Aqueous solvation of amphiphilic solutes: concentration and temperature dependent study of the ultrafast polarisability relaxation dynamics

An understanding of the influence of hydrophilic and hydrophobic interactions on the dynamics of solvating water molecules is important in a diverse range of phenomena. The polarisability anisotropy relaxation dynamics of aqueous solutions of the amphiphiles TBA (t-butyl alcohol) and TMAO (trimethylamine N-oxide) have been measured as a function of concentration and temperature. TMAO is shown to have a greater effect on the picosecond relaxation dynamics of water than TBA. This result is consistent with hydrophilic interactions being mainly responsible for the slowing down the polarisability relaxation in aqueous solutions. The room temperature Raman spectral densities of the two solutions are remarkably similar to that of bulk water, an effect which is tentatively ascribed to the formation of nanoscale structure in the solutions, allowing the formation of bulk-like water pools. The temperature dependent spectral density of TMAO remains similar to that of bulk water at all temperatures, while that for TBA shows a marked decrease in the amplitude of the response usually ascribed to a water-water stretch with increasing temperature. This is discussed in terms of the temperature dependent structure of TBA aggregates in solution.     

In situ infrared and EXAFS studies of an Ag cluster in zeolite X

The in situ measurements of infrared spectra and the Ag K-edge EXAFS spectra of the fully Ag⁺ exchanged zeolite X (Ag₈₆–X) were carried out from room temperature to 300 °C under vacuum. By evacuation at room temperature the O–H stretch vibration (ν(O–H)) mode around 3 μm disappears and the coordination number of oxygen around Ag, N_Ag–O, decreases due to removal of water molecules. The T–O asymmetric stretch (ν_as(T–O)) mode associated with zeolite framework oxygen appears around 10 μm. These infrared spectra are fitted by summing up Gaussian peaks. The positions of the main two peaks are 1000 and 1100 cm⁻¹ at room temperature. At 100 °C, a third infrared peak appears at around 955 cm⁻¹, the total N_Ag–O becomes small and the coordination number of Ag around Ag, N_Ag–Ag, is 0.5. These results suggest that Ag atoms change sites in the zeolite and play an important role as a precursor of the Ag clusters. At 300 °C, the peaks around 1000 and 1100 cm⁻¹ shift to 1050 and 1140 cm⁻¹, respectively, and N_Ag–Ag becomes 2.9, which indicates that the Ag clusters attached to the zeolite framework are stabilized at high temperature. When the zeolite with Ag clusters is exposed to atmosphere, it is found that: (1) the ν(O–H) mode around 3 μm appears again, (2) there are two main peaks (1000 and 1100 cm⁻¹) and a small peak around 856 cm⁻¹ and (3) the local structure of the Ag clusters formed at 300 °C never reverses.     

Dilectric and thermal properties of PEO doped with cadimium chloride salt

The main aim of this research is to investigate the effect of salt concentration on the dielectric properties(AC (σ_AC),permittivity（ε′）,dielectric loss（ε″）,and dielectric relaxation process) and melting behavior of polyethylene oxide （PEO）/CdCl₂ complexes.The dielectric study was carried out over a frequency range 10-335 kHz and a temperature range 25-45℃.The AC conductivity,permittivity and dielectric loss of the PEO/CdCl₂ complexes increase with increasing salt concentration and temperature.Also,it was found that the addition of CdCl₂ salt to PEO host reduced the melting temperature of PEO host.Dielectric results reveal that the relaxation process of these complexes is due to viscoelastic relaxation or non-Debye relaxation at room temperature.Additionally,it was found that relaxation behavior remained viscoelastic at different temperatures and salt concentrations.     

