Intermolecular Interactions at the Interface Quantified by Surface

Article pubs.acs.org/JPCC

Intermolecular Interactions at the Interface Quantified by SurfaceSensitive Second-Order Fermi Resonant Signals Kangzhen Tian,†,‡ Baixiong Zhang,† Shuji Ye,*,†,‡ and Yi Luo†,‡ †

Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics and ‡Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: Accurate determination of intermolecular interaction forces at the surface and the interface is essential to identify the nature of interfacial phenomena such as absorption, interfacial assembly, and specific ion effect, but it still represents a major technical challenge. In this study, we proposed a novel method to deduce the interfacial interaction forces by using surface-sensitive second-order Fermi resonant signals, generated in sum frequency generation vibrational spectroscopy (SFG-VS). By investigating the influence of lipid chain length and intermolecular distance on the Fermi resonant signals of phospholipid monolayer at the air/CaF2 surface and the air/water interface, a linear correlation between the Fermi resonant intensity ratio and the dominated interactions in the lipid monolayer has been observed. It implies that the amplitude of the intensity ratio can be used as an effective in situ vibrational optical ruler to characterize the total intermolecular interaction forces at the surface and the interface. Such a relationship further enables us to elucidate the specific ion effects on the interfacial interactions, allowing us to identify different contributions from van der Waals, electrostatic, and hydration interactions. This study clearly demonstrates the power of the second-order Fermi resonant signals for evaluating the interfacial interaction forces in situ and in real time.

1. INTRODUCTION

specific ion effects on the interfacial interactions can be well elucidated by this method in situ. Vibrational spectral features, Fermi resonant signals in particular, are highly sensitive to the small conformation change of a molecule induced by its interaction with the chemical surroundings.8−10 Here, Fermi resonant interaction, originating from a special state in the vibrational spectrum, is referred as an intramolecular coupling between a fundamental vibration (ν1) and an overtone band (2ν2) of another vibration.11 In general, the overtone band is extremely weak (even spectrally invisible) by itself. However, a resonance can be generated when the bands of ν1 and 2ν2 have the same symmetry and nearly the same energy. This resonance can result in much enhanced intensity of overtone band through the borrowing of the intensity from the fundamental band.10,12 In principle, the smaller the difference in the two vibrational levels is, the greater the resonance becomes. As a result, small changes in the frequency of the fundamental vibration may cause significant changes in the overtone spectral profile.10,12 In theory, the Fermi resonant interactions can be explained as a combined effect of molecular anharmonicity of an isolated molecule and the intermolecular interaction potential between the single molecule and its neighboring molecules.13−20 For the same isolated molecule, the molecular anharmonicity and the

Intermolecular interactions at the surface and the interface play a crucial role in controlling interfacial phenomena such as absorption and interfacial assembly. Yet the lack of effective techniques for determining such interaction forces has made it very difficult, if not impossible, to get in-depth insights into the nature of interfacial phenomena at the molecular level. Consequently, many scientific problems associated with the interaction forces at the interface remain elusive. For example, a molecular-level description of the molecular interactions responsible for the specific ion effects (also known as Hofmeister effects) has been the subject of a long debate,1−3 for which several conceptually different interaction models have been proposed, such as the local binding model, ionic partitioning model, and the dispersion model.1−5 To interpret the ion specific forces between ions and solvent quantitatively, ionic dispersion forces, or the concept of matching water affinities, or the balance between ion pairing and nonpolar attraction have been introduced theoretically.1−7 However, none of them can offer a satisfactory explanation for the experimental observations on a broad scale. In this study, we introduced a novel method to deduce the interfacial interactions by tuning the second-order Fermi resonant signals of methyl group. With the assumption of a simple model, a relationship between the amplitude of second-order Fermi resonant intensity ratio and the total intermolecular interaction forces at the interface has been successfully established. The © 2015 American Chemical Society

