Cascade diffusion of water molecules in a porous granular medium

Water vapor inleakage to a porous granular adsorbent was studied by diode laser spectroscopy. Stepping in the temporal variation of the diffusivity, probably associated with porous matrix space fractality, was detected. The H₂O molecular sensing technique is proposed as a tool for experimental study of the structure of porous media and processes occurring in them.     

An investigation into adsorption of simple organic and inorganic molecules on a silica gel surface

Adsorption of simple organic and inorganic molecules on an amorphized silica gel surface is studied within the framework of density functional theory in the approximation of periodic crystalline slabs. Adsorption of water, ammonia, acetone, and ethanol molecules is examined. Most probable adsorption sites of molecules on the surface are calculated and the adsorption energies and bond lengths of molecules on the silica gel surface are estimated.     

On the correlation between diffusion and self-diffusion processes of adsorbed molecules in a simple microkinetic model

Combining the formalism of transition state theory with an appropriate collisional term, a general relation between diffusion and self-diffusion coefficients (D and D¹ respectively) in zeolites is derived. For low pore filling factors, this relation reduces to Darken's equation, which is shown to coincide in this case with the simple equation $\text{D = (}\text{d}\text{dc}\text{)(D}{}^1\text{c)}$, where c denotes the concentration of adsorbate. Concerning the intracrystalline self-diffusion of ethane in NaCaA zeolites, it is shown, that up to pore filling factors θ ≈ 0.8, Darken's equation should be valid, whereas for higher concentrations considerable deviations from this simple relation may occur and the general formula, as derived in this paper, should be applied.     

Diffusion-controlled photoprocesses on chemically modified porous silica

Photoprocesses involving organic luminophores adsorbed on the surface of wide-porous silica chemically modified with n-alkyl hydrocarbons with different chain lengths are studied upon cw and pulsed laser excitation. The results of the study are interpreted within the framework of a diffusion model of acceleration of the exchange-resonance processes of triplet-triplet energy transfer and triplet-triplet homo-and hetero-annihilation of the interacting molecules.     

Probing four orders of magnitude of the diffusion time in porous silica glass with unconventional NMR techniques

The combined use of two unconventional NMR diffusometry techniques permits measurements of the self-diffusion coefficient of fluids confined in porous media in the time range from 100 microseconds to seconds. The fringe field stimulated echo technique (FFStE) exploits the strong steady gradient in the fringe field of a superconducting magnet. Using a standard 9.4 T (400 MHz) wide-bore magnet, for example, the gradient is 22 T/m at 375 MHz proton resonance and reaches 60 T/m at 200 MHz. Extremely short diffusion times can be probed on this basis. The magnetization grid rotating frame imaging technique (MAGROFI) is based on gradients of the radio frequency (RF) field. The RF gradients not necessarily need be constant since the data are acquired with spatial resolution along the RF gradient direction. MAGROFI is also well suited for unilateral NMR applications where all fields are intrinsically inhomogeneous. The RF gradients reached depend largely on the RF coil diameter and geometry. Using a conic shape, a value of at least 0.3 T/m can be reached which is suitable for long-time diffusion measurements. Both techniques do not require any special hardware and can be implemented on standard high RF power NMR spectrometers. As an application, the influence of the tortuosity increasing with the diffusion time is examined in a saturated porous silica glass.     

Shock compression of simple molecules

A simple approach is developed to calculate shock compression of simple molecules. This approach is based upon an accurate analytic representation of the Lennard-Jones fluids in conjunction with the Enskog theory, which is used to calculate the molecular diameter as a function of temperature along the Hugoniot. The model permits rapid, yet reliable calculations. It is applied to N₂, O₂, H₂, D₂, CH₄, CO, and CO₂. The results are tested by the comparison with experimental data and with other calculations. The computed Hugoniots agree reasonably with experimental results for many (but not all) simple molecules and are comparable to those of more complicated models.     

Surface diffusion of lead on tungsten

Field electron microscopy is used to study the surface diffusion of lead on tungsten. A simple method to measure rough values of the diffusion coefficient and its dependence on sub-monolayer coverage is described and tested. In the region around (001) the displacement energy found is about 1.30 eV/atom up to 10¹⁵ atoms/cm² where it decreases to 0.78 eV/atom. In the residual region except (110) this energy at 1.5×10¹⁴ atoms/cm² is 1.22 eV/atom, it decreases at 4 × 10¹⁴ atoms/cm² to 0.61 eV/atom and increases at 10¹⁵ atoms/cm² to 0.78 eV/atom. Corresponding values of the diffusion coefficient D and of the preexponential D₀ are given. The dependence of D on submonolayer coverage is discussed.     

