- Email: [email protected]
Editorial overview
597
Dynamic aspects of colloids and interfaces Editorial overview Alice Gast* and Brian Robinsont Addresses "Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025, USA; e-mail: [email protected] tSchool of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, UK; e-mail: [email protected] Current Opinion in Colloids & Interface Science 1997, 2:597-599 Electronic identifier: 1359-0294-002-00597 © Current Chemistry Ltd ISSN 1359·0294 Abbreviation TIR total internal reflection
In this section, we bring together articles discussing the dynamic aspects of suspensions, together with kinetic approaches to the investigation of reactions at interfaces. We include the study of both physical forces and chemical processes at interfaces. We begin with a discussion of measurements of the interactions between individual particles. These colloidal forces dictate the bulk properties of a suspension such as its phase behavior or rheology. Thus, we have articles on colloidal crystallization and suspension structure under flow. We then turn to another, different, complex suspension, this time involving air bubbles concentrated .into a foam. Here, it is shown that the interfacial forces also dictate a unique and interesting dynamic behavior. On the chemical side, contributions are concerned with the rate-enhancement effects that are observed for reactions taking place on or in micellar aggregates; there is also a progress report on the somewhat related topic of the use of organized media to enhance metal-ion extraction and recovery from aqueous streams. There is also a detailed review concerned with the application of NMR techniques to the study of processes involving micelles and other surfactant assemblies. Clearly, much of the interesting behavior found in colloidal suspensions and interfacial phenomena arises due to the long-range forces governing structure and dynamics. Direct measurement of these forces has revolutionized the field, as the surface-force apparatus [1,2) and force microscopies have allowed verification of many force laws. These measurements generally involve macroscopic surfaces inspected quasi-statically with the aid of delicate 'spring balance' techniques. One elegant method to measure forces on particles themselves involves the measurement of the light scattered from freely moving particles located near planar interfaces. This dynamic approach to the study of particle interactions is discussed in the review of TIR microscopy by Walz (pp 600-606). The optical phenomenon of TIR is used to probe the region
near a solid/liquid interface. Upon internal reflection, light penetrates the, fluid via an exponentially decaying evanescent wave. Colloidal particles suspended in the liquid scatter light when they encounter the evanescent wave, providing a measure of the proximity of the particles to the planar interface. From the intensity of scattered light, one can infer the particle probability distribution near the interface. This probability distribution then implies the interaction potential with the interface. Adding dynamic light scattering to this technique provides a direct measurement of the hindered mobility of particles near surfaces caused by the forces acting upon them. Thus, in one experiment, as discussed by \Valz, one can measure both interparticle forces and mobility associated with the same particle. One of the most striking and notable consequences of colloidal forces acting between monodisperse particles is the formation of macroscopic crystalline arrays. Such ordered structures produce striking iridescent materials such as opals [3) as a result of Bragg diffraction of visible light from the colloidal panicles. The colloidal size scale allows these crystals to be readily studied by optical microscopy and scattering techniques. Colloidal particles are also good models for molecular systems because of the variety of forces that can be readily induced in the system by the addition of electrolyte, polymer or solvents and the convenient time scales of the ordering process. The application of statistical mechanics to the understanding of colloidal crystallization has been another active area of research [4). Thus, the review by Palberg on colloidal crystallization dynamics (pp 607-614), covers an area of extensive activity over the past two years. The many activities highlighted include new synthetic approaches to creating novel hard and soft spheres, optical techniques to study crystal growth, and models of the growth process and crystallization in restricted geometries. Particularly noteworthy is the issue of the glass transition in hard spheres. The glass transition is defined as the point where diffusion ceases and crystals can no longer nucleate. The glass transition observed in colloidal polymethylmethacrylate particles in an essentially index matched fluid occurs within a well-defined volume fraction between 0.574 and 0.581 [5,6). In recent microgravity experiments, nucleation is. observed above this threshold (see [40] in Palberg's review). Dendritic growth of hard spheres is also observed under negligible gravitational fields. Recent studies of two-dimensional colloidal crystals build on a large body of work by Murray and van Winkle [7). Of interest is the question of the nature (first-order or continuous) of the melting transition in two
