Article pubs.acs.org/Langmuir
Origin of the Instability of Octadecylamine Langmuir Monolayer at Low pH Zaure Avazbaeva,† Woongmo Sung,† Jonggwan Lee,† Minh Dinh Phan,‡ Kwanwoo Shin,‡ David Vaknin,§ and Doseok Kim*,† †
Department of Physics and ‡Department of Chemistry and Institute of Biological Interfaces, Sogang University, Seoul 121-742, Korea § Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States S Supporting Information *
ABSTRACT: It has been reported that an octadecylamine (ODA) Langmuir monolayer becomes unstable at low pH values with no measurable surface pressure at around pH 3.5, suggesting significant dissolution of the ODA molecule into the subphase solution (Albrecht, Colloids Surf. A 2006, 284−285, 166−174). However, by lowering the pH further, ODA molecules reoccupy the surface, and a full monolayer is recovered at pH 2.5. Using surface sum-frequency spectroscopy and pressure−area isotherms, it is found that the recovered monolayer at very low pH has a larger area per molecule with many gauche defects in the ODA molecules as compared to that at high pH values. This structural change suggests that the reappearance of the monolayer is due to the adsorbed Cl− counterions to the protonated amine groups, leading to partial charge neutralization. This proposition is confirmed by intentionally adding monovalent salts (i.e., NaCl, NaBr, or NaI) to the subphase to recover the monolayer at pH 3.5, in which the detailed structure of the monolayer is confirmed by sum frequency spectra and the adsorbed anions by X-ray reflectivity.
1. INTRODUCTION It is well established that fatty acids (CnH2n+1COOH) with n ≤ 14 form unstable Langmuir monolayers at high pH values (in the range of 9 to 11) due to deprotonation,1,2 while those having longer alkyl chains form stable Langmuir monolayers at similar conditions.3−5 By contrast, octadecylamine (ODA, C18H37NH2), with its relatively long hydrocarbon chain is unstable as a Langmuir monolayer and soluble at pH ∼ 4 due to protonation of the amine groups.6 Remarkably, however, by lowering the pH further (pH ≤ 3), the ODA monolayer formation recovers, albeit with a surface pressure versus molecular area (π−A) isotherm that significantly differs from that at high pH values.6 Although it is apparent that protonation of the amine headgroup as pH is lowered increases the solubility of ODA and destabilizes the monolayer, the reappearance of stable monolayers below pH ∼ 3 has not yet been fully understood and requires further investigations. To find out the underlying mechanism of the above phenomenon, we investigated the ODA monolayer at different pH values by using sum-frequency generation (SFG) spectroscopy and X-ray reflectivity (XRR). As a second-order nonlinear optical response, SFG spectroscopy monitors electronic and vibrational transitions of the molecules at interfaces by virtue of broken inversion symmetry. As has been demonstrated, sumfrequency (SF) responses from interfacial water reflect the net surface charge density at the immediate interfacial region.7,8 © 2015 American Chemical Society
Taking advantage of changes in interfacial electron density due to the binding of anions, XRR is ideally suited to monitor the adsorbed counterions to the monolayer.9 These studies determine the structure of the ODA monolayer, the influence of this monolayer on the interfacial water molecules, and the adsorbed counterions at various pH values. We also explored the influence of adding monovalent salts to the solutions at low pH to examine their effect on the recovery of the monolayer.