Received: April 2, 2015 Revised: June 20, 2015 Published: June 24, 2015 16587

DOI: 10.1021/acs.jpcc.5b03204 J. Phys. Chem. C 2015, 119, 16587−16595

Article

The Journal of Physical Chemistry C

earlier publications.31,32 Unfortunately, the effect of Fermi resonant signals has been largely overlooked in the previous studies because of the serious overlap in frequency between the Fermi resonance and the asymmetric stretching mode of methyl group. In order to compensate for the Fermi resonant contribution, a correction factor of 1.33 has been employed to take into account the Fermi interaction in r+ and r+FR.33 It is not strictly correct to assume the Fermi interaction as a constant in a real study. To accurately detect the Fermi resonant signals, we used the multiplexed-polarization method to reduce the interference from the asymmetric stretching mode of CH3 group. Two SFG experimental geometries were used in our experiments (Figure S2). The geometry of CaF2/film interface was used to investigate the effect of lipid chain length on the PA lipid monolayer, whereas the geometry of air/DI water interface was used to study the influence of intermolecular distance and specific ions on the DLPC lipid monolayer. The experimental procedures at the air/DI water interface are similar to our earlier report.32 A computer-controlled KSV mini trough system was perpendicularly placed to the xz-plane of SFG geometry (Figure S2). A 20 μL of lipid solution was spread on a subphase of DI water (or salt solution) with an initial surface area of 243.0 cm2. It was then given 10−15 min to allow solvent evaporation and to reach surface pressure equilibrium. After that, the monolayer was compressed at a rate of 10 mm/min by moving two Teflon barriers to get a target surface pressure. Normally, at least 10 min was given to allow the lipid monolayer to reach equilibrium before polarized SFG spectra were collected. The SFG spectra from interfacial PA and DLPC lipid monolayer with different polarization combinations of ssp (s-polarized SFG output, s-polarized visible input, and ppolarized infrared input), 45°ΩVisp, and ppp were collected with the spectral resolution of 2 cm−1. All SFG spectra were averaged over 100 times at each point and normalized by the intensities of the input IR and visible beams. 2.3. Models of Lipid Molecular Interactions. It is wellknown that the interactions in the lipid monolayer are dominated by the total contributions from the electrostatic, van der Waals (vdW) interactions, headgroup hydration, chain configurations, and others.34−39 The interactions can be determined by measuring the intermolecular distance (rij) for lipid using surface pressure−area (π−A) isotherms. The vdW interactions between the lipid molecules principally rise from the interaction between alkyl chains, whereas the electrostatic interactions originate from the interactions between charged head groups. Theoretical models for these interactions have been given in many excellent literatures,34−39 which greatly help us to establish the correlation between the total interfacial molecular interactions and the second-order Fermi resonant signals. Following these models for the lipid molecule containing two linear alkyl chains, we can estimate the interactions of vdW, electrostatic, hydration, and chainconfiguration components. Here we defined the energy of the attractive interaction is negative while the energy of the repulsive interaction is positive. van der Waals (vdW) Interaction. To calculate the vdW interaction, some simplified assumptions have to be introduced to arrive at reasonable working models. Generally, each chain of a lipid molecule has a general formula of −(CH2)nCH3; therefore, the lipid chain may be simply considered as a cylinder of diameter d (nm) composed of CH2 groups spaced linearly at intervals of l = 0.127 nm, corresponding to the normal CH2−CH2 distance along an alkane chain. Considering

zero-order state energy will be the same. The intermolecular interaction energy between a single molecule and its neighboring molecules may be resolved into the summation of the contributions of electrostatic, induction, dispersion, and repulsion interactions.13−20 Thus, an alteration of Fermi resonant interaction can be used as an indication of a change in the balance of effective attractive and repulsive forces between the isolated molecule and its environments. The intensity ratio of the ν1 and 2ν2 bands (R2ν2/ν1 = I2ν2/Iν1) has been used for quantifying the intermolecular interaction by Raman spectra.10,12 We could anticipate that the second-order Fermi resonant signals, provided by surface-sensitive sum frequency generation vibrational spectroscopy (SFG-VS), might permit to obtain the interfacial interaction force. To materialize this idea, here we used phospholipids such as phosphatidic acid (PA) and phosphocholine as the models, to carry out the first detailed study on the correlation between the total interfacial molecular interactions and the second-order Fermi resonant signals. Phospholipids are important components of cell membrane and represent widely accepted models of macromolecular interactions.21 Furthermore, the Langmuir monolayer of phospholipid at the air/water interface has been frequently used as an excellent model system for lung surfactant, cell membrane, and two-dimensional materials at the asymmetric interface.22,23

