[Show abstract][Hide abstract] ABSTRACT: The surfactant sodium dodecyl sulphate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. We report a microscopic model for the interaction between the surfactant and water and between water molecules at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct sub-ensembles of water in the presence of this negatively charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O-H groups, i.e. localised O-H stretch vibrations, and those whose O-H stretch vibrations are delocalised, i.e. shared between multiple O-H bonds. The two sub-ensembles are coupled, with sub picosecond energy transfer occurring between them. This is markedly different from O-H bonds at the air - water interface, which are less heterogeneous, and indicates that the water molecules that interact with the surfactant headgroups have different hydrogen bonding properties from those interacting with the other water molecules.
Full-text · Article · Nov 2015 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: We present a combined experimental sum-frequency generation (SFG) spectroscopy and ab initio molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine N-oxide (AO) surfactant monolayers. Simulated O-H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the likewise zwitterionic AO interface. The positive peaks at the water/PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen-bond network to reach, with its (up-oriented) O-H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group on the interfacial water structure at the membrane interface, by stabilizing the water hydrogen-bond network.
Full-text · Article · Oct 2015 · Journal of Physical Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: Dispersing hydrophilic nanofillers in highly hydrophobic polymer matrices is widely used to tune the mechanical properties of composite material systems. The ability to control the dispersion of fillers is closely related to the mechanical tunability of such composites. In this work, we investigate the physical-chemical underpinnings of how simple end-group modification to one end of a styrene-butadiene chain modifies the dispersion of silica fillers in a polymer matrix. Using surface-sensitive spectroscopies, we directly show that polymer molecular orientation at the silica surface is strongly constrained for silanol functionalized polymers compared to nonfunctionalized polymers because of covalent interaction of silanol with silica. Silanol functionalization leads to reduced filler aggregation in composites. The results from this study demonstrate how minimal chemical modifications of polymer end groups are effective in modifying microstructural properties of composites by inducing molecular ordering of polymers at the surface of fillers.
[Show abstract][Hide abstract] ABSTRACT: We examine the temperature dependence of the interfacial molecular structure at the water-air interface by combining experimental and simulated sum-frequency generation (SFG) spectroscopy. The experimental SFG spectra of the OH-stretching mode show a decrease in the amplitude at [similar]3300 cm-1 with increasing temperature, while the 3700 cm-1 'free OH' SFG feature is insensitive to temperature changes. The simulated spectra are in excellent agreement with experiment. A comparison between interfacial SFG spectra and bulk infrared/Raman spectra reveals that the variation of the SFG signal due to the temperature change is not caused by a temperature-dependent OH bond orientation of the interfacial water molecules, but can be fully accounted for by the temperature dependence of the optical response of water. These results indicate that while the thickness of the interfacial region varies with temperature, the molecular organization of interfacial water at the water-air interface is surprisingly insensitive to temperature changes.
No preview · Article · Aug 2015 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Die Grenzfläche von Wasser zu Luft bietet eine Plattform für viele wichtige biologische, chemische und physikalische Prozesse. Die Wasser-Luft-Grenzfläche ist die verbreitetste und einfachste Grenzfläche und dient als Modellsystem für Wasser an hydrophoben Oberflächen. Die Aufklärung der mikroskopischen (<1 nm) Struktur und Dynamik von Wasser an der Wasser-Luft-Grenzfläche ist zum Verständnis der an der Wasseroberfläche auftretenden Prozesse unerlässlich. Das Netzwerk sehr starker intermolekularer Wechselwirkungen, der Wasserstoffbrücken (H-Brücken), ist an der Wassergrenzfläche unterbrochen. Ebenso ist dort die Dichte von Wasser verringert. Eine zentrale Frage bezüglich Wasser an Grenzflächen ist, inwieweit Struktur und Dynamik der Wassermoleküle von Unterbrechungen des H-Brückennetzwerks beeinflusst werden und sich dadurch von Wasser im Volumen unterscheiden. In diesem Aufsatz werden jüngste Fortschritte in der Untersuchung von Wasser an der Wasser-Luft-Grenzfläche mittels Laser-basierter oberflächenspezifischer Schwingungsspektroskopie diskutiert.
