Strong Cooperative Effect of Oppositely Charged Surfactant Mixtures

Letter pubs.acs.org/Langmuir

Strong Cooperative Effect of Oppositely Charged Surfactant Mixtures on Their Adsorption and Packing at the Air−Water Interface and Interfacial Water Structure Khoi T. Nguyen,†,‡ Tuan D. Nguyen,† and Anh V. Nguyen*,† †

School of Chemical Engineering, The University of Queensland, Brisbane QLD 4072, Australia School of Biotechnology, International University, Vietnam National University, Ho Chi Minh City, Vietnam

‡

S Supporting Information *

ABSTRACT: Remarkable adsorption enhancement and packing of dilute mixtures of water-soluble oppositely-charged surfactants, sodium dodecyl sulfate (SDS) and dodecyl amine hydrochloride (DAH), at the air−water interface were observed by using sum frequency generation spectroscopy and tensiometry. The interfacial water structure was also observed to be significantly influenced by the SDS-DAH mixtures, differently from the synergy of the single surfactants. Most strikingly, the obtained spectroscopic evidence suggests that the interfacial hydrophobic alkyl chains of the binary mixtures assemble differently from those of single surfactants. This study highlights the significance of the cooperative interaction between the headgroups of oppositely charged binary surfactant systems and subsequently provides some insightful observations about the molecular structure of the air−aqueous interfacial water molecules and, more importantly, about the packing nature of the surfactant hydrophobic chains of dilute SDS-DAH mixtures of concentration below 1% of the CMC.

1. INTRODUCTION Understanding surfactant adsorption and packing at the air− water interface and related interfacial water structure is critical to many applications, from separating KCl minerals using air bubbles for fertilizer production1 and making hydrates of carbon dioxide and natural gas for capture, storage, and transportation2 to consumer products.3 Adsorption isotherms of Gibbs, Langmuir, van der Waals, and Frumkin have been used to quantify the surfactant adsorption dynamics and equilibrium.3 The Gibbs adsorption isotherm provides the generic framework for analyzing the change in the surface tension by surfactant adsorption. The Langmuir isotherm is assumed to be valid for low surfactant concentration in the bulk solution, while the van der Waals and Frumkin isotherms are to be used for high surfactant concentration, which produces high surface coverage with strong cooperative interaction between hydrophobic chains. These models can provide useful information about the surfactant surface excess and packing, which are difficult to measure. However, the Gibbs framework has recently been challenged because of the assumption of adsorption saturation.4 Further experimental evidence of the molecular conformation and orientation of adsorbed surfactant molecules and interfacial water molecules appears to be critical to our understanding and applications of surfactant adsorption phenomena. Here vibrational sum frequency generation spectroscopy (SFG)5,6 and tensiometry were applied to obtain a strong cooperative effect of polar headgroups of anionic SDS (sodium dodecyl sulfate) and cationic DAH (dodecyl amine © 2014 American Chemical Society

hydrochloride) on their adsorption and packing at the air− aqueous interface and the interfacial water structure. For the first time, conformational information about interfacial surfactant molecules was elucidated at concentrations of as low as 40 μM (about 0.5% of the CMC, the critical micelle concentration). The cooperative interactions between these two headgroups were found to dominate the hydrophobic alkyl−alkyl interactions, leading to significant impacts on the interfacial water structure and the conformation of their hydrophobic alkyl C12 chains. Significantly, SDS-DAH mixtures were surprisingly observed to change the interfacial water structure in a fashion that would not arise from a linear combination of effects by the individual surfactants.

2. EXPERIMENTAL SECTION A series of SFG measurements (Supporting Materials) were carried out at the air−aqueous interface to gain spectroscopic insights into the interfacial water structure, as well as the hydrophobic regime of adsorbed SDS, DAH, and their mixtures. The O−H stretches of water were investigated in the 3000−3800 cm−1 region (Figure 1) while the C−H vibrational modes of the alkyl chain were studied in the 2800− 3000 cm−1 range (Figure 2) in both ssp (s-polarized SFG signal, spolarized input visible, and p-polarized input IR beam) and ppp polarization combinations. Received: January 20, 2014 Revised: June 6, 2014 Published: June 6, 2014 7047

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Figure 1. SFG water signal of different surfactant systems, all taken in the ssp polarization combination. The free OH peaks at 3700 cm−1 remain unchanged in all of the single systems (left) but disappear in the mixture systems (right). The spectral shifts of the icelike and liquidlike peaks are also noticeable. The vertical lines are a guide to the eye, emphasizing the spectral shifts.

