Alex G. F. de Beer

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.

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/.

Obtaining molecular orientation from second harmonic and sum frequency scattering experiments in water: angular distribution and polarization dependence

We present a method for determining molecular orientation from second-order nonlinear light scattering experiments. Our modeling shows that there is an optimal angular region, for which the scattering pattern is most sensitive to molecular orientation. We show that molecular orientation can be retrieved from measuring intensities at different polarization combinations, measuring the relative amplitudes of different vibrational modes of the same moiety and by analyzing the shape of the angular scattering pattern. We further show that for C(2v) and C(3v) point groups, the asymmetric stretch mode displays a higher sensitivity to molecular orientation than the corresponding symmetric mode. We have implemented the model in an interactive simulation program that may be found at http://www.mf.mpg.de/en/abteilungen/roke/simulation.html.

Theory of optical second-harmonic and sum-frequency scattering from arbitrarily shaped particles

We present a theoretical framework for the generation and scattering of second-harmonic and sum-frequency light from the surface of particles of arbitrary shape in the limit of low index of refraction contrast. For homogeneous and isotropic surfaces, light scattering can be described by a finite set of scattering functions. Selection rules regarding these scattering functions are presented. We also find that the scattering functions associated with achiral and chiral surfaces are directly related to the bulk and surface linear optical form factors, respectively. Finally, we derive explicit expressions for particles of ellipsoidal shape, for which we calculate angular scattering patterns as a function of particle orientation and for ensembles of particles.

Intermolecular Headgroup Interaction and Hydration as Driving Forces for Lipid Transmembrane Asymmetry

Variations between the inner and outer leaflets of cell membranes are crucial for cell functioning and signaling, drug-membrane interactions, and the formation of lipid domains. Transmembrane asymmetry can in principle be comprised of an asymmetric charge distribution, differences in hydration, specific headgroup/H-bonding interactions, or a difference in the number of lipids per leaflet. Here, we characterize the transmembrane asymmetry of small unilamellar liposomes consisting of zwitterionic and charged lipids in aqueous solution using vibrational sum frequency scattering and second harmonic scattering, label-free methods, specifically sensitive to lipid and water asymmetries. For single component liposomes, transmembrane asymmetry is present for the charge distribution and lipid hydration, but the leaflets are not detectably asymmetric in terms of the number of lipids per leaflet, even though geometrical packing arguments would predict so. Such a lipid transmembrane asymmetry can, however, be induced in binary lipid mixtures under conditions that enable H-bonding interactions between phosphate and amine groups. In this case, the measured asymmetry consists of a different number of lipids in the outer and inner leaflet, a difference in transmembrane headgroup hydration, and a different headgroup orientation for the interacting phosphate groups.

Label-free spectroscopic detection of vesicles in water using vibrational sum frequency scattering

Vibrational sum frequency scattering (SFS) has been used to study sub-micron, catanionic vesicles in solution. The vesicles were synthesized from a binary mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylsulfate (SDS) surfactants in deuterated water, which spontaneously assemble into thermodynamically stable vesicles. The stability of these vesicles is attributed to a surfactant concentration asymmetry between the inner and outer bilayer leaflets. This concentration asymmetry should be observable by SFS due to local inversion symmetry-breaking. The signal corresponding to the symmetric sulfate stretch mode of the SDS head group is observed at 1042 cm−1, indicating that there is indeed asymmetry in the local structure of the leaflets. The results indicate that it should be possible to measure the interfacial structure of liposomes in aqueous solution and study in situ processes like the binding of sugars and proteins that are important for many processes in biophysical chemistry.

The Presence of Ultralow Densities of Nanocrystallites in Amorphous Poly(lactic acid) Microspheres

Ultralow densities of crystalline nanospheres have been detected in amorphous polymer microspheres by utilizing the unique sensitivity of second-order nonlinear optical techniques to anisotropy. Vibrational sum frequency scattering (SFS) and X-ray diffraction (XRD) are used to quantify the crystallinity of amorphous poly(d,l-lactic acid) microspheres. While XRD does not display any crystallinity for the microspheres, SFS spectra and patterns are reminiscent of a heterogeneous microsphere that contains small crystalline domains. Nonlinear light scattering theory was used to model the data, and an average domain radius of 147 ± 65 nm was obtained. The degree of crystallinity (0.2%) was estimated by comparing XRD and SFS data obtained from the amorphous microspheres to similar data obtained from crystalline microspheres. We estimate a detection limit of 0.002% for SFS.

What interactions can distort the orientational distribution of interfacial water molecules as probed by second harmonic and sum frequency generation

Aqueous interfaces are omnipresent in nature. Nonlinear optical methods such as second harmonic and sum frequency generation (SHG/SFG) are valuable techniques to access molecular level information from these interfaces. In the interpretation of SHG and SFG data for both scattering and reflection mode experiments, the relation between the second-order hyperpolarizability tensor β(2), a molecular property, and the surface second-order susceptibility χ(2), a surface averaged property, plays a central role. To correctly describe the molecular details of the interface, it needs to be determined how molecules are oriented, and what the influence is of interfacial electrostatic fields and H-bonding on the orientational distribution. Here, we revisit the relations between β(2) and χ(2) and show, by means of a Boltzmann average, that significant energy differences are needed to generate measurable changes in the molecular orientational distribution at the interface. In practice, H-bonding and surface pressure such as applied in a Langmuir trough can be strong enough to alter the shape of the orientational distribution function of water. In contrast, electrostatic fields, such as those present in the Stern layer, will not have a significant impact on the shape of the orientational distribution function of water molecules.

Continuous Photobleaching to Study the Growth Modes of Focal Adhesions

We combine total internal reflection fluorescence microscopy with continuous photobleaching to study the growth mode of focal adhesions in rat embryonic fibroblasts (REF-52). We measured GFP-labelled β3 integrin, which exhibits a diffusion rate of 10–14–10–13 m2/s when it is not bound to a focal adhesion. We show that exchange of β3 integrin between focal adhesions and the surrounding membrane occurs only at the edges of the focal adhesion. We, therefore, conclude that focal adhesions show an edge-on/edge-off growth, in contrast to the bulk-on/bulk-off growth mode that is assumed in many theoretical treatments.