Heat Capacities: Liquids, Solutions and Vapoursby Emmerich Wilhelm (Editor), Lee Hansen (Contribution by), Trevor M Letcher (Editor), David Raal (Contribution by), Jean-Pierre Grolier (Contribution by)
The book contains the very latest information on all aspects of heat capacities related to liquids and vapours, either pure or mixed. The chapters, all written by knowledgeable experts in their respective fields, cover theory, experimental methods, and techniques (including speed of sound, photothermal techniques, brillouin scattering, scanning transitiometry, high resolution adiabatic scanning calorimetry), results on solutions, liquids, vapours, mixtures, electrolytes, critical regions, proteins, liquid crystals, polymers, reactions, effects of high pressure and phase changes. Experimental methods for the determination of heat capacities as well as theoretical aspects, including data correlation and prediction, are dealt with in detail. Of special importance are the contributions concerning heat capacities of dilute solutions, ultrasonics and hypersonics, critical behaviour and the influence of high pressure. This new book covers the wide range of topics in the field of heat capacities and vapours and as such is a key point of reference for undergraduates and graduates alike as well as researchers, academics and anyone working in the field or related areas.
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A Versatile Strategy for Creating Nanostructured Porous Materials
By An-Hui Lu, Dongyuan Zhao, Ying Wan
The Royal Society of ChemistryCopyright © 2010 An-Hui Lu, Dongyuan Zhao and Ying Wan
All rights reserved.
Principles of Nanocasting
1.1 Nanocasting Concept
Casting is a 6000-year-old manufacturing process. The oldest surviving casting is a copper frog from 3200 bc. In the casting process, a liquid or a fluid material is poured into a mold, which contains a hollow cavity of the desired shape, and is then allowed to solidify (Figure 1.1). The solid casting is then ejected or broken out to complete the process. Casting is most frequently used for making complex shapes that would be otherwise difficult or uneconomical to produce by other methods. The casting process is divided into two distinct subgroups: expendable and non-expendable mold casting. Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster and investment (lost-wax technique) moldings. Non-expendable mold casting differs from expendable processes in that the mold does not need to be reformed after each production cycle.
However, the above-mentioned old casting techniques are constrained to large size with a limitation in the centimeter range. If the casting procedure is scaled down to the nanometer scale, 'nanocasting' is a suitable word to describe this process. In recent years, nanocasting started emerging and developing with the demand for creating nanomaterial arrays and nanopores. Nanocasting can also be used for making complex shapes in nanoscale that cannot be prepared or fabricated by other methods. The earliest 'nanocasting' process was utilized to produce nanowire arrays using porous anodic aluminum oxide (AAO) as a template (Figure 1.2). However, the templating process is actually not a 'nanocasting' due to the fact that the diameter of AAO is normally in the sub-micrometer range. From this viewpoint, the first 'nanocasting' technique was adopted by Kyotani and coworkers. The 'nanocasting' technique was first adopted by Kyotani and other researchers to fabricate microporous carbons using ultra-stable zeolite Y (USY) as a hard template. They fabricated microporous carbons with large surface areas (>2000m2 g-1 and large microporosity via a nanocasting process. Microporous carbon replicas exhibited very high adsorption capacities.
In 1998, Göltner and coworkers first proposed the concept termed as 'nanocasting' in mesoporous materials. They used a mesoporous silica monolith with the interconnective pore system as a confined hard template via the 'two-step' nanocasting to prepare mesoporous organic polymer networks with well-defined nanostructure. From then on, the hard templating and nanocasting processes have attracted more and more attention and become one of the most important approaches for the synthesis of porous materials, especially mesoporous materials.
Microporous carbons cast by zeolite Y normally should have the negative structure of the template. Unfortunately, the produced carbons cannot fully duplicate their parent zeolites' structure due to the fact that the pore size of the zeolite template is too small for carbon precursors to infiltrate. Kyotani and coworkers had made tremendous efforts to control nanocasting; however, partial 'zeolite' framework structure with a low periodic regularity can only be observed in a small domain of the obtained carbon products. Another reason for unsuccessful duplication of zeolitic structures is the large amount of defects caused by crystalline aluminosilicates in amorphous carbons. Since this work, nanocasting has been successfully employed in the synthesis of ordered mesoporous materials. The earliest example could be traced back to 1999 when mesoporous carbons were synthesized by using mesoporous silica as a hard template via the nanocasting strategy (Figure 1.3). Two Korean research groups independently reported the nanocasting synthesis of mesoporous carbons. Thereafter, the synthesis of ordered mesoporous materials from nanocasting strategy became a hot topic in the research field of mesostructured materials, especially for non-silicious mesostructures.
