Article pubs.acs.org/JPCC
Interfacial Water Structure at Surfactant Concentrations below and above the Critical Micelle Concentration as Revealed by Sum Frequency Generation Vibrational Spectroscopy Khoi Tan Nguyen,*,† Anh V. Nguyen,*,† and Geoffrey M. Evans‡ †
School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia
‡
ABSTRACT: Interfacial water in close proximity to an adsorbed surfactant layer plays an important role in many areas. Here, we systematically investigate the interfacial water structure at the adsorption layer of cetyltrimethylammonium bromide (CTAB) from low concentrations to its critical micelle concentration. Our sum frequency generation (SFG) spectroscopy results show that, with increasing CTAB concentration, the water SFG signals first increase, reaching maximum intensities at 0.1 mM, and then drop, whereas the ppp SFG signals of the terminal-methyl asymmetric stretch reach maximum intensities and saturate at 1 mM, which is the critical micelle concentration of CTAB. Our analysis reveals that the interfacial water layer adopts the most orderly arrangement when the interfacial potential of the adsorption layer reaches saturation and not at the surfactant concentration of adsorption saturation. Most importantly, our SFG data provide direct evidence for the antagonistic effects of the interfacial potential and thickness compression of the electrostatic field of the surfactant adsorption layer, leading to the strongest water SFG signals at 0.1 mM. Below 0.1 mM, the increased interfacial potential can have a pronounced effect on the increase of the overall dipole moment of the interfacial water layers. Above 0.1 mM, the decreased Debye screening length significantly reduces the water dipole moment. Finally, the newly proposed adsorption model cannot explain our SFG results.
1. INTRODUCTION The critical micelle concentration (CMC) of a surfactant is the concentration at which the surface concentration of surfactant molecules reaches a level where they begin to self-aggregate to form micellular structures. An extensive amount of research has been devoted to determining the CMCs of surfactants.1−3 However, so far, little attention has been paid to characterizing the interfacial properties of water and adsorption layers beyond this point. Surfactant adsorption layers at the air/water interface have been studied by a variety of surface-specific methods including (dynamic) surface tension measurement, neutron reflectivity, and sum frequency generation (SFG) vibrational spectroscopy.4−9 Specifically, neutron reflectivity has been used to quantify excess nonionic surfactants at the air/water interface, whereas SFG spectroscopy has been extensively used to determine the degree of orderliness of adsorbed surfactant molecules. Whereas recent studies have highlighted the role of the water layers on the air/water interfacial properties,10,11 most studies have focused on characterizing the surfactant adsorption layers rather than the interfacial water layer at concentrations below and above the CMC. Therefore, it is necessary to obtain further information about the interfacial water layer using interface-specific spectroscopy. In this study, a second-order nonlinear spectroscopic technique, namely, SFG spectroscopy, was used to study both the surfactant adsorption © 2015 American Chemical Society
layer and the interfacial water molecules at pre- and post-CMC concentrations. Being a key parameter of surfactants, further knowledge of the CMC is essential to advancing a number of industrial processes including mineral flotation, especially that of soluble salt minerals.12−14
2. EXPERIMENTAL SECTION The experimental setup of the SFG system was described previously.15,16 Briefly, the visible and tunable IR beams were spatially and temporally overlapped on the solution interface. The visible beam was generated by frequency-doubling the fundamental output pulses (1064 nm, 10 Hz) of the 36-ps pulse width from an EKSPLA solid-state Nd:YAG laser. The tunable IR beam was generated by an EKSPLA optical parametric generation/amplification and difference frequency system based on lithium triborate (LBO) and AgGaS2 crystals. Fluctuations in the beam energies were only 3% standard deviation in the tunable IR beam and 1.5% in the visible beam. The quantities χ(2) ssp (s-polarized SFG, s-polarized visible, and ppolarized infrared polarization combination) and χ(2) ppp (ppolarized SFG, p-polarized visible, and p-polarized infrared Received: May 7, 2015 Revised: June 11, 2015 Published: June 15, 2015 15477
DOI: 10.1021/acs.jpcc.5b04416 J. Phys. Chem. C 2015, 119, 15477−15481
Article
The Journal of Physical Chemistry C
