Spin-probe ESR study on the dynamics of liquid molecules in the MCM-41 nanochannel: temperature dependence on 2-propanol and water

A spin-probe ESR study has been made on the dynamics of 2-propanol and water molecules in the nanochannel of MCM-41 at various temperatures. In the former system, 2-propanol is separated into two phases: one with molecules immobilized in the ESR time scale and the other with mobile ones, even at temperatures more than 40 degrees higher than the bulk melting point. In the case of water, on the other hand, only the "immobilized" water was detected at a temperature as high as 313 K. At higher temperature, spin-probe molecule undergoes anisotropic rotational diffusion to reduce resistance from the solvent molecules in the nanochannel. These results are explained in relation to the intermolecular network intensified in the nanochannel. Static as well as dynamic structures of these solutions have been discussed.     

Diffusion of guest molecules in MCM-41 agglomerates

The pulsed field gradient nuclear magnetic resonance method has been used to study self-diffusion of cyclohexane in a commercial MCM-41 material at different external gas pressures from zero to saturated vapor pressure. It is found that the effective diffusivities exhibit three different regions with increasing pressure: decrease at low pressures, a sudden drop at intermediate pressures, and increase at higher pressures. In addition, in the region of irreversible adsorption (hysteresis loop) the diffusivities are also found to differ on the adsorption and the desorption branches. A simple analytical model taking account of different molecular ensembles with different transport properties due to the complex architecture of the porous structure is developed which provides a quantitative prediction of the experimental data. The analysis reveals that the effective diffusivity is predominantly controlled by the adsorption properties of the individual mesoporous MCM-41 crystallites which, in combination with high transport rates, provide a simple instrument for fine tuning of the transport properties by a subtle variation of the external conditions.     

Determination of the sticking coefficient and scattering dynamics of water on ice using molecular beam techniques

The sticking coefficient for D(2)O impinging on crystalline D(2)O ice was determined for incident translational energies between 0.3 and 0.7 eV and for H(2)O on crystalline H(2)O ice at 0.3 eV. These experiments were done using directed molecular beams, allowing for precise control of the incident angle and energy. Experiments were also performed to measure the intensity and energy of the scattered molecules as a function of scattering angle. These results show that the sticking coefficient was near unity, slightly increasing with decreasing incident energy. However, even at the lowest incident energy, some D(2)O did not stick and was scattered from the ice surface. We observe under these conditions that the sticking probability asymptotically approaches but does not reach unity for water sticking on water ice. We also present evidence that the scattered fraction is consistent with a binary collision; the molecules are scattered promptly. These results are especially relevant for condensation processes occurring under nonequilibrium conditions, such as those found in astrophysical systems.     

Molecular dynamics study of translational and rotational diffusion in liquid ortho-terphenyl

NVT molecular dynamics simulations were performed on liquid o-terphenyl as a function of temperature in the range 320-480 K. Computed translational diffusion coefficients displayed the non-Arrhenius behavior expected of a fragile glass-forming liquid and were in good, semiquantitative agreement with experimental results. Rotational correlation functions calculated for various vectors within the molecule exhibited a very short time (0-1 ps) initial decay, followed by a reversal, which corresponds to free reorientation within the "solvent" cage prior to collision with a wall. Rotational correlation times of three orthogonal vectors fixed on the central benzene were close to equal at all temperatures, indicating nearly isotropic overall molecular reorientation. The average correlation times exhibited a non-Arrhenius temperature dependence and were in very good agreement with experimental values derived from 2D and 1H NMR relaxation times. Correlation times of vectors located on the lateral phenyl rings were used to calculate the "spinning" internal rotation diffusion coefficients, which were approximately twice as great as the overall rotational diffusion constants, indicating rapid internal rotation of the phenyl side groups over wide ranges of angle in the liquid.     

Neutron scattering study of water confined in periodic mesoporous organosilicas

A series of quasi-elastic neutron scattering measurements were performed using IN6 at the Institute Laue Langevin for a mesoporous organosilica material with phenyl functions, called phenyltriethoxysilane (PTES). The aim of the experiment was to study the diffusion dynamics of nano-scale water clusters inside the hydrophobic pores as a function of temperature and hydration. By fitting the Debye-Waller factor, the data show clearly the different behavior between water, both inside and outside the hydrophobic pores, which resembles bulk water. The mean thermal displacement 〈u²〉 of the external water increases with T almost linearly up to 353 K, while the internal water quickly reaches the maximum at T∼323 K, indicating the confinement by an averaged pore diameter of the porous organosilica.     

Interlayer water molecules in vanadium pentoxide hydrate. IX. Anisotropic translational diffusion leading to anisotropic ac conductivity

The anisotropy of the dynamic properties of interlayer water molecules along the a and b axes of vanadium pentoxide hydrate, orthorhombic V2O5.nH2O, was studied using quasielastic neutron scattering (QENS) in relation to the anisotropy of the ac conductivity. The QENS spectra were analyzed using a stretched exponential function and a Lorentzian function. Both methods showed that the double-layer water molecules along the b axis are more mobile than those along the a axis. The difference in mobility between the two axes is more pronounced using a Lorentzian function analysis. These facts suggest that the diffusion coefficient of water molecules along the b axis is larger than that along the a axis, which is closely related to the ac conductivity originating from proton hopping. The anisotropy of the dynamic motion of water molecules can be attributed to the shorter b-axis length (b=3.60 A), with respect to the longer and less regular repetition of the atomic arrangements along the a axis (42.34 A).     

