Schematic principle of vibrational sum-frequency generation (SFG) spectroscopy that we use in our group to study the molecular structure and charging at interfaces.
© B. Braunschweig 2021

Research Strategy and Concept

Fluid interfaces are present in various different soft matter materials where they dominate both microscopic and macroscopic properties and their functions. Increasing our level of understanding on the physical chemistry of fluid interfaces is, therefore, of great importance not only for molecular self-assembly, bio-macromolecules at interfaces, colloid science, and catalysis, but also in soft nanoscience. In addition to a basic understanding of the physics and chemistry at fluid interfaces there is great potential to use that knowledge in order to tune or to design new interface-controlled and hierarchical soft matter materials such as foams and emulsions with new properties like adaptive or responsive functions.
Prerequisite to master this challenge is to identify molecular building blocks such as simple surfactants but also macromolecules and their aggregates at liquid interfaces, which determine properties on larger length scales.
In our group, we use surface specific nonlinear optical spectroscopy such as sum-frequency generation (SFG) and second-harmonic scattering (SHS) orthogonal to existing soft matter research topics. In addition, we combine the results from SFG and SHS with complementary methods such as tensiometry (pendent drop), surface dilatational rheology (oscillating drop) and disjoining pres-sure isotherms using a thin-film pressure balance. From previous and current research of the group, three focal topics for research in the next years can be identified:

  • Development of experimental methods for interface spectroscopy
  • Adsorption, ion specific effects and structure property-relation in soft matter
  • Responsive interfaces and soft matter that react to light and temperature stimuli (Equilibrium vs Nonequilibrium properties, local vs global changes)
  • Electrode/Electrolyte Interfaces

Structure-property relations in aqueous foam, where macroscopic properties such as stability and bubble size distributions are controlled by the interfacial properties of surface-active molecular building blocks like surfactants, polymers, and proteins.
© B. Braunschweig and E. Weißenborn 2021
  • Responsive Interfaces and Materials with Remote Control

    In our work, we show that fluid interfaces can be a unique platform to test the responsiveness of switchable building blocks on a molecular level. That is because interfaces can be modified through adsorption and subsequent self-assembly. In addition, we have provided several examples on how interfacial properties can dominate properties on larger length scales including macroscopic behavior if prevailing structure-property relations like in aqueous foam exist. Because the above approach allows imprinting material properties by the molecular design of existing interfaces, the use of molecular switches at interfaces allows us to manipulate both interfacial and macroscopic properties through external stimuli like temperature and light.

    Photoswitchable Amphiphiles: In particular, light stimuli can trigger responses that are localized in both time and space and thus offer four-dimensional control of material properties. In future work, we propose to use amphiphilic molecular switches such as arylazopyrazole derivatives which are synthesized in our group with different electrostatic and dispersive interactions at the interface and the ability to change the latter under light irradiation with high yield.

    As a second building block, we are using thermoresponsive polymers such as hydroxypropyl cellulose (HPC). We are currently continuing this work where we investigate HPC mixtures with arylazopyrazole surfactants in order to achieve multi-responsive functions (here temperature and light).

    • M. Schnurbus, R.A. Campbell, J. Droste, C. Honnigfort, D. Glikman, P. Gutfreund, M. R. Hansen, B. Braunschweig, Photo-Switchable Surfactants for Responsive Air–Water Interfaces: Azo versus Ar-ylazopyrazole Amphiphiles, J. Phys. Chem. B 124, 6913–6923 (2020)
    • M. Schnurbus, M. Kabat, E. Jarek, M. Krzan, P. Warszynski, B. Braunschweig, Spiropyran Sulfonates for Photo and pH Responsive Air-Water Interfaces and Aqueous Foam, Langmuir 36, 6871–6879 (2020) (Invited)
    • C. Honnigfort, R.A. Campbell, J. Droste, P. Gutfreund, M.R. Hansen, B.J. Ravoo, B. Braunschweig, Unexpected Monolayer-to-Bilayer Transition of Arylazopyrazole Surfactants Facilitates Superior Pho-to-Control of Fluid Interfaces and Colloids, Chem. Sci. 11, 2085-2092 (2020).
    • E. Weißenborn and B. Braunschweig, Hydroxypropyl Cellulose as a Green Polymer for Thermo-Responsive Aqueous Foams, Soft Matter 15, 2876-2883 (2019)
    • M. Schnurbus, L. Stricker, B.J. Ravoo, B. Braunschweig, Smart Air-Water Interfaces with Arylazopy-razole Surfactants and Their Role in Photoresponsive Aqueous Foam, Langmuir 34, 6028-6035 (2018)

