Cooperative Effects of ZwitterionicâIonic Surfactant Mixtures on

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Cooperative Effects of Zwitterionic–Ionic Surfactant Mixtures on Interfacial Water Structure Revealed by Sum Frequency Generation Vibrational Spectroscopy Xuecong Pan, Fangyuan Yang, Shunli Chen, Xuefeng Zhu, and Chuanyi Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00178 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Cooperative Effects of Zwitterionic–Ionic Surfactant Mixtures on Interfacial Water Structure Revealed by Sum Frequency Generation Vibrational Spectroscopy Xuecong Pan,†,‡ Fangyuan Yang, † Shunli Chen, † Xuefeng Zhu*,† and Chuanyi Wang*,† †

Laboratory of Environmental Science and Technology, Xinjiang Technical Institute of Physics

and Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, 40-1 South Beijing Road, Urumqi 830011, Xinjiang, China ‡

Laboratory of Nanofiber Membrane Materials and Devices, Xinjiang University Institute of Science and Technology, 1 Xuefu Road, Akesu 843100, Xinjiang, China

ABSTRACT. Cooperative effects of a series of equimolar binary zwitterionic–ionic surfactant mixtures on interfacial water structure at the air–water interfaces have been studied by sum frequency generation vibrational spectroscopy (SFG-VS). For zwitterionic surfactant palmityl sulfobetaine (SNC16), anionic surfactant sodium hexadecyl sulfate (SHS) and cationic surfactant cetyltrimethylammonium bromide (CTAB) with the same length of alkyl chain, significantly enhanced ordering of interfacial water molecules was observed for the zwitterionic–anionic surfactant mixtures SNC16-SHS, indicating that SNC16 interacts more strongly with SHS than

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with CTAB due to the strong headgroup–headgroup electrostatic attraction for SNC16-SHS. Meanwhile, the SFG amplitude ratio of methyl and methylene symmetric stretching modes was used to verify the stronger interaction between SNC16 and SHS. The conformational order indicator increased from 0.64 for SNC16 to 7.17 for SNC16-SHS, but only 0.94 for SNC16-CTAB. In addition, another anionic surfactant sodium dodecyl sulfate (SDS) was introduced to study the influence of chain–chain interaction. Decreased SFG amplitude of interfacial water molecules for SNC16-SDS was observed. Therefore, both the headgroup–headgroup electrostatic interaction and chain–chain van der Waals attractive interaction of the surfactants play important role in enhancing the ordering of interfacial water molecules. The results provided experimental and theoretical bases for practical applications of the surfactants.

INTRODUCTION. Surfactants with amphiphilic property are widely used in industrial production and in our daily life such as detergents, cosmetics and lubricants.1 Usually, several surfactants are coexist in the products due to the enhanced surface activity arising from the strong synergistic interaction of distinct surfactants.2–4 Zwitterionic surfactants have both positively and negatively charged sites in the headgroups, thus the study on the intermolecular interaction of zwitterionic–ionic surfactant mixtures is meaningful for practical applications. Surfactants adsorbed at the air–water or oil–water interfaces could form self-assembly monolayer. A number of techniques have been employed to probe the properties of interfaces with surfactants, such as surface tension (ST), neutron reflectivity (NR), X-ray diffraction (XRD) and second-order nonlinear spectroscopy.5–12 Recently, the asymmetric synergy in the adsorption of zwitterionic–ionic surfactant mixtures at the air–water interface below and above the critical


