Understanding the Different Steps of Surfactant Adsorption at the Oil

Mar 21, 2016 - Probing the behavior of surfactants at oil–water interfaces is crucial to understand their functionality. In this work, we present de...
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Understanding the Different Steps of Surfactant Adsorption at the Oil−Water Interface with Second Harmonic Generation Wei Wu,†,‡ Hui Fang,†,‡ Fangyuan Yang,†,‡ Shunli Chen,† Xuefeng Zhu,† Qunhui Yuan,† and Wei Gan*,† †

Laboratory of Environmental Science and Technology, Xinjiang Technical Institute of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Probing the behavior of surfactants at oil−water interfaces is crucial to understand their functionality. In this work, we present detection of the adsorption of several common surfactants at the hexadecane−water interface with second harmonic generation (SHG) and zeta potential measurements. Water molecules were used as reliable indicators of the adsorption of ionic surfactants in SHG analysis. With the change of the interfacial potential monitored by both SHG and zeta potential measurements, unique information about the multiple steps involved in the adsorption of typical surfactants at the oil−water interface is provided. It was revealed that the adsorption of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) at the hexadecane−water interface is initialled by a step dominated by the adsorption of the hydrophobic part of the surfactant, and a latter step involves comparable contributions from both the hydrophobic part and the counterion. The adsorption free energies involved in the initial step can be quantitatively analyzed. In addition, the adsorption of two oil-soluble amphiphiles at the hexadecane−water interface was also studied. Analysis of the ionic strength dependent SHG signal at the hexadecane−water interface also reveals that the origin of the SHG emission is mainly the water molecules at the interfacial layer. The preferential orientation of water molecules is with the hydrogen atoms pointing to the oil phase.

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used as indicators of surfactant adsorption.32,42−46 It has been demonstrated that the adsorption isotherm of a surfactant at an interface can be obtained by monitoring the SFG amplitude from the methyl group at the end of the alkane chain31,32 or the sulfate headgroup of the surfactant.44 For such SFG-VS analysis, a relatively high bulk concentration is required to achieve a reliable adsorption isotherm, although it has been demonstrated that surfactant in a concentration range of tens of nM can significantly alter the SFG-VS spectra of the CCl4− water interface.43 SHG has also been successfully applied in the analysis of molecular adsorption, molecular structure, and photochemical reaction at the interfaces.3−7,9,21,22,47−49 However, using SHG to probe the adsorption of surfactants at the interfaces is not easy because in comparison with dye molecules or metallic materials that were generally studied, the hyperpolarizabilities of typical surfactant molecules are relatively small.7,9,21,22 It was demonstrated that molecules with an embedded chromophore were needed as SHG probes for such analyses.3−5,7,21 Surfactants at interfaces and the interactions between interfacial molecules can also be indirectly probed using dye molecules, as shown in the literature9 and our recent report.22 The adsorption of ionic surfactants at water-based interfaces was

INTRODUCTION The dynamics and molecular structure of interfaces in the presence of surfactants have been intensively investigated.1−23 Among the techniques used, second-order nonlinear spectroscopic techniques, i.e., second harmonic generation (SHG) and sum frequency generation−vibrational spectroscopy (SFG-VS), have attracted more and more attention because of their unique interface selectivity.4,6,7,9,11,13,18−20,22,24−29 Other techniques including interfacial tension measurements, ellipsometry, quasi-elastic laser scattering, neutron scattering, and X-ray scattering have also been applied to get information such as the interfacial excess of the surfactants, the interfacial depth, and the orientational order of the absorbed long chain molecules at the interfaces.1,2,8,10,12,15,16 As a powerful probe of the vibrational spectra of interfacial molecules, SFG-VS has been intensively applied in the study of surfactants at interfaces,6,11,13,18−20,29−32 benefiting from its capability of molecular recognition.6,33−40 In the analysis of the density and molecular conformation of surfactants at interfaces, generally the amplitude of the SFG signal from methyl and methylene groups and their ratio are measured.13,19,20,30−32 For example, long chain surfactants were observed to be more ordered and well-oriented at oil−water interfaces when their bulk concentrations approach the critical micelle concentration (CMC).13,31,41 The SFG signals from other molecular groups such as the sulfate or carboxylate headgroup in surfactants and the vibrational stretching modes of water molecules were also © XXXX American Chemical Society