Surface modification of porous poly(tetrafluoraethylene) film by a simple chemical oxidation treatment

A simple, inexpensive and environmental chemical treatment process, i.e., treating porous poly(tetrafluoroethylene) (PTFE) films by a mixture of potassium permanganate solution and nitric acid, was proposed to improve the hydrophilicity of PTFE. To evaluate the effectiveness of this strong oxidation treatment, contact angle measurement was performed. The effects of treatment time and temperature on the contact angle of PTFE were studied as well. The results showed that the chemical modification decreased contact angle of as-received PTFE film from 133 ± 3° to 30 ± 4° treated at 100 °C for 3 h, effectively converting the hydrophobic PTFE to a hydrophilic PTFE matrix. The changes in chemical structure, surface compositions and crystal structure of PTFE were examined by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), environmental scanning electron microscopy (ESEM), X-ray diffraction (XRD), respectively. It was found that the F/C atomic ratio decreased from untreated 1.65-0.10 treated by the mixture at 100 °C for 3 h. Hydrophilic groups such as carbonyl (CO) and hydroxyl (OH) were introduced on the surface of PTFE after treatment. Furthermore, hydrophilic compounds K_0.27MnO₂·0.54H₂O was absorbed on the surface of porous PTFE film. Both the introduction of hydrophilic groups and absorption of hydrophilic compounds contribute to the significantly decreased contact angle of PTFE.     

Adsorption and diffusion of F_2 molecules on pristine graphene

The adsorption and diffusion of F_2 molecules on pristine graphene are studied by using first-principles calculations.For the diffusion of F_2 from molecular state in gas phase to the dissociative adsorption state on graphene surface, a kinetic barrier is identified, which explains the inertness of graphene in molecular F_2 at room temperature, and its reactivity with F_2 at higher temperatures. Study of the diffusion of F_2 molecules on graphene surface determines the energy barrier along the optimal diffusion pathway, which conduces to the understanding of the high stability of fluorographene.     

One-step synthesis of dye-incorporated porous silica particles

Fluorescent nanoparticles have a variety of biomedical applications as diagnostics and traceable drug delivery agents. Highly fluorescent porous silica nanoparticles were synthesized in a water/oil phase by a microemulsion method. What is unique about the resulting porous silica nanoparticles is the combination of a single-step, efficient synthesis and the high stability of its fluorescence emission in the resulting materials. The key of the success of this approach is the choice of a lipid dye that functions as a surrogate surfactant in the preparation. The surfactant dye was incorporated at the interface of the inorganic silica matrix and organic environment (pore template), and thus insures the stability of the dye?Csilica hybrid structure. The resulting fluorescent silica materials have a number of properties that make them attractive for biomedical applications: the availability of various color of the resulting nanoparticle from among a broad spectrum of commercially dyes, the controllablity of pore size (diameters of ~5?nm) and particle size (diameters of ~40?nm) by adjusting template monomer concentration and the water/oil ratio, and the stability and durability of particle fluorescence because of the deep insertion of surfactant??s tail into the silica matrix.     

NMR of mercury in porous carbon and silica gel

The temperature dependences of the integrated intensity and of the Knight shift of ¹⁹⁹Hg NMR signals are measured for liquid and solid mercury introduced into porous carbon and silica gel. A decrease in the temperature of completion of crystallization and a small temperature hysteresis (from 4 to 9 K) between melting and crystallization are observed. The melting temperature of mercury in pores coincides with that in the bulk. The ¹⁹⁹Hg NMR signal from crystalline mercury under the condition of restricted geometry is observed for the first time. It is established that the Knight shift for liquid and crystalline mercury in pores is smaller than in the bulk.     

Surface diffusion of adsorbates

The review covers studies of the surface diffusion of adsorbed species on single crystal substrates. Experimental techniques employed in investigations of the surface diffusion of adsorbates are discussed. Emphasis is placed on the analysis of data obtained at considerable degrees of coverage. These results demonstrate a close interrelation between surface diffusion and phase transitions in adsorbed layers. Possible mechanisms of surface diffusion are considered taking into account the structure of adsorbed layers. Findings on the effect of electric fields on surface diffusion processes are reviewed.     

Surface diffusion on heterogeneous correlated substrates

The behaviour of the surface diffusion coefficient on an heterogeneous correlated surface is studied in the framework of the lattice-gas model. Using the multivariate adsorptive energy distribution function the statistical properties of the heterogeneous correlated surface can be appropriately described and the effect of the heterogeneity on the collective diffusion coefficient at finite coverage can be analized through a correlation length. Monte Carlo simulation is also performed to test the analytical results.     

Development of porous silica production by hydrothermal method

Abstract

A new process for porous silica production has been developed using a hydrothermal method. Hydrothermally synthesized calcium silicate was used as the starting material in this study, which was produced from a mixture of Ca(OH)₂ and amorphous silica (white carbon) under hydrothermal conditions of 140°C and 0.4 MPa, for 8 hours. The calcium silicate was subsequently treated with an acid solution, facilitating the leaching of Ca ions. After washing with pure water, the multant Ca²⁺ -free silica powder was allowed to dry. The Ca²⁺ -free silica powder was found to have an amorphous structure, with 0.9 ml/g pore volume, up to 610m²/g BET specific surface area, and an average 5 ～ 8 nm pore size. Our hydrothermal process is simple and low cost, and is anticipated to have numerous applications to the petrochemical industry.     

Nonlinear diffusion of water molecules in glycerol

The dependence of the concentration of water molecules in water vapor brought into contact with glycerol surface was measured in a time window of 10⁻¹–10³ s. An obvious nonlinearity of water molecule diffusion was found in a short time approximation (within a minute). A proposal is made with regard to the convective character of diffusion due to heat release during absorption.