Session T29: Soft Charged Interfaces: Advanced Experimental and Computational Techniques

Probing the reversible self-assembly of pH-responsive switchable surfactants using SFG spectroscopy

Stimuli-responsive amphiphiles, such as monoalkyl diamines, have garnered significant interest for their capacity to modulate the properties of oil-water systems through structural changes driven by external stimuli. This work presents a molecular-level examination of N-dodecylpropane-1,3-diamine (DPDA) as a pH-sensitive switchable surfactant, probed at air-water and dodecane-water interfaces. The study encompasses an analysis of adsorption, self-assembly, molecular organization, and surface activities of water, dodecane, and DPDA at different pH levels. Surface tension measurements and density functional theory-based free energy calculations complemented spectral data obtained using SFG spectroscopy. At neutral pH, uncharged surfactants from the oil phase migrated to the oil-water interface, undergoing protonation upon interaction with neighboring water molecules to form a charged surfactant layer. By adjusting the water pH, we fine-tuned the charge of adsorbed surfactants, their self-assembly, interfacial molecular conformation, and interface stability, leveraging the unique structure of DPDA with diamine. The formation of dicationic surfactant at pH < 7 resulted in a lower intensity ratio of the terminal CH₃ symmetric stretch (~ 2880 cm^-1) and CH₂ symmetric stretch (~ 2850 cm^-1). This observation indicates the presence of defects in alkyl chain conformations due to variations in the conformation of the propyl chain, which separated the two cationic centers of DPDA under acidic conditions. These findings suggest potentially enhancing the utilization of DPDA and similar switchable surfactants as effective chemical demulsifiers for oil-water emulsion separation.   

Probing the emergence of electrostatic attraction between like-charged phospholipid membranes

Counterintuitive electrostatic attraction between similarly charged macroions arises when multivalent counterions are present. Surprisingly, recent experiments have demonstrated that monovalent counterions alone can induce attraction among fully charged phospholipid membranes [1]. This phenomenon has been attributed to the low dielectric constant of interfacial water. To deepen our understanding of the interplay between hydration and electrostatic forces, we examined the interaction between an odd number of phospholipid layers deposited on a silica substrate. Specifically, at the air/solid interface, we controlled osmotic pressure by adjusting air relative humidity. Using the D17 neutron reflectometer at ILL (Grenoble, France) and ellipsometry, we measured the interlayer spacing for membranes with varying charged surface density. From this data, we deduced the disjoining pressure between the layers. Our findings reveal a continuous transition between a repulsive mean-field regime and an attractive strong coupling regime. This work represents the first experimental measurement of a crossover between weak and strong coupling, shedding light on the emergence of electrostatic attraction in water-confined layers.   

Functionalizing Nanochannels with Polyelectrolyte Polymer Brushes for Nanofluidic Applications

Nanochannels with responsive gating characteristics are attractive design elements in a variety of applications such as particle sorting, viral detection, and flow regulation. To achieve this, we employed selective functionalization of nanochannel sidewalls with poly(dimethylaminoethyl methacrylate) (PDMAEMA) as a pH-responsive polyelectrolyte polymer. We investigated the channel gating response to variations in solution pH and ionic strength by tracking conformational and structural changes of the PDMAEMA brush using specular and off-specular neutron reflectometry. Simultaneous fits of the specular and off-specular signals to a dynamical theory model showed that the polymer brush assumes a collapsed state under basic solution conditions – equivalent to an open gate. In contrast, measurements under acidic conditions revealed an expanded brush state, representing a partially closed gate. Additional variations in salt concentration resulted in further modifications to the brush conformation and channel gate width. These findings open new possibilities in polyelectrolyte polymer applications in tunable nanofluidics and lab-on-chips devices with advanced designs and improved functionality.   

Interfacial Reactions in Electrochemical Energy Systems: In-Situ Studies Using Synchrotron X-ray Techniques

For electrochemical systems such as batteries and fuel cells, the gas/solid and liquid/solid interfaces are critical parts where many important reactions take place. It is critical to understand the interfacial changes for the better design of efficient energy systems. In the past years we have used various in-situ and operando synchrotron-based X-ray techniques including scattering, spectroscopy and imaging to investigate the atomic and electronic structure, chemistry and compositions of numerous electrochemical interfaces in fuel cells, electrolyzers, lithium- sodium- and magnesium-batteries. In my talk, I will go through couple of case studies including electrocatalyst restructuring in oxygen evolution reaction for hydrogen production and interfacial morphological and structural changes during battery charging-discharging processes. We will also highlight the importance of using model thin film systems to obtain insights on interfacial reaction in electrochemical reactions.   

