水在NanZSM-5型分子筛中吸附的研究: 分子模拟

利用分子动力学(MD)模拟退火的方法和巨正则系综Monte Carlo模拟方法(GCMC)研究了水在NanZSM-5型分子筛中的吸附行为, 计算结果与文献中报道的实验结果吻合较好. 在此基础上, 进一步预测了水在不同硅铝比的NanZSM-5型分子筛中的吸附性质, 计算结果显示: 分子筛骨架上的硅铝比会显著影响水分子的吸附量和吸附等温线, 随着硅铝比的降低, 水的吸附量增加; 水分子的吸附位置是在钠离子和铝原子的周围, 平均每个钠离子周围吸附4个水分子, 而当水的吸附量增大时, 水分子与分子筛骨架上的氧原子之间发生了氢键作用; 在吸附量相同的条件下, 水的吸附热随着硅铝比的降低而升高.     

Adhesion between Silica Particle and Mica Surfaces in Water and Electrolyte Solutions

An atomic force microscope (AFM) is used to study the adhesion between a silica sphere and a mica plate in pure water and solutions of monovalent cations (LiCl, NaCl, KCl, and CsCl). It is found that the adhesive force depends not only on the electrolyte concentration but also on the hydration enthalpy of cations and the contact time of the particle on the surface. Possible mechanisms by which the observed phenomena can be explained consistently are discussed extensively. It is suggested that the adhesive force is closely related to the structure of the layer of cations and water molecules adsorbed on the surfaces: the strong adhesive force is obtained when highly hydrated cations (Li(+), Na(+)) are adsorbed to form a thick but weakly adsorbed layer, while the weak adhesive force is observed when poorly hydrated cations (Cs(+), K(+)) are adsorbed to form a thin but strongly adsorbed layer. Copyright 2000 Academic Press.     

Molecular dynamics simulation studies of the conformation and lateral mobility of a charged adsorbate biomolecule: implications for estimating the critical value of the radius of a pore in porous media

The conformations, the values of the lateral transport coefficient of a charged biomolecule (desmopressin) in the adsorbed layer and in the liquid layers above the adsorbed layer, the potential energies of the interaction between the biomolecules located in different liquid layers with the charged solid surface and with the biomolecules in the adsorbed layer, the potential energies of the interaction between water molecules in the hydration layers surrounding the conformations of the biomolecules in different layers, as well as the structure and number of hydration layers between the different conformations of desmopressin, were determined by molecular dynamics simulation studies. The results show that the lateral mobility of the adsorbed desmopressin is approximately equal to zero and the value of the lateral transport coefficient of the biomolecule in the liquid layers located above the adsorbed layer increases as the distance of the liquid layer from the charged solid surface increases. But the values of the lateral transport coefficient of the biomolecule in the liquid layers above the adsorbed layer are lower in magnitude than the value of the transport coefficient of desmopressin along the direction normal to the charged solid surface in the liquid phase located above the vacant charged sites of the solid surface, and these differences in the values of the transport coefficients have important implications with respect to the replenishment of the biomolecules in the inner parts of a channel (pore), the overall rate of adsorption, and the form of the constitutive equations that would have to be used in macroscopic models to describe the mechanisms of mass transfer and adsorption in the pores of adsorbent media. Furthermore, a novel method is presented in this work that utilizes the information about the sizes of the conformations of the biomolecule in the adsorbed layer and in the liquid layers above the adsorbed layer along the direction that is normal to the charged solid surface, as well as the number and size of the hydration layers along the same direction, and could be used to estimate the value of the lower bound of the linear characteristic dimension of a pore (i.e., pore radius) in porous adsorbent media (e.g., porous adsorbent particles; skeletons of porous monoliths) in order to realize effective transport and overall adsorption rate.     

Structure of electrolyte solutions sorbed in carbon nanospaces, studied by the replica RISM theory