598
Dynamic aspects of colloids and Interfaces
dimensions and the existence of the hexatic phase [8-tO]. This is an active area of experimental and theoretical research, which is also described in Palberg's review. Colloidal crystals, micellar solutions, polymers and liquid crystals all have common features of nonlinear flow behavior. The non-Newtonian rheology in these complex fluids arises largely from the interparticle interactions discussed above. The interparticle interactions influence suspension or solution microstructure which, in turn, dictates the rheological response. One of the best ways to probe these microstructural details is to carry out small angle scattering experiments under flowing conditions. Small angle neutron scattering is a particularly useful technique because of the ability to contrast match components of a complex fluid, for example, by chemical hydrogen/deuterium substitution to selectively study particular aspects of structure. The development of shearing cells at several of the world's neutron-scattering facilities has resulted in an explosion of research in this area in recent years. In a broad review of recent studies of macromolecular structure under shear, Hanley (pp 635-640) discusses colloidal suspensions, structured polymers, liquid crystals, surfaces and apparatus development. Some of the notable advances in the past two years include a study of binary mixtures of colloidal particles by Hunt and Zukoski (see [18] in Hanley's review). Much of the work on shearing colloidal crystals carries over into the study of structural polymers, where polymeric micelles order in much the same way as charged colloids [11]. One of the interesting features of small angle neutron scattering measurements on systems studied under flow conditions is the strong connection between structural information and theories to model rheological properties. The action of shear can influence the onset of a phase transition such as is found in anisotropic phases involving block copolymers and concentrated surfactant solutions. Even simpler systems of polymer molecules can be 'shear-induced' to undergo phase transitions, a process readily monitored via scattering. Finally, the ability to simultaneously measure rheological properties as well as structure has made new instrument development a key to advancement of the field of complex fluid rheology. Such new equipment is now available at various user facilities such as National Institute for Standards and Technology in Gaithersberg, Maryland, USA and Institut Laue Langevin in Grenoble, France. A foam is an important concentrated dispersion of a gas in a liquid. Foams arise in many contexts, ranging from shaving cream to ore purification [2]. Their ubiquity belies their complexity. Only very recent developments in light scattering have made the detailed study of foam dynamics feasible. Durian (pp 615-621) reviews the new advances in our understanding of this important area. In addition to the well known process of foam drainage, Durian highlights aspects of foam evacuation caused by thermal fluctuations. These dynamics are revealed in experiments where light
that is multiply scattered by a turbid system is evaluated for the inherent correlations between scattering events due to bubble motions. These dynamics are strongly linked to the rheological properties of the foam. Recent developments in the area of micellar catalysis are reviewed by Romsted, Bunton and Yao (pp 622-628). The similarities and differences with enzyme catalysis are discussed and it is pointed out that micelles can as easily inhibit as promote reactions. Qualitative prediction of these effects is generally possible, but there can still be difficulties with an appropriate theoretical interpretation of the kinetics, because this can require a detailed knowledge of partitioning of reactants both between the colloidal aggregates and the external phase, and internally within the aggregate itself. An alternative approach is to fall back on classical thermodynamics and activity coefficients, as described in [34] of Romsted er o/.'s review. It is also noted that the use