Influence of restricted environment and ionic interactions on water solvation dynamics

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

Chemical reactions in confined environments are important in areas as diverse as heterogenous catalysis, environmental chemistry and biochemistry, yet they are much less well understood than the equivalent reactions in either the gas phase or in free solution. The understanding of chemical reactions in solution was greatly enhanced by real time studies of model reactions, through ultrafast spectroscopy (especially when supported by molecular dynamics simulation). Here we review some of the efforts that have been made to adapt this approach to the investigation of reactions in confined media. Specifically, we review the application of ultrafast fluorescence spectroscopy to measure reaction dynamics in the nanoconfined water phase of reverse micelles, as a function of the droplet radius and the charge on the interface. Methods of measurement and modelling of the reactions are outlined. In all of the cases studied (which are focused on ultrafast intramolecular reactions) the effect of confinement was to suppress the reaction. Even in the largest micelles the result in the bulk aqueous phase was not usually recovered, suggesting an important role for specific interactions between reactant and environment, for example at the interface. There was no simple one-to-one correspondence with direct measures of the dynamics of the confined phase. Thus, understanding the effect of confinement on reaction rate appears to require not only knowledge of the dynamics of the reaction in solutions and the effect of confinement on the medium, but also of the interaction between reactant and confining medium.

Chemical Reaction Dynamics at Liquid Interfaces: A Computational Approach

Recent advances in experimental and theoretical studies of liquid interfaces provide remarkable evidence for the unique properties of these systems. In this review we examine how these properties affect the thermodynamics and kinetics of chemical reactions which take place at the liquid/vapor interface and at the liquid/liquid interface. We demonstrate how the rapidly varying density and viscosity, the marked changes in polarity and the surface roughness manifest themselves in isomerization, electron transfer and photodissociation reactions.

Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment

Vibrational relaxation and the rotational dynamics of water molecules encapsulated in reverse micelles (RMs) have been investigated by ultrafast infrared (IR) spectroscopy and two-dimensional IR (2D IR) spectroscopy. By changing the counterion of the hydrophilic headgroup in the RMs formed by Aerosol-OT (AOT) from Na⁺ to K⁺, Cs⁺ and Ca²⁺, we could determine the specific counterion effects on the rotational dynamics of water molecules. The orientational relaxation time constant of water decreases in the order Ca²⁺ > Na⁺ > K⁺ > Cs⁺. The SCN^- anionic probe and counterion can form ion pairs at the interfacial region of the RMs. The rotational dynamics of SCN^- anion significantly decreases because of the synergistic effects of confinement and the surface interactions in the interfacial region of the RMs. The results can provide a new understanding of the cationic Hofmeister effect at the molecular level observed in biological studies.

Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals

The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na⁺, K⁺, NMe₄⁺) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion-water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.

Counterion-Regulated Dynamics of Water Confined in Lyotropic Liquid Crystalline Morphologies

The dynamics of confined water is of fundamental and long-standing interest. In technologically important forms of confinement, such as proton-exchange membranes, electrostatic interactions with the confining matrix and counterions play significant roles on the properties of water. There has been recent interest on the dynamics of water confined to the lyotropic liquid crystalline (LLC) morphologies of Gemini dicarboxylate surfactants. These systems are exciting because the nature of confinement, for example, size and curvature of channels and surface functionality is dictated by the chemistry of the self-assembling surfactant molecules. Quasielastic neutron scattering experiments have shown an interesting dependence of the water self-diffusion constant, D_α, on the identity (denoted α) of the counterion: at high hydration, the magnitude of the water self-diffusion constant is in the order D_TMA < D_Na < D_K, where TMA, Na, and K refer to tetramethyl ammonium, sodium, and potassium counterions, respectively. This sequence is similar to what is seen in bulk electrolyte solutions. At low hydrations, however, the order of water self-diffusion is different, that is, D_Na < D_TMA < D_K. In this work, we present molecular dynamics simulations for the dynamics of water in the LLC phases of dicarboxylate Gemini surfactants. The simulations reproduce the trends seen in experiments. From an analysis of the trajectories, we hypothesize that two competing factors play a role: the volume accessible to the water molecules and the correlations between the water and the counterion. The excluded volume effect is the largest with TMA⁺, and the electrostatic correlation is the strongest with Na⁺. The observed trend is a result of which of these two effects is dominant at a given water to surfactant ratio.