1.2 Hard Templates: Ordered Mesoporous Materials
Porous materials have both continuous skeletons and voids that can be randomly arranged (disordered pore system) or highly regular (ordered pore system) with large surface areas. According to the definition of IUPAC (International Union of Pure and Applied Chemistry), porous materials can be divided into microporous (with pore diameter < 2nm), mesoporous (pore size in the range of 2–50 nm) and macroporous materials (pore size >50nm). Among them, microporous mesoporous materials are the most widely studied for applications of sensors, shape-selective catalysis, chemical separations and electronic applications. The presence of micro- or mesopore channels allows for molecules that are accessible to the large internal surface areas and the internal active sites, enhancing shape selectivity and sorption properties.
Generally, inorganic microporous materials like zeolites or molecular sieves are often in crystalline form with narrow pore-size distributions, large surface areas and high ion exchangeable properties, which make them suitable for application in adsorption and catalysis. However, the small pore size of zeolites limits their further application in heavy oil products and the synthesis/ separation of large molecules.
1.2.1 Synthesis of Mesoporous Materials
In spite of the considerable efforts that have been made toward making large and regular pore systems, ordered mesoporous materials still remained elusive until the discovery of MCM-41 in 1992. The researchers of Mobil Company first reported a family of mesoporous silicate molecular sieves (M41S) with large surface areas (up to 1400m2 g-1) and narrow pore-size distribution (in the range of 1.5–10nm). The MCM-41 materials process 2-D hexagonal mesostructures with uniform pore size, but their silicate pore walls are amorphous. MCM-41 can be easily synthesized under the 'hydrothermal condition' in the presence of alkyltrimethylammonium surfactant cations with an alkyl chain length ranging from 8 to 22 carbon atoms. The synthesis mechanism of MCM-41 was proposed, for the first time, as the true 'template' concept, which brought out a novel concept for the scientific domain. As a consequence, a large number of ordered mesoporous silicate materials like SBA-n, KIT-n, FDU-n and HMS-n with various structures have been obtained one after another following the 'surfactant templating' method.
1.2.2 Ordered Mesoporous Materials Prepared from Soft-templating
Soft-templating is defined as a process in which organic molecules serve as a 'mold' and around which a framework is built up. The removal of these organic molecules results in a cavity which retains the same morphology and structure of the organic molecules (Figure 1.4).
The 'soft' templates are usually in the molten or liquid state. Their macroscopic mechanical properties present 'soft' characteristics, such as the fluid property under certain conditions, which is very different with the casting molds. An ample variety of ordered structures can be formed by the soft-templating method through the non-covalence intermolecular interaction (short-distance repulsion and long-distance attraction), which is situated between the solid crystal structure and the liquid state structure. Another characteristic of soft templates is the resulting periodic structure restricted in the nanometer scale. Plenty of soft templates, including cationic, anionic and non-ionic surfactants, and mixed surfactant systems, such as cationic–cationic, cationic–anionic and non-ionic mixed surfactant systems, have been utilized to synthesize highly ordered mesoporous materials. During the soft-templating processes, the sol-gel or evaporation induced self-assembly (EISA) processes are typically involved in the synthesis of ordered mesoporous materials. This route is suitable for the preparation of mesoporous silica, because the hydrolysis and condensation rates of silicates can be easily controlled by adjusting reaction parameters such as pH value and temperature. Many excellent reviews have been reported for the use of the soft-templating method to synthesize ordered mesoporous materials. These papers can assist readers in understanding both the synthesis of ordered mesoporous materials by the self-assembly approach and also the contents of the following chapters.
1.2.3 Formation Mechanism
A large number of studies have been carried out to investigate the formation and assembly mechanism of periodical mesostructures and to understand the roles of surfactants. The initial liquid-crystal template mechanism was first proposed by Mobil's scientists, which covers almost all of the possibilities. On the basis of recent developments, two main pathways, i.e. cooperative self-assembly and the 'true' liquid-crystal templating process, are seemingly efficient to synthesize ordered mesostructures from the soft-templating method, as shown in Figure 1.4.