No preview · Article · Apr 2015 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: Dendrimeric macromolecules with defined shape and size are promising candidates to deliver a drug or DNA molecule into cells. In this work we study the influence of an amphiphilic polyphenylene dendrimer on a model cell membrane consisting of a condensed monolayer of the lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). A small surface pressure decrease is observed when the dendrimer solution is injected into the aqueous phase below the monolayer. X-ray reflectivity measurements show that the surface monolayer remains intact. The molecular scale picture is obtained with sum-frequency generation spectroscopy. With this technique we observe that the tails of the surfactant molecules become less ordered upon interaction with the amphiphilic polyphenylene dendrimer. In contrast, the water molecules below the DPPC layer become more ordered. Our observations suggest that electrostatic interactions between the negative charge of the dendrimer and the positively charged part of the DPPC headgroup keep the dendrimer located below the headgroup. No evidence of dendrimer insertion into the membrane has been observed. Apparently before entering the cell membrane, the dendrimer can stick at the hydrophilic part of the lipids.
[Show abstract][Hide abstract] ABSTRACT: GALA is a 30 amino acid synthetic peptide consisting of a Glu-Ala-Leu-Ala repeat and is known to undergo a reversible structural transition from a disordered to an α-helical structure when changing the pH from basic to acidic values. In its helical state GALA can insert into and disintegrate lipid membranes. This effect has generated much interest in GALA as a candidate for pH triggered, targeted drug delivery. GALA also serves as a well-defined model system to understand cell penetration mechanisms and protein folding triggered by external stimuli. Structural transitions of GALA in solution have been studied extensively. However, cell penetration is an interfacial effect and potential biomedical applications of GALA would involve a variety of surfaces, e.g., nanoparticles, lipid membranes, tubing, and liquid-gas interfaces. Despite the apparent importance of interfaces in the functioning of GALA, the effect of surfaces on the reversible folding of GALA has not yet been studied. Here, we use sum frequency generation vibrational spectroscopy (SFG) to probe the structural response of GALA at the air-water interface and IR spectroscopy to follow GALA folding in bulk solution. We combine the SFG data with molecular dynamics simulations to obtain a molecular-level picture of the interaction of GALA with the air-water interface. Surprisingly, while the fully reversible structural transition was observed in solution, at the water-air interface, a large fraction of the GALA population remained helical at high pH. This “stickiness” of the air-water interface can be explained by the stabilizing interactions of hydrophobic leucine and alanine side chains with the water surface.
Full-text · Article · Oct 2014 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The UV-induced cross-linking of methacryloxypropyl-terminated poly(dimethylsiloxane) oligomers was studied at the air-water-interface either in pure PDMS Langmuir monolayers or in mixed films containing cellulose acetate butyrate. Surface pressure-area isotherms, area measurement at constant surface pressure, Brewster angle microscopy observations and infrared-visible sum frequency generation (SFG) spectroscopy were combined to follow the evolution of the monolayers upon in-situ UV photoirradiation. For both systems, the mean area per repeat unit decreases with irradiation time reflecting the monolayer contraction. In addition, SFG measurements evidence the conversion of the methacrylate groups into unconjugated poly(methacrylate) ones. These results demonstrate PDMS cross-linking, leading to the formation of either a single PDMS network or a PDMS network entrapped in a CAB matrix. The network formation is accompanied by morphology changes as shown by Atomic Force Microscopy on the transferred monolayer. Indeed, filamentous structures appear on both pure and mixed preirradiated monolayers.
[Show abstract][Hide abstract] ABSTRACT: Wassermoleküle wechselwirken stark miteinander durch Wasserstoffbrücken. Diese effiziente intermolekulare Kopplung verursacht eine starke Delokalisierung der Molekülschwingungen im Inneren des Wassers. Wir untersuchen die intermolekulare Kopplung der Schwingungen an der Luft/Wasser-Grenzfläche und beobachten, dass die intermolekulare Kopplung 1) signifikant reduziert ist und 2) für verschiedene Wassermoleküle an der Grenzfläche stark variiert – während im Inneren des Wassers die Kopplung homogen ist. Für stark Wasserstoffbrücken-gebundene O-H-Gruppen ist die Kopplung etwa zweimal geringer als im Inneren des Wassers, aufgrund der geringeren Dichte in der grenzflächennahen Region. Für schwach Wasserstoffbrücken-gebundene O-H-Gruppen, die bei 3500 cm−1 absorbieren, ist diese Kopplung um einen weiteren Faktor ≈2 reduziert. Diese O-H-Gruppen werden den äußersten, jedoch Wasserstoffbrücken-gebundenen Wassermolekülen zugeordnet, deren andere O-H-Gruppe zur Gasphase zeigt. Trotz der geringen strukturellen Einschränkungen durch die umgebenden Wasserstoffbrücken auf diese Wassermoleküle ist – bemerkenswerterweise – deren Strukturrelaxation langsam und die intermolekulare Kopplung schwach.
