Sum-Frequency Generation Spectroscopy of an Adsorbed Monolayer

Article pubs.acs.org/JPCC

Sum-Frequency Generation Spectroscopy of an Adsorbed Monolayer of Mixed Surfactants at an Air−Water Interface Ankur Saha,† Hari P. Upadhyaya,† Awadhesh Kumar,*,† Sipra Choudhury,‡ and Prakash D. Naik† †

Radiation & Photochemistry Division and ‡Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai − 400 085, India ABSTRACT: The effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, and cetyltrimethylammonium bromide (CTAB), a cationic surfactant, at an air−water interface have been investigated using vibrational sum-frequency generation (VSFG) nonlinear optical spectroscopy. The work is focused on understanding the temporal evolution of aggregation behavior of the catanionic system of CTAB and SDS at the charged air−water interface. For the mixed surfactants, the VSFG intensity of the OH stretching bands decreases, whereas that of the CH stretching of alkyl chains increases with time. For the 1:1 ratio of surfactants, the VSFG intensity of the OH stretch vanishes much earlier than the complete growth of the CH stretching modes. Thus, the polar ordering of interfacial water molecules is faster than the time-evolution of alkyl chains of the catanionic system. The temporal growth of the complex between surfactants CTAB and SDS exhibited an induction time (up to ∼2000 s), followed by a rapid growth (∼30 s) and then a slow growth for hours. The effects of different compositions of the surfactants on the induction time and the adsorption kinetics have been investigated. Our results on VSFG are supported by measurements, employing the surface pressure-time (π−t) kinetics and Brewster angle microscopy (BAM). The nature of π−t curves and its dependence on composition of surfactants is qualitatively similar to VSFG temporal profiles, except for the absence of the rapid adsorption growth in the former. This difference in the adsorption kinetics is explained based on formation of surfactant domains, as detected by BAM, at the air−water interface.

1. INTRODUCTION Studies at surfaces and interfaces are very important, since an interfacial region between bulk phases consists of only a small fraction of the material, but frequently these are the sites of reactions, and hence dominate the macroscopic properties. However, probing a small interfacial region is quite challenging, because of interferences from the bulk media. One requires a surface selective technique to probe an interface, and avoid any contribution from the bulk phases. A number of techniques are being employed to study the microscopic structure and morphological properties of both insoluble (spread/Langmuir) and adsorbed (Gibbs) monolayers. Among these, the techniques based on second-order nonlinear optical processes, such as second harmonic generation (SHG) and sum-frequency generation (SFG), are more appealing. These are interface specific, sensitive, selective, and ideal to probe any buried interface accessible by light.1,2 These processes are forbidden under the electric-dipole approximation in the bulk of media having an inversion symmetry. At the interface between centrosymmetric media, the symmetry is broken and these SHG and SFG processes are allowed. These techniques can be applied to a broad range of surfaces and interfaces. The SFG technique is surface-sensitive and quite selective to probe vibrational spectra of molecules present solely at the interfaces.2,3 The SFG process is primarily a three-wave mixing with two input beams and the third signal beam. The two input © 2014 American Chemical Society

beams interact temporally and spatially in a medium at the interface, and the coherent SFG signal is generated due to second-order nonlinear susceptibility, χ(2). One of these two input beams is in the visible frequency region (generally fixed at 532 nm), and the other is in the IR frequency region with tunability. The SFG signal is generated at the frequency ωSFG, which is the sum of the visible (ωVIS) and IR (ωIR) input frequencies, i.e., ωSFG = ω VIS + ωIR (1) The signal beam direction is given by the phase-matching condition, k SFG = k VIS + kIR

(2)

where ki is the wave vector of the beam i. Since these input visible and IR beams interact at the interface at angles θVIS and θIR with the normal to the interface, the general phase-matching condition (eq 2) can be written as follows (eq 3), k SFG sin θSFG = k VIS sin θVIS + kIR sin θIR

(3)

A three-wave-mixing experiment is performed employing short (ns or ps) or ultrashort (fs) laser pulses, since the signal is Received: November 27, 2013 Revised: January 17, 2014 Published: January 21, 2014 3145

dx.doi.org/10.1021/jp411641z | J. Phys. Chem. C 2014, 118, 3145−3155

The Journal of Physical Chemistry C

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Figure 1. A schematic diagram of the experimental set up for VSFG spectroscopy. Letters L, I, P, R, and M stand for lens, iris, polarizer, reference material (ZnSe), and monochromator with PMT, respectively.

proportional to the reciprocal of the laser pulse width.4 Depending on the pulse duration, two main schemes are commonly employed to detect the vibrational SFG (VSFG) signal. For the short laser pulses, the IR wavelength is scanned across the vibrational band, and the signal is recorded for each wavelength after three stages of spatial, spectral and polarization filtration to remove the reflected input visible light. The spectral resolution is determined by the width of the IR laser, which is typically less than 1 cm−1 for nanosecond systems, and 1−20 cm−1 for ps systems. For femtosecond lasers, the IR frequency is not required to be scanned to obtain a vibrational spectrum because of a large bandwidth of such lasers (∼100 cm−1 for ∼100 fs). A spectrum can be measured over this interval of 100 cm−1 without scanning the IR frequency. This scheme is known as broadband SFG spectroscopy, which offers the advantage of a shorter acquisition time. The VSFG spectra, measured under different polarization geometries, can provide valuable information on polar orientation, molecular conformation, tilt angle of the adsorbate with the surface, etc. However, heterodyne-detected VSFG studies provide direct information on these structural parameters at an interface.5,6 Polar ordering of adsorbed molecules at the interfaces is important for surface modifications and chemical reactions. The VSFG has been employed even in real-time to investigate motion and transformation of molecules on surfaces in real-time, on a femtosecond time scale. The technique has been successfully used to elucidate the reorientational dynamics of surface molecules.7,8 We have employed the SFG technique to investigate adsorbed monolayer of mixed surfactants. Surfactants are widely used for several industrial applications. Hence, the adsorption of these molecules at interfaces, particularly air− water interface, is an important area of research in surface science. Recently, aqueous mixtures of cationic and anionic surfactants (catanionic mixtures) have received immense attention, particularly because of their higher surface activity than the individual components or tendency to form vesicle spontaneously.9−13 These catanionic mixtures have many promising bulk and interfacial properties. These mixtures can

