Water Self-diffusion Research Articles

Concentration dependence of water self-diffusion coefficients in dilute glycerol–water binary and glycerol–water–sodium chloride ternary solutions and the insights from hydrogen bonds

Water self-diffusion coefficients for glycerol–water binary and glycerol–water–sodium chloride ternary solutions at low glucose concentrations have been predicted using the mean square displacements method. It was found that the water self-diffusion coefficient decreases as the glycerol concentration increases. The reasons for the decrease have been analysed from the viewpoint of the hydrogen bonds.

Highly proton conducting sulfonic acid functionalized mesoporous materials studied by impedance spectroscopy, MAS NMR spectroscopy and MAS PFG NMR diffusometry

Proton conducting mesoporous Si-MCM-41 materials with different amounts of SO3H-groups were prepared by the co-condensation method and investigated by NMR spectroscopy, NMR diffusometry and impedance spectroscopy. The successful incorporation of mercaptopropyltrimethoxysilane (MPMS) into the mesoporous framework was proven by 29Si NMR measurements. The deconvolution of the 29Si MAS NMR spectra shows that Si atoms of the functionalizing silane are linked to the MCM-41 framework via three or two ≡Si–O–Si≡ bonds. 29Si MAS NMR proves that almost all MPMS was incorporated into the mesoporous framework, since 36.9% of the signal belongs to T groups, whereas the concentration of MPMS in the synthesis solution amounts 40%. 13C CP MAS spectroscopy confirms that the majority of the organic functional groups remained intact after the oxidation in 30 wt.-% H2O2. The proton conductivity was investigated by impedance spectroscopy (IS). Drastic differences were found for different degrees of functionalization. The maximum conductivity was found for the sample with maximum loading of sulfonic acid groups (40% functionalization) as σdc=0.19 S cm−1 at 413K. The large conductivity differences between 20% and 40% functionalization result in large differences in the diffusion coefficients of the charge carrier by application of the Nernst–Einstein equation. But only small differences of the water self-diffusion coefficients were measured by magic-angle spinning pulsed field gradient (MAS PFG) NMR diffusometry. The comparison of IS and MASPFG NMR results allows the determination of the Haven factor, which amounts 5 660 for 20% and 329 for 40% functionalization. We explain the proton conductivity in functionalized MCM-41 by structure diffusion. The drastic increase of conductivity (at 353K from 9.51 to 260×10−5Scm−1) from 20% to 40% functionalization is caused by the reduction of the activation energy of the charge relocation in a denser lattice of proton donator sites.

Some Anomalies in the Self-Diffusion of Water in Disordered Carbons

The unusual dynamics of water under confinement is important to a number of conventional and emerging industrial processes and to life in general; nevertheless, our understanding of this critical area is still in an early stage. Nanoporous carbons have provided a key material through which to investigate this dynamics; however, existing simulations have been based on the use of idealized slit pore carbons or carbon nanotubes with smooth energy landscapes and fail to capture the influence of structural disorder inherent to real carbons. We show here that the irregular structure of such carbons critically influences the dynamics and the mode of diffusion (single file, subdiffusion or Fickian). Our molecular dynamics simulations, using a realistic hydrophobic carbon model based on hybrid reverse Monte Carlo simulation of the structure of an activated carbon fiber, show the existence of a single-file diffusion mode between the ballistic and Fickian modes in the narrowest pore regions of this material, not seen in simulations using model 2-D slit pores. A rich variety of behavior is found in this subdiffusion regime, with the fits of time-dependent mean square displacement to the power law of time (∝tα), revealing that the exponent α varies significantly with temperature, especially at low temperatures (273–350 K). It reaches a minimum value of 0.5 at 298 K corresponding to the single-file diffusion regime and approaches unity at 610 K, corresponding to the Fickian mode. It is demonstrated that confinement effects lead to the experimentally observed non-Arrhenius behavior of the water dynamics at low temperatures (<350 K), for both the realistic carbon model and an idealized 2-D slit pore model, due to the transport of the adsorbed water as a large cluster in a water monolayer.

