Femtosecond OH Bending Dynamics of Water Nanopools Confined In

May 14, 2013 - In the nonionic case, the w0-independent librational frequencies with a small energy mismatch lead to shorter T1 times. For w0 ≥ 8, t...
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Femtosecond OH Bending Dynamics of Water Nanopools Confined In Reverse Micelles Rene Costard and Thomas Elsaesser* Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Str. 2 a, D-12489 Berlin, Germany ABSTRACT: The relaxation of OH bend excitations of H2O confined in reverse micelles is studied by femtosecond broadband pump−probe spectroscopy for the two ionic systems dioctyl sodium sulfosuccinate (AOT) and dioleoylphosphatidylcholine (DOPC) and for the nonionic tetraethylene glycol dodecyl ether (Brij-30). In the ionic AOT/ DOPC reverse micelles, the OH bending lifetime T1 decreases from T1 > 615 fs for a 3:1 ratio of water and AOT/DOPC molecules (w0 = 3) to T1 = 345 fs for a 16:1 ratio (w0 = 16). In contrast, H2O in Brij-30 reverse micelles shows a much shorter T1 = 400 fs at w0 = 2 which decreases to T1 = 250 fs at w0 = 8. OH bend relaxation proceeds mainly via librational overtones of the bend-excited water molecules with a rate correlating with the energy mismatch between the v = 1 OH bend state and the librational overtone. In the ionic systems, the lower librational frequencies at small w0 result in a larger mismatch and longer T1 times. In the nonionic case, the w0-independent librational frequencies with a small energy mismatch lead to shorter T1 times. For w0 ≥ 8, the energy flow into the first hydration shell of the bend-excited molecules makes an additional contribution to the relaxation rate in all systems.

1. INTRODUCTION The structure of liquid water, an extended network of water molecules linked by intermolecular hydrogen bonds, displays pronounced fluctuations in the ultrafast time domain between some 50 fs and several picoseconds. Intra- and intermolecular water vibrations are most sensitive probes of structural fluctuations and have been studied extensively by femtosecond infrared spectroscopy, theoretical calculations, and molecular dynamics simulations.1,2 There are three intramolecular vibrational modes of the water molecule, the asymmetric and symmetric OH stretch vibrations and the OH bend vibration at about half of the stretching frequencies.3 The intramolecular modes are complemented by intermolecular degrees of freedoms such as librations, i.e., hindered rotations, and translations. Femtosecond two-dimensional (2D) infrared spectroscopy and pump−probe experiments have mainly focused on the dynamics of OH or OD stretching excitations in H2O or HOD in H2O or D2O. The time scales and mechanisms of OH/OD stretch relaxation and energy dissipation into the network have been addressed for bulk water, small water pools confined in reverse micelles, and hydration layers interacting with biomolecules such as phospholipids, DNA and peptides.1,2,4−15 In contrast, the OH bending vibration of water which represents the intramolecular mode with the lowest frequency has been studied to a much lesser extent.16−20 Pump−probe experiments with bulk H2O have demonstrated vibrational (v = 1) lifetimes between 170 and 260 fs at T = 300 K which slightly increase with temperature.17−20 A decay into librational degrees of freedom represents the main relaxation channel. The excess energy released in this process is randomized within the water © XXXX American Chemical Society

network on a 1−2 ps time scale and gives rise to an increase of vibrational temperature by a few degrees Kelvin (“hot water ground state”).4,21,22 In-depth theoretical work has shown that the OH bending excitation decays predominantly into hindered rotations of the bend-excited water molecules.23−25 The relaxation rate is mainly determined by the centrifugal coupling between librational and OH bending motions and a 2:1 Fermi resonance of a librational overtone and the v = 1 OH bend state (Wintra in Figure 1e, ref 24). Energy transfer into the water shell around the bend-excited water molecule makes a second smaller contribution (Winter) to the overall relaxation rate. Depopulation of the v = 1 OH bend state is followed by rapid librational energy transfer into the environment. Both experiment and theory have shown that the lifetime of librational excitations is less than 100 fs.17,26 Librational frequencies in aqueous systems depend sensitively on the hydrogen bond structure. Recent work on librational absorption bands of water pools which were nanoconfined in reverse dioctyl sodium sulfosuccinate (AOT, Figure 1a) micelles has shown a substantial red-shift of the librational L2 band upon reducing the hydration level, i.e., the size of the water pool.27−29 This behavior has been attributed to the modified water structure at the water−surfactant interface, i.e., an increasing fraction of water molecules bound to the ionic headgroups of the surfactant molecules, interacting with the Special Issue: Michael D. Fayer Festschrift Received: April 11, 2013 Revised: May 10, 2013

