Re-orientation of water molecules in response to surface charge at surfactant interfaces

Low Reducing Potentials Enabled by CaF2-Supported Graphene Electrodes in High Impedance Solutions

We report electrochemical measurements using in situ Raman spectroscopy at graphene/D2O interfaces under extremely low applied potentials. Here, the hydrophobic and catalytically inert nature of graphene and the insulating nature of the deionized (DI) water enables potentials as low as Vapplied = -7 V vs Ag/AgCl to be applied without exceeding 200 μA/cm2 of current density. At higher currents, bubble formation (i.e., hydrogen evolution reaction) prohibits reliable spectra from being obtained from the electrode surface. Using CaF2 as the supporting substrate enables significantly lower reducing potentials to be reached compared to glass substrates, likely due to trapped charge and impurities in the glass substrate. G band Raman spectra taken under various applied electrochemical potentials exhibit a linear relationship between the G band shift (ΔωG) and the applied potential, with blueshifts as high as ΔωG = 18 cm-1. These large Raman shifts indicate a large change in the Fermi level of ΔEF = -0.43 eV for graphene electrodes in contact with water, favoring reduction half-reactions. Based on the solution resistance measurement, there is a VIR = 3.1 V voltage drop across the solution for D2O (when the applied potential was Vapplied = -7 V vs Ag/AgCl) and the effective reducing potential on the working electrode is Veffective = -3.9 V vs Ag/AgCl. We have also tested these graphene electrodes in ionic liquids [DEME][TFSI], which are limited to applied potentials above Vapplied = -2.7 V vs Ag/AgCl and a corresponding shift in the Fermi level ΔEF = -0.32 eV, indicating that pure water can provide a more robust electrolyte for reaching low reducing potentials than ionic liquids.

Momentum-dependent sum-frequency vibrational spectroscopy of bonded interface layer at charged water interfaces

Interface-specific hydrogen (H)-bonding network of water directly controls the energy transfer and chemical reaction pathway at many charged aqueous interfaces, yet to characterize these bonded water layer structures remains a challenge. We now develop a sum-frequency spectroscopic scheme with varying photon momenta as an all-optic solution for retrieving the vibrational spectra of the bonded water layer and the ion diffuse layer and, hence, microscopic structural and charging information about an interface. Application of the method to a model surfactant-water interface reveals a hidden weakly donor H-bonded water species, suggesting an asymmetric hydration-shell structure of fully solvated surfactant headgroups. In another application to a zwitterionic phosphatidylcholine lipid monolayer-water interface, we find a highly polarized bonded water layer structure associating to the phosphatidylcholine headgroup, while the diffuse layer contribution is experimentally proven to be negligible. Our all-optic method offers an in situ microscopic probe of electrochemical and biological interfaces and the route toward future imaging and ultrafast dynamics studies.

Molecular Orientation of Carboxylate Anions at the Water-Air Interface Studied with Heterodyne-Detected Vibrational Sum-Frequency Generation

The carboxylate anion group plays an important role in many (bio)chemical systems and polymeric materials. In this work, we study the orientation of carboxylate anions with various aliphatic and aromatic substituents at the water-air interface by probing the carboxylate stretch vibrations with heterodyne-detected vibrational sum-frequency generation spectroscopy in different polarization configurations. We find that carboxylate groups with small aliphatic substituents show a large tilt angle with respect to the surface normal and that this angle decreases with increasing size of the substituent. We further use the information about the orientation of the carboxylate group to determine the hyperpolarizability components of this group.

Water as a Link between Membrane and Colloidal Theories for Cells

This review is an attempt to incorporate water as a structural and thermodynamic component of biomembranes. With this purpose, the consideration of the membrane interphase as a bidimensional hydrated polar head group solution, coupled to the hydrocarbon region allows for the reconciliation of two theories on cells in dispute today: one considering the membrane as an essential part in terms of compartmentalization, and another in which lipid membranes are not necessary and cells can be treated as a colloidal system. The criterium followed is to describe the membrane state as an open, non-autonomous and responsive system using the approach of Thermodynamic of Irreversible Processes. The concept of an open/non-autonomous membrane system allows for the visualization of the interrelationship between metabolic events and membrane polymorphic changes. Therefore, the Association Induction Hypothesis (AIH) and lipid properties interplay should consider hydration in terms of free energy modulated by water activity and surface (lateral) pressure. Water in restricted regions at the lipid interphase has thermodynamic properties that explain the role of H-bonding networks in the propagation of events between membrane and cytoplasm that appears to be relevant in the context of crowded systems.