Excited-state dynamics of the medicinal pigment curcumin in a hydrogel

Curcumin is a yellow polyphenol with multiple medicinal effects. These effects, however, are limited due to its poor aqueous stability and solubility. A hydrogel of 3% octadecyl randomly substituted polyacrylate (PAAC18) has been shown to provide high aqueous stability for curcumin under physiological conditions, offering a route for photodynamic therapy. In this study, the excited-state photophysics of curcumin in the PAAC18 hydrogel is investigated using a combination of femtosecond transient absorption and fluorescence upconversion spectroscopy. The transient absorption results reveal a multiexponential decay in the excited-state kinetics with fast (1 ps & 15 ps) and slow (110 ps & ≈5 ns) components. The fast decay component exhibits a deuterium isotope effect with D₂O in the hydrogel, indicating that the 15 ps decay component is attributable to excited-state intramolecular hydrogen atom transfer of curcumin in the PAAC18 hydrogel. In addition, solvent reorganisation of excited-state curcumin is investigated using multiwavelength femtosecond fluorescence upconversion spectroscopy. The results show that the dominant solvation response (τ = 0.08 ps) is a fast inertial motion owing to the presence of bulk-like water in the vicinity of the hydrophobic octadecyl substituents of the PAAC18 hydrogel. The results also show an additional response with longer time constants of 1 and 6 ps, which is attributable to translational diffusion of confined water molecules in the three-dimensional, cross-linking network of the octadecyl substituents of PAAC18. Overall, we show that excited-state intramolecular hydrogen atom transfer and solvent reorganisation are major photophysical events for curcumin in the PAAC18 hydrogel.

An ion's perspective on the molecular motions of nanoconfined water: a two-dimensional infrared spectroscopy study

The vibrational population relaxation and hydration shell dynamics of the symmetric tricyanomethanide (TCM) anion are investigated in a sodium bis(2-ethylhexyl)sulfosuccinate reverse micelle as a function of the water pool radius. Two-dimensional infrared (IR) spectroscopy in combination with linear absorption and ultrafast IR pump-probe spectroscopy is utilized in this study. Spectroscopic measurements show that the anion has two bands in the 2160-2175 cm(-1) region, each with its own spectroscopic signatures. Analysis of the vibrational dynamics shows that the two vibrational bands are consistent with the anion located either at the interface or in the water pool. The sensitivity of the TCM anion to the environment allows us to unequivocally monitor the vibrational and hydration dynamics of the anion in those two different environments. A TCM anion located at the interface does not show any significant variation of the vibrational dynamics with the water pool size. On the contrary, the TCM anion inside the water pool exhibits a large and nonlinear variation of the vibrational lifetime and the frequency-frequency correlation time with the pool radius. Moreover, for the solvated anion in water pools of 49 Å in radius (W0 = 30), the vibrational lifetime reaches the values observed for the anion in bulk water while the frequency-frequency correlation time shows a characteristic time higher than that observed in the bulk. In addition, for the first time a model is developed and used to explain the observed nonlinear variation of the spectroscopic observables with the pool size. This model attributes the changes in the vibrational dynamics of the TCM anion in the water pool to the slow and radius-dependent water dynamics present in the confined environment of a reverse micelle.

Extracting 2D IR frequency-frequency correlation functions from two component systems