A

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molecules plays a minor role in the OH bend relaxation scenario. In the nonionic Brij-30 reverse micelles with a librational spectrum close to bulk H2O, the OH bend lifetime at w0 = 2 has a small value of 400 fs and shortens to 250 fs at w0 = 8.

2. EXPERIMENTAL TECHNIQUES In our experiments, H2O confined in dioctyl sodium sulfosuccinate (AOT, Figure 1a), dioleoylphophatidylcholine (DOPC, Figure 1b), or tetraethylene glycol dodecyl ether (Brij30, Figure 1c) reverse micelles (scheme in Figure 1d) was studied for hydration levels between w0 = 1 and 16 (concentration ratio w0 = [H2O]/[surfactant molecules]). The sample preparation has been discussed in detail elsewhere.31 In brief, 0.25 M solutions of AOT (Sigma-Aldrich, BioUltra >99%) in CCl4 (Riedel-de Haën), DOPC (Echelon Inc.) in benzene (Sigma-Aldrich, anhydrous 99.8%), and Brij-30 (BrijL4, Sigma-Aldrich) in cyclohexane (Merck) were prepared, and the respective amount of H2O was added. The samples were held between two 1 mm thick CaF2 windows with sample thicknesses between 50 and 200 μm, resulting in an optical density in the OH bending range of OD < 0.5. Linear infrared spectra of such samples were recorded with a Varian 640-IR spectrometer. Independently tunable pump and probe pulses of ∼150 fs duration were generated in home-built optical parametric frequency converters driven by the output of a regeneratively amplified Ti:sapphire laser (800 nm, repetition rate 1 kHz).32 The pump energy and the spectral bandwidth of the pulses had a value of 2 μJ and 100 cm−1, respectively. Pump−probe data were recorded with parallel and perpendicular linear polarization of pump and probe pulses. After interaction with the sample, the probe pulses were spectrally dispersed in a monochromator and detected by a 64-element MCT detector array. The nonlinear change of absorbance ΔA = −log(T/T0) of the sample was measured both as a function of pump−probe delay and of the frequency position within the spectrum of the probe pulses (T, T0: sample transmission with and without excitation of the sample). To avoid absorption from water vapor in air, the experimental setup was purged with nitrogen gas.

Figure 1. Molecular structures of (a) AOT, (b) DOPC, and (c) Brij30. (d) Schematic of an ionic reverse micelle structure containing a water pool (blue area; small blue/orange spheres: phosphate/choline groups). (e) Level scheme of OH bend relaxation in water. The total relaxation rate W1−0 = Wintra + Winter of the v = 1 state of the OH bend oscillator δOH consists of an intramolecular contribution Wintra involving relaxation via librational (lib) overtones of the bend-excited water molecule and an intermolecular contribution Winter involving energy transfer to the water molecules of the first solvation shell. The inset shows a schematic water molecule with the x-axis. Centrifugal coupling to the rotation around this axis makes the main contribution to Wintra.

Na+ counterions, and/or trapped in the layer of surfactant molecules. Molecular dynamics simulations have suggested that such structural changes compared to bulk water underlie the observed red-shift of librational absorption.28,29 In contrast, the L2 band of H2O confined in nonionic tetraethylene glycol dodecyl ether (Brij-30, Figure 1c) reverse micelles is close to that of bulk H2O, even at low hydration levels.30 A change of librational frequencies in nanoconfined water pools should also affect the OH bend relaxation, given the prominent role of librational degrees of freedom in depopulating the v = 1 state of OH bend oscillators. So far, the knowledge on OH bend relaxation in water nanopools has remained scarce. In this article, we present a detailed femtosecond pump−probe study of OH bend relaxation of H 2 O confined in AOT, dioleoylphosphatidylcholine (DOPC),12,21,31 and Brij-30 reverse micelles, covering a wide range of hydration levels. In the two ionic systems, the OH bend lifetime shortens substantially when going from a 1:1 ratio of water and surfactant molecules (w0 = 1) to a 16:1 ratio (w0 = 16). This shortening correlates with the decreasing energy mismatch between the librational overtones and the v = 1 state of the OH bending oscillators at higher hydration levels, resulting in an enhancement of the v = 1 decay rate. The very similar behavior of water in AOT and DOPC reverse micelles suggests a minor influence of the different hydrogen bond strengths of interfacial water in AOT and DOPC on the relaxation behavior. For DOPC reverse micelles, we also show that coupling to fingerprint vibrations of the phospholipid