How Far Is "Bulk Water" from Interfaces? Depends on the Nature of the Surface and What We Measure

Using systematic molecular dynamics (MD) simulations, we revisit the question: At what distance from an interface do the properties of "bulk water" get recovered? We have considered three different kinds of interfaces: nonpolar (hydrophobic; isooctane-water interface), charged (negative; AOT bilayer), and polar (zwitterionic; POPC bilayer). In order to interrogate the extent of perturbation of the interfacial water molecules as a function of the distance from the interface, we utilize a diverse range of structural and dynamical parameters. To capture the structural perturbations, we look into local density (translational order), local tetrahedral order parameter, and dipolar orientation of the water molecules. We also explore the anisotropic diffusion of the water molecules in the direction perpendicular to the interface as well as the planar diffusion parallel to the interface in a distance dependent manner. In addition, the orientational time correlation functions have been computed to understand the extent of slowdown in the rotational dynamics. As expected, the electrostatic field emanating from the charged AOT interface seems to have the highest long-range effect on the orientational order and dynamics of the water molecules, whereas specific interactions like hydrogen bonding and electrostatic interaction lead to significant trapping and kinetic slowdown for both AOT and POPC (zwitterionic) very close to the interface. Our analysis highlights that not only the length-scale of perturbation depends on the nature of the interfaces and specific interactions but also the type of water property that we measure/calculate. Different water properties seem to have widely different length-scale of perturbation. Orientational order parameters seem to be perturbed to a much longer length-scale as compared to translational order parameters. The global orientational order of water can be perturbed even up to ∼4-5 nm near the negatively charged AOT surface in the absence of any extra electrolyte. This observation has significant implication toward the interpretation of experimental measurements as well since different spectroscopic techniques would probe different parameters or water properties with possible mutual disagreement and inconsistency between different types of measurements. Thus, our study provides a broader and unifying perspective toward the aspect of "context dependent" structural and dynamical perturbation of "interfacial water".

Molecular orientation of small carboxylates at the water-air interface

We study the properties of formate (HCOO−) and acetate (CH3COO−) ions at the surface of water using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. For both ions we observe a response...

Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration

We study the bending mode of pure water and charged aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy. We observe a low (1626  cm^{-1}) and a high (1656  cm^{-1}) frequency component that can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, respectively. We thus demonstrate that probing the bending mode provides structural and quantitative information on both the surface and the bulk.

Local Mutations Can Serve as a Game Changer for Global Protein Solvent Interaction

Although it is well-known that limited local mutations of enzymes, such as matrix metalloproteinases (MMPs), may change enzyme activity by orders of magnitude as well as its stability, the completely rational design of proteins is still challenging. These local changes alter the electrostatic potential and thus local electrostatic fields, which impacts the dynamics of water molecules close the protein surface. Here we show by a combined computational design, experimental, and molecular dynamics (MD) study that local mutations have not only a local but also a global effect on the solvent: In the specific case of the matrix metalloprotease MMP14, we found that the nature of local mutations, coupled with surface morphology, have the ability to influence large patches of the water hydrogen-bonding network at the protein surface, which is correlated with stability. The solvent contribution can be experimentally probed via terahertz (THz) spectroscopy, thus opening the door to the exciting perspective of rational protein design in which a systematic tuning of hydration water properties allows manipulation of protein stability and enzymatic activity.

Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces

The origin of the sum-frequency generation (SFG) signal of the water bending mode has been controversially debated in the past decade. Unveiling the origin of the signal is essential, because different assignments lead to different views on the molecular structure of interfacial water. Here, we combine collinear heterodyne-detected SFG spectroscopy at the water-charged lipid interfaces with systematic variation of the salt concentration. The results show that the bending mode response is of a dipolar, rather than a quadrupolar, nature and allows us to disentangle the response of water in the Stern and the diffuse layers. While the diffuse layer response is identical for the oppositely charged surfaces, the Stern layer responses reflect interfacial hydrogen bonding. Our findings thus corroborate that the water bending mode signal is a suitable probe for the structure of interfacial water.