2. EXPERIMENTAL AND THEORETICAL SECTION 2.1. Materials. 1,2-Dilauroyl-sn-glycero-3-phosphate (sodium salt) (DLPA), 1,2-dimyristoyl-sn-glycero-3-phosphate (sodium salt) (DMPA), 1,2-dipalmitoyl-sn-glycero-3-phosphate (sodium salt) (DPPA), 1,2-distearoyl-sn-glycero-3-phosphate (sodium salt) (DSPA), and 1,2-dilauroyl-sn-glycerol-3- phosphocholine (DLPC) were purchased from Avanti Polar Lipids (Alabaster, AL). The lipid molecular structures are shown in Figure S1. DLPA, DMPA, DPPA, and DSPA were dissolved in mixed solvents of chloroform and methanol (with a volume ratio of 2:1) (Sinopharm Chemical Reagent Co., Ltd.). DLPC was dissolved in chloroform solvent. All of the lipid solutions were kept at −20 °C. CaF2 windows were purchased from Chengdu Ya Si Optoelectronics Co., Ltd. (Cheng Du, China) and were thoroughly cleaned using the standard procedures given in ref 24. Substrates were tested using SFG, and no signal from contamination was detected. The PA lipid monolayers were prepared on CaF2 window substrates using Langmuir− Blodgett (LB) methods with a KSV mini trough LB system at the surface pressure of 30 mN/m. The PA lipid monolayer was stored in dry air for more than 24 h before experiments. It has been confirmed by SFG that no water layer was tracked between CaF2 prisms and lipid monolayer. The salts of potassium sulfate (K 2 SO 4), potassium chloride (KCl), potassium bromide (KBr), potassium nitrate (KNO 3 ), potassium iodide (KI), potassium perchlorate (KClO4), and potassium thiocyanate (KSCN) were purchased from Aldrich with a purity of >99.8%. All of the salts were baked for more than 6 h to remove the organic impurities. 2.2. SFG-VS Experiments. SFG-VS, a second-order coherent optical technique, has been extensively applied to investigate the interfacial molecular structures of many molecules including alcohol, lipid, polymer, and protein by monitoring C−H stretching vibrations of both CH2 and CH3 groups.25−30 The theory and instruments related to the SFG have been introduced in many literatures,25−30 including our 16588


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headgroup charges, corresponding to a centered hexagonal lattice for the chains (Figure 1), the electrostatic lattice sums for the positive and negative charges arranged on the regular two-dimensional lattice yield the following expression for the Coulomb self-energy:

one lipid molecule surrounded by six close-packed neighboring cylinders (Figure 1), we can sum the vdW interaction of any

Edip = −NAe 2M /(εrε0 AL / 3 )

(4) 35,36

where M is the Madelung constant. In the low potential approximation, the electrostatic interaction is inversely proportional to rij. Hydration Interaction. The hydration energy is given approximately by36 E hyd = −NAAL (χ /ξ)(2kT /e w)2 [cosh(e wψho/2kT − 1)] tan(d w /ξ)

(5)

where ψho is the hydration potential, χ is the orientational susceptibility of the water polarization, ξ is the correlation length of the water polarization, ew is the effective charge associated with the water polarization, and dw is the water layer thickness of the hydration layer. The hydration interaction is a function of AL. 2.4. Fermi Resonance and Intermolecular Interaction. In theory, the vibrational Hamiltonian of a molecule in the condensed phase is given as10−20

Figure 1. Upper view of the lipid hexagonal packing. The circles represent the lipid molecular area, and the solid dot represents each hydrocarbon chain.

H = H0 + Ha + U

one CH2 group in the central molecule with all the CH2 groups in the six adjacent CH2 groups in the same plane and the other CH2 groups that are one and two planes higher and lower using eq 1:35 EvdW = 2nc

where H0 and Ha are the harmonic and anharmonic parts of the Hamiltonian of an isolated molecule and U is the intermolecular interaction potential between the single molecule and its neighboring molecules. The intermolecular interaction energy U between a single molecule and its neighboring molecules may be resolved into the summation of the contributions of electrostatic, induction, dispersion, and repulsion.15,16

2 ⎤ 5 3α0 hν NA ⎡ 6 12 12 + 2 + ...⎥ ⎢ + 2 6 4(4πε0)2 2 ⎣ d 6 ⎦ [d + l 2]3 [d + (2l)2 ]3

