Quantifying Double-Layer Potentials at Liquid-Gas Interfaces from Vibrational Sum-Frequency Generation.

Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential ϕ0 at liquid-gas interfaces. For this, mixed solutions of nonionic tetraethyleneglycol-monodecylether (C10E4) and cationic hexadecyltrimethylammonium bromide (C16TAB) surfactants were investigated using SFG spectroscopy and a thin-film pressure balance (TFPB). Derjaguin-Landau-Verwey-Overbeek analysis of disjoining pressure isotherms obtained with the TFPB technique provides complementary information on ϕ0, which we apply to validate the results from SFG spectroscopy. By using a single ϕ0 value, we can disentangle χ(2) and χ(3) contributions to the O-H stretching modes of interfacial water molecules in the SFG spectra. Having established the latter, we show that unknown double-layer potentials at the liquid-gas interface from solutions with different C16TAB/C10E4 mixing ratios can be obtained from an analysis of SFG spectra and are in excellent agreement with the complementary results from the TFPB technique.

Introduction

The interaction of ions, surfactants, or proteins can significantly change the surface chemistry of aqueous interfaces and is of great importance for a wide range of fundamental questions in the physical chemistry of interfaces as well as in applications.[1−4] In fact, there is a general interest to quantify the charging state of fluid interfaces in applications ranging from antibody formulation to oxide surfaces.[5−8] Particularly, in soft colloids, such as foams and emulsions, the electrostatic component can dominate the disjoining pressure Π,[9,10] which stabilizes bubbles or drops against coalescence.[9−12] Addressing as well as quantifying double-layer potentials ϕ0 at charged interfaces is, therefore, a necessity.

However, for investigations of aqueous interfaces, there exist only a few experimental techniques that can probe their charging state and provide quantitative information on ϕ0. For solid/liquid interfaces, classical methods such as the surface-force apparatus[13] and (colloidal probe) atomic force microscopy[14−16] have been used to provide information on ϕ0 by an analysis of force–distance curves and the application of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. For liquid/gas interfaces, the use of the thin-film pressure balance (TFPB) technique[17,18] is so far the only commonly accepted method for quantitative analysis but is also constrained to stable foam or emulsion thin films. In the present work, we address the possibilities to quantify ϕ0 at charged water interfaces using sum-frequency generation (SFG) and directly compare the results with a complementary method, which has lately attracted considerable interest[19−23] and traces back to the original work by Eisenthal and co-workers[24−27] using second-harmonic generation (SHG). In addition to SHG, the applicability of SFG for charge determination at interfaces was demonstrated.[7,21,28−31] However, validation on the limits and boundaries of SFG using a complementary technique such as the TFPB has hitherto not been presented but will be addressed in this work using ionic surfactant and nonionic surfactant mixtures to systematically change ϕ0 at the liquid–gas interface (Figure ). It is well-known that surfactants provide colloidal stability and stability of foams.[32] Particularly, mixtures of surfactants are used to modify the properties of interfaces,[33,34] foam films,[35] and thereby the properties of macroscopic foam.[34,36]

Figure 1

(a) Chemical structures and artistic sketches of C16TAB and C10E4 surfactants used in this study. Artistic sketch (not to scale) of (b) an air-charged liquid interface and (c) a foam film from mixed surfactant solutions showing key parameters of the two techniques employed in this manuscript: (b) sum-frequency generation spectroscopy and (c) thin-film pressure balance.

For example, addition of a (less soluble) co-surfactant to a surfactant solution provides increased interfacial dilational viscoelasticity.[32,33] With respect to charge-induced interfacial properties, mixed solutions of nonionic and ionic surfactants in different molar ratios can be used as a tool to obtain stable foam films, while simultaneously the charge density at the film interfaces and thus the electrostatic interactions in the films can be controlled in a well-defined way.[35]

On the basis of the double-layer theory of a charged air–water interface,[1] the interfacial region is composed of a few monolayers of molecules, followed by bulk like water in the so-called diffuse layer. However, neither the electric double-layer potential ϕ0 nor the corresponding surface charge of the liquid–gas interface are directly accessible by available methods and without formulation of a consistent molecular-level description of the interface. In this respect, much progress in molecular-level understanding of aqueous interfaces has been developed due to the fruitful feedback between experiments and simulations.[31,37−44]