No preview · Article · Jul 2014 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface-whereas in bulk water the coupling is homogeneous. For strongly hydrogen-bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near-surface region. For weakly hydrogen-bonded OH groups that absorb around 3500 cm(-1) , which are assigned to the outermost, yet hydrogen-bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen-bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.
Full-text · Article · Jul 2014 · Angewandte Chemie International Edition in English
[Show abstract][Hide abstract] ABSTRACT: In nature, aqueous solutions often move collectively along solid surfaces (for example, raindrops falling on the ground and
rivers flowing through riverbeds). However, the influence of such motion on water-surface interfacial chemistry is unclear.
In this work, we combine surface-specific sum frequency generation spectroscopy and microfluidics to show that at immersed
calcium fluoride and fused silica surfaces, flow leads to a reversible modification of the surface charge and subsequent realignment
of the interfacial water molecules. Obtaining equivalent effects under static conditions requires a substantial change in
bulk solution pH (up to 2 pH units), demonstrating the coupling between flow and chemistry. These marked flow-induced variations
in interfacial chemistry should substantially affect our understanding and modeling of chemical processes at immersed surfaces.
[Show abstract][Hide abstract] ABSTRACT: Water possesses an extremely high polarity, making it a unique solvent for salts. Indeed, aqueous electrolyte solutions are ubiquitous in the atmosphere, biology, energy applications and industrial processes. For many processes, chemical reactions at the water surface are rate determining, and the nature and concentration of the surface-bound electrolytes are of paramount importance, as they determine the water structure and thereby surface reactivity. Here we investigate the dynamics of water molecules at the surface of sodium chloride and sodium iodide solutions, using surface-specific femtosecond vibrational spectroscopy. We quantify the interfacial ion density through the reduced energy transfer rates between water molecules resulting from the lowered effective interfacial density of water molecules, as water is displaced by surface active ions. Our results reveal remarkably high surface propensities for halogenic anions, higher for iodide than for chloride ions, corresponding to surface ion concentrations several times that of the bulk.
Full-text · Article · Jun 2014 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: We introduce an infrared pump-terahertz probe technique to measure the thermalization dynamics of aqueous solutions with a time resolution <200 fs. This technique makes use of the sensitivity of the terahertz absorption to the temperature of the hydrogen bond network. The thermalization dynamics of different aqueous solutions are measured and compared to the dynamics inferred from ultrafast infrared pump-infrared probe measurements on the intramolecular stretch vibration of water. This technique can shed new light on important aspects of energy transfer and heat dynamics and is applicable to a wide range of systems.
[Show abstract][Hide abstract] ABSTRACT: We study the properties of water molecules adjacent to a hydrophobic molecular layer with vibrational sum-frequency generation spectroscopy. We find that the water molecules at D2O/hexane, D2O/heptane, and D2O/polydimethylsiloxane interfaces show an enhanced ordering and stronger hydrogen-bond interactions than the water molecules at a D2O/air interface. With increasing temperature (up to 80 °C) the water structure becomes significantly less ordered and the hydrogen bonds become weaker.
Full-text · Article · Feb 2014 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Interfaces of liquid water play a critical role in a wide variety of processes that occur in biology, a variety of technologies, and the environment. Many macroscopic observations clarify that the properties of liquid water interfaces significantly differ from those of the bulk liquid. In addition to interfacial molecular structure, knowledge of the rates and mechanisms of the relaxation of excess vibrational energy is indispensable to fully understand physical and chemical processes of water and aqueous solutions, such as chemical reaction rates and pathways, proton transfer, and hydrogen bond dynamics. Here we elucidate the rate and mechanism of vibrational energy dissipation of water molecules at the air-water interface using femtosecond two-color IR-pump/vibrational sum-frequency probe spectroscopy. Vibrational relaxation of nonhydrogen-bonded OH groups occurs at a subpicosecond timescale in a manner fundamentally different from hydrogen-bonded OH groups in bulk, through two competing mechanisms: intramolecular energy transfer and ultrafast reorientational motion that leads to free OH groups becoming hydrogen bonded. Both pathways effectively lead to the transfer of the excited vibrational modes from free to hydrogen-bonded OH groups, from which relaxation readily occurs. Of the overall relaxation rate of interfacial free OH groups at the air-H2O interface, two-thirds are accounted for by intramolecular energy transfer, whereas the remaining one-third is dominated by the reorientational motion. These findings not only shed light on vibrational energy dynamics of interfacial water, but also contribute to our understanding of the impact of structural and vibrational dynamics on the vibrational sum-frequency line shapes of aqueous interfaces.
