Water Nanopool Research Articles

Water Dynamics in Polyacrylamide Hydrogels.

Polymeric hydrogels have wide applications including electrophoresis, biocompatible materials, water superadsorbents, and contact lenses. The properties of hydrogels involve the poorly characterized molecular dynamics of water and solutes trapped within the three-dimensional cross-linked polymer networks. Here we apply ultrafast two-dimensional infrared (2D IR) vibrational echo and polarization-selective pump-probe (PSPP) spectroscopies to investigate the ultrafast molecular dynamics of water and a small molecular anion solute, selenocyanate (SeCN-), in polyacrylamide hydrogels. For all mass concentrations of polymer studied (5% and above), the hydrogen-bonding network reorganization (spectral diffusion) dynamics and reorientation dynamics reported by both water and SeCN- solvated by water are significantly slower than in bulk water. As the polymer mass concentration increases, molecular dynamics in the hydrogels slow further. The magnitudes of the slowing, measured with both water and SeCN-, are similar. However, the entire hydrogen-bonding network of water molecules appears to slow down as a single ensemble, without a difference between the core water population and the interface water population at the polymer-water surface. In contrast, the dissolved SeCN- do exhibit two-component dynamics, where the major component is assigned to the anions fully solvated in the confined water nanopools. The slower component has a small amplitude which is correlated with the polymer mass concentration and is assigned to adsorbed anions strongly interacting with the polymer fiber networks.

Isoquinoline based alkaloid chemosensor for detection of alkanes and subsequent fluorescence switching inside surfactant coated bio-mimicking nanocavity

In this article, an attempt has been taken to endeavour the physiochemical modulation of an isoquinoline based alkaloid sanguinarine (SANG) in various nonpolar (alkane) medium and inside surfactant coated bio-mimicking nanocavity. Notably, in different alkane medium, SANG undergoes cationic iminium to neutral alkanolamine conversion. This conversion generally occurs as an outcome of nucleophilic attack by water (or hydroxyl group) at the electron deficient centre of iminium form of SANG in water at pKa >7.2. Hence, such conversion in nonpolar mediumis surprising, as alkane cannot act as a nucleophile. After investing much effort behind theoretical and experimental techniques, we have directly monitored that ‘water of crystallization’ of the alkaloid itself take part in the intranucleophilic attack at the electron deficient centre in alkane medium, which switches the fluorescence from orange (iminium) to bluish-violet (alkanolamine) even detectable by naked eye. Notably, we have employed such fluorescence switching of SANG into the detection of alkane mixed with the drinking water, and we have noticed that SANG alkaloid may be potentially useful for the detection of hazardous alkane, like, hexane, heptane, octane, decane etc. from drinking water. This kind of fluorescence switching of organic cationic fluorophore in alkane medium is rare, and such switching would be highly interesting for the application in petrochemistry, environmental and geological sciences for development of sensors to detect alkanes and other nonpolar solvents. Moreover, in presence of anionic (AOT) surfactant in nonpolar solvent (n-heptane and benzene), the newly created alkanolamine form is found to enter inside the reverse micellar (RMs) nanocavity. Intriguingly, subsequent addition of water results in the return of alkaloid into its initial cationic iminium form by second conversion inside the reverse micellar water nanopool, exhibiting an isoemissive point at ~530nm.

Ultrafast Fluorescence Dynamics of Highly Stable Copper Nanoclusters Synthesized inside the Aqueous Nanopool of Reverse Micelles

Herein, we have reported a new strategy for the synthesis of highly stable fluorescent copper nanoclusters (CuNCs) with l-cysteine (Cys) as a protecting ligand within the water nanopool of reverse micelles (RMs). In the present work, efforts are also given to address the origin of excitation-dependent fluorescence spectral shift of CuNCs. From our experiments, we have elucidated that the broad fluorescence from CuNCs in RMs consists of two spectrally overlapped bands corresponding to the metal-core and surface states of CuNCs. The intrinsic emission of CuNCs distributed in shorter wavelength regions ( 470 nm) is caused by surface states and consists of a much broader emission because of the presence of numerous surface states. The trapping of excited electrons in the various surface states leads to the emission in the longer wavelength regions and is believed to be responsible for excitation-dependent em...

How Does Nanoconfinement within a Reverse Micelle Influence the Interaction of Phenazinium-Based Photosensitizers with DNA?