Received: November 17, 2015 Revised: March 7, 2016

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original alignment back. However, for a cylindrical cell, such height adjustment changes the refraction angle of the fundamental laser while it travels across the wall of the cell. Then the incident angle of the laser at the oil−water interface is changed accordingly and a deviation in SHG measurement is induced. The details of this discussion are presented in the Supporting Information. At the same time, choosing this square cell keeps the advantage of TIR geometry that greatly enhances the SHG or SFG emission, as was first demonstrated by Richmond’s group.31,54 For the SHG experiments, the pumping laser is a Ti− sapphire laser (Mira-900f) from Coherent. The pulse duration is around 130 fs. The energy of the laser used in the experiments is 100 mW which is adjusted by a neutral density filter. The wavelength was adjusted to 810 nm, and the repetition rate is 76 MHz. A high pass filter (>750 nm) was used to eliminate the second harmonic light in the laser or generated from the optics. The laser was then focused on the hexadecane/water interface with an incident angle of ∼70°. The signal in the TIR direction was passed through a band-pass filter (BG-39, 300−600 nm band-pass) and collected by a lens. The signal was directed to a monochromator (Andor SR-500I), and the SHG signal with the wavelength centered at 405 nm was detected by a photomultiplier tube (PMT, Hamamatsu R1527P) with voltage of −1000 V. The output from the PMT was amplified by a factor of 5 with a Stanford Research preamplifier (SR-445A) and analyzed by a photon counter (Stanford Research, SR400). The polarization of the incident laser was controlled with a polarizing beam splitter (Thorlabs, PBS202, 620−1000 nm, extinction ratio 1:1000) and a halfwave plate (Thorlabs, WPH05M-808, 808 nm). The SHG signal was also selected with a polarizing beam splitter (Thorlabs, PBS201, 420−680 nm, extinction ratio 1:1800 at 405 nm). In the experiment the SH signal and the fundamental laser were both p-polarized; i.e., the electric field of the light is parallel to the incident plane (the plane defined by the incident laser direction and the surface normal). This polarization setup was selected for the higher SHG intensity although the analysis in this report also works at other polarizations. After a variation of the liquid sample in either the oil phase or the aqueous phase, the oil−water interface was allowed to reach equilibrium, which generally takes 30−60 min, before the SHG signal is recorded for analysis. The experimental process for each curve shown in this work typically takes 5−10 h. For this reason, the fundamental laser was checked before and after each experiment to ensure the variation of the laser energy, which is typically less than 1%, will not affect the data analysis in this work. Furthermore, each curve shown in this work was averaged using data from 2 or 3 repetitions. Water (18.25 MΩ·cm) was supplied from a water purification system (Water Purifier, WP-UP-UV-20, Sichuan Water Technology Development Co. Ltd., China). Hexadecane (99%, Sigma-Aldrich) was purified by basic alumina columns with up to six passes following the procedure described in our previous reports.22,28 SDS (99%, Sigma-Aldrich) was recrystallized three times in a mixed solvent of ethanol (99.7%, Tianjing Baishi Chemical Co. Ltd., China) and water at a volume ratio of 9.5:0.5.22 CTAB (99%, Shanghai Shanpu Chemical Co. Ltd., China) was recrystallized three times in a mixed solvent of acetone (99.5% Tianjing Zhiyuan Reagent Co. Ltd., China) and ethanol at a volume ratio of 1:1.13,22 Oleic acid (99%, SigmaAldrich) and oleylamine (95%, Strem Chemicals) were used after being dissolved in hexadecane. Aqueous solution of

also studied using SHG by correlating the change of the SHG signal from interfacial water molecules with the amount of the adsorbed surfactant,17,50,51 an approach based on probing the interfacial potential through electric field induced second harmonic (EFISH) analysis. It is known that preferential adsorption of ionic species and their counterions induces an interfacial electrical double layer which can be differentiated to different layers based on the distribution of ions and the resulting electric potential. In order to describe the different layers, researchers have developed models including the Helmholtz model, Gouy−Chapman model, and Stern model.52 The decrease of the electric potential value with the increased distance from the interface can be calculated with theories such as Gouy−Chapman theory.52 Water molecules in the electrical double layer may be polarized and have changed second-order nonlinear efficiencies.50,53 In this work, we show that the adsorption of typical ionic surfactants, such as sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and other amphiphilic long-chain molecules, at the oil−water interface causes notable changes in the SHG intensity from the oil−water interface. This change can be correlated with the change of the interfacial potential induced by adsorption of the charged molecules. Based on these observations, the adsorption free energy of the surfactants at the oil−water interface can be deduced. The obtained information revealed that adsorption of SDS and CTAB at the oil−water interface is initiated by a step that has not been analyzed before. The whole adsorption process and the corresponding structural changes of the interfacial molecules can then be understood more clearly.

