Infrared Pump–Probe Study of Nanoconfined Water Structure in

Letter pubs.acs.org/JPCL

Infrared Pump−Probe Study of Nanoconfined Water Structure in Reverse Micelle Jooyong Lee,† Michał Maj,† Kyungwon Kwak,*,§ and Minhaeng Cho*,†,‡ †

Department of Chemistry, Korea University, Seoul 136-701, Korea Multidimensional Spectroscopy Laboratory, Korea Basic Science Institute, Seoul 136-713, Korea § Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea ‡

S Supporting Information *

ABSTRACT: The influence of nanoconfinement on water structure is studied with timeand frequency-resolved vibrational spectroscopy of hydrazoic acid (HN3) encapsulated in reverse micelle. The azido stretch mode of HN3 is found to be a promising infrared probe for studying the structure and local hydrogen-bond environment of confined and interfacial water in reverse micelle due to its narrow spectral bandwidth and large transition dipole moment. The results show a clear separation between the core and shell spectral components, making it advantageous over the previously studied infrared probes. The measured vibrational lifetimes appear to be substantially different for the interfacial and bulk-like environments but show no remarkable size dependency, which indicates that water structures around this IR probe are distinctively different in the core and shell regions. The influence of local hydrogen bond network in the first and higher solvation shells on the vibrational dynamics of HN3 is further discussed. SECTION: Liquids; Chemical and Dynamical Processes in Solution

U

the fact that the molecular ion’s vibrational frequencies are highly sensitive to local H-bonding interaction. However, they might tend to strongly interact with the negatively charged head groups of the surfactant molecules, which inevitably make small the population of anionic probe molecules in the interfacial region. Furthermore, their broad bandwidth makes it difficult to assign peaks to ionic IR probes in either interfacial or core region of the confined water pool. Consequently, a vibrational probe characterized by relatively narrow bandwidth and large transition dipole,30 which are prerequisites for clearly distinguishing two spectral components originating from the bulk and interfacial water environments, has long been sought. Here we show that the stretching vibration of the azido group in HN3 is a highly promising IR probe. The normalized infrared absorption spectra of HN3 in liquid water and in Aerosol OT (AOT) RMs of different size are shown in Figure 1. The spectrum of HN3 in bulk water shows a single symmetric peak at 2146 cm−1 that is well-described by a Lorentzian function with full width at half-maximum of 13.6 cm−1. Once the HN3 water solution comprises an inner part of RM, another spectral feature emerges in the red-side of the spectrum, which is the peak at 2132 cm−1 in the case of the smallest RM (w0 = 2); note that w0 (= [H2O]/[AOT]) is the molar ratio of water to AOT surfactant, and the diameter (in nm) of approximately spherical RM can be estimated by using

nderstanding the structure and dynamics of nanoconfined and interfacial water has long been an important research subject. In particular, it has been argued that biological water at the interface of proteins or membranes plays a crucial role in controlling protein’s stability, function, and even ion transport process.1−10 Various model systems have thus been considered to study the interfacial water dynamics on both solid and liquid surfaces using second- and higher-order nonlinear spectroscopic measurement methods, which are capable of providing information about spectral diffusion and reorientational dynamics of local probe molecules.11−23 Water confined in nanoscale reverse micelles (RMs) has been of particular interest due to the possibility of both studying water molecules located at the peripheral region of the nanopool and extracting information about the confinement effects on the bulk properties of water.24 A significant advantage of the RM system is that its size can be easily controlled by changing the concentration ratio of water to surfactant used to make each RM. A few vibrational probes have been used so far, which include the OD stretch mode of HOD molecule,25,26 stretch modes of pseudohalide27 and tricyanomethanide28 ions, and amide I′ mode of fully deuterated N-methylacetamide.29 Fayer, Levinger, and coworkers clearly demonstrated that water structure and dynamics in such confined environments can be directly studied using HDO molecule as an internal probe.25,26 Because of the broad line shape of the OD stretch band, however, the subsequent interpretation of the experimental results required an elaborate decomposition analysis method.25,26 Ionic molecules can be an alternative IR probe due to © 2014 American Chemical Society