An NMR study of sorption-diffusion properties of MF-4SK-carbon composite membranes in aqueous methanol solutions

The distribution of water and methanol and their self-diffusion coefficients (SDCs) in carbon-doped MF-4SK perfluorinated membranes have been studied by NMR. It has been shown that at a concentration of 12–35 wt % in the external solution, the methanol concentration inside the membrane is higher by a factor of 1.4–2 than that in the external solution and increases with increasing the carbon phase content. It has been found that SDC values for water and methanol in the membrane decrease with an increase in the C phase content. In a membrane with 23 wt % C, the SDC of methanol is reduced by a factor of 1.6. It has been shown that the SDC of methanol in these membranes is below that of water by a factor of 1.5. The activation energies of self-diffusion of water and methanol have been determined. It has been found that they remain almost unaffected by the C content and have close values of 19–20 kJ/mol, which are 2–4 kJ/mol above the activation energy of self-diffusion of pure water in these membranes.

Self-Diffusion of water in compressed hexagonal phases

Self-Diffusion of water in compressed hexagonal phases

Effect of water content on microstructures and oxygen permeation in PSiMA–IPN–PMPC hydrogel: a molecular simulation study

Grand canonical Monte Carlo combined with molecular dynamics simulation methods are used to investigate the microstructure and oxygen permeability in hydrogels containing interpenetrating poly(bis(trimethylsilyloxy)methylsilylpropyl glycerol methacrylate) (PSiMA) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) networks at varying water contents. Water molecules are classified into hydrophilic-hydration water, hydrophobic-hydration water and intermediate water according to their distribution in the hydrogel. As the hydrogel becomes increasingly swollen, three kinds of water molecules concurrently move faster. This leads to the increased water self-diffusion coefficient as the hydrogel's water content is increased. On the contrary, increasing water content in the hydrogel is associated with the decrease of oxygen permeability. This decrease is mainly attributed to the unique transport path of oxygen molecules in the specific hydrogel. Oxygen molecules mainly transport through the PSiMA networks. While the increasing amount of hydrophobic-hydration water molecules occupy oxygen's sorption sites within PSiMA. In addition, higher amount of hydrophobic-hydration water lessens the continuity of free volume in the silicone phase. As a consequence, both oxygen's solubility and diffusivity in the hydrogel are reduced. This contribution anticipates providing some guidance and inspiration for developments of next-generation hydrogel contact lenses.

A combination of the tree-code and IPS method to simulate large scale systems by molecular dynamics

An IPS/Tree method which is a combination of the isotropic periodic sum (IPS) method and tree-based method was developed for large-scale molecular dynamics simulations, such as biological and polymer systems, that need hundreds of thousands of molecules. The tree-based method uses a hierarchical tree structure to reduce the calculation cost of long-range interactions. IPS/Tree is an efficient method like IPS/DFFT, which is a combination of the IPS method and FFT in calculating large-scale systems that require massively parallel computers. The IPS method has two different versions: IPSn and IPSp. The basic idea is the same expect for the fact that the IPSn method is applied to calculations for point charges, while the IPSp method is used to calculate polar molecules. The concept of the IPS/Tree method is available for both IPSn and IPSp as IPSn/Tree and IPSp/Tree. Even though the accuracy of the Coulomb forces with tree-based method is well known, the accuracy for the combination of the IPS and tree-based methods is unclear. Therefore, in order to evaluate the accuracy of the IPS/Tree method, we performed molecular dynamics simulations for 32,000 bulk water molecules, which contains around 10(5) point charges. IPSn/Tree and IPSp/Tree were both applied to study the interaction calculations of Coulombic forces. The accuracy of the Coulombic forces and other physical properties of bulk water systems were evaluated. The IPSp/Tree method not only has reasonably small error in estimating Coulombic forces but the error was almost the same as the theoretical error of the ordinary tree-based method. These facts show that the algorithm of the tree-based method can be successfully applied to the IPSp method. On the other hand, the IPSn/Tree has a relatively large error, which seems to have been derived from the interaction treatment of the original IPSn method. The self-diffusion and radial distribution functions of water were calculated each by both the IPSn/Tree and IPSp/Tree methods, where both methods showed reasonable agreement with the Ewald method. In conclusion, the IPSp/Tree method is a potentially fast and sufficiently accurate technique for predicting transport coefficients and liquid structures of water in a homogeneous system.

