Correlating proton transfer dynamics to probe location in confined environments

The dramatic impact of differing environments on proton transfer dynamics of the photoacid HPTS prompted us to investigate these systems with two highly complementary methods: ultrafast time-resolved transient absorption and two-dimensional NMR spectroscopies. Both ultrafast time-resolved transient absorption spectroscopy and time-resolved anisotropy decays demonstrate the proton transfer dynamics depend intimately on the specific reverse micellar system. For w(0) = 10 reverse micelles formed with anionic AOT surfactant, the HPTS proton transfer dynamics are similar to dynamics in bulk aqueous solution, and the corresponding (1)H 2D NOESY NMR spectra display no cross peaks between HPTS and AOT consistent with the HPTS residing well hydrated by water in the interior of the reverse micelle water pool. In contrast, ultrafast transient absorption experiments show no evidence for HPTS photoinduced proton transfer reaction in reverse micelles formed with the cationic CTAB surfactant. In CTAB reverse micelles, clear cross peaks between HPTS and CTAB in the 2D NMR spectra show that HPTS embeds in the interface. These results indicate that the environment strongly impacts the proton transfer reaction and that complementary experimental techniques develop understanding of how location critically affects molecular responses.     

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.     

Variation of Microenvironment around Auramine O Molecules Doped in Silica Prepared from Tetramethoxysilane

Silica gel doped with Auramine O (AuO), which is known as a microscopic viscosity probe through fluorescence spectra, was prepared by the sol-gel method using tetramethoxysilane (TMOS) as a starting alkoxide. Fluorescence spectra from the starting solution to the dry gel were measured to pursue the variation of microscopic viscosity around the AuO molecules during the sol-gel process. The fluorescence intensity of AuO did not change at the gelation point, indicating that the microscopic viscosity around AuO did not change during the sol-to-gel transformation. The intensity was, however, increased largely in the drying process; the internal motion of AuO molecules became largely restricted by the surrounded silica matrix during the drying process.     

Environment-dependent ultrafast photoisomerization dynamics in azo dye

Azoaromatic dyes have been extensively investigated over the past decade due to their potential use in a variety of optical devices that exploit their ultrafast photoisomerization processes. Among the azoaromatic dyes, Disperse Red 19 is a commercially available azobenzene nonlinear optical chromophore with a relatively high ground-state dipole moment. In the present study, we used ultrafast time-resolved spectroscopy to clarify the dynamics of a push-pull substituted azobenzene dye. Solution and film samples exhibited different ultrafast dynamics, indicating that the molecular environment affects the photoisomerization dynamics of the dye.     

Temperature- and solvation-dependent dynamics of liquid sulfur dioxide studied through the ultrafast optical Kerr effect

The ultrafast dynamics of liquid sulphur dioxide have been studied over a wide temperature range and in solution. The optically heterodyne-detected and spatially masked optical Kerr effect (OKE) has been used to record the anisotropic and isotropic third-order responses, respectively. Analysis of the anisotropic response reveals two components, an ultrafast nonexponential relaxation and a slower exponential relaxation. The slower component is well described by the Stokes-Einstein-Debye equation for diffusive orientational relaxation. The simple form of the temperature dependence and the agreement between collective (OKE) and single molecule (e.g., NMR) measurements of the orientational relaxation time suggests that orientational pair correlation is not significant in this liquid. The relative contributions of intermolecular interaction-induced and single-molecule orientational dynamics to the ultrafast part of the spectral density are discussed. Single-molecule librational-orientational dynamics appear to dominate the ultrafast OKE response of liquid SO2. The temperature-dependent OKE data are transformed to the frequency domain to yield the Raman spectral density for the low-frequency intermolecular modes. These are bimodal with the lowest-frequency component arising from diffusive orientational relaxation and a higher-frequency component connected with the ultrafast time-domain response. This component is characterized by a shift to higher frequency at lower temperature. This result is analyzed in terms of a harmonic librational oscillator model, which describes the data accurately. The observed spectral shifts with temperature are ascribed to increasing intermolecular interactions with increasing liquid density. Overall, the dynamics of liquid SO2 are found to be well described in terms of molecular orientational relaxation which is controlled over every relevant time range by intermolecular interactions.     