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EXPERIMENTAL SECTION The SHG experimental setup used in this work is similar to that described in our recent report,28 with the main difference being that the sample holder was changed from a cylindrical cell to a square cell, as shown in Figure 1. During the experiments

Figure 1. Side view of the square sample cell used in the SHG experiment. The incident angle of the pumping laser was approximately 70° from the surface normal at the hexadecane−water interface, which is slightly larger than the total internal reflection (TIR) angle of the interface.

presented in this work, we noticed that the cylindrical cell, which is generally used to measure the SHG signal from liquid−liquid interfaces at total internal reflection (TIR) geometry, in some cases may cause deviation in the SHG signal. Briefly, the oil−water interface is normally not flat but curved due to the influence of interfacial tensions of the oil− water interface, the water−cell (fused silica) interface, and the oil−silica interface. The curvature changes when the interfacial tensions are altered by surfactant and the height of the oil− water interface under study also changes accordingly. This change is significant enough to cause misalignment in the SHG measurement and deviation in the recorded SHG signal. With a square cell, by adjusting the height of the cell one may bring the B

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information.58,59 The SHG signal generated from the oil−water interface can be expressed as

sodium hydroxide (NaOH, 99.99%, Sigma-Aldrich) was used to achieve a basic water phase. Sodium chloride (NaCl, >99%, Sigma-Aldrich) was calcined at 500 °C for 10 h and then filtered after being dissolved in water following the procedure in the literature.28,55,56 Glassware was cleaned with piranha solution (H2O2:H2SO4 with a volume ratio of 3:7), thoroughly rinsed with water, and dried before the experiment. Caution: piranha solution is strongly oxidizing and must be handled with care. The hexadecane/water emulsions were prepared with the procedure described previously.22 Briefly, 40 μL of hexadecane was mixed with 2 mL of water and stirred at 500 rpm in a thermostatic water bath for 15 min. Then it was sonicated (KQ300DE, 300 W, 40 kHz, Kunshan Ultrasonic Instrument Co. Ltd., China) for another 10 min with the temperature controlled at 25 ± 3 °C. The emulsion was then diluted 100 times with solution containing 1 mM NaCl for the experiments. The solution with 1 mM NaCl instead of water was used here to keep a relatively stable ionic strength for the zeta potential analysis. The diameter of the oil droplet in the emulsions is in the range of 200−300 nm. The average droplet diameter was monitored and found stable during the zeta potential measurements. The size and zeta potential of the oil droplet in the emulsions were measured by dynamic light scattering (DLS) using a ZS90 instrument (Malvern). The laser wavelength used was 633 nm, and the measuring angle was 90°. The measurement temperature was set as 25 °C.

ISHG ∝ |ESHG|2 = |Eoriginal + eiϕE induced|2

(1)