be used as substitutes for phospholipids,14 and as emulsion stabilizers.15 These can also be used to make foams with high stability.16 Cationic mixtures exhibit several complex behaviors.17,18 The properties of these mixtures strongly depend on the compositions, concentration of individual surfactants, the relative number of alkyl chains per surfactant, temperature, ionic strength of solution, etc.11,19,20 Therefore, the method of preparation governs the properties of these catanionic mixtures. We have investigated effects of various compositions in mixed surfactant solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, and cetyltrimethylammonium bromide (CTAB), a cationic surfactant, at air−water interface using vibrational sum-frequency generation (VSFG) nonlinear optical spectroscopy. The main objective of this study was to investigate formation process of the adsorbed monolayer and ordering of interfacial water molecules at the charged air−water interface in the catanionic system of CTAB and SDS with varying compositions in aqueous solutions, using time-dependent SFG studies.

2. EXPERIMENTAL SECTION 2.1. Vibrational Sum-Frequency Generation. VSFG spectroscopy is based on a coherent second order nonlinear process, which involves two input beams of fixed visible (ωvis) and tunable infrared frequencies (ωIR). The laser at ωvis is fixed at 532 nm, and that at ωIR is tuned in the 2.3−10.0 μm spectral range to measure the vibrational spectra at an interface. The visible beam is generated by frequency doubling of the fundamental output of a Nd:YAG laser (PL2241B, Ekspla, Lithuania). The tunable IR beam is generated in a difference frequency generator (DFG) by mixing the output (420−680 nm) of an optical parametric generator (OPG) with the fundamental output (1064 nm) of the Nd:YAG laser in a silver thiogallate (AgGaS2) crystal. The OPG (PG401, Ekspla) was pumped by the third harmonic (355 nm) beam of the Nd:YAG laser, using lithium triborate (LiB3O5) as a nonlinear crystal. These two input beams are passed through apertures, energy attenuators and polarizers, and finally loosely focused at the interface. The angles of incidence were kept at 55° and 60° for the IR and visible laser beams, respectively. The visible beam is 3146



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the air−water interface, by using the Gibbs adsorption isotherm equation. The Gibbs adsorption for a dilute solution of two surfactants can be written as follows:22

also passed through a delay line for proper temporal overlap of both the beams at the interface. The reflected IR and visible beams were blocked, and the SFG signal beam was separated and detected with a photomultiplier tube (PMT) after spatial, polarization and spectral filtering. Four different polarization schemes (ssp, sps, pss and ppp) are possible for SFG experiments, and these are denoted based on the polarization states of the SFG, visible and IR beams in the sequence. For example, the polarization ssp implies that the SFG and visible beams are s-polarized, whereas the IR beam is p-polarized. In all our experiments, the ssp polarization has been used. The schematics of the VSFG experimental set up is shown in Figure 1. Two different types of SFG experiments have been performed. For measuring the vibrational spectra at an interface, the IR wavelength was scanned in the appropriate range. The CH and OH vibrational stretching regions were scanned in the range of 2750−3000 cm−1 and 3000−3600 cm−1, respectively. Each scan was obtained with a spectral resolution of 1 or 2 and 2 or 4 cm−1 for the CH and OH spectral regions, respectively, and an average of 60 or 100 laser shots per experimental data. In another type of experiment, the temporal evolution of a vibrational band was measured. In this experiment, the frequency of the tunable IR laser beam was also fixed, at the maximum of a vibrational band, and the SFG signal was measured at different times by scanning the time delay. For the VSFG studies on mixed surfactants at the air−water interface, stock aqueous solutions of SDS (∼0.8 mM) and CTAB (∼0.8 mM) were prepared. For time-dependent measurements, a specific procedure was followed for making catanionic solutions. A known volume of one surfactant solution was added to a circular glass trough (50 mm in diameter, ∼14 mm in depth, and 27 mL volume) containing a fixed volume of water, and thoroughly stirred. Subsequently, the required volume of the second surfactant solution was added to the trough to achieve a desired molar ratio. Then, after light stirring for a short time, the interfacial region was probed immediately in either the CH or OH vibrational region. All additions of the required volume of the surfactant solutions were carried out using micro syringes. Individual surfactant concentration of each solution (1−35 μM) was much lower than its critical micelle concentration (CMC), 8.1 mM for SDS13 and 0.98 mM21 for CTAB. All the experiments were conducted in the circular glass trough, which was mounted on a six-axes mount for spatial overlapping of the laser beams at the interface (shown in Figure 1). In other than time-dependent experiments, usual premixed catanionic solution was used. 2.2. Surface Pressure-Time (π−t) Adsorption Kinetics. The π−t adsorption kinetics was recorded at room temperature of ∼298 K, using a platinum Wilhelmy plate microbalance with an accuracy of ±0.02 mN/m. It was equipped with a computer controlled KSV 5000 Langmuir double barrier Teflon trough. The time resolution of the surface pressure measurements was ≤1 s. The aqueous solutions of different compositions of CTAB and SDS were used to study the adsorption growth kinetics. A fixed volume of CTAB from the stock solution was added to the trough containing a known volume of Millipore water, and thoroughly mixed. Subsequently, a known volume of SDS was added to this solution to obtain the required composition, and mixed. Immediately after this, measurements on the π−t adsorption kinetics were initiated. The surface pressure or surface tension (γ) measurements also give information on the adsorbed amount of a surfactant at

dγ = RT (Γ1 d ln a1 + Γ2 d ln a 2)

