11642
J. Phys. Chem. C 2007, 111, 11642-11652
Structure and Hydration of Poly(ethylene oxide) Surfactants at the Air/Liquid Interface. A Vibrational Sum Frequency Spectroscopy Study Eric Tyrode,*,† C. Magnus Johnson,‡ Mark W. Rutland,† and Per M. Claesson† Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-100 44 Stockholm, Sweden, YKI, Institute for Surface Chemistry, Stockholm, Sweden, and DiVision of Corrosion Science, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-100 44 Stockholm, Sweden ReceiVed: January 11, 2007; In Final Form: April 16, 2007
Adsorption layers of penta(ethylene oxide) n-dodecyl ether (C12E5) at the air/liquid interface have been studied using the surface-sensitive technique vibrational sum frequency spectroscopy (VSFS). The CH and COC stretching vibrations of the surfactant molecule, as well as the OH stretching vibrations of the surface water molecules, have been targeted to obtain a comprehensive picture of the adsorption process. The concentration range studied comprises different adsorption regimes, starting from the neat water surface until attaining the saturated liquid expanded monolayer when approaching the critical micellar concentration (cmc). The surfactant molecules were found to first adsorb to the air-liquid interface with their hydrocarbon tails preferentially orientated close to the surface plane, surrounded by patches of unperturbed surface water. These patches were only seen to disappear at areas per molecule close to 65 Å2, coinciding with a sudden change in the orientation of the surfactant alkyl chains, which adopted a more upright configuration. Nonetheless, gauche defects in the hydrocarbon tails were observed along the whole concentration range, even above the cmc. Moreover, the poly(ethylene oxide) headgroup was seen to induce a significant structuring of the water molecules in direct proximity to the surfactant monolayer, despite being themselves substantially disordered. Comparison of the hydration fingerprint region is made with another non-ionic surfactant with a sugar-based headgroup. The temperature effect in the VSFS spectra of C12E4 and C12E8 solutions has also been considered, and the results are discussed in terms of the different models proposed to explain the peculiar temperature dependence of ethylene oxide-based surfactants and polymers in water.
Introduction Poly(ethylene oxide) alkyl ethers (CnEx) represent the most common class of non-ionic surfactants. Their properties can be tuned by varying the length of the ethylene oxide chain, the hydrocarbon chain, and/or by changing the temperature. Ethylene oxides have the unusual property that they become less soluble in water with increasing temperature, which is related to a less favorable interaction between water and the poly(ethylene oxide) (PEO) chain at elevated temperatures. The peculiarity of the interaction of ethylene oxide units with water is also emphasized by the fact that structurally similar polyethers, like polymethylene oxide and polypropylene oxide, are insoluble in water, while PEO chains are soluble in all proportions at moderate temperatures.1,2 The strong dependence on temperature of the interactions of PEO chains with water leads to phenomena such as clouding of micellar solutions and phase inversion of microemulsion systems at elevated temperatures.3,4 This is also manifested in the decreased repulsion between surfaces coated with this type of surfactant at higher temperatures.5 Moreover, they represent an interesting material in sensor applications because the protein adsorption onto surfaces coated with poly(ethylene oxide) is very low.6 The “protein-repellent” ability decreases with increasing temperature, indicating that this phenomenon is also mediated by the interaction with water. * Corresponding author. Fax: +46 8 208998. E-mail:
[email protected]. † Department of Chemistry, Royal Institute of Technology, and Institute for Surface Chemistry. ‡ Division of Corrosion Science, Royal Institute of Technology.
While these phenomena are well documented, there is no agreement behind the underlying molecular mechanism. It has been proposed that the surfactant headgroup simply becomes dehydrated at elevated temperatures,7-10 or, in contrast, that the hydration of the isolated headgroup remains at high temperature but the subtle balance between energy and entropy of dehydration changes in favor of association between headgroups at higher temperatures.11 Still another model has been proposed that focuses on conformational changes of the oligomeric headgroup with temperature, and this model explains the clouding phenomena as being due to the increasing population of nonpolar conformations at higher temperatures.12 Progress in this area has been slow, mainly because there has been no technique available for simultaneously measuring the hydration of the oligo(ethylene oxide) headgroup and its conformation. Here, we use the intrinsically surface sensitive technique vibrational sum frequency spectroscopy (VSFS), to study the adsorption behavior of poly(ethylene oxide) surfactants at the air-liquid interface and provide unique information about the orientation and structure of both the surfactant molecule and the water molecules in direct proximity hydrating the surfactant headgroup. VSFS has been used in the past to study the interaction of oligo(ethylene oxide) terminated thiols with water, primarily focusing on the CH stretching region. One important conclusion that emerged from those studies is the apparent randomization of the PEO chain when in contact with water.13-16 In the present study, we explore a missing part of the puzzle, specifically the changes observed in the structure and order of
10.1021/jp070246r CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007
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J. Phys. Chem. C, Vol. 111, No. 31, 2007 11643
water due to the presence of the PEO chains. In the first section of this Article, we describe the changes observed at the air/ liquid interface starting from pure water, as penta(ethylene oxide) n-dodecyl ether (C12E5) is added to solution. In the last section, the changes on the surface water structure as the temperature is varied from 10 to 40 °C are considered for two other members of this family of surfactants: C12E4 and C12E8. Principles of Vibrational Sum Frequency Spectroscopy Vibrational sum frequency spectroscopy (VSFS) is a secondorder nonlinear optical technique, which has an exquisite sensitivity to the order and conformation of molecules at surfaces and interfaces. The theory has been described in detail in many sources,17-21 and therefore only a summary of the most important concepts is given here. In this technique, two pulsed laser beams, one tunable in the IR region and one at a fixed frequency, usually in the visible region of the electromagnetic spectrum, are temporarily and spatially overlapped at an interface. The two beams excite interfacial molecules and a third beam is emitted, having a frequency equal to the sum of the frequencies (SF) of the two incoming beams, ωSF ) ωvis + ωIR. The process can be viewed as a resonant IR absorption followed by a non-resonant Raman anti-Stokes process, implying that to be SF active a vibrational mode must be both IR and Raman active. Moreover, being second order in nature, the SF process can only occur in noncentrosymmetric media under the electric dipole approximation. Thus, only molecules residing at the interface between two bulk centrosymmetric media will give rise to a SF signal. The SF intensity, ISF, is given by 2 ISF ∝ |χ(2)|2IvisIIRLSFG L2IRL2vis
(1)
where χ(2) is the macroscopic second-order nonlinear susceptibility, Ivis and IIR are the intensities of the visible and infrared beams, respectively, and LSFG, LIR, and Lvis are the Fresnel factors, which depend on the optical setup and the refractive indexes of the media involved in the process. χ(2) contains both an essentially frequency-independent nonresonant contribution, and resonant contributions originating from molecular vibrations. Furthermore, χ(2) is proportional to the molecularly averaged hyperpolarizability, 〈β(2)〉, which in turn is directly related to the product of the infrared transition dipole moment, µγ, and the Raman polarizability tensor element, RRβ. In media where the molecules lack a net orientation, 〈β(2)〉 ) 0, and thereby no SF signal is generated. When the infrared beam is in resonance with a vibrational mode, β(2) is resonantly enhanced, and thus a peak is generated in the surface vibrational spectrum, as described by eq 2, (2) ) βRβγ
