Melting and freezing of water in cylindrical silica nanopores

Freezing and melting of H(2)O and D(2)O in the cylindrical pores of well-characterized MCM-41 silica materials (pore diameters from 2.5 to 4.4 nm) was studied by differential scanning calorimetry (DSC) and (1)H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting point depression DeltaT(m) can be represented by the Gibbs-Thomson equation when the existence of a layer of nonfreezing water at the pore walls is taken into account. The DSC measurements also show that the hysteresis connected with the phase transition, and the melting enthalpy of water in the pores, both vanish near a pore diameter D* approximately equal to 2.8 nm. It is concluded that D* represents a lower limit for first-order melting/freezing in the pores. The NMR spin echo measurements show that a transition from low to high mobility of water molecules takes place in all MCM-41 materials, including the one with 2.5 nm pores, but the transition revealed by NMR occurs at a higher temperature than indicated by the DSC melting peaks. The disagreement between the NMR and DSC transition temperatures becomes more pronounced as the pore size decreases. This is attributed to the fact that with decreasing pore size an increasing fraction of the water molecules is situated in the first and second molecular layers next to the pore wall, and these molecules have slower dynamics than the molecules in the core of the pore.     

Correlation Between Structure and Transport Properties of Polymeric Membranes for Immunoisolation

Laboratory-made asymmetric polyurethane membranes designed for immunoisolation were investigated. Two types of EK and ES membranes were prepared in different spinning conditions. The membrane structure was characterised by the skin pore radius measurements using differential scanning calorimetry (DSC). Diffusive transport properties of membranes were determined by in vitro method for albumin and creatinine. The scanning electron microscopy (SEM) was applied to study the morphology of membranes. It has been found that the DSC technique is a useful tool for the evaluation of pore radii in the skin layer for PU membranes. Calculated pore radii were in the range from 1.95 to 2.47 nm for the EK and ES types. A correlation between the skin pore radii and the transport properties was not found in this case of investigated membranes. However, the transport properties data can serve for the estimation of selectivity of membranes. Thus, the selectivity of membranes for solutes of various molecular size was estimated from the D _m/D _w ratio of diffusion coefficients for albumin and creatinine. The SEM micrographs reveal the finger-like internal structure of capillary membranes, as well as various skin thickness. This revised version was published online in July 2006 with corrections to the Cover Date.     

羧基官能化SBA-15型介孔分子筛的小角X射线散射研究

使用小角X射线散射(SAXS)技术对共缩聚法制备的羧基官能化SBA-15介孔分子筛进行了表征,利用相关函数理论和弦长分布理论获得了羧基化分子筛的孔结构、界面和周期性信息,辅以常规的氮气吸附-脱附测试,并将基于Barrett-Joyner-Halenda(BJH)模型计算的孔结构与弦长分布理论得到的结果进行比较.结果表明:除了羧基引入量高达50%(摩尔分数,相对无机硅源)的样品外,随着羧基引入量的增加所有羧基官能化SBA-15均能保持较高的有序结构且有序度变化不大.所有样品的Porod曲线均无明显偏离,羧基基团倾向于进入介孔的骨架结构而不仅仅是键合在表面的Si—OH上.利用弦长分布法,无需建立模型,可以得到独立的孔壁厚度分布和孔径分布,尤其是其它方法无法给出的孔壁厚度分布,对于进一步认识有序介观结构材料有很大帮助.     

磁性金属镍纳米管的有效合成

以多孔氧化铝为模板, 三嵌段共聚物F127(EO108PO69EO108, EO: ethylene oxide, PO: propylene oxide)为添加剂, 采用电化学沉积技术, 制备了高度有序的磁性金属镍纳米管阵列. 该合成方法简单、有效、易操作, 特别是, 氧化铝模板的孔壁不需要进行任何修饰. 通过透射电子显微镜(TEM)、扫描电子显微镜(SEM)、X射线衍射(XRD)和振动样品磁强计(VSM)对产物的形貌、结构和磁学性质进行表征. X射线衍射(XRD)分析表明, 产物的晶型结构为面心立方. 利用透射电子显微镜研究了实验参数, 如电流密度、共聚物浓度和电沉积时间, 对产物形貌的影响, 结果表明, 镍纳米管的管壁厚度随着电流密度增大和电沉积时间的延长而变大, 但几乎不受F127浓度变化的影响. 以上的实验表明, 调节实验参数, 可以有效控制纳米管管壁的厚度. 磁性研究结果表明, 与块体镍相比较, 镍纳米管阵列表现出较大的矫顽力.     