Molecular modeling and simulation of water near model micelles: diffusion, rotational relaxation and structure at the hydration interface

This paper reports on molecular dynamics simulations of two hydrated micelles composed of C12E6 and LDAO surfactants. The simulations results provide a quantitative picture of the dynamics of the hydration water at the water/micelle interface. Both the residence time of water near the micelle surface and its retardation with respect to the bulk have been estimated. It is found that the water dynamics is radically different for the two micellar systems and depends on the physical nature of the micelle surface in contact with water. For C12E6 this interface is thicker and presents a stronger hydrophilic character than that of LDAO. Thus, in C12E6, surface water dynamics is 1-2 orders of magnitude slower than that of bulk water, compared with only 18% for the LDAO system. The simulations have also revealed the nature of the rotational landscape experienced by water at the micellar surface: In the C12E6 micelle water rotation occurs in a highly anisotropic space due to confinement of waters at the interface; in LDAO the rotational landscape is instead isotropic. These findings clearly indicate that the slowdown of interfacial water relaxation near complex micelles depends, case by case, on the structural properties of the interface itself, such as the ratio between hydrophobic/hydrophilic exposed regions and on the interface thickness and topography.     

Ultrafast intermolecular dynamics of liquid water: a theoretical study on two-dimensional infrared spectroscopy

Physical and chemical properties of liquid water are dominated by hydrogen bond structure and dynamics. Recent studies on nonlinear vibrational spectroscopy of intramolecular motion provided new insight into ultrafast hydrogen bond dynamics. However, our understanding of intermolecular dynamics of water is still limited. We theoretically investigated the intermolecular dynamics of liquid water in terms of two-dimensional infrared (2D IR) spectroscopy. The 2D IR spectrum of intermolecular frequency region (<1000 cm(-1)) is calculated by using the equilibrium and nonequilibrium hybrid molecular dynamics method. We find the ultrafast loss of the correlation of the libration motion with the time scale of approximately 110 fs. It is also found that the energy relaxation from the libration motion to the low frequency motion takes place with the time scale of about 180 fs. We analyze the effect of the hindered translation motion on these ultrafast dynamics. It is shown that both the frequency modulation of libration motion and the energy relaxation from the libration to the low frequency motion significantly slow down in the absence of the hindered translation motion. The present result reveals that the anharmonic coupling between the hindered translation and libration motions is essential for the ultrafast relaxation dynamics in liquid water.     

Ion–micelle interactions and the modeling of reactivity in micellar solutions of simple zwitterionic sulfobetaine surfactants

Aqueous micellar solutions of sulfobetaine surfactants provide a simple physical–chemical system for investigating the origin and consequences of the specific interactions of anions with a model zwitterionic interface. Studies of ground-state reaction kinetics in micellar solutions of these surfactants provide a window into some of the more intriguing aspects of both anion and cation interactions with a zwitterionic micelle surface. Recent molecular dynamics simulations paint a more physically reasonable picture of the zwitterionic micelle–water interface than that usually depicted in the literature and should contribute to our future understanding of the factors that contribute to specific anion binding.     

Ultrafast 2D IR anisotropy of water reveals reorientation during hydrogen-bond switching

Rearrangements of the hydrogen bond network of liquid water are believed to involve rapid and concerted hydrogen bond switching events, during which a hydrogen bond donor molecule undergoes large angle molecular reorientation as it exchanges hydrogen bonding partners. To test this picture of hydrogen bond dynamics, we have performed ultrafast 2D IR spectral anisotropy measurements on the OH stretching vibration of HOD in D(2)O to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment using restricted rotation models. From the inertial 2D anisotropy decay, we find that water molecules initiating from a strained configuration and relaxing to a stable configuration are characterized by a distribution of angles, with an average reorientation half-angle of 10°, implying an average reorientation for a full switch of ≥20°. These results provide evidence that water hydrogen bond network connectivity switches through concerted motions involving large angle molecular reorientation.     