Characterization of polymer electrolyte Nafion membranes: Influence of temperature, heat treatment and drying protocol on sorption and transport properties

We carried out systematic sorption, proton conductivity and water diffusion measurements in commercial Nafion N115 and Nafion NRE212 membranes. Because of the high dispersion of data in the literature, our primary objective was to gather a comprehensive set of experimental measurements and compare them with published results. Our simple, reproducible, experimental protocol allowed us to measure these important membrane properties over a wide range of experimental conditions. The membrane samples were heat-treated at different temperatures and the effect of thermal history on water sorption capacity and transport properties was investigated. We show that water uptake, proton conductivity and water self-diffusion coefficient decrease with increasing drying temperature. As a consequence, these variations must systematically be taken into account when comparing literature values to each other. The effect of temperature on sorption has also been investigated. The measurements show that the water sorption capacity of heat-treated (60 °C during 24 h) N115 and NRE212 membranes measured in vapor phase decreases with temperature while it increases in liquid phase. In membranes that were not heat-treated, however, the sorption in liquid water is independent of temperature.

Liquid Water: From Symmetry Distortions to Diffusive Motion

Water deviates from tetrahedral symmetry on different scales, creating "defects" that are important for its dynamics. In this Account, I trace the manifestations of these distortions from the isolated molecule through gas-phase clusters to the liquid phase. Unlike the common depiction, an isolated water molecule has a nonsymmetric charge distribution: although its positive charge is localized at the hydrogens, the negative charge is smeared between the lone-pair sites. This creates a "negativity track" along which a positive charge may slide. Consequently, the most facile motion within the water dimer is a reorientation of the hydrogen-bond (HB) accepting molecule (known as an "acceptor switch"), such that the donor hydrogen switches from one lone pair to the other. Liquid water exhibits asymmetry between donor and acceptor HBs. Molecular dynamics simulations show that the water oxygens accepting HBs from the central molecule are spatially localized, whereas water hydrogens donating HBs to it are distributed along the negativity track. This asymmetry is manifested in a wider acceptor- versus donor-HB distribution. There is a higher probability for a water molecule to accept one (trigonal symmetry) or three HBs than to donate one or three HBs. A simple model can explain semiquantitatively how these distributions evolve by distorting perfectly tetrahedral water. Just two reactions are required: the dissociation of a HB between a double-donor donating to a double-acceptor, D(2)···A(2), followed by a switching reaction in which a HB donor rotates its hydrogen between two double-acceptor molecules. The preponderance of D(2)···A(2) dissociation events is in line with HB "anticooperativity", whereas positive cooperativity is exhibited by conditional HB distributions: a molecule with more acceptor bonds tends to have more donor bonds and vice versa. Quantum mechanically, such an effect arises from intermolecular charge transfer, but it is observed even for fixed-charge water models. Possibly, in the liquid state this is partly a collective effect, for example, a more ordered hydration shell that enhances the probability for both acceptor and donor HBs. The activation energy for liquid water self-diffusion is considerably larger than its HB strength, pointing to the involvement of collective dynamics. The remarkable agreement between the temperature dependence of the water self-diffusion coefficient and its Debye relaxation time suggests that both share the same mechanism, likely consisting of coupled rotation and translation with collective rearrangement of the environment. The auto-correlation function of a hydrogen-bonded water molecule pair is depicted quantitatively by the solution of the diffusion equation for reversible geminate recombination, up to long times where the ubiquitous t(-3/2) power law prevails. From the model, one obtains the HB dissociation and formation rate coefficients and their temperature dependence. Both have a similar activation enthalpy, suggesting rapid formation of HBs with alternate partners, perhaps by the HB switching reaction involving the trigonal site. A detailed picture of how small fluctuations evolve into large-scale molecular motions in water remains elusive. Nonetheless, our results demonstrate how the plasticity of water can be traced to its asymmetric charge distribution, with duality between tetrahedral and trigonal ligation states.

Inorganic–organic membranes based on Nafion, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] nanoparticles. Part II: Relaxations and conductivity mechanism