where the SHG field (ESHG) is composed of two parts: one generated from the original hexadecane−water interface (Eoriginal) and another induced by surfactants at the interface (Einduced). The phase difference (ϕ) in the equation then determines if the two parts are constructively or destructively interfered, which was treated as 0 or 180°, respectively.50,60 The dominant trend of the increase in Figure 2a indicates that the phases of Eoriginal and Einduced caused by SDS are the same, while that caused by CTAB is the opposite. This is understandable because SDS and CTAB lead to opposite interfacial potentials at the oil−water interface. Also, the original hexadecane−water interface was negatively charged because of the adsorption of hydroxide ions, as has been discussed in our recent report22,28 and the references therein. Interpretation of the nonlinear optical emission from interfaces under electric potentials varies in the literature. However, they are all related to the change of the nonlinear emission from the interfacial water molecules. For SFG-VS studies, the varied SFG signal is generally attributed to reoriented water molecules.18,42,45 However, the change of the SHG intensity at water interfaces is mostly explained by the EFISH effect that originates from the third-order nonlinear efficiency (χ(3)) of the water molecules (Einduced ∝ χ(3)EωEωψ, with Eω being the electric field of the fundamental laser and ψ being the static interfacial potential), and the reorientation effect is excluded or ignored.50,51,61 These two effects may contribute to the SHG field with the same phase,50 and thus they are relatively difficult to distinguish. Recent experimental observations including phase sensitive (heterodyne-detected) SFG-VS studies demonstrated the reorientation and even flipflop of water molecules at the interfaces in the presence of surfactants with different charges,18,24,62,63 showing the possibility of a more complicated mechanism behind the changed nonlinear efficiency at the water-based interfaces rather than an EFISH effect only. Other than these interpretations, the change of the orientational structure of water molecules induced by hydrogen bonding has also been introduced, and it is considered to have the same direction in orienting water molecules as the electric potential.42 Despite the existence of different interpretations summarized above, it is clear that the increase of the SHG intensity from the hexadecane−water interface in the presence of SDS and the decrease caused by CTAB can be readily correlated with the adsorption of negatively charged DS− ions (long chain ionic part of SDS) and positively charged CTA+ ions (long chain ionic part of CTAB) at the interface, respectively. The effect induced by the variation of the ionic strength in the solution can be neglected because normally a salt concentration as tens or hundreds of mM is needed for an ionic strength effect large enough to induce a significant SHG change.17,50 Discussion in later part of this work also supports this justification. The variation of the interfacial density of the surfactants with the bulk concentration has been used to analyze the adsorption process with models such as the Langmuir model.47,52 However, based on the different interpretations, the introduced interfacial electric charge was treated as being proportional to the changed SFG signal62 or the exponential of the changed SHG field.50 This causes differences in the model analysis.

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RESULTS AND DISCUSSION Multiple Steps in the Adsorption of SDS and CTAB at the Hexadecane/Water Interface Revealed with SHG. As shown in Figure 2, with increasing SDS concentration, the SHG

Figure 2. SHG signal from the hexadecane−water interface at various SDS (left) and CTAB (right) concentrations in water. The data are normalized to the first point, which is typically 2000 counts/s. Inset: significant SHG intensity change in the presence of surfactants at low concentrations.

intensity from the hexadecane−water interface initially significantly increases and then gradually decreases. In contrast, with increasing concentration of CTAB, the SHG signal decreases to a value close to zero and then recovers to ∼30% of the original value, followed by a slight decrease. The observation of a change in the second-order nonlinear signal from interfaces induced by a change in the electric potential is not new. A similar effect has been reported in both SFG-VS and SHG studies.17,18,28,45,50,51,57 In the following, we use this significant SHG change to reveal the initial and following steps of the adsorption of surfactants at the oil−water interface, which has not been addressed clearly and cannot be analyzed without such detailed SHG experimental observations. The measured SHG intensity can be expressed as the square of the SHG field, which is a vector containing certain phase C

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previous report32 was at only the higher concentration range in our study. This means that we probed a different step in the adsorption process, which will be further discussed later. Adsorption of Oleic Acid and Oleylamine at the Hexadecane−Water Interface. The adsorption of two oilsoluble amphiphiles (oleic acid and oleylamine) at the oil− water interface was also investigated and used to compare the difference in the multiple steps of the adsorption and the effect of counterions. They were also found to adsorb at the oil− water interface and to create a sufficiently strong interfacial potential to significantly alter the SHG intensity. For oleic acid, the aqueous phase needs to be made basic to deprotonate oleic acid (a NaOH solution with 0.01 M concentration was used in this experiment). Otherwise, oleic acid will be mostly in the neutral form at the interface and barely change the SHG intensity from the interface. These SHG measurements were different from our previous report,28 where we observed an increase in the SHG signal from the same interface but in a cylinder cells. Here, the observations with a square cell in the TIR-SHG measurements are more accurate in principle, as discussed in the Supporting Information. Figure 4 shows the change of the SHG intensity from the hexadecane−water interface in the presence of oleic acid and

Fortunately, we found that the two treatments led to only small variations in the adsorption free energies of surfactants. As described in the Supporting Information, it is the relative change in the SHG field as a function of the surfactant concentration (rather than the amplitude of change in the SHG field) that determines the adsorption free energy of the surfactants at the oil−water interface, i.e., the concentration at which the surfactant substantially adsorbs matters. For this reason, the interfacial electric charge, i.e., the number of adsorbed ionic surfactant molecules, was simply treated as being proportional to the change of the SHG field (Einduced) from the interface. The square roots of the SHG intensities shown in Figure 2, which represent the SHG fields from the hexadecane−water interface under varied surfactant concentration (Eoriginal + Einduced), were calculated and used to plot Einduced against the bulk concentration of the surfactants (Figure 3). Since the data

