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Reactions in microheterogeneous media
Current Opinion in Colloid and Interface Science 8 (2003) 135–136
Editorial overview
Reactions in microheterogeneous media
There has been much interest in recent years in the use of microemulsions and other microcompartmentalized systems as media for chemical reactions. The ability of surfactants to self-assemble into structures of defined size and shape has been found to be useful in this respect. The nanosized structures formed by the surfactant aggregation and the surfactant-stabilized oil–water interface can be used as templates for various types of reactions. In this issue of Current Opinion in Colloid and Interface Science recent advances in the use of microheterogeneous media in six different areas are highlighted. Nanosized inorganic particles can be prepared from microemulsions, in particular microemulsions of waterin-oil type. The interest in the area derives from the wellknown fact that the properties of advanced materials are critically dependent on the microstructure of the sample. Control of size, size distribution and morphology of the individual grains or crystallites are of utmost importance in order to obtain the material characteristics desired. The use of microemulsions as a reaction media has been found to be straightforward and reliable. The paper by ´ M.A. Lopez-Quintela discusses several important issues related to this type of synthesis, such as variation of reaction parameters in order to control particle size and studies of the kinetics of the particle formation. An interesting development of recent origin is that of superlattices of nanoparticles. These are made from highly monodispersed particles and they can be obtained in one-, two- or three-dimensional arrangements. Mesoporous materials can be made from surfactant liquid crystals. In this case the liquid crystal serves as a direct template for the reaction and the mesoporous material comes as a replica of the lyotropic liquid crystal. The aqueous phase normally contains a precursor for the polymerisation reaction. Hydrolysis of the precursor is normally initiated by a pH change and the hydrolysis product formed is not stable but polymerises under the conditions used. The polymerisation-gelation that occurs in the aqueous phase leads to solidification of this phase. The mesoporous, inorganic material is subsequently obtained by washing away, or by burning off, the surfactant. An alternative way to make mesoporous materials is to use a micellar solution as starting formulation.
The mesoporous structure is then gradually formed during the course of the reaction as a result of surfactant self-assembly into well-defined structures as the reaction progresses. The overview by A.E.C. Palmqvist highlights recent advances in the synthesis of the materials. An interesting conclusion drawn by the author is that silica can be prepared in all the structures represented by the different types of surfactant liquid crystals and the type of structure formed is governed by the volume fractions of surfactant and hydrolyzed precursor. The same probably holds true for other oxides, such as alumina, titania, etc. but the synthesis of these materials has not yet been so carefully investigated. A special type of mesoporous materials, so-called mesoymacroporous inorganic oxide materials, can be made from solid foams, which, in turn, are prepared from highly concentrated emulsions. The solid foams are made by polymerising the continuous phase of highly concentrated water-in-oil emulsions. These foams have very high porosity and very low density and they can be used as a macroporous scaffolds for the preparation of solid materials with both a macroporous structure— obtained as a replica of the foam structure—and a mesoporous structure. The latter is a result of self-assembly of the amphiphile, a block copolymer, which is added to the foam together with the sol–gel solution. After the solution has gelled, the material is calcined to remove the organic matter. This process, and other applications of the highly concentrated emulsions, is presented in the overview by C. Solans et al. It is shown in the paper that the gel emulsions can be used as reaction media for various types of chemical and enzymatic syntheses. Hence, these emulsions can be seen as alternatives to microemulsions as compartmentalized media for chemical reactions. Microemulsions, vesicles, lyotropic liquid crystals and other self-organized surfactant systems can also be used as media for the preparation of organic polymers. Polymerisations in such systems can be of the direct templating type, in which case the morphology of the polymeric product resembles that of the template. Polymerization inside microemulsion droplets is an example of this. Alternatively, the polymerization may
1359-0294/03/$ - see front matter 䊚 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-0294Ž03.00023-2
136
Editorial overview