of the term 'catalysis' is inappropriate, since, unlike a true catalyst, the presence of micelles can change the equilibrium position of the reaction (i.e. the kinetics of the forward and backward reactions are not changed to the same extent) so it is not simply the energy barrier between reactants and products which is affected, but the free energy of the initial and final states. The dynamic (in the sense of loose) structure of micelles has so far restricted their use in applications where reactant specificity is required (following substrate specificity in enzyme-catalyzed reactions) but the recent interest in understanding vesicle systems, which are more rigid, suggests this could be a profitable line for future investigations. Cerichelli is a passionate advocate of the use of NMR techniques to study colloidal aggregates, and a sense of the scope of NMR in this field is given in his and Mancini's comprehensive review (pp 641-648). It was once the case that NIvlR tended to contradict other techniques, leading to lively discussions at conferences, but this is less so nowadays. Indeed, Cerichelli and Mancini stress the use of NIvlR in conjunction with other techniques to enable the maximum amount of information on the systems to be obtained. Finally, Stevens, Perera and Grieser (pp 629-634) describe recent advances in our understanding of the important industrial process of metal-ion extraction. An obvious approach is to use a negatively charged surface formed by surfactants to attract cations before extraction by an oil-soluble ligand, and a number of papers which relate to this approach are discussed. The overall extraction process is a complex series of steps, and it is generally not predictable which will be rate-limiting. In this context, it is becoming clear that the characterization of the transfer species, which is a metal-ligand-surfactant aggregate in the organic phase, is likely to be important, as there is evidence that large reverse micellar-type species can be formed, which would have very restricted movement
Editorial overview Gast and Robinson
through the organic phase. It is valuable in these kinetic studies [0 specifically interrogate the oil/water interface and hence eliminate as far as possible kinetic effects due [0 diffusion across stagnant liquid layers close [0 the oil/water interface. Some years ago, a rotating cell device was described, which looked [0 be highly promising for further development [12], bur this work does not seem [0 have been followed up. Progress, however, has been made using TIR techniques, as mentioned earlier in this overview, which specifically explore the interface region, and this approach is described in some detail in their article (pp 629-634). Progress in this area might well be stimulated by the imposition of legislative programmes in the area of 'clean technology', where removal of deleterious metal ions released into the environment is important. This will bring fresh challenges, in that extraction of low concentrations of metal ions (possibly selectively) will be needed. The choice of ligand is normally restricted and is dictated by cost considerations, but advances in our understanding and the development of new processes will depend on a fusion of the expertise of physical-inorganic chemists, membrane materials scientists and chemical engineers, working together in a systematic way.
References 1.
Israelachvili IN: Intermolecular & Surface Forces, edn 2. San Diego: Academic Press, Inc; 1992.
599
2.
Adamson AW, Gast AP: Physical Chemistry of Surfaces, edn 6. New York: John Wiley & Sons; 1997.
3.
Darragh PJ, Gaskin AJ, Sanders JV: Opals. Sci Am 1976, 234:84-95.
4.
Lowen H: Melting freezing and colloidal suspension. Phys Reports 1994, 237:249·324.
5.
Pusey PN: Colloidal dispersions. In Liquid, Freezing and Glass Transition. Edited by Hansen D, Levesque D, Zinn-Justin J. Amsterdam: Elsevier Science 1991 :340-342.
6.
van Megen W, Underwood SM: Change in crystallization mechanism at the glass transition of colloidal spheres. Nature 1993, 362:616-618.
7.
Murray CA, van Winkle DH: Experimental observation of two' state melting in a classical two-dimensional screened coulomb system. Phys Rev Lett 1987, 58:1200-1203.
8.
Kosterlitz JM, Thouless DJ: Ordering, metastability and phase transitions in two dimensional systems. J Phys C 1973, 6:1181-1203.
9.
Halperin BI, Nelson DR: Dislocation mediated melting in two dimensions. Phys Rev B 1979, 19:2457-2484.
10.
Young AP: Melting and the vector Coulomb 9as in two dimensions. Phys Rev B 1979, 19:1855-1866.
11.
Gast AP: Polymeric micelles. 2:258-263.
12.
Albery WJ, Choudhery RA, Fisk PR: Kinetics and mechanism of interfacial reactions in the solvent extraction of copper. Disc Faraday Soc 1984, 77:53-65.