Vibrational relaxation of CN stretch of pseudo-halide anions (OCN-, SCN-, and SeCN-) in polar solvents

The vibrational relaxation dynamics of pseudo-halide anions XCN- (X = O, S, Se) in polar solvents were studied to understand the effect of charge on solute-to-solvent intermolecular energy transfer (IET) and solvent assisted intramolecular vibrational relaxation (IVR) pathways. The T1 relaxation times of the CN stretch in these anions were measured by IR pump/IR probe spectroscopy, in which the 0-1 transition was excited, and the 0-1 and 1-2 transitions were monitored to follow the recovery of the ground state and decay of the excited state. For these anions in five solvents, H2O, D2O, CH3OH, CH3CN, and (CH3)2SO, relaxation rates followed the trend of OCN- > SCN- > SeCN-. For these anions and isotopes of SCN-, the relaxation rate was a factor of a few (2.5-10) higher in H2O than in D2O. To further probe the solvent isotope effect, the relaxation rates of S12C14N-, S13C14N-, and S12C15N- in deuterated methanols (CH3OH, CH3OD, CH3OH, CD3OD) were compared. Relaxation rate was found to be affected by the change of solvent vibrational band at the CN- stretching mode (CD3 symmetric stretch) and lower frequency regions, suggesting the presence of both direct IET and solvent assisted IVR relaxation pathways. The possible relaxation pathways and mechanisms for the observed trends in solute and solvent dependence were discussed.     

Ultrafast vibrational dynamics of interfacial water

We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.     

Poly-1-oxy-2-phenyltrimethylene as studied by 1H pulsed low resolution NMR: a possible oxygen scavenger

Aromatic polymers can adsorb a large amount of oxygen on their aromatic rings. In semicrystalline aromatic polymers, the amount of adsorbed oxygen is much larger in the amorphous phase than in the crystalline counterpart. As a consequence, fully amorphous aromatic polymers are more suitable as oxygen scavengers than semicrystalline polymers.A pulsed low resolution ¹H-NMR relaxation study on semicrystalline and fully amorphous poly-1-oxy-2-phenyltrimethylene (St–CO), deuteriated and not deuteriated on the backbone, is reported. T₁ relaxation values were measured at 30 and 57 MHz and compared with the values of two aromatic polymers previously studied, syndiotactic polystyrene (s-PS) and polyphenyleneoxide (PPO), both patented as oxygen scavengers. For all these polymers, using a set of equations, at each temperature, the amount of adsorbed oxygen was calculated.Very short T₁ values are observed at the low temperature point of PPO which is the best oxygen scavenger at low temperature, while St–CO adsorbs oxygen efficiently at room temperature. Thus, St–CO might be suitable to be used as an oxygen scavenger.In St–CO, a ¹H T_1ρ relaxation study on the rotating frame has also been performed. In the atactic copolymer, in the temperature range 150–160 K, a sharp transition was observed only in the presence of oxygen. In agreement with a previously given interpretation for analogous data, the observed transition might be related to low frequency motions present in low molecular weight components.     

O-H stretch in phenol and its hydrogen-bonded complexes: band position and relaxation pathways

Anharmonic force fields are a suitable means for identification of vibrational degrees of freedom responsible for the peculiar shape of molecular spectra and the existence of diverse relaxation pathways. In this contribution, we investigated interactions that govern the position of the O-H stretching band in phenol and its dimers with water and ammonia. Dominant couplings are identified, and the nature of relaxation channels is analyzed. The effect of hydrogen bonding on O-H stretching motion and vibrational energy redistribution time through intra- and intermolecular interactions is studied, and possible vibrational predissociation upon O-H stretch excitation is addressed. The results based on computed anharmonic force constants are in accord with the available experimental findings.     