The replica RISM theory is used to investigate the structure of electrolyte solutions confined in carbonized polyvinylidene chloride (PVDC) nanoporous material, compared to bulk electrolyte solution. Comparisons are made between the models of electrolyte solution sorbed in the carbonized PVDC material and a single carbon nanosphere in bulk electrolyte solution. Particular attention is paid to the chemical potential balance between the species of the sorbed electrolyte solution and the bulk solution in contact with the nanoporous material. As a result of the strong hydrophobicity of the carbonized PVDC material in the absence of activating chemical groups, the densities of water and ions sorbed in the material are remarkably low compared to those in the ambient bulk solution. The interaction between water molecules and cations becomes strong in nanospaces. It turns out that, in carbon nanopores, a cation adsorbed at the carbon surface is fully surrounded by the hydration shell of water molecules which separates the cation and the surface. Distinctively, an anion is adsorbed in direct contact with the carbon surface, which squeezes a part of its hydration shell out. The tendency increases toward smaller cations, which are characterized as "positive hydration" ions. In the bulk, cations are not hydrated so strongly and behave similarly to anions. The results suggest that the specific capacitance of an electric double-layer supercapacitor with nanoporous electrodes is intimately related to the solvation structure of electrolyte solution sorbed in nanopores, which is affected by the microscopic structure of the nanoporous electrode.     

Dilatometric study of the adsorption of heavy-metal cations on goethite

The specific volumes of adsorption of Cd, Co, Cu, Ni, Pb, and Zn on goethite determined by means of the dilatometric method are 21, 32, 32, 31, 31, and 42 cm3/mol, respectively, and are independent of pH. The effect of NaCl (up to 0.5 mol dm(-3)) on the specific volume of adsorption is rather insignificant. The specific volume of precipitation of corresponding hydroxides (determined experimentally and calculated) is about 60 cm3/mol. Apparently, the adsorbed heavy-metal cations lose half of their hydration water. The adsorption constant decreases as the pressure increases, and the effect becomes significant at pressures of > 10(7) Pa, i.e., more than 1 km of water column.     

Hydration structure and free energy of biomolecularly specific aqueous dications, including Zn2+ and first transition row metals

The hydration of some of the alkaline earth divalent metal cations and first row transition metal cations is considered within the quasi-chemical theory of solutions. Quantum chemical calculations provide information on the chemically important interactions between the ion and its first-shell water molecules. A dielectric continuum model supplies the outer-shell contribution. The theory then provides the framework to mesh these quantities together. The agreement between the calculated and experimental quantities is good. For the transition metal cations, it is seen that the ligand field contributions play an important role in the physics of hydration. Removing these bonding contributions from the computed hydration free energy results in a linear decrease in the hydration free energy along the period. It is precisely such effects that molecular mechanics force fields have not captured. The implications and extensions of this study to metal atoms in proteins are suggested.     

Phasenumwandlungserscheinungen in Lecithin/Wasser-Systemen

1. By means of differential-scanning-calorimetry the phase transition temperatures and -enthalpies were determined and evaluated for the three following lecithin/water systems: 1,2-dimyristoyl-lecithin/water; 1,2-dipalmitoyl-lecithin/water; 1,2-distearoyllecithin/water.
2. The preparation of the lecithin/water mixtures was made by adsorption of water from the gaseous phase. The adsorption isotherms were evaluated by the BET equation.
3. Four phase transitions were found for the monohydrates of the lecithins. The parameters depend systematically on the length of the alkyl residues.
4. In the heterogeneous two phase region the main-transition and the pre-transition occurred. The thermodynamical parameters of both transitions depend on the alkyl chain length.
5. Whole the results refer to the conclusion that the lecithin head group hydration is a stepwise process. The hydration of the first shell is finished if 5 to 6 molecules water per molecule lecithin are present, while the second hydration shell is complete when about 13 water molecules are adsorbed

     

Osmotic ensemble methods for predicting adsorption-induced structural transitions in nanoporous materials using molecular simulations

Osmotic framework adsorbed solution theory is a useful molecular simulation method to predict the evolution of structural transitions upon adsorption of guest molecules in flexible nanoporous solids. One challenge with previous uses of this approach has been the estimation of free energy differences between the solid phases of interest in the absence of adsorbed molecules. Here we demonstrate that these free energy differences can be calculated without reference to experimental data via the vibrational density of states of each phase, a quantity that can be obtained from molecular dynamics simulations. We show the applicability of this method through case studies of the swelling behaviors of two representative systems in which swelling upon adsorption of water is of importance: single-walled aluminosilicate nanotube bundles and cesium montmorillonite. The resulting predictions show that the aluminosilicate nanotube bundles swell significantly with increasing interstitial adsorption and that the layer spacing of cesium montmorillonite expands up to about 12.5 A?, giving good agreement with experiments. The method is applicable to a wide range of flexible nanoporous materials, such as zeolites, metal-organic frameworks, and layered oxide materials, when candidate structures can be defined and a force field to describe the material is available.     