give a replica of the template. The latter occurs, for instance, when a monomer polymerizes around the template structure and the template is subsequently removed (compare the formation of mesoporous materials from surfactant liquid crystals as templates). The paper by H.-P. Hentze and E.W. Kaler focuses on the direct templating, in particular the use of microemulsions, vesicles and lyotropic liquid crystals as microreactors for polymerisation. An important conclusion drawn by the authors is that for one-to-one templating to work well, it is important to balance the thermodynamic and the kinetic parameters of the reaction. Important parameters that must be under control are partitioning and diffusion of the monomer; exchange dynamics; template rigidity; and compatibility of the surfactant, monomer and polymer formed. With properly designed systems nanostructured polymers can be obtained and the technique allows for making very well defined materials, e.g. core-shell nanolattices or hollow nanop articles. Bioorganic synthesis can be made in microemulsions of water-in-oil type and in related systems. A large number of enzymes have been found to exhibit good stability and high activity in such systems, provided care is taken with regard to the choice of both solvent and surfactant. A drawback of the approach, compared to enzymatic reactions in surfactant-free systems, is that of work-up. Removal of the surfactant is usually not a trivial operation, although ways have been described that work relatively well, and this extra step, which can be quite cumbersome, probably limits the practical applicability of the technique. One must keep in mind, however, that there are applications, e.g. in the cosmetics area, where surfactant removal may not be necessary. In the overview by N.L. Klyachko and A.V. Levashov it is pointed out that synthesis in microemulsions could be of particular interest for reactions of high molecular weight materials. It has been shown that the size of the water droplet must be such that it can accommodate the two high molecular weight reactants at the same time. If the size is smaller than that, no reaction will occur. This opens up the possibility to discriminate between reactants of a different size simply by the size of the droplets of the microemulsion used as a reaction medium. The size of the droplets are governed by the water to surfactant molar ratio. An interesting development in recent years is the use of b-glycosidase for the synthesis of alkyl glycoside in water-in-oil microemulsions. In this reaction the hydrophobic reactant, octanol, was also the oil component of the microemulsion. Alkyl glycosides are of current interest as environmentally benign surfactants and the use of glycosidase-catalysed preparation methods may be practically useful. There is substantial interest in the use of microemulsions as media for organic reactions. The interest relates to the ability of microemulsions to solubilize both polar
and non-polar substances and to compartmentalize and concentrate reactants. It has been found that the use of microemulsions is an excellent way to circumvent the reagent incompatibility problem that one frequently encounters in organic synthesis, e.g. in the reaction between a lipophilic organic compound and an inorganic salt. The microemulsion approach can be regarded as an alternative to phase transfer catalysis, i.e. a two-phase system with added phase transfer agent. Quaternary ammonium salts and crown ethers are typical phase transfer catalysts. It has also been shown that the surfactant charge can influence the reaction rate. For instance, reactions involving an anionic species, which is common in nucleophilic substitution reactions, will be accelerated by the use of a cationic surfactant in the microemulsion formulation. In the paper by K. Holmberg it is shown that not only can a microemulsion be regarded as an alternative to phase transfer catalysis as a way to accelerate the reaction between two incompatible reactants, the two approaches can also be combined. Addition of a phase transfer agent to a microemulsion results in a considerable rate enhancement. This can be a practically important observation. A practical example of the use of microemulsions as media for organic synthesis is for food applications. Using food grade surfactants and oils, microemulsions offer very interesting possibilities for such processes. The most frequently studied food-related reaction in microemulsion is the Maillard reaction between sugars and amino acids. The regiospecificity of certain microemulsion-based reactions have been taken advantage of in recent studies on odor generation in thermal processing of foods. The use of microemulsions seems to result in new reaction pathways as compared to reactions under conventional conditions. Also enzymatic reactions with relevance to the food area can be performed in edible microemulsion formulations. Lipase-catalyzed transesterification of triglycerides is one such example. As is pointed out in the overview by N. Garti, there is, however, still a long way to go before microemulsions become established as reaction media in normal food processes. Microemulsions may be more practically useful for the synthesis of food additives, such as emulsifiers, colorants, antioxidants and, in particular, flavors. There are several examples of the use of microemulsions as media for such syntheses. Nissim Garti Hebrew University of Jerusalem, Casali Institute of Applied Chemistry, Givat Ram Campus, Jerusalem 91904, Israel Krister Holmberg Chalmers University of Technology, Department of Applied Surface Chemistry, ¨ SE-41296 Goteborg, Sweden