RRβµγ ωn - ωIR - iΓn
(2)
where ωn is a vibrational transition frequency, i is the imaginary unit, and Γn is the inverse relaxation time. Spectra can be gathered using different polarization combinations for the input and output fields. For isotropic planar surfaces like the surfactant systems studied in this Article, only three polarization combinations give rise to independent information: ssp, ppp, and sps, where the letters designate the polarization, whether parallel (p) or perpendicular (s) to the plane of incidence of the SF, visible, and infrared beams, respectively. Spectra at different polarization combinations can be used to extract information regarding the orientation of specific bonds at the surface, as well as identifying additional molecular
species, which are present at the interface but only observable under certain polarization combinations. Experimental Section (a) Materials. Penta(ethylene oxide) n-dodecyl ether (C12E5) was obtained from Nikkol and further purified in a high performance surfactant purification unit.22 Tetra(ethylene oxide) n-dodecyl ether (C12E4) and octa(ethylene oxide) n-dodecyl ether (C12E8) were obtained from Nikkol. The deuterated C12E5 species (dC12hE5) was purchased from Dr. R. K. Thomas (University of Oxford, UK). Measurements on C12E4, C12E8, and dC12hE5 were only performed above their respective critical micellar concentrations. and for this reason these compounds were used without further purification. The surfactant n-decylβ-D-maltopyranoside was obtained from Anatrace (Anagrade). The water used in the experiments was obtained from a Millipore RiOs-8 and Milli-Q PLUS 185 purification system, finally filtered through a 0.2 µm Millipak filter. The resistivity was 18.2 MΩ cm, while the total organic carbon content of the water used was monitored with a Millipore A-10 unit and did not exceed 4 ppb during any of the measurements. (b) Methods. Vibrational Sum Frequency Spectrometer. The VSFS setup has been described in detail elsewhere23 and will be only briefly outlined here. The system consists of a Nd:YAG laser (1064 nm, 20 Hz, 24 ps) from EKSPLA, which is used to pump an OPG/OPA (LaserVision), which generates the visible beam with a fixed wavelength of 532 nm and the tunable infrared beam with a frequency range extending from 1000 to 4200 cm-1. These two laser beams are overlapped in time and space at the surface in a copropagating geometry, with angles of incidence of 55° and 63° from the surface normal for the visible and IR beams, respectively. The generated SF beam is optically and spatially filtered, passed through a monochromator, and finally detected with a photomultiplier tube (PMT). The gated signal is integrated in a boxcar interfaced with a PC. The measured SF intensity is normalized by the energy of the visible and IR fields to account for power fluctuations, as well as for gas-phase absorption inside and outside the sample cell. All experiments are performed in a closed cell to avoid evaporation and contamination, which is particularly important for experiments at low surface coverage of surfactant molecules.24,25 The bottom of the measuring cell is immersed in a temperaturecontrolled bath, while the temperature inside the cell is monitored with a thermocouple placed inside a sealed glass capillary, which is in direct contact with the liquid under study. A minimum of 4 h was required to collect each of the spectra presented here. The experiments were repeated on at least three different occasions. Moreover, the spectrum of pure water was collected for reference at the beginning of each experiment in the same measuring cell. Thus, even though the intensities in the spectra shown in the CH and OH stretching region are presented in arbitrary units, they can be directly compared (the intensity of the free OH peak of pure water is equivalent to 10 au). The surfactant solutions were prepared by adding to pure water aliquots of stock solutions of C12E5. The concentration of these stock solutions was never below 15 µM. Solutions were subsequently stirred with a glass-coated stirrer placed inside the measuring cell, for an amount of time which varies depending on the concentration probed and reaching up to 30 min for concentrations below ∼1 µM. No measurable changes were observed between the first and last recorded spectrum for a given concentration, which, when considering the three polarization combinations gathered, was usually more than 15 h. Surface Tension. The surface tension (γ) was measured with a Kru¨ss K12 tensiometer, employing the Wilhelmy plate method.
11644 J. Phys. Chem. C, Vol. 111, No. 31, 2007
Tyrode et al.
Figure 2. Surface tension isotherm for C12E5 at 20 °C. Figure 1. VSF spectra of the neat water surface under the polarization combinations ssp, ppp, and sps. The ssp and ppp spectra have been offset for clarity by 2 and 1 arbitrary units (au), respectively. The spectra presented in this figure are based on data previously collected by the authors.24
The platinum plate was sand-blasted to ensure a zero contact angle at the three-phase contact line. All measurements were performed at 20.0 ( 0.2 °C. Results and Discussion (a) The Neat Water Surface. The pristine water surface is the starting point for our experiments and will be shortly described first. In Figure 1, the VSF spectra of water under the polarization combinations ssp, ppp, and sps are presented as a reference.24 The main features of the ssp spectrum, which is the most commonly reported spectrum of water in the literature,26-31 are a broad band extending on the lower frequency side, and a sharp peak centered at approximately 3700 cm-1. The latter is assigned to the uncoupled OH stretching vibration of water molecules with one OH bond protruding out into the gas phase and vibrating free from hydrogen bonds. The peak is commonly referred to as “free OH”. Only molecules present at the top monolayer can be responsible for this intensity, which demonstrates the surface sensitivity of VSFS. On the other hand, the broad band intensity arises from OH stretching vibrations of water molecules interacting with neighbors through hydrogen bonds. The frequency of the OH stretching vibration of water molecules is sensitive to the strength and coordination of the hydrogen bonds it forms, red shifting more than 600 cm-1 when going from the isolated water molecules in the gas phase to the tetrahedrally coordinated hydrogen-bonded molecules in ice. The number of different types of water species contributing to this broad band is still a source of debate,28,29,31 despite recent and encouraging advances in theoretical and computational methods to interpret the VSFS spectra.32-34 The simplest and broadly accepted approach has been to divide it into two main bands centered at ∼3200 and ∼3450 cm-1, respectively. The first of these is referred to as strongly hydrogen bonded or “ice-like” peak, due to its closeness in frequency to the OH vibrations of tetrahedrally coordinated water molecules on the surface of ice (∼3150 cm-1).35 The second is usually labeled as weakly hydrogen bonded or “liquidlike” peak and is associated with molecules vibrating in a more disordered hydrogen-bonded network, in analogy to the strongest feature observed in the Raman spectrum of liquid bulk water.36 In the ppp spectrum, the most evident feature is the strong peak at ∼3700 cm-1, which is assigned to the “free OH” vibration as described above, while in the sps spectrum no