and visible incident angles. The SFG C−H signals from the alkyl groups are not confused with the broad band arising from the interfacial water molecules as previously reported.18,19 The SFG signal of the methyl asymmetric stretching mode thus inherently describes the surfactant adsorption layer much more precisely than its symmetric counterpart. In line with a recent SFG study on surfactant adsorption at the air/water interface through its headgroups,20 our data showed that the SFG signal intensity of the methyl asymmetric mode increased monotonically with the CTAB concentration up to the CMC (Figure 2). Interestingly, the methyl
polarization combination) reflect the observed SFG intensities in the laboratory frame. We found that very small changes in the height of the air−water interface, as caused by inevitable evaporation during the experiments, significantly impacted the quality and sensitivity of the SFG signals collected. To overcome this experimental issue, we used a home-built He− Ne laser leveller to monitor and control the interface height. Cetyltrimethylammonium bromide (CTAB, CMC ≈ 1 mM) with a purity of >99% was purchased from Sigma-Aldrich and used as received. Freshly purified water (Ultrapure Milli-Q unit from Millipore, Billerica, MA) with a resistivity of 18.2 MΩ·cm was used to make all of the solutions in the experiments. A magnetic microstirrer was used for mixing for 10 s to ensure homogeneous distributions of the added surfactants. In the experiments, specific amounts of the CTAB stock solution (2 mM) were injected into a reservoir of 20 mL to achieve the desired surfactant concentrations. All experiments were carried out at room temperature of approximately 25 °C.
3. RESULTS AND DISCUSSION 3.1. SFG C−H Signals of the CTAB Adsorption Layer at the Air/Water Interface. CTAB contains an alkyl chain with four terminal methyl groups that gives rise to two main groups of SFG signals arising from the vibrational modes of the terminal methyl group and the trimethylammonium groups: for the terminal methyl, asymmetric mode (∼2970 cm−1), symmetric mode (∼2875 cm−1), and its Fermi resonance (∼2935 cm−1);8 for the trimethylammonium groups, asymmetric mode (∼3040 cm−1), symmetric mode (∼2985 cm−1), and its Fermi resonance (∼2935 cm−1).17 However, the SFG signals of the asymmetric mode of the trimethylammonium groups (∼3040 cm−1) were not detectable in the ppp polarization combination, which could be due to the orientation of the headgroups in the macroscopic frame or the intrinsically weak overall SFG hyperpolarizability (Figure 1). The ssp SFG signals of the symmetric stretching modes of
Figure 2. ppp SFG signals of the terminal-methyl asymmetric stretching mode at various CTAB concentrations. The CMC of CTAB is 1 mM. The ppp SFG signal intensities of the terminal-methyl asymmetric stretching mode are saturated at post-CMC concentrations.
asymmetric mode at 2970 cm−1 became detectable only when the CTAB concentration exceeded 0.1 mM, at which the surface potential was measured to reach saturation.21 It is noted that these results of ours are in apparent disagreement with the previous report on the SFG signals, which were for the methyl symmetric stretching mode (which was found to be saturated well before the CMC).19 We believe that the quickly saturated SFG signal of the symmetric mode reported earlier was caused by the inevitable constructive interference between the methyl symmetric stretch and the broad bands from the water O−H vibrational modes, as recently pointed out by Nihonyanagi et al.22 3.2. SFG O−H Signals of the Interfacial Water Layers upon CTAB Adsorption. We then measured the SFG spectrum of the O−H vibrational modes of the interfacial water molecules in the presence of varying CTAB concentrations (Figure 3). The results showed a strong SFG signal enhancement of the O−H stretches in the 3000−3600 cm−1 region at both pre- and post-CMC concentrations. It is worth noting that CTAB is a cationic surfactant that can create a strong electrostatic field upon its adsorption at the air/water interface. This electric field can influence the interfacial water orientation at the model interface.22 In this study, CTAB was chosen to avoid possible surfactant headgroup hydrolysis with the formation of nonionic surfactant at neutral pH (as dodecanol in the case of SDS), which otherwise might significantly affect the adsorption of the surfactants at the air/ water interface.23 The thickness of the water layer affected by the 10 mM CTAB electric field at the air/water interface was previously estimated to be around 3 nm (or about 10 water layers),18 which gives rise to a strong third-order susceptibility χ(3)
Figure 1. ppp SFG signals at the air/water interface of a 1 mM CTAB solution.