The center line slope (CLS) method is often used to extract the frequency-frequency correlation function (FFCF) from 2D IR spectra to delineate dynamics and to identify homogeneous and inhomogeneous contributions to the absorption line shape of a system. While the CLS method is extremely efficient, quite accurate, and immune to many experimental artifacts, it has only been developed and properly applied to systems that have a single vibrational band, or to systems of two species that have spectrally resolved absorption bands. In many cases, the constituent spectra of multiple component systems overlap and cannot be distinguished from each other. This situation creates ambiguity when analyzing 2D IR spectra because dynamics for different species cannot be separated. Here a mathematical formulation is presented that extends the CLS method for a system consisting of two components (chemically distinct uncoupled oscillators). In a single component system, the CLS corresponds to the time-dependent portion of the normalized FFCF. This is not the case for a two component system, as a much more complicated expression arises. The CLS method yields a series of peak locations originating from slices taken through the 2D spectra. The slope through these peak locations yields the CLS value for the 2D spectra at a given T(w). We derive analytically that for two component systems, the peak location of the system can be decomposed into a weighted combination of the peak locations of the constituent spectra. The weighting depends upon the fractional contribution of each species at each wavelength and also on the vibrational lifetimes of both components. It is found that an unknown FFCF for one species can be determined as long as the peak locations (referred to as center line data) of one of the components are known, as well as the vibrational lifetimes, absorption spectra, and other spectral information for both components. This situation can arise when a second species is introduced into a well characterized single species system. An example is a system in which water exists in bulk form and also as water interacting with an interface. An algorithm is presented for back-calculating the unknown FFCF of the second component. The accuracy of the algorithm is tested with a variety of model cases in which all components are initially known. The algorithm successfully reproduces the FFCF for the second component within a reasonable degree of error.

Solvation dynamics and proton transfer in nanoconfined liquids

Nanoconfined liquids are of interest because of both their fundamental properties and their potential utility in an array of applications. The structure and dynamics of the liquid can be dramatically impacted by the geometrical constraints and the interactions with the interface. Understanding the molecular-level origins of these changes and how they are determined by the characteristics of the confining framework is the subject of ongoing experimental and theoretical studies. The progress and remaining challenges in these efforts are reviewed in the context of solvation dynamics and proton transfer reactions, processes that are strongly affected by nanoscale confinement.

Water dynamics in small reverse micelles in two solvents: two-dimensional infrared vibrational echoes with two-dimensional background subtraction

Water dynamics as reflected by the spectral diffusion of the water hydroxyl stretch were measured in w(0) = 2 (1.7 nm diameter) Aerosol-OT (AOT)/water reverse micelles in carbon tetrachloride and in isooctane solvents using ultrafast 2D IR vibrational echo spectroscopy. Orientational relaxation and population relaxation are observed for w(0) = 2, 4, and 7.5 in both solvents using IR pump-probe measurements. It is found that the pump-probe observables are sensitive to w(0), but not to the solvent. However, initial analysis of the vibrational echo data from the water nanopool in the reverse micelles in the isooctane solvent seems to yield different dynamics than the CCl(4) system in spite of the fact that the spectra, vibrational lifetimes, and orientational relaxation are the same in the two systems. It is found that there are beat patterns in the interferograms with isooctane as the solvent. The beats are observed from a signal generated by the AOT/isooctane system even when there is no water in the system. A beat subtraction data processing procedure does a reasonable job of removing the distortions in the isooctane data, showing that the reverse micelle dynamics are the same within experimental error regardless of whether isooctane or carbon tetrachloride is used as the organic phase. Two time scales are observed in the vibrational echo data, ~1 and ~10 ps. The slower component contains a significant amount of the total inhomogeneous broadening. Physical arguments indicate that there is a much slower component of spectral diffusion that is too slow to observe within the experimental window, which is limited by the OD stretch vibrational lifetime.

Difference in screening effect of alkali metal counterions on H-AOT-based W/O microemulsion formation

The purpose of this study was to estimate the screening of electrostatic repulsions between the polar headgroups of AOT(-) by alkali metal counterions and to explore the relationships between the screening effect and the phase behavior of H-AOT-based W/O microemulsions. The screening effect was evaluated by means of critical micelle concentration (CMC) data using the pyrene 1:3 ratio method with aqueous solutions containing M-AOT (where M(+) = Li(+), Na(+), K(+), Rb(+) and Cs(+)) to form normal micelles, and by counterion binding constants, determined from plots of CMC versus counterion concentration. The order of the screening effect was found to be K(+) approximately = Rb(+) > Cs(+) > Na(+) > Li(+). Interestingly, the order does not follow the hydration size dependence of the alkali metal counterions. An aqueous MOH solution containing a given concentration/H-AOT/isooctane was emulsified at a water content (w(0) = [water]/[H-AOT]) of 10 to produce H-AOT-based W/O microemulsions. The phase behavior and size variation were investigated by FT-IR and DLS measurements. The emulsified mixture separates into two phases at lower MOH concentration due to an insufficient screening effect. When the concentration is increased to a level sufficient to intensify the screening effect, W/O microemulsions are formed without phase separation at lower KOH and RbOH concentrations compared to CsOH. A period of standing after the emulsification and a higher concentration of NaOH compared to KOH, RbOH, and CsOH are required to form W/O microemulsions. W/O microemulsions are not formed in the case of LiOH. These results indicate that the formation of a W/O microemulsion with H-AOT is strongly correlated with the order of the screening effect. A possible cause for the difference in the screening effect is proposed based on hydration of the polar headgroups and counterions, as evidenced by FT-IR spectral data, i.e., symmetrical sulfonate stretching and O-H stretching.