184.108.40.206 Cooperative Surfactant-templating Assembly with Inorganic Oligomers
This pathway is based on the interactions occurring between surfactants and inorganic species such as silicates to form inorganic–organic mesostructured composites. Silicate polyanions such as oligomers interact with positively charged groups of surfactants driven by Coulomb forces. The silicate species at the interface cooperatively polymerize and cross-link, and further change the charge density of inorganic layers. With the reaction, the cooperative arrangements of surfactants and the charge density between inorganic and organic species influence each other. Hence the compositions of inorgani–corganic hybrids differ to some degree. The matching of charge density at the surfactant/inorganic interfaces governs the assembly process. The final mesophase is the ordered 3-D arrangement with the lowest energy.
Elaborate investigations on mesoporous materials have been focused on understanding and utilizing the inorganic–organic interactions. Stucky and coworkers proposed four general synthetic routes which are S+ I-, S-I+, S+X-I+ and S-X+I- (S+ = surfactant cations, S-= surfactant anions, I+ = inorganic precursor cations, I- = inorganic precursor anions, X+ = cationic counterions and X- = anionic counterions), respectively. A series of ordered non-siliceous mesostructured solids were successfully prepared. Moreover, mesoporous silica can be synthesized under strong acidic conditions. To yield mesoporous materials, it is important to adjust the chemistry of surfactant heads which can fit inorganic components. Under a basic solution, silicate anions (I-) match with surfactant cations (S+) through Coulomb forces (S+I-). The assembly of polyacid anions and surfactant cations to 'salt'-like mesostructures also belongs to the S+I- interaction. The organic–inorganic assembly of surfactants and inorganic precursors with the same charge is also possible in which counter ions are necessary as the bridge. For example, in the syntheses of mesoporous silicates by the S+X-I+ interaction, S+ and I+ are cationic surfactants and precursors, and X- can be halogen ions (Cl-, Br- and I-). In a strong acid medium, the initial S+ X-I+ interaction through Coulomb forces or, more exactly, double-layer hydrogen bonding interactions gradually transforms to the (IX)-S+ interaction.
220.127.116.11 Liquid-Crystal Template Pathway
In this pathway, true or semi-liquid-crystal mesophases are involved in the soft- templating assembly to synthesize ordered mesoporous solids using high-concentration surfactants as templates. The condensation of inorganic precursors is improved by reaction of the confined growth around the surfactants, forming ceramic-like frameworks. After condensation, the organic templates can be removed by either calcination or extraction, or by other methods. The inorganic materials 'nanocast' the mesostructures, pore sizes and symmetries from the liquid-crystal scaffolds.
The high concentration of surfactant can be achieved by inducing solvent evaporation, therefore this process is called an EISA. The EISA method is believed to be a powerful and versatile strategy for the synthesis of ordered mesoporous materials with diverse components and various morphologies, especially for the efficacious synthesis of ordered mesoporous metal oxides. Detailed and systematic studies on the EISA-based syntheses of inorganic species-surfactant hybrid composite films have been carefully carried out by Sanchez and coworkers. Beginning with inorganic precursors of a low polymerization degree in volatile polar solvents, the assembly on an organic–inorganic interface can be improved and moldable organic–inorganic frameworks can be formed. Inorganic precursors further hydrolyze and cross-link during the solvent evaporation process. At the final stage of solvent evaporation, high-content surfactants form liquid-crystal phases in the presence of inorganic oligomers. The organized mesostructures are possibly transferred from the first generated disordered intermediate phase. Finally, ordered mesophases are solidified to form a rigid inorganic framework. Afterwards, the surfactants can be removed by calcination. The final mesostructures are affected by several factors such as hydrolysis rate, cross-linking degree and surfactant/precursor ratio, in a similar way to the syntheses in the aqueous systems. In addition, it was found that some apparently noticeable parameters, such as water concentration, processing humidity, evaporating temperature and rate, and substrates greatly influence the regularity of final materials.
An excellent example for the application of the EISA process is the synthesis of mesoporous carbon frameworks reported recently by Zhao's group. The synthesis procedure includes five major steps: the preparation of resol precursors, the formation of ordered hybrid mesophases by organic–organic self-assembly during the solvent evaporation, thermopolymerization of the resols around the templates to solidify the ordered mesophases, the removal of templates and carbonization of the resin polymer frameworks to the homologous carbons.