2. EXPERIMENTAL SECTION The ODA (∼99%, Aldrich) sample is prepared in chloroform (>99.9%, HLPC grade, Sigma-Aldrich) with typical concentrations of 1.0 mg/mL. After spreading from a stock solution and waiting for 10− 15 min to allow for chloroform evaporation, the monolayer is compressed at a rate of ∼1.0 Å2/min, and surface pressure is measured by a sensor (PS4, NIMA). pH values of the subphase solution is controlled by HCl and NaOH without additional buffer. NaCl (>99.5%, Sigma-Aldrich), NaBr (>99%, Sigma-Aldrich), and NaI (>99.5%, Sigma-Aldrich) are introduced into the solution as detailed below. At neutral or basic subphase pH conditions (pH ≥ 5.7), ODA molecules form a stable Langmuir monolayer, and area/molecule (together with temperature and surface pressure) is a good thermodynamic variable to specify this two-dimensional system. At Received: October 25, 2015 Revised: November 25, 2015 Published: November 30, 2015 13753
DOI: 10.1021/acs.langmuir.5b03947 Langmuir 2015, 31, 13753−13758
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Langmuir acidic conditions, however, quantifying the exact area per molecule is difficult due to partial dissolution of the ODA molecules. Thus, we labeled the x-axis of the π−A isotherm both in terms of the changing area (Atrough) of the trough as well as the nominal area/molecule Amol = Atrough /N, where N is the initial number of the molecules deposited which is the same for different pH values. The use of Amol allows easier comparison of the surface at low pHs with the surface at pH > 5.7 and can intuitively indicate partial dissolution or buildup of multilayers in the collapse regime of the isotherm. During the SFG experiments, the surface pressure of the monolayers is kept constant by a feedback loop that controls the position of the barrier in the Langmuir trough, which is kept at room temperature (20 °C).8 The SFG experiments are conducted by using a picosecond Nd:YAG laser (PL2143B, Ekspla, 1064 nm in wavelength, 16 mJ in pulse energy, 25 ps in pulse duration) together with a LiNbO3-based optical parametric generator/amplifier (OPG/OPA) system.8 The OPG/OPA system provides a tunable IR output from 2.5 to 4 μm, and the second harmonic beam (532 nm) is generated from a potassium titanyl phosphate (KTP) crystal. Typical energies are 1 mJ/pulse for the visible beam and ∼200 μJ/pulse for the tunable IR beam. The visible and the IR beams are incident on the samples at 45° and 60°, respectively, and overlap within a diameter of ∼200 μm on the sample surface. The SF output in the reflection direction is spatially and spectrally filtered and detected by a photomultiplier tube (R928, Hamamatsu). Typically, the spectra are taken at ∼5 cm−1 intervals, and each data point is averaged over at least 100 laser pulses. The spectra are normalized by the SF spectrum of a z-cut quartz. Input/output polarization combination of SSP (denote S-, S-, and P-polarized SF output, visible input, and IR input, respectively) is used for all measurements.8 X-ray reflectivity (XRR) measurements are performed by using the D8 Advance (Bruker AXS) and a ceramic Mo anode X-ray generator (Kα radiation, λ = 0.7107 Å) with incoming and reflected X-ray beams arranged for specular reflectivity from liquid surfaces. This specular goniometer setup enables monitoring of the liquid/air interface without disturbing the Langmuir trough during the measurements. The wavevector transfer, qz, is normal to the liquid-surface and is given by qz = (4π/λ) sin Φ, in which Φ denotes the angle between the incident beam and the surface. The Langmuir trough, with a working area of 30 × 10 cm2, is sealed in a Plexiglas box with Kapton-covered windows for in/out X-ray beams, and is placed on an antivibration table platform. More details about the XRR setup are described elsewhere.10 To extract the electron density (ED) profile from the XRR curves, a model of the vertical ED of the studied system is proposed from which the model reflectivity is calculated by the recursive Parratt formalism.11 The simplest model to describe the Langmuir monolayer is the twohomogeneous-layer model, representing the headgroup and the hydrocarbon tail, with specific thickness (d), electron density (ρ), and interfacial roughness (σ) for each layer.12 The two-layer ED model is found to be sufficient in fitting all the XRR measurements in this study. Typical uncertainties of the fitting parameters are estimated at 2% for ρ, and ±0.2 Å for σ and d. Since the footprint of the incident beam is very large at around the critical angle owing to the short wavelength of Mo Kα radiation, the intensity of the beam reflected from the sample is less than unity, thus making it difficult to achieve a good fit at this low qz region.