Editorial overview
Reactions in microheterogeneous media
There has been much interest in recent years in the use of microemulsions and other microcompartmentalized systems as media for chemical reactions. The ability of surfactants to self-assemble into structures of defined size and shape has been found to be useful in this respect. The nanosized structures formed by the surfactant aggregation and the surfactant-stabilized oil–water interface can be used as templates for various types of reactions. In this issue of Current Opinion in Colloid and Interface Science recent advances in the use of microheterogeneous media in six different areas are highlighted. Nanosized inorganic particles can be prepared from microemulsions, in particular microemulsions of waterin-oil type. The interest in the area derives from the wellknown fact that the properties of advanced materials are critically dependent on the microstructure of the sample. Control of size, size distribution and morphology of the individual grains or crystallites are of utmost importance in order to obtain the material characteristics desired. The use of microemulsions as a reaction media has been found to be straightforward and reliable. The paper by ´ M.A. Lopez-Quintela discusses several important issues related to this type of synthesis, such as variation of reaction parameters in order to control particle size and studies of the kinetics of the particle formation. An interesting development of recent origin is that of superlattices of nanoparticles. These are made from highly monodispersed particles and they can be obtained in one-, two- or three-dimensional arrangements. Mesoporous materials can be made from surfactant liquid crystals. In this case the liquid crystal serves as a direct template for the reaction and the mesoporous material comes as a replica of the lyotropic liquid crystal. The aqueous phase normally contains a precursor for the polymerisation reaction. Hydrolysis of the precursor is normally initiated by a pH change and the hydrolysis product formed is not stable but polymerises under the conditions used. The polymerisation-gelation that occurs in the aqueous phase leads to solidification of this phase. The mesoporous, inorganic material is subsequently obtained by washing away, or by burning off, the surfactant. An alternative way to make mesoporous materials is to use a micellar solution as starting formulation.
The mesoporous structure is then gradually formed during the course of the reaction as a result of surfactant self-assembly into well-defined structures as the reaction progresses. The overview by A.E.C. Palmqvist highlights recent advances in the synthesis of the materials. An interesting conclusion drawn by the author is that silica can be prepared in all the structures represented by the different types of surfactant liquid crystals and the type of structure formed is governed by the volume fractions of surfactant and hydrolyzed precursor. The same probably holds true for other oxides, such as alumina, titania, etc. but the synthesis of these materials has not yet been so carefully investigated. A special type of mesoporous materials, so-called mesoymacroporous inorganic oxide materials, can be made from solid foams, which, in turn, are prepared from highly concentrated emulsions. The solid foams are made by polymerising the continuous phase of highly concentrated water-in-oil emulsions. These foams have very high porosity and very low density and they can be used as a macroporous scaffolds for the preparation of solid materials with both a macroporous structure— obtained as a replica of the foam structure—and a mesoporous structure. The latter is a result of self-assembly of the amphiphile, a block copolymer, which is added to the foam together with the sol–gel solution. After the solution has gelled, the material is calcined to remove the organic matter. This process, and other applications of the highly concentrated emulsions, is presented in the overview by C. Solans et al. It is shown in the paper that the gel emulsions can be used as reaction media for various types of chemical and enzymatic syntheses. Hence, these emulsions can be seen as alternatives to microemulsions as compartmentalized media for chemical reactions. Microemulsions, vesicles, lyotropic liquid crystals and other self-organized surfactant systems can also be used as media for the preparation of organic polymers. Polymerisations in such systems can be of the direct templating type, in which case the morphology of the polymeric product resembles that of the template. Polymerization inside microemulsion droplets is an example of this. Alternatively, the polymerization may
1359-0294/03/$ - see front matter 䊚 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-0294Ž03.00023-2
136
Editorial overview