Figure 3. Change of SHG field (Einduced) from the oil−water interface with increasing surfactant concentration: (a) SDS and (b) CTAB. The Einduced values were obtained from the SHG intensity values shown in Figure 2. Smooth blue curves are fitting with the Langmuir model. Insets: magnified plots at low concentration range.

in Figure 2 were normalized to the first point, Eoriginal is always 1. Differing from the monotonously increasing interfacial absorption predicted by the Langmuir model, Einduced at both interfaces increases to a maximum and then slightly decreases. This should originate from the effect of counterions in the water phase, that is, the interaction of Na+ with the interface after the adsorption of DS− and the attraction of Br− to the interface after the adsorption of CTA+. A similar interaction between ionic surfactants and counterions has been studied by SFG-VS and SHG.17,64−67 The effect of monovalent ions on the headgroup of lipid layers at the water interface has also been experimentally and theoretically investigated.12,68 The significant increase of the interfacial potential with increasing concentration at very low surfactant concentrations, as well as the following gradual decrease, not only reveals the initial adsorption step of the hydrophobic part of the surfactants, which has not been described before, but also provides information about the electric charge and molecular structure of the interface during the adsorption process following this initial step. Based on the above analysis, adsorption of the hydrophobic part of the surfactants can be fitted with the Langmuir model with the data points measured at relatively low surfactant concentration range. The adsorption free energies of DS− and CTA+ at the hexadecane−water interface were determined to be −10.8 ± 0.4 and −11.4 ± 0.2 kcal/mol, respectively. The obtained adsorption free energy of SDS is notably higher than that reported in a previous SFG-VS study.32 There may be two reasons for this. First, as de Aguiar et al. reported, the molecular structure of SDS at the surface of an oil droplet in the emulsions is different from that at the planar oil−water interface.20 Second, the concentration range of SDS used in the

Figure 4. SHG signal from the hexadecane−water interface at various (a) oleic acid and (b) oleylamine concentrations in oil. The data are normalized to the first point. In the oleic acid experiment, the aqueous phase was with 0.01 M NaOH. Insets: magnified plots at low concentration range.

oleylamine in hexadecane at various concentrations. The adsorption of oleic acid at the interface increases the SHG intensity, similar to SDS adsorption, while oleylamine adsorption decreases the SHG intensity, similar to CTAB adsorption. This strongly supports that the change of the SHG intensity at the interface can be attributed to the induced positive or negative potential at the interface. It also excludes the possibility that the change in the SHG is from the long chains of the surfactants. We then analyzed Einduced from the interface. As shown in Figure 5, fitting leads to adsorption free energies at the hexadecane−water interface of −10.8 ± 0.1 and −10.8 ± 0.5 kcal/mol for oleic acid and oleylamine, respectively. The

Figure 5. Change of the SHG field from the hexadecane−water interface with increasing surfactant concentration: (a) oleic acid and (b) oleylamine. The solid curves are the data fitted with the Langmuir model. Insets: magnified plots at low concentration range. D