Size-dependent carrier dynamics in CdS nanoparticles by femtosecond visible-pump/IR-probe measurements

Ultrafast photoexcited carrier dynamics in CdS nanoparticles prepared by an AOT/n-heptane reversed micelle system were investigated by a femtosecond visible-pump/mid-IR probe technique. A mid-IR probe beam was found to mainly probe the ultrafast dynamics of photoexcited electrons in the conduction band. Dispersions of CdS nanoparticles with 8 different mean diameters from 2.9 to 4.1 nm were prepared by tuning the mole ratio between water and AOT (W = [H(2)O]/[AOT]) in the reversed micelle systems. The excited state lifetime strongly depended on the mean size of CdS nanoparticles with a maximum around a mean diameter of 3.5 nm. This result was explained by considering the balance between the carrier recombination rates via surface states and those via interior states. The relationship between the excited state lifetime and the size of CdS nanoparticles was drastically changed when the surface was terminated by thiol molecules.     

Water dynamics--the effects of ions and nanoconfinement

Hydrogen bond dynamics of water in highly concentrated NaBr salt solutions and reverse micelles are studied using ultrafast 2D-IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments performed on the OD hydroxyl stretch of dilute HOD in H(2)O. The vibrational echo experiments measure spectral diffusion, and the pump-probe experiments measure orientational relaxation. Both experimental observables are directly related to the structural dynamics of water's hydrogen bond network. The measurements performed on NaBr solutions as a function of concentration show that the hydrogen bond dynamics slow as the NaBr concentration increases. The most pronounced change is in the longest time scale dynamics which are related to the global rearrangement of the hydrogen bond structure. Complete hydrogen bond network randomization slows by a factor of approximately 3 in approximately 6 M NaBr solution compared to that in bulk water. The hydrogen bond dynamics of water in nanoscopically confined environments are studied by encapsulating water molecules in ionic head group (AOT) and nonionic head group (Igepal CO 520) reverse micelles. Water dynamics in the nanopools of AOT reverse micelles are studied as a function of size by observing orientational relaxation. Orientational relaxation dynamics deviate significantly from bulk water when the size of the reverse micelles is smaller than several nm and become nonexponential and slower as the size of the reverse micelles decreases. In the smallest reverse micelles, orientational relaxation (hydrogen bond structural randomization) is almost 20 times slower than that in bulk water. To determine if the changes in dynamics from bulk water are caused by the influence of the ionic head groups of AOT or the nanoconfinement, the water dynamics in 4 nm nanopools in AOT reverse micelles (ionic) and Igepal reverse micelles (nonionic) are compared. It is found that the water orientational relaxation in the 4 nm diameter nanopools of the two types of reverse micelles is almost identical, which indicates 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.     

Dynamics of nanoscopic water: vibrational echo and infrared pump-probe studies of reverse micelles

The dynamics of water in nanoscopic pools 1.7-4.0 nm in diameter in AOT reverse micelles were studied with ultrafast infrared spectrally resolved stimulated vibrational echo and pump-probe spectroscopies. The experiments were conducted on the OD hydroxyl stretch of low-concentration HOD in the H2O, providing a direct examination of the hydrogen-bond network dynamics. Pump-probe experiments show that the vibrational lifetime of the OD stretch mode increases as the size of the reverse micelle decreases. These experiments are also sensitive to hydrogen-bond dissociation and reformation dynamics, which are observed to change with reverse micelle size. Spectrally resolved vibrational echo data were obtained at several frequencies. The vibrational echo data are compared to data taken on bulk water and on a 6 M NaCl solution, which is used to examine the role of ionic strength on the water dynamics in reverse micelles. Two types of vibrational echo measurements are presented: the vibrational echo decays and the vibrational echo peak shifts. As the water nanopool size decreases, the vibrational echo decays become slower. Even the largest nanopool (4 nm, approximately 1000 water molecules) has dynamics that are substantially slower than bulk water. It is demonstrated that the slow dynamics in the reverse micelle water nanopools are a result of confinement rather than ionic strength. The data are fit using time-dependent diagrammatic perturbation theory to obtain the frequency-frequency correlation function (FFCF) for each reverse micelle. The results are compared to the FFCF of water and show that the largest differences are in the slowest time scale dynamics. In bulk water, the slowest time scale dynamics are caused by hydrogen-bond network equilibration, i.e., the making and breaking of hydrogen bonds. For the smallest nanopools, the longest time scale component of the water dynamics is approximately 10 times longer than the dynamics in bulk water. The vibrational echo data for the smallest reverse micelle displays a dependence on the detection wavelength, which may indicate that multiple ensembles of water molecules are being observed.     