Figure 1. π−A isotherms from the ODA monolayers showing the maximum achievable surface pressure decreases systematically with decreasing pH. Inset shows partial π−A isotherms in the pH range of 2 to 4 for which the monolayer disappears and reappears. All the isotherms were measured at 20 °C. The x-axis of area/molecule (Å2/ molecule) is only valid for the ODA monolayers at pH = 5.7.
become positively charged) at lower pH facilitates dissolution of molecules into the subphase. Upon further decrease of pH below 3.0, πmax increases, and at pH 2.5 (inset of Figure 1) the pressure increases at ∼60 cm2 of the trough area (corresponding to 24 Å2/molecule of the ODA monolayer in a stable Langmuir monolayer condition) followed by a stable plateau in the isotherm. Further pH lowering does not change the isotherm significantly compared to that at pH 2.5. We emphasize that, although the surface pressure at and around pH 3.5 cannot be maintained above ∼1 mN/m, some ODA molecules may still be present at the interface at very low surface density, such that compression results in pushing more molecules from the surface to the subphase. As we demonstrate below and in the Supporting Information by SFG and XRR, some ODA molecules at very low density are still present at the interface, somewhat reminiscent to the behavior of short chain surfactants (for instance, sodium dodecyl sulfate; SDS13). 3.2. SFG at Different pH Values. To find out the microscopic structures responsible for this phenomenon, the ODA monolayers at different subphase pH values have been investigated by surface SFG spectroscopy. Figure 2a shows the SFG spectra of the ODA Langmuir monolayer/water interfaces at different pHs. Each spectrum was obtained at the same experimental conditions and normalized by the same z-cut quartz SFG spectrum. Two sharp peaks are seen at pH ≥ 5.7. The one at 2875 cm−1 originates from the symmetric stretch of the terminal CH3 group of the alkyl chain, and the second at 2935 cm−1 from its Fermi resonance with its bending overtone together with the CH3 asymmetric stretch mode (∼2960 cm−1) at the shoulder.7 Unlike IR or Raman for which the spectra from long alkyl chains are dominated by methylene groups (CH2 symmetric at ∼2850 cm−1 and CH2 asymmetric at ∼2920 cm−1), the SFG spectrum is insensitive to the methylene moieties, as we observe for the ODA Langmuir monolayer at high pH (Figure 2a, pH ≥ 5.7). This is evidence that the alkyl chains of the ODA molecules in the monolayer at these pH values are in an all-trans conformation.14 At pH 3.5, SFG signals from CH3 and CH2 bands are hardly seen, while a broad peak centered at about 3200 cm−1 due to
3. RESULTS AND DISCUSSION 3.1. π−A Isotherms at Different pH Values. Figure 1 shows a series of π−A isotherms of the ODA Langmuir monolayers at different pH values of water. We find that the maximal achievable surface pressure (πmax, the pressure just before monolayer collapse) decreases systematically with decreasing pH, and at pH 3.5, a fully compressed monolayer hardly shows any finite pressure (∼1 mN/m), in agreement with a previous report.6 It is likely that the protonation of the ODA molecules (the headgroups of the ODA molecules 13754
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presence of some gauche defects in the alkyl chains of the molecules in the monolayer or the alkyl chains lying flat on the water surface. As the pH is lowered to 2.0, the SF signal in the CHx range increases further, indicating an increase in the density of the ordered ODA molecules at the surface as indicated by stronger peaks of the CH3 modes (Figure 2). Although the density of the ODA molecules is set similar to that of the monolayer at pH 5.7 (∼20 Å2), the SF signal from terminal methyl groups is weaker, while that from methylene groups is more appreciable, indicating that it is either due to the presence of gauche defects in the alkyl chains of the ODA molecules and/or due to a less dense monolayer. 3.3. Addition of Salts to pH 3.5 Water Subphase. As stated above, the behavior of ODA at pH 3.5 water is analogous to partially soluble short chain surfactants. It is well established that the presence of counterions in solutions affects both surface activities and bulk structures of the surfactants.17 We therefore set out to examine this hypothesis by adding various monovalent salts while maintaining the solutions at pH 3.5 (the pH value for which the monolayer disappears). Figure 3 shows
Figure 2. (a) SSP SFG spectra from the ODA monolayers at the air/ water interface at various pHs. The spectra were taken at 20 °C. For pH ≥ 5.7 the surface area per molecule was 20 Å2/molecule, and for pH ≤ 3.5, the same amount of ODA molecules was put onto the surface having the same trough area as those in higher pHs. For clarity, each spectrum is shifted vertically by 1. The inset shows the spectrum in the CHx region inside the box at pH 3.0. (b) SSP SFG spectra in the OH stretch region from the ODA Langmuir monolayers at pH 3.5 (magenta squares) and at pH 5.7 (violet circles) together with that of air/water interface (black solid line).