Water at charged interfaces

The ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the molecular picture of the interfacial organization and preferential alignment of water molecules, as well as the structure of water molecules and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aqueous interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which molecular properties are relevant, such as interfacial chemical conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.

Bursting the bubble: A molecular understanding of surfactant-water interfaces

Surfactant science has historically emphasized bulk, thermodynamic measurements to understand the microemulsion properties of greatest industrial significance, such as interfacial tensions, phase behavior, and thermal stability. Recently, interest in the molecular properties of surfactants has grown among the physical chemistry community. This has led to the application of cutting-edge spectroscopic methods and advanced simulations to understand the specific interactions that give rise to the previously studied bulk characteristics. In this Perspective, we catalog key findings that describe the surfactant-oil and surfactant-water interfaces in molecular detail. We emphasize the role of ultrafast spectroscopic methods, including two-dimensional infrared spectroscopy and sum-frequency-generation spectroscopy, in conjunction with molecular dynamics simulations, and the role these techniques have played in advancing our understanding of interfacial properties in surfactant microemulsions.

Assessing the Impact of Solvent Selection on Vibrational Sum-Frequency Scattering Spectroscopy Experiments

The development of vibrational sum-frequency scattering (S-VSF) spectroscopy has opened the door to directly probing nanoparticle surfaces with an interfacial and chemical specificity that was previously reserved for planar interfacial systems. Despite its potential, challenges remain in the application of S-VSF spectroscopy beyond simplified chemical systems. One such challenge includes infrared absorption by an absorptive continuous phase, which will alter the spectral lineshapes within S-VSF spectra. In this study, we investigate how solvent vibrational modes manifest in S-VSF spectra of surfactant stabilized nanoemulsions and demonstrate how corrections for infrared absorption can recover the spectral features of interfacial solvent molecules. We also investigate infrared absorption for systems with the absorptive phase dispersed in a nonabsorptive continuous phase to show that infrared absorption, while reduced, will still impact the S-VSF spectra. These studies are then used to provide practical recommendations for anyone wishing to use S-VSF to study nanoparticle surfaces where absorptive solvents are present.

Breakdown of classical paradigms in relation to membrane structure and functions

Updates of the mosaic fluid membrane model implicitly sustain the paradigms that bilayers are closed systems conserving a state of fluidity and behaving as a dielectric slab. All of them are a consequence of disregarding water as part of the membrane structure and its essential role in the thermodynamics and kinetics of membrane response to bioeffectors. A correlation of the thermodynamic properties with the structural features of water makes possible to introduce the lipid membrane as a responsive structure due to the relaxation of water rearrangements in the kinetics of bioeffectors' interactions. This analysis concludes that the lipid membranes are open systems and, according to thermodynamic of irreversible formalism, bilayers and monolayers can be reasonable compared under controlled conditions. The inclusion of water in the complex structure makes feasible to reconsider the concept of dielectric slab and fluidity.

Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra

Recently, there has been controversy over whether the HOH bend signal of water in the vibrational sum frequency generation (VSFG) spectrum arises from the conventional dipole mechanism or the quadrupole mechanism. Here, we show that the Im χ⁽²⁾ (the imaginary part of the second-order nonlinear susceptibility) spectra of the HOH bend mode of water at oppositely charged monolayer/water interfaces all exhibit positive bands, irrespective of the difference in the sign of the charge at the interface. Furthermore, it is found that the peak frequency of the HOH bend band substantially changes depending on the chemical structure of the charged headgroup located at the interface. These results demonstrate that the VSFG signal of the HOH bend vibration is generated from interfacial water with the interfacial quadrupole mechanism that is associated with the large field gradient of incident lights localized in a very thin region at the interface.

