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Five reviews on thin liquid films and five reviews on foams
Current Opinion in Colloid & Interface Science 15 (2010) 295–296
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s
Editorial overview
Five reviews on thin liquid films and five reviews on foams This is the second issue of the section “Thin Films and Foams” of Current Opinion in Colloid and Interface Science (COCIS). Practically no topic of the present 10 review articles overlaps the topics of the reviews published three years ago in our previous issue (COCIS, Vol. 13, No. 3, June 2008). This fact shows that both scientific areas – the thin liquid films and the foams – are rather vast and important areas of the colloid and interface science. The thin liquid films exist in all dispersion systems: foam films, emulsion films, films between colloid particles (nanoparticles!), asymmetric films at hetero-coagulation or at wetting phenomena, etc. Their study provides valuable information about the interactions in them and the kinetics of thinning and rupture, which determine the stability of the corresponding colloidal systems. Only one type of thin liquid film is relevant to the dispersion system foam—the foam film. On the other hand the foams are systems of great importance in industry and everyday life. In view of that we offer now to the reader two collections with equal number of review articles in each of these two large scientific areas: five reviews on thin liquid films and five reviews on foams. The group of thin liquid films opens with a review on the new developments of the theories of electrostatic and Van der Waals disjoining pressures. The subsequent four articles provide overviews of the recently published results about different types of thin liquid films, especially foam films, emulsion films, wetting films. Mainly the role of the adsorption layers in the films from solutions of polyelectrolytes, surfactants, polymers, natural substances, as well as the role of the disjoining pressures, are considered. The review by Boinovich considers the further progress in the theory of DLVO surface forces in thin liquid films beyond the classic DLVO theory. Due to increasing accuracy and a number of new experimental methods our understanding of the detailed mechanisms of the two components of disjoining pressure is significantly improved recently. However the theoretical analysis as well as numeric experiments and simulations are still highly demanded to provide better understanding of the experimental findings. The major recent advances are related to more accurate accounting for the ionic nature of the liquid film in the calculations of Van der Waals forces and for the impact of dispersion interaction of the ions with other ions or with confining phases on the pequliarities of the ion–electrostatic interactions in liquid films. There is essential progress in the description of Van der Waals and electrostatic surface forces and poorer understanding of the phenomena related to peculiarities of the molecular structure of liquids inside a thin liquid film, associated to both solvent and solute or surfactant molecules. The review article by Üzüm, Kristen, and von Klitzing addresses the influence of polyelectrolytes on the statics and dynamics of thin liquid films. Polyelectrolytes in any type of thin liquid film can be positioned either on the film's surfaces (adsorbed or grafted) or in the film's bulk. The transition between these two states mostly depends on the charge and concentration of the polymer and the thickness of the film. Chain 1359-0294/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2010.06.001
length also has an effect on the polyelectrolyte position and conformation, leading to a selective existence of short and long chains in the film's bulk. In most of the systems, ionic strength is observed to change the chain–chain and surface–chain interactions dramatically. The dynamics of foams can be understood by monitoring the stratification process, which leads to oscillatory disjoining pressure isotherms. Polyelectrolytes can possess stabilizing or destabilizing effects on the foam films depending on their ratio to the surfactant in the system. Specific type of polymers called polymeric surfactants can stabilize emulsion films as well as foam films in the absence of any additional surfactant. The article by Mileva is focused on most recent studies, in which the interconnections of interfacial-layer and foam–film properties are innate for the performance of the respective systems and have been clarified to highest possible degree. The potential of combining microscopic thin liquid film instrumentation and advanced tensiometry as investigation tools in the study of premicellar nanostructures in aqueous surfactant solutions is enlightened. These investigations cover fundamental issues as the initial onset of premicelles in single-surfactant solutions, explore intricate topics as the interplay of competitive and cooperative adsorption in mixtures and give hints for fine-tuning the design of various formulations to be applied in industrial applications. Different cases have been considered: foam films from aqueous solutions, containing (i) single surfactants; (ii) mixtures from surfactants; (iii) mixtures of polymers including proteins and low-molecular-mass surfactants. The interaction mechanism (both in the interface and in the bulk) depends on the nature of the protein and on the type of the surfactant, the concentration of the surfactant being a significant tuning parameter. The short review by Khristov and Czarnecki is dedicated to the thin emulsion films stabilized by natural and polymeric surfactants. If the vast practical application of emulsions is placed on the one side of a balance and the articles exploring oil/water/oil and water/oil/water emulsion films on the other, it will weighs in favor of the former. Detailed information derived from quantitative studies of such films is still surprisingly