the trimethyl groups were not detectable in this study because of the domination of the broad SFG band from the interfacial water layers. In this systematic study, we first measured the SFG spectrum of the asymmetric stretching mode of the alkyl terminal methyl group (Figure 1). This stretching mode was chosen because it is the dominant mode detectable in the ppp polarization combination, whereas the water O−H stretching modes are usually silent in the ppp polarization combination with our IR 15478
DOI: 10.1021/acs.jpcc.5b04416 J. Phys. Chem. C 2015, 119, 15477−15481
Article
The Journal of Physical Chemistry C
detectable by SFG spectroscopy. Our results show a nonmonotonic behavior of the SFG signal of the O−H stretching modes in response to variations in the CTAB concentration. The observations of the current study support earlier findings that the SFG intensity of the adsorbed surfactant headgroups positively correlates with surface tension measurements.20,24 Hence, the increase in positively charged ammonium groups must affect the interfacial water structure in a certain fashion that can be probed and interpreted by SFG spectroscopy. Our results show that the maximum SFG water signals were reached at a low CTAB concentration of around 0.1 mM, beyond which the interfacial water signal decreases as the CTAB concentration was increased up to and beyond the CMC (Figure 3). Whereas the maximum SFG water signals obtained at 0.1 mM surfactant concentration can be explained by the rapid increase in the interface potential within the CTAB concentration range of 0−0.1 mM,21 the reverse correlation between the SFG water signal intensity and the CTAB concentration beyond this concentration point is interesting and needs to be explained. We thus believe that our SFG data are able to provide important information on the interfacial water when the CTAB concentration exceeds 0.1 mM that cannot be obtained by surface potential measurements. It is worth mentioning that an analogous reverse correlation was also observed in the case when CTAB adsorbs at the negatively charged silica/liquid interface at high surfactant concentration.17 The present study could thus shed some light on this phenomenon and provide useful predictions of the interfacial properties of surfaces affected by interfacial water layers. Interfacial surfactant headgroups have been reported to attain a constant orientation over a wide concentration range from about 1 mM to above the CMC for SDS.20 Therefore, the growing population of CTAB at the air/water interface should, in principle, result in a continuous increase in the SFG interfacial water signals, at least in the range between 0.1 and 1.0 mM. However, in this study, we observed the opposite
Figure 3. ssp SFG signals of the interfacial water O−H vibrational modes at various CTAB concentrations. The arrows show the increasing (bottom panel) and decreasing (top panel) trends of the SFG signal intensities with increasing CTAB concentration. The CMC of CTAB is approximately 1 mM.