Rotational structure of water in a hydrophobic environment: carbon tetrachloride

Infrared spectroscopy has been used to probe the interaction between water and the hydrophobic solvent, carbon tetrachloride. At room temperature, water exists as monomers in carbon tetrachloride, presenting a system for studying the rotational properties of water free of strong hydrogen-bonding. The rotational structure suggests a very anisotropic motion consisting of essentially free rotation about the symmetry axis and highly hindered rotation about the two perpendicular axes of the asymmetric water molecule. The rotational lifetime is significantly shortened relative to gas-phase water. An upper limit of 0.93 ps is deduced from the spectrum. Interaction with carbon tetrachloride also slightly enhances the intensity of the symmetric stretch. The results are compared with results of interactions between water and the cations Li+, Na+, K+, and Cs+. It is concluded that the attractive interaction is between the oxygen of water and the electropositive carbon of carbon tetrachloride.     

半晶PBT在室温存放时的聚态结构变化

用DSC观察了不同冷却条件半晶PBT样品在室温存放时所发生的聚态结构变化。实验表明,在310～325K出现的热焓松弛峰的峰高和峰温随存放时间的变化与冷却条件有关。该峰表征试样中“硬无定形部分”的松弛。本文定量地描述了它的峰高与峰温对存放时间的依赖关系。发现水对松弛过程和随之发生的冷结晶有一定程度的抑制。室温存放过程中,晶态结构缺陷有,所减少并少量新的亚稳态晶体形成。     

The temperature dependences and methods for the functional representation of the rates of proton magnetic relaxation in aqueous solutions of electrolytes

The paper presents the results of temperature dependence measurements for the rate of proton relaxation (1/T ₁) in sea water with salinity 35‰ over the temperature range −22−+120°C at atmospheric pressure and in some salt solutions at two concentrations (0.5 and 1 mol/l). The possibility of approximating the temperature dependences of magnetic relaxation rates by various functions in pure water, sea water, and solutions of salts with various concentrations was studied. The parameters of these dependences and trends of their variations under the influence of salt components are reported. The most well grounded method for the functional representation of the temperature dependences of 1/T ₁ is the use of the sum of exponents with the number of terms depending on solution concentration. This representation takes into account structural changes in solutions as the concentration grows and corresponds to the Frenkel model of the thermal motion of molecules in aqueous solutions of electrolytes. The combined use of the parameters of the temperature dependences of the rate of relaxation represented by various functions can be a mutually augmenting method for studying the dynamic properties of aqueous solutions of electrolytes with low and moderate concentrations.     

Viscoelastic properties of Nafion at elevated temperature and humidity

Tensile stress–strain and stress relaxation properties of 1100 equivalent weight Nafion have been measured from 23 to 120 °C at 0–100% relative humidity. At room temperature, the elastic modulus of Nafion decreases with water activity. At 90 °C, the elastic modulus goes through a maximum at a water activity of ～ 0.3. At temperatures ≥90 °C, hydrated membranes are stiffer than dry membranes. Stress‐relaxation was found to have two very different rates depending on strain, temperature, and water content. At high temperature, low water activity, and small strain, the stress relaxation displays a maximum relaxation time with stress approaching zero after 10³–10⁴ s. Water absorption slows down stress‐relaxation rates. At high water activity, the maximum stress relaxation time was >10⁵ s at all temperatures. No maximum relaxation time was seen at T ≤ 50 °C. Increasing the applied strain also resulted in no observed upper limit to the stress relaxation time. The results suggest that temperature, absorbed water, and imposed strain alter the microstructure of Nafion inducing ordering transitions; ordered microstructure increases the elastic modulus and results in a stress relaxation time of >10⁵ s. Loss of microphase order reduces the elastic modulus and results in a maximum stress relaxation time of 10³–10⁴ s. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 11–24, 2009.     

Effect of temperature on the mechanical properties of casein-g-poly(acrylonitrile) films

Casein was grafted with acrylonitrile in a water medium using potassium persulphate, at 60°C. The reaction mixture of acrylonitrile grafted casein was converted into films by drying the reaction mixture over a mercury surface at room temperature. Stress-strain characteristics of these films were studied at different temperatures. Stress relaxation and mechanical hysteresis were also studied as a function of temperature. The results obtained from the above experiments show that the properties of the films change above 60°C. These results suggest that the changes in the films take place due to the effect produced by polyacrylonitrile [T_g of poly(acrylonitrile) (PAN) is 80°C] chains present in the composite. The morphological studies of the fractured samples suggest that at 40°C, failure takes place in the films by crazing and shear yielding mechanisms and the material is multiphasic in nature.     