Molecular Dynamics Simulation of Charged Clay Colloidal Assemblages

Interactions between charged clay colloidal particles play important roles in a range of geophysical phenomena including soil mechanics, sediment transport, debris flow, fault slip, and subsidence. Large-scale representation of these phenomena remains challenging because of a lack of appropriate consitutive relations for the mechanical properties of hydrated clay colloidal assemblages as a function of solid density, aqueous chemistry, and deformation history. We present recent efforts to predict these consistitutive relations using all-atom and coarse-grained molecular dynamics (MD) simulations of hydrated clay colloidal assemblages. Simulations span a range of scales from nanometers to micrometers and from individual charged clay particles (with explicit counterion clouds) to assemblages of thousands of clay particles. Our results provide insight into the sensitivity of clay mechanics to salinity and counterion type (Na vs. Ca).   

X-Ray Fluorescence Directly Reveals Ion Specificity near Ionizable Interfaces.

The Hofmeister effect refers to the drastically different effects that apparently similar ions (e.g., Cl^- and I^-) have on protein solubility and stability. This phenomenon has been intensively studied, but not only the underlying mechanisms remain obscure, but even the order of ions within the ‘Hofmeister series’ is still not well-established.

We have studied the interactions of monovalent ions with floating octadecylamine monolayers—amines are one of the major hydrophilic functional groups in proteins. We used X-Ray Fluorescence Near Total Reflection (XFNTR), which has the advantage of being both element-specific and surface-sensitive. It detects ions in the interfacial region directly, in contrast to indirect methods looking at the effects of ions on the surface films or on water molecules (SFG, FTIR, etc.). Our XFNTR results confirm ion-specific adsorption to amine groups at neutral solution pH, with the adsorbed ion population following the familiar Hofmeister series. Further, when the bulk solution pH is lowered, the observed ion specificity is lost, with all ions studied exhibit similar affinity to amines. This fading of the Hofmeister effect in response to pH changes may potentially explain the reported ‘alternation’ or sometimes even ‘reversal’ of the Hofmeister series under various specific conditions. A possible explanation for these observations will be discussed.   

From Pre to Post Reaction Conditions: The Transitioning Amino Acid Behavior at Air/Aqueous Interface under Direct Air Capture Conditions

To achieve global climate objectives, such as limiting the global temperature rise to less than 2C, the implementation of advanced negative emission technologies (NETs) is required. Among these, Direct Air Capture (DAC) using amino acids stands out due to lower volatility, nontoxicity, biodegradability, and scalability. Nonetheless, the optimization of DAC efficiency requires a comprehensive understanding of the CO₂ interaction mechanisms with amino acids at the air/aqueous interface. This study utilizes molecular dynamics simulations and Sum Frequency Generation (SFG) spectroscopy to characterize the interfacial behavior of three amino acids namely Valine, Leucine, and Phenylalanine, in both before and after CO₂ capture conditions. The study explores the outcomes of ion-generated competitive interactions on interfacial adsorption, orientation, and speciation, aiming to elucidate the fundamental processes that govern the direct air capture mechanism. Through this, we hope to inform the development of more effective interfacial systems for DAC, contributing to the reduction of atmospheric CO₂ and the mitigation of climate change.   

Electrotonic potentials couple short-term and long-term potentiation in lipid bilayer memristors and memcapacitors

Electrotonic potentials are associated with the passive conduction of ionic charges inside sensory receptor neurons and other excitable cells that do not involve voltage-dependent changes to membrane conductance. Also known as graded potentials, they do not propagate like voltage-dependent action potentials but attenuate rapidly, rendering them unsuitable for long-distance signaling. However, while action potentials can be transmitted over long distances, graded potentials conduct faster, enabling a higher bandwidth and information capacity. This allows for early signal processing optimization before encoding information in action potentials for distant transmission. In physiology, graded potentials involve activity-dependent modification of gap junctions, known as electrotonic couplings, which can be potentiated, just like chemically mediated postsynaptic action potentials, using the same electrical stimulation protocols that produce long-term synaptic potentiation (LTP) of long-term memory formation in the hippocampus. It has been shown that the effect that LTP has on electrotonic coupling in neurons is the result of increases in gap junction conductance.

