X-ray reflectivity (XR) and atomistic molecular dynamics (MD) simulations, carried out to determine the structure of the oil-water interface, provide new insight into the simplest liquid-liquid ...interface. For several oils (hexane, dodecane, and hexadecane) the XR shows very good agreement with a monotonic interface-normal electron density profile (EDP) broadened only by capillary waves. Similar agreement is also found for an EDP including a sub-Å thick electron depletion layer separating the oil and the water. The XR and MD derived depletions are much smaller than reported for the interface between solid-supported hydrophobic monolayers and water.
X-ray investigations reveal that the monolayers formed at the bulk alkanol-sapphire interface are densely packed with the surface-normal molecules hydrogen bound to the sapphire. About 30-35 °C above ...the bulk, these monolayers both melt reversibly and partially desorb. This system exhibits balanced intermolecular and molecule-substrate interactions which are intermediate between self-assembled and surface-frozen monolayers, each dominated by one interaction. The phase behavior is rationalized within a thermodynamic model comprising interfacial interactions, elasticity, and entropic effects. Separating the substrate from the melt leaves the monolayer structurally intact.
We have used anomalous small-angle X-ray scattering (ASAXS) to investigate the spatial correlation of counterions with a macroion consisting of polyelectrolyte brushes. The spherical polyelectrolyte ...brush is composed of a solid poly(styrene) core of 68 nm radius with a dense grafting of linear poly(acrylic acid) chains and rubidium counterions. ASAXS allows one to evaluate the spatial distribution of counterions by exploiting the selective variation in contrast near the absorption edge of rubidium. The deduced spatial distribution of the counterions shows that they are closely correlated to the polymer chains of the macroion. The correlation between ions and polymer chains in these brushes is much stronger than that found in linear polyelectrolytes.
The knowledge of the mechanism used by vectors to gain access to cell interiors is key to the development of effective drug delivery tools for different pathologies. The role of the initial ...interaction with the membrane bilayer is widely recognized, although not fully understood. We use neutron reflectivity experiments and internalization studies with cells to reveal the extent of interaction of dendrimers functionalized with the peptide gH625 with biomimetic membranes. We further investigate the internalization by use of Caco-2 cells for assessing the membrane permeability properties of the peptide-dendrimer construct. Neutron reflectivity allowed for the hypothesis that the peptide-dendrimer is able to pass across the bilayer which was confirmed
via
permeability studies. We find that gH625-dendrimers interact more strongly with cholesterol containing membranes. The advances in our understanding of the mechanism of drug uptake are extremely useful to push further the design of new drug delivery systems.
The knowledge of the mechanism used by vectors to gain access to cell interiors is key to the development of effective drug delivery tools for different pathologies.
This paper reports a time-resolved small-angle X-ray scattering study of in situ Stöber silica synthesis. The hydrolysis reaction is initiated by rapidly mixing equal amounts of alcoholic solutions ...of ammonia and tetraethyl orthosilicate, using a stopped-flow device coupled to a flow-through capillary cell. Measurements covered the scattering wave vector (q) range of 0.02 ≤ q ≤ 6 nm-1 and time (t) range of 0.1 ≤ t ≤ 1000 s. The combination of high sensitivity, low background, and high dynamic range of the experimental setup permitted observation of the primary particles of nucleation. During the entire growth process, the measured scattered intensity can be adequately described by a sphere scattering function weighted by a Schultz size distribution function. At the early stages of growth, the fitted radius increased linearly with time, subsequently crossing over to a smaller exponent of between 1/3 and 1/2. The observed behavior is consistent with an aggregation process involving primary particles of a few nanometers in size.
The effect of hard X-ray radiation on the structure and electrostatics of solid-supported lipid multilayer membranes is investigated using a scanning Kelvin probe (SKP) integrated with a high-energy ...synchrotron beamline to enable in situ measurements of the membranes’ local Volta potential (V p) during X-ray structural characterization. The undulator radiation employed does not induce any detectable structural damage, but the V p of both bare and lipid-modified substrates is found to undergo strong radiation-induced shifts, almost immediately after X-ray exposure. Sample regions that are macroscopically distant (∼cm) from the irradiated region experience an exponential V p growth with a characteristic time constant of several minutes. The V p variations occurring upon periodic on/off X-ray beam switching are fully or partially reversible depending on the location and time-scale of the SKP measurement. The general relevance of these findings for synchrotron-based characterization of biomolecular thin films is critically reviewed.