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micelle concentration (CMC) was analyzed by ST and NR measurements.9 The CMC, surface compositions, surface excess and excess free energy were compared for distinct surfactant mixtures. For surface-selective second-order nonlinear spectroscopy, in contrast to second harmonic generation (SHG), sum frequency generation vibrational spectroscopy (SFG-VS) can provide more interface information on the molecular level.13–17 The SFG signals from either the alkyl chains or the headgroups of surfactants as well as the interfacial water molecules have been reported to investigate the characteristics of interfaces with surfactants. The SFG signals from methyl and methylene groups of the alkyl chain have been used to reveal the conformational order caused by gauche defects,18–20 the interfacial density, the orientation,6, 21–22 the effects of halide co-ions on the adsorption of surfactants,23 the electrostatic interaction with proteins24 and so on. The SFG signals from the headgroups have also been used to probe the adsorption behaviors of surfactants.25 Furthermore, based on the surfactant–water interaction, both the adsorption of surfactants and the interfacial water structure have been analyzed by the SFG signals from the interfacial water molecules.1, 26 Usually, more than one region of SFG signals are measured to obtain the comprehensive characteristics of interfaces with surfactants. Although many kinds of individual surfactants (nonionic, zwitterionic, anionic, and cationic surfactants) and catanionic system have been investigated by SFG-VS, work on the zwitterionic-ionic system is still limited. The interactions of anionic surfactant sodium dodecyl sulfate (SDS) at the millimolar and micromolar level with insoluble zwitterionic lipid 1,2-dipalmitoyl-sn-glycero-3phosphocholine (DPPC) at the air−water interface have been studied, respectively.27–28 To the best of our knowledge, molecular level information by SFG-VS study on the water-soluble zwitterionic-ionic surfactants mixtures has not yet reported so far.


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In this study, a series of binary zwitterionic–ionic surfactant mixtures have been studied by sum frequency generation vibrational spectroscopy (SFG-VS) to show the cooperative effects on interfacial water structure. Firstly, zwitterionic surfactant palmityl sulfobetaine (SNC16), anionic surfactant sodium hexadecyl sulfate (SHS) and cationic surfactant cetyltrimethylammonium bromide (CTAB) with the same length of alkyl chain were explored. The SFG spectra in the OH region were applied to show the ordering of interfacial water molecules. In contrast to the individual surfactants, significantly enhanced ordering was observed for SNC16-SHS. The results implied that zwitterionic surfactant SNC16 interacted more strongly with anionic surfactant SHS than with cationic surfactant CTAB. Subsequently, the SFG spectra in the CH region were measured to verify the stronger interaction for SNC16-SHS than SNC16-CTAB. Finally, another anionic surfactant sodium dodecyl sulfate (SDS) with the same headgroup but shorter alkyl chain was introduced to demonstrate the contribution of chain-chain interaction. The weaker SFG intensity of interfacial water molecules for SNC16-SDS than that for SNC16-SHS suggested that weaker chain–chain van der Waals attractive interaction also resulted in poorer ordering of interface water molecules. MATERIALS AND METHODS. Sample Preparation. Sodium dodecyl sulfate (SDS) (≥99%) was purchased from Sigma-Aldrich. Sodium hexadecyl sulfate (SHS) (99%) was purchased from Alfa Aesar. Cetyltrimethylammonium bromide (CTAB) (99%) was purchased from J&K Scientific Ltd. Palmityl sulfobetaine (SNC16) (>98%) were purchased from TCI Shanghai. The molecular structures of the surfactants were shown in Scheme 1(a). All the surfactants were used without further purification. Ultrapure water (18.2 MΩ·cm) was used for solution preparation and dilution. Individual surfactant solution with high concentration (0.2 mM for SHS and 1 mM for other surfactants) was prepared, then a specific volume of the solution was injected into a


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circular reservoir with a certain volume of ultrapure water to obtain 12.18 mL solution of binary zwitterionic–ionic surfactant mixtures (1:1) or individual surfactants with the total bulk concentration of 15 µM for sum frequency generation (SFG) measurements.

Scheme 1. (a) Molecular structures of the surfactants. (b) Illustrated structures of the zwitterionic–ionic surfactant mixtures with the same length of alkyl chain at the air–water interface.

Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) System. The setup of the SFG-VS system was similar to that reported elsewhere.29 The femtosecond laser (35 fs, 4 mJ, 800 nm, 1 kHz) from a commercial Ti:Sapphire regenerative amplifier system (Spitfire Ace, Spectra-Physics) was split into three beams. The optical parametric amplifier and difference frequency mixing system (Topas-Prime, TP8U1C3, Light Conversion) pumped by 1.5 mJ of the femtosecond laser generated infrared (IR) pulses (2600–20000 nm). One-fourth of the


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femtosecond laser passed through a pulse stretcher and a time delay device, then the modulated laser was used as VIS pulse and overlapped with the IR pulse temporally and spatially on the sample to generate SFG signal. The residual 1.5 mJ of the femtosecond laser was prepared for pumping samples which was not used in this study. Pulse stretching was obtained by a pulse shaper,29–30 which consisted of a reflective diffraction grating (GR25-1208, 1200 L/mm, Thorlabs), a cylindrical lens, a slit and a reflection mirror. The bandwidth of VIS pulse in the experiments was about 10 cm-1. Time delay was controlled by a motorized translation stage (TLSM, Zaber). The polarizations of VIS pulse and SFG signal were both controlled by an achromatic half-wave plate and a polarizing beamsplitter cube (Thorlabs). The incident angle of VIS pulses was 45°, and the IR pulse had an incident angle of 55°. After attenuating the VIS pulse by double 750 nm shortpass filters, the SFG signal was focused into monochromator (SR500, Andor) by a lens of 60mm, and then recorded by an electron-multiplied charge-couple device (EMCCD) camera (Newton, Andor) at -65 °C. In the experiments, the OH vibrational modes of water were studied in the region of 3050–3550 cm-1, and the CH vibrational modes of alkyl chain were studied in the region of 2750–3050 cm-1, both in the ssp (the polarization for SFG signal, VIS pulse and IR pulse, respectively) polarization combinations. The SFG intensities were normalized by that of a z-cut quartz crystal with the thickness of 1 mm. RESULTS AND DISCUSSION. In order to investigate the interaction between zwitterionic surfactant and ionic surfactant with the same length of alkyl chain and their cooperative effects on the interfacial water structure, zwitterionic surfactant SNC16 was chosen as a model and mixed with anionic surfactant SHS or cationic surfactant CTAB at 1:1 molar ratio, forming binary zwitterionic–anionic surfactant mixture SNC16-SHS and zwitterionic–cationic surfactant mixture SNC16-CTAB, respectively. The SFG spectra at the air–water interfaces with binary


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zwitterionic–ionic surfactant mixtures and individual surfactants in the OH region were shown in Figure 1. Compared with neat water, significant increase of the SFG intensities of a broad band from 3050 cm-1 to 3550 cm-1 was observed for all the studied surfactant systems. For the individual surfactants, the SFG intensity for SNC16 was comparable to that for SHS, and stronger than that for CTAB. For the binary zwitterionic–ionic surfactant mixtures, the SFG intensity for SNC16-SHS was much stronger than those for the individual surfactants, while the SFG intensity for SNC16-CTAB was stronger than that for CTAB and weaker than that for SNC16. Although the assignment of this broad band was controversial,22, 26, 31–33 it was acknowledged that the SFG signal of interface water molecules in this region was assigned to two OH stretching modes at around 3200 cm-1 and 3450 cm-1 which were attributed to icelike interfacial water structure with high degree of hydrogen bond order and liquidlike interfacial water structure with weak hydrogen bond, respectively. The significant increases of the SFG intensities were due to enhanced ordering of interfacial water molecules induced by the electrostatic field of the charged headgroups. Surfactants with amphiphilic property adsorbed at the air–water interface and formed self-assembly monolayer. Charged headgroups of the ordered surfactants generated strong surface electrostatic field and aligned the interface water molecules, then the contribution to the second-order nonlinear susceptibility34–35 corresponding to the OH stretching modes of hydrogen bond increased, which resulted in enhanced SFG intensity. For the individual surfactants, the anionic surfactant oriented water molecules with hydrogens pointing up toward the air phase, while the cationic surfactant aligned the water molecules with hydrogens pointing down toward the liquid phase.36 The zwitterionic surfactant with both positively charged segment and negatively charged segment in the headgroup caused opposite orientation of the water molecules without significantly flipping the water molecules.26 The