(1)

where nc is the length of each lipid hydrocarbon chain. α0 and ε0 are the polarizability and the vacuum permittivity, respectively. NA is Avogadro’s number. Here for CH2 groups, α0/4πε0 =1.84 × 10−30 m3 and hν = 1.67 × 10−18 J.35 The interaction is multiplied by a factor of 2nc because there are two chains in each molecule, and the length of each chain is nc. The interactions between −CH2 groups in the same molecule are not considered because only intermolecular energies are estimated, and hence the interaction is multiplied by 5/6. The vdW interaction is inversely proportional to rij6.35,36 Electrostatic Interaction. The electrostatic interaction energy (Eel) can be calculated according to the electrostatic free energy estimated from the Gouy−Chapman diffuse doublelayer theory.34−39 2

(6)

U = ϕiel + ϕi ind + ϕidis + ϕi rep

(7)

If Ha + U is taken as a perturbation, the energy En of the nth vibrational state to second order and its wave function |ψn⟩ to first order are given by eqs 8 and 9.14,15 En = En0 + ⟨ψn|Ha + U |ψn⟩ +

∑′

|⟨ψμ|Ha + U |ψn⟩0 |2 En0 − Eμ0

μ

(8) 2

|ψn⟩ = |ψn⟩0 +

∑′

|⟨ψμ|Ha + U |ψn⟩0 | En0 − Eμ0

μ

2

Eel = 2NAAL (kT /e) εrε0 8N A e C /εrε0kT [cosh(eψel /2kT ) − 1] (2)

|ψμ⟩0 (9)

where E0n and |ψn⟩0 are the unperturbed energy and wave function, respectively, of the nth vibrational state. The part of the energy due to the interaction is

where AL is the mean molecular area per lipid molecule (MMA). ψel is the surface potential. εr is the relative dielectric constant of electrolyte solution, k is Boltzmann’s constant. C is the bulk ionic strength of the 1:1 electrolyte solution and equals to 10−7 M for pure water. ψel is the surface potential and can be determined from the surface charge density (σ) using the Gouy−Chapman theory.34−39 zeψel ez σ= = 8NAεrε0kTC sinh AL 2kT (3)

δEn = ⟨ψn|U |ψn⟩0 +

∑′

2Re⟨ψn|Ha|ψμ⟩0 ⟨ψμ|U |ψn⟩0 + |⟨ψn|U |ψμ⟩0 |2

μ

En0 − Eμ0 (10)

where Re means the real part of the complex quantity, and the frequency shift (Δ) of the transition |ψi⟩ → |ψf⟩ is Δ(|ψi⟩ → |ψf ⟩) = δE|ψf ⟩ − δE|ψi⟩

(11)

A nonzero value of ⟨ψn|Q|ψn⟩ describes the displacement of the average geometry of the molecule in the vibrational state |ψn⟩. The displacement caused by the intermolecular interaction is

The electrostatic interactions between the dipoles of phospholipids with zwitterionic headgroups can also contribute to the total interaction. For an open hexagonal lattice of the 16589


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The Journal of Physical Chemistry C δQ = 2Re ∑ ′

⟨ψn|Q |ψμ⟩0 ⟨ψμ|U |ψn⟩0

plane of the incidence is termed as p polarization while the one perpendicular to the plane of the incidence is s polarization. Under the SFG experimental geometry, the p-polarized light can be resolved into surface electric fields in both the x- and zaxes at the surface, whereas the s-polarized light has a component solely in the y direction. It is therefore possible to find one set of the polarization angles of the SFG signal, visible, and IR laser beams to satisfy χ(2) eff,as = 0 after considering the Fresnel coefficient constants. In this case, the effect of CH3 asymmetric stretching signals on the Fermi resonance can be eliminated since the asymmetric signals were suppressed. Details on the method are given in the Supporting Information. Here, one of the solutions for the polarization angles (Ω) of the SFG signal, visible, and IR laser beams to satisfy χ(2) eff,as = 0 is ΩSF = 45°, ΩVis = 74°−76°, and ΩIR = 0°. To confirm this analysis, we measured the SFG spectra of DSPA film using the polarization combination of 45°ΩVisp (Figure 2). The ssp and

En0 − Eμ0

μ

+ 2Re ∑ ′

⟨ψn|Ha|ψμ⟩0 ⟨ψμ|Q |ψk⟩0 ⟨ψk|U |ψn⟩0 (En0 − Eμ0)(En0 − Ek0)

μ

(12)