obvious resonant features are observed in the whole OH stretching range. The data at different polarization combinations allow calculation for example of the average tilt of the OH bond that protrudes out in the vapor phase (35° from the surface normal26,30), but it also provides additional information about the nature of the different species responsible for a certain spectral feature. The fact that almost no intensity is observed in the spectral region below 3500 cm-1 in the ppp and sps spectra indicates that the intensity observed in this region in the ssp spectrum essentially originates from symmetric stretching vibrations of water and/or hydrogen-bonded uncoupled OH vibrations,24 a statement which is in accordance with recent theoretical calculations.37,38 Curiously, no evidence is found in the VSF spectra for water molecules with both hydrogen bonds free (called “acceptor only”) at the surface of water, a structural moiety that has been suggested by recent X-ray experiments under off-equilibrium conditions (expanding jet in a vacuum)39 and ab initio calculations.40 The absence of signal in the VSF spectra for this species, which should give rise to two sharp bands close to 3650 and 3750 cm-1 (symmetric and antisymmetric gas-phase OH stretching bands), could be due to a lack of net orientation, to its total absence under equilibrium conditions, or to an unfavorable orientation with the molecular dipole moment close to the surface plane. This latter option is fairly unlikely because if both hydrogen atoms were on the surface plane they would be likely to form hydrogen bonds with the underlying water molecules. The VSF spectra of Figure 1 reveal that the structure of liquid water at the surface is particularly ordered. This structure has been referred to as ice-like,41 although it is certainly not as ordered as ice, as evidenced by the significant intensity at ∼3450 cm-1 in the ssp spectrum. Nonetheless, careful VSF experimental studies at ice interfaces have suggested that the liquid water surface, which is structurally different from the quasiliquid layer on the ice surface, has a higher degree of order than that observed at the ice surface close to the melting point.35 (b) C12E5 Concentration Dependence. General ObserVations. Upon addition of a surface active component, the liquid water surface is quickly perturbed. In Figure 2, the surface tension isotherm of C12E5 at 20 °C is shown. The observed decrease in surface tension is a consequence of the adsorption of surfactant molecules to the air/liquid interface. In the low surface density region, which is referred to as the Henry range, the average molecular area is considerably greater than that of the molecule itself, whereby the interaction between surfactant
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J. Phys. Chem. C, Vol. 111, No. 31, 2007 11645
Figure 3. VSF spectra of a 0.07 µM C12E5 solution under the polarization combinations ssp, ppp, and sps. Below each spectrum the corresponding spectrum of pure water is shown (black and unfilled symbols). The ppp and ssp spectra of C12E5 very closely follow those for water, whereas a weak broad band at ∼3600 cm-1 is seen in the C12E5 sps spectrum indicating the onset of the disruption of the water surface structure. The ssp and ppp spectra have been offset for clarity by 3 and 1.5 arbitrary units, respectively.
TABLE 1: Molecular Areas and Surface Pressures for Different C12E5 Solutions C12E5 concentration (µM)
area per molecule (Å2)
surface pressure (mN/m)
0.07 0.19 0.44 0.81 1.8 21 90
∼800 ∼300 ∼90 ∼68 ∼58 ∼51 ∼50
0.4 1.4 3.8 7.7 14 31 42
molecules can be neglected.42 In the particular case of C12E5, this regime corresponds to concentrations below ∼0.1 µM and areas larger than ∼400 Å2 per surfactant molecule. Toward the end of the Henry range, the rapid change in slope in the surface tension isotherm indicates a marked decrease in the area per molecule and the commencement of interactions between the surfactant molecules. After this transitional regime, the liquid expanded phase is formed, which is a surface-covering layer consisting of a coherent liquid-like hydrocarbon tail phase region and a domain where headgroups and water mix. For C12E5 the liquid expanded phase appears at concentrations above ∼1 µM, which corresponds to areas per molecules smaller than ∼70 Å2. The break in the surface tension curve observed at 71 µΜ corresponds to the critical micellar concentration for C12E5 at 20 °C. At the cmc the area per molecule is ∼50 Å2. The molecular areas and surface pressures (II ) γ0 - γ) for the different concentrations shown in the spectra below are collected in Table 1. The molecular areas are estimated using the Gibbs equation (eq 3) based on the data shown in Figure 2.
area per molecule ) -
(
NAv dγ RT d ln c
)
-1
(3)
where NAv, R, T, and c refer to Avogadro’s number, the gas constant, the temperature, and the concentration of surfactant solution, respectively. Henry Range. The VSF spectra of C12E5 solutions obtained in the Henry concentration range under the ssp and ppp polarization combinations do not show any measurable difference from those of pure water. However, in the sps polarization combination, which was featureless for water, a weak broad
band at ∼3600 cm-1 is readily observed. In Figure 3, such an example is shown for a 0.07 µM solution (Π ≈ 0.45 mN/m). At this low concentration, the molecular area is large (∼800 Å2) and neither the adsorbed surfactant nor its effect on the water surface structure is detectable, besides the weak band observed in the sps spectrum. It has been proposed that this intensity originates from OH stretching vibration of water molecules in a non-donor configuration close to hydrocarbon tails.24 This fact introduces the peculiarity that the weakest of the three polarization combination spectra of pure water seems to be the most sensitive to the presence of hydrocarbon contaminants at very low surface densities. Transitional Range. Surface Patches. Spectral features characteristic of the surfactant molecule start to appear in the VSF spectra at concentrations close to ∼0.1 µΜ, which corresponds to surface molecular areas of approximately 400 Å2. In Figure 4, the changes in the VSF spectra with concentration in the transitional concentration range are shown. The CH stretching vibrations of the surfactant alkyl chain give rise to the peaks observed between 2800 and 3000 cm-1, while the contribution from the surfactant headgroup is negligible in this region (vide infra). The intensities above 3000 cm-1 are thought to originate exclusively from the OH stretching vibrations of water molecules. As discussed in detail in a later section, the contribution from the terminal hydroxyl group of the EO chain is considered to be minute because of the random orientation of the headgroup, particularly at the end of the chain. Examining in detail the CH stretching region, the first intensity that appears in the spectra is centered at ∼2850 cm-1 and is assigned to the symmetric methylene stretch (labeled d+), in accordance with previous studies43,44 (spectra showing in detail the CH stretching region can be found in the Supporting Information). The presence of this mode indicates the existence of gauche defects, because in an all-trans hydrocarbon chain configuration the methylene groups would be in a locally centrosymmetric environment, effectively rendering the CH2 vibrations SF inactive.45 The d+ mode is the strongest peak in the CH region for the three polarization combinations in the concentration range that extends up to 1 µM. Most of the other evident peaks in this region are also associated with methylene vibrations, which include the asymmetric CH2 stretch (d-) at ∼2885 cm-1 observed in both the sps and the ppp spectra, and the Fermi resonance of the symmetric methylene mode (dFR+) at ∼2915 cm-1 in the ssp spectra. These assignments are based on recent VSFS studies on alcohols with different chain lengths, which dispersed doubts in previous assignments.21,46 Examining the selection rules for vibrational sum frequency spectroscopy for methylene stretches, the significant intensity observed in the sps polarization combination for the d+ mode indicates that the molecular backbone has a preferred twist about its axis. However, a quantitative analysis of the methylene intensities as a function of orientation may only be carried out under very restrictive assumptions, because the 11 different methylene groups in the surfactant chain with different preferred orientations have essentially the same resonant frequencies, and this issue will thus not be discussed any further here (the contributions from the methylenes stretches of EO headgroup are thought to be negligible as discussed in detail in a later section). On the other hand, the analysis of the terminal methyl group is more specific because there is only one methyl group in the surfactant chain, and its resonant frequencies are well separated from those of the methylene modes. The theoretical framework for calculating the orientation of the methyl group is well established.20,47-49 Nonetheless, in the concentration range
11646 J. Phys. Chem. C, Vol. 111, No. 31, 2007
Figure 4. VSF spectra of 0.19, 0.44, and 0.81 µM solutions of C12E5 under the polarization combinations ssp (A), ppp (B), and sps (C). The spectrum of water for the different polarization combinations is also shown for reference. Spectra showing in detail the CH stretching region can be found in the Supporting Information (Figure A1).