The major focus of the present work lies in exploring the influence of nanoconfinement within aerosol-OT (AOT) reverse micelles on the binding interaction of two phenazinium-based photosensitizers, namely, phenosafranin (PSF) and safranin-O (SO), with the DNA duplex. Circular dichroism and dynamic light-scattering studies reveal the condensation of DNA within the reverse micellar interior (transformation of the B-form of native DNA to ψ-form). Our results unveil a remarkable effect of the degree of hydration of the reverse micellar core on the stability of the stacking interaction (intercalation) of the drugs (PSF and SO) into DNA; increasing size of the water nanopool (that is, w0) accompanies decreasing curvature of the DNA duplex structure with the consequent effect of increasing stabilization of the drug:DNA intercalation. The marked differences in the dynamical aspects of the interaction scenario following encapsulation within the reverse micellar core and the subsequent dependence on the size of the water nanopool are also meticulously explored. The differential degrees of steric interactions offered by the drug molecules (presence of methyl substitutions on the planar phenazinium ring in SO) are also found to affect the extent of intercalation of the drugs to DNA. In this context, it is imperative to state that the water pool of the reverse micellar core is often argued to approach bulk-like properties of water with increasing micellar size (typically w0 ≥ 10), so that deviation from the bulk water properties is likely to be minimized in large reverse micelles (w0 ≥ 10). On the contrary, our results (particularly quantitative elucidation of micropolarity and dynamical aspects of the interaction) explicitly demonstrate that the bulk-like behavior of the nanoconfined water is not truly achieved even in large reverse micelles.

Water Librations in the Hydration Shell of Phospholipids.

The hydrophilic phosphate moiety in the headgroup of phospholipids forms strong hydrogen bonds with water molecules in the first hydration layer. Time-domain terahertz spectroscopy in a range from 100 to 1000 cm-1 reveals the influence of such interactions on rotations of water molecules. We determine librational absorption spectra of water nanopools in phospholipid reverse micelles for a range from w0 = 2 to 16 waters per phospholipid molecule. A pronounced absorption feature with maximum at 830 cm-1 is superimposed on a broad absorption band between 300 and 1000 cm-1. Molecular dynamics simulations of water in the reverse micelles suggest that the feature at 830 cm-1 arises from water molecules forming one or two strong hydrogen bonds with phosphate groups, while the broad component comes from bulk-like environments. This behavior is markedly different from water interacting with less polar surfaces.

Dynamics of Water Confined in Reversed Micelles: Multidimensional Vibrational Spectroscopy Study

Here we perform a comprehensive study of ultrafast molecular and vibrational dynamics of water confined in small reversed micelles (RMs). The molecular picture is elucidated with two-dimensional infrared (2D IR) spectroscopy of water OH stretch vibrations and molecular dynamics simulations, bridged by theoretical calculations of linear and 2D IR vibrational spectra. To investigate the effects of intermolecular coupling, experiments and modeling are performed for isotopically diluted (HDO in D2O) and undiluted (H2O) water. We put a separation of water inside RMs into two subensembles (water-bound and surfactant-bound molecules), observed by many before, on a solid theoretical basis. Water molecules fully attached to the lipid interface ("shell" water) are decoupled from one another and from the central water nanopool ("core" water). The environmental fluctuations are largely "frozen" for the shell water, while the core waters demonstrate much faster dynamics but still not as fast as in the bulk case. A substantial nanoconfinement effect on the dynamics of the core water is observed after disentanglement of the shell water contribution, which is fully confirmed by the simulations of 2D IR spectra. Current results provide new insights into interaction between biological objects like membranes or proteins with the surrounding aqueous bath, and highlight peculiarities in vibrational energy redistribution near the lipid surface.

Spectral features of 4,4-diaminodiphenyl sulfone in anionic and cationic inverted micelles

This Letter reports the photophysics of 4,4-diaminodiphenyl sulfone in anionic and cationic reverse micelles (RM). Decreased fluorescence intensity with significant blue shift of intramolecular charge transfer (ICT) fluorescence maximum and longer emission lifetime of the entrapped probe in motionally restricted water nano-pool of anionic RM could be observed. Bi-exponential decay in anionic RM appears due to polarity heterogeneity in the pool. In a nut shell, experiments establish that the probe is anchored in the micellar head group region of anionic RM vis-à-vis far away from the core in cationic RM. Increasing pool size show the opposite effect to the probe.