Figure 3. π−A isotherms from the ODA monolayers on water without salt (magenta), 3 mM NaCl (red), 3 mM NaBr (blue), and 3 mM NaI (black) at pH 3.5. The isotherm of the ODA monolayer at pH 2.5 (gray) is shown for comparison. Together with surface area of the trough (cm2) in the upper x-axis, area/molecule (Å2/molecule) corresponding to the stable ODA monolayer is shown in the lower xaxis.
the OH stretch vibration of the interfacially oriented water molecules is observed. This shows almost complete disappearance of the ODA molecules from the surface, consistent with the π−A isotherm in Figure 1. However, this relatively strong OH signal (compared to that of pure water15) is evidence of interfacial charges due to the presence of residual ODA molecules at the interface that induces the alignment of water dipoles. For comparison, we plotted an SF spectrum of the neat water surface together with those of the ODA monolayers at pH 5.7 and pH 3.0 in Figure 2b. At neutral pH, OH peak strengths for ODA/water and for air/water interfaces look comparable except for some difference in the band shape. It has been known that water molecules under the different functional groups (for example, long chain fatty acid, bovine serum albumin (BSA), or fatty alcohol Langmuir monolayers) exhibit different hydrogen bonding networks.7,16 At pH 3.0, a more complex SFG spectrum, in the typical spectral range of the CHx groups, emerges, signaling the reappearance of the ODA monolayer. The spectrum has a weak peak at ∼2850 cm−1 generally associated with the CH2 groups, indicating the
the π−A isotherms from ODA on water at pH 3.5, with 3 mM NaCl, NaBr, and NaI added to keep the total halide concentration comparable to that of Cl− in pH 2.5. As shown in Figure 3, the addition of all three counterions recovers the surface pressure to yield isotherms that are similar to that obtained at pH 2.5 (see Figure 1). Whereas the isotherms of NaCl and NaBr in solution are not significantly different from that for pH 2.5, the addition of NaI affects the collapse of the monolayer at larger molecular area of ∼26 Å2/molecule compared to ∼20 Å2/molecule for NaCl and NaBr. This is likely due to the larger size of I− anion and its higher surface propensity than Cl− and Br,−18−20 making I− anions penetrate into (and even bind to) the charged headgroups. These results support the notion that the anion (Cl−, Br−, or I−) association with the headgroup of ODA makes it more charge neutral and thus more favorable for the hydrophobic interactions to overcome the hydrophilic ones. This association of the monovalent ions is thus responsible for the recovery of the 13755
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Langmuir surface pressure at lower pH (∼ pH 2.5). We speculate that with the addition of much lower concentrations of divalent or trivalent ions, the monolayer will also recover at pH 3.5. Figure 4 shows the SFG spectra from ODA monolayers on water at pH 3.5 with and without added 3 mM salts at the
Realizing the role of counterions in the recovery of the ODA monolayer, the SF spectra of the ODA monolayers in Figure 2 can be understood following the two processes as subphase pH is lowered: (1) protonation of the amine headgroups with increasing proton concentration, and (2) stabilization of monolayer by adsorbed Cl− counterion. At basic and neutral pH conditions (pH 11.0−5.7), the spectral shape in the CH range indicates liquid condensed (LC) phase of the ODA monolayer. Although subphase pH crosses the pKa value of the individual amine headgroup (∼10.6)23 the weak OH band shows no noticeable change,7 implying that the number of protonated amines is still small. At pH 3.5, ODA molecules start to be ionized and a substantial amount of ODA molecules dissolve into the bulk. Part of the ODA molecules