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ordering of interfacial water molecules was dependent on both the charged headgroup and the concentration of surfactant,1, 37 which influenced the electrostatic field at the air–water interface. It was known that ionic surfactant induces much more ordered water structure at the air–water interface than zwitterionic surfactant at CMC with the similar interfacial density because of the stronger electrostatic field of the charged headgroup.36 When the concentration of surfactant was far below the CMC, the interfacial density of surfactants increased as the concentration increased, and resulted in enhanced electrostatic field at the air–water interface, leading to enhanced SFG intensity of the OH stretching modes. With the same length of alkyl chain, the headgroup– headgroup electrostatic repulsion of ionic surfactants was the possible reason for the lower surface activity of SHS and CTAB. The chosen concentration of surfactants was close to the CMC38 of SNC16, and far below the CMCs37,

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of SHS and CTAB, leading to the lower

interfacial density of SHS and CTAB. Hence, the relative SFG intensities of the OH stretching modes for SNC16, SHS and CTAB were determined by both the charge characteristic of their headgroups and the concentration in contrast to their CMCs. The much higher interfacial density of SNC16 as verified by the surface tension measurements (Table S2 in the Supporting Information) induced comparable SFG intensity between SNC16 and SHS, while the low interfacial density (high CMC) of CTAB may induce the lower SFG intensity. For the equimolar binary zwitterionic–ionic surfactant mixtures, the SFG intensity of the OH stretching modes for zwitterionic–anionic surfactant mixture SNC16-SHS was much stronger than that for zwitterionic–cationic surfactant mixture SNC16-CTAB, indicating that SNC16 (zwitterionic surfactant) interacted more strongly with SHS (anionic surfactant) than with CTAB (cationic surfactant). The result was in agreement with the reported much higher interaction parameter or excess free energy of adsorption for zwitterionic–anionic surfactant mixture


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obtained by surface tension measurements.9, 40–41 The interaction between zwitterionic surfactant and ionic surfactant included both hydrophobic chain–chain interaction (van der Waals attractive interaction) and headgroup–headgroup interaction (electrostatic interaction). With the same length of alkyl chain, the observed different interaction mainly resulted from the headgroup– headgroup interaction. Several possible explanations have been proposed as following. Some supported that electrostatic attractions for both systems should be comparable, and the difference was due to the different structures of mixed monolayers.40 The positively charged segment of zwitterionic surfactant SNC16 was located close to the air–water interface, thus attracted the negatively charged headgroup of anionic surfactant and repelled the positively charged headgroup of cationic surfactant. Some suggested that because of the larger size of the trimethylammonium headgroup of cationic surfactant, the positive charge could not discriminate well between the positive and negative sites in the headgroup of zwitterionic surfactant, causing weaker interaction.9 Some showed that for the zwitterionic surfactant with sulfobetaine headgroup, the size of the positive segment was significantly larger than that of the negative segment, leading to cationic-like characteristic.26 Therefore, SNC16 interacted more strongly with SHS than with CTAB. Despite the uncertainty in the interactive way of zwitterionic and ionic surfactants, both the local and global interaction of headgroups may contribute to the strong headgroup–headgroup electrostatic attraction for SNC16-SHS and repulsion for SNC16-CTAB. Electrostatic attraction is one of the non-covalent interactions for forming supra-amphiphile,42-44 which was introduced to explain the ordering of interfacial water molecules. For SNC16-SHS, SNC16 and SHS were suggested to self-assemble into double-chain anionic-like supraamphiphile with higher surface activity which can be supported by the reported low CMC of surfactant mixture with the same headgroups but different alkyl chains.9 As discussed above, the


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increase of both the interfacial density of surfactants and the charge of headgroup enhanced the electrostatic field at the air–water interface, as shown in scheme 1(b), and then significantly strengthened SFG signal of the hydrogen-bonded interfacial water molecules was observed. For SNC16-CTAB, the weak interaction between SNC16 and CTAB made it difficult to form supraamphiphile. The competitive adsorption of SNC16 and CTAB may increase the charge of headgroup, but the contribution was small due to the weaker surface activity of CTAB. As previously observed by surface tension and neutron reflection measurements, the fraction of ionic surfactants at the air–water interface is less than zwitterionic surfactants for equimolar mixtures.9 Furthermore, the moderate interfacial density of surfactants supported by the reported CMC result9 induced moderate SFG signal of SNC16-CTAB with respect to SNC16 and CTAB. Namely, the effect of zwitterionic–cationic surfactant mixture on interface water structure was similar to those of the individual surfactants.