Molecular dynamics study of anisotropic translational and rotational diffusion in liquid benzene

Equilibrium NPT and NVT molecular dynamics simulations were performed on liquid benzene over an extended range of temperature (from 260 to 360 K) using the COMPASS force field. Densities and enthalpies of vaporization (from cohesive energy densities) were within 1% of experiment at all temperatures. tumbling and spinning rotational diffusion coefficients, D(perpendicular) and D(parallel), computed as a function of temperature, agreed qualitatively with the results of earlier reported experimental and computational investigations. Generally, it was found that D(parallel)/D(perpendicular) approximately 1.4-2.5 and the activation energy for tumbling was significantly greater than for spinning about the C6 axis [Ea(D(perpendicular)) = 8.1 kJ mol(-1) and Ea(D(parallel)) = 4.5 kJ mol(-1)]. Calculated translational diffusion coefficients were found to be in quantitative agreement with experimental values at all temperatures [deviations were less than the scatter between different reported measurements]. In addition, translational diffusion coefficients were computed in the molecule-fixed frame to yield values for Dxy (diffusion in the plane of the molecule) and Dz (diffusion perpendicular to the plane). It was found that the ratio Dxy/Dz approximately 2.0, and that the two coefficients have roughly equal activation energies. This represents the first atomistic molecular dynamics study of translational diffusion in the molecular frame.     

Entanglement in polystyrene detected by translational diffusion of photo-labeled molecules

The contribution of polymer diffusion studies to clarifying the entanglement problem in polymer chain liquids is reviewed. In particular, we present a phenomenological equation for a critical molecular weight M_C as a function of the diffusant and matrix molecular weights, respectively, and compare it with recent theories.     

Structure of methanol confined in MCM-41 investigated by large-angle X-ray scattering technique

Large-angle X-ray scattering (LAXS) measurements over a temperature range from 223 to 298 K have been made on methanol confined in mesoporous silica MCM-41 with two different pore diameters, 28 A (C14) and 21 A (C10), under both monolayer and capillary-condensed adsorption conditions. To compare the structure of methanol in the MCM-41 pores with that of bulk methanol, X-ray scattering intensities for bulk methanol in the same temperature range have also been measured. The radial distribution functions (RDFs) for the monolayer methanol samples showed that methanol molecules are strongly hydrogen bonded to the silanol groups on the MCM-41 surface, resulting in no significant change in the structure of adsorbed methanol with respect to the pore size and temperature. On the other hand, the RDFs for the capillary-condensed methanol samples showed that hydrogen-bonded chains of methanol molecules are formed in both pores. However, the distance and number of hydrogen bonds estimated from the RDFs suggested that hydrogen bonds between methanol molecules in the pores are significantly distorted or partly disrupted. It has been found that the hydrogen bonds are more distorted in the smaller pores of MCM-41. With decreasing temperature, however, the hydrogen-bonded chains of methanol in the pores were gradually ordered. A comparison of the present results on methanol in MCM-41 pores with those on water in the same pores revealed that the structural change with temperature is less significant for confined methanol than for confined water.     

Permeability of micelles in surfactant-containing MCM-41 silica as monitored by embedded dye molecules

Mesoporosu (MCM-41 type) silica containing surfactant-embedded Congo Red has been prepared and tested against gas phase HCl and ammonia, as well as solutions of ionic species; it is shown that the hybrid (organic-inorganic) material is permeable to both gases and ionic species, and can act as a pH indicator and as a selective chelating agent.     

纳米MCM-41组装DBC-偶氮胂的研究

以纳米MCM-41孔道为模版,组装制备了二溴对氯偶氮胂(DBC-ASA)纳米簇合物。采用化学分析、粉末X-射线衍射、77 K N2吸附-解吸附、红外光谱、固体扩散漫反射光谱及发光光谱表征了制备的(MCM-41)-(DBC-ASA)主-客体纳米复合材料。结果表明,DBC-ASA成功地组装在了主体MCM-41中,并且处于主体分子筛材料的纳米孔道中。(MCM-41)-(DBC-ASA)纳米复合材料具有作为发光材料的应用前景。     

Nuclear dynamics and spectator effects in resonant inelastic soft x-ray scattering of gas-phase water molecules

The electronic structure of gas-phase H(2)O and D(2)O molecules has been investigated using resonant inelastic soft x-ray scattering (RIXS). We observe spectator shifts for all valence orbitals when exciting into the lowest three absorption resonances. Strong changes of the relative valence orbital emission intensities are found when exciting into the different absorption resonances, which can be related to the angular anisotropy of the RIXS process. Furthermore, excitation into the 4a(1) resonance leads to nuclear dynamics on the time scale of the RIXS process; we find evidence for vibrational coupling and molecular dissociation in both, the spectator and the participant emission.     