remaining at the surface have disordered alkyl chains due to small packing density (as seen from Figure S1), while their protonated headgroups work to reorient adjacent water molecules. Further lowering of the pH (pH ≤ 3.0) increases the amount of Cl− counterions, which weakens the hydrophilic effect by partially neutralizing the headgroup and brings ODA to the surface. Since the recovered monolayer contains bound Cl− ions, positive charge of the amine groups is partially screened resulting in slight decrease of the OH intensity at pH 2.0. The XRR from ODA at pH 3.5 of water showing minute presence of the ODA film at the interface is provided in the Supporting Information and is consistent with the conclusion from aforementioned SFG measurements above. Even though the isotherms and the SFG results support the proposed mechanism of the anion binding to restabilize the monolayer, they do not provide direct proof, as they do not probe the inorganic anions directly. By contrast, the XRR curves in Figure 5 for the ODA samples with added salts into the subphase show clear modulations, indicating the formation of intact ODA monolayers. In more detail, we find that the XRR curves differ in two respects depending on salt in the solution. First, there is an increase in the reflectivity in the low qz range of 0.04−0.20 Å−1 (inset in Figure 5a), in the order Cl− < Br− ∼ I−, consistent with the presence of anion binding at the headgroup region. Second, the minimum in the reflectivity shifts to larger qz values, suggesting the formation of a thinner monomolecular layer as the atomic number of the anion increases. The thinning of the layer requires quantitative modeling of the reflectivity curves with ED profile as shown in Figure 5b and the variable parameters as listed in the SI. For Br− and I− in the solution,
Figure 4. SFG signal from the ODA/water interface at pH 3.5 with added salts. All spectra were taken at 20 °C and at the same trough area of 56 cm2 (corresponding to 20 Å2/molecule of stable ODA monolayers at pH ≥ 5.7).
trough area of 56 cm2 (∼20 Å2/molecule of the ODA monolayer in neutral and basic pH conditions). Whereas the SF spectrum without salt shows very weak signal in the CHx range, there is a relatively strong OH signal from water due to remnant ODA molecules that are fully protonated. Upon addition of salts, the SF intensity in the CHx range increases overall, indicating the increase in surface density of the ODA molecules. However, the SFG intensity for NaI in the CHx range is about 3 times weaker than those for which NaCl or NaBr are added. The weaker signal in the CHx range for NaI is consistent with the fact that the monolayer is not stable at the surface area of the trough that corresponds to molecular area smaller than ∼26 Å2/molecule of the stable ODA monolayer as shown in the π−A isotherms in Figure 3 as well as with a recent report that has shown that the adsorbed I− disrupts positively charged monolayers’ structure and water polarization the most.19,21 Indeed, the NaI shows the lowest OH signal, indicating that the affinity of I− to the charged amine group is strongest among the three halide anions. This anion dependence on the monolayer structure and the OH spectra is consistent with recent studies.19,22
Figure 5. (a) XRR results for the ODA monolayers at pH 3.5 with additional 3 mM salts (NaCl, NaBr, and NaI) in the subphase. The inset is the magnification of the data in the qz range of 0.04−0.2 Å−1. The solid lines are least-squares fits to the data with the model electron density profiles shown in panel b at 25 mN/m. 13756
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Figure 6. Schematic interfacial structures of the ODA molecules, interfacial water, and the adsorbed counterions at (a) pH > 5.7, (b) pH ∼ 3.5, (c) pH < 3.0.