Combined small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) is a powerful technique for the study of materials at length scales ranging from atomic/molecular sizes (a few angstroms) to the mesoscopic ...regime (∼1 nm to ∼1 µm). A set‐up to apply this technique at high X‐ray energies (E > 50 keV) has been developed. Hard X‐rays permit the execution of at least three classes of investigations that are significantly more difficult to perform at standard X‐ray energies (8–20 keV): (i) in situ strain analysis revealing anisotropic strain behaviour both at the atomic (WAXS) as well as at the mesoscopic (SAXS) length scales, (ii) acquisition of WAXS patterns to very large q (>20 Å−1) thus allowing atomic pair distribution function analysis (SAXS/PDF) of micro‐ and nano‐structured materials, and (iii) utilization of complex sample environments involving thick X‐ray windows and/or samples that can be penetrated only by high‐energy X‐rays. Using the reported set‐up a time resolution of approximately two seconds was demonstrated. It is planned to further improve this time resolution in the near future.
The effect of a double hydrophilic block-copolymer additive (made of polyaspartic acid and polyethyleneglycol, pAsp(10)-b-PEG(110)) on the initial formation of calcium carbonate from a supersaturated ...salt solution has been studied in situ by means of time-resolved synchrotron small-angle X-ray scattering (SAXS). A stopped-flow cell was used for rapidly mixing the 20 mM aqueous reactant solutions of calcium chloride and sodium carbonate. In reference measurements without polymer additive the very rapid formation of primary, overall spherical CaCO(3) particles with a radius of ca. 19 nm and a size polydispersity of ca. 26% was observed within the first 10 ms after mixing. A subsequent, very rapid aggregation of these primary particles was evidenced by a distinct upturn of the SAXS intensity at smallest angles. During the aggregation process the size of the primary particles remained unchanged. From an analysis of the absolute scattering intensity the mass density of these particles was determined to 1.9 g/cm(3). From this rather low density it is concluded that those precursor particles are amorphous, which has been confirmed by simultaneous wide-angle X-ray diffraction measurements. Upon adding 200 pm of the block-copolymer no influence on the size, the size polydispersity and morphology of the primary particles, nor on the kinetics of their formation and growth, was found. On the other hand, the subsequent aggregation and precipitation process is considerably slowed down by the additive and smaller aggregates result. The crystalline morphology of the sediment was studied in situ by WAXS ca. 50 min after mixing the reactants. Several diffraction rings could be detected, which indicate that a transformation of the metastable, amorphous precursor particles to randomly oriented vaterite nanocrystallites has taken place. In addition, a few isolated Bragg spots of high intensity were detected, which are attributed to individual, oriented calcite microcrystals that nucleated at the wall of the capillary.
Phase contrast x-ray imaging is a powerful technique for the detection of low-contrast details in weakly absorbing objects. This method is of possible relevance in the field of diagnostic radiology. ...In fact, imaging low-contrast details within soft tissue does not give satisfactory results in conventional x-ray absorption radiology, mammography being a typical example. Nevertheless, up to now all applications of the phase contrast technique, carried out on thin samples, have required radiation doses substantially higher than those delivered in conventional radiological examinations. To demonstrate the applicability of the method to mammography we produced phase contrast images of objects a few centimetres thick while delivering radiation doses lower than or comparable to doses needed in standard mammographic examinations (typically approximately 1 mGy mean glandular dose (MGD)). We show images of a custom mammographic phantom and of two specimens of human breast tissue obtained at the SYRMEP bending magnet beamline at Elettra, the Trieste synchrotron radiation facility. The introduction of an intensifier screen enabled us to obtain phase contrast images of these thick samples with radiation doses comparable to those used in mammography. Low absorbing details such as 50 microm thick nylon wires or thin calcium deposits (approximately 50 microm) within breast tissue, invisible with conventional techniques, are detected by means of the proposed method. We also find that the use of a bending magnet radiation source relaxes the previously reported requirements on source size for phase contrast imaging. Finally, the consistency of the results has been checked by theoretical simulations carried out for the purposes of this experiment.
The microstructure and dynamics of a colloidal system interacting via short-ranged interparticle potential is studied by ultra-small-angle x-ray scattering and x-ray photon correlation spectroscopy. ...A colloidal gas-liquid type transition is induced when the short-ranged attractive interactions attain sufficient magnitude. The development of liquidlike structure is preceded by a systematic transition in the particle dynamics from diffusive to constrained motion and then completely frozen behavior. This demonstrates the existence of a jamming transition induced by strong short-ranged attractive interactions even at low packing fractions.
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