Using the droplet interface bilayer (DIB) platform, we have developed both memristors (memory resistors) and memcapacitors (memory capacitors) as artificial synapses, consisting of diphytanoylphosphatidylcholine (DPhPC) lipid bilayers doped with antimicrobial peptide ion channels (alamethicin and gramicidin), and have demonstrated both short-term (msec to sec, STP) and long-term (minutes to hours, LTP) potentiation in DIB membranes. Here, we describe how the electrical training protocols that induce LTP in pure lipid bilayer memcapacitors can couple to STP in memristors consisting of lipid bilayers that contain peptides, through long-term modulation of the ion channel conductances. We go on to discuss the implications these coupled dynamical membrane models have for increased understanding of information processing in the brain, and for discovering novel functionality in soft matter-based memelements for neuromorphic computational devices.   

Thermodynamics of structured liquids

The assembly of nanoparticles at a liquid-liquid interface results in so-called structured liquids:

liquid droplets with solid-like properties. The origins of the solid-like response of these droplets is

rooted in the membrane-like elasticity of the interface that is generated by the nanoparticle assembly.

These materials have often been assumed to be out of equilibrium, with the sluggish dynamics of the

interfacial nanoparticles resulting in a “jammed” interface. Here, we explore the thermodynamics

of these interfacial assemblies by systematically including the effects of interfacial elasticity in the

thermodynamics of liquid and nanoparticle mixtures. Our findings indicate that elasticity acts as a

stabilizing force against emulsification. While the effective surface tension dominates the stability

criteria of large droplets, elasticity plays a significant role with decreasing droplet size and, in the

event of emulsification, can radically increase the size of the resulting microdroplets. In addition to

our theoretical analysis, we conduct molecular dynamics simulations to extract the elastic properties

of the interface. Our findings suggest that structured liquids may in fact be thermodynamically

stable states rather than arrested nonequilibrium materials.   

Effects of the hydrophobic fluid on particle jamming in Pickering systems at air/water and oil/water interfaces

The impact of interfacial properties on emulsion stability is key to designing systems with controlled mechanical properties. Utilizing strongly adsorbed solid particles at the interface, Pickering systems are shown to enhance the stability of emulsions and foams. Many interfacial studies have characterized the interfacial effects of the aqueous fluid properties and particle chemistry on these systems. Additionally, changes in the hydrophobic fluid are shown to impact particle and surfactant behavior. In this work, the interfacial mechanical properties are characterized at fluid/fluid interfaces to determine the impact of the hydrophobic fluid on solid particle jamming. A model system of CTAB and SiO₂ complexes is used to deliver particles to a/w and o/w interfaces in controlled experiments using a microtensiometer platform. Results show that the hydrophobic phase impacts the effective areal coverage of particles at the interface. When subject to nonlinear compression cycles, interfacial jamming is impacted by particle wettability and electrostatic interactions between adsorbed particles. This work highlights how changes to the hydrophobic fluid introduce further complexities to the interfacial properties, and these results further the understanding solid particle interactions at different fluid interfaces for controlled design of Pickering systems.   

Understanding surfactant mixture adsorption towards the next generation fluorine-free firefighting foams.

The primary ingredient in the current aqueous film forming foams (AFFFs) is per- and polyfluoroalkyl substances (PFAS) which poses a threat to environment and human health. A better understanding of surfactant's role in foam properties can lead to new formulations and elimination of PFAS in firefighting foams. In this work, a mixture of commercial alkyl polyglycoside and siloxane-based surfactants are studied which have shown promising fire suppression characteristics on various fuel-water interfaces. Dynamic surface tension of these systems at fluid-fluid interfaces are measured across length scales to understand the curvature dependence of surfactant transport to bubble surfaces, using pendant drop and capillary pressure micro-tensiometer (CPM) methods. Both the alkyl polyglycoside and siloxane surfactants present a characteristic two-step adsorption over CPM when the area of the interface is held constant, which we explain as due to the polydispersity in the commercial product. To test this explanation, we characterize mixtures of pure surfactants and under some conditions, observe the same two-step behavior in mixtures on the CPM. This work lays a solid foundation in providing key properties responsible in the study of surfactant transport of the next generation fluorine-free firefighting foams.