dissolve into water at these pH values, as the protonation of the ODA headgroup is close to complete, making the hydrophilic interaction at the headgroup region stronger. It is well known that the bulk pKa value of fatty amine is ∼10, thus it is expected that the amine groups are protonated (NH3+) at neutral pH (∼6) conditions. However, our SFG experiment in Figure 2, does not show significant change of water OH intensity in neutral and basic pH region (5.7−11), indicating that the ionization of the amine groups is not complete. Such behavior is expected from the collective behavior of surface charge that effectively lowers the “surface pKa”. As the abrupt dissolution of the ODA monolayer and increased water SF intensity occurs together in lower pH, we conclude that ionization of the amine group is responsible for these phenomena below pH 5. This is consistent with a second harmonic generation study of the longer fatty amine docosyl amine CH3(CH2)21NH2 that demonstrated how the effective pKa of amine surface shifts to much lower values from that of the individual amine in water.27 Recently, heterodyne-detected electronic SFG (HD-ESFG) spectroscopy monitored the depletion of protons at the positively charged air/water interface and observed differences in the dissociation curves of pH sensitive chromophores at the interface and in the bulk.28,29 These studies are also relevant to the question regarding longer-chain fatty-amines. In this study, we also find that the reappearance of the monolayer by lowering the pH to 2.5 is due to the neutralization of the headgroup by the adsorption of the increased density of Cl− in the subphase. To test this, we add monovalent halide salts (NaCl, NaBr, and NaI) to the pH 3.5 solution to show that the ODA monolayer recovers, albeit with some differences among the different halides. This is clear evidence that the screening of the charge at the headgroup by the anion is responsible for reducing the hydrophilic interaction and for driving the molecule back to the surface. We also find that among the three halogen ions, I− has the strongest effect on the structure of the monolayer consistent with recent observations.22
the ED in the headgroup region is significantly higher than that of the Cl−, as expected from the atomic numbers. However, whereas the ED at the chain region is practically the same for the NaCl and NaBr solutions, it is significantly lower for the NaI solution. This indicates that the hydrocarbon chains in the presence of I− in the solution are not densely packed, and presumably the chains are tilted and have gauche defects. This is consistent with the SF spectra in Figure 4, which shows smaller signals and appreciable CH2 signals only for NaI. We note that the EDs associated with the headgroup layer on salt subphases are higher than that ODA on water, clearly indicating the presence of anion adsorption (Figure 5b). Assuming the adsorption ratio between anion and amine is 1:1, the calculated ED values for NH3 + Cl, NH3 + Br, NH3 + I, are 0.48, 0.68, and 0.66 e−/Å3, respectively, compared to the calculated ED values for H2O and NH3+ are 0.33 and 0.26 e−/Å3. The fact that the ED of the monolayer on I is slightly lower than that of Br is likely due to the fact that the molecular area of ODA is larger on the I subphase compared to that on Br. This is consistent with the conclusion regarding the ED of the chains discussed above. The structural change of the monolayer, interfacial water, and the counterions are schematically illustrated in Figure 6: (a) The ODA molecules are almost charge-neutral, and the monolayer is stable at pH > 5.7. (b) As the pH is lowered, the amine headgroups are protonated, and the ODA molecules either dissolve into the water subphase or are positioned loosely