(4)

where R is the gas constant, T is the absolute temperature, Γ1 and Γ2 are the surface (excess) concentrations of the two surfactants at the interface, and a1 and a2 are their respective activities in the solution phase. From this equation, one can write expressions for Γ1 and Γ2 as given in eq 5, after substituting molar concentrations for activities in a dilute solution. Γ1 =

1 ⎛ −∂γ ⎞ 1 ⎛ −∂γ ⎞ ⎜ ⎟ and Γ2 = ⎜ ⎟ RT ⎝ ∂ ln C1 ⎠C RT ⎝ ∂ ln C2 ⎠C 2

1

(5)

where C1 and C2 are bulk molar concentrations of the two surfactants. Thus, the value of Γ1(Γ2) can be obtained from the slope of the plot between γ versus ln C1 (ln C2), by holding the concentration C2 (C1) constant. 2.3. Brewster Angle Microscopy. A Brewster angle microscope (BAM, model: Nanofilm_ep3bam, Accurion, Germany) was employed for microscopic observation of the surface morphology. It was mounted to the computerinterfaced microbalance to probe the surface morphology at a particular surface pressure. The BAM was equipped with a green laser (532 nm, 50 mW) emitting p-polarized light, which is not reflected off from the air−water interface at the Brewster angle of ∼53.1°. The Brewster angle was optimized by measuring the reflectivity of pure water with the angle of incidence; the angle corresponding to the lowest reflectivity was selected as the Brewster angle for further measurements. The presence of a condensed monolayer phase leads to a change in the refractive index and thus, to a measurable change in reflectivity. The lateral resolution of the microscope was ∼2 μm. The images captured by a CCD camera were digitized and processed for obtaining good quality of BAM pictures. 2.4. Materials. CTAB (Sigma-Aldrich, purity 99%) and SDS (Sigma-Aldrich, purity 99%) were commercial products, which were used without any further purification. The subphase water with surface tension of 71.9 mN m−1 and resistivity of 18.2 MΩ at 298 K was prepared employing a Millipore apparatus (Millipore, France). Absence of any hydrocarbon impurity in water was ascertained with VSFG measurement.

3. RESULTS AND DISCUSSION 3.1. SDS or CTAB Solution at the Interface. VSFG spectra of water show broad, but weak, peaks at ∼3200 cm−1 and ∼3400 cm−1, due to the OH symmetric vibrational stretching frequencies of hydrogen-bonded water molecules. In addition, a sharp peak is observed at 3750 cm−1 due to the OH vibrational frequency of free water molecules. This sharp peak is not shown in the spectra, since it is not discussed further and it will extend the range of the abscissa (wavenumbers) leading to congestion. In the presence of either SDS or CTAB aqueous solution, we observed a significant enhancement in the intensities of the OH stretching modes of hydrogen-bonded interfacial water molecules (shown in Figure 2). The observed enhancement is attributed to ordering of interfacial water molecules induced by the large electrostatic field due to the charged surfactants.13 This explanation is supported by VSFG spectra of mixed surfactants of SDS and CTAB (vide inf ra), which are oppositely charged. With the 1:1 composition of SDS 3147



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Table 1. Assignment of Vibrational Frequencies in VSFG Spectra of Surfactants at an Air−Water Interface under SSP Polarizationa observed vibrational frequencies/cm‑1 assignment of vibrational frequencies CH2 symmetric stretch, νs(CH2) CH3 symmetric stretch, νs(CH3) CH2 antisymmetric stretch, νa(CH2) CH3 Fermi-resonance, ν(CH3−FR)

SDS

CTAB

1:1-Complex

2857−2850

2848

2846

2881−2879

2876

2876

∼2924

2920−2914

--

2947−2944

--

2940

a

With increased bulk concentration of surfactants, some vibrational frequencies shift to lower values.

the mixed surfactants due to the neutralization of opposite charges on these surfactants. With the 1:1 ratio of CTAB and SDS, opposite charges on these surfactant molecules are canceled, resulting in the disappearance of OH peaks (shown in Figure 3). One can assume that for this composition,

Figure 2. VSFG spectra of neat water and aqueous solutions of CTAB and SDS at the air−water interface, showing enhancement of OH stretching modes of water.

and CTAB, opposite charges on these surfactant molecules are canceled resulting in very much reduced intensities of the OH peaks. Similarly, the vibrational region of the CH stretching frequencies at the air−water interface was probed to investigate the conformational order of the alkyl chain of surfactant molecules. The frequencies of CH2 stretching modes are understood to be conformation sensitive and can be correlated empirically with the conformational order (trans/gauche ratio) of the alkyl chains.23,24 A shift of these frequencies toward lower wavenumbers or decrease in their intensities with respect to the CH3 stretching modes, or completely absence of these frequencies of CH2 modes suggest highly ordered conformation of the methylene chains. With increasing bulk concentration of SDS (up to 1.6 mM) and CTAB (up to 0.2 mM), intensities of vibrational bands increased due to an increase in their surface concentrations. However, a decrease in vibrational frequencies of vs(CH2) from 2857 to 2850 cm−1 was observed with increasing bulk concentration of SDS, indicating an increase in the conformational order of the alkyl chain, such as a decrease in the gauche and increase in trans conformers. The va(CH2) band of SDS at ∼2924 cm−1 could not be differentiated well from the Fermi-resonance CH3 band, v(CH3−FR). The wavenumber of 2924 cm−1 is characteristic of melting of the methylene chains, having a number of gauche conformers, whereas that of ∼2918 cm−1 is characteristic of highly ordered conformations with all trans conformations. Similarly, with only CTAB solution, vibrational peaks have been observed at 2848 cm−1, 2876 cm−1 and ∼2917 (2920−2914) cm−1, and intensities of all VSFG peaks increased with the bulk concentration of CTAB. The frequencies in the CH region of the VSFG spectra are listed in Table 1. 3.2. Mixed Surfactants at the Interface. A significant change was observed in the OH spectral region of the mixed surfactants. The VSFG intensities of OH stretching modes decrease as one moves toward the 1:1 molar ratio of CTAB and SDS surfactants, and finally vanishes for this composition. The observed enhancement in the intensities of the OH stretching modes in the presence of either CTAB or SDS, attributed to ordering of interfacial water molecules induced by the large electrostatic field due to the charged surfactants, gets reduced in