Water electrification: Principles and applications

Deep engineering of liquid water by charge and impurity injection, charged support, current flow, hydrophobic confinement, or applying a directional field has becoming increasingly important to the mankind toward overcoming energy and environment crisis. One can mediate the processes or temperatures of molecular evaporation for clean water harvesting, HO bond dissociation for H₂ fuel generation, solidification for living-organism cryopreservation, structure stiffening for bioengineering, etc., with mechanisms being still puzzling. We show that the framework of "hydrogen bonding and electronic dynamics" has substantiated the progress in the fundamental issues and the aimed engineering. The segmental disparity of the coupled hydrogen bond (O:HO or HB with ":" being lone pair of oxygen) resolves their specific-heat curves and turns out a quasisolid phase (QS, bound at -15 and 4 °C). Electrification shows dual functionality that not only aligns, orders, polarizes water molecules but also stretches the O:HO bond. The O:HO segmental cooperative relaxation and polarization shift the QS boundary through Einstein's relation, ΔΘ_Dx ∝ Δω_x, resulting in a gel-like, viscoelastic, and stable supersolid phase with raised melting point T_m and lowered temperatures for vaporization T_V and ice nucleation T_N. The supersolidity and electro structure ordering provide additional forces to reinforce Armstrong's water bridge. QS dispersion and the secondary effect of electrification such as compression define the T_N for Dufour's electro-freezing. The T_V depression, surface stress disruption, and electrostatic attraction raise Asakawa's molecular evaporability. Composition of opposite, compatible fields eases the HO dissociation and soil wetting. Progress evidences not only the essentiality of the coupled O:HO bond theory but also the feasibility of engineering water and solutions by programmed electrification.

Spiropyran Sulfonates for Photo- and pH-Responsive Air-Water Interfaces and Aqueous Foam

Responsive foams and interfaces are interesting building blocks for active materials that respond and adapt to external stimuli. We have used the photochromic reaction of a spiropyran sulfonate surfactant to render interfacial, rising bubbles as well as foaming properties active to light stimuli. In order to address the air-water interface on a molecular level, we have applied sum-frequency generation (SFG) spectroscopy which has provided qualitative information on the surface excess and the interfacial charging state as a function of light irradiation and solution pH. Under blue light irradiation, the surfactant forms a closed ring spiro form (SP), whereas under dark conditions the ring opens and the merocyanine (MC) form is generated. Using SFG spectroscopy, we show that at the interface, different pH conditions of the bulk solution lead to changes in the interfacial charging state. We have exploited the fact that the MC surfactant's O-H group can be deprotonated as a function of pH and used that to tune the molecules net charge at the interface. In fact, SFG spectroscopy shows that with increasing pH the intensity of the O-H stretching band from interfacial water molecules increases, which we associate to an increase in surface net charge. At a pH of 5.3, irradiation with blue light leads to a reversible decrease of O-H intensities, whereas the C-H intensities were unchanged compared to the corresponding intensities under dark conditions. These results are indicative of changes in the surface net charge with light irradiation, which are also expected to influence the foam stability via changes in the electrostatic disjoining pressure. In fact, measurements of the foam stabilities are consistent with this hypothesis and show higher foam stability under dark conditions. At pH 2.7 this behavior is reversed as far as the surface tension and surface charging as well as the foam stability are concerned. This is corroborated by rising bubble experiments, which demonstrated an unprecedented reduction of ∼30% in bubble velocity when the bubbles were irradiated with blue light compared to the velocity of bubbles with the surfactants in the dark state. Clearly, the light-triggered changes can be used to control foams, rising bubbles, and fluid interfaces on a molecular level which renders them active to light stimuli.

Unsaturated Lipid Accelerates Formation of Oligomeric β-Sheet Structure of GP41 Fusion Peptide in Model Cell Membrane

Membrane fusion of the viral and host cell membranes is the initial step of virus infection and is catalyzed by fusion peptides. Although the β-sheet structure of fusion peptides has been proposed to be the most important fusion-active conformation, it is still very challenging to experimentally identify different types of β-sheet structures at the cell membrane surface in situ and in real time. In this work, we demonstrate that the interface-sensitive amide II spectral signals of protein backbones, generated by the sum frequency generation vibrational spectroscopy, provide a sensitive probe for directly capturing the formation of oligomeric β-sheet structure of fusion peptides. Using human immunodeficiency virus (HIV) glycoprotein GP41 fusing peptide (FP23) as the model, we find that formation speed of oligomeric β-sheet structure depends on lipid unsaturation. The unsaturated lipid such as POPG can accelerate formation of oligomeric β-sheet structure of FP23. The β-sheet structure is more deeply inserted into the hydrophobic region of the POPG bilayer than the α-helical segment. This work will pave the way for future researches on capturing intermediate structures during membrane fusion processes and revealing the fusion mechanism.