Energies of Intermolecular Interaction and Hydration of Benzene Derivatives Molecules Adsorbed on Active Carbon from an Aqueous Solution

Energy heterogeneity of an adsorbent surface does not affect the isotherm of adsorption from aqueous solutions. Therefore, the balance of the changes in the Gibbs energy resulted from the adsorption of sparingly water-soluble substances shows that, in the whole range of filling of the adsorption phase, the pattern of the adsorption isotherm depends on the difference between the energy of the interaction of adsorbed molecules with each other and the energy of their hydration needed to displace water molecules from the adsorption phase. The standard adsorption energy of molecules of benzene and its derivatives from aqueous solutions and the difference between the energy of their interaction in the adsorption phase and the energy of their hydration are determined by the extrapolation of adsorption isotherms of these substances to the standard conditions ( 0, C 0) and to the conditions of maximal approach of the adsorbed molecules to each other ( 1). The hydration energy of the molecules of benzene and its derivatives is calculated based on the proportionality of this energy to the sum of the concentration potentials of the components in a saturated solution, where the proportionality coefficient is equal to the number of water molecules interacting with one organic molecule. Calculated energies of phenol and aniline hydration are equal to the energies of H-bonds of phenol (OH···OH₂) and amino (NH···OH₂) groups. Hydration energies of phenol and aniline derivatives vary according to the effect of a substituent in the benzene ring on the H-bond energy. Negative values of hydration energy of polar hydrophobic molecules result from their hydrophobic effect on water structure. The interaction energy of hydrated and dehydrated adsorbed molecules, whose benzene ring planes are oriented in parallel to the carbon surface, is found from the calculated hydration energies. For H-bond-forming molecules, this energy is equal to the energy of one H-bond formed upon the surface dimerization. The energy of repulsion between polar hydrophobic molecules of benzene derivatives depends on the vertical component of the dipole moment, and is the higher the larger the polar group volume.     

Chemical composition and ion exchange properties of zeolites

Series of exchange selectivity for alkali cations in zeolites change in a regular manner in dependence on the ratio (z) between the number of particles capable of solvating cations in the frameworks of zeolites (molecules of zeolite water, one-fourth of the framework oxygen ions) and the number of zeolite cations. For z=5–6 zeolites have maximum selectivity for large cations, for z<3–4 they selectively absorb sodium, while if z<2 they selectively absorb lithium. An analysis is given of the reasons of this dependence of the ion exchange selectivity of zeolites on the chemical composition of their frameworks. It is shown that the free energy of solvation of cations in zeolites is less than the free energy of their hydration. The effect of transformation of the selectivity series of zeolites can be connected with the contribution, on the part of the effect of redistribution of water between the solid and liquid phases of the ion exchange system, to the free energy of ion exchange reactions, which differs in dependence on the composition and structure of the crystals.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 378–382, May–June, 1990.     

Self-localization of polyacrylic acid molecules on polar ZnO(0001)-Zn surfaces

The adsorption of single polyacrylic acid (PAAc) molecules was investigated on stepped hydroxide-stabilized polar ZnO(0001)-Zn surfaces using atomic force microscope (AFM) topography and force distance spectroscopy. Stepped surfaces of ZnO(0001)-Zn were prepared by a wet chemical etching procedure and PAAc molecules were adsorbed from aqueous NaClO(4) solutions. AFM single molecule topography studies could be utilized to show that polyacrylic acid molecules specifically adsorb on the non-polar (10-10) step edge faces at low ionic strengths. The radius of gyration of the dissolved PAAc in aqueous solution was measured by means of static light scattering experiments yielding a radius of gyration of R(g)=136 nm at pH 7.4 in 50 mM NaClO(4)/NaOH solution, which is in good agreement with the size of the adsorbed PAAc molecules as measured using AFM. The obtained results could be rationalized in terms of binding-site configurations at step edges and the effect of the chemical environment on both local electric double layer charge and molecular conformation of the PAAc molecules. The point of zero charge of the ZnO(10-10) surface was measured with chemical force microscopy to be pH(PZC)=10.2 ± 0.2. The specific adsorption of polyacrylic acid at non-polar ZnO step-edges can be explained by coordinative bonds formed between the carboxylic acid group and the Zn-surface atoms. On the hydroxide stabilized polar surface only weak hydrogen bonds can be formed in addition to van-der-Waals forces. Thus a "diffusion and trapping" mechanism keeps the adsorbed PAAc molecules mobile on the ZnO(0001)-Zn surface terraces due to small interaction forces until they are trapped at the (10-10) step faces by stronger coordinative bonds from the carboxylic groups to zinc atoms located in the first atomic layer of the crystal structure.     