The temperature dependence of the solubility of oxygen in sea water according to the pulsed NMR data

The temperature dependences of the solubility of oxygen in distilled and sea water were obtained over the temperature range 0–90°C on the basis of proton spin—lattice relaxation measurements in degassed and not degassed samples. The temperature dependence of O₂ solubility is approximated by the sum of exponential functions according to not only relaxation measurement results but also literature data. The temperature dependence of the solubility of O₂ in distilled water can be written as the sum of two exponential functions with the predominance of the first component over the low-temperature interval. The best approximation for sea water has the form of the sum of three exponential functions, the relative contributions of the second and third components being approximately equal. The conclusion was drawn from the temperature dependences obtained that the main mechanism of the solution of oxygen in water was the filling of ice-like formation voids.     

On the triplet state of poly(N-vinylcarbazole)

Triplet state properties including transient triplet absorption spectrum, intersystem crossing yields in solution at room temperature and phosphorescence spectra, quantum yields and lifetimes at low temperature as well as singlet oxygen yields were obtained for poly(N-vinylcarbazole) (PVK) in 2-methyl-tetrahydrofuran (2-MeTHF), cyclohexane or benzene. The results allow the determination of the energy value for the lowest lying triplet state and also show that triplet formation and deactivation is a minor route for relaxation of the lowest excited singlet state of PVK. In addition, they show the triplet state is at higher energy than reported heavy metal dopants used for electrophosphorescent devices, such that if this is used as a host it will not quench their luminescence.     

Radiation deterioration in mechanical properties and ion exchange capacity of Nafion N117 swelling in water

The base form Nafion N117 samples swelling in the oxygen-saturated distilled water were irradiated with γ-rays or with electron beams at various doses up to 1600 kGy at room temperature or at 343 K to obtain detailed information on the effect of oxygen on the radiation deterioration in the Nafion mechanical properties and in the ion exchange capacity. The contribution of the radical reactions where oxygen molecules did not participate was dominant for the radiation deterioration in the mechanical properties of the Nafion N117 membrane swelling in the oxygen-saturated distilled water at room temperature. The effect of oxygen molecules was not significant due to the little oxygen concentration in the distilled water even though oxygen was saturated in the water. The Nafion N117 membrane irradiated with γ-rays at 343 K became a weak and brittle material, since the rise of the temperature activated the reactions. Oxygen molecules, in contrast, have negligible part in the reactions at 343 K due to their lower solubility in the water. We observed the negligible effect of the radiation sources as well as of the dose rate below 10 kGy/h on the radiation deterioration in Nafion N117 mechanical properties at room temperature. The irradiation of the Nafion membrane with γ-rays or with electron beams is one of the acceleration tests of the Nafion degradation. We made clear that the dissolved fluorine measured using a fluoride ion meter is a hopeful index for the in situ judging of the Nafion mechanical deterioration. The ion exchange capacity of the membrane irradiated up to 1600 kGy was the 20% decrease of the initial capacity at room temperature. Nafion side chains terminating with –SO₃⁻ M⁺ groups were radiation-durable compared with the backbone.     