3. RESULTS Figure 2 displays the linear infrared absorption spectra of the (a, b) DOPC, (c) AOT, and (d) Brij-30 reverse micelles in the fingerprint range for different values of w0. For DOPC, the absorption of the solvent benzene was subtracted. In Figure 2a, the pronounced vibrational bands around 1250, 1650, and 1740 cm−1 are due to the asymmetric (PO2)− stretch vibration of DOPC, the OH bend vibration of H2O, and the carbonyl stretch vibration of DOPC. With increasing w0, the phosphate vibration undergoes a shift to lower frequency which has been analyzed in detail elsewhere.31 The strong spectral feature around 1480 cm−1 is dominated by a contribution from the solvent benzene. Because of the very high oscillator strength of this band, the subtraction of the absorption of neat benzene from the infrared spectrum of the DOPC samples is imperfect, leaving this feature in the difference spectrum. In Figure 2b, the OH bend absorption of H2O in the DOPC reverse micelles is plotted between 1500 and 1800 cm−1. The corresponding spectrum of H2O in the AOT reverse micelles is shown in Figure 2c and displays the OH bend and the carbonyl stretch B

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Figure 2. Linear infrared absorption spectra of water in different reverse micelle systems for hydration levels of w0 = 1 (black lines), w0 = 3 (red lines), w0 = 8 (green lines), and w0 = 16 (blue lines). (a) Infrared spectra of water in DOPC reverse micelles. The spectra display the carbonyl stretch absorption of DOPC around 1740 cm−1, the OH bend absorption of H2O between 1580 and 1680 cm−1, a very strong band of the solvent benzene at 1480 cm−1, and the asymmetric (PO2)− stretch band around 1250 cm−1. The latter undergoes a redshift with increasing hydration level. (b) OH bend absorption of water in DOPC reverse micelles for hydration levels from w0 = 1 to w0 = 16. (c) OH bend absorption of water in AOT reverse micelles for different hydration levels. The band around 1730 cm−1 represents the carbonyl stretch absorption of AOT. Dash-dotted line: intensity spectrum of the femtosecond pump pulses. (d) OH bend absorption of water in Brij30 reverse micelles for w0 = 2, 8, and 16. The black line gives the spectrum of dry Brij-30 reverse micelles.

Figure 3. Transient OH bend spectra of H2O in AOT (left column) and DOPC (right column) reverse micelles for hydration levels of (a, b) w0 = 3, (c, d) w0 = 8, and (e, f) w0 = 16. Nonlinear signals caused by the benzene solvent are subtracted in (b). The normalized nonlinear change of absorbance ΔA = −log(T/T0) (T, T0: sample transmission with and without excitation) is plotted as a function of probe frequency. The spectra were taken with parallel linear polarization of pump and probe pulses for delay times between 200 fs and 5 ps (symbols). The arrows in (a) mark the frequency positions at which the transients plotted in Figure 4 were recorded. Absolute values of ΔA are given in Figure 4.