Adsorption d'eau sur la kaolinite: Influence de la nature des sites actifs

Water adsorption on kaolinite is a specific cooperative adsorption which does not satisfy the fundamental hypothesis of the BET theory.The adsorption isotherms on different homoionic samples show the effect of the hydration energy of the active sites (exchangeable cations) on quantitative adsorption data.The corresponding calorimetric curves present a maximum which characterizes interactions in the adsorbed phase. A relationship is apparent between these interactions and the electric field or the polarizability of the fixed cation, these factors determining the nature of the bond between the surface and cation.From the experimental data, we may propose an approximative value for the number of molecules which compose the primary hydration sheath of the active sites.     

Hydration of a synthetic clay with tetrahedral charges: a multidisciplinary experimental and numerical study

The interaction of water with a synthetic saponite clay sample, with a layer charge of 1 per unit cell (0.165 C m(-2)), was investigated by following along water adsorption and desorption in the relative pressure range from 10(-6) to 0.99 (i) the adsorbed amount by gravimetric and near-infrared techniques, (ii) the basal distance and arrangement of water molecules in the interlayer by X-ray and neutron diffraction under controlled water pressure, and (iii) the molecular structure and interaction of adsorbed water molecules by near-infrared (NIR) and Raman spectroscopy under controlled water pressure. The results thus obtained were confronted with Grand Canonical Monte Carlo (GC/MC) simulations. Using such an approach, various well-distinct hydration ranges can be distinguished. In the two first ranges, at low water relative pressure, adsorption occurs on external surfaces only, with no swelling associated. The next range corresponds to the adsorption of water molecules around the interlayer cation without removing it from its position on top of the ditrigonal cavity of the tetrahedral layer and is associated with limited swelling. In the following range, the cation is displaced toward the mid-interlayer region. The interlamellar spacing thus reached, around 12.3 A, corresponds to what is classically referred to as a "one-layer hydrate," whereas no water layer is present in the interlayer region. The next hydration range corresponds to the filling of the interlayer at nearly constant spacing. This leads to the formation of a well-organized network of interlayer water molecules with significant interactions with the clay layer. The structure thus formed leads to a complete extinction of the d001 line in D2O neutron diffraction patterns that are correctly simulated by directly using the molecular configurations derived by GC/MC. The next range (0.50 < P/P0 < 0.80) corresponds to the final swelling of the structure to reach d spacing values of 15.2 A (usually referred to the "two-layer hydrate"). It is associated with the development of a network of liquidlike water molecules more structured than in bulk water. The final hydration range at high relative pressure mainly corresponds to the filling of pores between clay particles.     

Hydration of calixarene-containing polymers

The hydration of cross-linked polymers containing tetramethylcalix[4]resorcinarene and tetraphenylcalix[4]resorcinarene was studied using the isopiestic, calorimetric, and MNDO/PM3 method. Adsorption of water vapor by calixarene-containing polymers leads to type II isotherms according to the Brunauer—Deming—Deming—Teller classification. In the framework of the Aranovich model of polymolecular adsorption, the monolayer capacity and pure heat of adsorption were calculated. The first monolayer is formed from 3—4 water molecules adsorbed due to hydrogen bonding with OH groups of calixarenes. The integral thermodynamic functions of hydration of the calixarene-containing polymers in water at 298 K were determined.     

Hydration of Cl<Superscript>–</Superscript> ion in a planar nanopore with hydrophilic walls. 1. Molecular structure

Computer simulation has been employed to obtain equilibrium molecular configurations, as well as spatial and angular distributions of water molecules, under the action of the field of a single-charged chlorine anion in a model planar nanopore with structureless walls at room temperature. A detailed many-body model of intermolecular interactions calibrated in accordance with experimental data relative to the free energy of hydration in water vapor has been used. The effect of the hydrophilicity of the walls on the ion hydration shell consists in its disintegration into two parts, i.e., molecules retained exclusively due to the interactions with the ion and those adsorbed on the walls. In the regime of strong interactions with the walls, two relatively stable states arise with asymmetric distribution of molecules between opposite walls. The existence of the two metastable states destabilizes the position of ions inside a pore and is expected to accelerate their adsorption on the walls.     