Although previous studies already used a combination of SFG and TFPB,[12,22] the authors applied SFG mainly to address the structure of the interface but not to extract quantitative information on ϕ0. In fact, the relation between the SFG intensity of O–H stretching bands and ϕ0 has not been discussed quantitatively in these works. In a recent work by Dreier et al.,[23] SHG spectroscopy was combined with the vibrating plate capacitor method on a lipid monolayer to extract ϕ0. However, the authors concluded that the two approaches were dominated by different molecular moieties and effects. Including SFG in their analysis, Dreier et al.[23] showed how the SHG signals were dominated by contributions of interfacial water molecules, the lipids, as well as by hyper-Rayleigh scattering, whereas the response from the vibrating plate capacitor was attributed directly to the lipid carbonyl groups. In fact, Dreier et al.[23] pointed out that the interpretation of ϕ0 from SHG spectroscopy and its comparison with other methods need to be carefully analyzed and that a comparison at least between the latter two methods is highly challenging.

We have addressed air–water interfaces from mixed solutions that contained the nonionic surfactant tetraethyleneglycol-monodecylether (C10E4) and the cationic surfactant hexadecyltrimethylamonium bromide (C16TAB) (Figure a). In this system, C16TAB is the charge-determining species at the interface and its surface excess thus controls ϕ0. Thermodynamic models describe the adsorption behavior of mixed surfactant systems; however, separating the surface excess from the nonionic and ionic species is not that simple.[45,46] Both surfactants were chosen because of their known surface properties[47−50] and their close critical micelle concentrations, which are ∼0.9[47] and ∼0.7 mM[48] for C16TAB and C10E4, respectively. Our strategy demonstrates how the change in the O–H amplitude (AOH) from interfacial H2O can be used to determine ϕ0 quantitatively, whereas these results are corroborated with the results obtained from the TFPB technique (Figure ).

Figure 4

Comparison of ϕ0 determined by applying DLVO theory to fit the disjoining pressure isotherms, as obtained from a thin-film pressure balance (TFPB, circles in blue) with results from SFG spectroscopy taking into account the phase between χS(2) and χS(3) versus mole fraction of C16TAB. All data points correspond to samples with the total surfactant concentration [C16TAB] + [C10E4] and the ionic strength fixed to 0.7 mM. The solid line guides the eye.

Theory

Vibrational SFG spectroscopy is a three-photon process where an infrared (IR) and a visible (VIS) beam are overlapped in time and space at the interface, generating the sum-frequency beam (ωSF = ωIR + ωVIS). The electric field of the induced second-order polarization at the sum frequency (SF) is proportional to the effective second-order polarization of the interface: P(2)(ωSF) ∝ χeff(2)EIREVIS. At the air–water interface, the SF signal arises from both surface and bulk contributions (Figure b).[51] The former contribution depends on the second-order electric susceptibility χS(2), which is dominated by the molecular structure of interfacial molecules.[20,52−54] The latter is caused by the static electric field EDC in the electric double layer (EDL) perpendicular to the interface (z) and originates from adsorbed molecules that carry a net charge (surfactants,[55] proteins,[12] polyelectrolytes, etc.) and depends on the third-order electric susceptibility χS(3). In the EDL, reorientation and polarization of interfacial water molecules are possible.[26,28,56−59] The effective second-order electric susceptibility χeff(2) of the charged air–water interface can be expressed by[20]here, the χS,EDL(2) term accounts for the probing depth and the phase mismatch Δk of the fundamental and SF waves at charged interfaces (EDC ≠ 0), which reflects possible interference effects at low ionic strengths.[20,21,57] Using the Gouy–Chapman model and the Debye–Hückel approximation to solve the Poisson–Boltzmann equation, we can write the double-layer potential ϕ(z) = ϕ0 e–κ. Using the latter equation, one can obtain the relation between the χeff(2) and ϕ0 by integration of eq (7,20,21,30,56,60)To use the SF intensity I(ωSF) for quantification of ϕ0, direct application of eq is, however, not possible because of the three unknown parameters: χS(2), χS(3), and ϕ0. Obviously, disentangling these parameters is necessary and we have, therefore, varied ϕ0 systematically, studied the changes in SF signals, and combined our analysis with thin-film pressure balance measurements.

