An experimental study of gas adsorption on fractal surfaces

The validity of the fractal versions of the FHH and BET theories for describing the adsorption of butane and nitrogen on a variety of partially dehydroxylated silica surfaces has been tested. The fractal dimensions obtained from adsorption data have been compared with those obtained completely independently using SAXS. It was found that the fractal dimensions obtained from butane adsorption isotherms, using both the fractal FHH and fractal BET theories, agreed well with the corresponding values obtained from SAXS over overlapping length scales. However, in general, a systematic deviation between the fractal dimension obtained from nitrogen adsorption and that obtained from SAXS was observed. The fractal dimensions obtained from nitrogen adsorption were consistently larger than those obtained from SAXS, which is the opposite of what has often been found in the literature. It has been suggested that the differences in the suitability of the adsorption theories tested to describe butane and nitrogen adsorption is due to the significant difference between the interaction strengths of these two different molecules with silica surfaces. A modified theory that can account for the discrepancy between the fractal dimensions obtained from nitrogen adsorption and SAXS has been proposed. The implications of the new theory for the accuracy of nitrogen adsorption BET surface areas for silicas are discussed.     

Synthesis of Microporous Silica Spheres

Spherical microporous silica powders with a narrow size distribution have been prepared by a precipitation technique involving the hydrolysis reaction of a silicon alkoxide in ethanol. The formation of the important microporosity has been investigated following two templating methods: the co-hydrolysis and condensation of two alkoxides, one of which presents porogen function, and the adsorption of an organic compound (glycerol) as the porogen. In both processes, the organic porogen is removed by a simple calcination. In the first method, the addition of more than 20 mol% of the porogen alkoxide, necessary for generating enough microporosity, disturbs completely the condensation process resulting in microporous, nonuniform silica particles of large size distribution. The best result has been obtained with the glycerol method where submicrometer-sized silica spheres with a very narrow size distribution and about 40 vol% porosity have been synthesized. The presence of glycerol during the synthesis considerably affects the precipitation mechanism, resulting in a larger mean particle size. The use of an aggregative growth model has successfully been employed to explain the effect of the porogen during particle formation. The precipitation mechanism of silica involves the aggregation between particles of similar size until a critical size is reached, resulting in a uniform particle size distribution. In the presence of glycerol, it has been shown that a second aggregative growth between still-nucleating primary particles and large particles occurred with increasing reaction time. This second aggregative growth appears at an intermediate stage of the precipitation process and is due to both the precipitation of smaller primary particles and the destabilization of the colloidal stability of the system. This explains why the final particle size reached in this system is larger compared to silica particles synthesized without glycerol and shows how glycerol is incorporated in the silica particles. The synthesis of silica microporous spheres of narrow size distribution, by varying particle size and porosity, should yield a wide range of aqueous silica slurries for particular chemical mechanical polishing applications. Copyright 2000 Academic Press.     

用吸附数据对硅胶表面的分形分析

根据一种中孔硅胶对甲酸、乙酸、丙酸和四氯化碳蒸气吸附等温线的单层区域和毛细凝结区域的数据以及自四氯化碳稀溶液中吸附系列脂肪醇的结果计算了该硅胶表面分形维数D.除了由吸附四氯化碳等温线毛细凝结区域所得D值低于2外,由其它吸附数据得出的该硅胶的分维D近似相等(D=2.06±0.05).中孔硅胶低D值可用其孔结构特性解释.     

Fractal Characterization of Silica Sol Prepared by the Sol-Gel Method: From the Sol Formation to the Flocculation Process

The fractal characterization of silica particles prepared by the sol-gel method was obtained; from the beginning of the sol-gel synthesis to the aggregation process of these particles by adding metal ions in solution, the fractal dimension was determined. At the beginning of the sol-gel process, unstable structures were formed due, essentially, to the auto-catalytic nature of the sol-gel condensation reactions; these particles are fractal structures with a fractal exponent corresponding to a reaction limited aggregation regime. As the time proceeds, the reactants are consumed approaching the system to equilibrium, stabilizing the size of the silica particles. The silica sol can be flocculated by adding metal ions in solution. The fractal exponent for the aggregation process was determined, obtaining a value corresponding to a diffusion limited aggregation regime.     