Figure 4. Possible mechanisms explaining the SFG water signal decrease with increasing CTAB concentrations: (a) new adsorption model with the formation of subsurface centrosymmetric aggregates that decrease the SFG signal intensity and (b) electrical compression of the interfacial water layer by the interfacial charge and the reduction of the Debye length with increasing surfactant (electrolyte) concentration. The size and direction of the arrows magnify the relative magnitudes and directions of the overall dipole moment of the water layers below the surfactant adsorption layers and around the surfactant aggregate. The mechanism in panel a is not supported by our SFG results. 15479
DOI: 10.1021/acs.jpcc.5b04416 J. Phys. Chem. C 2015, 119, 15477−15481
Article
The Journal of Physical Chemistry C
compression of the electric field of the adsorption layer. The available experimental results (Table 1) also show that the interfacial potential at 0.1 mM CTAB was very close to saturation. Below this concentration, both the interfacial potential and the Debye length affect the electric field and the water SFG signals, as discussed previously, with the effect of the increased interfacial potential being more pronounced than the effect of the double-layer thickness compression. According to the electrostatic double-layer (Debye−Hückel) theory of screening of interface electrical fields, the number of oriented water layers being electrostatically driven starts to decrease with increasing CTAB concentration, leading to an inevitable decrease of the third-order susceptibility χ(3), as experimentally observed in this study (Figure 3). When the CTAB concentration is less than 0.1 mM, the surfactant population at the interface is small, as reflected by surface tension data and Figure 2, and thus is not sufficient to produce an oriented interfacial water layer with a maximum degree of orderliness. Therefore, the SFG interfacial water signals were observed to be directly proportional to the CTAB concentration. It was also reported that there was no discernible SFG signals from the SDS headgroups detected at a comparable surface tension (∼64 mN/m),20 which implies either an insufficient surfactant surface coverage or a randomly oriented surfactant adsorption layer. It was observed in this study that the water “free O−H” dangling mode30 at 3700 cm−1 disappeared when the CTAB concentration exceeded 0.1 mM (data not shown). We suspect that this observation does not mean that the air/ water interface is saturated at 0.1 mM CTAB, but rather implies a well-ordered interfacial water layer that is entirely dictated by the ammonium headgroups. To verify the influence of the solution ionic strength on the Debye length, we measured the SFG interfacial water signals of 0.1 mM CTAB with increasing solution ionic strengths, controlled with NaBr salt (Figure 5). The data shown in Figure 5 are in excellent agreement with those shown in Figure 2. The difference in the absolute SFG signal intensity observed between Figures 2 and 5 can be explained by the decreasing CTAB solubility with the presence of NaBr and the possible incomplete dissociation of the surfactant headgroups.
trend, which underpins an interesting phenomenon taking place in the adsorption process. Typically, an SFG signal decrease implies either a decrease in the number density or an increase in the centrosymmetry of the system. The latter interpretation might apply in many cases when surfactant concentrations exceed the CMC, as the formation of micelles creates a centrosymmetric arrangement with the surfactant adsorption layer at the air/water interface, as illustrated in Figure 4a. A conventional Gibbs adsorption approach cannot be used to explain our observed decrease in SFG water signal in the CTAB concentration range of 0.1−1.0 mM, as there should not be any micelles in the solution at these concentrations.25 Indeed, the classical Gibbs adsorption model was recently shown to be unsuitable for describing the adsorption of soluble surfactants, because the existence of a sublayer of surfactant aggregates/ bilayers has been observed extensively by independent techniques.21,26−28 If these surfactant sublayers do exist, then our SFG data on the interfacial water signals in the CTAB range of 0.1−1.0 mM can be qualitatively explained as illustrated in Figure 4a, whereby surfactant aggregates/clusters or bilayers are treated as micelles. However, when the CTAB concentration reaches the CMC, the deformation of the surfactant aggregates/clusters into micelles should lead to an abrupt change in the SFG interfacial water signals, which was not observed in this study. We therefore rule out this rationale as a viable interpretation of our SFG data. A possible reason for our SFG water signal decrease is the weakening of dipole moments of water molecules within the interfacial region, due to the reduction of the effective electrostatic length of the adsorption layer field (known as the Debye length, lD), as illustrated in Figure 4b. The electrostatic field is present in the close proximity of the adsorption layer within a distance of lD. The Debye length decreases with increasing electrolyte (surfactant) concentration or, equivalently, solution ionic strength, I. For 1:1 electrolyte solutions, one can write29 lD =