populated v = 1 state. The width of the v = 0 to 1 absorption decrease is very similar in the two cases. For H2O in DOPC reverse micelles, the crossing points between the negative and positive signals shift from 1635 cm−1 at w0 = 3 to 1624 cm−1 at w0 = 16. With increasing delay time, the two components decay simultaneously because of the depopulation of the v = 1 state. At a delay time of 5 ps, an almost complete decay of the absorption changes is found for w0 = 3, while the spectra for w0 = 8 and 16 display an absorption increase (ΔA > 0) at high probe frequencies. The maximum of this positive ΔA is slightly red-shifted compared to the maximum of the negative absorption changes occurring at earlier delays. The enhanced absorption is a hallmark of a heated ground state of the water pools, generated by redistributing the excess energy released in the OH bend decay.22 Time traces of the change of OH bend absorption at fixed probe frequencies are shown in Figure 4 for AOT (left column) and DOPC reverse micelles (right column). We subtracted the benzene contribution for low water content (w0 = 3) in DOPC. The time-dependent v = 1 to 2 absorption (open squares) is

absorption. The dash-dotted line represents the intensity spectrum of the pump pulses applied in the femtosecond experiments. Figure 2d shows the OH bend absorption of H2O in the Brij-30 reverse micelles. Transient pump−probe spectra are presented in Figure 3 for AOT (left column) and DOPC reverse micelles (right column) for (a, b) w0 = 3, (c, d) w0 = 8, and (e, f) w0 = 16 and delay times between 200 fs and 5 ps. A background of the benzene solvent was subtracted for w0 = 3 in the DOPC case. At early delay times, the spectra display an enhanced absorption (positive absorption change) at low probe frequencies which is due to the v = 1 to 2 absorption of the OH bend oscillator, and a reduced absorption (negative absorption change) at higher probe frequencies in the range of the v = 0 to 1 transition. The latter signal is caused by the bleaching of the v = 0 ground state and stimulated emission from the transiently C

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Figure 5. (a, b) Transient OH bend spectra and time traces for H2O in Brij-30 reverse micelles at a hydration level of w0 = 2 (symbols). A single exponential fit of the decay of the v = 1 to 2 absorption (red line in Figure 5b) gives a v = 1 lifetime of 400 fs. (c, d) Same of H2O in Brij-30 reverse micelles at w0 = 8. The v = 1 lifetime shortens to 250 fs. Figure 4. Time evolution of the transient OH bend absorption recorded at fixed probe photon energies of Epr = 1600 cm−1 (open squares) and 1650 cm−1 (solid circles). The left column shows data for water in AOT reverse micelles at hydration levels of (a) w0 = 3, (c) w0 = 8, and (e) w0 = 16, while the right column (b, d, f) shows the results for DOPC reverse micelles. Nonlinear signals caused by the benzene solvent are subtracted in (b). Red solid lines: Calculated single exponential decays with the time constants indicated.

state. Pump−probe data on the OH bend response after OH stretch excitation at w0 = 1 have been presented in ref 12 for DOPC reverse micelles. The transient spectra are similar to the OH bend spectra for w0 = 3 (Figure 3b), and time-resolved transients give an OH bend lifetime of 800 fs. From measurements with parallel and perpendicular linear polarization of pump and probe pulses, we derived the pump− probe anisotropy r(t) = (ΔA∥(t) − ΔA⊥(t))/(ΔA∥(t) + 2ΔA⊥(t)), where ΔA∥(t) and ΔA⊥(t) are the absorption changes measured with parallel and perpendicular polarizations. In all cases, we found an anisotropy of r(t) > 0.3 of the v = 1 to 2 absorption and the v = 0 to 1 bleaching within the v = 1 lifetime. At delay times longer than 2 ps, the anisotropy of the enhanced absorption of the vibrationally hot ground state shows values close to zero for w0 = 8 and 16. We conclude that resonant energy transfer of OH bend excitations between different H2O molecules plays a minor role. In the hot ground state, energy redistribution has extinguished the preferential polarization of the initial OH bend excitation. Transient OH bend spectra and time traces of H2O in Brij-30 reverse micelles are summarized in Figure 5. The data taken at a hydration level of w0 = 2 (Figure 5a,b) display a v = 1 decay with a lifetime of T1 = 400 fs, substantially shorter than in the w0 = 3 data for the ionic micelles (cf. Figure 4a,b). Upon increasing the hydration level to w0 = 8 (Figure 5c,d), the v = 1 lifetime shortens to T1 = 250 fs, a value within the 170−260 fs range of OH bend lifetimes reported for bulk H2O.17−19 It should be noted that the spectra in Figure 5c exhibit a very pronounced enhanced red-shifted absorption of the OH bend vibration at picosecond delay times. This behavior is due to the vibrationally hot ground state in the water pool and is very similar to bulk H2O.17,18,22