Anomalous Acid-Base Equilibria in Biologically Relevant Water Nanopools

Water plays a crucial role in many principal biological phenomena such as enzymatic catalysis and proton pumping through a membrane protein channel. Moreover, in biological systems, water is usually contained in a small pocket of a membrane, and such confined water, which is generally called a water nanopool, shows peculiar properties differing considerably from the properties of bulk water. The confinement effect and the enclosing interfacial surfaces of waterpools are the main factors to determine the properties, such as polarity, viscosity, and H-bonding ability, of water nanopools. For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (e = 78.5) but similar to that of an alcohol (e = 30-40). On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems. Thus, for better understanding of cellular dynamics, it is necessary to investigate the properties of a biologically relevant water nanopool as a biomimetic system of water confined in a cell membrane. In this regard, water nanopools confined in reverse micelles, which are formed by surfactant molecules having polar headgroups pointing inward and dispersed in hydrocarbon solvents, can be good model systems of biological water. It is a unique feature of reverse micelles that they can make nonpolar media to solubilize a large amount of water by encapsulating water molecules in their inner polar cores; reverse micelles are surrounded by a layer of surfactant molecules such as Aerosol-OT (sodium 1,4-bis-2-ethylhexylsulfosuccinate, AOT) and immersed in a nonpolar solvent, so that nanometer-sized droplets of a polar solvent such as water are formed inside. The polar headgroups of surfactant molecules point inward toward a polar solvent pool, and the alkyl chains of the surfactant molecules point outward toward a nonpolar solvent. About 20 surfactant molecules of AOT form a reverse micelle having a diameter of 3.0 nm above the critical concentration of 1 mM in a hydrocarbon solvent such as n-heptane. By adding water to the AOT solution, microemulsions of nanometer-sized water droplets surrounded by AOT molecules are formed, and the size of water nanopools confined in AOT reverse micelles increases as the concentration of water increases. In n-heptane, the diameters in nanometers of the waterpools have been reported to be about 0.3w0, in which w0 is the molar ratio of water to AOT. The catalytic properties of water nanopools depend strongly on the hydration extent of reverse micelles and on the solvation structure of reactants relevant to the polarity of waterpools confined in reverse micelles. The peculiar structure of waterpools contained in reverse micelles, which is in conjunction with the solvation ability and the acid-base activity of waterpools, is of major importance to understand the reactivity of biological water confined in a membrane. In aqueous solutions, while the acidity of water can be adjusted by pH, the prototropic dissociation of solutes would vary with pH. Thus, to know whether solutes have a cationic form or an anionic form at a given pH, the pKa value of the solute should be determined. Many researchers have reported numerical values of pKa, which are deduced in empirical ways, in waterpools confined in reverse micelles. As already mentioned above, water is the major medium of biological systems, and biological proton relay usually occurs through a H-bonded water chain. Accordingly, water molecules act as proton donors or acceptors, and the activity of water plays a key role to determine the rate constant of proton transport. In addition, biological water which is confined in a membrane shows characteristic properties distinctive from the properties of bulk water. Thus, we consider that waterpools confined in reverse micelles are good biomimetic systems to investigate the properties of biological water. Hydroxyquinolines, having two prototropic groups of acidic enol and basic imine, are extensively explored because they are good experimental models to investigate biological

Dynamics of Water at the Interface in Reverse Micelles: Measurements of Spectral Diffusion with Two-Dimensional Infrared Vibrational Echoes

Water dynamics inside of reverse micelles made from the surfactant Aerosol-OT (AOT) were investigated by observing spectral diffusion, orientational relaxation, and population relaxation using two-dimensional infrared (2D IR) vibrational echo spectroscopy and pump-probe experiments. The water pool sizes of the reverse micelles studied ranged in size from 5.8 to 1.7 nm in diameter. It is found that spectral diffusion, characterized by the frequency-frequency correlation function (FFCF), significantly changes as the water pool size decreases. For the larger reverse micelles (diameter 4.6 nm and larger), the 2D IR signal is composed of two spectral components: a signal from bulk-like core water, and a signal from water at the headgroup interface. Each of these signals (core water and interfacial water) is associated with a distinct FFCF. The FFCF of the interfacial water layer can be obtained using a modified center line slope (CLS) method that has been recently developed. The interfacial FFCFs for large reverse micelles have a single exponential decay (∼1.6 ps) to an offset plus a fast homogeneous component and are nearly identical for all large sizes. The observed ∼1.6 ps interfacial decay component is approximately the same as that found for bulk water and may reflect hydrogen bond rearrangement of bulk-like water molecules hydrogen bonded to the interfacial water molecules. The long time offset arises from dynamics that are too slow to be measured on the accessible experimental time scale. The influence of the chemical nature of the interface on spectral diffusion was explored by comparing data for water inside reverse micelles (5.8 nm water pool diameter) made from the surfactants AOT and Igepal CO-520. AOT has charged, sulfonate head groups, while Igepal CO-520 has neutral, hydroxyl head groups. It is found that spectral diffusion on the observable time scales is not overly sensitive to the chemical makeup of the interface. An intermediate-sized AOT reverse micelle (water pool diameter of 3.3 nm) is analyzed as a large reverse micelle because it has distinct core and interface regions, but its core region is more constrained than bulk water. The interfacial FFCF for this intermediate-sized reverse micelle is somewhat slower than those found for the larger reverse micelles. The water nanopools in the smaller reverse micelles cannot be separated into core and interface regions. In the small reverse micelles, the FFCFs are biexponential decays to an offset plus a fast homogeneous component. Each small reverse micelle exhibits an ∼1 ps decay time, which may arise from local hydrogen bond fluctuations and a slower, ∼6-10 ps decay, which is possibly due to slow hydrogen bond rearrangement of noninterfacial water molecules or topography fluctuations at the interface.