Figure 2. Top: ssp SFG signal of alkyl chain C−H stretches in the single and binary surfactant systems. Bottom: Illustration of SDS (left) and DAH (right) water interface interactions.

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signal collected in this 3000−3800 cm−1 region mainly comes from the layer of interfacial water molecules, and the spectral features can be explained by their level of ordering and the hydrogen bonding. Rationally, when adsorbed at the air− aqueous interface, the positively charged amine headgroups of DAH are likely to reorient the water molecules so that their oxygen atoms stay closer to the interface. Being a strong hydrogen bond donor, the amine headgroups are likely to form hydrogen bonds with water. In addition, infrared spectroscopy and neutron diffraction measurements21,22 showed only slight deviations of the amine H−N−H and the water H−O−H angles from 109.5o as predicted by the VSEPR (valence shell electron pair repulsion) theory, implying that hydrogen atoms from the amine group are able to behave in a manner similar to those in water. Consequently, an ordered self-assembly of DAH initiates the formation of a well-defined structure resembling the hexagonal ice lattice structure. It was observed from our SFG measurement that at a low surfactant concentration of 0.04 mM, DAH did not pack a full monolayer at the air− aqueous interface as evidenced by the presence of the free O− H bonds at 3700 cm−1 (Figure 1c). Even so, the existence of the hexagonal structure is indisputable, and the interfacial water molecules at this low DAH concentration follow both orientation fashions. To further validate the hypothesis of enhancing the interfacial water hexagonal ice lattice structure of DAH, we carried out an experiment in which hexadecyl trimethylammonium bromide (CTAB) was used instead of DAH. Being unable to form direct hydrogen bonds with water, CTAB was observed to affect interfacial water molecules electrostatically, in a manner similar to that of SDS (Supporting Information, Figure S3). Thus, the enhancement in the OH spectral region observed with CTAB can be attributed to the electrostatic field at the interface.13 Once this electrostatic field was reduced by using a mixture of CTAB/SDS (0.02 mM/0.02 mM), the O−H signal predictably decreased significantly (Figure S3, Supporting Information). It is noted that due to different relative scales used to report the SFG intensities in this paper and the literature13 there is an apparent difference in the electrostatic field cancellation caused by the oppositely charged headgroups of SDS and DAH at similar concentrations. Indeed, if similar scales are used, then the results are reconciled. In principle, the cancellation of the electrostatic field depends on the charge and dipole balances which are a function of many factors including the surfactant concentration ratio and could not completely vanish for all concentration ratios. Structural differences in SDS and DAH were reflected in their different interaction schemes with the interfacial water molecules (Figure 2). SDS possesses an anionic sulfate headgroup which keeps the hydrogen atoms of water closer to the surface. Despite the fact that the sulfate headgroup also has a tetrahedral geometry, only one of its four oxygen atoms can form hydrogen bonds with water. The adsorbed SDS molecules, therefore, are unable to facilitate the enhancement of the water icelike property, which is a characteristic of DAH. However, at a low concentration of 0.04 mM, SDS can still greatly enhance the orderliness of water (Figure 1b). The interpretation of the adsorption of a binary surfactant system requires careful consideration. In a surfactant system of 0.02 mM SDS/0.04 mM DAH, the peak is dominant at 3600 cm−1 while it almost disappears at 3150 cm−1. This observation indicates that an intense cooperative interaction does occur between the SDS sulfate and DAH amine headgroups, and this interaction happens very quickly (within minutes, spectra not