Preparation and characterization of CuO/SiO2 and NiO/SiO2 with bimodal pore structure by sol-gel method

CuO/SiO₂ and NiO/SiO₂ with bimodal pore structure were prepared by sol-gel reactions of Tetra-methoxysilane (TMOS) and the respective metal nitrate in the presence of poly (ethylene oxide) (PEO) with an average molecular weight of 10 000 and the catalyst of acetic acid. In this process, the interconnected macroporous morphology was formed when transitional structures of spinodal decomposition were frozen by the sol-gel transition of silica. The addition of copper and nickel into the silica-PEO system had a negligible effect on the morphology formation. In gel formation, it was found that NiO crystalline sizes in the samples increased with decreasing Si/Ni molar ratio. It was considered that PEO interacted with both silica and nickel cations. In the CuO/SiO₂ with the presence of PEO_, CuO crystalline sizes were larger than those of NiO/SiO_2. It was considered that there was no obvious interaction between the Cu cation and PEO, most of the copper ions in wet silica gel were present in the outer solution. They easily aggregated as copper salts in the drying process of wet gel and decomposed into CuO particles in heating. While in the CuO/SiO₂ with the absence of PEO, the Cu was selectively entrapped as small particles in the gel skeleton due to the interaction between Cu aqua complex and silica gel network.     

Fractal dimensions of silica gels generated using reactive molecular dynamics simulations

We have used molecular dynamics simulations based on a three-body potential with charge transfer to generate nanoporous silica aerogels. Care was taken to reproduce the sol-gel condensation reaction that forms the gel backbone as realistically as possible and to thereby produce credible gel structures. The self-similarity of aerogel structures was investigated by evaluating their fractal dimension from geometric correlations. For comparison, we have also generated porous silica glasses by rupturing dense silica and computed their fractal dimension. The fractal dimension of the porous silica structures was found to be process dependent. Finally, we have determined that the effect of supercritical drying on the fractal nature of condensed silica gels is not appreciable.     

Dynamic scaling properties of TeO2-based gels

Homogeneous, transparent, and mechanically rigid gels have been successfully synthesized in the tellurium isopropoxide-isopropanol-citric acid and water system. The sol to gel transition and the gels microstructure have been studied by using small angle X-ray scattering (SAXS) experiments. For any value of the two key synthesis parameters, which are the citric acid ratio and the alkoxide concentration, very small Te-rich elementary particles, about 1-1.5 nm in radius, form immediately when the water is added, leading to colloidal sols. During gelation, these elementary particles stick progressively together to build up fractal aggregates by a pure hierarchical aggregation process which has been identified as a reaction-limited cluster aggregation (RLCA) mechanism. The SAXS curve analysis, based on scaling concepts, shows that the gelling network exhibits a time and length scale invariant structure factor characterized by self-similarity. This self-similarity is also displayed for a wide range of chemical compositions and the gel microstructures only differ in their fractal aggregate size according to the tellurium isopropoxide concentration as well as the citric acid ratio.     

Ammonia Catalyzed Hydrolysis-Condensation Kinetics of Tetraethoxysilane/Dimethyldiethoxysilane Mixtures Studied by <Superscript><Emphasis Type="Bold">29</Emphasis></Superscript>Si NMR and SAXS

In-situ ²⁹Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the ammonia catalyzed hydrolysis and condensation of the mixed systems of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) dissolved in methanol. With ammonia catalysis, the hydrolysis reaction orders for TEOS and DDS in the mixed systems remained first order, which is similar to that observed for their corresponding single silane component precursor systems. The hydrolysis rate constant for TEOS in the mixed systems was larger than that of TEOS in the single silane component precursor systems. Meanwhile, the hydrolysis rate constants of DDS in the mixed precursor systems were smaller than those of DDS in the single silane component precursor systems. The hydrolysis and condensation kinetics showed more compatible hydrolysis-condensation relative rates between TEOS and DDS, which remarkably affected the final microstructure of the resulting silica particles. Small angle X-ray scattering (SAXS) experiments showed a typical double fractal structure in the particulate networks.     

Structural characteristics of silica sonogels prepared with additions of isopropyl alcohol

Wet silica gels with approximately 1.4 x 10(-3) mol SiO2/cm3 and approximately 92 vol % liquid phase were obtained from sonohydrolysis of tetraethoxysilane (TEOS) with different additions of isopropyl alcohol (IPA). The IPA/TEOS molar ratio R was changed from 0 to 4. Aerogels were obtained by supercritical CO2 extraction. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The wet gels exhibit mass fractal structure with fractal dimension increasing from D approximately 2.10 to D approximately 2.22, characteristic length xi decreasing from approximately 9.5 to approximately 6.9 nm, as R increases from 0 to 4, and an estimated characteristic length for the primary silica particles lower than approximately 0.3 nm. The supercritical process apparently eliminates a fraction of the porosity, increasing the mass fractal dimension and shortening the fractality domain in the mesopore region. The fundamental role of isopropyl alcohol on the structure of the resulting aerogels is to decrease the porosity and the pore mean size as R changes from pure TEOS to R = 4. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure, with correlated mass fractal dimension Dm approximately 2.7 and surface fractal dimension Ds approximately 2.3, as inferred from SAXS and nitrogen adsorption data.     

