Development of Heterodyne-Detected Total Internal Reflection

Article Cite This: J. Phys. Chem. C 2017, 121, 25206-25214

pubs.acs.org/JPCC

Development of Heterodyne-Detected Total Internal Reflection Vibrational Sum Frequency Generation Spectroscopy and Its Application to CaF2/Liquid Interfaces Naoki Takeshita, Masanari Okuno, and Taka-aki Ishibashi* Department of Chemistry, School of Pure and Applied Sciences, University of Tsukuba 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan S Supporting Information *

ABSTRACT: We present heterodyne-detected total internal reflection vibrational sum frequency generation (HD-TIR VSFG) spectroscopy for CaF2/liquid interfaces. With this technique, absolute orientations at solid/liquid interfaces can be determined by measuring complex χ(2) spectra of the buried interfaces. We applied the technique to CaF2/sodium dodecyl sulfate (SDS) solution interfaces, and directly determined the polar orientations of the water molecules and surfactants at the interface based on the phase information on VSFG spectra, which is unavailable in conventional homodynedetected TIR VSFG spectroscopy. The reorientation of the interfacial water molecules was observed as a change of the sign of Im[χ(2)] depending on the surfactant concentration.

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INTRODUCTION Vibrational sum frequency generation (VSFG) spectroscopy is a powerful method for measuring vibrational spectra of interfacial species. It relies on a second-order nonlinear optical process, and thus VSFG signals can be generated only from systems without centrosymmetry, where the second-order nonlinear susceptibility (χ(2)) is nonzero. The technique can be applied to any interface accessible by light even if it is buried between bulk media. Buried interfaces, such as solid/liquid interfaces, play important roles in both basic sciences and technological applications.1 VSFG spectroscopy is the only currently available technique that can provide vibrational spectra of buried interfaces with interface specificity. When one or two of the probe laser beams for SFG spectroscopy shine a sample interface with incidence angles larger than their critical angles, total internal reflection (TIR) occurs and the VSFG signal intensity is enhanced by several orders of magnitude.2,3 VSFG spectroscopy in such TIR geometries (TIR VSFG) has been extensively used to probe buried interfaces such as interfaces between chemically modified glass prisms and aqueous solutions, and has given illuminating insight into molecular structures and dynamics with high sensitivity.4−7 Recently, phase measurement techniques have been introduced into VSFG spectroscopy. With the techniques, the phase of VSFG susceptibility can be obtained. They are called phasesensitive VSFG8−10 and multiplex heterodyne-detected (HD) VSFG.11−14 Especially HD-VSFG is advantageous in terms of signal acquisition efficiency, because a spectral domain interferogram for a wide spectral region can be recorded in one © 2017 American Chemical Society

measurement with a multichannel detector and converted into a complex χ(2) spectrum through Fourier analysis. Conventionally, in VSFG spectroscopy, the VSFG light intensity from the sample is measured. The VSFG light intensity is proportional to the absolute square of the VSFG susceptibility of the sample, |χ(2)|2. Therefore, the sign of the susceptibility, which reflects the polar orientation of interfacial molecules, is lost. It is often difficult to interpret |χ(2)|2 spectra due to the interference among several vibrational resonances and nonresonant background. Indeed, previous interpretations for the air/water interface proposed by conventional homodynedetected VSFG spectroscopy have been recently refuted by experimental measurements of the sign of χ(2).15−17 Nevertheless, the sign information can be sometimes retrieved by vibrational band shape analysis of VSFG intensity spectra in favorable cases. To achieve this, the sample should contain an element such as a substrate or vibrational bands, whose susceptibilities including their signs are known in advance, and the spectral quality, such as the wavenumber resolution and signal-to-noise ratio, should be good enough to permit such reliable analysis.18 In HD-VSFG, a spectrum with phase information is measured by utilizing the interference between the SFG field from the sample and a light field of the same frequency with a well-defined phase, called local oscillator (LO). The phase of χ(2) of the sample is finally calibrated by comparing the spectrum with that Received: August 17, 2017 Revised: October 2, 2017 Published: November 7, 2017 25206