shown in Figure 4, the intensities related to the terminal CH3 group are not readily observed. Only the symmetric methyl stretch (r+) can be seen as a shoulder in the ssp spectrum of the 0.81 µM solution at ∼2870 cm-1, while the asymmetric methyl stretch (r-), expected at ∼2960 cm-1, is absent in all spectra. The intensity of the sum frequency beam is dependent on the number of molecules that contribute to the signal, and also on their average orientation. The low intensities observed for the r+ mode could in principle be linked to the relatively high molecular areas, which span from approximately 300 to 68 Å2 for the concentrations shown in Figure 4. However, comparison with the spectra of Figure 5 for higher concentrations shows that the intensities of the r+ and r- modes are
Tyrode et al.
Figure 5. VSF spectra of 1.8, 21, and 90 µM solutions of C12E5 under the polarization combinations ssp (A), ppp (B), and sps (C). The spectrum of water for the different polarization combinations is also shown for reference. Spectra showing in detail the CH stretching region can be found in the Supporting Information (Figure A2).
increasing much faster than the surface density. As a consequence, the low intensity of the r+ mode and the absence of the r- mode at the low C12E5 concentrations illustrated in Figure 4 can be explained in terms of the conformationally disordered hydrocarbon chains having their terminal methyl group essentially randomly oriented at low concentrations. The chain tilt, which is also a useful parameter for modeling the disordered surfactant monolayers, can only be roughly estimated from the VSF spectra. It has been proposed that a random orientation of the terminal methyl group may imply that the hydrocarbon chain lies close to the surface plane.43 Nonetheless, additional information is required to confirm this
Structure and Hydration of PEO Surfactants hypothesis. Neutron reflection studies of monolayers of C12Ex surfactants adsorbed to the air-water interface have shown that the alkyl part of the layer becomes thinner with decreasing concentration,50-52 which supports the idea of alkyl chains lying flat in the concentration range considered here. Moreover, recent surface tension studies of sugar surfactants have indicated that the annihilation of air/water interface upon adsorption of a surfactant tail in the Henry range represents the most important contribution to the local free energy of the system, which implies that a vast majority of hydrocarbon chains will indeed reside in the plane of the surface.53 The OH stretching region of the spectra of Figure 4 also provides interesting information. Focusing first on the “free OH” vibration at ∼3700 cm-1, it is seen that the peak intensity decreases with increasing concentration while the peak position remains constant in both the ssp and the ppp polarization combinations. The presence of the sharp free OH peak signifies that ordered water molecules are vibrating in direct contact with air, implying that patches of essentially unperturbed water are still to be found in between islands of surfactants molecules. The faster decline observed in the ppp spectra is indicative of small changes in the average orientation of the “free OH” bond, in particular a slight increase of a few degrees in the tilt angle with respect to the surface normal or simply a broadening of the orientation.24 This finding reveals that even the structure in the water patches is also slightly perturbed by the adsorbed surfactant, in contrast to what was observed in the adsorption of acetic acid molecules to the air/water interface.30 Another clear spectral feature observed in the OH stretching region in the ppp and sps spectra of Figure 4 is the broad band centered at ∼3580 cm-1, which intensifies with increasing concentration. As mentioned above, this band was assigned to non-donor water molecules in close proximity to the hydrocarbon tails, and as the number of adsorbed surfactant molecules increases the number of these water species is expected to rise. Because VSFS is a coherent technique, the SF signal carries information in both its phase and its magnitude.13 Following the changes of phase with respect to a known reference allows the absolute orientation of the specific bond or molecule under scrutiny to be obtained.54 In the experiments presented here, despite not specifically measuring the phase of the SF signal, the closeness in frequency between the non-donor and free OH modes allows their relative phases to be estimated by considering the effect of interference on the spectral features. Because the phase of a particular mode changes by 180° at the resonant frequency, two vibrational modes are considered to have the same phase, if destructive interference is observed in the region between the two center frequencies. In the ppp spectra of Figure 4, such a case is observed between the free OH and the broad band centered at ∼3580 cm-1, indicating that both modes have the same phase. This result provides additional support for the proposed non-donor assignment, because the water molecules responsible for the two bands must necessarily have their hydrogen atoms directed away from the bulk solution. At lower frequencies, the attention is directed toward the broad band observed in the ssp spectra of Figure 4, which in broad terms resembles that observed for pure water. However, we argue that the intensity seen in this region results from two main contributions: water molecules present in the unperturbed patches of water forming the structured layer at the interface that supports the free OH configuration,24 and also from those hydrating the surfactant headgroup. As the surfactant concentration is raised, the free OH peak declines and the contribution to this broad band from water molecules of the hydration shell
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11647 consequently increases. It is noteworthy that at these low concentrations the band is strongest at ∼3200 cm-1, which is characteristic of tetrahedrally coordinated water molecules in ice. This point will be further developed below after discussing the higher concentration range. Liquid Expanded Region up to the cmc. When increasing the concentration above ∼1 µM, some significant changes are observed in the VSF spectra as shown in Figure 5. The first evident change occurs in the CH stretching region where the features corresponding to the methylene stretching vibrations are seen to suddenly intensify after which they remain approximately constant up to the cmc, despite a small rise in the surface density (a detailed view of the CH stretching region can be found in the Supporting Information). Actually the dmode at ∼2885 cm-1 is even seen to decrease in the ppp and sps spectra at the cmc. Moreover, the resonant features characteristic of the methyl group, which were almost completely absent in the spectra taken at lower concentrations, now appear in the three polarization combinations: r+ at 2870 cm-1 and the Fermi resonance (r+FR) of the symmetric CH3 stretch with an overtone of the methyl bend at ∼2940 cm-1 in the ssp spectra, and the r- at 2960 cm-1 in the ppp and sps spectra. These results are interpreted as a sudden change in the orientation of the hydrocarbon tail, as it moves away from the surface plane adopting a more upright configuration. In an alltrans configuration like the one adopted by fatty alcohols, the methylene modes are completely absent in the SF spectra because they reside in a local centrosymmetric environment.44 The significant intensity of the d+ mode in the SF spectra of C12E5, even at concentrations above the cmc, evidences the presence of gauche defects in the saturated monolayer. The ratio between the fitted r+ and d+ amplitudes has been proposed as a useful indicator of the degree of conformational order in the monolayer.44 This ratio is seen to increase when increasing the concentration from 1.8 µM, indicating a slight increase in the order of the hydrocarbon chains as the cmc is approached. These results in the CH stretching region for areas per molecule smaller than 65 Å2 correlate with those presented previously by Goates et al.,43 where spectra were gathered exclusively in the ssp polarization combination. In those studies, it was also shown that the number of gauche defects and degree of orientation of the alkyl chains were directly linked to the surface density, and not to the number of ethylene oxide units of the headgroup. The observed changes in the CH stretching range, in particular those related to the CH3 vibrations, are closely coupled with the disappearance of the sharp free OH peak at ∼3700 cm-1 at ∼1 µM, which also indicates the end of the concentration range at which patches of unperturbed water are seen at the surface, and the ultimate formation of a surface covering monolayer characteristic of the liquid expanded phase. As the sharp free OH peak, characteristic of the ordered pure water surface, diminishes in intensity and finally disappears, a broader and considerable weaker band centered at approximately the same frequency is seen to arise in the ssp spectra. The larger breadth indicates a significantly broader average orientation of possibly uncoupled OH oscillators of water molecules in close proximity to the hydrocarbon tails of the C12E5. We believe this latter band is associated with the disrupted water molecules that give rise to the “non-donor” band at ∼3580 cm-1 in the sps and ppp polarization combinations. The intensity of the “non-donor” band was seen to increase when compared to the lower concentrations presented in Figure 4, remaining afterward essentially constant up to the cmc.