Fermi resonance occurs when a fundamental vibration mode (ν1) coupled with an overtone band (2ν2) of the same symmetry. According to eqs 6−12, the interaction between the fundamental and the overtone band can be also viewed as a perturbation of the original noninteracting system.10−20 The resulting observed frequency separation Δ = 2ν2 − ν1 has the relationship10−20 W = ⟨ψν0|Ha + U |ψ20ν ⟩ = 1

2

Δ2 − Δ0 2 /2

(13)

where Δ0 = (2ν2)0 − (ν1)0 is the wavenumber separation between the unperturbed harmonic levels in the absence of Fermi resonance. W is the Fermi coupling coefficient. The intensity ratio of the two bands in Fermi resonance R2ν2/ν1 in Raman spectra is given by R 2Raman ν2 / ν1 =

I2Raman ν2 IνRaman 1

=

Δ − Δ0 Δ + Δ0

(14)

here Iν1 is the intensity of the fundamental band and I2ν2 is the intensity of the combination or overtone band of the Raman spectra; both values refer to the bands perturbed by Fermi resonance. The equation was deduced under the simplified assumption that the intensity of the unperturbed combination or overtone is zero. According to eqs 13 and 14, R2ν2/ν1 is a very important quantity and very sensitive to Ha + U.10−20 Because Ha is the anharmonic part of the Hamiltonian of an isolated molecule, Ha will be the same for the same isolated molecule. In addition, the zero-order state energy will be the same for the same isolated molecule. Therefore, R2ν2/ν1 can be utilized to characterize the intermolecular interactions in condensed phase.12−20 Because SFG-VS is a second-order coherent optical technique, R2ν2/ν1 is given as eq 15. R 2SFG ν2 / ν1 =

χ2SFG ν 2

χνSFG 1

=

Δ − Δ0 Δ + Δ0

Figure 2. 45°ΩVisp SFG spectra of DSPA film on CaF2 window prepared at the surface pressure of 30 mN/m.

ppp polarization combinations are the most often used configurations for monitoring the C−H stretch vibrations of the alkyl chain.25−30 In general, the ssp spectra are contributed by the signals from the CH2 symmetric (∼2850 cm−1), CH3 symmetric stretch (∼2880 cm−1), CH2 asymmetric stretch (∼2920 cm−1), Fermi resonance of CH3 group (∼2940 cm−1), and CH3 asymmetric stretch (∼2965 cm−1).40−44 The Fermi resonant signals always seriously overlap with the signals from the CH3 asymmetric stretch.40−44 By tuning the angle of ΩVis, we can modulate the phase of the CH3 asymmetric stretch, producing a combination of ssp and ppp spectra. As indicated by Figure 2, two isolated peaks from the Fermi resonance (∼2940 cm−1) and the asymmetric stretch of CH3 group (∼2965 cm−1) are observed in the region of 2930−2970 cm−1 for ΩVis ≤ 74°, while the ∼2940 and ∼2965 cm−1 peaks fuse together and become inseparable for ΩVis ≥ 80°. As expected from the theoretical analysis, the signals from the CH3 asymmetric stretching are almost suppressed for ΩVis = 74°− 76°, resulting in a single peak at ∼2940 cm−1 in the region of 2930−2970 cm−1. 3.2. Effect of Lipid Chain Length. After obtaining the polarization angle of ΩVis in which the signals from the CH3 asymmetric stretch mode are suppressed, we turned to collect the spectra of the lipids with different chain length (DLPA, DMPA, DPPA, and DSPA) with the polarization combination of ssp, ppp, and 45°76°p. The lipid monolayer was deposited on the cleaned CaF2 window surface using the Langmuir− Blodgett method at the surface pressure of 30 mN/m. The

(15)

According to eq 13, we can have Δ = (4W + Δ0 ) , therefore R2v2/v1 can be expressed as 2

R 2ν2 / ν1 =

Δ − Δ0 = Δ + Δ0

2 1/2

4W 2 + Δ0 2 − Δ0 4W 2 + Δ0 2 + Δ0

(16)

3. RESULTS AND DISCUSSION 3.1. Method To Accurately Determine Fermi Resonance by Suppressing Asymmetric Signal of Methyl Group. It is essential to develop a method to accurately detect the Fermi resonant signals without the interference from the asymmetric stretching mode of methyl group. As described in detail elsewhere,25−30 the SFG susceptibility tensor elements χijk (i, j, k = x, y, z) of the asymmetric stretch mode of a methyl (2) (2) group have a relationship of χ(2) zzz,as = −2χxxz,as = −2χyyz,as. In light of the polarization theory, a polarized light can be split into p and s polarizations. The component polarized parallel to the 16590