DOI: 10.1021/acs.jpcc.7b08212 J. Phys. Chem. C 2017, 121, 25206−25214

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION Multiplex HD-TIR VSFG Spectrometer. HD-TIR VSFG spectroscopy was conducted on our multiplex HD-VSFG spectrometer that has been described elsewhere.21,22 Schematics of the optical configuration of the HD-TIR VSFG spectrometer are shown in Figure 1a. A Ti:sapphire regenerative amplifier

obtained in the same experimental condition from a reference sample whose χ(2) is known. In HD-VSFG spectroscopy, we obtain the χ(2) spectrum from a Fourier component corresponding to the product of the SFG field and LO. As a result, LO can serve as an “amplifier” of the SFG field of the sample, enhancing the sensitivity of VSFG. HD-VSFG was recently applied to interfaces buried in water; one interface was an interface between an octadecyltrichlorosilane (OTS) monolayer on fused silica and water,13 and the other was a buried silica/water interface.19 In the former study, the spectral changes of the CH stretching bands of alkyl chains due to the contact with water were observed. The latter study revealed the origin of the doublet shape of the hydrogen-bonded OH stretching band of water and the polar orientations of water molecules at different pHs. 19 A phase-sensitive VSFG spectrometer recently developed based on a nonlinear interferometer14 is an attractive alternative to obtain complex χ(2) spectra of buried interfaces because the interferometer may facilitate positioning a reference sample properly; however, this method has not been applied to buried interfaces yet. The combination of a TIR geometry with HD-VSFG is promising because it is expected to give rich and direct information on buried interfaces with high sensitivity. However, HD-TIR VSFG had not been realized partly due to a difficulty in the implementation of a phase reference sample. Another difficulty is that, in the case of TIR geometries, the Fresnel coefficients relevant to linear and nonlinear optics become complex numbers. Therefore, in order to obtain χ(2) spectra that can be directly connected to molecular structures and orientations, several complex optical factors should be properly eliminated from the effective susceptibility that depends on the experimental setup. We have overcome the difficulties and successfully performed HD-TIR VSFG spectroscopy. In this paper, we describe the instrumentation and the principle of measurement in detail, and then present an application to the interfaces between CaF2 and aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS). Richmond and co-workers intensively studied CaF2/aqueous solutions of SDS by conventional homodyne-detected TIR VSFG spectroscopy.20 They observed the OH stretching band of water molecules as a function of the SDS concentration. With the increase of the concentration, its intensity first decreased to almost zero, and then increased. They ascribed the observed intensity change to the reorientation of interfacial water molecules that was induced by surface charge reversal. The surface charge of CaF2 is positive at nearly neutral pH conditions due to the slight dissolution of F− ion of the solid surface without SDS. They interpreted that the charge was neutralized and then became negative by the adsorption of the anionic surfactant molecules on the surface. They also found that the interference between the OH stretching and the CH stretching bands in homodyne |χ(2)|2 spectra changed from destructive to constructive with increasing the SDS concentration. The change of the interference is consistent with the presumed water reorientation if we assume that the phases of the CH bands were unchanged irrespective of the SDS concentration. Note that the assumption has not been experimentally verified. Here, we directly determined the absolute phase of the OH and CH bands by HD-TIR VSFG spectroscopy. We experimentally confirmed that the interpretation and assumption of Richmond and coworkers on CaF2/aqueous solutions of SDS were correct.

Figure 1. (a) A schematic of HD-TIR VSFG setup. (b) Three-layer model.

(Legend Elite, Coherent) was used to generate laser pulses centered at 800 nm with a pulse duration of 100 fs. The amplifier produced ∼3.5 mJ of energy/pulse at the repetition rate of 1 kHz. The output was divided into two. Two-thirds of the output was introduced into a narrow-band second-harmonic generator (SHBC, Coherent) to generate spectrally narrow 400 nm (∼5 ps, ∼8 cm−1) light. The output of the second-harmonic generator was used to pump a white-light-seeded optical parametric amplifier (TOPAS-400 ps-WL, Coherent) to obtain a narrowband visible beam (wavelength, 764 nm; bandwidth, ∼10 cm−1). The other one-third of the fundamental output was used to pump a white-light-seeded optical parametric amplifier (TOPAS-800 fs, Coherent) to obtain a broad-band IR beam with a central wavelength of ∼3300 nm (∼3000 cm−1) with a full width at halfmaximum (fwhm) of 200 cm−1. The IR and visible beams were overlapped in a thin y-cut quartz plate (10 μm thick) to generate a broad-band sum frequency generation light as a local oscillator (LO). The transmitted visible and IR probe beams, and the LO were refocused onto a sample interface. Among the three incident lights, only the LO was passed through a fused silica plate (1.5 mm thick) between the quartz plate and the sample to delay in time. The LO and the SFG from the sample were passed through a polarization analyzer for selecting the probe polarization. Then, they were introduced into a polychromator (TRIAX550, Horiba Jovin Yvon, 1200 grooves/mm grating) after a prism monochromator (CT25-UV, JASCO) and interfered with each other in the frequency domain. The interfered light was finally detected by a LN-cooled CCD detector (LN/CCD-1340/400-EB, Roper Scientific). When we conducted HD-VSFG measurements at a TIR geometry, a CaF2 hemicylindrical prism (20 mm in diameter, Pier Optics) was used and the interface between the bottom flat surface of the prism and liquid or air was monitored. The probe beams that entered the prism through the round surface of the prism irradiated the bottom face. DPPC LB Monolayer. A Langmuir−Blodgett (LB) monolayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC; 25207