CUff
Opin Coli Interface Sci 1997,
Dynamic aspects of colloids and interfaces Editorial overview Alice Gast* and Brian Robinsont Addresses "Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025, USA; e-mail: [email protected] tSchool of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, UK; e-mail: [email protected] Current Opinion in Colloids & Interface Science 1997, 2:597-599 Electronic identifier: 1359-0294-002-00597 © Current Chemistry Ltd ISSN 1359·0294 Abbreviation TIR total internal reflection
In this section, we bring together articles discussing the dynamic aspects of suspensions, together with kinetic approaches to the investigation of reactions at interfaces. We include the study of both physical forces and chemical processes at interfaces. We begin with a discussion of measurements of the interactions between individual particles. These colloidal forces dictate the bulk properties of a suspension such as its phase behavior or rheology. Thus, we have articles on colloidal crystallization and suspension structure under flow. We then turn to another, different, complex suspension, this time involving air bubbles concentrated .into a foam. Here, it is shown that the interfacial forces also dictate a unique and interesting dynamic behavior. On the chemical side, contributions are concerned with the rate-enhancement effects that are observed for reactions taking place on or in micellar aggregates; there is also a progress report on the somewhat related topic of the use of organized media to enhance metal-ion extraction and recovery from aqueous streams. There is also a detailed review concerned with the application of NMR techniques to the study of processes involving micelles and other surfactant assemblies. Clearly, much of the interesting behavior found in colloidal suspensions and interfacial phenomena arises due to the long-range forces governing structure and dynamics. Direct measurement of these forces has revolutionized the field, as the surface-force apparatus [1,2) and force microscopies have allowed verification of many force laws. These measurements generally involve macroscopic surfaces inspected quasi-statically with the aid of delicate 'spring balance' techniques. One elegant method to measure forces on particles themselves involves the measurement of the light scattered from freely moving particles located near planar interfaces. This dynamic approach to the study of particle interactions is discussed in the review of TIR microscopy by Walz (pp 600-606). The optical phenomenon of TIR is used to probe the region
near a solid/liquid interface. Upon internal reflection, light penetrates the, fluid via an exponentially decaying evanescent wave. Colloidal particles suspended in the liquid scatter light when they encounter the evanescent wave, providing a measure of the proximity of the particles to the planar interface. From the intensity of scattered light, one can infer the particle probability distribution near the interface. This probability distribution then implies the interaction potential with the interface. Adding dynamic light scattering to this technique provides a direct measurement of the hindered mobility of particles near surfaces caused by the forces acting upon them. Thus, in one experiment, as discussed by \Valz, one can measure both interparticle forces and mobility associated with the same particle. One of the most striking and notable consequences of colloidal forces acting between monodisperse particles is the formation of macroscopic crystalline arrays. Such ordered structures produce striking iridescent materials such as opals [3) as a result of Bragg diffraction of visible light from the colloidal panicles. The colloidal size scale allows these crystals to be readily studied by optical microscopy and scattering techniques. Colloidal particles are also good models for molecular systems because of the variety of forces that can be readily induced in the system by the addition of electrolyte, polymer or solvents and the convenient time scales of the ordering process. The application of statistical mechanics to the understanding of colloidal crystallization has been another active area of research [4). Thus, the review by Palberg on colloidal crystallization dynamics (pp 607-614), covers an area of extensive activity over the past two years. The many activities highlighted include new synthetic approaches to creating novel hard and soft spheres, optical techniques to study crystal growth, and models of the growth process and crystallization in restricted geometries. Particularly noteworthy is the issue of the glass transition in hard spheres. The glass transition is defined as the point where diffusion ceases and crystals can no longer nucleate. The glass transition observed in colloidal polymethylmethacrylate particles in an essentially index matched fluid occurs within a well-defined volume fraction between 0.574 and 0.581 [5,6). In recent microgravity experiments, nucleation is. observed above this threshold (see [40] in Palberg's review). Dendritic growth of hard spheres is also observed under negligible gravitational fields. Recent studies of two-dimensional colloidal crystals build on a large body of work by Murray and van Winkle [7). Of interest is the question of the nature (first-order or continuous) of the melting transition in two