scarce. It may be said that certain stirring in that direction is becoming noticeable, evidenced also with the new methods and techniques developed in the recent years. The first part of this article summarizes recently published results on o/w/o emulsion films stabilized by polymeric surfactants. Two types of surface forces, which determine the film's stability, have been distinguished, namely: DLVO forces at low electrolyte concentrations and non-DLVO-forces at high electrolyte concentrations. Non-DLVO-forces are mainly steric surface forces of the brush-to-brush or loop-to-loop interaction type. The second part deals with w/o/w emulsion films stabilized by natural surfactants present in crude oil, mainly asphaltenes. The article by Saramago presents an overview of the studies on thin liquid wetting films reported during the last five years. A description of the most used experimental techniques for measurement of the disjoining pressure and the film thickness is provided. The
296
Editorial overview
novelty in experimental grounds is the use of atomic force microscopy to measure disjoining pressures in films of highly viscous liquids. The state-of-art of the surface forces acting in wetting films from different liquids and solutions are reviewed. The recent advances involve studies of the stability conditions of films obtained from complex fluids (polymer solutions, lubricants, liquid crystals) and the modification of the solid surface to control the film stability. The change of pH and electrolyte concentration allow identification of the role of electrostatic interactions. Studies of films from polymer solutions yield information on steric forces due to the polymer chains adsorbed. The problem of dewetting is the focus of several papers. The topics of the reviews in the group of foams are rather specific: monodisperse foams, non-aqueous foams, aqueous protein foams, Ostwald ripening of foams, foam accumulation and separation. Reviews on these topics are rarity up to now or do not exist at all (not only in COCIS). That is why, unlike the articles on thin liquid films where the prevailing number of references is published recently, much more references in the group of foams, especially in the last two articles, are older (over 25% published in the 20th century). Certainly the older papers are discussed in connection with the new results. The very comprehensive, detailed, and informative review article by Drenckhan and Langevin considers the monodisperse foams, including one, two and three dimensional foams. Even though monodisperse foams are in some sense simplified foams, their physical and physico-chemical properties are everything else but trivial—and in many cases still poorly understood. It turned out that this subject is very rich in recently published papers. On the other hand, as far as the authors are aware, no review on the general topic of monodisperse foams exists up to date, although it is really emerging as an important subject in the fundamental and applied foam research. That is why this review article is unusually long for a COCIS review and it contains an unusually large number of references. Most major recent advances in the knowledge on monodisperse foams are centred around the ability to generate them with excellent control of bubble sizes down to a few micrometers thanks to the development of appropriate micro- and millifluidic techniques. As a natural consequence, monodisperse liquid and solid foams are playing an increasingly important role. Nevertheless monodisperse foams have been much less exploited than they should have been in view of the control over the foam properties which they provide. The review article by Friberg addresses the non-aqueous foams. The multi-phase approach to clarify the fundamentals of the stability of foams from non-aqueous liquids has been further developed. The fundamentals affecting the stability of non-aqueous foams are discussed, calling attention to the fact that petroleum based surfactants do not reduce surface tension of common oils and, as a consequence, surface tension is not an operational factor for the stability. With surface forces excluded, the stability of foams from one-phase non-aqueous liquids depends solely on drainage rate, i.e. rheological properties of the film material per se. However, the vast majority of non-aqueous foams emanate from multi-phase condensed media, necessitating a multiphase approach to obtain a useful prediction of foam stability for real systems. In the multi-phase approach the liquid film vehicle is combined with particles of lyotropic liquid crystals and solids, of which the latter provide superior stabilization. The two-phase liquid/liquid crystal line of attack for the liquid oils variety has been extended to foams stabilized by solid particles, in turn leading to progress in the fundamental science of solid metal foams and plastic foams. Wierenga and Gruppen present in their review new views on foams from protein solutions. The molecular properties of proteins and their foam forming and stabilizing properties are typically linked to the adsorption kinetics and the interfacial properties, as well as to the properties of the foam films. However there is not yet a “unifying” theory on protein stabilized foams. There is a lack of quantitative parameters and rules, thus the advance in understanding of protein