11648 J. Phys. Chem. C, Vol. 111, No. 31, 2007
Figure 6. VSF spectra of 8.6 µM solutions of n-decyl β-D-maltopyranoside (C10Maltoside) under the ssp polarization combination. The spectrum of water is also included for reference. The concentration shown corresponds to the range where patches of unperturbed surface water are still observed (cmc at 2200 µM). Note the striking difference in the bonded OH range when compared to that shown in Figures 4 and 5 for C12E5.
In the ssp spectra of Figure 5, interesting changes are also observed in the bonded OH region (3000-3500 cm-1), the spectral features of which mainly originate from water molecules interacting with the surfactant headgroup.24 With the disappearance of the sharp free OH peak a rapid change in the overall shape of bonded OH band is observed, where an increase in the intensity in the ∼3400 cm-1 region is clearly noticed, only slightly increasing at higher concentrations. At this point, it is worth drawing attention to the significance of this band in the VSF spectra. As mentioned above, only molecules with a preferred orientation give rise to the SF signal. The intensity observed in this region corresponds to ordered water molecules hydrating the ethylene oxide headgroup. In contrast to ionic surfactants where the electrostatic surface field is known to induce an enhanced ordering of the interfacial water molecules55-57 effectively increasing the intensity of the bonded OH band, the band observed in the C12E5 case results from short range interactions of water molecules with the EO chain. In Figure 6, the ssp spectrum of another non-ionic surfactant with a sugar headgroup (n-decyl β-D-maltopyranoside) is shown to emphasize the peculiarity of the bonded OH band observed for surfactants with a poly(ethylene oxide) headgroup. The concentration of 8.6 µM corresponds to the end of the transition range or surface micellar phase where patches of unperturbed water are observed, as can be deduced by the presence of the free OH peak in the spectrum.53 We argue that the sharp free OH intensity is the result of the structuring and cooperative interactions of a certain number of water molecules in the surrounding liquid, and not due to single isolated molecules. When solute molecules are adsorbed, these domains are perturbed, and the intensity of the free OH peak as well as the bonded OH band from water molecules supporting this structure are expected to decrease. This is the case of the C10Maltoside spectrum shown in Figure 6 where the decrease in the intensity observed in the bonded OH region is correlated with the decrease of the free OH (this situation is also observed for other non-charged adsorbates at the air/liquid interface like formic and acetic acid23,58). The decreased intensity in the bonded OH region at this concentration does not imply that there is no water molecules hydrating the sugar headgroup, but simply that most of these water molecules have no net orientation and as such are not detectable in the VSF spectra. This situation is in stark
Tyrode et al. contrast to the one noticed for C12E5. The hydration of the maltoside headgroup will be discussed in a forthcoming publication. Accordingly, the strong intensity observed in the bonded OH region of Figures 4 and 5 indicates that the ethylene oxide chain is inducing a significant structuring of the water molecules in the hydrating layer. The overall shape of the band is similar to that of pure water, where two major contributing bands can be distinguished centered at ∼3250 and ∼3400 cm-1, supporting the proposal of Kjellander11 that an ethylene oxide chain, unlike chains of other polyethers, fits into the dynamic structure of water. As mentioned above, the frequency of the OH stretching vibrations depends on the number and strength of hydrogen bonds formed by the water molecules contributing to the measured intensity. Therefore, the structured layer is seen to be constituted by ordered tetrahedrally coordinated water molecules similar to the ones observed in bulk ice and also by ordered molecules forming weaker hydrogen bonds. At concentrations below 1 µM the stronger hydrogen-bonded configuration seems to be preferred, while at higher concentrations it is the weaker hydrogen-bonded band that dominates. This finding is rationalized as follows: at low concentrations where patches of free water are still observed at the surface, the PEO chains are less constrained and are able to adopt a configuration with a larger number of strongly hydrogen-bonding water molecules in the hydration shell. As the concentration is increased, the space available to each headgroup and corresponding hydration shell diminishes, promoting a less strongly hydrogen-bonding structure. Nonetheless, in the ppp spectra at high concentrations a weak band appears around ∼3130 cm-1, which is even lower than the center frequency observed in ice, clearly indicating that water molecules forming strong hydrogen bonds also take part in the structured hydration shell. It is important to point out that the structured shell around the EO groups is not static and should be considered as a dynamic cage formation, which is constantly forming and rearranging. This will become evident when considering the orientation of the poly(ethylene oxide) headgroup, as discussed in the following section. (c) Poly(ethylene oxide) Headgroup. In the previous discussion of the CH stretching vibrations, the contributions from the methylene groups of the headgroup, which are almost as many as those in the alkyl chain, were deliberately ignored. The methylene stretching modes of the EO groups are expected at essentially the same frequencies as those from the hydrocarbon chain, and in principle there is no direct way to separate these contributions in the measured spectra of the fully perprotonated surfactant. To resolve this dilemma, SF spectra of a C12E5 isotope with the alkyl chain fully deuterated (d-C12E5) were measured at a concentration above the cmc (saturated monolayer). The heavier mass of the deuterium atom displaces the stretching vibrations of the methylene and methyl modes of the surfactant tail to considerably lower frequencies (CD stretching region spans from 2050 to 2250 cm-1), allowing them to be spectrally differentiated from the CH2 modes of the headgroup, which should appear in the CH stretching range (2800-3000 cm-1). In Figure 7, the VSF spectra of a 130 µM solution of d-C12E5 taken under the polarization combinations ssp, ppp, and sps are shown. In clear contrast to the C12E5 spectra of Figure 5, no resonant features are observed in the ppp and sps spectra, while only a very weak intensity at ∼2850 cm-1 (d+ mode) is discerned in the ssp spectrum. The interpretation of these results is that the PEO headgroup is randomly oriented at the liquidair interface, in particular when considering previous Raman
Structure and Hydration of PEO Surfactants
Figure 7. VSF spectra of 130 µM solution of d-C12E5 under the polarization combinations ssp, ppp, and sps at 10 and 30 °C. The ssp and ppp spectra have been offset for clarity by 3 and 6 au, respectively. The absolute intensity is normalized to the free OH peak of pure water (10 au) and can be directly compared to the previous spectra.