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3.3. Influence of Intermolecular Distance. To further confirm the relationship between the intensity ratio (R2ν2/ν1) and the dominant intermolecular interactions, we investigated the ratio change of DLPC lipid monolayer at the air/DI water interface following the mechanical compression. DLPC has been used as a model to investigate the ionic effect on the vdW reduction.47 Figure 4A and Figure S5 show the 45°76°p SFG

45°76°p spectra at the CaF2/film interface are given in Figure 3A. We put the ssp and ppp spectra in the Supporting

Figure 3. (A) 45°76°p SFG spectra of PA Langmuir monolayer deposited at CaF2 windows. (B) Intensity ratio (R2ν2/ν1) is plotted as a function of lipid chain length. (C) Intensity ratio is plotted as a function of vdW interactions.

Information (Figure S3) for references. The 45°76°p spectra of PA monolayer films at air/film interface are dominated by three peaks at ∼2852, 2880 (ν1), and 2940 cm−1 (2ν2). Generally, the existence of gauche defects on the alkyl chains will result in the rise of the signal at ∼2852 cm−1 (from methylene groups).40−44 A very weak signal of the ∼2852 cm−1 peak is detected, indicating the tails of the lipid monolayers totally form all-trans configuration at CaF2/film interface.40,41,45 To quantitatively analyze the Fermi resonant interactions, we fit the SFG spectra using a standard procedure, eq S2.25−30 Figure 3B plots the value of R2ν2/ν1 as a function of lipid chain length. The value of R2ν2/ν1 nonlinearly decreases with the lipid chain length increasing. To gain insight into the dependence of Fermi resonant interactions on the lipid chain length, we determined the interactions of the lipid monolayer in terms of the intermolecular distance (rij) for PA lipid by measuring surface pressure−area (π−A) isotherms. According to the π−A isotherms of the lipid monolayer at the air/DI water interface (Figure S4), we can calculate the interactions according to eqs 1−5. Normally, the neat CaF2 surface is positively charged. When the negatively charged PA lipid monolayer was deposited on the CaF2 windows, the CaF2 windows deposited by PA lipid could let the surface less charged,46 leading to a smaller contribution from the electrostatic interaction. Therefore, mainly the vdW interaction changes for the DLPA, DMPA, DPPA, and DSPA lipid monolayer at the CaF2/lipid interface. To simplify the calculation, we assumed that lipid films at the CaF2/lipid interface have the same molecular density with the lipid monolayers at air/water interface at the surface pressure of 30 mN/m. The parameters and vdW interaction are given in Table S1. Figure 3C plots the intensity ratio (R2ν2/ν1) as a function of vdW interactions. It is evident that the intensity ratio (R2ν2/ν1) increases when the absolute value of vdW interactions reduces. Moreover, a linear correlation is clearly observed between the intensity ratio (R2ν2/ν1) and the dominated vdW interactions.

Figure 4. (A) 45°76°p SFG spectra of DLPC Langmuir monolayer at the air/DI water interface at different surface pressures. The intensity ratios (R2ν2/ν1) of DLPC lipid monolayer at the air/DI water interface are plotted as a function of (B) surface pressures, (C) rij2, (D) 1/rij, (E) 1/rij2, (F) 1/rij6, and (G) the vdW interaction energy.

spectra of DLPC Langmuir monolayer at the air/DI water interface at different surface pressures. It is clearly seen that the ratios (R2ν2/ν1) decrease with the surface pressures increasing (Figure 4B). To identify different contributions from the intermolecular interactions, we plot the ratios (R2ν2/ν1) against different power of the intermolecular distance between lipid chains: rij2 (Figure 4C), 1/rij (Figure 4D), 1/rij2 (Figure 4E), and 1/rij6 (Figure 4F). It can be observed that the ratios (R2ν2/ν1) are linearly dependent on 1/rij6, implying that the vdW interaction dominates the contributions to the change of R2ν2/ν1 (Figure 4G), since the hydration, electrostatic, chain-configuration, and charge-dipole interactions are a function of rij2, 1/ rij, 1/rij, and 1/rij2, respectively.35,36 DLPC is a neutral lipid; hence the surface charge density is close to zero. Therefore, the 16591