Water dynamics in large and small reverse micelles: from two ensembles to collective behavior

The dynamics of water in Aerosol-OT reverse micelles are investigated with ultrafast infrared spectroscopy of the hydroxyl stretch. In large reverse micelles, the dynamics of water are separable into two ensembles: slow interfacial water and bulklike core water. As the reverse micelle size decreases, the slowing effect of the interface and the collective nature of water reorientation begin to slow the dynamics of the core water molecules. In the smallest reverse micelles, these effects dominate and all water molecules have the same long time reorientational dynamics. To understand and characterize the transition in the water dynamics from two ensembles to collective reorientation, polarization and frequency selective infrared pump-probe experiments are conducted on the complete range of reverse micelle sizes from a diameter of 1.6-20 nm. The crossover between two ensemble and collective reorientation occurs near a reverse micelle diameter of 4 nm. Below this size, the small number of confined water molecules and structural changes in the reverse micelle interface leads to homogeneous long time reorientation.

Geometry and nanolength scales versus interface interactions: water dynamics in AOT lamellar structures and reverse micelles

To determine the relative importance of the confining geometry and nanoscopic length scale versus water/interface interactions, the dynamic interactions between water and interfaces are studied with ultrafast infrared spectroscopy. Aerosol OT (AOT) is a surfactant that can form two-dimensional lamellar structures with known water layer thickness as well as well-defined monodispersed spherical reverse micelles of known water nanopool diameter. Lamellar structures and reverse micelles are compared based on two criteria: surface-to-surface dimensions to study the effect of confining length scales, and water-to-surfactant ratio to study water/interface interactions. We show that the water-to-surfactant ratio is the dominant factor governing the nature of water interacting with an interface, not the characteristic nanoscopic distance. The detailed structure of the interface and the specific interactions between water and the interface also play a critical role in the fraction of water molecules influenced by the surface. A two-component model in which water is separated into bulk-like water in the center of the lamellar structure or reverse micelle and interfacial water is used to quantitatively extract the interfacial dynamics. A greater number of perturbed water molecules are present in the lamellar structures as compared to the reverse micelles due to the larger surface area per AOT molecule and the greater penetration of water molecules past the sulfonate head groups in the lamellar structures.

Ultrafast dynamics in reverse micelles

Recent advances in ultrafast laser technology have spurred investigations of microheterogeneous solutions. In particular, researchers have explored details of reverse micelles (RMs), which present isolated droplets of polar solvent sequestered from a continuous nonpolar phase by a surfactant layer. This review explores recent studies utilizing a variety of ultrafast laser techniques to uncover details about structure and dynamics in various RMs. Using ultrafast vibrational spectroscopy, researchers have probed hydrogen-bond dynamics and vibrational energy relaxation in RMs. These studies have developed our understanding of reverse micellar structure, identifying varying water environments in the RMs. In a plethora of experiments employing probe molecules, researchers have explored the confined environment presented by RMs and their impact on a range of chemical reactions. These studies have shown that confinement, rather than the specific interactions with surfactants, is an important factor determining the impact of the reverse micellar environment on the chemistry.