Figure 8. VSF spectra of C12E4, C12E5, and C12E8 at concentrations above the cmc collected under the ssp polarization combination at 20 °C in the C-O-C stretching region. (The SF intensity arbitrary units in this figure are not comparable to those presented for the CH and OH stretching regions.)
measurements,59,60 which indicate that most of the ethylene segments are found in a gauche conformation and consequently should be SF active. Similar conclusions have also been reported in the literature for d-C12E3 adsorbed on a hydrophobic ODT/ Au surface,13 and oligo(ethylene oxide) terminated thiols.15 The weak intensity in the ssp spectrum of the CH region could be explained by an incomplete deuteration of the hydrocarbon chain (the isotopic purity in the chain is 98%) or by some small degree of order in the headgroup itself. Although omitted for clarity in Figure 7, the OH stretching region showed no major differences from those presented in Figure 5 for the fully perprotonated C12E5 at 20 °C. The importance of the temperature measurements will be discussed in the last section. To further explore the poly(ethylene oxide) headgroup orientation, VSF spectra were also gathered in the spectral region corresponding to the C-O-C stretching vibrations of the ether bonds. In Figure 8, the ssp spectra of C12E4, C12E5, and C12E8 solutions above the respective cmc’s at 20 °C are shown. The appearance of this weak band centered at ∼1085 cm-1 assigned to the C-O-C stretch61,62 indicates that the headgroup has some preferred orientation. The fact that the intensity actually diminishes with increasing number of EO units, which within experimental error scales with the increasing area per molecule at cmc, supports the view that the signal arises from a very
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11649
Figure 9. VSF spectra of neat water under the polarization combination ssp acquired at 10 and 40 °C.
small section of the poly(ethylene oxide) headgroup. This section is most likely the first EO segment attached to the alkyl chain. The experimental finding indicating that the poly(ethylene oxide) headgroup at the air/water interface is essentially randomly orientated is in disagreement with recent molecular dynamic simulations of C12E5 at the air-water interface,63,64 where the EO chains were depicted as ordered rods aligned perpendicular to the surface plane. Our results suggest that the theoretical picture may not yet be complete. Another interesting fact that stems from the spectra presented above is that while the water molecules hydrating the surfactant headgroup are oriented forming an intricate structured layer involving hydrogen bonds of different strength and coordination, the EO chains themselves have no net orientation behaving as in an isotropic bulk. (d) Temperature Dependence. As briefly discussed in the Introduction, ethylene-oxide based surfactants and polymers have the unusual property that they phase separate when the temperature is increased. Before exploring the effect of temperature on the VSF spectra of ethylene oxide based surfactants, it is important to consider the effect of this variable in the spectra of the neat water surface. In Figure 9, the VSF spectra of pure water taken under the ssp polarization combination at two different temperatures are shown. The overall shape of the bands is not seen to change over the temperature range considered. Previous VSFS experiments carried out in a wider temperature range65 showed that at higher temperatures, a decrease in the lower frequency side (3200 cm-1) and an increase on the “liquidlike” band centered at ∼3400 cm-1 are observed, which evidence a disordering of the surface structure at higher temperatures. In Figure 10, the VSF spectra of C12E4 and C12E8 solutions above their respective cmc’s are shown, under the polarization combinations ssp, ppp, and sps for 10 and 40 °C. The cloud point or onset of phase separation for 1% weight solutions of these two surfactants is approximately 10 and 80 °C, respectively,66 and should be slightly higher for the concentrations used here. Moreover, the molecular areas of the adsorbed surfactants are essentially invariant with temperature in the range considered;67 therefore, any variation in the spectral features can be directly linked to changes in the preferred orientation of the adsorbed molecules. The scale allows the OH stretching region to be examined in detail. Nonetheless, the major differences between the two different surfactants reside in the CH stretching range, where the C12E4 surfactant tail is seen to adopt a more upright configuration with fewer gauche defects than the C12E8 counterpart (a detailed view of the CH stretching
11650 J. Phys. Chem. C, Vol. 111, No. 31, 2007
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Figure 10. VSF spectra of C12E4 and C12E8 solutions above the respective cmcs under the polarization combinations ssp, ppp, and sps at two different temperatures: 10 and 40 °C. Legend: 9 free OH peak of pure water; red filled circle, ssp 10 °C; red open circle, ssp 40 °C; blue filled triangle, ppp 10 °C; blue open triangle, ppp 40 °C; green filled triangle, sps 10 °C; green open triangle, sps 40 °C. Spectra showing in detail the CH stretching region can be found in the Supporting Information (Figure A3).