Molecular dynamics simulation study on the transient response of solvation structure during the translational diffusion of solute

The transient response function of the density profile of the solvent around a solute during the translational diffusion of the solute is formulated based on the generalized Langevin formalism. The resultant theory is applied to both neat Lennard-Jones fluids and cations in liquid water, and the response functions are obtained from the analysis of the molecular dynamics simulations. In the case of the self-diffusion of Lennard-Jones fluids, the responses of the solvation structures are in harmony with conventional pictures based on the mode-coupling theory, that is, the binary collision in the low-density fluids, the backflow effect from medium to high density fluids, and the backscatter effect in the liquids near the triple point. In the case of cations in water, the qualitative behavior is strongly dependent on the size of cations. The pictures similar to simple dense liquids are obtained for the large ion and the neutral molecule, while the solvent waters within the first solvation shell of small ions show an oscillatory response in the short-time region. In particular, the oscillation is remarkably underdumped for lithium ion. The origin of the oscillation is discussed in relation to the theoretical treatment of the translational diffusion of ions in water.     

An ab initio molecular dynamics study on hydrogen bonds between water molecules

The quantitative estimation of the total interaction energy of a molecular system containing hydrogen bonds (H bonds) depends largely on how to identify H bonding. The conventional geometric criteria of H bonding are simple and convenient in application, but a certain amount of non-H bonding cases are also identified as H bonding. In order to investigate the wrong identification, we carry out a systematic calculation on the interaction energy of two water molecules at various orientation angles and distances using ab initio molecular dynamics method with the dispersion correction for the Becke-Lee-Yang-Parr (BLYP) functionals. It is shown that, at many orientation angles and distances, the interaction energies of the two water molecules exceed the energy criterion of the H bond, but they are still identified as H-bonded by the conventional "distance-angle" criteria. It is found that in these non-H bonding cases the wrong identification is mainly caused by short-range interaction between the two neighbouring water molecules. We thus propose that, in addition to the conventional distance and angle criteria of H bonding, the distance d(H···H) between the two neighbouring hydrogen atoms of the two water molecules should also be taken as a criterion, and the distance r(O···H) between the hydrogen atom of the H-bond donor molecule and the oxygen atom of the acceptor molecule should be restricted by a lower limit. When d(H···H) and r(O···H) are small (e.g., d(H···H) < 2.0 ? and r(O···H) < 1.62 ?), the repulsion between the two neighbouring atoms increases the total energy of the two water molecules dramatically and apparently weakens the binding of the water dimer. A statistical analysis and comparison of the numbers of the H bonds identified by using different criteria have been conducted on a Car-Parrinello ab initio molecular dynamics simulation with dispersion correction for a system of 64 water molecules at near-ambient temperature. They show that the majority of the H-bonds counted by using the conventional criteria combined with the d(H···H) criterion and the restriction of r(O···H) match what is identified by the binding energy criteria (e.g., E ≤ -10 kJ/mol), while some of them still have a binding energy that exceeds the energy criterion, indicating that the complicated quantum effects in H bonding can only be described by the three geometric parameters to a certain extent.     

Adsorption of organic molecules on the highly ordered MCM-41 sorbent modified by different amounts of melamine

Thermodynamic characteristics of adsorption of organic molecules on the ordered organosilylated composite MCM-41 with grafted trimethylsilyl groups (MMet) and modified by melamine deposited on the surface were studied by inverse gas chromatography. Mole changes in the internal energy and entropy of adsorption, the dispersion and specific components of the value proportional to the Helmholtz absorption energy, and the contribution of the methyl group to the heat of adsorption were calculated from the specific retention volumes. The change in the retention volumes was established to be determined by the dispersion component of the Helmholtz energy of adsorption, whereas the energy of specific interactions remains almost unchanged. After the adsorbent surface was coated by the second and third melamine layers, the changes in ?ΔU and ?ΔS for adsorption of the members of the homologous series of n-alkanes are so small that lie within admissible measurement inaccuracies. This phenomenon can be explained by the regularities of alkane adsorption in slit-like pores of the supramolecular structure of melamine.     

Adsorption and structure of hydrocarbons in MCM-41: a computational study

The adsorption of linear, branched, and cyclic hydrocarbons in MCM-41 is studied using Configurational Bias Monte Carlo simulations. A new computational model for MCM-41 is proposed which, although simple, is able to predict adsorption isotherms which are in agreement with the scarce experimental data. The structure of the adsorbed phase is analyzed and found to be similar to that of studies using small, hard spheres trapped in pores. The adsorption of mixtures is investigated, and the adsorption hierarchy is discussed. The structure of the adsorbed mixture is revealed and shows that all components of the mixture exhibit structure, even if they are only adsorbed in small quantities. Finally, the model is modified to include surface roughness and the effect on the adsorption isotherms and structure of the adsorbed phase is discussed.