Periodic Reporting for period 4 - SUPERFOAM (Structure-Property Relations in Aqueous Foam and Their Control on a Molecular Level)
Periodo di rendicontazione: 2018-07-01 al 2020-02-29
Foams are ubiquitous in our daily lives be it as milk foam or as heat insulation. The various technological applications range from lightweight materials, waste water treatment to the recycling of rare earth metals via ion flotation, just to mention a few. The vast number of possibilities for the use of foam in industrial processes and products originates from a unique tunability of its optical, mechanical as well as chemical properties. This makes foams to an exciting object of current interdisciplinary research.
Foams are hierarchical materials and as such they are greatly affected by the arrangement and distribution of gas bubbles on a macroscopic scale as well as on thickness and composition of lamella on a mesoscopic scale. Although they are hidden in the bulk, liquid-gas interfaces are a building block of foams with overwhelming importance as their properties on a molecular scale can easily dominate hierarchical elements on larger length scales. Thus, composition, conformation and intermolecular interactions of a few molecular layers at liquid-gas interfaces - that are ubiquitous in aqueous foam - can decide whether the produced macroscopic foam has the desired properties or not. This is not only determined by the actual composition of an interface, but depends dramatically on the interactions of interfacial molecules, ions and water molecules. Making foam - particularly from aqueous solutions – is surprisingly easy as one simply has to lower the water’s surface tension e.g. by additions of amphiphiles such as proteins or surfactants. However, in most cases the produced foam is inherently unstable. The latter is actually the bottle-neck in foam formulation which is usually performed purely empirically, because starting from a specially engineered interface; one has to control the actual driving forces throughout the entire hierarchical chain. For that reason, understanding and controlling foam properties with a bottom-up approach is a major challenge in current research.
In order, to put foam formulation and also our knowledge on foams on a molecular basis we need to perform in situ studies and resolve interfaces on a molecular level. For that reason, our goal is to identify molecular building blocks which are comprised of solutes, ions and solvents at interfaces and their interactions that make the most stable foams or drive other foam properties. Once identified and understood, we can use these building blocks to break the ground for new ways in foam formulation and related fields.
Foams are hierarchical materials and as such they are greatly affected by the arrangement and distribution of gas bubbles on a macroscopic scale as well as on thickness and composition of lamella on a mesoscopic scale. Although they are hidden in the bulk, liquid-gas interfaces are a building block of foams with overwhelming importance as their properties on a molecular scale can easily dominate hierarchical elements on larger length scales. Thus, composition, conformation and intermolecular interactions of a few molecular layers at liquid-gas interfaces - that are ubiquitous in aqueous foam - can decide whether the produced macroscopic foam has the desired properties or not. This is not only determined by the actual composition of an interface, but depends dramatically on the interactions of interfacial molecules, ions and water molecules. Making foam - particularly from aqueous solutions – is surprisingly easy as one simply has to lower the water’s surface tension e.g. by additions of amphiphiles such as proteins or surfactants. However, in most cases the produced foam is inherently unstable. The latter is actually the bottle-neck in foam formulation which is usually performed purely empirically, because starting from a specially engineered interface; one has to control the actual driving forces throughout the entire hierarchical chain. For that reason, understanding and controlling foam properties with a bottom-up approach is a major challenge in current research.
In order, to put foam formulation and also our knowledge on foams on a molecular basis we need to perform in situ studies and resolve interfaces on a molecular level. For that reason, our goal is to identify molecular building blocks which are comprised of solutes, ions and solvents at interfaces and their interactions that make the most stable foams or drive other foam properties. Once identified and understood, we can use these building blocks to break the ground for new ways in foam formulation and related fields.
We have addressed surfactant proteins and polyelectrolytes and their equilibrium as well as non-equilibrium structures at air/water interfaces and their charging state and derived for these systems structure-property relations.
For polyelectrolytes where air/water interfaces were modified by oppositely charged poly(sodium 4-styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) polyelectrolyte/surfactant mixtures and which were studied on a molecular level with vibrational sum-frequency generation (SFG), tensiometry, and other surface methods. In order to deduce structure property relations, our results on interfacial PSS-/CnTA+ complexes (with n=12, 14 and 16) were compared to the stability and structure of macroscopic foam as well as to the bulk. PSS-/CTA+ complexes start to replace free CTA+ surfactants at the interface and thus reduce the interfacial electric and once fully screened at the interface, hydrophobic complexes dominate the interface and where they tend to aggregate. As a consequence, adsorbate layers with the highest film thickness, surface pressure and dilatational elasticity are formed. These surface layers provide much higher stabilities and foamabilities of polyhedral macroscopic foams. Mixtures around this concentration show precipitation after a few days, while their surfaces are in a local equilibrium state. Low concentrations result in a significant decrease in surface pressure and a complete loss in foamability. However, SFG and surface dilatational rheology provide strong evidence for the existence of PSS-/CTA+ complexes at the interface. At high concentrations, air-water interfaces are dominated by an excess of free polyelectrolytes.