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penetration): the interaction is strong for the chaotropic ions (on the right side of Cl−), whereas it is negligible for the ions of SO42− and Cl−.39 This interaction between the ion and the lipid can cause two changes:47 weakening the attractive interaction (such as vdW interaction) and strengthening the repulsive interaction through the electrostatic charging and salt exclusion. The total interaction energy in salt conditions is higher than the one in water, resulting in a larger R2ν2/ν1 in salts. To show the specific ion effect on the interfacial interactions, we first determine the vdW interaction (Tables S3 and S4) using the same procedure as described above. As shown in Figure 6A, it is also observed that the intensity ratios (R2ν2/ν1) linearly increase with the vdW interaction (Figure 6A), indicating that the screening of the vdW interaction is ion specific. However, when the ratios (R2ν2/ν1) with and without salts are plotted as a function of the vdW interaction into the same figure, a kink appears clearly (Figure 6B). The slope of the fitting line in Figure 6A is 6.1 times the slope of the fitting line in Figure 4C, which implies that other repulsive interactions dominate the specific ion effects, rather than the screening of the vdW interaction alone. According to the traditional view, the binding (or association, or affinity, or penetration) of anions into the lipid can increase the repulsive electrostatic interaction, which is correlated with the surface potentials and Debye screening length. It is known that the Debye length remains the same for the same ionic strength. The effectiveness of the anions in reducing the surface potential of the membrane follows the order of SCN− > ClO4− > I− > NO3− > Br− > Cl− > SO42−.51,52 Thus, the ions on the right side of the Hofmeister series give rise to higher electrostatic interaction. Therefore, the electrostatic interaction can be one of the repulsive interactions in the salt conditions. On the other hand, the addition of the salts can increase the mean molecular area per lipid molecule (MMA) (Figures S8 and S9) to enhance the hydration interaction that is linearly dependent on the MMA. It is worth noting that if a new interaction (Enew= −19.41 + 19.96AL) is added to the vdW and electrostatic interactions in the salt conditions, the ratios (R2ν2/ν1) with and without salts against the dominated intermolecular interactions can be fitted with a straight line (Figure 6C). Because the new interaction (Enew) is linearly dependent on the MMA, it most likely originates from the hydration interaction (Ehyd) and the chain configuration free energy (Econf). Among the common interactions, the Ehyd has a linear relation with the MMA (eq 5) while Econf obeys the relationship of C0 + C1 exp(C2AL) +

vdW interactions (Table S2) are the dominant interactions at the air/DI water interface, and the electrostatic interaction is thus negligible. This result is in accordance with those widely observed in the neutral molecular solid at high pressure where the ratios (R2ν2/ν1) decrease rapidly with pressure increasing.48−50 A direct mechanical compression on the Langmuir monolayer is a straightforward analogue of hydrostatic compression in three dimensions (3D). The 2D or 3D compression can both induce the changes in the intermolecular distance (rij) and the conformations of flexible alkyl chains. 3.4. Influence of Specific Ions. After establishing the above experimental protocol, we then started to apply this method to elucidate the effect of different ions on the interfacial interactions. The studied salts include K2SO4, KCl, KBr, KNO3, KI, KClO4, and KSCN. Two ion strengths (20 and 100 mM) were considered here. Figure 5A shows the 45°76°p SFG

Figure 5. (A) 45°76°p SFG spectra of DLPC Langmuir monolayer at the air/salt solution (100 mM) interface at the surface pressures of 10 mN/m. (B) Intensity ratios (R2ν2/ν1) of DLPC lipid monolayer at the air/salt solution (○ for 100 mM and ▲ for 20 mM) interface at the surface pressures of 10 mN/m.

spectra of DLPC Langmuir monolayer at the air/salt solution (ion strength = 100 mM) interface at the surface pressures of 10 mN/m. Other 45°76°p SFG spectra are put in Figures S6 and S7. It is found that the intensity ratio (R2ν2/ν1) follows the well-known Hofmeister series (Figure 5B). Compared to the value in DI water condition, the ions on the right side of the Hofmeister series lead to a higher R2ν2/ν1, while the ions of SO42− and Cl− have smaller effects on R2ν2/ν1. In addition, the ratio of R2ν2/ν1 at the ion strength of 100 mM is larger than the value at the ion strength of 20 mM. Such results originate from the ionic specific interaction (binding, association, affinity, or