POLARIZATION STUDY OF 5(E)-STYRYL-1,3-DIMETHYL URACIL IN DIISOOCTYL SULFOSUCCINATE (AEROSOL-OT) REVERSED MICELLE SOLUTIONS

The degree of polarization around the water pool generated in aerosol- OT/hexane/water reversed micelle solutions was studied by monitoring the depolarization of solubilized probe fluorescence. The 5(E)-styryI-1, 3-dimethyl uracil (SDU) used in this study was moderately polar and proved to be solubilized between the interface and hydrocarbon phases in reversed micelle solution by a fluorescence quenching study with hydrophilic and hydrophobic quenchers. The degree of polarization monitored by SDU was parallel with that obtained from the fluorescence depolarization study of hydrophylic probes solubilized in the water pool and interface. A dramatic increase in microviscosity was observed when ω value became small.     

Femtosecond time-resolved electronic sum-frequency generation spectroscopy: a new method to investigate ultrafast dynamics at liquid interfaces

We developed a new surface-selective time-resolved nonlinear spectroscopy, femtosecond time-resolved electronic sum-frequency generation (TR-ESFG) spectroscopy, to investigate ultrafast dynamics of molecules at liquid interfaces. Its advantage over conventional time-resolved second harmonic generation spectroscopy is that it can provide spectral information, which is realized by the multiplex detection of the transient electronic sum-frequency signal using a broadband white light continuum and a multichannel detector. We studied the photochemical dynamics of rhodamine 800 (R800) at the air/water interface with the TR-ESFG spectroscopy, and discussed the ultrafast dynamics of the molecule as thoroughly as we do for the bulk molecules with conventional transient absorption spectroscopy. We found that the relaxation dynamics of photoexcited R800 at the air/water interface exhibited three characteristic time constants of 0.32 ps, 6.4 ps, and 0.85 ns. The 0.32 ps time constant was ascribed to the lifetime of dimeric R800 in the lowest excited singlet (S(1)) state (S(1) dimer) that is directly generated by photoexcitation. The S(1) dimer dissociates to a monomer in the S(1) state (S(1) monomer) and a monomer in the ground state with this time constant. This lifetime of the S(1) dimer was ten times shorter than the corresponding lifetime in a bulk aqueous solution. The 6.4 ps and 0.85 ns components were ascribed to the decay of the S(1) monomer (as well as the recovery of the dimer in the ground state). For the 6.4 ps time constant, there is no corresponding component in the dynamics in bulk water, and it is ascribed to an interface-specific deactivation process. The 0.85 ns time constant was ascribed to the intrinsic lifetime of the S(1) monomer at the air/water interface, which is almost the same as the lifetime in bulk water. The present study clearly shows the feasibility and high potential of the TR-ESFG spectroscopy to investigate ultrafast dynamics at the interface.     

Excess electron relaxation dynamics at water/air interfaces

We have performed mixed quantum-classical molecular dynamics simulations of the relaxation of a ground state excess electron at interfaces of different phases of water with air. The investigated systems included ambient water/air, supercooled water/air, Ih ice/air, and amorphous solid water/air interfaces. The present work explores the possible connections of the examined interfacial systems to finite size cluster anions and the three-dimensional infinite, fully hydrated electron. Localization site analyses indicate that in the absence of nuclear relaxation the electron localizes in a shallow potential trap on the interface in all examined systems in a diffuse, surface-bound (SB) state. With relaxation, the weakly bound electron undergoes an ultrafast localization and stabilization on the surface with the concomitant collapse of its radius. In the case of the ambient liquid interface the electron slowly (on the 10 ps time scale) diffuses into the bulk to form an interior-bound state. In each other case, the excess electron persists on the interface in SB states. The relaxation dynamics occur through distinct SB structures which are easily distinguishable by their energetics, geometries, and interactions with the surrounding water bath. The systems exhibiting the most stable SB excess electron states (supercooled water/air and Ih ice/air interfaces) are identified by their characteristic hydrogen-bonding motifs which are found to contain double acceptor-type water molecules in the close vicinity of the electron. These surface states correlate reasonably with those extrapolated to infinite size from simulated water cluster anions.     