介孔分子筛COAPSO的合成、表征及其环己烷氧化催化性能

采用一步晶化法合成了具有McM-41介孔分子筛结构的CoAPSO分子筛.用X射线粉末衍射(XRD)、元素分析(ICP)、氮气吸附、透射电镜(TEM)、紫外可见漫反射光谱(UV-Vis)和热重分析(TG)等对样品进行了表征.合成的样品热稳定性高,孔径大约在2.5 nm左右.钴原子以四配位形式进入介孔孔壁中;随着钴含量增加,样品的孔径减小,孔壁增厚.在环己烷氧化反应中,所合成的CoAPSO分子筛显示出较高的催化活性和环己酮选择性;当钴含量为0.34%时,单位钴原子上环己烷的转化数达到420.5.     

Pore size distribution of micelle-templated silicas studied by thermoporosimetry using water and n-heptane

Thermoporosimetry, i.e., DSC measurements of melting point depression of water and heptane confined in mesopores, has been used for determination the pore size distribution of several mesoporous silicas synthesized with the use of micelle templates. Porosity of these materials was additionally characterized by low-temperature nitrogen adsorption and quasi-equilibrated thermodesorption of nonane. The pore size distributions obtained using the water thermoporosimetry were similar to those determined using the other methods, but the pore size values found for the narrow pore materials were underestimated by ca 1?nm. Too large pore sizes obtained for the wide pore silica from heptane thermoporosimetry were attributed to nonlinear dependence of the melting point depression on the reciprocal of the pore size.     

Wettability of polypropylene capillary membranes during the membrane distillation process

Studies of membrane wettability in the membrane distillation process were performed with the application of hydrophobic capillary membranes made of polypropylene. Three kinds of Accurel PP membranes (Membrana GmbH, Germany) differing in the diameter of capillaries and pores as well as in the wall thickness were used. It was confirmed that membranes with lower wall thickness and larger pore size provide higher yields of the process. The studies demonstrated that the pores of used membranes located close to the external surface of capillaries are several times larger than those located inside the membrane wall. Based on air permeability measurements it was found that external surface of the membranes with such large pores was completely wetted by water after 50–80 h of membrane distillation. However, the pores located inside the wall with the diameter below 1 μm were not wetted and electrical conductivity of the obtained distillate was maintained at the level of 3–6 μS cm⁻¹.     

Modification of electrochemical properties of pore wall of track-etched mica membranes

The electrochemical properties of the pore wall of track-etched mica membranes are modified (a) by covalent binding of positively and negatively charged groups, and (b) by adsorption of cationic and amonic polyelectrolytes. The electrochemical properties of the pore wall are characterized by measurements of membrane potential, electrical conductivity and streaming potential.By these methods it is possible to change the sign of the surface charge density of the pore wall and to increase its absolute value by a factor of about 30 compared with that of the unmodified pore wall. Changes of electrochemical properties of the pore wall are desirable in studies of negative osmosis and incongruent electrolyte transport in membranes with known pore structure.     

NMR Study of Pore Surface and Size in a Mesoporous Material FSM-16

Surface structure, pore size distribution and pore wall thickness of a mesoporous material FSM-16 have been studied by X-ray powder diffraction (XRD), ^lH and ²⁹Si MAS NMR and ¹H liquid-state NMR, and by applying surface silylation as a probe. Concentrations of surface hydroxyl groups for FSM-16 are estimated from ²⁹Si and ¹H MAS NMR, which are about 3×l0²¹ g^-1, corresponding to approximately 3 nm^-2. O₂ molecules contribute to ²⁹Si spin-lattice relaxation of Q² and Q³ as well as Q⁴, suggesting thin wall thickness. ¹H MAS NMR spectra indicate the presence of isolated and hydrogen-bonded hydroxyl groups. Both hydroxyl groups are silylated, where the silylated fraction is about 50%. The spatial distribution of surface hydroxyl groups is estimated from the line width in ¹H static spectra. A rather homogeneous distribution is demonstrated in one of the samples. The sample with less homogeneous distribution has a larger affinity for moisture. Pore size and pore wall thickness were determined by ¹H NMR measurements on water saturated FSM-16 samples, which are in good agreement with literature values obtained by N₂ adsorption isotherms and transmission electron micrographs on a similar sample. In benzene saturated samples, a non-freezing surface layer of benzene is much thicker than that of water, which indicates a stronger interaction between benzene and the FSM-16 surface.     