Figure 3. VSFG spectra of both the CH and OH regions in aqueous solution of individual (∼0.4 mM) and mixed (1:1) surfactants of CTAB and SDS at the air−water interface; the OH modes almost disappeared in the mixed solution. The CH region of SDS does not show any peak due to its low bulk concentration.

approximately half of the interfacial water molecules are oriented with oxygen atoms toward the bulk water phase, and the other half are oriented with oxygen atoms toward the air. This results in null orientation of interfacial water molecules, leading to disappearance of the OH peaks. For other compositions, the OH peak intensity is less than that observed for the individual component. Like the OH spectral region, a significant change was also observed in the CH spectral region of the mixed surfactants. VSFG spectra for different compositions of mixed surfactants are shown in Figure 4. The spectra of the CTAB-rich 9:1 ratio and the SDS-rich 1:9 ratio are almost similar to CTAB and SDS spectra, respectively. Due to low concentration of SDS, no CH peaks from SDS could be observed for the 1:9 ratio. However, the same low concentration of CTAB in the 9:1 composition gives CH peaks from CTAB. With CTAB-rich ratios of 9:1 and 3148



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the 1:1 composition of these two surfactants were probed at 2876 cm−1 and 2940 cm−1. Similarly, the OH spectral region was probed at ∼3240 cm−1. Premixed solutions of both these surfactants do not show VSFG peaks in the OH spectral region (as mentioned earlier), due to neutralization of charges on the surfactants. Therefore, a different procedure for the measurement was adopted as described in the Experimental Section. A known volume (from the stock solution) of one surfactant (CTAB) solution was added to the trough containing a known volume of water, and properly stirred. Subsequently, the required volume of the second surfactant solution (SDS) was added to the equilibrated solution in the trough, and the time delay scan was immediately started at a fixed wavelength in the CH or the OH spectral region. 3.3.1. Temporal Growth at 2876 cm−1. A typical temporal profile of the mixed surfactant shows an induction time after which a fast growth is observed, followed by a slow growth for hours. These profiles strongly depend on the total bulk concentration of surfactants, and their compositions at a particular total bulk concentration. Figure 5a depicts the

Figure 4. VSFG spectra of the CH region of mixed CTAB and SDS solutions with different compositions at the air−water interface. The total bulk concentration of surfactants in this measurement was about 0.35 mM.

7:3, the absorption at ∼2946 cm−1 is not observed, because of a destructive interference between the OH and the CH3 Fermiresonance modes in CTAB.13 However with the 1:1 ratio, a strong new peak at ∼2940 cm−1, close to v(CH3−FR) peak at 2946 cm−1 of SDS, is observed. The spectral features of the 1:1 composition are very much different from other compositions. For the 1:1 composition, the intensity of the vs(CH2) band at ∼2850 cm−1 is reduced and that for the va(CH2) band at ∼2917 cm−1 (due to CTAB) is negligibly small. But the intensities of the CH3 modes are very much enhanced, and are the maximum among all compositions. In addition, the vs(CH2) band at ∼2850 cm−1 has shifted to a lower wavenumber of 2846 cm−1. Even the new peak at ∼2940 cm−1 has shifted to lower wavenumber by about 3 cm−1, and this peak can be assigned to the ν(CH3−FR) band of a complex between CTAB and SDS surfactant molecules. Assignment of the vibrational frequencies in VSFG spectra of surfactants is given in Table 1, based on literature data.13,25 Thus, the observed VSFG spectra from the mixed surfactant system are dominated by the vs(CH3) and CH3 Fermi-resonance bands positions, suggesting highly ordered conformation and low gauche conformation with respect to individual surfactant component. Similar highly ordered conformation is observed in SFG studies for mixed surfactants of SDS and dodecylammonium chloride (DAC).13 Thus, SFG results on both the OH and CH spectral regions suggest formation of an insoluble 1:1 complex between CTAB and SDS molecules, and thus these molecules are retained at the surface. These complex molecules are formed to the greater extent for the 1:1 ratio of the surfactants. The IR external reflection (IER) spectroscopy of similar catanionic mixed surfactants, SDS and CPC (cetylpyridinium chloride) reveals not only highly ordered conformation but the 1:1 composition for the adsorbed monolayer.20 SFG studies reveal conformational ordering not only for both the charged molecules in a mixed solution, but in a mixed solution of charged and neutral molecules as well.26 3.3. Time-Dependent Measurements. Time-dependent VSFG spectra were measured to understand the dynamics of the 1:1 complex formation between CTAB and SDS mixed components. Different bulk concentrations (1−35 μM each) of

Figure 5. Temporal evolution of the catanionic mixture at 2876 cm−1 between CTAB and SDS for (a) the 1:1 ratio at different total bulk concentrations (μM), and (b) different ratios at the total bulk concentration of 6 μM.