For proteins we have done work on beta-lactoglobulin (BLG) and bovine serum albumin adsorption layers at air-water interfaces which were also studied with SFG and other surface methods to determine the interfacial charging state as a function of ion concentration (trivalent, divalent and monovalent) and electrolyte pH. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy – from the ubiquitous air-water interface to thin-foam films to macroscopic foam and clear structure-property relation are shown to exist in these systems. In fact, for the interaction of ions with BLG proteins, foam film measurements showed the formation of common black films modified by protein aggregates which become dominant at suitable conditions (pH, ionic strength) as micrographs of foam films clearly show. The presence of aggregates increases the stability of foam films and that of macroscopic foam due to Pickering effects, where the aggregates can form presumably a gel-like layer in the lamella which is their preferred location within foam as was demonstrated by fluorescence imaging of foams stabilized with labeled BSA proteins.
In case of surfactants, we have initially studied the mechanism of charge regulation and specific ion interaction at the air-water interfaces using SFG and foam film analysis where we used the experimentally determined disjoining pressure to study the surfactants double-layer potential and their dissociation degrees. This was done by using a new thin-film method which deploys IR interferometry to determine the thickness of the water core unambiguously. Furthermore, we can apply this method to more advanced surfactants that are responsive to external stimuli such as light and pH. In fact, active interfaces and foams that can respond to external stimuli such as light or temperature and change their chemistry on demand. For that we have shown that the photo-isomerization of surfactants such as arylazopyrazole (AAP) derivatives results in an unprecedented monolayer-to-bilayer transition upon photo-switching. This fast and reversible structural transformation helped to explain exceptionally large changes in the surface tension and surface excess upon photo-switching, and was rationalized in terms of the changing amphiphilic distribution of the molecules with respect to their conformations. We went on to show that this new mechanism can have strong effects on macroscopic length scales with superior photo-control of aqueous foams.
For polyelectrolytes where air/water interfaces were modified by oppositely charged poly(sodium 4-styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) polyelectrolyte/surfactant mixtures and which were studied on a molecular level with vibrational sum-frequency generation (SFG), tensiometry, and other surface methods. In order to deduce structure property relations, our results on interfacial PSS-/CnTA+ complexes (with n=12, 14 and 16) were compared to the stability and structure of macroscopic foam as well as to the bulk. PSS-/CTA+ complexes start to replace free CTA+ surfactants at the interface and thus reduce the interfacial electric and once fully screened at the interface, hydrophobic complexes dominate the interface and where they tend to aggregate. As a consequence, adsorbate layers with the highest film thickness, surface pressure and dilatational elasticity are formed. These surface layers provide much higher stabilities and foamabilities of polyhedral macroscopic foams. Mixtures around this concentration show precipitation after a few days, while their surfaces are in a local equilibrium state. Low concentrations result in a significant decrease in surface pressure and a complete loss in foamability. However, SFG and surface dilatational rheology provide strong evidence for the existence of PSS-/CTA+ complexes at the interface. At high concentrations, air-water interfaces are dominated by an excess of free polyelectrolytes.
For proteins we have done work on beta-lactoglobulin (BLG) and bovine serum albumin adsorption layers at air-water interfaces which were also studied with SFG and other surface methods to determine the interfacial charging state as a function of ion concentration (trivalent, divalent and monovalent) and electrolyte pH. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy – from the ubiquitous air-water interface to thin-foam films to macroscopic foam and clear structure-property relation are shown to exist in these systems. In fact, for the interaction of ions with BLG proteins, foam film measurements showed the formation of common black films modified by protein aggregates which become dominant at suitable conditions (pH, ionic strength) as micrographs of foam films clearly show. The presence of aggregates increases the stability of foam films and that of macroscopic foam due to Pickering effects, where the aggregates can form presumably a gel-like layer in the lamella which is their preferred location within foam as was demonstrated by fluorescence imaging of foams stabilized with labeled BSA proteins.
In case of surfactants, we have initially studied the mechanism of charge regulation and specific ion interaction at the air-water interfaces using SFG and foam film analysis where we used the experimentally determined disjoining pressure to study the surfactants double-layer potential and their dissociation degrees. This was done by using a new thin-film method which deploys IR interferometry to determine the thickness of the water core unambiguously. Furthermore, we can apply this method to more advanced surfactants that are responsive to external stimuli such as light and pH. In fact, active interfaces and foams that can respond to external stimuli such as light or temperature and change their chemistry on demand. For that we have shown that the photo-isomerization of surfactants such as arylazopyrazole (AAP) derivatives results in an unprecedented monolayer-to-bilayer transition upon photo-switching. This fast and reversible structural transformation helped to explain exceptionally large changes in the surface tension and surface excess upon photo-switching, and was rationalized in terms of the changing amphiphilic distribution of the molecules with respect to their conformations. We went on to show that this new mechanism can have strong effects on macroscopic length scales with superior photo-control of aqueous foams.
Using several different systems, we have shown that structure-property relations from the molecular scale of the air-water interface to foam films and bubbles to the macroscopic foam exist and can be used to tailor foam properties. This enables even remote control of foam properties when responsive/active building blocks are used. Information and knowledge that is gained in this project now offers new ways to tailor foams. Future research will extend to adaptive foams, which show self-learning properties e.g. Pavlovian-type of learning via a conditioned response.