region can be found in the Supporting Information). This effect has been directly linked to the higher surface density of C12E4 monolayer and not to the difference in the length of the EO chain.43 On the other hand, only a small decrease in the overall intensities is observed in the OH bonded region, in particular for the ssp polarization combination when the temperature is raised by 30 °C. A similar behavior with temperature was observed for C12E5 solutions (data not shown). In the literature, three different mechanisms have been proposed to rationalize the anomalous behavior of EO containing compounds with increasing temperature.68 In the conformational model, the peculiar behavior of EO chains is attributed to a change in their conformation with temperature.12 At low temperatures, the preferred conformation around the C-C bonds is of a gauche type, while at high temperatures the trans conformation is favored. As demonstrated in the previous sections, VSFS is particularly sensitive to the conformational order of hydrocarbon chains and should be able to provide direct evidence to support this model. However, when increasing the temperature from 10 to 30 °C, no changes were observed in the CH stretching of the d-C12E5 spectra shown in Figure 7 (within experimental error no changes with temperature were observed for the C-O-C stretching vibration either). These results do not necessarily indicate that the changes proposed by the conformational model do not occur, because the lack of a preferred orientation of the whole headgroup could explain the absence of a change. Nonetheless, we argue that the fact that the headgroups are significantly disordered while the water molecules surrounding them are clearly ordered may add support to the converse argument, that changes in the conformation of the headgroup are in fact induced by changes in the hydration layer. The other two models explain the clouding phenomena from the water perspective. However, they fundamentally differ in the origin of this behavior. The first of these models11,69 (Kjellander’s model) proposes that the hydration of the PEO chain is similar to the one observed for nonpolar solutes, where they are surrounded by a water clathrate structure with the added advantage that they can form hydrogen bonds with the ether oxygens. The solubility at low temperatures is explained by a fine balance between the enthalpy and entropy term. As temperature is increased, the entropy term augments faster and the PEO chains associate. In this model, it is the actual hydration that drives the phase separation. The second model is based on a hydrogen-bond mechanism, and it is proposed that the
surfactant headgroup simply becomes dehydrated at elevated temperatures.7-10 The water molecules in this model are not described to form any particular structure around the PEO chains. The VSF spectra shown in this Article demonstrate that the water molecules hydrating the headgroup are ordered forming a structured layer surrounding the PEO chains. The small changes observed in the ssp spectra as the temperature is increased by 30 °C indicate some disordering of this structure at higher temperatures but no major breakdown of the hydration shell. These facts are consistent with Kjellander’s model. Summary and Conclusions Vibrational sum frequency spectroscopy has provided unique information about the adsorption behavior of ethylene oxidebased surfactants, in particular C12E5. In the 2D gas phase or Henry concentration range, no differences can be observed in the measured spectra from those of pure water under the ppp and ssp polarization combinations. It is not until the end of the Henry range when the molecular areas are close to 400 Å2 that measurable changes are detected in the three polarization combinations studied. The surfactant molecules are then inferred to lie on the surface with their hydrocarbon tails close to the surface plane. This preferred conformation is observed to be essentially maintained until areas per molecules close to ∼65 Å2. In this surface density range (400-65 Å2) the sharp “free OH” peak in the spectra indicates that patches of unperturbed surface water are present on the surface. The observed results are consistent with the formation of disk-like “surface micelles” with a flat orientation of the amphiphiles at low surface concentrations, which have been proposed in the literature.25,53,70,71 The disappearance of the free OH peak at ∼65 Å2 coincides with a rapid change in the orientation of surfactant tails, which adopt a more upright configuration as the surface covering liquid expanded layer is formed. Further increasing the concentration promotes a reduction in the number of gauche defects in the monolayer. However, a significant number of gauche defects remain even above the cmc. Experiments targeting the C-O-C stretching region in conjunction with spectra taken for an isotope of C12E5 having a perdeuterated alkyl chain lend support to the conclusion that most of the headgroup is essentially randomly oriented with probably only the first EO segment attached to the alkyl chain displaying some preferred orientation. This is in contrast to
Structure and Hydration of PEO Surfactants previous results where the orientational changes of smaller surfactant headgroups were targeted, in particular ionic surfactants with sulfate72 and carboxylate groups (with a fluorocarbon chain),25 where they were shown to remain ordered even at low surface densities, displaying only small orientation changes along the adsorption process. In the OH stretching region, also interesting results are observed. The random orientation of the headgroup allows deducing that the terminal OH of the ethylene oxide chain does not contribute to the OH bands, and only water molecules are responsible for the intensity observed in this region. The spectra reveal that the hydration shell of the surfactant headgroup is particularly ordered and structured, especially when compared to other nonionic surfactants with sugar-based headgroups. Three major bands centered at 3130, 3250, and 3400 cm-1 can be distinguished from the spectra. The first two bands are characteristic of strong hydrogen-bonded, highly coordinated water molecules, while the latter is typical of more weakly bonded water molecules like those observed in bulk water. It is important to recall that only molecules with a preferred orientation give rise to the SF signal, and as a consequence the water molecules responsible for the spectral features observed, which participate in the formation of this complex cage structure around the EO headgroup, are ordered. The spectra presented here also suggest that at low concentrations, when the poly(ethylene oxide) chains do not interact significantly with each other, the hydrating structure involves more highly coordinated water molecules than those observed at higher concentrations. Interestingly, the results presented here indicate that while the EO headgroup has no preferred orientation, the water molecules surrounding this polar group are significantly oriented. Moreover, interference between the free OH band and the 3580 cm-1 band observed in the ppp and sps spectra indicates that both bands have the same phase, adding support to the previous proposed “non-donor” assignment for this band.24 Experiments at different temperatures show no indication of a change in orientation of the headgroup, as proposed by the conformational model. The ordered structure observed around the poly(ethylene oxide) headgroups and the small disordering of this hydrating structure at higher temperatures support the model presented by Kjellander to explain the phase behavior of PEO-based surfactants and polymers. Acknowledgment. Financial support from the Swedish Research Council (VR) and the Swedish Council for Strategic Research (Stiftelsen fo¨r strategisk forskning - SSF) is gratefully acknowledged. M.W.R. is a VR research fellow and also thanks the Biofibre Materials Center (BIMAC) at KTH. We thank Dr. A. Kumpulainen for valuable discussions and for kindly reviewing this manuscript. We also thank J. Iruthayaraj (KTH) for collecting some of the surface tension data. Supporting Information Available: Spectra showing the CH stretching region in detail. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gaylord, N. G. Polyacetals and other polyethers; Interscience: Newark, 1963; Vol. 13. (2) Bailey, F. E., Jr.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (3) Shinoda, K.; Arai, H. J. Phys. Chem. 1964, 68, 3485. (4) Tiddy, G. J. T. Phys. Rep. 1980, 57, 3. (5) Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2735.