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from the sample interface, respectively. T is the time delay between them introduced by the 1.5 mm thick fused silica plate between the y-cut quartz and the sample. First, the raw spectrum is transformed into the time domain interferogram by the inverse Fourier transformation. Then, the interference component that corresponds to the fourth term in eq 1 is extracted by multiplying a filter function. The filter function used in this study was

Figure 2) was used as a sample for test measurements. The details of the preparation of DPPC monolayer are as follows. DPPC was

⎧1 (t ≤ −1.7 ps) ⎪ ⎪ ⎡ π (t + 1.0 ps) ⎤ ⎪ 0.54 + 0.46 cos⎢ ⎥ A (t ) = ⎨ 0.7 ps ⎦ ⎣ ⎪ ⎪ ( −1.7 ps < t < −1.0 ps) ⎪ ⎩ 0 (t ≥ −1.0 ps)

Figure 2. Molecular structure of DPPC.

purchased from Avanti Polar Lipids Inc. and chloroform (spectroscopic grade) was purchased from Nacalai Tesque. All reagents were used as received. Ultrapure water with a resistivity of 18.2 MΩ cm provided from an arium 611 UV (Sartorius) was used as the subphase. The CaF2 prism was soaked in toluene for 1 h, was rinsed with acetone, methanol, and water, and then was set in an ozone chamber for 3 min to eliminate possible surface contaminations. A DPPC monolayer was deposited on the prism surface with a KSV Minitrough 2000 (KSV, Finland) at room temperature (25 °C). The trough was made of Teflon and the size was 364 × 75 mm2. First, a Langmuir monolayer of DPPC was prepared by spreading 1 mg/mL chloroform solution onto a surface of water in the trough. After waiting 10 min to allow the solvent to fully evaporate, the monolayer was compressed at a speed of 10 mm/min. Then the DPPC monolayer was transferred to the prism surface by the vertical dipping method at a dipping speed of 10 mm/min and at a surface pressure of 30 mN/m. CaF2/SDS Solution Interface. SDS was purchased from Wako, and dissolved into ultrapure water with a resistivity of 18.2 MΩ cm provided from the arium 611 UV (Sartorius). The CaF2 prism was cleaned in the same way as described in the preparation of the DPPC LB monolayer. After cleaning, the prism was attached to a home-built Teflon cell that contained a liquid sample via an O-ring made of Viton for TIR VSFG measurement. Principle of Measurement and Spectral Analysis. In heterodyne-detected VSFG spectroscopy, the following steps are generally taken to obtain the correct χ(2) spectra of a sample from which molecular structures and orientations can be directly discussed: (1) the extraction of the signal field spectrum of a sample from a observed fringe pattern in the spectral domain, (2) the phase calibration to remove extrinsic factors, such as the optical layout of the sample part using a reference sample, and (3) the elimination of the contribution of the Fresnel factors and reflectivities from the spectrum. While the first point is no different whether a sample is measured in a TIR geometry or not, the second and third points are more problematic in HD-TIR VSFG than in an external reflection geometry in the air: the choice of the reference is not so trivial, and the Fresnel factors and reflectivities are complex numbers in a TIR geometry. The detailed procedure of obtaining a correct χ(2) spectrum in the SSP polarization combination (the SFG, visible, and IR beams are set to be S-, S-, and P-polarized) is as follows. The measured total intensity (I) in the heterodyne detection is expressed as12,23

The filtered interferogram is then transformed back to the frequency domain by the Fourier transformation. This gives a complex spectrum of Ẽ sampleẼ r,LO exp(−iωT). The obtained complex spectrum can be expressed in terms of the YYZ component of χ(2) of the sample (χ(2) YYZ,sample) as ̃ ̃ * exp( −iωT ) Esample Er,LO (2) ∝ iFSSP,sampleχYYZ Ẽ Ẽ r* E ̃ * exp( −iωT ) ,sample VIS IR S,LO,sample i,LO

(3)

where Ẽ VIS and Ẽ IR are the Fourier components of the electric field amplitude of the visible and IR probe laser at the interface, rS,LO,sample is the reflectivity of the S-polarized LO (rS,LO) at the sample interface, and FSSP,sample is the Fresnel factor of the SFG signal in the SSP combination. In this paper, we assume the three-layer model (Figure 1b) composed of two centrosymmetric media 1 and 2, and an interfacial layer. In the model, the visible and IR probe beams and the LO beam impinge on the interface from medium 1, the SFG beam (Ẽ sample) generates at the interface in the reflected direction, and the LO beam reflected propagates in medium 1. FSSP,sample and rS,LO,sample are expressed as24,25 FSSP,sample = L YY (ωSFG) L YY (ω VIS) LZZ (ωIR ) sin θ1,IR

rS,LO,sample =

n1 cos θ2 − n2 cos θ1 n1 cos θ2 + n2 cos θ1

(4)