    Left: Light-switchable polymer surfactant interaction using a thermoresponsive polymer and a photoswitchable surfactant. / Right: Monolayer to bilayer transition of arylazopyrazole surfactants and characterization with sum-frequency generation (SFG) and neutron reflectometry (NR).
    © Left: Reproduced from Weißenborn et al. Chem. Commun., 2021, DOI: 10.1039/D1CC02054 with permission from The Royal Society of Chemistry / Right: Adapted with permission from Honnigfort et al. Chem. Sci., 2020, 11, 2085 DOI: 10.1039/C9SC05490A - Published by The Royal Society of Chemistry
  • Switchable wetting property using photoresponsive moieties that decorate either the solid-liquid or the liquid-gas interfaces.
    © B. Braunschweig and C. Honnigfort

    Molecular Self-Assembly at Solid Surfaces and Structure-Property Relations and Wetting Dynamics of Organic Thin Films with Photo-Switches

    We study dynamic wetting on photo-switchable substrates. Such “smart surfaces” can reversibly change their wetting properties are of great interest for applications such as self-cleaning surfaces, microfluidics or tunable lenses just to mention a few. However, even for passive surfaces that do not change their molecular properties when they are in contact with a liquid, a quantitative description the wetting dynamics is still a challenging issue. That is in particularly true when changes on the molecular scale due to adaptation of the substrate in different chemical environments or due to photo-switching of the substrate need to be taken into account as well. Previously, we have shown that even very simple soft self-assembled monolayers that are composed of long alkyl chains (e.g. C12 or C18), show a different molecular structure when they are wetted or dewetted from a polar solvent. For that reason, we believe that an understanding of the wetting dynamics of photo-switchable substrates requires molecular level information on the substrate dynamics and structure changes under different light conditions as well as different chemical environments.

    • C. Meltzer, H. Yu, W. Peukert, B. Braunschweig, Molecular structure of octadecylphosphonic acids during their self-assembly on α-Al2O3(0001), Phys. Chem. Chem. Phys. 20, 19382-19389 (2018) (PCCP Themed Hot Articles)
    • A. Pathak, A. Bora, B. Braunschweig, C. Meltzer, H. Yan, P. Lemmens, W. Daum, J. Schwartz and M. Tornow, Nanocylindrical confinement imparts highest structural order in molecular self-assembly of organophosphonates on aluminum oxide, Nanoscale 9, 6291-6295 (2017)
    • M. A. Frank, C. Meltzer, B. Braunschweig, W. Peukert, A. R. Boccaccini, S. Virtanen, Functionalization of steel surfaces with organic acids: influence on wetting and corrosion behavior, Appl. Surf. Sci. 404, 326 (2017)
    • C. Meltzer, H. Dietrich, D. Zahn, W. Peukert and B. Braunschweig, Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir−Blodgett Transfer, Langmuir 31, 4678 (2015)
    • C. Meltzer, J. Paul, H. Dietrich, C.M. Jäger, D. Zahn, T. Clark, B. Braunschweig and W. Peukert; Indentation and Self-Healing Mechanisms of a Self-Assembled Monolayer - A Combined Experimental and Modeling Study; J. Am. Chem. Soc., 136, 10718 (2014)
    • A. Rumpel, M. Novak, J. Walter, B. Braunschweig, M. Halik and W. Peukert. Tuning the molecular or-der of C60 functionalized phosphonic acid monolayers; Langmuir, 27, 15016–15023 (2011).