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.

Probing the Inner Cavities of Hydrogels by Proton Diffusion

The reversible proton dissociation and geminate recombination of a photoacid was studied in peptide-based hydrogels over a large concentration range of the self-assembled peptide nanotube (PNT) network. 8-Hydroxypyrene-1,3,6-trisulfonate trisodium salt (HPTS) was used as the photoacid. We found that the proton transfer to the solvent rate is not affected by the presence of the fibrous structures. However, the proton geminate recombination process is affected by the PNT walls. We used the photoprotolytic reversible geminate recombination model to analyze the time-resolved emission data in the hydrogel samples. Our assumptions lead to the suggestion that the hydrogels form water nanopools of the order of 100 A.

Water dynamics at neutral and ionic interfaces

The orientational dynamics of water at a neutral surfactant reverse micelle interface are measured with ultrafast infrared spectroscopy of the hydroxyl stretch, and the results are compared to orientational relaxation of water interacting with an ionic interface. The comparison provides insights into the influence of a neutral vs. ionic interface on hydrogen bond dynamics. Measurements are made and analyzed for large nonionic surfactant Igepal CO-520reverse micelles (water nanopool with a 9-nm diameter). The results are compared with those from a previous study of reverse micelles of the same size formed with the ionic surfactant Aerosol-OT (AOT). The results demonstrate that the orientational relaxation times for interfacial water molecules in the two types of reverse micelles are very similar (13 ps for Igepal and 18 ps for AOT) and are significantly slower than that of bulk water (2.6 ps). The comparison of water orientational relaxation at neutral and ionic interfaces shows that the presence of an interface plays the dominant role in determining the hydrogen bond dynamics, whereas the chemical nature of the interface plays a secondary role.

Ground-State Proton Transfer of 7-Hydroxyquinoline Confined in Biologically Relevant Water Nanopools

The ground-state reverse proton transfer of 7-hydroxyquinoline catalyzed by water confined in AOT reverse micelles has been investigated by measuring time-resolved transient-absorption spectra and kinetic profiles. The transfer time is profoundly retarded in water nanopools compared with that in bulk water (26 μs) although it diminishes with the size increase of the water nanopool. The spectral-shift time of tautomeric transient absorption agrees well with the proton transfer time. The probe molecule is subject to the local gradient of polarity, whose magnitude is altered with the sizes of water nanopools. Accordingly, the observations made in this study indicate the multidimensional character of reaction coordinates, in which solvent polarization coupled to charge transfer plays a seminal role in the control of overall proton-transfer dynamics. The retardation of proton transfer in water nanopools is ascribed to the increased formation energy of a charge-transferred optimal configuration, which is prereq...

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.

Water at the surfaces of aligned phospholipid multibilayer model membranes probed with ultrafast vibrational spectroscopy.