(5)

where n1 and n2 are the refractive indices of media 1 and 2, respectively. θ1’s for the visible and IR probe beams are the incident angles of the respective beams, and θ1 for the SFG beam is obtained using the phase matching condition at the interface: n1(ωSFG)ωSFG sin θ1,SFG = n1(ω VIS)ω VIS sin θ1,VIS + n1(ωIR )ωIR sin θ1,IR

(6)

θ2’s can be obtained using Snell’s law: n2(ω) sin θ2 = n1(ω) sin θ1

(7)

LYY(ω) and LZZ(ω) can be calculated using the following equations:

̃ |2 = |Esample ̃ ̃ |2 + Esample ̃ * Er,LO ̃ I ∝ |Etotal |2 + |Er,LO exp(iωT ) ̃ ̃ * exp(− iωT ) + Esample Er,LO

(2)

2n1(ω) cos θ1 n1(ω) cos θ1 + n2(ω) cos θ2

(8)

⎛ n1(ω) ⎞2 2n2(ω) cos θ1 LZZ (ω) = ⎜ ⎟ n1(ω) cos θ2 + n2(ω) cos θ1 ⎝ n′(ω) ⎠

(9)

L YY (ω) =

(1)

where Ẽ sample and Ẽ r,LO are the Fourier components of the electric fields of the SFG signal from the sample and that of LO reflected 25208


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Figure 3. (a) A schematic of the HD-TIR VSFG measurement of a DPPC monolayer deposited on the surface of the CaF2 hemicylindrical prism. (b) Raw |Ẽ total|2 spectra obtained from the DPPC monolayer/air interface and CaF2/silver interface. (c) Complex spectrum obtained by dividing the Ẽ sampleẼ *LO exp(−iωT) spectra of the DPPC monolayer by the Ẽ silverẼ *LO exp(−iωT) spectra of the silver. (d) Complex χ(2) YYZ spectrum obtained by dividing the complex spectrum in (c) by the factor of exp(−57°i) that includes the phases of the Fresnel coefficients, reflectivities, and the χ(2) YYZ of the CaF2/silver interface as well as the Fresnel coefficient and reflectivities of the CaF2/air interface. (e) A schematic of the HD-VSFG measurement in the external reflection geometry of the DPPC monolayer on the CaF2 hemicylindrical prism. (f) χ(2) YYZ spectrum of the DPPC monolayer obtained in the external reflection geometry. The dotted lines in (d) and (f) are the results of the spectral fitting analysis using the model function composed of Lorentz functions and constant. The details of the fitting are provided in the Supporting Information.

where n′ is the refractive index of the interfacial layer. In this study, we calculated n′ using the following expression:26 n′ =

In the case of HD-TIR VSFG measurements for CaF2 interfaces, a thin silver film (∼200 nm thick) sputtered on a part of the bottom surface of the prism was used as the reference sample. The target sample spectrum Ẽ sampleẼ *LOexp(−iωT) and the reference spectrum Ẽ silverẼ ′LO * exp(−iωT) were alternately obtained by changing the positions of the prism along its axis of the hemicylinder. Dividing the Ẽ sampleẼ LO * exp(−iωT) by the Ẽ silverẼ ′LO * exp(−iωT) gives

n12 + n2 2 + 4 2(n1−2 + n2−2 + 1)

(10)

Note that FSSP,sample becomes complex and its absolute value substantially increases when θ1 is near the critical angle. This is the principle of the signal enhancement in TIR VSFG spectroscopy.3,27 ̃ ̃ LOexp(−iωT) In order to extract χ(2) YYZ,sample from the EsampleE* spectrum, we need a reference spectrum, Ẽ referenceẼ *LO exp(−iωT), measured on a reference sample whose χ(2) YYZ,sample is known. The optical path length between the media for LO generation (y-cut quartz) and a sample must be the same for the target and reference samples; otherwise the phases of Ẽ LO * , Ẽ VIS, ̃ and EIR cannot be canceled out. In the measurement of the air/ solid and air/liquid interfaces, a z-cut quartz crystal is usually used as the reference sample. It is not difficult to set the surface of the z-cut quartz at the same position as that of the target sample with an accuracy of a few micrometers. However, the air/quartz interface cannot be used for the determination of χ(2) YYZ,sample of solid/liquid interfaces, since the optical path lengths of the incident beams in the solid media are necessarily different from those in the air.