mapped at Epr = 1600 cm−1, while the transients recorded at Epr = 1650 cm−1 (solid circles) reveal the time evolution of the v = 0 to 1 absorption decrease and the build-up of the enhanced absorption of the heated water ground state. At slightly negative delay times and around delay zero, the transients are affected by the coherent coupling of pump and probe fields in the sample and the cell windows, leading to a strong modulation of the signal. For time delays longer than 200 fs, pump and probe pulses interact sequentially with the sample and the decay of the v = 1 to 2 absorption reflects the population decay of the v = 1 state. This decay becomes substantially faster with increasing hydration level w0. For AOT reverse micelles, the monoexponential fitting curves (red lines) give decay times of (a) T1 = 615 fs at w0 = 3, (c) T1 = 440 fs at w0 = 8, and (e) T1 = 345 fs at w0 = 16. The corresponding values for DOPC reverse micelles are (b) T1 = 730 fs at w0 = 3, (d) T1 = 440 fs at w0 = 8, and (f) T1 = 345 fs at w0 = 16, i.e., very similar to the AOT case. For both AOT and DOPC reverse micelles, the initial recovery of the time-resolved absorption decrease (solid circles) gives time constants in agreement with the v = 1 lifetimes derived from the enhanced absorption. At w0 = 8 and 16, this kinetic component is followed by a sub-picosecond absorption increase with a time constant similar to the v = 1 decay time, reflecting the build-up of the hot water ground D

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group, while water−water hydrogen bonds are absent. Interfacial water forms hydrogen bonds with the (SO3)− headgroups and with other water molecules, while ion−dipole interaction between the Na+ ions and water molecules leads to a geometry with the water oxygens closer to the Na+ than the water hydrogens. Such mechanisms result in an orientation and organization of water molecules in the interfacial layer.33,34 For w0 ≤ 5, practically all water molecules interact with the interface. At high w0 values, a bulk-like water pool is formed in the interior which is electrically shielded from the (SO3)− groups by the counterions and the interfacial water layer. In the bulk-like pool, water molecules form close to four hydrogen bonds, similar to bulk water.28 The simulations of ref 28 have shown that water molecules closer to the interface display substantially reduced frequencies of their librations, caused by the influence of the ionic headgroups and counterions as well as by the lack of water−water hydrogen bonding. The structure of DOPC reverse micelles has not been analyzed in the same detail as the AOT system. There are, however, numerous studies of the structure of hydrated phospholipid interfaces.37−39 In DOPC molecules, the positively charged choline group is part of the molecular structure, in contrast to the separate Na+ counterion of AOT. While hydrogen bonds between the choline group and water are absent, the attractive interaction between the choline N+ ion and the water oxygens nearby leads to an orientation of water hydrogens away from the choline group,37,39 similar to Na+ hydration in AOT reverse micelles. At low hydration levels of phospholipids (w0 = 1), individual water molecules form a hydrogen bond with the ionic phosphate heads and the carbonyl groups of the phospholipid structure.37 With increasing hydration level w0, local water shells build up around the phosphate groups where the (PO2)− subunits accept hydrogen bonds from up to six water molecules. For DOPC reverse micelles, the gradual formation of phosphate water shells from w0 = 2 to 8 has been mapped via the continuous red-shift of the asymmetric (PO2)− stretching vibration with increasing w0.31 The phosphate−water hydrogen bonds are stronger than water−water hydrogen bonds in H2O. At higher hydration level (w0 = 16), the water pools around the phosphate heads become fully interconnected, and a bulk-like water pool is established in the interior of the reverse micelle. It is important to note that, even at w0 = 16, the structural heterogeneity of water in both AOT and DOPC reverse micelles is much more pronounced than in bulk liquid H2O and that all existing linear and nonlinear femtosecond infrared studies of reverse micelle systems average over the different structural components. The formation and structure of water-filled Brij-30 reverse micelles have been studied by electron spin resonance, dynamic light scattering, fluorescence methods, and time-resolved solvation and infrared experiments.40−43 In cyclohexane solution, spherical Brij-30 reverse micelles are formed with a diameter that grows with the size of the water pool in their interior. The boundary between the water pool and the Brij-30 interface is considered less defined than that in the ionic reverse micelles, resulting in a partially nonuniform hydration of the Brij-30 molecules. 4.2. Mechanisms of OH Bend Relaxation. We now discuss the relaxation pathways of the OH bend vibration by considering three different interactions: (i) the centrifugal coupling to librational motions of the bend-excited water molecules, (ii) couplings to other water molecules in the first

Figure 6. (a) Transient spectra (w0 = 1) of the DOPC carbonyl vibrations for different pump−probe delays. (b) Time traces taken at two frequency positions for w0 = 1 and w0 = 16.