相变调温微胶囊的制备

采用原位聚合法用三聚氰胺-甲醛树脂包覆正十八烷,制备出相变微胶囊．利用扫描电镜和差示扫描量热仪对微胶囊试样的表面形貌和热物理性能进行了研究．实验结果表明：制备的相变微胶囊表面光滑,平均粒径2．84μm,平均壁厚0．41μm．     

Surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses: curvature effects observed by 1H NMR

We have measured the thickness of the pre-molten surface layer that appears at the interface of octamethylcyclotetrasiloxane (OMCTS) to the matrix in controlled pore glasses with pore diameters ranging 7.5-73 nm. Except for the glass with the largest pores, the layer thickness data for different pore diameters fall on a single master curve when plotted versus Tm - T, where Tm is the size-dependent volume melting point of the pore-confined OMCTS. Hence, at a single temperature, the surface layer thickness depends strongly on the curvature of the pore wall and therefore that of the solid-liquid interface. For temperatures where it exceeds two monolayers, the layer thickness depends logarithmically on Tm - T; for the glass with the largest pores, this turns into a power law with the exponent -1/2. The results are interpreted in terms of a continuous model of the solid-liquid interface with an arbitrary curvature. Because OMCTS is a weakly polar molecule with close to spherical shape, our data also lend themselves to Lennard-Jones type simulations.     

X-ray diffraction structure analysis of MCM-48 mesoporous silica

The structure of MCM-48 mesoporous silicate materials has been fully characterized from X-ray diffraction data by applying recently developed methods of mesostructure analysis and full-profile refinement. The pore wall thickness of both as-made and calcined MCM-48 was determined with high precision to be 8.0(1) Angstrom. No regular variations of the wall thickness were detected, but its density was found to be ca. 10% higher in the low-curvature regions. The surfactant density in the pores was assessed around 0.6-0.7 g/cm(3) and was found to have a distinct minimum in the pore center similar to that detected previously in MCM-41. A new extended model function of the density distribution in MCM-48 is proposed on the basis of the structural features that were revealed.     

Evaluation of thermoporometry for characterization of mesoporous materials

The accuracy of thermoporometry (TPM) in terms of the characterization of SBA-15 is examined based on a model that classifies the water in the mesopores into two different types: freezable pore water, which can form cylindrical ice crystals, and nonfreezable pore water, which cannot undergo a phase transition during a differential scanning calorimetry (DSC) measurement. Applying the empirical relationship between the sizes of the ice crystals formed in the mesopores and the solidification temperature of the freezable pore water to a thermogram (a recording of the heat flux during the solidification of the freezable pore water) yielded a size distribution of the ice crystals. The size of the ice crystals increased slightly with repetitive freezing, indicating that the mesopores were enlarged by formation of the ice crystals. Adding the thickness, t(nf), of the nonfreezable pore water layer to the ice crystal-size distribution calculated from the thermogram allowed for the determination of the porous properties of SBA-15. The porous properties attained from TPM experiments were compared with the results attained through the combination of Ar gas adsorption experiments and nonlocal density functional theory (NLDFT) analysis. The porous properties determined by TPM were confirmed to be quite sensitive to the t(nf) value.     

Effects of crosslinker density on the polymer network structure in poly‐N,N‐dimethylacrylamide hydrogels

To investigate the effects of crosslinker density on the properties of hydrogels, compression tests, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Raman measurements were performed on poly‐N,N‐dimethylacrylamide hydrogels. The results of the compression tests showed that the Young's modulus increases as the crosslinker density increases. To understand the mechanism of the change in the mechanical properties, the structures of the polymer networks and water and the molecular vibrations were analyzed using SEM, DSC, and Raman methods. From the SEM images, it was found that the porosity estimated from the mesh size and cell density increases with increasing crosslinker density. In addition, the DSC and Raman results show that the thickness of the bound water increases as the porosity increases, although the density of the polymer chains in the porous wall remains nearly constant. The increase in the number density of polymer chains can be one of the mechanisms contributing to the increase in the mechanical strength of the hydrogels at lower crosslinker density below 5 mol %, as proposed by previous studies. At higher crosslinker density, however, the number density of polymer chains does not increase with increasing crosslinker density. The present results suggest that the bound water plays an important role in strengthening the hydrogel. The water structure may be one of the dominant factors governing the chemical and physical properties of hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1017–1027     

In-situ cryo-SEM investigation of porous structure formation of chitosan sponges

Aiming at better understanding of porous structure formation unique in-situ experiments on freeze-drying the chitosan-based solvents were carried out by cryo-SEM method. It allowed us to visualize the lyophilization process and to follow the formation of the porous structure in the chitosan sponge. It was clearly shown that the chitosan solutions just after freezing possess an evident microstructure consisting of the walls of the future pores and of the frozen solvent filling the space between them. The diameter of the pores and their wall thickness are controlled be pre-freezing temperature. An incorporation of nanofillers (chitin nanofibrils and montmorillonite) into the chitosan solution did not affect the pore size and wall thickness. However, they can promote the formation of the layered structure of the pore walls.     