Molecular Structure and Modeling of Water-Air and Ice-Air Interfaces Monitored by Sum-Frequency Generation

From a glass of water to glaciers in Antarctica, water-air and ice-air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water-air and ice-air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces.

Structure and Dynamics of Nanoconfined Water Between Surfactant Monolayers

The properties of nanoconfined water arise in direct response to the properties of the interfaces that confine it. A great deal of research has focused on understanding how and why the physical properties of confined water differ greatly from the bulk. In this work, we have used all-atom molecular dynamics (MD) simulations to provide a detailed description of the structural and dynamical properties of nanoconfined water between two monolayers consisting of an archetypal ionic surfactant, cetrimonium bromide (CTAB, [CH₃(CH₂)₁₅N(CH₃)₃]⁺Br^-). Small differences in the area per surfactant of the monolayers impart a clear effect on the intrinsic density, mobility, and ordering of the interfacial water layer confined by the monolayers. We find that as the area per surfactant within a monolayer decreases, the mobility of the interfacial water molecules decreases in response. As the monolayer packing density decreases, we find that each individual CTAB molecule has a greater effect on the ordering of water molecules in its first hydration shell. In a denser monolayer, we observe that the effect of individual CTAB molecules on the ordering of water molecules is hindered by increased competition between headgroups. Therefore, when two monolayers with different areas per surfactant are used to confine a nanoscale water layer, we observe the emergence of noncentrosymmetry.

Insight into the Mechanisms Driving the Self-Assembly of Functional Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers

Polymer-stabilized liquid/liquid interfaces are an important and growing class of bioinspired materials that combine the structural and functional capabilities of advanced synthetic materials with naturally evolved biophysical systems. These platforms have the potential to serve as selective membranes for chemical separations and molecular sequencers and to even mimic neuromorphic computing elements. Despite the diversity in function, basic insight into the assembly of well-defined amphiphilic polymers to form functional structures remains elusive, which hinders the continued development of these technologies. In this work, we provide new mechanistic insight into the assembly of an amphiphilic polymer-stabilized oil/aqueous interface, in which the headgroups consist of positively charged methylimidazolium ionic liquids, and the tails are short, monodisperse oligodimethylsiloxanes covalently attached to the headgroups. We demonstrate using vibrational sum frequency generation spectroscopy and pendant drop tensiometery that the composition of the bulk aqueous phase, particularly the ionic strength, dictates the kinetics and structures of the amphiphiles in the organic phase as they decorate the interface. These results show that H-bonding and electrostatic interactions taking place in the aqueous phase bias the grafted oligomer conformations that are adopted in the neighboring oil phase. The kinetics of self-assembly were ionic strength dependent and found to be surprisingly slow, being composed of distinct regimes where molecules adsorb and reorient on relatively fast time scales, but where conformational sampling and frustrated packing takes place over longer time scales. These results set the stage for understanding related chemical phenomena of bioinspired materials in diverse technological and fundamental scientific fields and provide a solid physical foundation on which to design new functional interfaces.

Unveiling Heterogeneity of Interfacial Water through the Water Bending Mode

The water bending mode provides a powerful probe of the microscopic structure of bulk aqueous systems because its frequency and spectral line shape are responsive to the intermolecular interactions. Furthermore, interpreting the bending mode response is straightforward, as the intramolecular vibrational coupling is absent. Nevertheless, bending mode has not been used for probing the interfacial water structure, as it has been yet argued that the signal is dominated by bulk effects. Here, through the sum-frequency generation measurement of the water bending mode at the water/air and water/charged lipid interfaces, we demonstrate that the bending mode signal is dominated not by the bulk but by the interface. Subsequently, we disentangle the hydrogen-bonding of water at the water/air interface using the bending mode frequency distribution and find distinct interfacial hydrogen-bonded structures, which can be directly related to the interfacial organization of water. The bending mode thus provides an excellent probe of aqueous interfacial structure.