  • Reactions at Electrode/Electrolyte Interfaces

    In this project, we use our expertise in nonlinear optical scattering to study electrocatalytic reactions in situ and partly also in operando. Currently we have a strong focus on the catalysis of CO2 in room temperature ionic liquids (ILs). In CO2 catalysis, ILs have been shown to yield low overpotentials and reduction reactions that can occur at potentials close to the theoretical thermodynamic potential. This makes ionic liquids highly interesting electrolytes for CO2 catalysis. However, in comparison to aqueous electrolytes, where extensive studies exist, we have just started to understand CO2 reduction reactions (CO2RR) on catalyst surfaces in contact with ionic liquids. For that, our group has established new in situ and in operando spectroscopies that can provide detailed molecular level information on CO2RR. So far, our work was focused mostly on Pt electrode surfaces which were the ideal choice to understand the molecular origin of the low overpotentials for CO2RR. In the future, we want to go beyond the fundamentally useful but for real world applications irrelevant Pt catalysts to new and much more relevant Cu and Cu/Au surface alloys have not been studied in ionic liquids in detail yet, but are expected to yield C1 products like HCOOH and possibly also C2 products. Several work packages will concentrate on delivering molecular level information from in situ/in operando spectroscopy of CO2 reaction pathways for Cu electrodes, while we also plan to extract the products from CO2RR in a continuous process. This will be done in a separate working package that is run parallel to the other addressing more fundamental aspects of CO2RR in ionic liquids.

    • A. Kemna and B. Braunschweig, Potential-Induced Adsorption and Structuring of Water at Pt(111) Electrode Surfaces in Contact with an Ionic Liquid, J. Phys. Chem. Lett. 11, 7116-7121 (2020)
    • B. Ratschmeier, A. Kemna, B. Braunschweig, Role of H2O for CO2 Reduction Reactions at Plati-num/Electrolyte Interfaces in Imidazolium Room-Temperature Ionic Liquids, ChemElectroChem 7, 1765-1774 (2020)
    • A. Kemna, N. García Rey and B. Braunschweig, Mechanistic Insights on CO2 Reduction Reactions at Platinum/[BMIM][BF4] Interfaces from In Operando Spectroscopy, ACS Catalysis 8, 6284-6292 (2019)
    • B. Braunschweig, P. Mukherjee, J. L. Haan and D. D. Dlott, Vibrational sum-frequency generation study of the CO2 electrochemical reduction at Pt/EMIM-BF4 solid/liquid interfaces, J. Electroanalytical Chem. 800, 144-150 (2017)
    • B. Braunschweig and Andrzej Wieckowski, Surface Spectroscopy of Pt(111) Single-Crystal Electro-lyte Interfaces with Broadband Sum-Frequency Generation, J. Electroanal. Chem. 716, 136-144 (2014)
    • B. Braunschweig, D. Hibbitts, M. Neurockand, A. Wieckowski, Electrocatalysis: a Fuel Cell and Sur-face Science Perspective, Catalysis Today 202, 197-209 (2013)

    Left: Lowering of the activation barrier for CO2 reduction reactions at the electrode/electrolyte interface in the presence of imidazolium cations like [BMIM]. / Right: Sum-frequency generation spectroscopy for the characterization of electrochemical reactions at the electrode/electrolyte interface. Here we present the proposed structure of an ionic liquid electrolyte with mM water concentrations at the Pt(111) surface.
    © Left: B. Braunschweig and B. Ratschmeier / Right: Reprinted with permission from J. Phys. Chem. Lett. 2020, 11, 17, 7116–7121. Copyright 2020 American Chemical Society
  • Interface characterization using sum-frequency generation spectroscopy to address surface charging and ion specific effects of macromolecules and surfactants at liquid interfaces.
    © Reproduced from Langmuir 2018, 34, 39, 11714–11722. CC-BY-NC-ND license

    Surfactants and Macromolecules at Electrified Interfaces

    Particularly, ions are of great importance as they can specifically bind to charge determining groups of proteins such as carboxylates or in analogous way bind to surfactants or polyelectrolytes. In addition, ions within the electric double layer of a charged interface can screen the interfacial electric field that is produced by the presence of e.g. proteins or surfactants and their net charge. Ion partitioning at interfaces can also directly inhibit bubble coalescence. Thus, specific ion adsorption due to ion pairing and ion partitioning can be applied to tune interfacial intermolecular interactions by either screening the interfacial electric field but also by enforcing a charge reversal upon the interface. The effects of ions on the molecular structure of neat water surfaces are equally im-portant and will be addressed in a similar way. In general, the issue of ions at aqueous interfaces impacts a long-standing problem on the origin of salt effects on the solubility of proteins and other macromolecules or colloidal particles which was originally invoked by Hofmeister. A particular focus is on the effects of ions on surface charge, molecular order and structure of surface adsorbates (proteins, polyelectrolytes, surfactants and their mixtures) since the presence of ions can significantly change intermolecular interactions at the interface from attractive to repulsive.