Two classes of hybrid inorganic–organic proton-conducting membranes consisting of Nafion and either [(ZrO2)·(HfO2)0.25] or [(SiO2)·(HfO2)0.28] nanofiller are investigated to elucidate their relaxations and conductivity mechanism and are labeled [Nafion/(ZrHf)x] and [Nafion/(SiHf)x], respectively. The membranes are studied by dynamic mechanic analysis (DMA) and broadband electric spectroscopy (BES). The latter technique allows a determination of the direct current ionic conductivity (σDC) and the proton diffusion coefficient (DH+). Pulse-field-gradient spin-echo nuclear magnetic resonance experiments (PFGSE-NMR) are carried out to determine the water self-diffusion coefficients (DH2O). DH+ and DH2O are correlated to obtain insight on the conductivity mechanism of the proposed materials. Results indicate that the nanofiller particles play a major role in the proton conduction mechanism of the proposed materials. It is demonstrated that the basic [(ZrO2)·(HfO2)0.25] nanoparticles form Nafion–nanofiller dynamic cross-links with high ionic character. These cross-links improve the mechanical properties and enhance the overall proton conductivity of the membranes at low humidification levels owing to an efficient delocalization of the protons. In [Nafion/(SiHf)x] membranes, the dynamic cross-links occur due to dipole–dipole interactions between the side groups of the Nafion host polymer and the quasi-neutral [(SiO2)·(HfO2)0.28] nanoparticles. These cross-links significantly reduce the delocalization of the protons, which decreases the overall conductivity of materials.

Segregation of hydroxide ions to an ice surface

Hydroxide ions that are initially buried within an ice film segregate to the ice film surface at elevated temperatures. This process was observed by conducting experiments with an ice film constructed with a bottom H(2)O layer and an upper D(2)O layer, with an excess of hydroxide ions trapped at the H(2)O/D(2)O interface as they were generated by Na hydrolysis. The transport of hydroxide ions from the interfacial layer to the surface was examined as a function of time using a low energy sputtering method. The progress of the H/D exchange reaction in surface water molecules was also monitored with the Cs(+) reactive ion scattering technique. At 90 K, only a small portion of buried hydroxide ions moved to the surface in the form of OD(-) species. This was due to hydroxide transport via proton hopping through a D(2)O layer, 3 BL thick, in the surface region. At 135 K, at which point water self-diffusion is active in the ice film, the majority of the buried hydroxide ions segregated to the surface after ∼1 h. Both OH(-) and OD(-) species were produced at the surface, at an OH(-)/OD(-) population ratio ≥1. Based on kinetic measurements for the transport of OH(-) and OD(-) species and the H/D exchange of surface water molecules, we concluded that the major transport channel for hydroxide ions in this regime is the migration of molecular hydroxide species. H/D exchange reactions also occur between surface hydroxide ions and water molecules. No evidence was observed for the occurrence of the hop-and-turn process at 135 K, although it is known as an important mechanism of proton transport in ice.

Effect of temperature on structure and water transport of hydrated sulfonated poly(ether ether ketone): A molecular dynamics simulation approach

The effects of temperature on hydrated sulfonated poly(ether ether ketone) are studied using molecular dynamics. Three different temperature conditions (298 K.15 K, 323.15 K, and 353.15 K) with two different water contents (10 wt. % and 20 wt. %) are simulated. Analyzing the pair correlation functions, it is found that there is limited temperature effect on the distribution and solvation of the sulfonate groups. The structure factor analysis shows that the temperature dependence of the nanophase-segregated morphology is not significant in the simulated temperature range. On the contrary, the structure factors S(q) at ∼30 Å (q = ∼0.2 Å−1) and ∼13 Å (q = ∼0.5 Å−1) clearly increase with water content, indicating that the development of water channels is mostly affected by the water content. Within such water phase in the nanophase-segregated structure, the internal structure of water phase becomes more developed with decreasing temperature and increasing water content. By analyzing the mean square displacement of the water molecules, it is also found that self-diffusion of water is enhanced with the increasing temperature. From the observation that the activation energies calculated from such temperature dependency are very similar (Ea = 25.7 kJ/mol and Ea = 24.9 kJ/mol for 10 wt. % and 20 wt. %, respectively), it is inferred that the extent of the structural change in the water phase as a function of temperature is very similar between the 10 wt. % water content and the 20 wt. % water content. Compared to the bulk water (13.2 kJ/mol) and the water in Nafion (16.7-18.9 kJ/mol), it is confirmed that more nanophase-segregation enhances water transport through the membrane.

Water Self-Diffusion in Glassy and Liquid Maltose Measured by Raman Microscopy and NMR

Raman microscopy and isotope labeling have been used for the first time to measure water self-diffusion in carbohydrate glasses. Together with pulsed-gradient stimulated-echo NMR, this method yielded the self-diffusion coefficients of water in amorphous maltose over 8 orders of magnitude, from the liquid to the glassy state. There are consistencies and major differences between our data and those obtained by evaporative drying. Water diffusion is remarkably fast in maltose glasses, decouples from maltose diffusion, and is not strongly affected by the glass transition.