̃ ̃ * exp( − iωT ) Esample Er,LO E ̃ E ̃ ′ * exp( − iωT ) silver r,LO

=

=

* FSSP,samplerS,LO,sample (2) * χYYZ FSSP,silverrS,LO,silver ,silver

(2) χYYZ ,sample

* FSSP,samplerS,LO,sample

(2) χYYZ ,sample

(2) * χYYZ FSSP,silverrS,LO,silver ,silver

× exp(i{Arg[FSSP,sample] − Arg[rS,LO,sample] (2) − Arg[FSSP,silver] + Arg[rS,LO,silver] − Arg[χYYZ ]}) ,silver

(11)

χ(2) YYZ,silver

where FSSP,silver, rS,LO,silver, and are the Fresnel coefficient, reflectivity, and the YYZ suceptibility component of the CaF2/ silver interface, respectively. “Arg” is the function that returns the 25209


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Figure 4. (a) LYY,VIS (θ1,IR = 60°), (b) LZZ,IR (θ1,VIS = 71.8°), (c) FSSP (θ1,IR = 60°), and (d) rS,LO for the CaF2/air interface as a function of the incidence angles, respectively.

bottom surface of a CaF2 hemicylindrical prism in air. DPPC monolayers prepared by the vertical dipping method are oriented with their terminal methyl groups pointing toward the air.22 We compared the χ(2) YYZ,sample spectrum of the monolayer in a TIR geometry (Figure 3a) with the probe laser beams coming from the solid side to that in an external reflection geometry (Figure 3e) with the beams coming from the air side. The two spectra should have the same spectral shape with opposite signs. Indeed, the experimental results showed that they did, indicating the procedure by which we deduce the χ(2) YYZ,sample spectrum from the row |Ẽ total|2 spectrum observed in the TIR geometry is correct. The DPPC monolayer/air and CaF2/silver interfaces were measured in the TIR geometry (θ1,VIS = 71.8° and θ1,IR = 60°) and in the SSP polarization combination (Figure 3a). Figure 3b shows the raw |Ẽ total|2 spectra obtained from the DPPC monolayer and silver, respectively. Both spectra show fine fringe structures originating from the third and fourth terms of eq 1. * exp(−iωT) spectrum of DPPC monolayer and the The Ẽ sampleẼ LO * exp(−iωT) spectrum of silver were extracted from the Ẽ silverẼ LO raw |Ẽ total|2 spectra by the procedure described in the * exp(−iωT) by Experimental Section. Dividing Ẽ sample Ẽ LO Ẽ silverẼ *LOexp(−iωT) gave the complex spectrum shown in Figure 3c. The Fresnel factors and reflectivities for the CaF2/air and CaF2/silver interfaces were calculated using eqs 4−10. Parts a, b, c, and d of Figure 4 show LYY,VIS, LZZ,IR, FSSP, and rS,LO for the CaF2/air interface as a function of the incidence angles, respectively. Parameters used in the calculation are listed in Table 1. The Fresnel coefficients in the measurements were calculated to be

phase of a complex number. The Fresnel factors and reflectivities of the sample and silver substrate can be calculated using eqs 4−10 with appropriate optical constants. The phase of χ(2) YYZ,silver was estimated by a HD-VSFG measurement of a silver film formed on a part of a CaF2 planar substrate (1 mm thick). A CaF2 substrate/air interface, whose χ(2) YYZ,sample can be assumed to be real positive, was used as the reference sample. The phase of χ(2) YYZ,silver was determined to be 153° for the IR region of 2800−3000 cm−1. The absolute value of χ(2) YYZ,silver was not estimated since the susceptibility of CaF2/air is unknown. This means that we use the susceptibility of the sample relative to the absolute value of the susceptibility of the CaF2/silver interface as the ordinate of χ(2) YYZ,sample spectra in this paper. The relative susceptibility is expressed as follows: (2) χYYZ ,sample (2) |χYYZ | ,silver

=

̃ ̃ * exp(− iωT ) FSSP,silverrS,LO,silver * Esample Er,LO * * ̃ Er,LO ̃ ′ exp(− iωT ) FSSP,samplerS,LO,sample Esilver

× exp( −i{Arg[FSSP,sample] − Arg[rS,LO,sample] (2) ]}) − Arg[FSSP,silver] + Arg[rS,LO,silver] − Arg[χYYZ ,silver

(12)

Extensions to Polarization Combinations Other than SSP. We have described the method to obtain Cartesian component χ(2) YYZ of azimuthally isotropic interfaces from HDTIR VSFG observations in the SSP polarization combination. The procedure can be applied in much the same way to measurements in the SPS and PSS combinations to get χ(2) YZY and χ(2) ZYY components, respectively. In the case of PPP measurements, the procedure to analyze the observed data becomes more complicated than in the SSP, SPS, and PSS cases because several (2) (2) (2) Cartesian components (χ(2) XXZ, χXZX, χZXX, and χZZZ) contribute observed data in the PPP measurements. The outline of the procedures and the related Fresnel factors of the SPS, PSS, and PPP measurements are given in the Supporting Information.