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Figure 1. SFG spectra (dots) in the ssp polarization at the air–water interfaces with (a) zwitterionic–anionic surfactant mixture and individual surfactants and (b) zwitterionic–cationic surfactant mixture and individual surfactants in the OH region. The normalized SFG intensity for SNC16-SHS was divided by five for preferable comparisons. The solid lines were shown as a guide to the eye. Though with the same length of alkyl chain, enhanced headgroup–headgroup electrostatic interactions of surfactants can increase the chain–chain van der Waals attractive interactions, and then give rise to a higher level of conformational order for the alkyl chain.22 Therefore, the SFG spectra at the air–water interfaces with binary zwitterionic–ionic surfactant mixtures and individual surfactants in the CH region were measured (as shown in Figure 2) to analyze the conformational order of the alkyl chain for further verification of the different interactions between zwitterionic and ionic surfactants. For SHS and CTAB, due to the low concentration with both low interfacial density and high orientation angle to the surface normal of the alkyl chain, no detectable SFG signal was observed in the CH region. For SNC16 and SNC16-CTAB, two obvious peaks at ~2855 cm-1 and ~2880 cm-1 as well as a broad band from 2900 cm-1 to 2970 cm-1 were observed. For SNC16-SHS, the SFG intensity of the peak at ~2855 cm-1 decreased significantly and obvious interference between the CH stretching modes of the surfactants and the OH stretching modes of interface water was observed. The Lorentzian line shape fittings of the SFG spectra were achieved by taking the icelike hydrogen bonding mode into account. The peak at ~2855 cm-1 was assigned to methylene symmetric stretch (d+)45–48 resulting from the gauche defects in the alkyl chain. The peak at ~2880 cm-1 was assigned to methyl symmetric stretch (r+). The ratio between the SFG amplitudes of r+ and d+ (A (r+) / A (d+)) in the ssp polarization combinations has been proposed as an indicator of the conformational order for the


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alkyl chain.5, 19 The fitted parameters for conformational order indicators were shown in Table 1. In comparison with SNC16, the SFG amplitudes of methyl symmetric stretch (r+) for SNC16-SHS and SNC16-CTAB respectively increased remarkably and slightly, probably because of the more tilted alkyl chain of SNC16 in contrast to SHS and CTAB.9 The different degrees of increase may verify the stronger interaction of SNC16 with SHS than with CTAB, which increased the interfacial density of SNC16-SHS supra-amphiphiles and efficiently reduced the gauche defects, then resulted in the higher conformational order. The weak interaction between SNC16 and CTAB can also decrease the gauche defects marginally.

Figure 2. SFG spectra (dots) in the ssp polarization at the air–water interfaces with binary zwitterionic–ionic surfactant mixtures and individual surfactants in the CH region and their Lorentzian line shape fittings (lines). The spectra were offset vertically for clarity. Table 1. Fitted Parameters for the Conformational Order Indicators.