Preparation and Structure of Microporous Silica Membranes

Silica sols have been prepared in an alcoholic solution by hydrolysis and condensation of TEOS (tetra-ethyl-ortho-silicate) molecules as a function of water and nitric acid concentration. The polymers are intended as precursors for ceramic, gas separation membranes. These molecules show fractal behavior as determined by SAXS (Small Angle X-ray Scattering). Microporosity of dried and calcined silica polymers is determined by N₂-adsorption at 77 K. Fractal dimension and porosity increase with increasing acid concentration. Both the sol structure and the drying kinetics determine the porosity values. N₂-adsorption isotherms are not very suitable for the determination of pore size distributions of microporous silica.     

Adsorption Properties of Silica Gel Prepared from a Decationized Sol

In accordance with the IUPAC classification, the shape of the capillary condensation hysteresis and type of the sorption isotherms of the vapors of water and tetrachloromethane by silica gel are identified, the latter being prepared from a decationized silica sol in the presence of aluminum chloride. It is found that the adsorption and structural properties of the silica gel depend on the concentration of silicon(IV) oxide and Al/Si molar ratio, as well as on the type of the adsorbate used. It is shown that the effect of aluminum chloride is manifested mainly in the spatial rearrangement of the particles of silicon(IV) oxide into dense agglomerates followed by their aggregation with the conservation of internal porosity. This leads to the appearance of narrow slitlike mesopores and to a decrease in or the stabilization of the average pore radius.     

Role of solvents in the gelation process: can light scattering studies shed some light?

Sol–gel reactions continue to be of interest for the preparation of nanostructured materials. Two chemical reactions that are important in the sol–gel process are the hydrolysis and condensation reactions. The rate of the these two reactions are affected by a number of factors such as reaction pH, temperature, humidity, amount of water, type of alkoxide, molar ratio of alkoxide to water, and nature of solvent. Moreover, there is a physical process, that of particle aggregation that is also important in the overall gelation process. The role of solvents in these chemical and physical processes is still not very clear. In order to clarify the role of solvents in the gelation process, small angle light scattering studies (SALS) were carried out. A model system chosen was a colloidal silica solution that contained preformed silica particles of 10–15 nm in diameter. SALS studies indicate that gelation times are independent of the nature of solvent.     

In situ physical or covalent trapping of phthalocyanine macrocycles within porous silica networks

An effective way of trapping phthalocyanines inside porous silica has been achieved when an aqueous solution of these macrocyclic species is reacted in situ with silicon alkoxide. The resultant porous gel network can encapsulate a myriad of metal phthalocyanine molecules at relatively high concentrations and, most importantly, in a disaggregated way. By employing this method metal sulfophthalocyanines of Fe, Co, Ni, Cu, Al, Eu or Sm can be encapsulated within SiO₂ xerogels. Here, the chemical entrapment of phthalocyanines inside silica gel pore networks is accomplished by the attachment of bifunctional groups to the walls of these substrates; while one of these moieties is directly linked to the macrocycle end groups, the other one is covalently bonded to the silanol groups resting on the SiO₂ walls. Furthermore, when the proper concentrations of phthalocyanine species, H₂O, silicon alkoxide, and HCl are reacted together, it is possible to obtain monolithic translucent silica xerogels. This latter property provides straightforward evidence of the innate fluorescence of the trapped macrocycles. The average size of the cavities encapsulating the macrocyclic molecules range from 2.0 to 3.6 nm; the precise size depends on the cation present in the complex and on the identity and position of the substituent groups at the periphery of the macrocycle. When the silica network is prepared from standard and/or organo-substituted alkoxides, the aggregation, degradation or stability of the macrocyclic species trapped in silica cavities depends on the nature of the alkyl group attached to the pore walls.     

冲洗色谱特征点法测定表面分形维数初探

在用非线性气相色谱测填料表面分形维数中,首次运用气固色谱（ＥＣＰ法（ｅｌｕｔｉｏｎ ｂｙ ｃｈａｒａｃｔｅｒｉｓｔｉｃ ｐｏｉｎｔｓ ｍｅｔｈｏｄ）,测定了正戊烷、正已烷、正庚烷３种烷烃在不同颗粒大小的硅胶表面的吸附等温线,通过拟合Ｌａｎｇｍｕｉｒ方程,求出单分子饱和吸附容量,并采用单分子层吸附的分形模型,求得硅胶的发形维数,提供了一种测定表面分形维数的新方法。     

Rheological Properties of Silica Suspensions in Aqueous Solutions of Block Copolymers and Their Water-Soluble Components

Dynamic moduli of fumed silica suspensions in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers and PEO homopolymers were measured as a function of surface coverage. Since the block copolymers and PEO are adsorbed on the silica surface through hydrogen bonding between the ether oxygen and the silanol group on the silica surface, the interaction between the silanol groups, which is dominant for the aggregation of silica particles, should be prohibited. Dynamic moduli in the silica suspensions were strongly related to the stability of the silica suspensions and the block copolymer, and the longest PEO portion was useful for stabilizing the silica particles. However, the PEO homopolymer did not support stability of the silica particles, suggesting that chain conformation of the PEO portion in the block copolymer is different from that for the PEO homopolymer. Copyright 2001 Academic Press.     