    Field effects can lead to substantial changes in the SFG spectrum of interfacial water molecules and can be used to track changes that are associated with different surface charging conditions. For these studies, we will systematically vary the salt and its concentration in the electrolyte so that we can compare series of SHG/SFG experiments from hard ions to soft ions. As we perform these studies with different molecules, that have quite different charge determining groups e.g. amines, carboxylate or sulfates, we can also address ion pairing at these groups as well as ion partitioning at hydrophobic parts of modified liquid interfaces. Here, we combine SFG spectroscopy with measurements in a thin-film pressure balance were the disjoining pressure in foam films can be determined. In fact, we could very recently demonstrate that these two methods provide complementary information (see García Rey et al. in JPCC (2019) below).

    • M. E. Richert, G. Gochev and B. Braunschweig, Specific Ion Effects of Trivalent Cations on the Struc-ture and Charging State of β-Lactoglobulin Adsorption Layers, Langmuir (under review in revised form)
    • M. García Rey, E. Weißenborn, F. Schulze-Zachau, G. Gochev, B. Braunschweig, Quantifying Dou-ble-Layer Potentials at Liquid-Gas Interfaces from Vibrational Sum-Frequency Generation, J. Phys. Chem C 123, 1279-1286 (2019)
    • F. Schulze-Zachau, S. Bachmann, B. Braunschweig, Effects of Ca2+ Ion Condensation on the Mo-lecular Structure of Polystyrene Sulfonate at Air-Water Interfaces, Langmuir 34, 11714-11722 (2018)
    • S. Streubel, F. Schulze-Zachau, E. Weißenborn, B. Braunschweig, Ion Pairing and Adsorption of Azo Dye/C16TAB Surfactants at the Air−Water Interface, J. Phys. Chem. C 121, 27992−28000 (2017)
    • B. Braunschweig, F. Schulze-Zachau, E. Nagel, K. Engelhardt, S. Stoyanov, G. Gochev, Khr.Khristov, E. Mileva, D. Exerowa, R. Miller and W. Peukert, Specific effects of Ca2+ ions and molecular structure of β-lactoglobulin interfacial layers that drive macroscopic foam stability, Soft Matter 12, 5995 (2016)
    • F. R. Beierlein, T. Clark, B. Braunschweig, K. Engelhardt, L. Glas, W. Peukert, Carboxylate Ion Pair-ing with Alkali-Metal Ions for β-Lactoglobulin and Its Role on Aggregation and Interfacial Adsorption, J. Phys. Chem. B 119, 5505 (2015)

  • Second-harmonic scattering (SHS) for characterization of interfaces in colloidal systems. Here the charging state of amphoteric particles was studied with SHS.
    © Reprinted with permission from J. Phys. Chem. C 2014, 118, 19, 10033–10042. Copyright 2014 American Chemical Society

    Interface Characterization in Colloids with Nonlinear Optical Scattering Methods

    A molecular understanding of nanoparticle surfaces and their functionalization is of great importance not only from fundamental aspects but is also highly demanded in specific applications. However, the mechanism and the kinetics of nanoparticle surface modification are still widely unknown on a molecular level, which is due to a lack of in situ techniques with high temporal resolution. In our present work we were able to follow the adsorption kinetics e.g. of thiolates on citrate stabilized Au nanoparticles by applying second-harmonic light scattering (SHS) and have separated in situ adsorption on defect and terrace sites which was hitherto not accessible.

    We expect that our research will help to optimize functionalization of Au nanoparticle surfaces and that the mechanistic understanding which we provide can be transferred also to other nanoparticle systems. Furthermore, we have applied SHS to study double layer charging and the orientation of water molecules at surfaces of polymer particles. Changing the pH has a dramatic effect on the surface potential which can be in many cases tuned from positive to negative values and has allowed us to record changes in SHS signals while the isoelectric point of the particles is being crossed. The SHS signal is a function of two contributions which are directly related to the first molecular layer, its orientation, and to a second electric field induced contribution of the unidirectional field within the interfacial electric double layer. Through additional charge screening experiments we measured the surface charge density for positively as well as for negatively charged particle surfaces and confirmed the isoelectric point, where an increase in ionic strength had little effect on the SHS intensity. Furthermore, we have determined the net orientation of water molecules directly adsorbed to the particle surface from pH-dependent changes in the relative phase of the two SHS contributions. Using SHS we can also identify morphology changes where we could provide new information on the growth of Au nanoshells with an in situ study that has used a combination of in situ SHS and UV-Vis spectroscopy and ex situ scanning electron microscopy.