Experimental Section

Sample Preparation

C16TAB (BioUltra, Sigma-Aldrich, >99%) and C10E4 (Bachem, Switzerland, 98.8%) were used as received. Mixed surfactant solutions were prepared with a total concentration Csurf = 0.7 mM but different [C16TAB]/[C10E4] molar ratios. The ionic strength of the solutions I = 0.7 or 500 mM was adjusted with NaCl (BioXtra, Sigma-Aldrich, 99.9%). Further details about the samples’ composition and preparation are given in the Supporting Information.

Broadband Sum-Frequency Generation Spectrometer

Münster Ultra-fast Spectrometer for Interfacial Chemistry (MUSIC) is a user-friendly broadband sum-frequency spectrometer. MUSIC is composed of a Spectra Physics Solstice-Ace Ti:sapphire amplifier seeded by a Ti:sapphire fs-oscillator and pumped by a Q-switched laser (Ascend 60, 520 nm, 1 kHz, 32 W). The oscillator emits at a wavelength of 794 nm and has a bandwidth of 22 nm (Spectra Physics MaiTai SP, 84 MHz, 770 mW). The Solstice-Ace regenerative amplifier delivers approximately 7 W (1 kHz, at 796 nm) of average power. The uncompressed beam is divided by an internal (50:50) beam splitter. One beam is subsequently guided into an internal, and the second beam to an external compressor. A total power of 3.2 W of the amplified and compressed femtosecond beam (796, 18 nm bandwidth) pumps an optical parametric amplifier (OPA, Light conversion TOPAS Prime) with a subsequent unit for noncollinear difference frequency generation (NDFG) of the OPAs’ idler and signal photons. The NDFG unit generates broadband femtosecond IR pulses that are tuneable from 2.5 to 20 μm. The broadband IR has >300 cm–1 full width at half-maximum bandwidth. An air-spaced etalon (SLS OPTics LTD, FSR 12.4 nm at 735 nm, R = 94.5%) was inserted into the external compressor to generate the VIS narrowband pulse centered at 804.1 nm and with a bandwidth of 4 cm–1. Etalon side bands are removed by beam blocks, which are also placed inside the external compressor.

For the SFG, the VIS and IR pulses overlap in time and space at 55 and 60° incident angles, respectively. The mean circularized focused beam diameters (1/e2) were 530 and 260 μm for the VIS and IR beams, using a VIS lens f′ = +500 mm (N-BK7, Thorlabs LA1908-780) and an IR lens f′ = +100 mm (ZnSe, AMSTechnologies ZC-PX-25-100 BB-AR coated).

The SFG photons were collected by a collimator lens f′ = +200 mm (N-BK7, Thorlabs LA1979-A) and focusing lens f′ = +100 mm (LA1509-A), directing the SFG beam into a spectrograph (Kymera-328i-D2-SIL, Andor) using a 1200 g/mm grating imaged into an electron-multiplied charge-couple device EMCCD (Andor Newton, Du97P-BVF). A short pass filter with the cutoff at 763 nm (AHF, F76-789) was used to filter the visible beam.

For the liquid–gas interface, all spectra were recorded using s-polarized SF, s-polarized visible, and p-polarized IR beams, that is, in the ssp polarization configuration. The polarization optics used are a half-wave plate for the VIS (Bernhard Halle Nachfl RZQ 2.15.0795) and a combination of an achromatic half-wave plate (Bernhard Halle Nachfl RAC 4.2.15) with a Glan polarizing prism of calcite (Bernhard Halle Nachfl, PGL 12). The Glan polarizing prism matches the polarization of the spectrograph-grating, being sensitivity-independent of the selected SFG polarization during the experiment.

The IR beam path and the sample-automatized stage are enclosed in a compressed air-purged box. The relative humidity inside the box is <3%.

The SFG spectra were taken scanning five IR frequencies from 2700 to 3700 cm–1. The spectra were stitched together and normalized with the SFG signal of an air-plasma-cleaned Au film that served as a reference. The SFG air–liquid spectra were performed with a glass Petri dish, containing 2.5 mL of the liquid. The glass was previously soaked in a mixed solution of sulfuric acid with Nochromix overnight, rinsed extensively with Milli-Q water, and dried with N2 afterward.