Study on the rapid deswelling mechanism of comb-type N-isopropylacrylamide gels

The shrinking mechanism of comb-type grafted poly(N-isopropylacrylamide) gel was investigated by fluorescence spectroscopy and small-angle X-ray Scattering (SAXS). The SAXS reveals that the microdomain structure with characteristic dimension of 460 Å is developed in the comb-type grafted poly(N-isopropylacrylamide) gel during the shrinking process. Fluorescence spectroscopy together with SAXS observation suggests that the freely mobile characteristics of the grafted chains are expected to show the rapid dehydration to make tightly packed globules with temperature, followed by the subsequent hydrophobic intermolecular aggregation of the dehydrated graft chains. The dehydrated grafted chains created the hydrophobic cores, which enhance the hydrophobic aggregation of the networks. These aggregations of the NIPA chains contribute to an increase in void volume, which allow the gel having a pathway of water molecules by the phase separation.     

The decomposition of gaseous products of the chemical transport reaction between H<Subscript>2</Subscript>O<Subscript>2</Subscript> and ZnO on the surface of silica gel

The decomposition of gaseous products of the chemical transport reaction that occurs in the interaction between H₂O₂ vapor and ZnO was studied on the surface of silica gel. At the initial stage of the decomposition of the intermediate complex formed in the chemical transport reaction between H₂O₂ and ZnO, the specific surface area of the sorbent increases noticeably. The pore size distribution maximum simultaneously shifts toward smaller values. The opposite effect is observed as the time of holding silica gel in a flow of gaseous products of the chemical transport reaction between H₂O₂ and ZnO increases. The treatment of the silica sorbent by the intermediate complex formed in the reaction between H₂O₂ and ZnO substantially influences the fractal dimension of its surface.     

Effect of synthesis conditions on the microstructure of TEOS derived silica hydrogels synthesized by the alcohol-free sol–gel route

Silica matrices synthesized from a pre-hydrolysis step in ethanol followed by alcohol removal at low pressure distillation, and condensation in water, are suitable for encapsulation of biomolecules and microorganisms and building bioactive materials with optimized optical properties. Here we analyze the microstructure of these hydrogels from the dependence of I(q) data acquired from SAXS experiments over a wide range of silica concentration and pH employed in the condensation step. From the resulting data it is shown that there is a clear correlation between the microscopic parameters—cluster fractal dimension (D), elementary particle radius (a) and cluster gyration radius (R)—with the attenuation of visible light when the condensation step proceeds at pH < 6. At higher pHs, there is a steep dependence of the cluster density (~R ^D−3) with the condensation pH, and non-monotonous changes of attenuance are less than 20%, revealing the complexity of the system. These results, which were obtained for a wide pH and silica concentration range, reinforce the idea that the behavior of gels determined in a restricted interval of synthesis variables cannot be extrapolated, and comparison of gelation times is not enough for predicting their properties.     

Insight on Structural Change in Sol–Gel-Derived Silica Gel with Aging under Basic Conditions for Mesopore Control

Structural rearrangement of sol–gel-derived silica gel by aging under basic conditions was investigated using small angle X-ray scattering (SAXS) and ²⁹Si nuclear magnetic resonance (NMR). A wet silica gel prepared under acidic conditions had a fractal nature, and many unreacted silanols remained on the surface. During aging of the gel in ammonia solution, additional Si—O—Si bonds rapidly formed, whereas the change in mesoscale structure gradually proceeded. This result was compared with that of simulation modeling the Ostwald ripening, i.e. dissolution from positive curvature and reprecipitation on negative curvature. In the simulation, structural change in a cluster from fractal nature to particle aggregates was well visualized in 2-dimmensional square lattice. Both scattering profiles calculated from the model clusters and the change in average coordination number of monomers in the cluster well agreed with the experimental SAXS and NMR results, respectively. This agreement strongly ensures us that structural change by aging under basic conditions proceeds through the Ostwald ripening. The mesopore size as well as mesopore volume in calcined silica gel is determined by the shrinkage degree during drying and calcination. The sample aged in basic solution restrains the shrinkage because of the growth of particulate structure, and retains large size and volume of pores.