    • R. Dinkel, J. Jakobi, A.R. Ziefuß, S. Barcikowski, B. Braunschweig, W. Peukert, Role of Citrate and NaBr at the Surface of Colloidal Gold Nanoparticles during Functionalization, J. Phys. Chem. C 122, 27383-27391 (2018)
    • R. Dinkel, W. Peukert and B. Braunschweig, In situ spectroscopy of ligand exchange reactions at the surface of colloidal gold and silver nanoparticles, J. Phys.: Cond. Mat. 29, 133002 (2017) Invited Re-view
    • R. Dinkel, B. Braunschweig, W. Peukert, Fast and slow ligand exchange at the surface of colloidal gold nanoparticles, J. Phys. Chem. C 120, 1673 – 1682 (2016)
    • C. Sauerbeck, B. Braunschweig and W. Peukert, Surface charging and interfacial water structure of amphoteric colloidal particles, J. Phys. Chem. C 118, 10033-10042 (2014)
    • C. Sauerbeck, M. Haderlein, B. Schürer, B. Braunschweig, W. Peukert and R.N. Klupp Taylor, Shed-ding Light on the Growth of Au Nanoshells, ACS Nano 8, 3088-3096 (2014)
  • © Prof. Braunschweig

    ERC Grant SUPERFOAM

    ERC Grant SUPERFOAM

    Foams are ubiquitous in our daily lives be it as milk foam on our cappuccino or as heat insulation of the building we live in.
    The various important technological applications range from lightweight materials, waste water treatment to recycling of rare earth metals via ion flotation just to mention a few.
    The vast number of possibilities to use foam in industrial processes and products originate from a unique tunability of optical, mechanical as well as chemical properties. This makes foams to an exciting object of current interdisciplinary research.

    Although making aqueous foam is surprisingly easy as one “simply” has to lower the water’s surface tension by additions of surface active molecules, the foam is in most cases inherently unstable. Furthermore, foam formulation is usually performed purely empirically because the actual driving forces must be controlled on several length scales that reach down to the molecular level. For that reason understanding and controlling foam properties with a bottom-up approach is a major challenge in current research.

    Structure-Property Relationship

    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.
    Thus composition, conformation and intermolecular interactions of a few molecular layers at liquid-gas interfaces – that are ubiquitous in aqueous foam – determine properties throughout the entire hierarchical chain.

    Structure-Property Relationship
    © B. Braunschweig and E. Weißenborn 2021

    Methodology of the SUPERFOAM project

    In order, to put foam formulation and also our knowledge on foams on a molecular basis we need to characterize the latter in situ and 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, we can use these building blocks to break the ground for new ways in foam formulation and related fields. Specifically, from this project we will get a library of structure-property relationships (SUPER) from which we can select molecular ingredients, tailor their interactions and thus generate FOAM based on molecular control.

    Experimental Approach

    The main focus of the ERC Starting Grant project SUPERFOAM will be on the molecular structure of interfaces and in situ charac-terization of interfaces. These experiments are a nucleation point for further studies on larger length scales - lamella, bubbles and macroscopic foam - which will be performed with solutions of identical composition. The approach will enable us to trace foam properties, bubble coalescence and lamella properties back to the actual molecular structures defining them. For that reason, we will divide the project into small work packages, which we can handle experimentally. At the conclusion of this project, we can unite the individual parts into a single concept on how molecular structures at gas-water interfaces should look like in order to make foam with the desired properties.

    Interfaces

    • Structure
    • Composition
    • Charging
    • Adsorption kinetics of surface active molecules

    Foam films and rising bubbles

    • Composition
    • Disjoining Pressure

    Macroscopic Foam

    • Bubble size distribution
    • Stability & more