Water penetration in nylon 6,6: Absorption,desorption, and exchange studied by NMR microscopy

The uptake of water by nylon 6,6 [42DB Adipure (trade name of Dupont Canada Inc.)] at 100°C has been monitored by a combination of one-dimensional proton NMR spectroscopy, relaxation time (T₁ and T₂) measurements and proton microscopic NMR imaging techniques. The relaxation times of the water absorbed into the nylon matrix are very short at room temperature, (T₂ < 1 ms and T₁ ≈ 1 s) indicating that the water is located in a highly restricted environment and suggesting that strong interactions exist between the absorbed water and the polymer. The diffusion profiles measured at room temperature indicate that the diffusion of water into nylon 6,6 at 100°C is Case I Fickian diffusion. The spatial dependence of the T₂ relaxation time constant and its variation with the water content was also examined. The results reveal that both T₂ and T₂^* decrease toward the center of the sample in samples that have a concentration gradient of sorbed water. In fully saturated samples, no spatial dependence was observed. The overall values of T₂ and T₂^* are also observed to increase as a function of exposure time. An evaluation of the desorption process at room temperature and at 100°C was performed. A continuous, exponentially decreasing solvent profile was observed for the desorption process which again indicates Case I Fickian kinetics. The exchange process of external bulk and atmospheric water with deuterium oxide (D₂O) saturated nylon rods has also been studied using the microscopic imaging technique. © 1993 John Wiley & Sons, Inc.     

Time scales and pathways of vibrational energy relaxation in liquid CHBr3 and CDBr3

Molecular dynamics simulations are used in conjunction with Landau-Teller, fluctuating Landau-Teller, and time-dependent perturbation theories to investigate energy flow out of various vibrational states of liquid CHBr3 and CDBr3. The CH stretch overtone is found to relax with a time scale of about 1 ps compared to the 50 ps rate for the fundamental. The relaxation pathways and rates for the CD stretch decay in CDBr3 are computed in order to understand the changes arising from deuteration. While the computed relaxation rate agrees well with experiments, the pathway is found to be more complex than anticipated. In addition to the above channels for CH(D) stretch relaxation that involve only the hindered translations and rotations of the solvent, routes involving off-resonant and resonant excitations of solvent vibrational modes are also examined. Finally, the decay of energy from low frequency states to near-lying solute states and solvent vibrations are studied.     

Dielectric permittivity and relaxation of aqueous cesium iodide solutions

The high-frequency dielectric permittivity and losses of CsI solutions were studied at 288–323 K in the range of water dielectric permittivity dispersion (7–25 GHz). The low-frequency electrical conductivity of these solutions was measured, and ionic losses at high frequencies were calculated. The Debye or Cole-Cole relaxation model was used for describing the spectra. The low-frequency limits of these relaxation region were calculated, which are the static dielectric constants ɛ_S and well as dielectric relaxation times (τ) and activation enthalpies (ΔH _ɛ⁺⁺). The ɛ_S values decrease in going from water to a solution. In concentrated solutions, the slope of the plot of ɛ_S versus temperature become zero. The decrease in τ and gDH _ɛ⁺⁺ is evidence of the structure-breaking effect of ions on water. At elevated temperatures (313 K), the decrease in τ is minimal. At 323 K, τ slightly increases in going from water to a solution.     

Microwave dielectric properties of aqueous potassium iodide solutions as a function of temperature

The complex dielectric permittivity of aqueous KI solutions was studied for molalities of 0.50–4.01 m and temperatures of 288–323 K in the region of water dielectric permittivity dispersion. The values of high-frequency of dielectric permittivity (ε) and dielectric losses (ε″) were obtained at seven frequencies ranging between 7.5 and 25 GHz. The low-frequency electrical conductivity of the aforementioned solutions was measured for calculating ionic losses. A single relaxation process is observed in these solutions, fitted by the Debye or Cole-Cole equation with small distribution parameters. The static dielectric constant and dielectric relaxation time were studied as functions of temperature and concentration; the activation enthalpy of dielectric relaxation was calculated. The temperature dependence of the static dielectric constant was found to disappear in highly concentrated solutions. The structure-breaking effect on water caused by K⁺ and I⁻ ions was affirmed, this effect disappearing in going to elevated temperatures.