εε0kBT 2e 2NAI
(1)
where ε0 is the permittivity of free space, ε is the dielectric constant, kB is the Boltzmann constant, T is the absolute temperature, NA is Avogadro’s number, and e is the elementary charge. A shorter Debye length means that fewer interfacial water layers are affected by the electric field, leading to weaker water SFG signals. The strength of the electric field also depends on the interfacial potential of the adsorption layer. However, the available experimental results (Table 1) show that, at CTAB concentrations greater than 0.1 mM, the interfacial potential does not change significantly. Therefore, we believe that the decrease in the water SFG signals was caused by the
4. CONCLUSIONS In summary, using SFG spectroscopy to detect both the surfactant and interfacial water signals, we have successfully
Table 1. Typical Values of the Debye Length (Eq 1) and Interfacial Potential (Interpolated from the Literature21) as a Function of CTAB Concentration C (mM)
lD (nm)
ΔV (mV)
0.001 0.01 0.1 1
307 97 31 10
12 150 420 460
Figure 5. ssp SFG signals of the interfacial water O−H vibrational modes of 0.1 mM CTAB at increasing ionic strengths. 15480
DOI: 10.1021/acs.jpcc.5b04416 J. Phys. Chem. C 2015, 119, 15477−15481
Article
The Journal of Physical Chemistry C
(14) Hancer, M.; Celik, M. S.; Miller, J. D. The Significance of Interfacial Water Structure in Soluble Salt Flotation Systems. J. Colloid Interface Sci. 2001, 235, 150−161. (15) Nguyen, K. T.; Nguyen, T. D.; Nguyen, A. V. Strong Cooperative Effect of Oppositely Charged Surfactant Mixtures on Their Adsorption and Packing at the Air−Water Interface and Interfacial Water Structure. Langmuir 2014, 30, 7047−51. (16) Nguyen, K. T.; Nguyen, A. V. In Situ Investigation of Halide Co-Ion Effects on SDS Adsorption at Air−Water Interfaces. Soft Matter 2014, 10, 6556−63. (17) Tyrode, E.; Rutland, M. W.; Bain, C. D. Adsorption of CTAB on Hydrophilic Silica Studied by Linear and Nonlinear Optical Spectroscopy. J. Am. Chem. Soc. 2008, 130, 17434−17445. (18) Gragson, D. E.; Richmond, G. L. Investigations of the Structure and Hydrogen Bonding of Water Molecules at Liquid Surfaces by Vibrational Sum Frequency Spectroscopy. J. Phys. Chem. B 1998, 102, 3847−3861. (19) Gragson, D. E.; McCarty, B. M.; Richmond, G. L. Ordering of Interfacial Water Molecules at the Charged Air/Water Interface Observed by Vibrational Sum Frequency Generation. J. Am. Chem. Soc. 1997, 119, 6144−6152. (20) Johnson, C. M.; Tyrode, E. Study of the Adsorption of Sodium Dodecyl Sulfate (SDS) at the Air/Water Interface: Targeting the Sulfate Headgroup Using Vibrational Sum Frequency Spectroscopy. Phys. Chem. Chem. Phys. 2005, 7, 2635−2640. (21) Nakahara, H.; Shibata, O.; Rusdi, M.; Moroi, Y. Examination of Surface Adsorption of Soluble Surfactants by Surface Potential Measurement at the Air/Solution Interface. J. Phys. Chem. C 2008, 112, 6398−6403. (22) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Direct Evidence for Orientational Flip-Flop of Water Molecules at Charged Interfaces: A Heterodyne-Detected Vibrational Sum Frequency Generation Study. J. Chem. Phys. 2009, 130, 204704-1−204704-5. (23) Yoon, R. H.; Yordan, J. L. Zeta-Potential Measurements on Microbubbles Generated Using Various Surfactants. J. Colloid Interface Sci. 1986, 113, 430−438. (24) Hore, D. K.; Beaman, D. K.; Richmond, G. L. Surfactant Headgroup Orientation at the Air/Water Interface. J. Am. Chem. Soc. 2005, 127, 9356−9357. (25) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (26) Nakahara, H.; Shibata, O.; Moroi, Y. Examination of Surface Adsorption of Sodium Chloride and Sodium Dodecyl Sulfate by Surface Potential Measurement at the Air/Solution Interface. Langmuir 2005, 21, 9020−9022. (27) Nakahara, H.; Shibata, O.; Moroi, Y. Examination of Surface Adsorption of Cetyltrimethylammonium Bromide and Sodium Dodecyl Sulfate. J. Phys. Chem. B 2011, 115, 9077−9086. (28) Humphry-Baker, R.; Gratzel, M.; Moroi, Y. Pyrene Fluorescence at Air/Sodium Dodecyl Sulfate Solution Interface. Langmuir 2006, 22, 11205−11207. (29) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, U.K., 1989. (30) Miranda, P. B.; Shen, Y. R. Liquid Interfaces: A Study by SumFrequency Vibrational Spectroscopy. J. Phys. Chem. B 1999, 103, 3292−3307.