Table 1. Parameters Used To Calculate the Fresnel Coefficients of the CaF2/Air Interface

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VIS

RESULTS AND DISCUSSION Validity of the Phase Measurement. The validity of the phase correction procedure was assessed by measuring a Langmuir−Blodgett (LB) monolayer of 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC; Figure 2) deposited on the 25210

IR

SFG, LO

λ (nm) nCaF228

764 1.431

3400 1.415

624 1.433

nair nsilver29

1 1.571 + 20.0i

1 1.667 + 20.6i

1 0.132 + 3.91i


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Figure 5), and SiO2/aqueous solution of a cationic surfactant, hexadecyltrimethylammonium bromide.33,34

FSSP,CaF2 /air = 0.96 exp(173°i) rS,LO,CaF2 /air = 1.00 exp( −122°i)

FSSP,CaF2 /silver = 1.03 × 10−3 exp(32°i) Figure 5. Molecular structure of SDS.

rS,LO,CaF2 /silver = 0.99 exp( −166°i)

We measured complex χ(2) spectra of CaF2/SDS aqueous solution interfaces. The concentrations of solutions were set to be 0, 0.04, 0.1, 0.4, 0.8, and 8 mM. The pHs of the solutions were 5.9−6.2. We carried out the HD-TIR VSFG measurements in the order of SDS concentration from lowest to highest. The incident angles of visible and IR lights (θ1,vis, θ1,IR) in the measurement were set to be 73.5 and 65°, respectively. The TIR condition was fulfilled with the visible angle, which was larger than the critical angle of 68° for the visible probe (764 nm). Figure 6 shows FSSP for the CaF2/water interface as a function of the incident angle of the visible light. Parameters used in the calculation are listed in Table 2.

With the calculated phases of the Fresnel coefficients and the phase of χ(2) YYZ,silver obtained in the earlier section (153°) ⎡ ⎤ * FSSP,CaF2 /airrS,LO,CaF 2 /air ⎥ = − 57° Arg⎢ (2) ⎢F ⎥ * ⎣ SSP,CaF2 /silverrS,LO,CaF2 /silverχYYZ ,CaF2 /silver ⎦

Therefore, dividing the complex spectrum in Figure 3c by the factor of exp(−57°i) gives a complex spectrum which is proportional to χ(2) YYZ spectrum of the DPPC monolayer in the TIR geometry (Figure 3d). The obtained χ(2) YYZ spectrum in the TIR geometry is dominated by five vibrational bands. According to previous studies,30 the positive bands at 2878 and 2939 cm−1 are assigned to the CH3 symmetric stretching (CH 3 (ss)) and Fermi resonance (CH3(FR)) modes, and the negative band at 2960 cm−1 is assigned to the CH3 antisymmetric stretching (CH3(as)) mode of terminal methyl groups of DPPC, respectively. The two positive bands at 2847 and 2902 cm−1 are attributed to the CH2 symmetric stretching bands of methylene groups split due to the Fermi resonance.30 The existence of the methylene bands in the spectrum indicates that the LB monolayer contained some gauche defects. The signs of the three methyl CH stretching bands indicate the average orientation of methyl groups with the H atoms pointing downward, i.e., toward the air. This is the expected orientation of the DPPC monolayer prepared by the vertical dipping method. This suggests that the χ(2) YYZ spectrum of the DPPC monolayer in the TIR geometry is correctly deduced by the method that we have described in the Experimental Section. Furthermore, the same DPPC monolayer was also measured in an external reflection geometry illustrated in Figure 3e. In this geometry, the Ẽ quartzẼ *LO exp(−iωT) spectrum, obtained from the air/z-cut quartz interface, was used as a reference. In TIR and external reflection geometries, we observed the identical monolayer placed in the two opposite orientations. Thus, the obtained χ(2) YYZ spectra in these two geometries should show identical vibrational bands with the opposite phases. Figure 3f shows the obtained χ(2) YYZ spectrum measured in the external reflection geometry. In fact, the χ(2) YYZ spectrum in the external reflection geometry did show a spectral shape almost identical to that in the TIR geometry except that their signs were opposite. This strongly confirms that HD-TIR VSFG spectroscopy developed in this study has a capability for providing an accurate χ(2) YYZ spectrum in the TIR geometry. Application to CaF2/SDS Solution Interface. Ionic surfactants in aqueous solutions adsorb on charged solid surfaces via electrostatic interactions and form aggregates via hydrophobic interactions between surfactant alkyl groups. The aggregates show various structures depending on the strength of the electrostatic and hydrophobic interactions.31−35 In particular, it has been proposed that interfacial charge reverses depending on the surfactant concentration, resulting in orientational flip-flop of interfacial water molecules for CaF2/aqueous solution of an anionic surfactant, sodium dodecyl sulfate (SDS;

Figure 6. FSSP for the CaF2/water interface as a function of the incident angle of the visible light. The incidence angle of IR beam was set to be 65°.