Full-text · Article · Nov 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: We study the effects of ions and hydrophobic molecular groups on the orientational dynamics of water using THz dielectric relaxation (THz-DR) and polarization-resolved femtosecond infrared (fs-IR) pump-probe spectroscopy. We measure the dynamics of water in solutions of NaI, NaCl, CsCl, guanidinium chloride (GndCl) and tetramethyl guanidinium chloride (TMGndCl) of different the static dipoles of their surrounding water molecules. With fs-IR we find that concentrations. With THz-DR we observe that strongly hydrated cations align the OD groups that form hydrogen bonds to halide anions reorient with two distinct time constants of 2 +/- 0.3 ps and 9 +/- 1 ps. The fast process is assigned to a wobbling motion of the OD group that keeps the hydrogen bond with the anion intact. The amplitude of this wobbling motion depends on the nature of both the anion and the counter cation. The replacement of four of the six hydrogen atoms of the weakly hydrated cation guanidinium by hydrophobic methyl groups leads to an exceptionally strong slowing down of the water dynamics. Hydrophobic groups thus appear to have a much stronger effect on the dynamics of water than ions. These findings give new insights in the mechanism of protein denaturation by GndCl and TMGndCl.
Full-text · Article · Jun 2013 · Faraday Discussions
[Show abstract][Hide abstract] ABSTRACT: Vibrational sum-frequency generation (VSFG) spectra of the amide-I band of proteins can give detailed insight into biomolecular processes near membranes. However, interpreting these spectra in terms of the conformation and orientation of a protein can be difficult, especially in the case of complex proteins. Here we present a formalism to calculate the amide-I infrared (IR), Raman and VSFG spectra based on the protein conformation and orientation distribution. Based on the protein conformation, we set up the amide-I exciton Hamiltonian for the backbone amide modes that generate the linear and nonlinear spectroscopic responses. In this Hamiltonian, we distinguish between nearest-neighbor and non-nearest-neighbor vibrational couplings. To determine nearest-neighbor couplings we use an ab initio 6-31G +(d) B3LYP-calculated map of the coupling as a function of the dihedral angles. The other couplings are estimated using the transition-dipole coupling model. The local-mode frequencies of hydrogen-bonded peptide bonds and of peptide bonds to proline residues are redshifted. To obtain realistic hydrogen-bond shifts we perform a molecular dynamics simulation in which the protein is solvated by water. As a first application, we measure and calculate the amide-I IR, Raman and VSFG spectra of cholera toxin B subunit docked to a model cell membrane. To deduce the orientation of the protein with respect to the membrane from the VSFG spectra, we compare the experimental and calculated spectral shapes of single-polarization results, rather than comparing the relative amplitudes of VSFG spectra recorded for different polarization conditions for infrared, visible and sum-frequency light. We find that the intrinsic uncertainty in the interfacial refractive index - essential to determine the overall amplitude of the VSFG spectra - prohibits a meaningful comparison of the intensities of the different polarization combinations. In contrast, the spectral shape of most of the VSFG spectra is independent of the details of the interfacial refractive index, and provides a reliable way of determining molecular interfacial orientation. Specifically, we find that the symmetry axis of the cholera toxin B subunit is oriented at an angle of 6° ± 17° relative to the surface normal of the lipid monolayer, in agreement with five-fold binding between the toxin's five subunits and the receptor lipids in the membrane.
Full-text · Article · Apr 2013 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: Capillary adhesion of microparticles was analytically calculated, modelled by finite element method (FEM) simulations and measured. The effects of elastic deformation and liquid adsorption were analyzed. By means of an atomic force microscope, we measured the force between a silica bead of 2 μm radius and a planar polydimethylsiloxane surface (Young's modulus E = 1 MPa) in the presence of ethanol at different vapor pressures. Results were compared to adhesion forces measured on a silicon wafer. Independent of the sample elastic modulus experiments showed a monotonous decrease of capillary forces with increasing ethanol partial vapor pressure for P/Psat > 0.2, where Psat is the saturation vapor pressure. However, adhesion forces on the soft surface were much stronger than on the rigid silicon wafer. In order to explain the experimental results, a previous developed theory (Soft Matter, 2010, 6, 3930) was extended to take into account vapor adsorption of ethanol. Analytical calculations were compared to results of FEM simulations where the detailed deformation of the elastic support close to the meniscus was explicitly taken into account.