at the interface. This conformation makes the interface positively charged, and the water molecules at the interfaces are aligned following the electric field. (c) At the lower pH, the Cl− ions bind to the protonated amine headgroups to neutralize the headgroups, which facilitates the monolayer formation as less ODA molecules leave the surface. As the surface is occupied by ODA with counterions, the monolayer is less densely packed as compared to the case of high pH. As a related phenomenon, the role of halide anions on the structure of Langmuir monolayers consisting of quaternary ammonium molecules having different molecular structures has been reported.23 Inclusion of a neutral or negatively charged surfactant may prevent dissolution of the ODA molecules, as reported in similar systems.24−26
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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.langmuir.5b03947. Evidence of remnant ODA at pH 3.5 and fitted result of XRR from ODA on various subphases (PDF)
4. CONCLUSION We have demonstrated that ODA does not form a stable monolayer on a water surface in the pH 3−4 range due to significant dissolution of the molecules into the subphase, as has been shown previously.6 However, although the π−A isotherms do not show appreciable increase in pressure at this pH range, sum-frequency generation spectroscopy and X-ray reflectivity find that remnant molecules are present at the interface, albeit at a very low density compared to the densely packed ODA monolayer at neutral pH. Most molecules
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AUTHOR INFORMATION
Corresponding Author
*E-mail address:
[email protected]. Notes
The authors declare no competing financial interest. 13757
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(19) Sung, W.; Wang, W.; Lee, J.; Vaknin, D.; Kim, D. Specificity and variation of length scale over which monovalent halide ions neutralize a charged interface. J. Phys. Chem. C 2015, 119, 7130−7137. (20) Piatkowski, L.; Zhang, Z.; Backus, E. H.; Bakker, H. J.; Bonn, M. Extreme surface propensity of halide ions in water. Nat. Commun. 2014, 5, 4083. (21) Gurau, M. C.; Lim, S. M.; Castellana, E. T.; Albertorio, F.; Kataoka, S.; Cremer, P. S. On the mechanism of the Hofmeister effect. J. Am. Chem. Soc. 2004, 126, 10522−10523. (22) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Counterion effect on interfacial water at charged interfaces and its relevance to the Hofmeister series. J. Am. Chem. Soc. 2014, 136, 6155−6158. (23) Lide, D. R. Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995. (24) Ge, A.; Peng, Q.; Wu, H.; Liu, H.; Tong, Y.; Nishida, T.; Yoshida, N.; Suzuki, K.; Sakai, T.; Osawa, M.; Ye, S. Effect of Functional Group on the Monolayer Structures of Biodegradable Quaternary Ammonium Surfactants. Langmuir 2013, 29, 14411− 14420. (25) Ge, A.; Wu, H.; Darwish, T. A.; James, M.; Osawa, M.; Ye, S. Structure and Lateral Interaction in Mixed Monolayers of Dioctadecyldimethylammonium Chloride (DOAC) and Stearyl Alcohol. Langmuir 2013, 29, 5407−5417. (26) Vaknin, D.; Bu, W. Neutrally Charged Gas/Liquid Interface by a Catanionic Langmuir Monolayer. J. Phys. Chem. Lett. 2010, 1, 1936− 1940. (27) Zhao, X.; Ong, S.; Wang, H.; Eisenthal, K. B. New method for determination of surface pKa using second harmonic generation. Chem. Phys. Lett. 1993, 214, 203−207. (28) Yamaguchi, S.; Bhattacharyya, K.; Tahara, T. Acid-base equilibrium at an aqueous interface: pH spectrometry by heterodyne-detected electronic sum frequency generation. J. Phys. Chem. C 2011, 115, 4168−4173. (29) Kundu, A.; Yamaguchi, S.; Tahara, T. Evaluation of pH at charged lipid/water interfaces by heterodyne detected electronic sum frequency generation. J. Phys. Chem. Lett. 2014, 5, 762−767.