In an independent experiment, we measured the transient behavior of the carbonyl stretching vibrations of DOPC which give rise to the strong band at 1740 cm−1. The maximum of the pump spectrum was tuned to 1750 cm−1, resulting in a negligible overlap with the OH bend absorption. In Figure 6a, we show the transient spectra of the carbonyl excitation at w0 = 1 for delay times between 300 fs and 5 ps. The maximum of the transient v = 1 to 2 absorption (positive ΔA) is red-shifted by approximately 25 cm−1 from the negative peak of the v = 0 to 1 absorption change, due to the anharmonicity of the carbonyl stretch oscillators. Both absorption changes decay with a very similar biexponential kinetics with time constants of 140 fs and 1.4 ps (Figure 6b). Both the shape of the transient spectra and the kinetics are independent from the hydration level for w0 = 1 to 16. It is important to note that the overall decay of the carbonyl excitation is substantially slower than the OH bend decays at w0 ≥ 3.

4. DISCUSSION 4.1. Structure of the Reverse Micelle Systems. Before addressing the OH bend relaxation mechanisms in detail, we briefly discuss the molecular structure of water nanopools in AOT reverse micelles, of water layers interacting with phospholipids, and of hydrated Brij-30 reverse micelles. Water structure in AOT reverse micelles has been studied by a variety of experimental methods and by MD simulations.6−10,27−30,33−36 In larger water pools (w0 > 10), a first species trapped at the interface, a second (interfacial) species strongly influenced by interactions with the charged interface containing the (SO3)− headgroups and the neighboring Na+ counterions, and a third inner bulk-like water core have been distinguished. Trapped water molecules are more or less exclusively coordinated with an AOT headgroup and/or a carbonyl E

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mechanism has been invoked to rationalize the moderate increase of the OH bend lifetime in bulk H2O from 170 fs at T = 300 K to 250 fs at T = 350 K.20 With increasing temperature, the librational potential becomes softer and the librational frequencies decrease. In bulk H2O, the maximum of the steadystate L2 librational band shifts from approximately 700 to 640 cm−1 upon increasing the temperature from T = 300 to 350 K.46 Much stronger red-shifts of the L2 band have been found for small water pools confined in AOT reverse micelles. In ref 27, a red-shift ΔνL2 ≈ 170 cm−1 of the L2 fundamental peak from 670 cm−1 at w0 = 20 to 500 cm−1 at w0 = 1 has been reported. As discussed above, this behavior has been attributed to the increasing fraction of interfacial and trapped water molecules which are part of a structure significantly different from bulk water.28,29 A very similar modification of hydrogen bond structure occurs in hydrated phospholipids and, in particular, the DOPC reverse micelles studied here. The concomitant lowering of librational frequencies should result in a strong reduction of the tcf Fourier amplitude at the OH bend frequency ω0 which may be estimated from the Fourier transforms of the rotational kinetic energy tcf in bulk H2O. Such tcf spectra show a frequency width of the librational overtone contributions of the order of 500 cm−1 (Figure 11 of ref 24) with only the x-axis librational tcf displaying a significant amplitude at the OH bend frequency ω0. A downshift of the overtone contributions of the order of 2ΔνL2 reduces the tcf amplitudes at ω0 significantly and leads to a concomitant decrease of the v = 1 relaxation rate Wintra. We attribute the measured decrease of the OH bend relaxation rate, i.e., the increase of the OH bend lifetime T1 with decreasing w0 in both AOT and DOPC reverse micelles mainly to this mechanism. The substantial difference between the OH bend lifetimes of T1 > 615 fs for H2O in AOT/DOPC reverse micelles at w0 ≤ 3 and T1 = 400 fs for H2O in Brij-30 reverse micelles at w 0 = 2 supports this conclusion independently. A reduction of w0 also diminishes the intermolecular decay rate into the first solvation shell around the OH bend excited water molecule (mechanism ii), simply because such a well-defined and closed first water shell is absent for w0 smaller than approximately 5. The fact that the measured OH bend lifetimes are practically identical for AOT and DOPC reverse micelles at the same hydration level suggests that the librational potentials and frequencies of interfacial water are very similar in the two systems with a different ionic structure and arrangement. To quantify this qualitative picture, detailed MD simulations including a calculation of OH bend relaxation rates are required in a wide range of hydration levels. Such work is beyond the scope of this experimental report. Intermolecular couplings between the OH bend oscillators of water and fingerprint modes of AOT or DOPC molecules are expected to be significantly smaller than the centrifugal coupling within the OH bend excited water molecules. Nevertheless, there could be a limited excitation transfer to AOT or DOPC modes in the same frequency range and a contribution to the overall OH bend decay. For instance, the carbonyl (CO) stretch mode is frequency-upshifted by 90 cm−1 relative to the OH bend vibration, i.e., by less than kT = 200 cm−1 for T = 300 K. The DOPC pump−probe data in Figure 6 clearly demonstrate an overall carbonyl stretch decay which is independent of w0 and slower than the OH bend decays shown in Figure 4. Moreover, we did not observe an OH bend response after exciting the carbonyl stretch. Thus,

