete cancellation occurs between the x-component of the electric field in the incident and reflected waves at the surface: only an electric field in the z-directed remains. Gold is a poorer reflector in the visible. p-Polarized light at 532 nm, the frequency of our visible laser, results in an electric field at the interface with both x and z components. Two components of x ( ~thus ) contribute to the nonlinear polarization at the interface.
The Journal of Physical Chemistry, Vol. 97, No. 28, I993 7143
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The phases, € 1 and €2, of x ( ~ ) ~ ~ and ~ , NxR( ~ ) ~ ~ ~are , N different R and, more importantly, E , and E, are not in phase at the interface, due to the complex refractive index of the gold substrate. The expression for the intensity of emitted light, analogous to eq 3, thus contains four cross-terms involving x ( ~ ) Rand x ( ~ ) N R ,each with a slightly different phase relationship between the nonlinear susceptibilities. The relative phase of x ( 2 ) and ~ x ( 2 )observed ~ ~ in a ppp-polarized S F spectrum thus depends not only on the sign of X ( ~ ) Rbut also on the relative magnitudes of x(2)zzz,Rand X(~),~~,R, which depend in turn on the details of the orientational distribution of the molecules a t the interface. Any possible ambiguity in our conclusion regarding the orientation of the dodecanol molecules can be removed if we compare sum-frequency spectra in which the infrared laser is p-polarized while the visible laser and the emitted light at the sum-frequency are s-polarized. Such spectra are much weaker than those acquired with all fields p-polarized, due to the large degree of cancellation of the incident and reflected fields of the visible laser, but do have the advantage of depending on only one component of the susceptibility, x ( ~ ) The ~ ~ ssp-polarized ~ . spectrum of a monolayer of ODT on gold in contact with pure water (Figure 4A) is described by the phase 6 = 7712 at resonance, as before, and by a nonresonant phase e l = - ~ / 3throughout the spectrum. The aqueous solution of dodecanol in contact with a hydrophobic surface (Figure 4B) exhibits the same value of €1, but now with 6 = - n / 2 on resonance. The inversion in the sign of x ( ~ ) Rconfirms that the methyl groups of the dodecanol are oriented toward the hydrophobic surface. The orientation of dodecanol at the interface between water and a hydrophobic solid contrasts sharply with recent reports on adsorption from nonpolar liquids. Scanning tunneling microscopy has revealed that octadodecanol dissolved in an organic solvent adsorbs on the basal planes of graphite (a hydrophobic solid) with the alkyl chains lying parallel to the surface,I6 and a very recent study suggests that a flat orientation also occurs a t the interface between graphite and liquid dodecan01.l~ We have studied the absorption of a range of anionic, cationic, and neutral surfactants at hydrophobic surfaces. In every case the phase of the sum-frequency resonances indicates that the methyl groups are oriented away from the aqueous phase. The
2900 Infrared Wavenumber (cm-')
3000
Figure 4. Sum-frequency spectra in the C-H stretching region with the electric fields of the visible and the emitted sum-frequency light s-polarized and the infrared laser p-polarized. (A) Monolayer of octadecanethiol on gold in contact with water. (B) Monolayer of djroctadecanethiol on gold in contact with a saturated aqueous solution of dodecanol.
sum-frequency spectra do, however, show great variety depending
on the area per molecule at the surface. These spectra will be discussed in a subsequent paper.
Acknowledgment. We are grateful to Unilever Research (Port Sunlight Laboratory), the SERC, and the Royal Society for their generous support. References and Notes (1) Langmuir, I. J . Am. Chem. SOC.1917, 39, 1848-1906. (2) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.;Wiley: New York, 1990; Chapters 3 and 4. Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966; Chapter 4. (3) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippmann-Krayer, P.; Mbhwald, H. Thin Solid Films 1988, 159, 17-28. (4) Simister, E. A,; Lee, E. M.; Thomas, R. K.; Penfold, J. J . Phys. Chem. 1992, 96, 1373-1382. (5) For a recent review, see: Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992,43, 6 2 7 6 6 1 . (6) Shen, Y. R. PrinciplesofNonlinear Optics; Wiley: New York, 1984. (7) Bain, C. D.; Davies, P. B.; Ong, T.H.; Ward, R. N.; Brown, M. A. Lungmuir 1991, 7, 1563-1566. (8) Kemnitz, K.; Bhattacharyya, K.; Hicks, J. M.;Pinto,G. R.; Eisenthal, K. B.; Heinz, T.F. Chem. Phys. Lett. 1986, 131, 285-290. Harris, A. L.;
Chidsey, C. E. D.; Levinos, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987, 141, 3SC-356. (9) Superfine, R.; Huang, J. Y.; Shen, Y. R. Opt. Lett. 1990, IS, 12761278. (10) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989, 111, 321-335. (11) Ong, T. H. PhD Thesis, Cambridge, 1993. (12) Ong, T.H.; Davies, P. B.; Bain, C. D. Lungmuir, in press. (13) Theseresonancesareonlyobservedinthepresenceof the bulksolution.
If the hydrophobic substrate is removed from the dodecanol solution, the adsorbed monolayer of dodecanol redissolves leaving a clean surface. (14) Dick, B.; Gieruslski, A.; Marowsky, G.; Reider, G. A. Appl. Phys. 1985,838, 107-116. (15) Dubois, L. H.; Nuzzo, R. G . Annu. Rev. Phys. Chem. 1992, 43, 437463. (16) Rabe, J. P.; Buchholz, S.Science (Washington, D.C.) 1991, 253, 424-427. (17) Yeo, Y. H.; McGonigal, G. C.; Thomson, D. J. Lungmuir 1993, 9, 649-65 1.