SNC16 SNC16-SHS SNC16-CTAB

A(r+) 1.12 2.15 1.51

A(d+) 1.75 0.30 1.63

A(r+)/ A(d+) 0.64 7.17 0.94


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Strong cooperative effect of zwitterionic–anionic surfactant mixture SNC16-SHS on interface water structure has been observed by the remarkably enhanced SFG intensity of OH stretching modes. The headgroup–headgroup electrostatic attraction of SNC16 and SHS was suggested to play a dominant role. Nevertheless, the van der Waals attractive interaction between the alkyl chains was also important. Another anionic surfactant SDS with the same headgroup but shorter alkyl chain was introduced to study the influence of chain length on the surface activity of zwitterionic–anionic supra-amphiphile and the ordering of interfacial water molecules. As shown in Figure 3, compared with SHS at the same concentration, the lower surface activity of SDS with shorter alkyl chain generated weaker SFG signal in the OH region. Similarly, the SFG signal of SNC16-SDS was also weaker than that of SNC16-SHS, indicating the weaker interaction of SNC16 with SDS than with SHS and the lower surface activity of SNC16-SDS, which were attributed to the weaker chain–chain van der Waals attractive interaction. Therefore, the chain length of anionic surfactant influenced the surface activity of zwitterionic–anionic supraamphiphile. Both the headgroup–headgroup electrostatic attraction and the chain–chain van der Waals attractive interaction of the zwitterionic–anionic surfactant mixture can enhance the ordering of interfacial water molecules.


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Figure 3. SFG spectra (dots) in the ssp polarization at the air–water interfaces with binary zwitterionic–anionic surfactant mixtures and individual surfactants in the OH region. The solid lines were shown as a guide to the eye. CONCLUSIONS. The cooperative effects of a series of binary zwitterionic–ionic surfactant mixtures on interfacial water structure at the air–water interfaces were explored and compared by SFG-VS. The interactions between zwitterionic and ionic surfactants with the same length of alkyl chain were analyzed in the OH and CH regions. Strong interactions between zwitterionic and anionic surfactants and weak interactions between zwitterionic and cationic surfactants were observed. Both the headgroup–headgroup electrostatic interaction and the chain–chain van der Waals attractive interaction in the mixtures influenced the adsorption of the surfactants and then induced the different ordering of the interfacial water molecules as supported by the SFG intensities. These results provided experimental and theoretical bases for further study on surfactant mixtures and the application of surfactants in industrial production as well as daily life. Further work on zwitterionic–ionic surfactant mixtures at various concentrations and compositions can provide deep insight into the interaction between different surfactants. ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. The fitting results of SFG spectra in the CH region and surface tension measurements. (PDF) AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected] or [email protected] ORCID ID: 0000-0002-7146-115X Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The National Natural Science Foundation of China (21403292 and 21403293); The Postdoctoral Scientific Research Foundation of Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (21403292 and 21403293) and the Postdoctoral Scientific Research Foundation of Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. ABBREVIATIONS SFG-VS, sum frequency generation vibrational spectroscopy; SNC16, palmityl sulfobetaine; SHS, sodium hexadecyl sulfate; CTAB, cetyltrimethylammonium bromide; SDS, sodium dodecyl sulfate; ST, surface tension; NR, neutron reflectivity; XRD, X-ray diffraction; CMC, critical micelle concentration; SHG, second harmonic generation; SFG, sum frequency generation; EMCCD, electron-multiplied charge-couple device.


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Table of Contents/Abstract Graphic.

Cooperative Effects of Zwitterionic-Ionic Surfactant Mixtures on Interfacial Water Structure at Air–Water Interface


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Scheme 1. (a) Molecular structures of the surfactants. (b) Illustrated structures of the zwitterionic–ionic surfactant mixtures with the same length of alkyl chain at the air–water interface. 214x251mm (96 x 96 DPI)


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Figure 1. SFG spectra (dots) in the ssp polarization at the air–water interfaces with (a) zwitterionic–anionic surfactant mixture and individual surfactants and (b) zwitterionic–cationic surfactant mixture and individual surfactants in the OH region. The normalized SFG intensity for SNC16-SHS was divided by five for preferable comparisons. The solid lines were shown as a guide to the eye. 312x426mm (96 x 96 DPI)


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Figure 2. SFG spectra (dots) in the ssp polarization at the air–water interfaces with binary zwitterionic–ionic surfactant mixtures and individual surfactants in the CH region and their Lorentzian line shape fittings (lines). The spectra were offset vertically for clarity. 312x220mm (96 x 96 DPI)


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Figure 3. SFG spectra (dots) in the ssp polarization at the air–water interfaces with binary zwitterionic– anionic surfactant mixtures and individual surfactants in the OH region. The solid lines were shown as a guide to the eye. 312x220mm (96 x 96 DPI)


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