foams seems to progress slowly. This is by some authors attributed to the complexity of the system, but the literature also shows that certain ideas seem to resist change. There are some interesting articles that offer a different point of view. The review provides an insight in the different ways in which the proteins in foamed systems are described. Recent results show that protein adsorption and subsequent changes in interfacial properties could be described in colloidal terms such as net charge, exposed hydrophobicity and size of the proteins. This description would decrease the gap between the description of protein and non-protein adsorbed layers and enables the comparison of interfacial layers of surfactants, proteins and particles. The review by Stevenson is focused on the inter-bubble gas diffusion in foams. Ostwald ripening of gas/liquid foam is a process in which large bubbles grow at the expense of small bubbles because pressure differences drive inter-bubble gas diffusion. The rather overlooked theory of Robert Lemlich, for relatively wet foam, is revisited, its analogy with Ostwald ripening of metal grains is discussed, and this is put into context recent advances. Lemlich's theory assumes that the rate of disproportionation is governed by gas mass transfer, whereas recent work has suggested that it may be mechanically resisted by contraction and expansion of gas/liquid interfaces. The exact generalisation to three-dimensions of the von Neumann's two-dimensional grain growth theory has the potential to enable predictions of the evolution of gas/liquid foam due to interbubble gas diffusion, if the starting topology is known. Developments in tomographies that can image foam and other experimental techniques have the potential to enable this model to be verified. The single bubble technique of Pitois may prove invaluable in investigating these processes. The article by Kruglyakov considers the theoretical and experimental studies devoted to the peculiarities of foam adsorption accumulation and separation of substances dissolved (in molecular, ionic, or micellar form) in the liquid phase of a dry foam. It is demonstrated that the most efficient method for improvement of all characteristics of accumulation remains the foam pressure drop technique. The analytical dependences of these characteristics on the structural parameters of the foam, and their interdependences are analysed and quantitative relationships have been obtained. The dependences of the accumulation coefficient on the expansion factor, on the foam dispersity, and on the capillary pressure in a polyhedral foam, both for individual surfactants and their mixtures are analysed. The reasons for accumulation restrictions of the foam concentration method are discussed. A theoretical analysis of the processes taking place in dynamic foam shows the possibility of different modes of surfactant accumulation within top foam layer. A new approach to modeling continuous foam accumulation with external reflux is considered. Examples of partial separation of surfactants in foam are given. We hope that the present review articles, together with those in the previous issue (published in 2008) of section “Thin Films and Foams”, have already composed a considerable collection of 21 reviews which cover many important topics in these two scientific areas. Certainly other significant topics are still missing. The future issues of this section of Current Opinion in Colloid and Interface Science should fill up the gap.
Dimo Platikanov University of Sofia, Bulgaria Corresponding author. E-mail address: [email protected]fia.bg. Dotchi Exerowa Bulgarian Academy of Sciences, Sofia, Bulgaria E-mail address: [email protected].
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s
Editorial overview
Five reviews on thin liquid films and five reviews on foams This is the second issue of the section “Thin Films and Foams” of Current Opinion in Colloid and Interface Science (COCIS). Practically no topic of the present 10 review articles overlaps the topics of the reviews published three years ago in our previous issue (COCIS, Vol. 13, No. 3, June 2008). This fact shows that both scientific areas – the thin liquid films and the foams – are rather vast and important areas of the colloid and interface science. The thin liquid films exist in all dispersion systems: foam films, emulsion films, films between colloid particles (nanoparticles!), asymmetric films at hetero-coagulation or at wetting phenomena, etc. Their study provides valuable information about the interactions in them and the kinetics of thinning and rupture, which determine the stability of the corresponding colloidal systems. Only one type of thin liquid film is relevant to the dispersion system foam—the foam film. On the other hand the foams are systems of great importance in industry and everyday life. In view of that we offer now to the reader two collections with equal number of review articles in each of these two large scientific areas: five reviews on thin liquid films and five reviews on foams. The group of thin liquid films opens with a review on the new developments of the theories of electrostatic and Van der Waals disjoining pressures. The subsequent four articles provide overviews of the recently published results about different types of thin liquid films, especially foam films, emulsion films, wetting films. Mainly the role of the adsorption layers in the films from solutions of polyelectrolytes, surfactants, polymers, natural substances, as well as the role of the disjoining pressures, are considered. The review by Boinovich considers the further progress in the theory of DLVO surface forces in thin liquid films beyond the classic DLVO theory. Due to increasing accuracy and a number of new experimental methods our understanding of the detailed mechanisms of the two components of disjoining pressure is significantly improved recently. However the theoretical analysis as well as numeric experiments and simulations are still highly demanded to provide better understanding of the experimental findings. The major recent advances are related to more accurate accounting for the ionic nature of the liquid film in the calculations of Van der Waals forces and for the impact of dispersion interaction of the ions with other ions or with confining phases on the pequliarities of the ion–electrostatic interactions in liquid films. There is essential progress in the description of Van der Waals and electrostatic surface forces and poorer understanding of the phenomena related to peculiarities of the molecular structure of liquids inside a thin liquid film, associated to both solvent and solute or surfactant molecules. The review article by Üzüm, Kristen, and von Klitzing addresses the influence of polyelectrolytes on the statics and dynamics of thin liquid films. Polyelectrolytes in any type of thin liquid film can be positioned either on the film's surfaces (adsorbed or grafted) or in the film's bulk. The transition between these two states mostly depends on the charge and concentration of the polymer and the thickness of the film. Chain 1359-0294/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2010.06.001