598
Dynamic aspects of colloids and Interfaces
dimensions and the existence of the hexatic phase [8-tO]. This is an active area of experimental and theoretical research, which is also described in Palberg's review. Colloidal crystals, micellar solutions, polymers and liquid crystals all have common features of nonlinear flow behavior. The non-Newtonian rheology in these complex fluids arises largely from the interparticle interactions discussed above. The interparticle interactions influence suspension or solution microstructure which, in turn, dictates the rheological response. One of the best ways to probe these microstructural details is to carry out small angle scattering experiments under flowing conditions. Small angle neutron scattering is a particularly useful technique because of the ability to contrast match components of a complex fluid, for example, by chemical hydrogen/deuterium substitution to selectively study particular aspects of structure. The development of shearing cells at several of the world's neutron-scattering facilities has resulted in an explosion of research in this area in recent years. In a broad review of recent studies of macromolecular structure under shear, Hanley (pp 635-640) discusses colloidal suspensions, structured polymers, liquid crystals, surfaces and apparatus development. Some of the notable advances in the past two years include a study of binary mixtures of colloidal particles by Hunt and Zukoski (see [18] in Hanley's review). Much of the work on shearing colloidal crystals carries over into the study of structural polymers, where polymeric micelles order in much the same way as charged colloids [11]. One of the interesting features of small angle neutron scattering measurements on systems studied under flow conditions is the strong connection between structural information and theories to model rheological properties. The action of shear can influence the onset of a phase transition such as is found in anisotropic phases involving block copolymers and concentrated surfactant solutions. Even simpler systems of polymer molecules can be 'shear-induced' to undergo phase transitions, a process readily monitored via scattering. Finally, the ability to simultaneously measure rheological properties as well as structure has made new instrument development a key to advancement of the field of complex fluid rheology. Such new equipment is now available at various user facilities such as National Institute for Standards and Technology in Gaithersberg, Maryland, USA and Institut Laue Langevin in Grenoble, France. A foam is an important concentrated dispersion of a gas in a liquid. Foams arise in many contexts, ranging from shaving cream to ore purification [2]. Their ubiquity belies their complexity. Only very recent developments in light scattering have made the detailed study of foam dynamics feasible. Durian (pp 615-621) reviews the new advances in our understanding of this important area. In addition to the well known process of foam drainage, Durian highlights aspects of foam evacuation caused by thermal fluctuations. These dynamics are revealed in experiments where light
that is multiply scattered by a turbid system is evaluated for the inherent correlations between scattering events due to bubble motions. These dynamics are strongly linked to the rheological properties of the foam. Recent developments in the area of micellar catalysis are reviewed by Romsted, Bunton and Yao (pp 622-628). The similarities and differences with enzyme catalysis are discussed and it is pointed out that micelles can as easily inhibit as promote reactions. Qualitative prediction of these effects is generally possible, but there can still be difficulties with an appropriate theoretical interpretation of the kinetics, because this can require a detailed knowledge of partitioning of reactants both between the colloidal aggregates and the external phase, and internally within the aggregate itself. An alternative approach is to fall back on classical thermodynamics and activity coefficients, as described in [34] of Romsted er o/.'s review. It is also noted that the use of the term 'catalysis' is