dependence of the temporal profiles of VSFG intensity at 2876 cm−1 on the total bulk concentration for the 1:1 ratio of the surfactants. The induction time increases with a decrease in the total bulk concentration, and it could be observed up to ∼2000 s for the bulk concentration of 1.6 μM. The growth kinetics also become faster at increased bulk concentration. The induction time and the growth kinetics also depend on the relative ratios of the surfactants. Figure 5b shows temporal profiles of catanionic solutions prepared with different molar ratios (CTAB:SDS = 2:6, 3:5, 4:4(1:1), 5:3, and 6:2), keeping the total bulk concentration fixed (∼6 μM). The induction times of the compositions 2:6 and 6:2 are almost the same within the experimental error. Similarly, the induction times of 3:5 and 5:3 ratios are almost the same, but relatively shorter than that of the ratios 2:6 and 6:2. The ratio 1:1 has the 3149



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shortest induction time, and the fastest growth kinetics. Thus, the induction time gets longer, and the growth kinetics gets slower as the composition shifts away from the 1:1 composition. These results suggest that the initial fast growth is due to adsorption of the catanionic complex at the interface, which forms a monolayer and keeps on slowly growing and getting saturated depending on the bulk concentration and the composition of the mixed surfactants. This result is not surprising, since the complex is expected to be present in the highest fraction at a 1:1 molar ratio of the two surfactants.27 The induction time for formation of the fast component is related to the frequency of encounters between two surfactant molecules of opposite charges, SDS and CTAB, leading to the adsorption of sufficient number density of complex molecules at the interface for the SFG detection. This frequency is the highest for the 1:1 ratio, implying the shortest induction time. The figure shows that the initial growth intensity is greater for a CTAB-rich composition, because of greater surface activity of CTAB than SDS.19 Thus, the CTAB molecules have a greater probability of residing at the interface than the SDS molecules, and these interfacial CTAB molecules can attract the SDS molecules and pull them from the bulk to form the complex at the interface. It is pertinent to point out that, in addition to surfactant composition, the induction time strongly depends on the experimental conditions, such as the order of addition of surfactant solution to the trough. In all the results discussed so far, CTAB is added first followed by SDS for the measurements. However, for the 1:1 ratio, if SDS is added first followed by CTAB, the induction time is lengthened for the same composition and under similar experimental conditions. This trend is also mainly due to greater surface activity of CTAB than SDS. However, the dependence of both the induction time and the adsorption kinetics on the bulk concentration and relative ratio of surfactants remains similar. The intensities of the CH stretching bands of the complex for the 1:1 ratio keep on growing for a longer time, signifying an increase in the conformational order of the alkyl chain as the complex stabilizes during formation, and adsorption at the interface. However, the time evolution observed for the OH stretching frequency of interfacial water molecules at ∼3140 cm−1 is significantly different. Under similar experimental conditions, the intensity of the OH stretching band is reduced to zero much earlier (shown in Figure 6). We should keep in mind that the VSFG intensity depends on both the number density and the orientation of adsorbed species at the interface.2 Moreover, the intensity of the OH band is due to the presence of the charges on the head groups of the surfactant molecules. Thus, the results suggest that the complex formation starts with an interaction between the head groups of the charged surfactants present at the interface and subsurface. In the process, the alkyl chains of both the surfactant molecules in the complex are brought closer to each other. Subsequently, the alkyl chains rearrange and further come closer increasing their conformational order, which leads to an increase in the CH intensity. The formation of monolayer because of adsorption from the bulk at the interface continues until saturation. In the bulk solution, both free surfactant molecules and their neutral catanionic complex exist. Since the OH VSFG intensity is decreased much earlier with the CH intensity still increasing, the adsorption of CTAB and SDS is simultaneous at the interface, or the catanionic complex from the bulk phase is getting directly adsorbed gradually at the interface to form the

Figure 6. Temporal evolution of the OH (3140 cm−1) and CH (2876 and 2940 cm−1) vibrational bands of the complex between CTAB and SDS for the 1:1 ratio.

monolayer. Recently, in studies using IER spectroscopy on adsorption of mixed surfactant solutions of cetylpyridinium chloride (CPC) and SDS, it was shown that the crystallization and adsorption of these surfactants occur simultaneously at the air−solution interface.20 Their argument was based on the observation of simultaneous increase in the intensity of νa(CH2) band intensity in the IER spectra of adsorbed monolayer and decrease in the frequency from 2928 to 2918 cm−1 with time (up to 3 h). In our SFG experiments, the νa(CH2) band intensity did not increase with time, rather there was an initial decrease in the intensity, which remained almost constant thereafter. The mechanism of the simultaneous adsorption of both the cationic and anionic surfactants at the air−water interface can explain the decrease in the intensity of the OH band and the initial increase in that of the CH band until the time when the former is completely vanished. However, this mechanism cannot further explain the increase in the VSFG intensity of the CH bands at 2876 and 2940 cm−1 until saturation. These results suggest that the initial adsorption (until the OH intensity vanishes) can be due to both the mechanisms of simultaneous adsorption of individual surfactants and adsorption of the catanionic complex from the bulk. However, the later temporal increase in the VSFG intensity of the CH bands is predominantly due to the direct adsorption of the complex from the subsurface and ordering of alkyl chains of the adsorbed surfactants with time. This proposed mechanism of adsorption entails greater adsorption of free surfactant molecules than the complex. Similar assumption of faster adsorption of free molecules than amphiphiles forming the bilayers was assumed to explain the measured surface tension.28 3.3.2. Time-Dependent Spectral Features. The timedependent VSFG spectra of different compositions of CTAB and SDS were recorded. For the 1:1 ratio with the total bulk concentration of 6.0 μM (Figure 7), vibrational bands at ∼2850 and ∼2924 cm−1 due to CH2 symmetric and asymmetric stretch, respectively, could be seen during the first scan of ∼14 min. In addition, bands at ∼2876 and 2940 cm−1 were observed due to CH3 stretch. These features pertain mostly to CTAB with its interaction with SDS forming a 1:1 insoluble complex. In the subsequent scan, the 2850 band shifts from 2851 to 2846 cm−1 with slightly increased intensity, which remains unchanged thereafter. The weaker band intensity at 2924 cm−1 band is almost completely reduced, and overlapped with 3150