Reactive dynamics in confined liquids: ultrafast torsional dynamics of auramine O in nanoconfined water in aerosol OT reverse micelles

The effects of confinement on the ultrafast torsional reaction of auramine O in aqueous solution are investigated through ultrafast fluorescence up-conversion with 50 fs time resolution. The aqueous solution is confined in nanoscale water droplets by an ionic surfactant. The torsional motion is orders of magnitude slower in the confined droplets than in bulk aqueous solution. The dynamics become faster with increasing radius of the nanodroplet but never reach the bulk value, even when the radius is as large as 10 nm. Time-dependent fluorescence spectra were constructed and subsequently analyzed using a one-dimensional generalized Smoluchowski equation. An accurate description of the data was achieved using a time-dependent diffusion coefficient. This is suggested to arise because the medium friction reflects dynamics on a broad range of time scales spanning the reaction dynamics. The friction recovered suggests strongly hindered motion in the confined droplet and can be qualitatively related to solvation dynamics measured in AOT, consistent with auramine O torsional dynamics being accompanied by intramolecular charge redistribution.

Temperature-dependent solvation dynamics of water in sodium bis(2-ethylhexyl)sulfosuccinate/isooctane reverse micelles

In this paper, for the first time, we report a detailed study of the temperature-dependent solvation dynamics of a probe fluorophore, coumarin-500, in AOT/isooctane reverse micelles (RMs) with varying degrees of hydration (w0) of 5, 10, and 20 at four different temperatures, 293, 313, 328, and 343 K. The average solvation time constant becomes faster with the increase in w0 values at a particular temperature. The solvation dynamics of a RM with a fixed w0 value also becomes faster with the increase in temperature. The observed temperature-induced faster solvation dynamics is associated with a transition of bound- to free-type water molecules, and the corresponding activation energy value for the w0 = 5 system has been found to be 3.4 kcal mol-1, whereas for the latter two systems, it is approximately 5 kcal mol-1. Dynamic light scattering measurements indicate an insignificant change in size with temperature for RMs with w0 = 5 and 10, whereas for a w0 = 20 system, the hydrodynamic diameter increases with temperature. Time-resolved fluorescence anisotropy studies reveal a decrease in the rotational restriction on the probe with increasing temperature for all systems. Wobbling-in-cone analysis of the anisotropy data also supports this finding.

Behavior of acetyl modified amino acids in reverse micelles: A non-invasive and physiochemical approach

The well-characterized, monodisperse nature of reverse micelles formed by sodium bis-(2-ethylhexyl)sulfosuccinate/water/isooctane and their usefulness in assimilating compounds of varied interests have been exploited to investigate the effect of acetyl modified amino acids (MAA) viz., N-acetyl-L-glycine (NAG), N-acetyl-L-aspartic acid (NAA) and N-acetyl-L-cysteine (NAC), on the water pool and physiochemical properties. Non-invasive techniques such as FTIR and UV-vis absorption spectroscopy have been employed to analyze the interactions of MAA with core water and the AOT headgroup. The micropolarities on both sides of AOT interface have further been investigated by UV-vis absorption probes, methyl orange (MO) and methylene blue (MB). The dynamics of water and temperature induced percolation process have also been studied. The MAA molecules have been found to assist the process with the increase in water content where as a contrary behavior has been observed with the increase in temperature. Conductivity results have been further rationalized in terms of scaling equations, which delineate the dynamic nature of the percolation process. The results have also been analyzed in the light of activation energy of the percolation process and thermodynamics of droplet clustering.

The effect of changing the microstructure of a microemulsion on chemical reactivity

A kinetic study was carried out on various solvolytic reactions in water/ NH4OT /isooctane microemulsions. The NH4OT surfactant is a derivative of the sodium salt of bis(2-ethylhexyl) sulfosuccinate (NaOT or AOT), where the Na+ counterion has been replaced by NH4+. The counterion substitution effects the phase diagram of the system, and therefore, NH4OT-based microemulsions with high water content reaching values of W = 350 (W = [H2O]/[NH4OT]) can be obtained. The presence of high W values suggests a transition in the microemulsion microstructure from water-in-oil (w/o) to oil-in-water (o/w), as was confirmed by conductivity and 1H NMR self-diffusion measurements. The interpretation of the kinetic studies in terms of pseudophase formalism allows us to analyze the effect of the microemulsion on chemical reactivity, regardless of its microstructure. It has been confirmed that the values of the solvolytic rate constants at the interphase of oil-in-water microemulsions are similar to those obtained for aqueous SDS systems, showing that the hydration degree of the interphase of the oil-in-water microemulsions is independent of W. The influence of the surfactant counterion on the solvolytic rate constants was analyzed by comparing HOT-, NaOT-, and NH4OT-based microemulsions. An important influence on the rate constants caused by the changes in the structural properties of water has been observed as was confirmed by the water 1H NMR signals.