Thin-Film-Pressure-Balance (TFPB) Technique

To obtain disjoining pressure isotherms Π(d), we utilize the porous plate technique, developed by Mysels[61] and refined by Exerowa.[18] The experimental setup is schematically shown in Figure S6. In our experiments, a foam film is created in a 2 mm hole of a porous glass plate, which is fused to a glass capillary tube (1 mm inner diameter). The film holder is enclosed in a gastight pressure cell with two CaF2 windows. The chamber is equipped with a reservoir filled up with the sample solution, avoiding evaporation of water from the foam film. The pressure is controlled by a syringe pump (Legato210, kdScientific) and measured by a pressure transducer (A-10, 0–50 mbar, WIKA Germany).

The disjoining pressure Π in the film is calculated as followswhere Pg and Pr are the chamber and external pressures, respectively; γ, the surface tension; r, the radius of the glass tube’s channel; Δρ, the density difference between surfactant solution and air; g = 9.81 m/s2; and hc, the height of solution in the glass tube above the film (see Figure S6). The third and fourth terms in the equation stand for the capillary pressure and the hydrostatic pressure in the capillary, respectively.

For a visual access of the foam film, a camera is mounted at the top of the microscope. Between the objective and the pressure chamber, a beam splitter is used to couple the focused broadband measurement beam from a deuterium–halogen lamp (AvaLight-D-S-Bal, Avantes). The transmission spectra T(λ) allow to calculate the thickness of the film, as described in the Supporting Information, using a fiber-optical spectrometer (AVASPEC-ULS3648-USB2-UA-25, Avantes).

Results and Discussion

In our procedure, we have recorded SFG spectra for different [C16TAB]/[C10E4] mixtures (Figure ) and we compare the spectra of the mixtures at a constant total surfactant concentration ([C16TAB] + [C10E4]) of 0.7 mM and constant total ionic strengths ([C16TAB] + [C10E4] + [NaCl]). As ionic strengths we have chosen low 0.7 mM and high 500 mM values for each surfactant mixing ratio. To clarify the composition of our samples, we refer to Table S1 in the Supporting Information.

Figure 2

SFG spectra for different [C16TAB]/[C10E4]/[NaCl] mixing ratios with a constant total surfactant concentration of Csurf = 0.7 mM. The mole percentage of the mixtures was as indicated in the figure. Ionic strength was fixed by adding NaCl to (a) I = 0.7 mM and to (b) I = 500 mM. (c) and (d) show a close-up of the C–H bands in (a) and (b) respectively. All spectra were recorded using s-polarized SF, s-polarized VIS, and p-polarized IR beams that are in the ssp polarization configuration.

To analyze the changes of O–H bands, we fit our spectra in Figure , accounting for the nonresonant χNR(2) and the resonant contributions, e.g., due to C–H and O–H vibrational modes. We use a combination of Lorentzian functions for the C–H stretching modes (2750–3000 cm–1) and a Voigt function for the O–H bands (2950–3700 cm–1), which accounts for both homogeneous and inhomogeneous line broadening.[62] See the Supporting Information for further description and the fitting parameters.

The amplitudes from the O–H stretching bands dominate the spectra in Figure a and appear as two separate broad bands centered at 3250 and 3400 cm–1. These bands have been previously attributed to O–H stretching modes of tetrahedrally coordinated (low-frequency branch) and nontetrahedrally coordinated (high-frequency branch) interfacial H2O molecules.[29,54,63] Tahara and co-workers[28,64] concluded from phase-resolved SFG spectroscopy at cetyltrimethylammonium bromide/water interfaces without additions of C10E4 that the O–H bands have negative amplitudes. The latter is direct proof that the interfacial water molecules exhibit a net orientation with the water’s hydrogens pointing away from the interface toward the bulk solution (H-down orientation). Figure a clearly demonstrates that not only does the intensity of O–H bands decrease as the [C16TAB]/[C10E4] molar ratio is decreased but also a change in the shape of the O–H bands can be noticed from a close inspection of Figure a,b. This change in the O–H spectrum is likely associated to a change in the water’s surface chemistry due to changes in the surface excess of C16TAB relative to C10E4. The latter change in the surface excess can be seen most clearly by an inspection of the surfactants’ C–H bands, which we show in more detail in Figure c,d. We observed methyl as well as methylene stretching bands centered at ∼2880 and ∼2850 cm–1, respectively. In particular, the presence of strong methylene bands indicates a substantial number density of gauche conformations at the interface and that not all molecules are in an all-trans state. The latter would give rise to local inversion symmetry and thus negligible CH2 contributions to the SFG spectra.[64,65]