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2940 cm−1 continuously shifts toward the lower wavenumbers (from 2942 to 2939 cm−1) with compositions getting richer in CTAB, the frequencies for the 3:2 and 7:1 compositions are almost similar to the 1:1 ratio. Thus, the CTAB-rich composition is probably more ordered. For the weak and broad bands at ∼2920 (mainly due to CTAB) and 2850 cm−1, it is difficult to make out the shift in frequencies. However, the former band intensity shows an increase and the latter almost unchanged in the intensity, with compositions richer in CTAB. On the other hand, the major band at 2876 cm−1 remains almost unchanged. However, the VSFG intensities of all the CH stretching bands increase with increased bulk concentration of surfactants with the 1:1 molar ratio. The band at 2940 cm−1 slightly shifts toward the lower wavenumbers (from 2942 to 2939 cm−1) with increasing bulk concentration (3 to 50 μM). Thus, higher ordering is observed for greater total bulk concentration. 3.4. Surface Pressure-Time (π−t) Adsorption Kinetics. The π−t adsorption kinetics of the mixed surfactants CTAB and SDS were measured to support the results on timedependent SFG studies, which suggest formation of the 1:1 catanionic complex due to electrostatic interaction between the two oppositely charged head groups. The growth kinetics of the CH region of the complex shows an induction time after which there is a fast growth followed by slow growth for hours. The growth is interpreted as an increase in the surface concentration and realignment of the alkyl chains of the complex. The π−t adsorption kinetics were measured for different molar ratios of CTAB to SDS with the total bulk concentration kept constant. These π−t adsorption curves (depicted in Figure 8) are

Figure 7. Time-dependent VSFG spectra of the CH region of mixed CTAB and SDS solutions (1:1 ratio, total bulk concentration = 6 μM) at the air−water interface.

the 2940 cm−1 band. The 2940 cm−1 peak due to the ν(CH3− FR) appears at 2942 cm−1 and finally shifts to 2940 cm−1. Thus, within about 30 min, the VSFG intensity due to the CH2 groups of both surfactants at the interface has acquired almost a constant value in the complex. Within similar time (Figure 6), VSFG intensity in the OH region is almost completely vanished, implying orientation of interfacial water molecules is complete. This further suggests that orientation of charged head groups of interfacial surfactant molecules is almost complete. However, intensities of the vibrational bands at 2876 and 2940 cm−1 due to the CH3 stretch in the complex kept on increasing with time (more than 2 h). Thus, rearrangement of alkyl chains and adsorption of catanionic molecules are continued for much longer time than the orientation of charged head groups of the interfacial surfactant molecules. Similar observation of faster orientation of interfacial water molecules than that of alkyl chains of surfactants is reported.13 Similar time-dependent VSFG spectra of the CH-region have been measured for different compositions of the surfactants. For composition of CTAB:SDS = 5:3, the CTAB-rich solution has initial spectrum with higher intensity of νs(CH2) band at 2850 cm−1, which decreased and subsequently remained constant with time. These facts imply more ordered structure of the catanionic complex. For the 3:5 composition, the band at 2850 cm−1 did not decrease with time and remained constant. The 2940 cm−1 peak also appears at 2942 cm−1 after ∼30 min. Thus, at any particular time, the 3:5 composition shows less ordered cationic complex than the 5:3 composition, implying the CTAB-rich composition is more ordered. 3.3.3. Equilibrated Adsorption VSFG Spectra. The VSFG intensities of vibrational bands evolve differently with time, depending on the compositions (as discussed earlier). The VSFG spectra of various compositions of CTAB and SDS were measured after more than 17 h, when the aggregation of surfactants was complete. The intensities for all the compositions (with the same total bulk concentration of 6.0 μM) remained almost the same within the experimental error, except for the ratio CTAB:SDS of 1:7, which shows a variation . The spectral features of different compositions exhibit some regular trend with respect to the band frequency. The band at

Figure 8. The π−t adsorption kinetics for the catanionic mixture of CTAB and SDS in different ratios with the total concentration of 6 μM.

qualitatively similar to the temporal profiles of SFG, except for the absence of the fast growth component in the former. Like SFG temporal profiles, the π−t adsorption curves also show induction time, which corresponds to the minimum surface concentration required to change the surface pressure, and slow adsorption kinetics, but not the fast growth component. The induction times in the SFG and surface pressure measurements for a particular solution are not quantitatively the same, due to a difference in the sensitivity of these instruments. However, 3151



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the dependence of the induction time on different experimental variables is qualitatively similar in both the cases, and the measurements are reproducible, within the experimental error, for a particular method. A large variation in the surface pressure with time is not due to individual surfactants, but rather is due to catanionic surfactants, since a large variation is observed only for a surfactant close to its cmc,29 and not in dilute solution of the present work. A significant increase in the surface pressure caused by the catanionic mixture is mainly due to the enhanced surface coverage through the strong attractive electrostatic interaction.30 During the formation of an adsorbed Gibbs monolayer of catanionic molecules, the surface pressure continuously increases, without showing any obvious inflection point. At first the surface pressure increases rapidly and almost linearly, and then slowly until saturation, implying that the dynamic adsorption process has reached an equilibrium state. The growth kinetics is the fastest for the 1:1 ratio, and becomes faster with increasing CTAB concentration for other ratios. Since CTAB is more surface-active than SDS, CTAB molecules mostly reside at or near the surface. These cationic CTAB molecules attract the anionic SDS molecules pulling them from the subphase to the surface in the process of minimizing their electrostatic force. This finally leads to the formation of catanionic complex structures, with mostly 1:1 composition, at the interface leading to an increase in the surface pressure. This explains why bulk concentration of CTAB controls the adsorption kinetics of mixed surfactants CTAB and SDS. However, after a long time, when the system is equilibrated, the final surface pressure converges to almost the same value of ∼35 mN/m, except for the CTAB: SDS ratios of 1:7 and 7:1, which show a variation. In the second set of experiment, the π-t adsorption kinetics were measured for the 1:1 composition of CTAB and SDS with varying total bulk concentrations of 4, 8, 32, and 60 μM. With increasing bulk concentration, the induction time decreases, the growth kinetics become faster, and the value of saturation pressure increases (31−39 mN/m). These π-t curves are depicted in Figure 9. These results on surface pressure are in qualitative agreement with that on SFG. Kinetic adsorption for