The initial homogenous solution is prepared by dissolving triblock copolymers and resol precursors in ethanol. Resol precursors have a low molecular weight (M = 200-500), and are water- and alcohol-soluble. These kinds of precursors have plenty of hydroxyl groups (-OH) which can strongly interact with the PEO segment of triblock copolymer templates via hydrogen bonds. The assembly of phenolic resins and copolymer templates occurs readily to form ordered mesostructures without macrophase separation. The preferential evaporation of ethanol progressively enriches the concentration of block copolymers and drives the organization of resol–copolymer composites into ordered liquid-crystalline mesophases. The mesostructures are dependent upon the hydrophilic/hydrophobic ratios in the resol-surfactant mesophases. The resol nanoparticles with sizes less than 1 nm are self-organized around the amphiphilic block copolymer templates instead of water. Cross-linkage of the resol nanoparticles may not be involved in this step, because resols are difficult to polymerize at room temperature under neutral conditions. Furthermore, the ordered resol–copolymer mesophases are thermopolymerized by the cross-linkage of resols, but without changing mesostructures. The 3-D network resin polymer nanostructures are formed via benzyl hydroxyl groups and non-substituted ortho- or para-carbons of benzene rings with covalent bonds. The PEO-PPO-PEO copolymer templates with high oxygen contents exhibit low thermal stability. By comparison, the stability of phenolic resins is much higher owing to the 3D network structures constructed by covalent bonds. Mesoporous polymers with large porosities are therefore obtained upon calcination at 350-500°C to remove templates. Further increasing the temperature leads to a framework transformation from polymers to carbons with ordered homologous mesostructures. This EISA strategy skillfully avoids the cooperatively assembling process between the precursor and surfactant template which facilitates the surfactant-templating assembly. It is quite different with the cooperative formation assembly mechanism, where the surfactant-templating assembly and polymerization of inorganic oligomers occur cooperatively and simultaneously.
1.2.4 Typical Ordered Mesoporous Silicas
18.104.22.168 2-D Hexagonal Mesoporous Silica Structures
The 2-D mesostructured materials with hexagonal symmetry are the most easily produced, the classical products being MCM-41, FSM-16, SBA-3 and SBA-15. The ideal models of basic structures are hexagonally closed packing cylindrical pore channels, belonging to the space group p6mm. Figure 1.5A shows the hexagonally symmetric pore arrays.
MCM-41 is one of the most simple and fully researched mesoporous silicates which can be synthesized in a wide range in a basic solution (pH = 9 - 13, optimal 11.5). The most used surfactant templates are cationic surfactants, i.e. long-chain alkyl ammonium, Cn H2n+1N(CH3)3+X- (n = 8-22, X = Br, Cl).
Excerpted from Nanocasting by An-Hui Lu, Dongyuan Zhao, Ying Wan. Copyright © 2010 An-Hui Lu, Dongyuan Zhao and Ying Wan. Excerpted by permission of The Royal Society of Chemistry.
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Professor Emmerich Wilhelm is Vice-Head and Head of the Institute of Physical Chemistry at the University of Wien, Vienna, Austria. Prior to that, he was Visiting Professor at Wright State University, Dayton, Ohio, USA, the UniversitÚ Blaise Pascal, Clermont-Ferrand, France and the University of Salzburg, Salzburg, Austria. For many years, Professor Wilhelm was a member of the Editorial Boards of Solubility Data Series (IUPAC), Fluid Phase Equilibria, Materials Chemistry and Physics, International DATA Series A, Thermochimica Acta and an Associate Member, and/or National Representative, and/or Member of the IUPAC Commission I.2: Thermodynamics and the IUPAC Commission V.8: Solubility Data (Gas Solubilities). He was also a Member of the Board of Directors of the European Society of Applied Physical Chemistry, US Calorimetry Conference and the International Council on Materials Education. Professor Wilhelm has published over 160 papers in peer reviewed journals in the areas of thermodynamics, physical chemistry of solutions, was Co-Organizer of 20 international scientific conferences and has given more than 110 Plenary/Keynote/Invited Lectures. He has been the recipient of the Felix-Kuschenitz Award (Thermophysics), Austrian Academy of Science, the Huffman Award, Calorimetry Conference, USA and the Doctor Honoris Causa, UniversitÚ Blaise Pascal, Clermont-Ferrand, France.
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