solvation shell which are linked by hydrogen bonds to the bend-excited molecules, and (iii) couplings to fingerprint vibrations of DOPC, in particular the carbonyl stretching mode and vibrations of the phosphate head groups. In bulk H2O, the measured v = 1 lifetime of the OH bend vibration of 170 fs at T = 300 K has been explained by a combination of mechanisms (i) and (ii) (Figure 1e, refs 23 and 24). The predominant channel is a decay via the v = 2 overtones of librations of the bend-excited molecules (mechanism (i)). In a semiclassical calculation of the relaxation rate, the rotational degrees of freedom have been treated classically and assigned to the dynamic environment, exerting a fluctuating force on the OH bend oscillator. In this limit, the relaxation rate Wintra from the v = 1 to the v = 0 OH bend state is given by44 Wintra = C·|Hcen|2 ·

∞

∫−∞ dt exp(−iω0t )⟨f (0)f (t )⟩

where C is a constant, Hcen is the matrix element of the centrifugal coupling term taken with the wave function of the v = 1 and v = 0 OH bend states,3,24 ω0 is the frequency of the OH bend v = 0 to 1 transition, and ⟨f(0)f(t)⟩ the time correlation function (tcf) of the fluctuating force. The Fourier transform of this function gives its amplitude at the relevant transition frequency ω0. In bulk H2O, there exists a 2:1 Fermi resonance between the v = 1 OH bend state and the overtone of the highly localized libration around the molecular x-axis with the lowest moment of inertia. This axis is parallel to the line connecting the two hydrogen atoms of the water molecule (cf. Figure 1e). For this libration, the large coupling element Hcen and the significant Fourier amplitude of the overtone at the OH bend frequency ω0 lead to a calculated OH bend relaxation rate of the order of Wintra ≈ 2.5 ps−1, corresponding to a v = 1 decay time of 1/Wintra ≈ 400 fs. The subsequent redistribution of excess energy within the librational manifold occurs within tens of femtoseconds.26,45 Overall, this channel accounts for about 2/3 of the total OH bend relaxation rate W 1−0 = W intra + W inter . Mechanism (ii) provides an intermolecular relaxation channel with a rate of Winter ≈ 0.33W1−0. Here, energy is mainly transferred to the two nearest neighbors, forming a hydrogen bond with the oxygen atom of the bend-excited molecules.24 The OH bend relaxation behavior of H2O in Brij-30 reverse micelles is fully in line with the scenario for bulk water. We recall that the librational L2 band of H2O in Brij-30 reverse micelles is close to the L2 band of bulk H2O30 with a frequency position independent of the degree of hydration. At low hydration levels where a closed first solvation shell around the bend excited molecule is absent, the OH bend lifetime should then be close to 1/Wintra ≈ 400 fs. This is exactly what we observe in our measurements, giving a 400 fs OH bend lifetime at w0 = 2 (Figure 5a,b). At the higher hydration level of w0 = 8 where the OH bend excited molecules are embedded in a water environment, the lifetime shortens to 250 fs, close to the theoretically predicted lifetime of 1/W1−0 = 1/(Wintra + Winter) ≈ 1/(1.5Wintra) = 270 fs. We now turn to H2O in the ionic reverse micelle systems AOT and DOPC. The decay rate via mechanism (i) depends sensitively on the Fourier amplitude of the tcf at the OH bend frequency. A reduction of frequency of the librational fundamental and, correspondingly, its overtone results in a smaller amplitude and, thus, a reduced relaxation rate, i.e., a longer v = 1 lifetime of the OH bend vibration. This F