ionic surfactant systems has been discussed well both experimentally and theoretically.29,31 The initial fast growth observed in SFG measurement, but not in surface pressure measurements, can be explained by the nonlinear nature of the SFG technique, in which the SFG intensity is directly proportional to the square of density of the surfactant molecules at the interface.32−34 This discrepancy can be rationalized, if we assume that surface aggregate domains are formed due to the adsorption of catanionic surfactant molecules. These domains have larger surface density than the continuous phase due to free surfactant molecules. Since the SFG intensity scales with the number of surfactant molecules per unit surface area, the domains dominate the SFG intensity. Therefore, the SFG signal is observed when these domains are formed; and the domain formation is expected to depend on the relative composition of CTAB and SDS and the total surfactant concentration. By contrast, the surface pressure continuously increases without showing a fast rise because adsorption of any free/catanionic surfactant molecules at the interface displaces water molecules and thereby reduces the high surface free energy. A discrepancy in the concentration dependence of the SFG field and the surface tension at the air−solution interface could be rationalized similarly by proposing formation of large surface aggregate domains.35 To ascertain the domain formation, we have measured the Brewster angle microscopy (BAM) images. 3.5. BAM Images. We could not observe BAM images of individual surfactants, CTAB and SDS. These surfactant molecules are reported to form homogeneous and dilute monolayers that do not measurably change the reflective properties of the surface.12,36 However, the BAM images of mixed surfactants could be observed due to formation of aggregates. Figure 10 shows the BAM images of the catanionic Gibbs monolayer, for the 1:1 ratio of CTAB and SDS with the total bulk concentration of 6 μM, to illustrate the phase coexistence of a fluidlike and a condensed phase, which are represented by dark region and bright domains, respectively. The bright domains consist of distinct and almost circular structures with a typical diameter of ∼3 μm. These bright domains are observed only after the induction period (image B). As time progresses, the surface density of these bright domains increases (image C), and they start combining mainly laterally (up to 2−3 units), acquiring a linear structure. At the same time the diameter of these bright domains also increases because of fusion (images D and E). After a long time (3 days), these bright domains fuse to form much larger irregular-shaped aggregates (diameter >150 μm). With higher bulk concentration, appearance of bright domains and their aggregation start much earlier. For the total bulk concentration of 60 μM of catanionic surfactant (1:1 ratio), much extensive irregularshaped aggregates could be observed only after 9 h. Appearance of these bright domains is responsible for observation of the fast component of the VSFG signal. Some more studies on the dynamics of the adsorption at the interface and the growth of the aggregates are required to assign the nature of the aggregates observed in our work. 3.6. Adsorption Kinetics. Adsorption of neutral molecules to the surface is mainly diffusive, whereas that of charged surfactant systems is generally limited by an energy barrier due to electrostatic effects.28 Thus, adsorption at the interface is expected to lead to an exponential growth. Since our measured temporal profile of VSFG is not fit to a single exponential

Figure 9. The π−t adsorption kinetics for the catanionic mixture of CTAB and SDS in the 1:1 ratio with different total bulk concentrations of 4, 8, 32, and 60 μM. 3152



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probably because of its fast growth. However, the ratio of the intensities (b/c) of the first exponential to the second one varies systematically with the composition; it is the greatest for the 1:1 composition (∼0.70) and decreases (up to ∼0.25) on deviation to either side of this composition. A typical fit for the 1:1 composition with the total bulk concentration of 6.0 μM is shown in Figure 11. Similar fit to the two exponential functions

Figure 11. Time evolution of the νs(CH3) band at 2876 cm−1 of the complex between CTAB and SDS surfactants (1:1 composition, total bulk concentration=6 μM) at the air−water interface. The solid curve represents a typical fit to the eq 6.

for the 1:1 composition with increasing total concentration from 1.6 to 40 μM leads to a decrease in the characteristic time τ2 from 6550 to 610 s. Similar to earlier work,37 the fast process with τ1 can be assigned to the surfactant molecules (either free or catanionic complex) being driven toward the interface by electrostatic attraction, whereas the second slow process can correspond to the adsorption of the catanionic complex and the rearrangement of the adsorbed catanionic molecules at the interface. In addition to the rearrangement of adsorbed molecules at the interface, the retardation of adsorption in the second slow process can also be due to steric or electrostatic barrier.38,39 Two similar distinct time processes have been observed in the surface tension measurement due to the adsorption of charged polymers at the air−water interface.40 They have assigned the first characteristic time τ1 to the diffusion/initial adsorption of the polymer from the aqueous solution, and the second τ2 to the reorganization of the preadsorbed polymer. Their τ1 values are an order of magnitude greater than the present work, but τ2 values are of the same order. Even acid−base complex formation for fatty amines under ionic conditions has been reported41 to have similar long relaxation time as our measured τ2. A similar double exponential fit to the measured temporal evolution of the surface pressure of catanionic surfactants is not good. An empirical function (eq 7) has been used to fit the dynamic surface tension, γ(t) of the ionic surfactants and catanionic systems.27