give a replica of the template. The latter occurs, for instance, when a monomer polymerizes around the template structure and the template is subsequently removed (compare the formation of mesoporous materials from surfactant liquid crystals as templates). The paper by H.-P. Hentze and E.W. Kaler focuses on the direct templating, in particular the use of microemulsions, vesicles and lyotropic liquid crystals as microreactors for polymerisation. An important conclusion drawn by the authors is that for one-to-one templating to work well, it is important to balance the thermodynamic and the kinetic parameters of the reaction. Important parameters that must be under control are partitioning and diffusion of the monomer; exchange dynamics; template rigidity; and compatibility of the surfactant, monomer and polymer formed. With properly designed systems nanostructured polymers can be obtained and the technique allows for making very well defined materials, e.g. core-shell nanolattices or hollow nanop articles. Bioorganic synthesis can be made in microemulsions of water-in-oil type and in related systems. A large number of enzymes have been found to exhibit good stability and high activity in such systems, provided care is taken with regard to the choice of both solvent and surfactant. A drawback of the approach, compared to enzymatic reactions in surfactant-free systems, is that of work-up. Removal of the surfactant is usually not a trivial operation, although ways have been described that work relatively well, and this extra step, which can be quite cumbersome, probably limits the practical applicability of the technique. One must keep in mind, however, that there are applications, e.g. in the cosmetics area, where surfactant removal may not be necessary. In the overview by N.L. Klyachko and A.V. Levashov it is pointed out that synthesis in microemulsions could be of particular interest for reactions of high molecular weight materials. It has been shown that the size of the water droplet must be such that it can accommodate the two high molecular weight reactants at the same time. If the size is smaller than that, no reaction will occur. This opens up the possibility to discriminate between reactants of a different size simply by the size of the droplets of the microemulsion used as a reaction medium. The size of the droplets are governed by the water to surfactant molar ratio. An interesting development in recent years is the use of b-glycosidase for the synthesis of alkyl glycoside in water-in-oil microemulsions. In this reaction the hydrophobic reactant, octanol, was also the oil component of the microemulsion. Alkyl glycosides are of current interest as environmentally benign surfactants and the use of glycosidase-catalysed preparation methods may be practically useful. There is substantial interest in the use of microemulsions as media for organic reactions. The interest relates to the ability of microemulsions to solubilize both polar
and non-polar substances and to compartmentalize and concentrate reactants. It has been found that the use of microemulsions is an excellent way to circumvent the reagent incompatibility problem that one frequently encounters in organic synthesis, e.g. in the reaction between a lipophilic organic compound and an inorganic salt. The microemulsion approach can be regarded as an alternative to phase transfer catalysis, i.e. a two-phase system with added phase transfer agent. Quaternary ammonium salts and crown ethers are typical phase transfer catalysts. It has also been shown that the surfactant charge can influence the reaction rate. For instance, reactions involving an anionic species, which is common in nucleophilic substitution reactions, will be accelerated by the use of a cationic surfactant in the microemulsion formulation. In the paper by K. Holmberg it is shown that not only can a microemulsion be regarded as an alternative to phase transfer catalysis as a way to accelerate the reaction between two incompatible reactants, the two approaches can also be combined. Addition of a phase transfer agent to a microemulsion results in a considerable rate enhancement. This can be a practically important observation. A practical example of the use of microemulsions as media for organic synthesis is for food applications. Using food grade surfactants and oils, microemulsions offer very interesting possibilities for such processes. The most frequently studied food-related reaction in microemulsion is the Maillard reaction between sugars and amino acids. The regiospecificity of certain microemulsion-based reactions have been taken advantage of in recent studies on odor generation in thermal processing of foods. The use of microemulsions seems to result in new reaction pathways as compared to reactions under conventional conditions. Also enzymatic reactions with relevance to the food area can be performed in edible microemulsion formulations. Lipase-catalyzed transesterification of triglycerides is one such example. As is pointed out in the overview by N. Garti, there is, however, still a long way to go before microemulsions become established as reaction media in normal food processes. Microemulsions may be more practically useful for the synthesis of food additives, such as emulsifiers, colorants, antioxidants and, in particular, flavors. There are several examples of the use of microemulsions as media for such syntheses. Nissim Garti Hebrew University of Jerusalem, Casali Institute of Applied Chemistry, Givat Ram Campus, Jerusalem 91904, Israel Krister Holmberg Chalmers University of Technology, Department of Applied Surface Chemistry, ¨ SE-41296 Goteborg, Sweden