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11651 (6) Gombotz, W. R.; Wang, G.; Horbett, T. A.; Hoffman, A. S. J. Biomed. Mater. Res. 1991, 25, 1547. (7) Goldstein, R. E. J. Chem. Phys. 1984, 80, 5340. (8) Matsuyama, A.; Tanaka, F. Phys. ReV. Lett. 1990, 65, 341. (9) de Gennes, P.-G. C. R. Acad. Sci. Paris 1991, 313, 1117. (10) Bekiranov, S.; Bruinsma, R.; Pincus, P. Europhys. Lett. 1993, 24, 183. (11) Kjellander, R. J. Chem. Soc., Faraday Trans. 2 1982, 78, 2025. (12) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962. (13) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (14) Zolk, M.; Eisert, F.; Pipper, J.; Herrwerth, S.; Eck, W.; Buck, M.; Grunze, M. Langmuir 2000, 16, 5849. (15) Balamurugan, S.; Ista, L. K.; Yan, J.; Lopez, G. P.; Fick, J.; Himmelhaus, M.; Grunze, M. J. Am. Chem. Soc. 2005, 127, 14548. (16) Wang, R. Y.; Himmelhaus, M.; Fick, J.; Herrwerth, S.; Eck, W.; Grunze, M. J. Chem. Phys. 2005, 122, 164702/1. (17) Bain, C. D. Non-linear optical techniques; Surfactant Science Series; Dekker: New York, 1999; Vol. 83, p 335. (18) Shen, Y. R. Sum frequency generation (SFG) spectroscopy. In Nonlinear Spectroscopy for Molecular Structure Determination; Field, R. W., Hirota, E., Maier, J. P., Tsuchiya, S., Eds.; Blackwell Science: Oxford, UK, 1998; p 249. (19) Richmond, G. L. Chem. ReV. 2002, 102, 2693. (20) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. ReV. B: Condens. Matter Mater. Phys. 1999, 59, 12632. (21) Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B.-H. Int. ReV. Phys. Chem. 2005, 24, 191. (22) Lunkenheimer, K.; Pergande, H. J.; Krueger, H. ReV. Sci. Instrum. 1987, 58, 2313. (23) Johnson, C. M.; Tyrode, E.; Baldelli, S.; Rutland, M. W.; Leygraf, C. J. Phys. Chem. B 2005, 109, 321. (24) Tyrode, E.; Johnson, C. M.; Kumpulainen, A.; Rutland, M. W.; Claesson, P. M. J. Am. Chem. Soc. 2005, 127, 16848. (25) Tyrode, E.; Johnson, C. M.; Rutland, M. W.; Day, J. P. R.; Bain, C. D. J. Phys. Chem. C 2007, 111, 316. (26) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Phys. ReV. Lett. 1993, 70, 2313. (27) Wei, X.; Shen, Y. R. Phys. ReV. Lett. 2001, 86, 4799. (28) Liu, D.; Ma, G.; Levering, L. M.; Allen, H. C. J. Phys. Chem. B 2004, 108, 2252. (29) Raymond, E. A.; Richmond, G. L. J. Phys. Chem. B 2004, 108, 5051. (30) Tyrode, E.; Johnson, C. M.; Baldelli, S.; Leygraf, C.; Rutland, M. W. J. Phys. Chem. B 2005, 109, 329. (31) Gan, W.; Wu, D.; Zhang, Z.; Feng, R.-r.; Wang, H.-f. J. Chem. Phys. 2006, 124, 114705/1. (32) Buch, V. J. Phys. Chem. B 2005, 109, 17771. (33) Perry, A.; Neipert, C.; Kasprzyk, C. R.; Green, T.; Space, B.; Moore, P. B. J. Chem. Phys. 2005, 123, 144705/1. (34) Morita, A. J. Phys. Chem. B 2006, 110, 3158. (35) Wei, X.; Miranda, P. B.; Zhang, C.; Shen, Y. R. Phys. ReV. B: Condens. Matter Mater. Phys. 2002, 66, 085401/1. (36) Carey, D. M.; Korenowskli, G. M. J. Chem. Phys. 1998, 108, 2669. (37) Perry, A.; Ahlborn, H.; Space, B.; Moore, P. B. J. Chem. Phys. 2003, 118, 8411. (38) Morita, A.; Hynes, J. T. Chem. Phys. 2000, 258, 371. (39) Wilson, K. R.; Cavalleri, M.; Rude, B. S.; Schaller, R. D.; Nilsson, A.; Pettersson, L. G. M.; Goldman, N.; Catalano, T.; Bozek, J. D.; Saykally, R. J. J. Phys.: Condens. Matter 2002, 14, L221. (40) Kuo, I. F. W.; Mundy, C. J. Science 2004, 303, 658. (41) Shen, Y. R. Solid State Commun. 1998, 108, 399. (42) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982. (43) Goates, S. R.; Schofield, D. A.; Bain, C. D. Langmuir 1999, 15, 1400. (44) Bell, G. R.; Bain, C. D.; Ward, R. N. J. Chem. Soc., Faraday Trans. 1996, 92, 515. (45) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. ReV. Lett. 1987, 59, 1597. (46) Lu, R.; Gan, W.; Wu, B.-h.; Zhang, Z.; Guo, Y.; Wang, H.-f. J. Phys. Chem. B 2005, 109, 14118. (47) Hirose, C.; Akamatsu, N.; Domen, K. J. Chem. Phys. 1992, 96, 997. (48) Bell, G. R.; Li, Z. X.; Bain, C. D.; Fischer, P.; Duffy, D. C. J. Phys. Chem. B 1998, 102, 9461. (49) Lu, R.; Gan, W.; Wu, B.-H.; Chen, H.; Wang, H.-F. J. Phys. Chem. B 2004, 108, 7297. (50) Lu, J. R.; Hromadova, M.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2417. (51) Lu, J. R.; Su, T. J.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. J. Phys. Chem. B 1997, 101, 10332. (52) Lu, J. R.; Li, Z. X.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2408.
11652 J. Phys. Chem. C, Vol. 111, No. 31, 2007 (53) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C.; Tyrode, E. C.; Johnson, C. M. Langmuir 2005, 21, 305. (54) Shen, Y. R.; Ostroverkhov, V. Chem. ReV. 2006, 106, 1140. (55) Gragson, D. E.; McCarty, B. M.; Richmond, G. L. J. Am. Chem. Soc. 1997, 119, 6144. (56) Ostroverkhov, V.; Waychunas, G. A.; Shen, Y. R. Chem. Phys. Lett. 2004, 386, 144. (57) Miranda, P. B.; Du, Q.; Shen, Y. R. Chem. Phys. Lett. 1998, 286, 1. (58) Tyrode, E. C. Vibrational Sum-Frequency Spectroscopy studies at the Air-Liquid Interface. Ph.D. Thesis, Royal Institute of Technology (KTH), Stockholm, 2005. (59) Gaufres, R.; Sportouch, S.; Ammour, J. E.; Maillols, J. J. Phys. Chem. 1990, 94, 4635. (60) Marinov, V. S.; Nickolov, Z. S.; Matsuura, H. J. Phys. Chem. B 2001, 105, 9953. (61) Colthup, N.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, 1990. (62) Enriquez, E. P.; Granick, S. Colloids Surf., A 1996, 113, 11.
Tyrode et al. (63) Kuhn, H.; Rehage, H. J. Phys. Chem. B 1999, 103, 8493. (64) Kuhn, H.; Rehage, H. Colloid Polym. Sci. 2000, 278, 114. (65) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 3292. (66) Jonsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: London, 1998. (67) Penfold, J.; Staples, E.; Tucker, I.; Thompson, L.; Thomas, R. K. J. Colloid Interface Sci. 2002, 247, 404. (68) Karlstrom, G.; Lindman, B. Phase Behavior of Nonionic Polymers and Surfactants of the Oxyethylene Type in Water and in Other Polar Solvents. In Organized Solutions: Surfactants in Science and Technology; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker: New York, 1992; p 49. (69) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2053. (70) Israelachvili, J. Langmuir 1994, 10, 3774. (71) Meister, A.; Kerth, A.; Blume, A. J. Phys. Chem. B 2005, 109, 6239. (72) Johnson, C. M.; Tyrode, E. Phys. Chem. Chem. Phys. 2005, 7, 2635.