Roles of 2D Liquid in Reduction of the Glass-Transition Temperature of Thin Molecular Solid Films

To clarify the properties of a liquidlike phase formed on the surface, self-diffusion of molecules in monolayer and multilayer films has been investigated by measuring their uptake behaviors in porous Si substrates using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. It is found that self-diffusion of water, ethanol, and 3-methylpentane in the first monolayer commences at temperatures of ca. 110, 85, and 50 K, respectively, which are considerably lower than the corresponding glass-transition temperatures (Tg) in the bulk. The surface mobility of water appears to be not influenced by hydrogen bonds with substrates: The molecules form droplets on the H-terminated Si substrate at 110 K, whereas they start to diffuse into pores of the OH-terminated Si substrate at the same temperature. The ethanol and 3-methylpentane molecules tend to diffuse into pores of both hydrophilic and hydrophobic Si substrates, although a small number of much slower diffusers remain on the surface. Multilayer films are also incorporated into pores at temperatures well below Tg because of sequential diffusion of the topmost-layer molecules. The origins of nanoconfinement effects on reduction of Tg are discussed based on these findings.

Is the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate [emim][MeSO 3] capable of rigidly binding water?

The binary system of water and the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate, [emim][MeSO 3], was carefully studied with the initial hypothesis that water might be tightly bound to the ionic liquid up to water mole fractions of 0.5, which would explain why water has been observed to seemingly be deactivated for interfering with water sensitive chemical reactions. Measurement results as a function of composition (generally 0–0.9 water mole fractions) and temperature (generally 40–85 °C) were obtained for heat capacity, heats of dissolution, density, viscosity, conductivity, as well as NMR measurements on diffusion, proton chemical shift and T 1 relaxation times of cation, anion and water. The combined results do not confirm our initial hypothesis. An activation energy analysis showed that the same barrier hinders translational motion (self-diffusion) of anion, cation and water, as well as momentum transfer (viscosity) and the water T 1 relaxation. While the activation energy was observed to be linearly dependent on the mol fraction composition, most of the measured physicochemical properties show linear or near linear dependencies to the composition expressed in mass%. A further detailed analysis of the combined experimental data is supportive that the ionic liquid medium remains highly structured even when loaded with water to very high mole fractions. The main structural motif is hypothesized to contain nonpolar domains in close resemblance to micellar aggregation. Thus, the deactivation of the water in chemical reactions may rather be explained by a highly structured ionic liquid framework keeping water physically separated from the reactant.

Effect of Solutes and Matrix Structure on Water Mobility in Glycerol−Agar−Water Gel Systems: A Nuclear Magnetic Resonance Approach

Nuclear magnetic resonance spectroscopy (NMR) has been widely used to determine water molecular mobility in food systems. This study aimed to examine the effects of matrix structure and solutes on the dynamics of water molecules in model mixed systems, glycerol-agar-water gels, using low- and high-resolution NMR. Simple models to explain water relaxation rates and self-diffusion coefficients in mixed systems were developed using the experimental values obtained for the individual binary systems (glycerol-water solutions and agar-water gels). The spin-lattice relaxation of mixed systems was influenced by interactions of both glycerol and agar with water, while the spin-spin relaxation of mixed systems was dominated by the interaction of agar with water. Water diffusion was influenced by not only molecular interactions between all components but also the gel matrix structure. These models are able to differentiate the effect of solutes from that of matrix structure on water molecular dynamics.

Hydration properties and proton exchange in aqueous sugar solutions studied by time domain nuclear magnetic resonance