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stable ionic strength for the analysis. The zeta potential curve in the presence of SDS with no NaCl was also measured (data not shown), and it was found to show the same trend as previously reported results32 with only an offset to a smaller potential. This might be because we used purified hexadecane, which has a low level of impurities, as we recently reported.28 The effect of salt addition on the zeta potential analysis and the adsorption of SDS at the oil−water interface are beyond the scope of this study and will be discussed in future work. The net surface charge density shown as blue in Figure 6 was calculated with the Graham equation, which is described in detail in the Supporting Information. The surface charge density confirms the trend revealed by the SHG measurements for the low and medium surfactant concentration ranges, with the only difference being when the SDS and CTAB concentration approach the CMC (∼8 and ∼0.8 mM, respectively). It has been reported that there is a difference between the molecular structures at the planar oil−water interface and the oil−water interface in the emulsion.20,32 It was also pointed out that SHG and zeta potential methods probe the interfacial potential at different sides of the shear zone.53 However, the consistency of the general trend in the potential curves in the low and medium surfactant concentration ranges confirms the reliability of the above SHG analyses. Notable increases are observed in the charge density curves at the high surfactants concentration ranges (Figure 6) but not observed in the SHG curves (Figure 3). It has also been reported that SFG-VS signal from interfacial SDS does not drastically increase as the bulk SDS concentration approaches its CMC.31,32 The absence of a drastic increase in either our SHG measurements or previous SFG-VS studies seems opposite with the increasing surfactant density at the oil−water interface. The formation of micelles that is centrosymmetric may explain why the SFG-VS signals from the CH stretching modes of SDS increase much less than the significantly increased surfactant densities. However, the SHG emission from the interface is less likely to be influenced by the formation of micelles. The mechanism behind the fact that the SHG and zeta potential measurements diverge at high surface charge is still not very clear to us. However, we believe that changed distribution of different ionic species at different layers of the interface caused by counterion adsorption may play a major role. In the step following the initial adsorption step, the increasing interfacial density of the surfactants at the oil− water interface with its bulk concentration has been confirmed by surface tension,1 SFG-VS,13,31,32 neutron scattering,10 and Xray scattering techniques.16 SFG-VS revealed the decrease of gauche structures and the increase of chain ordering at higher surfactants concentration.13,31,32 The increased SFG-VS signal also shows the orientation of surfactants at concentrations much less than the CMC should adopt a parallel arrangement rather than an antiparallel bilayer arrangement at this stage. However, charge−charge repulsion between the head groups makes parallel ordering energetically less favorable. The SHG observations and zeta potential measurements clearly reveal that counterions play an important role in the compensation of the interfacial electric charge and favor further adsorption of ionic surfactants. Finally, the absence of a drastic change in the previously reported SFG-VS signal compared with the significant increase of the surface charge density at the oil− water interface at SDS concentrations close to the CMC show that the antiparallel bilayer arrangement may exist with the formation of hemimicelle or micelle structures. The formation

adsorption free energy of molecules from the oil phase to the oil/water interface has not been extensively investigated. This investigation provides information regarding the free energies of adsorption and solvation of oleic acid and oleylamine at the oil−water interface. Differing from the SDS and CTAB experiments, the change of the SHG field at the hexadecane−water interface in the presence of oleic acid and oleylamine show monotonically increasing. This is because of the absence of a strong counterion effect. For SDS and CTAB, the concentration of the counterions increases with increasing surfactant concentration in the water phase. For the oleic acid and oleylamine cases, there are only trace amount of counterions from the protonation or deprotonation of the absorbed surfactants on the interface. Supporting from Zeta Potential Measurements and Further Discussion on Multiple Steps in SDS and CTAB Adsorption. Previous investigations of the adsorption of SDS and CTAB at oil−water interfaces have led to information about their interfacial structures.1,13,20,31,41 The generally accepted increase of the interfacial surfactant density with increasing bulk concentration below the CMC seems to contradict the slightly decreasing Einduced in our experiments (Figure 3) because the latter indicates a small decrease in the interfacial electric potential. This shows that counterions play an important role in the adsorption of surfactants at the oil− water interface after the initial adsorption step. In the initial adsorption step, the hydrophobic interaction dominates the adsorption process; i.e., the adsorbed species are mainly the ionic surfactants. After the interface is covered with a certain amount of surfactant, charge−charge repulsion between the adsorbed ionic surfactants, attraction of counterions, and interaction between the tail groups of the surfactants and sometimes chains of oil molecules make the following adsorption step more complicated. In this step, the adsorption of the ionic surfactants and their counterions both make important contributions to the adsorption process and thus cause an increase in the interfacial surfactant concentration and an almost unchanged (or even slightly decreased) interfacial electric potential. Our observations clearly show that this latter step is more complicated than previously thought. Therefore, the initial adsorption step provides a better estimation of the free energy change for the adsorption of ionic surfactants at the oil−water interface from Langmuir model analysis because in this step the interactions between the adsorbed species are relatively weak. We measured the zeta potential at the oil droplet surface in the hexadecane−water emulsion with increasing concentrations of CTAB and SDS, as shown in Figure 6. Please note that 1 mM NaCl was added to the emulsions to maintain a relatively

Figure 6. Zeta potential at the surface of oil droplets in hexadecane− water emulsions in the presence of various concentrations of (a) SDS and (b) CTAB (red). The calculated surface charge density is also plotted in blue. E