inappropriate, since, unlike a true catalyst, the presence of micelles can change the equilibrium position of the reaction (i.e. the kinetics of the forward and backward reactions are not changed to the same extent) so it is not simply the energy barrier between reactants and products which is affected, but the free energy of the initial and final states. The dynamic (in the sense of loose) structure of micelles has so far restricted their use in applications where reactant specificity is required (following substrate specificity in enzyme-catalyzed reactions) but the recent interest in understanding vesicle systems, which are more rigid, suggests this could be a profitable line for future investigations. Cerichelli is a passionate advocate of the use of NMR techniques to study colloidal aggregates, and a sense of the scope of NMR in this field is given in his and Mancini's comprehensive review (pp 641-648). It was once the case that NIvlR tended to contradict other techniques, leading to lively discussions at conferences, but this is less so nowadays. Indeed, Cerichelli and Mancini stress the use of NIvlR in conjunction with other techniques to enable the maximum amount of information on the systems to be obtained. Finally, Stevens, Perera and Grieser (pp 629-634) describe recent advances in our understanding of the important industrial process of metal-ion extraction. An obvious approach is to use a negatively charged surface formed by surfactants to attract cations before extraction by an oil-soluble ligand, and a number of papers which relate to this approach are discussed. The overall extraction process is a complex series of steps, and it is generally not predictable which will be rate-limiting. In this context, it is becoming clear that the characterization of the transfer species, which is a metal-ligand-surfactant aggregate in the organic phase, is likely to be important, as there is evidence that large reverse micellar-type species can be formed, which would have very restricted movement
Editorial overview Gast and Robinson
through the organic phase. It is valuable in these kinetic studies [0 specifically interrogate the oil/water interface and hence eliminate as far as possible kinetic effects due [0 diffusion across stagnant liquid layers close [0 the oil/water interface. Some years ago, a rotating cell device was described, which looked [0 be highly promising for further development [12], bur this work does not seem [0 have been followed up. Progress, however, has been made using TIR techniques, as mentioned earlier in this overview, which specifically explore the interface region, and this approach is described in some detail in their article (pp 629-634). Progress in this area might well be stimulated by the imposition of legislative programmes in the area of 'clean technology', where removal of deleterious metal ions released into the environment is important. This will bring fresh challenges, in that extraction of low concentrations of metal ions (possibly selectively) will be needed. The choice of ligand is normally restricted and is dictated by cost considerations, but advances in our understanding and the development of new processes will depend on a fusion of the expertise of physical-inorganic chemists, membrane materials scientists and chemical engineers, working together in a systematic way.
References 1.
Israelachvili IN: Intermolecular & Surface Forces, edn 2. San Diego: Academic Press, Inc; 1992.
599
2.
Adamson AW, Gast AP: Physical Chemistry of Surfaces, edn 6. New York: John Wiley & Sons; 1997.
3.
Darragh PJ, Gaskin AJ, Sanders JV: Opals. Sci Am 1976, 234:84-95.
4.
Lowen H: Melting freezing and colloidal suspension. Phys Reports 1994, 237:249·324.
5.
Pusey PN: Colloidal dispersions. In Liquid, Freezing and Glass Transition. Edited by Hansen D, Levesque D, Zinn-Justin J. Amsterdam: Elsevier Science 1991 :340-342.
6.
van Megen W, Underwood SM: Change in crystallization mechanism at the glass transition of colloidal spheres. Nature 1993, 362:616-618.
7.
Murray CA, van Winkle DH: Experimental observation of two' state melting in a classical two-dimensional screened coulomb system. Phys Rev Lett 1987, 58:1200-1203.
8.
Kosterlitz JM, Thouless DJ: Ordering, metastability and phase transitions in two dimensional systems. J Phys C 1973, 6:1181-1203.
9.
Halperin BI, Nelson DR: Dislocation mediated melting in two dimensions. Phys Rev B 1979, 19:2457-2484.
10.
Young AP: Melting and the vector Coulomb 9as in two dimensions. Phys Rev B 1979, 19:1855-1866.
11.
Gast AP: Polymeric micelles. 2:258-263.
12.
Albery WJ, Choudhery RA, Fisk PR: Kinetics and mechanism of interfacial reactions in the solvent extraction of copper. Disc Faraday Soc 1984, 77:53-65.
CUff
Opin Coli Interface Sci 1997,