Molecular Dynamics Simulation of the Excited-State Dynamics of Bacteriorhodopsin

Abstract— The excited-state dynamics of bacteriorhodopsin was studied by molecular dynamics simulation. For the purpose of suppressing large displacement of amino acid residues on the surface of bacteriorhodopsin, positional restraints were imposed on these residues. A new method was developed to investigate the movement of amino acid residues upon photoexcitation and their role on the ultrafast photoisomerization of the chromophore. The structural change of bacteriorhodopsin was then traced up to 200 fs, i.e. until the formation of the intermediate I. We found that when all the conjugated bonds of the chromophore were allowed to twist freely in the excited state, many bonds including the C13=C14 bond twist in large scale within 100 fs. When only the C13=C14 bond and the single bonds were allowed to twist freely, the twisting took place at most 20° within 200 fs. From these results, it is claimed that a special potential surface is provided for the C13=C14 bond twisting by the protein environment in the course of the isomerization reaction, giving rise to the specific, ultrafast photoisomerization of bacteriorhodopsin. As a trace of such a mechanism, we observed that several functionally important residues incuding Asp85, Asp212 and Tyr185 responded quickly to the photoexcitation of the chromophore.     

Microscopic inhomogeneity and ultrafast orientational motion in an organic photovoltaic bulk heterojunction thin film studied with 2D IR vibrational spectroscopy

Two-dimensional infrared vibrational spectroscopy is used to examine conformational inhomogeneity and ultrafast orientational motion within local environments of an organic photovoltaic bulk heterojunction thin film. The bulk heterojunction material consists of a mixture of the electron donor poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-(1-cyanovinylene)phenylene] (CN-MEH-PPV) and the electron acceptor [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). PCBM species reside in a distribution of environments within large domains of the molecules that cause their C=O stretch modes to be inhomogeneously broadened. The molecular inhomogeneity also results in frequency dependent vibrational relaxation dynamics. The butyric acid methyl ester group of PCBM undergoes ultrafast wobbling-in-the-cone orientational motion on the 110 fs time scale within a cone semiangle of 29 degrees . The vibrational dynamics are sensitive metrics of molecular order in the material and have implications for charge mobility and degradation phenomena in organic photovoltaic devices. This report represents the first study of organic photovoltaic materials using ultrafast two-dimensional infrared vibrational spectroscopy.     

Molecular dynamics study of the structure and mechanism of formation of molecular associates in aqueous solutions of surfactants

The process of micelle formation in an aqueous solution of the surfactant was simulated by the computer experiment. It was established by the molecular dynamics method that micelles are formed through the formation of premicellar associates of the surfactant. The practical absence of an attraction between single molecules of sodium pentadecyl sulfonate (SPDS) and premicellar associates dissolved in water was shown for a SPDS—water system. The function of the radial distribution of Na⁺ counterions towards polar groups of SPDS molecules in water and on the surfaces of micelles and premicellar associates was studied by the molecular dynamics method. The presence of dissociated and non-dissociated polar groups of the SPDS molecular on the micelle surface was found. The data obtained are consistent with the existence concepts on micelle formation processes.     

Fluorescence Dynamics of Coumarin C522 as a Function of Micelle Confinement along with Cyclodextrin Supramolecular Complex Formation