Received: August 16, 2014 Accepted: September 19, 2014 Published: September 19, 2014 3404

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Because the asymmetric azido stretch band of HN3 is in the same frequency region as the combination band of intramolecular bending and intermolecular librational modes of water, the heating contribution is mainly originated from the relaxation of water combination modes. This process was also observed by Chieffo et al., and its vibrational lifetime was estimated to be as fast as 140 ± 15 fs.41 Consequently, the complete relaxation producing local heat occurs on the time scale of 840 ± 100 fs.41 From this time constant at which the combination band relaxes, it is possible to describe our heatinginduced PP signal with a simple growing term, which in turn greatly simplified our fitting analyses. The vibrational lifetime of the azide stretch mode in bulk water is thus measured to be 2.32 ± 0.30 ps, which is identical to that reported by Houchins et. al.42 The azido stretch mode lifetime of HN3 confined in the smallest RM (w0 = 2) consisting almost entirely of the IR probe molecules located in the peripheral region is found to be 6.48 ± 0.20 ps. The decay of the isotropic signal in both cases is wellfitted by a single exponential function. The fact that the vibrational lifetime of HN3 in the interfacial region is significantly longer than that in the core water phase is consistent with the observation that the HN3 molecules at the AOT−water interface do not form strong H bonds with water molecules so that its frequency is red-shifted by ∼15 cm−1 in comparison with that in bulk water.43 The biexponential fitting analyses with variable amplitudes and decay constants were performed for the transient PP signals of the intermediate RM systems (2 < w0 < 40) comprising of substantial contributions of both core and shell peaks. We found that the vibrational lifetimes of the two (core and shell) components do not show a notable size dependence. To confirm this observation, we performed another simple analysis, where the azido stretch mode lifetimes of HN3 molecules in the bulk and w0 = 2 RM are assumed to be our basis lifetimes that are constant regardless of the size of RM. Then, we fitted the relative amplitudes of the two components with two exponential functions with these fixed time constants of 2.32 and 6.48 ps. (See the Supporting Information for fitting quality.) The analysis results with this assumption are shown in Figure 2, where the reconstructed eigenspectra of the core and shell components from the fitted relative amplitudes are plotted. The resulting IR PP spectra of the two components are found to be consistent with other observations made here. First, to examine the validity of the present biexponential fitting method with two fixed lifetimes for core and shell components, we compare the relative intensity (population) ratio between the core and shell components based on the FTIR fits with those obtained from the IR PP data. As can be seen in Figure 3, the two results are in good agreement with each other, and the relative population of the core component shows an increasing pattern as RM size increases. This suggests that despite the spectral shift (∼5 cm−1) of core component observed in the FTIR spectra the vibrational lifetime of the HN3 in the core part of the RM does not change much, whereas that in the shell remains the same for the same reason, as previously described. Now, assuming that (i) the shape of RM is approximately spherical, (ii) the shell thickness is constant d for all RMs, and (iii) the population of shell and core components are directly proportional to the corresponding volumes, and using the experimental results in Figure 3, we could estimate the average thickness of the interfacial water region to be ∼0.62 nm. (See the fitted lines in the bottom panel in Figure 3.) In fact, this value is very close to the thickness (0.5 nm) of hydration layer

Figure 1. Size-dependent FTIR spectra of HN3 molecule in reverse micelle system. Two peaks originating from the molecules located in the core (2146 cm−1) and shell (2132 cm−1) regions of RMs are clearly separated in frequency.