ACKNOWLEDGMENTS This research is supported by the National Research Foundation Grant No. 2011-0017435. K.S. acknowledges financial support by the Midcareer Researcher Program (2011-0017539), funded by the Ministry of Science, ICT & Future Planning, Korea. Z.A. acknowledges help from Peter V. Pikhitsa of Seoul National University. The work at Ames Laboratory was supported by the Department of Energy, Office of Basic Energy Sciences, under Contract Number DE-AC0207CH11358.
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REFERENCES
(1) Spink, J. A. Ionization of monolayers of fatty acids from C14 to C18. J. Colloid Sci. 1963, 18, 512−525. (2) Heikkila, R. E.; Deamer, D. W.; Cornwell, D. G. Solution of fatty acids from monolayers spread at the air-water interface: identification of phase transformations and the estimation of surface charge. J. Lipid Res. 1970, 11, 195−200. (3) Johann, R.; Vollhardt, D. Texture features of long-chain fatty acid monolayers at high pH of the aqueous subphase. Mater. Sci. Eng., C 1999, 8−9, 35−42. (4) Brzozowska, A. M.; Duits, M. H. G.; Mugele, F. Stability of stearic acid monolayers on artificial sea water. Colloids Surf., A 2012, 407, 38− 48. (5) Kundu, S.; Langevin, D. Fatty acid monolayer dissociation and collapse: Effect of pH and cations. Colloids Surf., A 2008, 325, 81−85. (6) Albrecht, O.; Matsuda, H.; Eguchi, K. Main and tilt transition in octadecylamine monolayers. Colloids Surf., A 2006, 284−285, 166− 174. (7) Miranda, P. B.; Du, Q.; Shen, Y. R. Interaction of water with a fatty acid Langmuir film. Chem. Phys. Lett. 1998, 286, 1−8. (8) Sung, W.; Seok, S.; Kim, D.; Tian, C. S.; Shen, Y. R. Sumfrequency spectroscopic study of Langmuir monolayers of lipids having oppositely charged headgroups. Langmuir 2010, 26, 18266− 18272. (9) Wang, W.; Sung, W.; Ao, M.; Anderson, N. A.; Vaknin, D.; Kim, D. Halide ions effects on surface excess of long chain ionic liquids water solutions. J. Phys. Chem. B 2013, 117, 13884−13892. (10) Kim, J. H.; Yim, S. Y.; Oh, M. K.; Phan, M. D.; Shin, K. Adsorption behaviors and structural transitions of organic cations on an anionic lipid monolayer at the air−water interface. Soft Matter 2012, 8, 6504−6511. (11) Parratt, L. G. Surface Studies of Solids by Total Reflection of XRays. Phys. Rev. 1954, 95, 359−369. (12) Phan, M. D.; Shin, K. Effects of cardiolipin on membrane morphology: a Langmuir monolayer study. Biophys. J. 2015, 108, 1977−1986. (13) Knock, M. M.; Bain, C. D. Effect of Counterion on Monolayers of Hexadecyltrimethyl-ammonium Halides at the Air−Water Interface. Langmuir 2000, 16, 2857−2865. (14) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Sum-frequency vibrational spectroscopy of a Langmuir film: Study of molecular orientation of a two-dimensional system. Phys. Rev. Lett. 1987, 59, 1597−1600. (15) Du, Q.; Freysz, E.; Shen, Y. R. Surface Vibrational Spectroscopic Studies of Hydrogen Bonding and Hydrophobicity. Science 1994, 264, 826−828. (16) Wang, J.; Buck, S. M.; Chen, Z. Sum Frequency Generation Vibrational Spectroscopy Studies on Protein Adsorption. J. Phys. Chem. B 2002, 106, 11666−11672. (17) Dong, B.; Li, N.; Zheng, L.; Yu, L.; Inoue, T. Surface Adsorption and Micelle Formation of Surface Active Ionic Liquids in Aqueous Solution. Langmuir 2007, 23, 4178−4182. (18) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M. Electron spectroscopy of aqueous solution interfaces reveals surface enhancement of halides. Science 2005, 307, 563−566. 13758
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