We report on the mechanism of growth of mesoporous silica (SBA-15, plane group p6m). In situ studies of the formation using ultrasmall angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) covering length scales from 5 to 10 000 Å, complemented with UV−vis and transmission electron microscopy (TEM), provide unique data on particle growth coupled with information regarding the progression of the mesostructure formation and the micellar evolution.
The self-assembly of the trifluoroacetate salt of the short peptide (ala)6-lys (A6K) in water has been investigated by cryo-transmission electron microscopy and small-angle X-ray scattering. For concentrations below ca. 12%, the peptide does not self-assemble but forms a molecularly dispersed solution. Above this critical concentration, however, A6K self-assembles into several-micrometer-long hollow nanotubes with a monodisperse cross-sectional radius of 26 nm. Because the peptides carry a positive charge, the nanotubes are charge-stabilized. Because of the very large aspect ratio, the tubes form an ordered phase that presumably is nematic.
We have characterized the lipid chain freezing in dilute aqueous vesicle dispersions of the cationic lipid dioctadecyldimethylammonium bromide (DODAB) using wide and small angle X-ray scattering, solid state NMR, DSC, turbidity and density measurements. The lipids freeze in two steps. Above 40 C the chains are fluid and the lipids are in a so-called liquid-crystalline state. When cooling below 40 C, the lipids form a gel phase where the chains stretch, the molecules are more densely packed and most molecular degrees of freedom are frozen, or at least dramatically slowed down. In the gel phase, the chain packing is still disordered, while the chain mobility is significantly reduced. From NMR data we further conclude that also the molecular rotational diffusion around the molecular long axis is quenched. Slow chain reorientation may occur, but then as individual reorientations of the separate chains. When cooling further below 36 C, crystalline ordering of the chains is obtained, resulting in a further increased packing density. We refer to this state as the subgel phase. The transitions are reversible. However, the formation of the ordered subgel is very slow for temperatures near the melting point. In fact, the gel phase can be supercooled by almost 20 C for considerable time. From analyzing this transition in terms of classical nucleation we obtain an estimate of the intra-bilayer interfacial tension between the gel phase and the growing subgel domains of 2 mN m À1 . Materials and methods MaterialsDioctadecyldimethylammonium bromide (DODAB, Fig. 1) with purity greater than 99% was used as purchased from Acros Organics (Belgium). Dioctadecyldimethylammonium chloride (DODAC) with purity greater than 97% was used as purchased from Alfa Aesar (Germany). Deuterium oxide (D 2 O) with purity
A small-angle X-ray scattering (SAXS) set-up has recently been developed at beamline I711 at the MAX II storage ring in Lund (Sweden). An overview of the required modifications is presented here together with a number of application examples. The accessible q range in a SAXS experiment is 0.009-0.3 A(-1) for the standard set-up but depends on the sample-to-detector distance, detector offset, beamstop size and wavelength. The SAXS camera has been designed to have a low background and has three collinear slit sets for collimating the incident beam. The standard beam size is about 0.37 mm x 0.37 mm (full width at half-maximum) at the sample position, with a flux of 4 x 10(10) photons s(-1) and lambda = 1.1 A. The vacuum is of the order of 0.05 mbar in the unbroken beam path from the first slits until the exit window in front of the detector. A large sample chamber with a number of lead-throughs allows different sample environments to be mounted. This station is used for measurements on weakly scattering proteins in solutions and also for colloids, polymers and other nanoscale structures. A special application supported by the beamline is the effort to establish a micro-fluidic sample environment for structural analysis of samples that are only available in limited quantities. Overall, this work demonstrates how a cost-effective SAXS station can be constructed on a multipurpose beamline.
Small-angle neutron scattering (SANS) measurements are reported on a sterically stabilized, coreshell colloidal system using contrast variation. Aqueous dispersions of polystyrene particles bearing grafted poly(ethylene glycol) (PEG) have been studied over a large range of particle concentrations and two different solvent conditions for the PEG polymer. SANS data are analyzed quantitatively by modeling the particles as core-shell colloids. In a good solvent and under particle contrast conditions, an effective hard-sphere interaction captures excluded-volume interactions up to high concentrations. Contrast variation, through isotopic substitution of both the core and solvent, expedite a detailed study of the PEG layer, both in the dilute limit and as a function of the particle concentration. Upon diminishing the solvent quality, subtle changes in the PEG layer translate into attractions among particles of moderate magnitude.
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