Our aim is to doubly confine a molecule of coumarin C522 in a host–guest supramolecular complex with β‐cyclodextrin in a reverse sodium dioctyl sulfosuccinate (AOT) micelle using nonpolar n‐heptane and polar water solvents. Varying the volumes of coumarin C522 and β‐cyclodextrin dissolved in water allows us to control the water‐pool diameters of the reverse micelle in n‐heptane with values of w=3, 5, 10, 20, and 40, where w is the ratio of water concentration to AOT concentration in n‐heptane. To study the fluorescence dynamics of coumarin C522, the spectral steady‐state and time‐resolved dependences are compared for the two systems coumarin C522(water)/AOT(n‐heptane), denoted C522/micelle, and coumarin C522/β‐cyclodextrin(water)/AOT(n‐heptane), referred to as C522/CD/micelle. The formation of the supramolecular host–guest complex CD–C522 is indicated by a blue shift, but in the micelle, the shift is red. However, the values of the fluorescence maxima at 520 and 515 nm are still way below the value of 535 nm representing bulk water. The interpretation of the red shift is based on two complementary processes. The first one is the confinement of CD and C522 by the micelle water pool and the second is the perturbation of the micelle by CD and C522, resulting in an increase of the water polarity. The fluorescence spectra of the C522/micelle and C522/CD/micelle systems have maxima and shoulders. The shoulder intensities at 440 nm, representing the C522 at n‐heptane/AOT interface, decrease as the w values decrease. This intensity shift suggests that the small micelle provides a stronger confinement, and the presence of CD shifts the equilibrium from n‐heptane towards the water pool even more. The fluorescence emission maxima of the C522/micelle and C522/CD/micelle systems for all w values clearly differentiate two trends for w=3–5, and w=10–40, suggesting different interaction in the small and large micelles. Moreover, these fluorescence maxima result in 7 and 13 nm differences for w=3 and w=5, respectively, and provide the spectral evidence to differentiate the C522 confinement in the C522/micelle and C522/CD/micelle systems as an effect of the CD molecule, which might be interpreted as a double confinement of C522 in CD within the micelle. The ultrafast decay in the case of w=3 ranges from 9.5 to 16 ps, with an average of 12.6 ps, in the case of the C522/micelle system. For C522/CD/micelle, the ultrafast decay at w=3 ranges from 9 to 14.5 ps, with an average of 11.8 ps. Increasing w values (from 10 to 40) result in a decrease of the ultrafast decay values in both cases to an average value of about 6.5 ps. The ultrafast decays of 12.6 and 11.8 ps for C522/micelle and C522/CD/micelle, respectively, are in the agreement with the observed red shift, supporting a double confinement in the C522/CD/micelle(w=3) system. The dynamics in the small and large micelles clearly show two different trends. Two slopes in the data are observed for w values of 3–5 and 10–40 in the steady‐state and time‐resolved data. The average ultrafast lifetimes are determined to be 12.6 and 6.5 ps for the small (w=3) and the large (w=40) micelles, respectively. To interpret the experimental solvation dynamics, a simplified model is proposed, and although the model involves a number of parameters, it satisfactory fits the dynamics and provides the gradient of permittivity in the ideal micelle for free water located in the centre (60–80) and for bound water (25–60). An attempt to map the fluorescence dynamics of the doubly confined C522/CD/micelle system is presented for the first time.     

The effect of the counterion on water mobility in reverse micelles studied by molecular dynamics simulations

In this study, mobility and structure of water molecules in Aerosol OT (bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles with water content w0 = 5 and Na+, K+, Cs+ counterions have been explored with molecular dynamics (MD) simulations. Using the Faeder/Ladanyi model (J. Phys. Chem. B, 2000, 104, 1033) of the reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function, FS(Q,t), for water hydrogen atoms that could be measured in a quasielastic neutron scattering experiment. Separate intermediate scattering functions FRS(Q,t) and FCMS(Q,t) were determined for rotational and translational motion. We find that the decay of FCMS(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior arises from decreased water mobility for molecules close to the interface and from confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay, which is consistent with relatively rapid restricted rotation and slower rotational relaxation over the full angular range. Rotational relaxation is anisotropic, with the O-H bond short-time rotational mobility considerably higher than that of the molecular dipole. This behavior is related to the decreased density of water-water hydrogen bonds in the vicinity of the interface compared to core or bulk water. We find that the interfacial mobility of water molecules is quite different for the three counterion types, but that the core mobility exhibits weak counterion dependence. Differences in interfacial mobility are strongly correlated with structural features, especially ion-water coordination, and the extent of disruption by the counterions of the water hydrogen bond network.