length also has an effect on the polyelectrolyte position and conformation, leading to a selective existence of short and long chains in the film's bulk. In most of the systems, ionic strength is observed to change the chain–chain and surface–chain interactions dramatically. The dynamics of foams can be understood by monitoring the stratification process, which leads to oscillatory disjoining pressure isotherms. Polyelectrolytes can possess stabilizing or destabilizing effects on the foam films depending on their ratio to the surfactant in the system. Specific type of polymers called polymeric surfactants can stabilize emulsion films as well as foam films in the absence of any additional surfactant. The article by Mileva is focused on most recent studies, in which the interconnections of interfacial-layer and foam–film properties are innate for the performance of the respective systems and have been clarified to highest possible degree. The potential of combining microscopic thin liquid film instrumentation and advanced tensiometry as investigation tools in the study of premicellar nanostructures in aqueous surfactant solutions is enlightened. These investigations cover fundamental issues as the initial onset of premicelles in single-surfactant solutions, explore intricate topics as the interplay of competitive and cooperative adsorption in mixtures and give hints for fine-tuning the design of various formulations to be applied in industrial applications. Different cases have been considered: foam films from aqueous solutions, containing (i) single surfactants; (ii) mixtures from surfactants; (iii) mixtures of polymers including proteins and low-molecular-mass surfactants. The interaction mechanism (both in the interface and in the bulk) depends on the nature of the protein and on the type of the surfactant, the concentration of the surfactant being a significant tuning parameter. The short review by Khristov and Czarnecki is dedicated to the thin emulsion films stabilized by natural and polymeric surfactants. If the vast practical application of emulsions is placed on the one side of a balance and the articles exploring oil/water/oil and water/oil/water emulsion films on the other, it will weighs in favor of the former. Detailed information derived from quantitative studies of such films is still surprisingly scarce. It may be said that certain stirring in that direction is becoming noticeable, evidenced also with the new methods and techniques developed in the recent years. The first part of this article summarizes recently published results on o/w/o emulsion films stabilized by polymeric surfactants. Two types of surface forces, which determine the film's stability, have been distinguished, namely: DLVO forces at low electrolyte concentrations and non-DLVO-forces at high electrolyte concentrations. Non-DLVO-forces are mainly steric surface forces of the brush-to-brush or loop-to-loop interaction type. The second part deals with w/o/w emulsion films stabilized by natural surfactants present in crude oil, mainly asphaltenes. The article by Saramago presents an overview of the studies on thin liquid wetting films reported during the last five years. A description of the most used experimental techniques for measurement of the disjoining pressure and the film thickness is provided. The
296
Editorial overview
novelty in experimental grounds is the use of atomic force microscopy to measure disjoining pressures in films of highly viscous liquids. The state-of-art of the surface forces acting in wetting films from different liquids and solutions are reviewed. The recent advances involve studies of the stability conditions of films obtained from complex fluids (polymer solutions, lubricants, liquid crystals) and the modification of the solid surface to control the film stability. The change of pH and electrolyte concentration allow identification of the role of electrostatic interactions. Studies of films from polymer solutions yield information on steric forces due to the polymer chains adsorbed. The problem of dewetting is the focus of several papers. The topics of the reviews in the group of foams are rather specific: monodisperse foams, non-aqueous foams, aqueous protein foams, Ostwald ripening of foams, foam accumulation and separation. Reviews on these topics are rarity up to now or do not exist at all (not only in COCIS). That is why, unlike the articles on thin liquid films where the prevailing number of references is published recently, much more references in the group of foams, especially in the last two articles, are older (over 25% published in the 20th century). Certainly the older papers are discussed in connection with the new results. The very comprehensive, detailed, and informative review article by Drenckhan and Langevin considers the monodisperse foams, including one, two and three dimensional foams. Even though monodisperse foams are in some sense simplified foams, their physical and physico-chemical properties are everything else but trivial—and in many cases still poorly understood. It turned out that this subject is very rich in recently published papers. On the other hand, as far as the authors are aware, no review on the general topic of monodisperse foams exists up to date, although it is really emerging as an important subject in the fundamental and applied foam research. That is why this review article is unusually long for a COCIS