6 research outputs found
About the Role of Surfactants on the Magnetic Control over Liquid Interfaces
The behavior of magnetically responsive
aqueous Fe(III) surfactant
solutions at liquid interfaces is analyzed. Such surfactants attracted
much attention, because of the ability to manipulate interfaces by
magnetic fields without any use of magnetic nanoparticles. A detailed analysis of the surface
properties proves that the mixing of paramagnetic electrolyte solution
with anionic, cationic and nonionic surfactants yields the similar
magnetic response and no effect of the surfactant charge can be observed.
We conclude that the observed magnetic shiftability of interfaces
is caused by a combination of the paramagnetic behavior of the bulk
liquid and a reduction of the surface tension. Thus, this work gives
an alternative interpretation of the properties of “magnetic
surfactants” compared to the ones claimed in the literature
Structure and Phase Behavior of Archaeal Lipid Monolayers
We report X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD) measurements of archaeal bipolar tetraether lipid monolayers at the air–water interface. Specifically, Langmuir films made of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at three different temperatures, i.e., 68, 76, and 81 °C, were examined. The dependence of the structure and packing properties of PLFE monolayers on surface pressure were analyzed in a temperature range between 10 and 50 °C at different pH values. Additionally, the interaction of PLFE monolayers (using lipids derived from cells grown at 76 °C) with the ion channel peptide gramicidin was investigated as a function of surface pressure. A total monolayer thickness of approximately 30 Å was found for all monolayers, hinting at a U-shaped conformation of the molecules with both head groups in contact with the interface. The monolayer thickness increased with rising film pressure and decreased with increasing temperature. At 10 and 20 °C, large, highly crystalline domains were observed by GIXD, whereas at higher temperatures no distinct crystallinity could be observed. For lipids derived from cells grown at higher temperatures, a slightly more rigid structure in the lipid dibiphytanyl chains was observed. A change in the pH of the subphase had an influence only on the structure of the lipid head groups. The addition of gramicidin to an PLFE monolayer led to a more disordered state as observed by XRR. In GIXD measurements, no major changes in lateral organization could be observed, except for a decrease of the size of crystalline domains, indicating that gramicidin resides mainly in the disordered areas of the monolayer and causes local membrane perturbation, only
On the Spontaneous Formation of Clathrate Hydrates at Water–Guest Interfaces
The formation of hydrates, cage-like water-gas structures,
is of
tremendous importance both in industries and research. Although of
major significance, the formation process is not completely understood
so far. We present a comprehensive study of hydrate formation at liquid–liquid
interfaces between water and isobutane, propane, carbon dioxide, and
at the liquid–gas interface between water and xenon. We investigated
the structure of these interfaces under quiescent conditions in situ
by means of X-ray reflectivity measurements both inside and outside
the zone of hydrate stability. At the interfaces between water and
liquid alkanes, no evidence for a structural change was found. In
contrast, the accumulation of guest molecules inside nanothick interfacial
layers was observed at the water–xenon and liquid–liquid
water–CO<sub>2</sub> interfaces. We show that only those systems
initially exhibiting such guest-enriched interfacial layers developed
into macroscopic gas hydrates within our observation times (∼12
h). Therefore, these layers act as triggers for the spontaneous formation
of macroscopic hydrates
Solid-Supported Lipid Multilayers under High Hydrostatic Pressure
In this work, the structure of solid-supported
lipid multilayers
exposed to increased hydrostatic pressure was studied <i>in situ</i> by X-ray reflectometry at the solid–liquid interface between
silicon and an aqueous buffer solution. The layers’ vertical
structure was analyzed up to a maximum pressure of 4500 bar. The multilayers
showed phase transitions from the fluid into different gel phases.
With increasing pressure, a gradual filling of the sublayers between
the hydrophilic head groups with water was observed. This process
was inverted when the pressure was decreased, yielding finally smaller
water layers than those in the initial state. As is commonly known,
water has an abrasive effect on lipid multilayers by the formation
of vesicles. We show that increasing pressure can reverse this process
so that a controlled switching between multi- and bilayers is possible
Subsurface Influence on the Structure of Protein Adsorbates as Revealed by in Situ X-ray Reflectivity
The adsorption process of proteins to surfaces is governed
by the
mutual interactions among proteins, the solution, and the substrate.
Interactions arising from the substrate are usually attributed to
the uppermost atomic layer. This actual surface defines the surface
chemistry and hence steric and electrostatic interactions. For a comprehensive
understanding, however, the interactions arising from the bulk material
also have to be considered. Our protein adsorption experiments with
globular proteins (α-amylase, bovine serum albumin, and lysozyme)
clearly reveal the influence of the subsurface material via van der
Waals forces. Here, a set of functionalized silicon wafers enables
a distinction between the effects of surface chemistry and the subsurface
composition of the substrate. Whereas the surface chemistry controls
whether the individual proteins are denatured, the strength of the
van der Waals forces affects the final layer density and hence the
adsorbed amount of proteins. The results imply that van der Waals
forces mainly influence surface processes, which govern the structure
formation of the protein adsorbates, such as surface diffusion and
spreading
Adsorption Behavior of Lysozyme at Titanium Oxide–Water Interfaces
We
present an in situ X-ray reflectivity study of the adsorption
behavior of the protein lysozyme on titanium oxide layers under variation
of different thermodynamic parameters, such as temperature, hydrostatic
pressure, and pH value. Moreover, by varying the layer thickness of
the titanium oxide layer on a silicon wafer, changes in the adsorption
behavior of lysozyme were studied. In total, we determined less adsorption
on titanium oxide compared with silicon dioxide, while increasing
the titanium oxide layer thickness causes stronger adsorption. Furthermore,
the variation of temperature from 20 to 80 °C yields an increase
in the amount of adsorbed lysozyme at the interface. Additional measurements
with variation of the pH value of the system in a region between pH
2 and 12 show that the surface charge of both protein and titanium
oxide has a crucial role in the adsorption process. Further pressure-dependent
experiments between 50 and 5000 bar show a reduction of the amount
of adsorbed lysozyme with increasing pressure