Proton NMR relaxation rates (R1 and R2) were measured in aqueous solutions of sucrose, d-glucose and d-fructose with increasing concentration. The measurements were carried out using Bruker PC 20 NMR Process Analyzer. Inversion recovery and CPMG pulse sequences are used for the measurement of relaxation rates. Results show that the values of relaxation rate increase as the concentration of the sugar is increased. The relaxation rate appears to be higher for sucrose solutions as compared to glucose or d-fructose solutions. These results were discussed on the basis of molecular association between sugar and water molecules through hydrogen bonding. The water self diffusion coefficient was measured in these sugar solutions by using pulse field gradient NMR method. As expected, the water self-diffusion coefficient was reduced with increased sugar concentrations. The results of translational mobility exhibited a higher mobility for fructose than glucose or sucrose in aqueous solutions.The dependence of R2 on the inter-pulse delay of the CPMG sequence gives information on the proton exchange mechanism involved. The mechanism of exchange was studied using R2 with increasing inter pulse delay of 0.05–2.0ms in aqueous solutions of 10%, 20% and 35% (w/v) of the above sugar solutions. From the plots of relaxation rates (R2) versus the 90°–180° pulse spacing it was possible to calculate the proton exchange rate (kb) of the different sugar solutions. Relaxation rates show characteristic variations with CPMG pulse spacing which can be interpreted on the basis of chemical exchange between solute and solvent molecules.The experimental results namely relaxation rates and CPMG pulse spacing data show the importance of water interactions with sweet molecules and this can lead to a better understanding of the effect of hydration water in taste chemoreception.

Human aquaporin 4 gating dynamics in dc and ac electric fields: A molecular dynamics study

Water self-diffusion within human aquaporin 4 has been studied using molecular dynamics (MD) simulations in the absence and presence of external ac and dc electric fields. The computed diffusive (p(d)) and osmotic (p(f)) permeabilities under zero-field conditions are (0.718 ± 0.24) × 10(-14) cm(3) s(-1) and (2.94 ± 0.47) × 10(-14) cm(3) s(-1), respectively; our p(f) agrees with the experimental value of (1.50 ± 0.6) × 10(-14) cm(3) s(-1). A gating mechanism has been proposed in which side-chain dynamics of residue H201, located in the selectivity filter, play an essential role. In addition, for nonequilibrium MD in external fields, it was found that water dipole orientation within the constriction region of the channel is affected by electric fields (e-fields) and that this governs the permeability. It was also found that the rate of side-chain flipping motion of residue H201 is increased in the presence of e-fields, which influences water conductivity further.

NMR studies of proton transport in fuel cell membranes at sub-freezing conditions

Water uptake activities and transport properties are critical for water management in fuel cell membranes. In this work, three perflourosulfonic acid (PFSA) fuel cell membranes, including Nafion®-117 and two Gore membranes, were evaluated at different relative humidity controlled conditions. These fuel cell membranes were studied using variable temperature 1H spin–lattice relaxation times (T1) and pulsed field gradient (PFG) NMR techniques in the temperature range of 298 to 239 K. Water self-diffusion coefficients and proton transport activation energies in the fuel cell membranes were obtained from the PFG-NMR experiments. The results show that the water self-diffusion coefficients increase with increasing hydration level, and decrease with decreasing temperature. The water molecular motion is significantly slowed at low temperatures; however, the water molecules in these membranes are not frozen, even at 239 K. The water uptake activity and diffusivity in these membranes were compared as a function of temperature and hydration level. At the same temperature and hydration level, the water self-diffusion coefficients of two Gore fuel cell membranes are higher than that of Nafion®-117. This is attributed to the lower EW of the Gore membranes. The presence of an expanded polytetrafluoroethylene (ePTFE) reinforcing layer in the membrane also has an impact on water diffusivity.

Hyper-mobility of water around actin filaments revealed using pulse-field gradient spin-echo 1H NMR and fluorescence spectroscopy

This paper reports that water molecules around F-actin, a polymerized form of actin, are more mobile than those around G-actin or in bulk water. A measurement using pulse-field gradient spin-echo 1H NMR showed that the self-diffusion coefficient of water in aqueous F-actin solution increased with actin concentration by ∼5%, whereas that in G-actin solution was close to that of pure water. This indicates that an F-actin/water interaction is responsible for the high self-diffusion of water. The local viscosity around actin was also investigated by fluorescence measurements of Cy3, a fluorescent dye, conjugated to Cys 374 of actin. The steady-state fluorescence anisotropy of Cy3 attached to F-actin was 0.270, which was lower than that for G-actin, 0.334. Taking into account the fluorescence lifetimes of the Cy3 bound to actin, their rotational correlation times were estimated to be 3.8 and 9.1 ns for F- and G-actin, respectively. This indicates that Cy3 bound to F-actin rotates more freely than that bound to G-actin, and therefore the local water viscosity is lower around F-actin than around G-actin.