3. RESULTS AND DISCUSSION As shown in Figure 1, there are generally two SFG peaks observable in the 3000−3800 cm−1 region for neat water. There is one narrow peak centered at around 3700 cm−1 and one broad continuum spanning from 3000 to 3600 cm−1. The narrow peak at 3700 cm−1, commonly assigned to the free O− H, is present in all spectra for neat water and single surfactant systems but disappears in all spectra for binary mixtures. The origin of the broad peak is still debatable: some believe that this continuum arises from the dynamic fluctuation of water molecules while others support the hypothesis that it is due to multiple hydrogen bond species coexisting among the surface water molecules.7−9 In our analysis (Supporting Information), two major peaks at around 3180 and 3450 cm−1 were used in the fitting of the water spectra in the 3000− 3800 cm−1 range, featuring the icelike and disordered (liquidlike) character, respectively.10,11 For the analysis of the hydrophobic tails, the vibrational modes of the terminal methyl CH3 symmetric (at 2876 and 2936 cm−1 collected in ssp polarization) and antisymmetric stretches (at 2965 cm−1 collected in ppp polarization) were chosen due to their fairly isolated spectral spacing, which allows for more accurate fitting results.12 Adding either of the surfactants to neat water produced two distinctive phenomena. First, there was an expected dramatic increase in the SFG signal intensity, explained by the formation of a more ordered water structure at the interface. Second, there was a clear separation of the broadly spanning peak into two major peaks as shown in Figure 1b,c. Interestingly, this broad peak splits in a different manner for SDS and DAH. With DAH, the dominant peak was centered at 3150 cm−1, while the two icelike and liquidlike peaks appeared to have comparable intensities to SDS. It is noted that the SFG O−H spectral region of SDS solutions has been controversially reported in the literature. For instance, Gragson et al.13 observed a dominant peak at 3150 cm−1 (with 0.05 mM SDS), while Nihonyanagi et al.14,15 recently showed a dominant peak at 3450 cm−1 (with 0.5 mM SDS) which is quite similar to the peak at that frequency in Figure 1b. It is difficult to understand the reasons for this discrepancy without experimental details or reference SFG spectra in the O−H regime of neat water. One of the possible reasons that lead to this disagreement is the spectral distortion caused by the infrared beam energy distribution associated with each frequency in the spectral region of interest. Covering both C−H and O−H regimes from 2800 to 3600 cm−1, the broad SFG spectrum13 might have suffered from the distortion and/or dispersion of the tunable IR beam. In our study, the IR walk-off was eliminated by the normalization and calibration of the IR energy and SFG signal using z-cut quartz. Our SFG O−H spectral region of neat water was then carefully checked with the spectra in the literature16−18 (Supporting Information, Figures S4−S6), and the identical optical alignment was used in all measurements reported here. Initially, the differences between the DAH and SDS spectral features in the O−H regime were thought to be due to the N− H stretch of the amine headgroups contributing to the SFG signal in this spectral range. However, it is still debatable whether the peak at 3300 cm−1 (if observable) arises from the amine or the backbone amide.19,20 Also, the peak at 3300 cm−1 was absent in the ssp spectrum of DAH in water (0.04 mM) (Figure 1c). Therefore, it is reasonably assumed that the SFG 7049

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shown). It is evident that the headgroups of SDS and DAH may interact with each other and create a new hydrogen bonding scheme that is invisible under SFG. It is worth mentioning that the absence of the electrostatic field may not diminish the hydrogen bonds entirely. The puzzling peak at 3600 cm−1 was also observed in the case of organic films (DPPC and DPPE) on an aqueous surface and was reported to be the free O−H stretch being red-shifted.11 However, due to the discrepancy in their peak widths, we suggest that there still be some complex yet weak hydrogen bonding scheme among the interfacial water molecules. Importantly, the narrow peak for the free O−H bonds is no longer visible in all of the SFG spectra (Figure 1d−f). It is possible that the mixed surfactant molecules self-assembled to form a relatively dense monolayer that fully covers the interface, shielding the interfacial water molecules from the air phase. (This new experimental evidence of the strong cooperative effect on adsorption at very dilute concentration can further invalidate the implementation of the Langmuir adsorption isotherm for surfactant mixtures.) Even though, the interfacial water molecules maintain a certain level of order, the well-defined tetrahedral hydrogen bonding structure totally collapses. Most strikingly, the C−H signals from the alkyl chain of this binary surfactant system suggest that the intimate interaction between SDS and DAH occurs not only among their charged headgroups but also among their hydrophobic tails. This intimate interaction happens in such a way that the surfactant alkyl tails adopt a higher level of conformational order, as evident by the negligible gauche defect reflected by the small methylene symmetric stretch SFG peak (CH2−SS) at 2850 cm−1 (Figure 2). This can be interpreted by the fact that surfactant molecules now adhere to each other primarily by their oppositely charged headgroups, not by the hydrophobic interaction among their alkyl chains anymore. In a single surfactant system, it was reported earlier using X-ray photoelectron spectroscopy that the interfacial depth of the surfactant headgroups does not adopt a single value but follows a distribution instead.23 On the contrary, the observed strong headgroup−headgroup interaction and the highly ordered alkyl chains suggest a surfactant packing scheme in which the surfactant headgroups are located within a fairly narrow interfacial depth in the case of the binary surfactant systems being studied. Additionally, the indiscernible SFG signals from the C−H vibrational modes could be explained by either the horizontal orientation of the alkyl chain or the low surfactant surface excess. By contrast, in a binary surfactant system, the SFG signals in the C−H region were greatly enhanced by at least 20-fold (Figure 2a), indicative of a major change in both the surface excess and the packing scheme of the surfactant hydrophobic tails. It is noted that the surface pressure of the binary SDS-DAH system increased noticeably from 0.8 mN/m to approximately 44.0 mN/m upon the addition of the second surfactant species (Figure 3), which is indicative of a very strong enhancement in the surfactant surface excess. This strong enhancement can be explained by the neutralization of the electrostatic field among surfactant headgroups, leading to the cancellation of the repulsion among the headgroups carrying the same charge. In this binary surfactant system, the orientation analysis of the terminal methyl group suggests an alkyl chain tilt angle of 11 ± 1° to the surface normal (Supporting Information). It is likely that this reorientation of the surfactant alkyl chain occurs to optimize the space and energy so that the air−water interface can accommodate more surfactant molecules, which is