Preparation of Porous Hollow SiO2 Spheres by a Modified St?ber Process Using MF Microspheres as Templates

Using the surface charged and acid dissolvable melamine formaldehyde (MF) microspheres as sacrificial hard templates, silica coated MF core?Cshell composite microspheres, denoted as MF@SiO₂, were synthesized via a surfactant-assisted sol?Cgel process by using tetraethyl orthosilicate (TEOS) as silica source. Hollow SiO₂ spheres with mesoporous shells were then obtained after selective removal of the MF cores and the pore directing surfactant by hydrochloric acid etching or calcinations in air. Interesting shrinkage phenomena were observed in both the hollow products derived from hydrochloric acid etching and calcinations. The influence of the ratio of MF sphere to TEOS and the removal method of the MF core on the size of the hollow spheres, the shell thickness and the shell surface roughness have been studied. The composition, the thermal stability, the morphology, the surface area and pore size distribution, the wall thickness and adsorption properties of the hollow spheres derived from hydrochloric acid etching and calcinations were also investigated and compared based on the FTIR, SEM, TEM, TGA, Nitrogen adsorption?Cdesorption and spectrophotometer techniques or measurements.     

介孔分子筛HMS-Al、HMS-Et的制备与表征

HMS[1]是六方结构的介孔二氧化硅分子筛,由中性表面活性剂和中性无机物通过氢键自组装途径合成,具有较大的比表面积和较大的孔容,孔径大小均一。相对于介孔分子筛MCM-41[2]来说,HMS具有较厚的骨架内壁和较丰富的表面羟基,在制备过程中所用模板剂(十六胺)可通过乙醇萃取的方法脱     

Facile synthesis of high quality mesoporous SBA-15 with enhanced control of the porous network connectivity and wall thickness

Convenient and commercially viable synthesis conditions are described, providing efficient and reproducible control of pore connectivity and pore wall thickness for the synthesis of high quality SBA-15 mesoporous silica.     

Enthalpy and interfacial free energy changes of water capillary condensed in mesoporous silica, MCM-41 and SBA-15

The effect of confinement on the solid-liquid phase transitions of water was studied by using DSC and FT-IR measurements. Enthalpy changes upon melting of frozen water in MCM-41 and SBA-15 were determined as a function of pore size and found to decrease with decreasing pore size. The melting point also decreased almost monotonically with a decrease in pore size. Analysis of the Gibbs-Thomson relation on the basis of the thermodynamic data showed that there were two stages of interfacial free energy change after the constant region, i.e., below a pore size of 6.0 nm: a gradual decrease down to 3.4 nm and another decrease after a small jump upward. This fact demonstrates that the simple Gibbs-Thomson relation, i.e., a linear relation between the melting point change and the inverse pore size, is limited to the range not far from the melting point of bulk water. FT-IR measurements suggest that the decrease in enthalpy change and interfacial free energy change with decreasing pore size reflect the similarity of the structures of both liquid and solid phases of water in smaller pores at lower temperatures.     

Structural studies of water in hydrophilic and hydrophobic mesoporous silicas: an x-ray and neutron diffraction study at 297 K

Water confined in a sol-gel network has been characterized by x-ray and neutron diffraction for two samples of mesoporous silica: one with a hydrophilic character (a nonmodified one) and another with a hydrophobic character (a modified one with a methylated internal pore surface). The pore size has been previously characterized [J. Jelassi et al., Phys. Chem. Chem. Phys. 134, 1039 (2010)] to have a mean pore diameter of approximately 55 A?. The diffraction measurements presented in this paper have been made at room temperature [293 K] for a filling factor of 0.45, giving a mean thickness of 8-9 A? for the water layer. The results show that the local order of the confined water molecules in the intermediate region of 3-6 A? is significantly different from that of the bulk water and also for the two different environments. For the hydrophilic sample, the siloxyl groups at the surface modify the water structure through the effects of interfacial hydrogen-bonding, which influences the orientational configuration of local water molecules and creates a modified spatial arrangement in the pore. In the case of the hydrophobic sample, there is no specific interaction with the pore wall, which is primarily van der Waals type, and the water molecules at the interface are differently oriented to create a hydrogen-bonded network linked more directly to the rest of the water volume. In the present circumstances, the thickness of the water layer has a relatively small dimension so that the interpretation of the measured diffraction pattern is not as straightforward as for the bulk liquids, and it is necessary to consider the effects of diffraction-broadening from a distributed sample volume and also the contribution from cross-terms that remain after conducting a "wet-minus-dry" analysis procedure. These analytic difficulties are discussed in the context of the present measurements and compared with the work of other groups engaged in the study of water confined in different environments. The present results, again, emphasize the complexity influencing the properties of water in a confined geometry and the strong influence of surface interactions on its behavior.