χ(2) YYZ spectra of the interface were obtained in the same manner as that of the DPPC monolayer on CaF2 in the air from the raw |Ẽ total|2 spectrum recorded in the TIR geometry. In the case of the CaF2/aqueous solution interface, due to the dispersion of the complex refractive index of water in the OH stretching region, the IR-frequency dependence of the Fresnel coefficient of the CaF2/water interface (FSSP,CaF2/water) cannot be ignored. Therefore, we used the phase correction factor calculated for each frequency of the data point in the measured spectra. The refractive index of CaF2 at each frequency was calculated using a dispersion formula in the literature,28 and those of water and silver at each frequency were obtained by interpolating literature values29,36 with the cubic spline curves. Figure 7 shows the obtained χ(2) YYZ spectra of CaF2/SDS solutions with various concentration interfaces. The Im[χ(2) YYZ] spectrum of the CaF2/neat water and CaF2/SDS solution interfaces at the concentrations of 0.04 and 0.1 mM showed a broad negative feature around 3250 cm−1. The band is ascribed to OH stretching modes of water molecules oriented by the electric field of the positive charge of the solid surface; F− ions at the CaF2 surface slightly dissolve into the aqueous phase, resulting in a positively charged surface.33 The band was assigned to water molecules tetrahedrally coordinated due to the surface charge of CaF2.20,37 This assignment needs to be reconsidered. The hydrogen-bonded water at air/water,38 SiO2/water,39 and CaF2/water37 interfaces exhibits a double peak feature in the 3000−3600 cm−1 region. Formerly, the lower frequency peak around 3200 cm−1 was assigned to the coupled, symmetric OH 25211


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The Journal of Physical Chemistry C Table 2. Parameters Used To Calculate the Fresnel Coefficients of CaF2/Water and CaF2/Silver Interfaces VIS

IR

SFG, LO

λ (nm) θ1 (deg) nCaF228

764 − 1.431

3300 65 1.416

620 − 1.433

nH2O36

1.330 + 1.60 × 10−7i

1.450 + 0.367i

1.332 + 1.36 × 10−7i

0.145 + 5.00i

1.571 + 20.0i

0.131 + 3.88i

nsilver

29

Figure 8. Schematics of the orientations of water and SDS molecules at (a) lower (0−0.04 mM), (b) intermediate (0.1 mM), and (c) higher (0.4−8 mM) SDS concentrations.

molecules should be due to the positively charged CaF2 surface. The amplitude of the OH stretching band at ∼3250 cm−1 decreased with increasing the concentration from 0 to 0.1 mM, indicating that the negative charge of the SDS headgroup adsorbed at the interface canceled the positive charge at the CaF2 surface (Figure 8b). In the conventional homodyne-detected VSFG studies, several assumptions were required to determine absolute orientations of molecules from interference patterns of vibrational bands or nonresonant background. With HD-TIR VSFG spectroscopy developed in this study, the absolute orientations of molecules can be directly determined from the sign of vibrational bands in Im[χ(2) YYZ] spectra, without any assumption. The sign of the OH stretching band changed from negative to positive at the higher SDS concentrations (0.4, 0.8, and 8 mM). This suggests that the orientation of water molecules at the interface flips as previously reported (Figure 8c).20 The reorientation of water molecules at the interface is probably due to the charge reversal of the interface; SDS molecules cancel the positive charge of the CaF2 interface and then the excess amount of SDS molecules adsorbs.20 Now, we focus on the CH stretching bands in 2800−3000 cm−1 of SDS aggregates (Figure 7b). Along with the decrease in intensity of the negative OH band, several bands appeared in the region of 2800−3000 cm−1. They were assigned to the CH stretching modes of the alkyl chains of SDS molecules. These bands show that SDS molecules adsorbed on the interface at 0.1 mM, which is 2 orders of magnitude smaller than the critical micelle concentration (cmc) of SDS (=8 mM). At the SDS concentration of 0.1 mM, two weak positive bands were observed at ∼2876 and ∼2938 cm−1. The two bands can be attributed to the symmetric stretching modes of the terminal methyl groups (CH3(ss)) split by a Fermi resonance.2 The amplitudes of bands in the CH stretching region increased with the concentration (0.4−8 mM). This indicates that the number of SDS molecules adsorbed on the interface increased with the concentration. A positive weak band at ∼2972 cm−1 can be ascribed to the antisymmetric stretching mode of methyl groups. A negative band at ∼2855 cm−1 and a positive band at ∼2904 cm−1 are assigned to the stretching modes of methylene groups.2 The signs of the methylene bands were difficult to determine by a fitting analysis of the conventional |χ(2)|2 spectra,20 but they were unambiguously determined in Im[χ(2) YYZ] spectra of the CaF2/SDS solution interface for the first time in this study.