DOI: 10.1021/acs.jpcc.5b11278 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C of antiparallel bilayer arrangement was also observed at solid− water interfaces.2,23 However, as mentioned before, further studies are needed to reach a clear mechanism of the unchanged SHG intensity. The adsorption of long chain ions at an oil−water interface, the effect of counterions in the solution, and different isotherms used for the analysis of the adsorption have been discussed by Davies as early as in 1958.69,70 In relatively recent years, smallangle neutron scattering experiment10 and electron paramagnetic resonance experiment with spin-labeled surfactants71 have been used for the study of SDS adsorption at oil−water interfaces. SFG-VS signal from the methyl groups of SDS adsorbed at a flat oil−water interface29 or the curved interface in hexadecane−water emulsions32 were also measured for the adsorption isotherm analysis. It was demonstrated in the literature that the Langmuir model could be successfully applied for the analysis of the adsorption isotherms of different molecules/ions at air−liquid, liquid−liquid, and solid−liquid interfaces.6,8,22,29,32,49,72,73 In this work, analysis based on the Langmuir model not only leads to adsorption free energy of several long chain ions at the hexadecane−water interface but also reveals the initial adsorption process at oil−water interface that has not been observed experimentally. Though, the different steps in surfactants adsorption at other interfaces have been discussed before.74,75 The measured zeta potential at the surface of oil droplets in hexadecane−water emulsions at neutral acidity is approximately −40 mV. This result is consistent with previous reports22,76 and is in line with our recent work on the adsorption of hydroxide ions at the hexadecane−water interface.28 The negative interfacial potential then suggests a net orientation of water molecules with hydrogen atoms pointing to the oil phase.28 This also supports the observation in this work that SHG signal increases with the adsorption of negative ions at the hexadecane−water interface; i.e., the phase of the initial SHG signal at the hexadecane−water interface is the same as the phase of the SHG signal induced by the adsorption of negative ions at the interface and opposite that of the field induced by positive ion adsorption. Although it has been argued that factors other than hydroxide ion adsorption, such as the existence of long chain carboxylic acids in the oil phase,76 can also induce a negative interfacial potential, we have shown that the impurities in the hexadecane sample has been substantially reduced by our purification process.28 Recent heterodynedetected SFG experiments reported by Bakker’s group77 also suggest an orientation of water molecules at oil−water interfaces with their hydrogen atoms pointing to the oil phase, which is consistent with our observation at the hexadecane−water interface. In that report, a specifically designed sample cell78 was used to form oil−water interfaces from saturated vapor of hexane or heptane. This greatly reduces the possibility of contamination at the oil−water interface. So the observations in their reports also support hydroxide adsorption at oil−water interfaces. Ionic Strength Effect and Discussion on the Origin of the Interfacial SHG Emission. The ionic strength effect on the SHG signal from the hexadecane−water interface was also investigated. As shown in Figure 7, the original SHG emission from the pure hexadecane−water interface and the enhanced SHG emission in the presence of SDS and oleic acid are subjected to notable influence from ionic strength (salt concentration) of the aqueous phase. On the other hand, the almost totally quenched interfacial SHG emission in the

Figure 7. Change of the SHG intensity from the hexadecane−water interface with increasing NaCl concentration in the presence of different surfactant in aqueous or oil phase: (a) none; (b) with 50 μM SDS; (c) with 0.5 μM CTAB; (d) with 20 μM oleic acid and 0.01 M NaOH; (e) with 10 μM oleylamine. For curves in (b)−(e), the first point was measured from the pure hexadecane−water interface, while the rest are measured after the addition of surfactant. All data points are normalized to the first point.

presence of positively charged ions (CTAB and oleylamine) at the interface are only weakly affected by the varied ionic strength. As has been discussed above as well as in our recent report,28 the origin of the SHG emission from the hexadecane−water interface is still not clear. From evidence in SHG and SFG-VS studies, both the EFISH effect and the oriented water molecules in interfacial layer may contribute. Eisenthal’s χ(3) treatment that considers the altered interfacial potential caused by ionic strength changes based on Gouy−Chapman theory has been generally used for the analysis of the SHG emission as a function of salt concentration.50,51,61 In such analysis, a fixed surfactant concentration at the interface and the fact that EFISH dominates the SHG emission are preferred to ensure the accuracy of the treatment.50 Because these prerequisites may not be satisfied here, we shall discuss the influence of the ionic strength and the counterions at the interface and analyze the origin of the SHG signal qualitatively. The decrease of the SHG intensity in the presence of salt shown in Figure 7a, which is more accurate than our previous data measured with a cylindrical sample cell28 (see Supporting Information), indicates a preferential adsorption of Na+ ions at the hexadecane−water interface with increased ionic strength. The adsorption of the Na+ ions in the diffuse layer screened the electrostatic potential of the interface. This is understandable since data in the literature79 and the zeta potential measurements above have shown that the original hexadecane−water interface has a negative potential, similar to the SDS and oleic acid occupied interface. The similar trend of decreasing SHG intensity observed in Figure 7a,b,d supports this interpretation of counterion adsorption induced by charge−charge attraction. As an explanation of the weakly affected SHG intensity shown in Figure 7c,e, the adsorption of positively charged CTAB or F