the empirical formula dwp = 0.29 w0 + 1.1, which was obtained from the viscosity measurements.31 On the basis of our quantum chemistry calculation and IR studies on vibrational solvatochromism of azido stretch mode in water, the Hbonding interaction of azido group with surrounding water molecules induces a frequency blue shift.32−38 Thus, the fact that the frequency of shell HN3 molecules is lower than that of core HN3 molecules by ∼15 cm−1 suggests that the HN3 molecules in the water−AOT interfacial region do not or weakly form H-bonding interaction with neighboring water molecules. The clear peak separation (Figure 1) allows us to distinguish the two components originating from HN3 molecules dissolved in bulk-like and interfacial water environments. Despite the fact that these two components have often been described in terms of the two-state model as core and shell,29,25,26 there remains a controversy about the validity of such a simple two-state assumption. What is particularly noteworthy here is that the two components appear to behave differently under the change in the reverse micelle size. The frequency and line width of interfacial HN3 peak at ∼2132 cm−1 do not depend on RM size. This suggests that the HN3 molecules located at the water− AOT interface show strong interaction specificity. That is to say, the nature of the intermolecular interactions in the shell part is well-defined and remains the same for every RM. The bulk-like (core) component shows a substantial red shift of ∼5 cm−1 (Figure 3), and its line shape becomes broad and close to a Gaussian function as the RM size (or confinement) decreases (increases). (See the Supporting Information.) These findings are particularly important because they indicate that contrary to the shell part the water hydrogen-bond structure and local environment around an IR probe in the reverse micelle core water is affected by the confinement effects. To elucidate how strongly the nanoconfinement affects the dynamic properties of the studied vibrational probe, we carried out a series of RM size-dependent IR pump−probe (PP) experiments to measure vibrational lifetimes of HN3 molecules in the core and shell regions of RM separately. (See the Supporting Information for the complete set of the isotropic IR PP spectra and transient absorption decay signals.) In the cases of the HN3 molecules in bulk water as well as in large RMs, the azido IR PP signal does not completely decay to zero due to the local heating contribution to the PP signal.39,40 3405

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identified in the infrared absorption spectra. The core part of the RM has been found to be affected by spatial confinement, resulting in changes in vibrational frequencies and spectral line shape, whereas the water structure in shell region is not. The polarization-controlled IR pump−probe spectroscopy revealed that the vibrational relaxation process of HN3 molecules in the shell part occurs on much slower time scale, indicating weak or no H-bonding interaction with water molecules. Furthermore, the vibrational lifetime of the HN3 molecules in the core region shows a weak dependence on RM size. The present work therefore supports the simple core−shell model, and the Hbonding environment around the IR probe is significantly different for the two regions. It is believed that despite the notable influence of the nanoconfinement on the water H-bond network structure local interaction of water molecules with the probe remains relatively unchanged. Currently, a further investigation using both classical and QM/MM (quantum mechanical/molecular mechanical) molecular dynamics simulation methods is being carried out to elucidate the interplay of local H-bonding environment around this IR probe with intermolecular vibrational energy relaxation process and mechanism. We anticipate that the present work would shed light onto how different nanoconfined, interfacial, or biological water structures are from that of bulk water.

Figure 2. Frequency-resolved IR PP spectra at Tw = 0.2 ps and the eigenspectra obtained with two-component analysis method. (See the Supporting Information for the complete set of IR PP data and detailed description on fitting analysis method.) The sum of two eigenspectra was presented as black open circles to compare with the IR PP spectra.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental methods. The entire set of IR and frequency-resolved pump−probe spectra are shown with the details of fitting results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.K.). *E-mail: [email protected] (M.C.). Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS This work was supported by grants from the NRF of Korea (nos. 20110020033 and 2013013156) to M.C. and from the NRF of Korea (no. 2013R1A1A1010130) to K.K. All IR pump−probe measurements were performed by using the multidimensional spectroscopy facility in the Seoul center of Korea Basic Science Institute (KBSI).

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Figure 3. Frequency shifts of both core and shell components derived from pseudo-Voigt profile-fitted FTIR spectra and frequency-resolved IR PP data are presented on top. The bottom plot presents the relative population of HN3 molecules located in the core and shell regions of the reverse micelle. The populations obtained from fitting the FTIR and IR PP data are presented together for comparison.

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