We can now link the O–H amplitude |AOH,low| at I = 0.7 mM to the effective interfacial susceptibility χeff(2) for different mixture ratios (Figure a). We recall that the latter is dependent on the double-layer potential ϕ0, as discussed above (eq ).In eq , factors f1 and f2 change with the Debye length κ–1 but have negligible variation for the range of IR wavelengths used in our experiments (Figure S3).[7,21] However, at high ionic strength (I > 200 mM), a substantial screening of the EDL takes place and decreases χS(3) contributions to negligible values (κ ≫ Δk, f1 → 1 and f2 → 0).[27] This is consistent with recent works[7,66] and consequently χeff(2) depends only on χS(2) contributions at high ionic strength. Having said this, we can now determine the first unknown parameter χS(2) directly from our fitting results to the SFG spectra at high ionic strength.In the second step, we can eliminate the second free parameter by introducing (for a single mixing ratio) a known double-layer potential (ϕ0,TFPB). This allows us to calculate |χS(3)| from the previously determined |χS(2)| and |AOH,low|, which we retrieve from our fits to the SFG spectra at low and high ionic strengths (Figure a). However, a prerequisite for this analysis is that we need to include a phase δ between χS(2) and χS(3). At the charged water–air interface, the average molecular dipole orientation is perpendicular to the interface (the direction depends on positive or negative surface charge). Since a theoretical study of χS(3) of the air–water interface[49] shows how the phase between χS(2) and χS(3) can have opposite signs, we consider both cases: 0 and π. After expansion of eq that is presented in detail in the Supporting Information, the following expression can be derivedFrom our fits to the SFG spectra in Figure , we determine |AOH,high| and |AOH,low|. Inserting eqs and 7 in 8, one can simplify eq towhere ϕ0,TFPB from the TFPB is used for “calibration” or determination of the second unknown |χS(3)|. Considering the two possible scenarios where the phase δ between the susceptibilities χS(2) and χS(3) is either 0 or π, eq can be rewritten as followsObviously, for the different phases, we will get according to eq also a different value for |χS(3)|. Having established the χS(3) for one system (here, one mixing ratio of C16TAB/C10E4), we can determine the double-layer potentials for unknown mixing ratios from an analysis of SFG spectra, using the following expression, derived from eq To determine the double-layer potential as discussed above, we have to consider the following points: (i) χS(2) necessarily depends on the surfactant mixing ratio, which accounts for the structural changes of the interface. (ii) We assume χS(2) has the same spectra at low and high ionic strengths for a given mixing ratio. This is also corroborated by the study of Urashima et al.[66] (iii) χS(3) spectra are independent of the surfactant mixing ratio, as was also pointed out earlier by Wen et al.[20]

We now discuss the point (ii) from the viewpoint of soft interfaces that are composed of different surfactants, which can change their composition with ionic strength. For nonionic surfactants, variations in the ionic strength do not lead to changes in the adsorption behavior. In contrast, the surface activity of ionic surfactants increases with ionic strength as the electrostatic repulsion between the charged surfactants is reduced. This leads to an increase of the surfactants’ surface excess and to a decrease of the surfactants’ critical micelle concentration.[67−69] However, studies of ionic/nonionic surfactant mixtures have shown that for [ionic]/[nonionic] molar mixing ratios <1, like in our work, the surface properties are dominated by the nonionic species.[35,70] Moreover, we measured the surface tensions γ of C16TAB/C10E4 mixtures and found that γ varies only within a few mN/m if the mixing ratio is constant but the ionic strength is varied (see Figure S1). Therefore, surface tension indicates that variations in the total surface excess are very small as a function of ionic strength. Consequently, these results from surface tensiometry corroborate our earlier assumption that χS(2) has a similar spectrum at different ionic strengths.

As we will discuss in more detail below, at least for the system used in our study, the double-layer potentials, as determined from SFG, are in excellent agreement with those from an analysis of disjoining pressure isotherms of the same system but in a foam film (Figure ).

