Temperature Effects and Activation Barriers in Aqueous Proton-Uptake Reactions.

Aqueous proton transfer reactions are fundamental in biology and chemistry, yet kinetics and mechanisms of strong base-weak acid reactions are not well understood. In this work, we present a temperature-dependent reaction kinetic study of the water-soluble photobase actinoquinol, in the presence and absence of succinimide, a weak acid reaction partner. We study the temperature dependence of the reaction and connect the observed dynamics to the reaction's thermodynamics.

Cholesterol Changes Interfacial Water Alignment in Model Cell Membranes.

The nanoscopic layer of water that directly hydrates biological membranes plays a critical role in maintaining the cell structure, regulating biochemical processes, and managing intermolecular interactions at the membrane interface. Therefore, comprehending the membrane structure, including its hydration, is essential for understanding the chemistry of life. While cholesterol is a fundamental lipid molecule in mammalian cells, influencing both the structure and dynamics of cell membranes, its impact on the structure of interfacial water has remained unknown.

Orientational Behavior and Vibrational Response of Glycine at Aqueous Interfaces.

Aqueous glycine plays many different roles in living systems, from being a building block for proteins to being a neurotransmitter. To better understand its fundamental behavior, we study glycine's orientational behavior near model aqueous interfaces, in the absence and presence of electric fields and biorelevant ions. To this purpose, we use a surface-specific technique called heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG).

Observation of Electrostatically Driven Surface Adsorption in Mixed Surfactant Systems.

We employed heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy to obtain a molecular-level understanding of the interaction between the anionic surfactant sodium dodecyl ammonium sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). We observed that these surfactants show a strong cooperative effect on their adsorption to the water-air interface. Even at bulk concentrations 1000 times lower than the critical micelle concentrations of SDS and DTAB, a nearly complete surface surfactant layer is observed when both surfactants are present.

Observation of a Two-Dimensional Hydrophobic Collapse at the Surface of Water Using Heterodyne-Detected Surface Sum-Frequency Generation.

We study the effect of sodium chloride (NaCl) on the properties of the interface of water and the surfactant dodecyl sulfate (DS) using heterodyne-detected vibrational sum-frequency generation spectroscopy. We find that the signal of the O-H stretch vibrations of oriented water molecules at the interface is highly nonlinearly dependent on the NaCl concentration. This nonlinear dependence is explained by a combination of screening of the electric field of surface-bound DS ions pointing into the bulk and screening of the Coulomb repulsion between the headgroups of the DS ions in the surface plane.

Observing Aqueous Proton-Uptake Reactions Triggered by Light.

Proton-transfer reactions in water are essential to chemistry and biology. Earlier studies reported on aqueous proton-transfer mechanisms by observing light-triggered reactions of strong (photo)acids and weak bases. Similar studies on strong (photo)base-weak acid reactions would also be of interest because earlier theoretical works found evidence for mechanistic differences between aqueous H and OH transfer.

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.

Observation of Strong Synergy in the Interfacial Water Response of Binary Ionic and Nonionic Surfactant Mixtures.

Interfacial vibrational footprints of the binary mixture of sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (CE) were probed using heterodyne detected vibrational sum frequency generation (HDVSFG). Our results show that in the presence of CE at CMC (70 μM) the effect of SDS on the orientation of interfacial water molecules is enhanced 10 times compared to just pure surfactants. The experimental results contest the traditional Langmuir adsorption model predictions.

Direct Probing of Vibrational Interactions in UiO-66 Polycrystalline Membranes with Femtosecond Two-Dimensional Infrared Spectroscopy.

UiO-66 is a benchmark metal-organic framework that holds great promise for the design of new functional materials. In this work, we perform two-dimensional infrared measurements on polycrystalline membranes of UiO-66 grown on c-sapphire substrates. We study the symmetric and antisymmetric stretch vibrations of the carboxylate groups of the terephthalate linker ions and find that these vibrations show a rapid energy exchange and a collective vibrational relaxation with a time constant of 1.

Molecular orientation of small carboxylates at the water-air interface.

We study the properties of formate (HCOO) and acetate (CHCOO) ions at the surface of water using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. For both ions we observe a response of the symmetric () and antisymmetric () vibrations of the carboxylate group. The spectra further show that for both formate and acetate the carboxylate group is oriented toward the bulk, with a higher degree of orientation for acetate than for formate.

Correction: The molecular structure of the surface of water-ethanol mixtures.

Correction for 'The molecular structure of the surface of water-ethanol mixtures' by Johannes Kirschner , , 2021, , 11568-11578, DOI: 10.1039/D0CP06387H.

Ultrafast vibrational dynamics of aqueous acetate and terephthalate.

We study the vibrational population relaxation and mutual interaction of the symmetric stretch (ν) and antisymmetric stretch (ν) vibrations of the carboxylate anion groups of acetate and terephthalate ions in aqueous solution by femtosecond two-dimensional infrared spectroscopy. By selectively exciting and probing the ν and ν vibrations, we find that the interaction of the two vibrations involves both the anharmonic coupling of the vibrations and energy exchange between the excited states of the vibrations. We find that both the vibrational population relaxation and the energy exchange are faster for terephthalate than for acetate.

Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent.