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ionic strength dependence of the interfacial SHG signal, we deduced that the SHG emission from the hexadecane−water interface are mainly from the contribution of interfacial water molecules.

oleylamine molecules at the interface may alter the negative interface potential to a smaller value close to zero or even a small positive value. Although accurate interfacial potential at flat oil−water interfaces cannot be determined in our work, from Gouy−Chapman theory (see Graham equation in Supporting Information), a near zero interfacial charge, i.e., near zero interfacial potential, is less sensitive to the ionic strength variation. The fact that the interfacial SHG emission is close to zero near zero interfacial potential also suggests that the original SHG emission at a pure hexadecane−water interface is most likely from the water molecules. The zeta potential of the surface of the oil droplets in hexadecane−water emulsions is approximately −40 mV, indicating a negative interfacial potential at the pure hexadecane−water interface in study. It is still hard to differentiate whether the SHG emission is from the χ(3) contribution known as the EFISH effect or the χ(2) contribution of the oriented water molecules. There are still subtle difference in curves shown in Figure 7a,b,d. In Figure 7a, approximately 40% of the original SHG emission quenched by the addition of 0.1 M NaCl. In Figure 7b,d, only approximately 20% of the original SHG emission can be quenched by 0.1 M NaCl. To get a 40% decrease in the SHG emission, a NaCl concentration of 0.5 M is needed. This relatively weak ionic strength effect for SDS and oleic acid cases is also understandable. In the absence of NaCl, the adsorbed SDS or oleic acid molecules at the oil−water interface are subjected to charge−charge repulsion from each other, which is a barrier for further adsorption of the ionic surfactant. In the presence of NaCl, the counterion (Na+) adsorption weakens this barrier and favors the adsorption of more ionic surfactant. A similar effect has been reported in the literature19 and observed in our recent work.22 For the pure hexadecane−water interface, the negative interfacial potential is believed to be induced by hydroxide ion adsorption.28,79 The less abundant hydroxide ions in the system may prevent its further adsorption at the oil−water interface. In a word, the varied causes of the electric potential at the oil−water interface may be the reason for the subtle difference in the ionic effect shown in Figure 7a,b,d.

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ASSOCIATED CONTENT

S Supporting Information *

. This material is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11278. The flaw of cylindrical cells in TIR-SHG measurements and the elimination of the flaw with square cells, the fitting of the SHG data, and the calculation of the interfacial charge density with the Graham equation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.G.). Present Address

H.F.: Department of Chemistry, Temple University, Philadelphia, PA 19122. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “1000 Talent Program” (The Recruitment Program of Global Experts), the “One Hundred Talents Project Foundation Program” of the Chinese Academy of Sciences, the National Natural Science Foundation of China (21273277 and 21473249), and the “Young Creative Sci-Tech Talents Cultivation Project of Xinjiang Uyghur Autonomous Region” (2013711016 and 2013711012).

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REFERENCES

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CONCLUSIONS By probing the SHG signal at the hexadecane−water interface, we investigated the complete adsorption process of common surfactants (e.g., SDS and CTAB) at the oil−water interface. Adsorption begins with an initial step that has not been analyzed before. The following adsorption step with an increase of both ionic surfactants and their counterions at the interface was also analyzed with both SHG and zeta potential measurements. We also studied the adsorption of two oilsoluble amphiphiles (oleic acid and oleylamine) at this oil− water interface. This work provides unique information for understanding the behavior of common surfactants at the oil− water interface. The simple protocol presented here is ready to be used for the study of the interfacial adsorption of other ionic surfactants. On the basis of the SHG analysis, we obtained adsorption free energies at the hexadecane−water interface of −10.8 ± 0.4 kcal/mol for DS−, −11.4 ± 0.2 kcal/mol for CTA+, −10.8 ± 0.1 kcal/mol for oleic acid, and −10.8 ± 0.5 kcal/mol for oleylamine. This adsorption free energy is mainly determined by the initial step in the adsorption isotherm, where the interactions between the adsorbed species and the counterions are relatively weak. At last, on the basis of the analysis of the G

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