demonstrated that the interfacial water layer adopts the most orderly arrangement at a surfactant concentration of around 0.1 mM, at which the surfactant surface coverage has not yet reached saturation. Our SFG data provide direct evidence for the continued reduction (up to and beyond the CMC) of the overall dipole moment of the interfacial water layers when the CTAB concentration exceeds 0.1 mM, as represented by the reduced Debye−Hückel screening length rather than the newly proposed adsorption model. We believe that this study provides important insights into the wide range of surfactant applications in which information about the interfacial water layer is crucial.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (K.T.N.). *E-mail:
[email protected] (A.V.N.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Projects funding schemes (Projects LE0989675 and DP1401089).
■
REFERENCES
(1) Jumpertz, T.; Tschapek, B.; Infed, N.; Smits, S. H. J.; Ernst, R.; Schmitt, L. High-Throughput Evaluation of the Critical Micelle Concentration of Detergents. Anal. Biochem. 2011, 408, 64−70. (2) Nesmerak, K.; Nemcova, I. Determination of Critical Micelle Concentration by Electrochemical Means. Anal. Lett. 2006, 39, 1023− 1040. (3) Paillet, S.; Grassl, B.; Desbrieres, J. Rapid and Quantitative Determination of Critical Micelle Concentration by Automatic Continuous Mixing and Static Light Scattering. Anal. Chim. Acta 2009, 636, 236−241. (4) Harper, K. L.; Allen, H. C. Competition between DPPC and SDS at the Air−Aqueous Interface. Langmuir 2007, 23, 8925−8931. (5) Li, P. X.; Thomas, R. K.; Penfold, J. Limitations in the Use of Surface Tension and the Gibbs Equation To Determine Surface Excesses of Cationic Surfactants. Langmuir 2014, 30, 6739−6747. (6) Menger, F. M.; Rizvi, S. A. A. Relationship between Surface Tension and Surface Coverage. Langmuir 2011, 27, 13975−13977. (7) Hore, D. K.; Beaman, D. K.; Parks, D. H.; Richmond, G. L. Whole-Molecule Approach for Determining Orientation at Isotropic Surfaces by Nonlinear Vibrational Spectroscopy. J. Phys. Chem. B 2005, 109, 16846−16851. (8) Casson, B. D.; Bain, C. D. Phase Transitions in Mixed Monolayers of Cationic Surfactants and Dodecanol at the Air/Water Interface. J. Phys. Chem. B 1999, 103, 4678−4686. (9) Noziere, B.; Baduel, C.; Jaffrezo, J. L. The Dynamic Surface Tension of Atmospheric Aerosol Surfactants Reveals New Aspects of Cloud Activation. Nat. Commun. 2014, 5, 3335. (10) Fan, Y. B.; Chen, X.; Yang, L. J.; Cremer, P. S.; Gao, Y. Q. On the Structure of Water at the Aqueous/Air Interface. J. Phys. Chem. B 2009, 113, 11672−11679. (11) Nguyen, C. V.; Phan, C. M.; Ang, H. M.; Nakahara, H.; Shibata, O.; Moroi, Y. Molecular Dynamics Investigation on Adsorption Layer of Alcohols at the Air/Brine Interface. Langmuir 2015, 31, 50−56. (12) Dobias, B. Adsorption, Electrokinetic, and Flotation Properties of Minerals above the Critical Micelle Concentration. ACS Symp. Ser. 1986, 311, 216−224. (13) Miller, J. D.; Veeramasuneni, S.; Yalamanchili, M. R. Recent Contributions to the Analysis of Soluble Salt Flotation Systems. Int. J. Miner. Process 1997, 51, 111−123. 15481
DOI: 10.1021/acs.jpcc.5b04416 J. Phys. Chem. C 2015, 119, 15477−15481