Charged molecular films on brownian particles: structure, interactions, and relation to stability

The interfacial film of physically adsorbed ionic amphiphilic molecules on submicron particles dispersed in water was studied by a combination of surface tension measurements, laser light scattering (LLS) and high-shear experiments in a microchannel. General features in the structure and morphology of the molecular film are identified and understood in the framework of the two-step Langmuir adsorption model deduced from the adsorption isotherm. On the basis of this approach, the phase transitions and structural ordering of the film at the solid-liquid interface are analyzed in detail. A novel methodology based on high-shear aggregation experiments subsequently analyzed by means of LLS is proposed and turns out to be able to provide significant information on the phase transitions and structural arrangements of the adsorbed molecules (in substantial agreement with the adsorption isotherm model) as well as on the resulting interactions. Particularly important for applications is the result that, with no added salt, the films on two particles can adhere/fuse, leading to aggregation as long as an uncovered (hydrophobic) patch is present (unsaturated molecular layers). In the opposite case of fully developed layers, by analyzing the mechanism of shear aggregation of charged particles in the low-salt limit, we show that, when the hydrophobic attraction is absent, short-range hydration repulsive forces dominate over Derjaguin-Landau-Verwey-Overbeek (DLVO) forces and adhesion can never be achieved even upon application of very high collision energies. Consistently, a lower limiting boundary for the hydration interaction is calculated and found to be in agreement with data in the literature.     

Pressure-induced hydration and order-disorder transition in a synthetic potassium gallosilicate zeolite with gismondine topology

Two high-pressure phases of a potassium gallosilicate with a gismondine framework (K-GaSi-GIS) were characterized using Rietveld refinements of in-situ high-pressure, high-resolution synchrotron X-ray powder diffraction data. The observed response of the K-GaSi-GIS framework under hydrostatic pressure is a gradual flattening of the so-called "double crankshaft" structural chain units. At pressures below 1.0(1) GPa, additional water molecules from the hydrostatic pressure-transmitting medium are inserted into the potassium-water guest network ("pressure-induced hydration") resulting in a "super-hydrated" high-pressure phase I. As the flattening of the double crankshaft structural units in the GIS framework continues above 1.6 GPa, the ellipticity of the cross-linking 8-ring windows is reduced below a certain threshold, and a disordering of the potassium-water guest structure along the 8-ring channel, characteristic of a disordered high-pressure phase II, is observed. The concerted framework distortion and guest network disordering accommodates the increased hydration level while maintaining the seven-fold coordination environment of the potassium cations to framework oxygen atoms and water molecules. We have thus established the atomistic details of a guest-host order-disorder transition under pressure-induced hydration conditions in a zeolite with GIS framework and compared it to other zeolites during pressure-induced hydration. We find that the structural changes mediated by the extra-framework cations and their coordination environment under PIH conditions are at the core of these different mechanisms and are driving the changes in the ellipticity of pore openings, order-disorder and disorder-order transitions, and framework distortions.     

Numerical modeling of adsorption on microporous adsorbents

Numerical modeling (Monte Carlo method) of the state of water adsorbed in the -cages of the Na and K forms of type A zeolite has shown that the energy parameters of adsorption are determined mainly by interaction of the adsorbed water with the exchange cations. It has been established that the difference between Na⁺ and K⁺ in effective radius has a very pronounced influence on the character of the interaction between adsorbed water and the zeolite framework. The distribution of energy of water interaction with the framework of the Na form is bimodal, but unimodal for the K form. The mean energy of water-molecule interaction with the framework in the case of the K form is practically independent of the degree of pore filling (coverage). At all degrees of filling, there is practically no hydrogen bonding between molecules of adsorbed water.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1724–1728, August, 1989.     

Fragile structured layers on surfaces in highly concentrated solutions of electrolytes of various valencies

In the present study a dynamic mode of atomic force microscopy for force measurements was employed to investigate the hydration repulsion force between charged surfaces in highly concentrated electrolyte solutions of NaCl, MgCl₂ and LaCl₃. A strong dependence of this repulsive force on the approaching rate of surfaces, the prehistory of their contact and the valency of cations was demonstrated. The phenomena were strongly pronounced in the cases of high scan rates, large surfaces and cations of high valency. The results obtained indicate that a fragile structure composed of water molecules, ions and hydrated ions exists outside of the primary layer of water molecules and ions adsorbed firmly on surfaces.