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface.

Bulk Response of Carboxylic Acid Solutions Observed with Surface Sum-Frequency Generation Spectroscopy.

We study the molecular properties of aqueous acetic acid and formic acid solutions with heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). For acid concentrations up to ∼5 M, we observe a strong increase of the responses of the acid hydroxyl and carbonyl stretch vibrations with increasing acid concentration due to an increase of the surface coverage by the acid molecules. At acid concentrations >5 M we observe first a saturation of these responses and then a decrease.

Water reorientation dynamics in colloidal water-oil emulsions.

We study the molecular-scale properties of colloidal water-oil emulsions consisting of 120-290 nm oil droplets embedded in water. This type of emulsion can be prepared with low concentrations of surfactants and is usually kinetically stable. Even though colloidal water-oil emulsions are used ubiquitously, their molecular properties are still poorly understood.

Direct Observation of the Orientation of Urea Molecules at Charged Interfaces.

Dissolving urea into water induces special solvation properties that play a crucial role in many biological processes. Here we probe the properties of urea molecules at charged aqueous interfaces using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. We find that at the neat water/air interface urea molecules do not yield a significant sum-frequency generation signal.

Accelerated Vibrational Energy Relaxation of Water in Alkaline Environments.

We observe that hydrated hydroxide ions introduce an additional relaxation channel for the vibrational relaxation of the OD vibrations of HDO molecules in aqueous NaOH solutions. This additional relaxation path involves resonant (Förster) vibrational energy transfer from the excited OD vibration to OH stretch vibrations of hydrated OH complexes. This energy transfer constitutes an efficient mechanism for dissipation of the OD vibrational energy, as the accepting OH stretch vibrations show an extremely rapid subsequent relaxation with a time constant of <200 fs.

Molecular Structure and Surface Accumulation Dynamics of Hyaluronan at the Water-Air Interface.

Hyaluronan is a biopolymer that is essential for many biological processes in the human body, like the regulation of tissue lubrication and inflammatory responses. Here, we study the behavior of hyaluronan at aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Low-molecular-weight hyaluronan (∼150 kDa) gradually covers the water-air interface within hours, leading to a negatively charged surface and a reorientation of interfacial water molecules.

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.

Emergence of Electric Fields at the Water-C12E6 Surfactant Interface.

We study the properties of the interface of water and the surfactant hexaethylene glycol monododecyl ether (C12E6) with a combination of heterodyne-detected vibrational sum frequency generation (HD-VSFG), Kelvin-probe measurements, and molecular dynamics (MD) simulations. We observe that the addition of the hydrogen-bonding surfactant C12E6, close to the critical micelle concentration (CMC), induces a drastic enhancement in the hydrogen bond strength of the water molecules close to the interface, as well as a flip in their net orientation. The mutual orientation of the water and C12E6 molecules leads to the emergence of a broad (∼3 nm) interface with a large electric field of ∼1 V/nm, as evidenced by the Kelvin-probe measurements and MD simulations.

Structure and Dynamics of a Temperature-Sensitive Hydrogel.

Polyisocyanotripeptides (TriPIC) are biomimetic polymers which consist of a β-helical backbone stabilized by hydrogen bonds between amide groups. Their oligoethylene glycol side chains give aqueous TriPIC solutions a thermoresponsive behavior: at 50 °C the solution becomes a hydrogel. In this paper we study the molecular structure and water dynamics of TriPIC aqueous solutions while undergoing gelation using FT-IR spectroscopy and polarization-resolved femtosecond infrared spectroscopy (fs-IR).

The molecular structure of the surface of water-ethanol mixtures.

Mixtures of water and alcohol exhibit an excess surface concentration of alcohol as a result of the amphiphilic nature of the alcohol molecule, which has important consequences for the physico-chemical properties of water-alcohol mixtures. Here we use a combination of intensity vibrational sum-frequency generation (VSFG) spectroscopy, heterodyne-detected VSFG (HD-VSFG), and core-level photoelectron spectroscopy (PES) to investigate the molecular properties of water-ethanol mixtures at the air-liquid interface. We find that increasing the ethanol concentration up to a molar fraction (MF) of 0.

Disaccharide Residues are Required for Native Antifreeze Glycoprotein Activity.

Antifreeze glycoproteins (AFGPs) are able to bind to ice, halt its growth, and are the most potent inhibitors of ice recrystallization known. The structural basis for AFGP's unique properties remains largely elusive. Here we determined the antifreeze activities of AFGP variants that we constructed by chemically modifying the hydroxyl groups of the disaccharide of natural AFGPs.

Accelerated Hot-Carrier Cooling in MAPbI Perovskite by Pressure-Induced Lattice Compression.

Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC.

Strong Reduction of the Chain Rigidity of Hyaluronan by Selective Binding of Ca Ions.

The biological functions of natural polyelectrolytes are strongly influenced by the presence of ions, which bind to the polymer chains and thereby modify their properties. Although the biological impact of such modifications is well recognized, a detailed molecular picture of the binding process and of the mechanisms that drive the subsequent structural changes in the polymer is lacking. Here, we study the molecular mechanism of the condensation of calcium, a divalent cation, on hyaluronan, a ubiquitous polymer in human tissues.