Figure 10. The upper panel shows the π−t adsorption curve for the 1:1 ratio of CTAB and SDS with total concentration of 6 μM. The lower panels show the BAM images of the catanionic monolayer at different times (A) Pure water or solution in the induction period. (B− F) 800 s, 1800 s, 3600 s, 12600 s and after 3 days.

growth, we attempted to fit the VSFG intensity (IVSFG) vs time data with two exponential functions: (IVSFG)1/2 = a + b exp( −t /τ1) + c exp( −t /τ2)

(6)

where a (= b + c), b, and c are constants, and τ1 and τ2 are the characteristic times. Since the VSFG intensity is proportional to the square of the number of interfacial molecules being probed, the surfactant density is obtained by taking its square root. This fitting equation is based on that used for two-step adsorption kinetics by Raposo, et al.37 The temporal profiles of VSFG signal at 2876 cm−1 for different ratios of CTAB and SDS, with the total concentration fixed, were a reasonably good fit to this biexponential equation. Both exponentials correspond to firstorder kinetics. The first exponential is characterized by a short characteristic time (τ1) of approximately 25−90 s, whereas the second exponential is characterized by a long characteristic time (τ2) of 2000−8500 s, which corresponds to the rate of ∼5.0 × 10−4 to 1.2 × 10−4 s−1. For the second component, the fastest rate is for the 1:1 molar ratio, with the rate decreasing on deviation to either side of this composition. We could not find a similar systematic correlation between the rate of the first exponential (characteristic time τ1) and the composition,

⎛ t ⎞m =⎜ ⎟ γ(t ) − γe ⎝ τ1 ⎠ γ0 − γ(t )

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(7)



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and SDS surfactants, the induction time decreases (from ∼2000 s to nil) and the adsorption kinetics become faster. Among all the compositions with the same total bulk concentration, the 1:1 ratio exhibits the smallest induction time and the fastest adsorption kinetics. Under similar conditions, the OH peak decays much faster, whereas the CH peaks keep on growing for a longer time. The slow growth can be due to rearrangement of the complex forming a monolayer. The π−t adsorption kinetics also exhibit qualitatively similar behavior as the VSFG temporal profile with respect to the induction time and slow growth and their dependence on the bulk concentration. However, the fast growth could not be observed in the π−t curves. The presence of the fast growth in the VSFG profile is explained based on the formation of surface aggregate domains due to the adsorption of catanionic surfactant molecules. The BAM images clearly show the presence of the bright domains in the catanionic mixture after the induction time.

In this equation, γ0 is surface tension of pure water, γe is the equilibrium surface tension, m is a nondimensional parameter, and τ1 is the characteristic relaxation time. On the basis of this function, recently another better empirical function (eq 8) was used,42 and successfully tested,28,42 for the catanionic systems. γ0 − γ(t ) γ(t ) − γe

=

m ⎛ t ⎞n ⎤ 1 ⎡⎛ t ⎞ ⎢⎜ ⎟ + ⎜ ⎟ ⎥ 2 ⎢⎣⎝ τ1 ⎠ ⎝ τ1 ⎠ ⎥⎦

(8)

In this equation, two nondimensional parameters m and n are used. In our experiments, we have attempted to fit the surface pressure-time adsorption (π−t) kinetics with these equations. In most of the cases, the eq 8 is found to be a better fit, with two characteristic times τ1 and τ2, to the measured curves. However, in most cases, the difference between the values of τ1 and τ2 is small, except for CTAB:SDS ratio of 5:3, and the adsorption kinetics can be represented by only one characteristics time τ within the experimental error. This difference between τ1 and τ2 decreases with the total bulk concentration. The τ values decrease with increasing total bulk concentration of the 1:1 ratio of the two surfactants, and were found to be 2100 ± 200, 850 ± 100, and 320 ± 50 s for the total bulk concentration of 6.0, 33.0, and 68.0 μM, respectively. These characteristic times are reasonably in good agreement with the τ2 values obtained from the VSFG measurements. The τ values for CTAB:SDS ratio of 1:1, 3:5, and 2:6 are 2100 ± 200, 4600 ± 200, and 10000 ± 200 s, respectively, for the total bulk concentration of 6.0 μM. However, τ1 and τ2 values for the 5:3 ratio are 1920 ± 50 and 3370 ± 100 s, respectively. The relaxation time τ1 can correspond to the adsorption of free surfactants or the catanionic molecules, whereas τ2 can correspond to the adsorption of these catanionic molecules from the bulk and realignment of these adsorbed molecules at the interface.

■

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

■

REFERENCES

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4. CONCLUSIONS We have investigated the effects of various compositions in mixed surfactant solutions of SDS and CTAB, at an air−water interface, using VSFG nonlinear optical spectroscopy. In the presence of either SDS or CTAB aqueous solution, we observed a significant enhancement in the intensities of the OH stretching modes (broad peaks at ∼3200 cm−1 and ∼3400 cm−1) of interfacial water molecules in VSFG spectra, due to ordering of these molecules induced by the large electrostatic field of the charged surfactants. However, with a mixed surfactant solution of SDS and CTAB in the 1:1 ratio, opposite charges on these surfactant molecules get canceled and consequently the OH peaks disappear. For other compositions, the VSFG intensity of OH stretch is less than that of an individual component. The experimental results suggest formation of an insoluble complex between two surfactants, which has nearly 1:1 composition. Vibrational region of the CH stretching frequencies at the air−water interface was also probed to investigate the time evolution of the alkyl chain of the surfactant molecules in the insoluble 1:1 complex. The complex is characterized by two major vibrational peaks at 2876 and 2940 cm−1. The temporal profiles show an induction time before the appearance of these two peaks of the complex. Subsequently, there is a fast growth (within ∼30s) followed by a slow growth up to more than 3 h. The induction time and the growth kinetics strongly depend on the total bulk concentration and the relative compositions. With increasing bulk concentrations of the 1:1 ratio of CTAB 3154



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