We now want to discuss our results from the thin-film pressure balance, which are used to obtain χS(3) (eq ), as well as for direct verification of the potentials calculated by SFG (eq ). To determine ϕ0 from disjoining pressure isotherms Π(d) of foam films, the separation d between the two charged surfaces needs to be determined (Figure c). Π arises from the balance between electrostatic force, dispersion, and other intermolecular forces.[13,18] In our case, we consider only the repulsive electrostatic Πel and the attractive part ΠvdW of the van der Waals forces.[9,10] Analysis of the experimentally determined disjoining pressure isotherms provides access to ϕ0. The generalized equation[13,18] for Π can be expressed for the case of monovalent (molecular) ions as followsEquation can be now used to fit the experimental data Π(d) with ϕ0 as the only free parameter (Supporting Information). For the Hamaker constant A, we used 3.7 × 10–20 J for the air–water–air system.[13] Note that the film thickness measured by optical methods is an equivalent thickness heq that differs from the real physical thickness d of the water core film, because the thickness of the adsorbate layer contributes to heq. Although d ≈ heq is often assumed for simplicity,[22] a more detailed analysis involves the exact value of d = heq – Δhcorr, where Δhcorr is a correction factor.[18,50,71] We found a value of 3.2 ± 0.6 nm for Δhcorr, wherein the standard deviation is due to approximations for the thickness and the refractive index of the adsorbate layer which are used in the data analysis (Supporting Information). Figure shows a comparison between theory and experiment for three selected surfactant [C16TAB]/[C10E4] mixing ratios. The results in Figure demonstrate that decreasing the C16TAB concentration causes a progressive shift of the Π(d) isotherms toward smaller film thicknesses d, which already points to a decrease in ϕ0, as one would also intuitively expect.

Figure 3

Disjoining pressure isotherms Π(d) of foam films for [C16TAB]/[C10E4] mixing ratios with constant ionic strengths I of 0.7 mM and a total concentration Csurf of 0.7 mM. The mole percentage of each component of the mixtures was as indicated in the figure. Symbols: experimental data, solid lines: the best fit with eq .

Figure shows the results of our analysis from both methods (SFG and TFPB) for ϕ0, at the different mixing ratios, where n refers to the molar fraction of C16TAB. From a close comparison of Figure , we find an excellent agreement between the two complementary methods, which also brings strong support to our model and the above-mentioned assumptions for SFG analysis. Comparing our work with the recent report by Dreier et al.,[23] where SHG spectroscopy was combined with a vibrating plate capacitor method, our approach to combine SFG with a TFPB provides a better overlap between the latter techniques because they both depend strongly on the double-layer potential.

To test a possible bias of our procedure that could be caused by the initial choice of ϕ0,TFPB in eq , we calculated ϕ0, using different initial potentials ϕ0,TFPB for calibration. From the latter, we calculated ϕ0, as explained above and present the mean values for the different initial conditions in Figure . This comparison is omitted for clarity in Figure but is shown in Figure S4 of the Supporting Information. In addition, we demonstrate in Figure S5 of the Supporting Information that our analysis is also independent of the choice of the O–H band. A separate analysis of the O–H stretching bands at 3250 and 3450 cm–1 provides again an excellent agreement with the results from the TFPB. In addition, the choice of the phase between χS(2) and χS(3) being either 0 or π does not change the results significantly (Figure ). A close comparison of all data sets in Figures and S4, S5 shows excellent agreement, which we take as further evidence for the robustness of our procedure, as well as for the feasibility of SFG spectroscopy for the determination of double-layer potentials at aqueous interfaces.

Summary and Conclusions

In summary, we demonstrated in this letter the ability of SFG spectroscopy to determine quantitatively double-layer potentials at liquid–gas interfaces by validating our results with complementary measurements of the disjoining pressure in foam films using a TFPB. For that, we have systematically changed ϕ0 by changing the bulk mixing ratio of nonionic C10E4 and cationic C16TAB surfactants, with the latter surfactant serving as the charge-determining species. Results from SFG spectroscopy on double-layer potentials are in excellent agreement with foam film results. The detailed analysis of the foam film data provided reliable ϕ0 values, which were used as initial inputs to obtain the a priori unknown values for third-order contributions to the electric susceptibility. We point out that compared with measurements with a TFPB, SFG can be seen as a fast and time-efficient method for addressing the charging state of fluid interfaces (1 day for all mixing ratios in SFG versus 1 day per mixing ratio for TFPB). Therefore, our new strategy, which uses the latest developments in the analysis of second-order optical spectroscopies, provides a reliable and validated method to measure ϕ0, which also accounts for the changes in the interfacial molecular structure. Furthermore, we expect that the procedure we describe can be applied to solid/liquid interfaces as well.[31]