Local environments of coumarin dyes within mesostructured silica-surfactant nanocomposites

The local environments surrounding dye molecules were studied with use of coumarin dyes in a mesostructured silica-surfactant nanocomposite, which was formed in a porous alumina membrane by a surfactant-templated method and has an average pore diameter of 3.4 nm. Coumarin dyes, such as coumarin 480 (C480), coumarin 343 (C343), and propylamide coumarin 343 (PAC343), were extracted into the silica-surfactant nanocomposite and time-resolved fluorescence spectra of these dyes were examined. C480 and C343 show slow dynamic Stokes shifts and the decay curve can be fitted by a biexponential function. The decay-time constants obtained from the fitting are almost identical for C480 and C343: 0.87 and 7.5 ns for C480, and 0.86 and 7.6 ns for C343. In contrast to these two coumarin dyes, short decay-time constants (0.50 and 4.8 ns) were obtained for PAC343 in the silica-surfactant nanocomposite. These results indicate that the local environments of C480 and C343 are almost identical but different from that of PAC343. By considering the origin of the dynamic Stokes shift and the mesostructure of the silica-surfactant nanocomposite, the location and microenvironment of coumarin dyes within the silica-surfactant nanocomposite are discussed.

Ultrafast Hydration Dynamics in the Lipidic Cubic Phase: Discrete Water Structures in Nanochannels

We report here our studies of hydration dynamics of confined water in aqueous nanochannels (approximately 50 A) of the lipidic cubic phase. By systematically anchoring the hydrocarbon tails of a series of tryptophan-alkyl ester probes into the lipid bilayer, we mapped out with femtosecond resolution the profile of water motions across the nanochannel. Three distinct time scales were observed, revealing discrete channel water structures. The interfacial water at the lipid surface is well-ordered, and the relaxation dynamics occurs in approximately 100-150 ps. These dynamically rigid water molecules are crucial for global structural stability of lipid bilayers and for stabilization of anchored biomolecules in membranes. The adjacent water layers near the lipid interface are hydrogen-bonded networks and the dynamical relaxation takes 10-15 ps. This quasi-bound water motion, similar to the typical protein surface hydration relaxation, facilitates conformation flexibility for biological recognition and function. The water near the channel center is bulklike, and the dynamics is ultrafast in less than 1 ps. These water molecules freely transport biomolecules near the channel center. The corresponding orientational relaxation at these three typical locations is well correlated with the hydration dynamics and local dynamic rigidity. These results reveal unique water structures and dynamical motions in nanoconfinements, which is critical to the understanding of nanoscopic biological activities and nanomaterial properties.

Tracking Water's Response to Structural Changes in Nafion Membranes

As the water content of Nafion membranes increases, the local environments of water molecules change due to reorganization of the pendant side chains in the hydrophilic domains. Changes in local structure as a function of water content are studied by measuring the IR spectra and the vibrational lifetimes of the hydroxyl stretch of dilute HOD in H(2)O. The main features of the IR spectra are fit well by a weighted sum of the spectra of bulk water and almost dry Nafion, suggesting a two-environment model. An additional small peak on the high frequency side of the main band associated with non-hydrogen-bonded water embedded in the polymer near the interface is analyzed quantitatively as a function of the membrane water content. The spectra of this peak show that a significant reorganization of the interfacial region occurs when the water content of the membrane exceeds the threshold for ion conduction. Vibrational excited state population relaxation times (lifetimes) of the main band lengthen substantially as the water content of the membrane is decreased. The population decays are not single exponentials and indicate that multiple ensembles of water molecules exist, and the characteristics of the individual ensembles change with water content. This is in contrast to the spectra of the main water absorption band, which is only sensitive to two classes of water molecules.