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highly relevant for understanding biochemical reactions in aqueous environments.

coupling to the carbonyl stretch vibration makes a minor contribution to the observed OH bend relaxation. The DOPC asymmetric (PO2)− stretch mode (and the AOT asymmetric sulfonate stretch mode) around 1250 cm−1 which has a lifetime of 300 fs is a second candidate for population transfer. Here, however, the large energy mismatch of 400 cm−1 to the OH bend vibration makes a sub-picosecond population transfer highly improbable. A systematic study of H2O monomers with a similar energy mismatch between their OH bend frequency and the C−H/C−D bending frequencies of the surrounding solvents has revealed OH bend lifetimes between 4.8 and 40 ps that were correlated with the energy gap.47 This time scale of relaxation is much longer than the decay kinetics observed here. On a picosecond time scale, the transient OH bend spectra and the time traces display pronounced contributions from the hot ground state of the water nanopools. At hydration levels w0 ≥ 6, a water pool exists in the interior of the reverse micelles and represents the main sink of excess energy released in vibrational relaxation, as has been demonstrated in detail in refs 12 and 21. At lower hydration levels, essentially all water molecules interact with the surfactant molecules and are interconnected only partially. Nevertheless, clear signatures of a hot water ground state in local water pools around the headgroups of DOPC at w0 = 3 have been reported in ref 21. In parallel, however, energy transfer from the water molecules into the vibrational manifold of the surfactant molecules occurs with characteristic time constants of a few picoseconds, as has been observed for DOPC at w0 = 1.31

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 247051. We acknowledge valuable discussions with Rossend Rey, Barcelona.

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REFERENCES

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5. CONCLUSIONS In conclusion, we have shown that the lifetime of the OH bend vibration of water confined in ionic AOT and DOPC reverse micelles depends strongly on the size of the water pool. For low hydration of DOPC phospholipid reverse micelles (w0 = 1), the OH bend lifetime has a value of 800 fs. This value which represents an average over all molecular arrangements in the spatially inhomogeneous water pools decreases continuously to 345 fs upon increasing w0 up to 16. A very similar behavior is found for H2O in AOT reverse micelles. As the OH bending relaxation proceeds mainly via librations of the bend-excited water molecules, the longer lifetimes at small w0 are attributed to the larger energy mismatch between the librational overtones and the v = 1 state of the bend excited water molecules. This mismatch results in a weaker Fourier amplitude of the fluctuating rotational force at the OH bend frequency and, thus, a reduced rate of population relaxation. Such effects are absent for H2O in nonionic Brij-30 reverse micelles which show a librational spectrum close to bulk H2O even at small w0. Here, the OH bend lifetime of 400 fs observed at w0 = 2 is in agreement with theoretical predictions for the intramolecular decay channel via librational overtones, and the lifetime shortening to 250 fs at w0 = 8 is due to the additional energy transfer to the first water shell around the OH bend-excited molecules. Thus, all our results provide strong and independent support for the prominent role of the intramolecular OH bend relaxation channel in H2O which is complemented by intermolecular energy transfer at sufficiently high hydration levels. In contrast, couplings of the OH bend vibration to fingerprint modes of DOPC molecules, in particular the carbonyl stretch and asymmetric (PO2)− stretch vibrations, play a minor role for OH bend relaxation. Our results allow for a detailed understanding of vibrational relaxation and its specific pathways in nanoconfined water, processes which are G

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