Figure 7. (a) χ(2) YYZ spectra of the CaF2/neat water and CaF2/SDS solution interfaces. (b) Enlarged plot of Im[χ(2) YYZ] spectra in the CH stretching region.

stretching mode of water molecules in a highly ordered, tetrahedrally coordinated environment, and the higher frequency peak at ∼3400 cm−1 was assigned to those in a more disordered hydrogen bonding environment.37−39 However, recent VSFG studies using isotopically diluted water disproved this interpretation. It has been shown that the double peaks observed at the air/water15,40 and SiO2/water19 interfaces correspond to the OH symmetric stretching mode split by the Fermi resonance. Although isotopically dilute experiments have not yet been conducted on the CaF2/water interface, it is likely that the band observed in this study at ∼3250 cm−1 should be also assigned to the lower frequency component of the hydrogen-bonded OH stretching bands split by the intramolecular coupling of vibrational modes, rather than that of the tetrahedrally coordinated water molecules. The amplitude and sign of the OH stretching band at ∼3250 cm−1 varied with the SDS concentration, indicating the variations of the charge at the interfacial region. The negative sign of the OH band at lower SDS concentrations (0, 0.04 mM) clearly shows that water molecules were oriented with their hydrogen atoms toward bulk solution (Figure 8a). The alignment of water 25212


Article

The Journal of Physical Chemistry C Author Contributions

The structure of the SDS aggregates formed at the interface is discussed based on the signs of the methyl CH stretching bands. VSFG is forbidden in media with the inversion symmetry in the electronic dipole approximation; therefore, the clearly observed CH stretching bands at the higher SDS concentrations suggest that the SDS aggregates formed at the CaF2/solution interface had asymmetric structures across the plane parallel to the interface. Moreover, the positive sign of CH3(ss) and CH3(FR) bands at ∼2876 and ∼2938 cm−1 indicates that the methyl groups are on average aligned with their hydrogen atoms pointing down toward the bulk solution. At concentrations near the cmc, surfactants are expected to form a bilayer structure so that the exposure of the hydrophobic tails to water is avoided and the repulsion among the charged headgroups is decreasd.20 One interpretation consistent with our observation is that SDS molecules at the CaF2/SDS solution interface form an asymmetric bilayer structure composed of the disordered lower leaflet (distal leaflet) and the ordered upper one (proximal leaflet), which gives CH3 stretching bands.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This study is partially supported by JSPS KAKENHI Grants (JP26104504, JP16H00821 (Innovative Areas 2503) for T.I., JP15K17802 for M.O.) and by the Sasakawa Scientific Research Grant from The Japan Science Society for M.O. Notes

The authors declare no competing financial interest.

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CONCLUSION In this study, we have realized HD-TIR VSFG spectroscopy for CaF2/liquid interfaces for the first time. We employed a sputtered silver film on the CaF2 substrate as the reference of the phase, and the phase of the susceptibility of the CaF2/silver interface was calibrated by that of the CaF2/air interface. The procedure to deduce the VSFG susceptibility χ(2) of interfacial molecular layers by correcting the Fresnel factors and reflectivities has been described. This method enables us to obtain the sign of χ(2) and determine the absolute molecular orientations at solid/liquid interfaces with high sensitivity. The validity of the phase obtained in the method has been confirmed by a measurement of the DPPC monolayer in the air. The method was applied to the CaF2/SDS aqueous solution interface. From the measured complex χ(2) spectra, we directly determined the absolute orientations of the water molecules and surfactants at the interface, which could not be achieved by conventional homodyne-detected VSFG without any precondition. In particular, we observed the orientational flip-flop of the interfacial water molecules, which depends on the surfactant concentration, as the unambiguous sign of Im[χ(2)]. The present method may probably be applicable to solid/liquid interfaces other than CaF2/liquid interfaces as long as the material of the solid is transparent to the visible and IR probe lights. HD-TIR VSFG spectroscopy is expected to give rich and direct information on complex systems at solid/liquid interfaces with excellent sensitivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08212. Detailed discussion on HD-TIR VSFG spectroscopy in the SPS, PSS, and PPP polarization combinations, and fitting analysis of χ(2) spectra of a DPPC monolayer measured in TIR and external reflection geometries (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Taka-aki Ishibashi: 0000-0003-0000-547X 25213


Article

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Development of Heterodyne-Detected Total Internal Reflection

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