review and it contains an unusually large number of references. Most major recent advances in the knowledge on monodisperse foams are centred around the ability to generate them with excellent control of bubble sizes down to a few micrometers thanks to the development of appropriate micro- and millifluidic techniques. As a natural consequence, monodisperse liquid and solid foams are playing an increasingly important role. Nevertheless monodisperse foams have been much less exploited than they should have been in view of the control over the foam properties which they provide. The review article by Friberg addresses the non-aqueous foams. The multi-phase approach to clarify the fundamentals of the stability of foams from non-aqueous liquids has been further developed. The fundamentals affecting the stability of non-aqueous foams are discussed, calling attention to the fact that petroleum based surfactants do not reduce surface tension of common oils and, as a consequence, surface tension is not an operational factor for the stability. With surface forces excluded, the stability of foams from one-phase non-aqueous liquids depends solely on drainage rate, i.e. rheological properties of the film material per se. However, the vast majority of non-aqueous foams emanate from multi-phase condensed media, necessitating a multiphase approach to obtain a useful prediction of foam stability for real systems. In the multi-phase approach the liquid film vehicle is combined with particles of lyotropic liquid crystals and solids, of which the latter provide superior stabilization. The two-phase liquid/liquid crystal line of attack for the liquid oils variety has been extended to foams stabilized by solid particles, in turn leading to progress in the fundamental science of solid metal foams and plastic foams. Wierenga and Gruppen present in their review new views on foams from protein solutions. The molecular properties of proteins and their foam forming and stabilizing properties are typically linked to the adsorption kinetics and the interfacial properties, as well as to the properties of the foam films. However there is not yet a “unifying” theory on protein stabilized foams. There is a lack of quantitative parameters and rules, thus the advance in understanding of protein
foams seems to progress slowly. This is by some authors attributed to the complexity of the system, but the literature also shows that certain ideas seem to resist change. There are some interesting articles that offer a different point of view. The review provides an insight in the different ways in which the proteins in foamed systems are described. Recent results show that protein adsorption and subsequent changes in interfacial properties could be described in colloidal terms such as net charge, exposed hydrophobicity and size of the proteins. This description would decrease the gap between the description of protein and non-protein adsorbed layers and enables the comparison of interfacial layers of surfactants, proteins and particles. The review by Stevenson is focused on the inter-bubble gas diffusion in foams. Ostwald ripening of gas/liquid foam is a process in which large bubbles grow at the expense of small bubbles because pressure differences drive inter-bubble gas diffusion. The rather overlooked theory of Robert Lemlich, for relatively wet foam, is revisited, its analogy with Ostwald ripening of metal grains is discussed, and this is put into context recent advances. Lemlich's theory assumes that the rate of disproportionation is governed by gas mass transfer, whereas recent work has suggested that it may be mechanically resisted by contraction and expansion of gas/liquid interfaces. The exact generalisation to three-dimensions of the von Neumann's two-dimensional grain growth theory has the potential to enable predictions of the evolution of gas/liquid foam due to interbubble gas diffusion, if the starting topology is known. Developments in tomographies that can image foam and other experimental techniques have the potential to enable this model to be verified. The single bubble technique of Pitois may prove invaluable in investigating these processes. The article by Kruglyakov considers the theoretical and experimental studies devoted to the peculiarities of foam adsorption accumulation and separation of substances dissolved (in molecular, ionic, or micellar form) in the liquid phase of a dry foam. It is demonstrated that the most efficient method for improvement of all characteristics of accumulation remains the foam pressure drop technique. The analytical dependences of these characteristics on the structural parameters of the foam, and their interdependences are analysed and quantitative relationships have been obtained. The dependences of the accumulation coefficient on the expansion factor, on the foam dispersity, and on the capillary pressure in a polyhedral foam, both for individual surfactants and their mixtures are analysed. The reasons for accumulation restrictions of the foam concentration method are discussed. A theoretical analysis of the processes taking place in dynamic foam shows the possibility of different modes of surfactant accumulation within top foam layer. A new approach to modeling continuous foam accumulation with external reflux is considered. Examples of partial separation of surfactants in foam are given. We hope that the present review articles, together with those in the previous issue (published in 2008) of section “Thin Films and Foams”, have already composed a considerable collection of 21 reviews which cover many important topics in these two scientific areas. Certainly other significant topics are still missing. The future issues of this section of Current Opinion in Colloid and Interface Science should fill up the gap.
Dimo Platikanov University of Sofia, Bulgaria Corresponding author. E-mail address: [email protected]fia.bg. Dotchi Exerowa Bulgarian Academy of Sciences, Sofia, Bulgaria E-mail address: [email protected].