Figure 3. Increase in surface pressure of the SDS/DAH surfactant system. An amount of 0.04 mM SDS was added at 0 s. The initial fast rise starting at around 100 s was caused by the addition of the SDS stock solution to the reservoir. A further addition of 0.04 mM DAH at 1860 s sharply increased the surface pressure. Reversing the surfactant addition order (0.04 mM DAH before 0.04 mM SDS) yielded a similar surface pressure profile.

experimentally confirmed by surface pressure measurements. A second binary surfactant system of 0.04 mM SDS/0.04 mM DAH was also studied. It was observed that at this concentration ratio there existed icelike hydrogen bonding among the water molecules. These hydrogen bonds are possibly dictated by the noninteracting adsorbed SDS molecules at the interface, similar to the single SDS system. Assuming that the alkyl chains of the interacting surfactant molecules follow a single distribution, they align pretty much vertically with respect to the interface with a tilt angle similar to that of the above binary system (result not shown). It is a reasonable assumption that the noninteracting SDS molecules selfassemble in the same way that they do in the single surfactant system and hence are invisible under SFG. Predictably, a significant increase in the DAH concentration (up to 0.24 mM) of this binary surfactant system brings back the hexagonal ice lattice structure of the interfacial water structure (Figure 1f). However, it does not significantly alter the conformational properties of the hydrophobic tails of the interacting surfactant molecules.

4. CONCLUSIONS AND OUTLOOK Our study shows different effects of single and binary surfactant systems of SDS and DAH on their adsorption and packing and the water structure at the air−aqueous interface. We discuss the conformation and self-assembly properties of the surfactant hydrophobic tails and polar headgroups and characterize, in situ, the strong cooperative interaction of the components in binary SDS-DAH surfactant systems. These results present a tremendous opportunity for further interface studies in modeling and controlling the self-assembly and adsorption of surfactants in a wide range of fields, especially novel gas hydration technologies for the capture, storage, and transportation of CO2 and natural gas.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental setup and procedure. This material is available free of charge via the Internet at http://pubs.acs.org. 7050

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and acid solution investigated by an exciton model. J. Chem. Phys. 2007, 127 (). (18) Raymond, E. A.; Tarbuck, T. L.; Brown, M. G.; Richmond, G. L. Hydrogen-bonding interactions at the vapor/water interface investigated by vibrational sum-frequency spectroscopy of HOD/H2O/ D2O mixtures and molecular dynamics simulations. J. Phys. Chem. B 2003, 107, 546−556. (19) Clarke, M. L.; Wang, J.; Chen, Z. Conformational changes of fibrinogen after adsorption. J. Phys. Chem. B 2005, 109, 22027−22035. (20) Mermut, O.; Phillips, D. C.; York, R. L.; McCrea, K. R.; Ward, R. S.; Somorjai, G. A. In situ adsorption studies of a 14-amino acid leucine-lysine peptide onto hydrophobic polystyrene and hydrophilic silica surfaces using quartz crystal microbalance, atomic force microscopy, and sum frequency generation vibrational spectroscopy. J. Am. Chem. Soc. 2006, 128, 3598−3607. (21) Denisov, G. S.; Kuzina, L. A.; Shchepkin, D. N. Stretching Vibrations of Amino Group and Inter Intramolecular Hydrogen-Bond in Anilines. Croat. Chem. Acta 1992, 65, 89−100. (22) Temleitner, L.; Pusztai, L.; Schweika, W. The structure of liquid water by polarized neutron diffraction and reverse Monte Carlo modelling. J. Phys.: Condens. Mater. 2007, 19, 335207. (23) Wang, C. Y.; Morgner, H. Molar concentration-depth profiles at the solution surface of a cationic surfactant reconstructed with angle resolved X-ray photoelectron spectroscopy. Appl. Surf. Sci. 2011, 257, 2291−2297.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +61 7 3365 4199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s LIEF and DP Projects funding schemes (project numbers LE0989675 and DP140101089).

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