Sylvie Roke

The orientation and charge of water at the hydrophobic oil droplet-water interface.

We established the charge and structure of the oil/water interface by combining ζ-potential measurements, sum frequency scattering (SFS) and molecular dynamics simulations. The SFS experiments show that the orientation of water molecules can be followed on the oil droplet/water interface. The average water orientation on a neat oil droplet/water interface is the same as the water orientation on a negatively charged interface. pH dependent experiments show, however, that there is no sign of selective adsorption of hydroxide ions. Molecular dynamics simulations, both with and without intermolecular charge transfer, show that the balance of accepting and donating hydrogen bonds is broken in the interfacial layer, leading to surface charging. This can account for the negative surface charge that is found in experiments.

Nonlinear Light Scattering and Spectroscopy of Particles and Droplets in Liquids

Nano- and microparticles have optical, structural, and chemical properties that differ from both their building blocks and the bulk materials themselves. These different physical and chemical properties are induced by the high surface-to-volume ratio. As a logical consequence, to understand the properties of nano- and microparticles, it is of fundamental importance to characterize the particle surfaces and their interactions with the surrounding medium. Recent developments of nonlinear light scattering techniques have resulted in a deeper insight of the underlying light-matter interactions. They have shed new light on the molecular mechanism of surface kinetics in solution, properties of interfacial water in contact with hydrophilic and hydrophobic particles and droplets, molecular orientation distribution of molecules at particle surfaces in solution, interfacial structure of surfactants at droplet interfaces, acid-base chemistry on particles in solution, and vesicle structure and transport properties.

Protons and Hydroxide Ions in Aqueous Systems.

Understanding the structure and dynamics of waters constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

The Interfacial Tension of Nanoscopic Oil Droplets in Water Is Hardly Affected by SDS Surfactant

Surfactants such as sodium dodecylsulfate (SDS) can reduce the interfacial tension between bulk water and bulk n-hexadecane by 42 mN/m. Although reduction of interfacial tension should also take place on the interface of nanoscopic oil droplets in water, vibrational sum frequency scattering experiments indicate otherwise. In these measurements we have directly measured the adsorption of SDS onto hexadecane oil droplets with an average radius of 83 nm. We find that the interfacial density of adsorbed SDS is at least 1 order of magnitude lower than that at a corresponding planar interface. The derived maximum decrease in interfacial tension is only 5 mN/m.

Surface structure of sodium dodecyl sulfate surfactant and oil at the oil-in-water droplet liquid/liquid interface: a manifestation of a nonequilibrium surface state.

We present sum frequency scattering spectra on kinetically stabilized emulsions consisting of nanoscopic oil droplets in water, stabilized with sodium dodecyl sulfate (SDS). We have measured the interfacial structure of the alkyl chains of the surfactant molecules, the alkyl chain of the oil molecules, the weakly dispersive D(2)O response, and the interference between SDS and the oil. We find a big difference in chain conformation: SDS has many chain defects, whereas the oil has very few. Our spectra are interpreted to originate from a surface structure with oil molecules predominantly oriented parallel with respect to the plane of the interface. The SDS headgroup is surrounded by water molecules. The SDS alkyl tail is in a disordered state and partially in contact with water. Such a conformation of surfactant occupies a surface area of several hundreds of squared angstroms.

Nonlinear Optical Spectroscopy of Soft Matter Interfaces

Soft matter consists of complex molecules that can undergo drastic structural transformations under mild changes of chemical and physical conditions. Since a wide variety of chemical, physical and biological processes occur at soft matter interfaces, they can exhibit complex behavior. This is even more so for interfaces of colloidal soft matter since the relative amount of interface material increases by orders of magnitude. Herein, we focus on new developments that enable us to obtain detailed molecular structural changes in the topmost molecular layers of soft matter interfaces composed of complex bio-molecules. In particular, the possibilities to probe interfaces of colloidal soft matter systems are discussed.

Luminescence of Exchange Coupled Pairs of Transition Metal Ions

The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for Mn 2+ -Mn 2+ . Cr 3+ -Cr 3+ , Cr 3+ -Gd 3+ , and Mn 2+ -Gd 3+ pairs. Two effects are reported: shortening of the lifetime of the spin-forbidden emission and a shift of emission lines or hands to a longer wavelength. The shortening of the lifetime of the spin-forbidden emission is important in Mn 2+ doped phosphors. For application of Mn 2+ -doped phosphors in displays the long lifetime of the Mn 2+ emission is a problem. Shortening of the lifetime is possible by exchange coupling with Mn 2+ neighbors, or possibly other paramagnetic neighbors. The results reported here show that the lifetime shortening depends on the exchange-coupling parameter J. The strongest coupling is observed in Cr 3+ -Cr 3+ pairs and the lifetime shortening is strong (e.g., in LaAlO 3 :Cr 3+ J = 63 cm 1 and the lifetime is about 30 times shorter for the pair emission). The coupling in Mn 2+ -Mn 2+ and Cr 3+ -Gd 3+ pairs is weak. Typically, values for J are around I cm 1 and the lifetime is reduced by a factor of 2-10 (e.g., J is 1.0 cm 1 for exchange coupling between Cr 3+ and Gd 3+ ; the lifetime of the spin-forhidden Cr 3+ emission is 14 ms in GdAlO 3 compared to 56 ms in LaAlO 3 ). The lifetime shortening for the Mn 2+ emission is not related to the observed red shift for the pair emission. For the Mn 2+ -Gd 3+ pair the exchange interaction is very weak and no significant lifetime shortening could he measured for Gd 3+ -Mn 2+ pairs (24 ms for the Mn 2+ emission in YF 3 vs. 22 ms in GdF 3 ).

Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water

Ions induce changes in the H-bond network of water that extend by >20 nm, vary for H2O and D2O, and lead to surface tension anomalies. Electrolytes interact with water in many ways: changing dipole orientation, inducing charge transfer, and distorting the hydrogen-bond network in the bulk and at interfaces. Numerous experiments and computations have detected short-range perturbations that extend up to three hydration shells around individual ions. We report a multiscale investigation of the bulk and surface of aqueous electrolyte solutions that extends from the atomic scale (using atomistic modeling) to nanoscopic length scales (using bulk and interfacial femtosecond second harmonic measurements) to the macroscopic scale (using surface tension experiments). Electrolytes induce orientational order at concentrations starting at 10 μM that causes nonspecific changes in the surface tension of dilute electrolyte solutions. Aside from ion-dipole interactions, collective hydrogen-bond interactions are crucial and explain the observed difference of a factor of 6 between light water and heavy water.

Water-Mediated Ion Pairing : Occurrence and Relevance

We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li(+) and Na(+) and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems.

Direct comparison of phase-sensitive vibrational sum frequency generation with maximum entropy method: Case study of water

We present a direct comparison of phase sensitive sum-frequency generation experiments with phase reconstruction obtained by the maximum entropy method. We show that both methods lead to the same complex spectrum. Furthermore, we discuss the strengths and weaknesses of each of these methods, analyzing possible sources of experimental and analytical errors. A simulation program for maximum entropy phase reconstruction is available at: http://lbp.epfl.ch/.