The dynamics of water at the surface of artificial membranes composed of aligned multibilayers of the phospholipid dilauroyl phosphatidylcholine (DLPC) are probed with ultrafast polarization selective vibrational pump-probe spectroscopy. The experiments are performed at various hydration levels, x = 2 - 16 water molecules per lipid at 37 degrees C. The water molecules are approximately 1 nm above or below the membrane surface. The experiments are conducted on the OD stretching mode of dilute HOD in H 2O to eliminate vibrational excitation transfer. The FT-IR absorption spectra of the OD stretch in the DLPC bilayer system at low hydration levels shows a red-shift in frequency relative to bulk water, which is in contrast to the blue-shift often observed in systems such as water nanopools in reverse micelles. The spectra for x = 4 - 16 can be reproduced by a superposition of the spectra for x = 2 and bulk water. IR Pump-probe measurements reveal that the vibrational population decays (lifetimes) become longer as the hydration level is decreased. The population decays are fit well by biexponential functions. The population decays, measured as a function of the OD stretch frequency, suggest the existence of two major types of water molecules in the interfacial region of the lipid bilayers. One component may be a clathrate-like water cluster near the hydrophobic choline group and the other may be related to the hydration water molecules mainly associated with the phosphate group. As the hydration level increases, the vibrational lifetimes of these two components decrease, suggesting a continuous evolution of the hydration structures in the two components associated with the swelling of the bilayers. The agreement of the magnitudes of the two components obtained from IR spectra with those from vibrational lifetime measurements further supports the two-component model. The vibrational population decay fitting also gives an estimation of the number of phosphate-associated water molecules and choline-associated water molecules, which range from 1 to 4 and 1 to 12, respectively, as x increases from 2 to 16. Time-dependent anisotropy measurements yield the rate of orientational relaxation as a function of x. The anisotropy decay is biexponential. The fast component is almost independent of x, and is interpreted as small orientational fluctuations that occur without hydrogen-bond rearrangement. The slower component becomes very long as the hydration level decreases. This component is a measure of the rate of complete orientational randomization, which requires H-bond rearrangement and is discussed in terms of a jump reorientation model.

Confinement or the nature of the interface? Dynamics of nanoscopic water.

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

Proton Transport and the Water Environment in Nafion Fuel Cell Membranes and AOT Reverse Micelles

The properties of confined water and diffusive proton-transfer kinetics in the nanoscopic water channels of Nafion fuel cell membranes at various hydration levels are compared to water in a series of well-characterized AOT reverse micelles with known water nanopool sizes using the photoacid pyranine as a molecular probe. The side chains of Nafion are terminated by sulfonate groups with sodium counterions that are arrayed along the water channels. AOT has sulfonate head groups with sodium counterions that form the interface with the reverse micelle's water nanopool. The extent of excited-state deprotonation is observed by steady-state fluorescence measurements. Proton-transfer kinetics and orientational relaxation are measured by time-dependent fluorescence using time-correlated single photon counting. The time dependence of deprotonation is related to diffusive proton transport away from the photoacid. The fluorescence reflecting the long time scale proton transport has an approximately t-0.8 power law decay in contrast to bulk water, which has a t-3/2 power law. For a given hydration level of Nafion, the excited-state proton transfer and the orientational relaxation are similar to those observed for a related size AOT water nanopool. The effective size of the Nafion water channels at various hydration levels are estimated by the known size of the AOT reverse micelles that display the corresponding proton-transfer kinetics and orientational relaxation.

Probing the Behavior of Confined Water by Proton-Transfer Reactions

The picosecond dynamics of a bifunctional and H-bonding molecule, 7-hydroxyquinoline (7HQ), has been studied in a reverse micelle with increasing water content. The fluorescence kinetics has a complex behavior as the water content is changed. All reactions are irreversible, and a two-step mechanism is invoked to explain the observations. H2O/D2O exchange and excitation energy effects show that the second step has a higher barrier and that the corresponding reaction occurs through tunneling. The results clearly indicate two regimes of water nanopool behavior switching at W0 approximately 5 (W0 = [water]/[surfactant]). Water collective dynamics explains these observations. The lower fluidity of confined water within the reverse micelle with respect to normal bulk water alters the related H-bond network dynamics and therefore is responsible for the slower proton-transfer processes.

Testing the Core/Shell Model of Nanoconfined Water in Reverse Micelles Using Linear and Nonlinear IR Spectroscopy

A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.

Excited-State Double Proton Transfer of 7-Azaindole in Water Nanopools

The excited-state proton-transfer dynamics of 7-azaindole occurring in the water nanopools of reverse micelles has been investigated by measuring time-resolved fluorescence spectra and kinetics, as well as static absorption and emission spectra, with varying water content and isotope. 7-Azaindole molecules are found to exist in the bound-water regions of reverse micelles. The rate constant and the kinetic isotope effect of proton transfer are smaller than those in bulk water although both increase with the size of the water nanopool. The retardation of proton transfer in the bound regions is attributed to the increased free energy of prerequisite solvation to form a cyclically H-bonded 1:1 7-azaindole/water complex.