Figure 6. Intensity ratios of DLPC lipid monolayer at th air/solution interface as a function of the interactions. (A) The case at air/salt solution (100 mM) interface at the surface pressures of 10 mN/m. (B) All the intensity ratio with and without salt were plotted together. (C) Relationship between R2ν2/ν1 of DLPC monolayer and the dominated interactions with addition of new interaction (Enew= −19.41 + 19.96AL) to the results with salt. 16592


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The Journal of Physical Chemistry C C3AL.35,36,38,39 Actually, it has been suggested that the hydration effect must be responsible for ion specificity.53−56 For example, the opposite anion dependence of lamellar swelling between POPC and POPE can be explained as a result of disturbance on water structure around the membranes.53 This result further illustrates the power of using the amplitude of the second-order Fermi resonant intensity ratio to evaluate the total intermolecular interaction forces at interface in situ. These findings also provide many important parameters that can help the theorists to further improve the models proposed for the specific ion effects. It is worth mentioning that although we have established a good correlation between the intensity ratio (R2ν2/ν1) and the total intermolecular interactions, there are some concerns that the orientation of lipid molecules at the air/water interface may affect the ratio of R2v2/v1. To address the concern, we used the case at the air/salt solution (100 mM) and at the air/DI water interface with the surface pressures of 10 mN/m as examples and further determined the orientation of the terminal methyl group of DLPC lipid monolayer. The molecular orientation of methyl group can be determined using the measured ppp and (2) ssp spectral intensity ratios (χ(2) ppp(CH3,as)/χssp (CH3,ss)), which is the common procedures that most people did in the literatures.57,58 The detailed method has been given elsewhere57,58 and will not be repeated here. It is evident that the orientations of the terminal methyl group at the air/DI water interface and the air/salt solution interface change very small (θ = 21 ± 1°) (Table S5). On the other hand, the signals of the symmetric peak and the Fermi peak were collected in the same spectra using the same polarization combination. Because these two vibrational modes possess similar symmetry, therefore the influence of the orientation of the terminal methyl group on the value of the ratio R2v2/v1 will be very small. In addition, although the gauche defect (at 2850 cm−1) is not negligible in Figure 3A and Figure 4A, the intensity ratio of 2850 cm−1/2875 cm−1 is independent of the ionic types and does not follow the same trend of ratio R2v2/v1 (Figure S12). Earlier studies indicated that the effect of gauche defect on the alkyl chain can be described into the change of the length of the chain, namely, the effective carbon number (nc) in eq 1.35,59 In our results, the intensity ratio of 2850 cm−1/2875 cm−1 keeps almost constant (Figure S12). Thus, the existence of gauche defect will affect the values of VdW interaction in a systematic way.

exceptional ability of our method will enable to clarify the scientific problems associated with the interfacial interaction forces (for example, interfacial assembly, nanoparticle interaction) in situ and in real time.

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ASSOCIATED CONTENT

S Supporting Information *

Details about experimental procedure and method of accurately probing Fermi resonant signals, orientation of the terminal methyl group in different salt solution, Figures S1−S12, and Tables S1−S5. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b03204.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 086-(551)63603462; Fax 086-(551)6360-3462 (S.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21273217, 91127042, and 21421063), Fundamental Research Funds for the Central Universities (WK2340000064 and WK2030020023), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB01020200), and the Key Research Program of the Chinese Academy of Sciences.

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REFERENCES

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4. CONCLUSION We have established an experimental protocol that can estimate the total intermolecular interactions at interface using secondorder Fermi resonant signals. The linear correlation between the intensity ratio and the dominated interactions of the PA and PC lipid monolayer strongly implies that the amplitude of the intensity ratio can be used as an effective vibrational optical ruler for characterizing molecular interaction at interfaces. It has been seen that this method is able to elucidate the specific ion effect on the interfacial forces in situ. The observed relationships will help to improve theoretical models for the specific ion effects. Since the intermolecular interaction is a direct function of the intermolecular distances, for example, vdW interactions is inversely proportional to rij6,35,36 our proposed experimental protocol might be applied to determine the change of the average molecular distance in angstrom scale, a challenge for all other existing methods. It is clear that the 16593


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