Microemulsion samples of a polyoxyethylene trisiloxane surfactant, water, and 1-decanol are investigated using pulsed field gradient NMR and small-angle neutron scattering (SANS) to determine the solution structure. The surfactant/decanol weight ratio has been kept constant at values of 10:1, 8:1, and 6:1 under variation of water content. The temperature was 32 degrees C for the measurement series at the weight ratio of 10:1 to avoid phase separation at high water content. Also, aqueous surfactant solution samples have been investigated as a function of composition and temperature. Water-rich samples consist of micelles that are close to spherical at very low surfactant concentration and grow into anisometric, that is, oblate formed aggregates, at higher surfactant (or surfactant and decanol) concentration. The aggregates grow with increasing temperature, most probably due to dehydration of the hydrophilic groups. In a concentration range around 50 wt % water, the systems form bicontinuous structures. SANS data are used to estimate surfactant film properties using a model developed for interpretation of neutron scattering data from related systems.
Systems of platelet-shaped nanostacks of the synthetic clay Na-fluorohectorite, suspended in saline solutions of various salt concentrations, exhibit a rich phase behavior with up to four phases coexisting in a single sample tube. They are studied here using small-angle x-ray scattering: the anisotropy of the obtained images is quantified, and, together with x-ray absorption measurements, this provides a precise determination of the phase boundaries, as well as a measure of the orientational ordering of the clay colloids in the various gel phases. The coexistence of different phases results from a sedimentation-induced vertical gradient in particle fraction. Quantitative relation of the vertical coordinate to the clay particle fraction in these samples allows determination of a phase diagram for these Na-fluorohectorite systems, as a function of the particle fraction and salt concentration.
A series of polystyrene–clay nanocomposites, based on two natural clay types (Na–Montmorillonite and Hectorite) and two synthetic clays (Laponite and Li–Fluorohectorite), were prepared via in situ intercalative polymerization after surface modification with an organic ammonium cation (CTAB). The structural characteristics of the organically modified clays as well as the nanocomposites were investigated by means of wide-angle X-ray scattering (WAXS), and the thermal properties were studied with TGA. In the organically modified clays, the silicate interlayer spacing increases, and the magnitude seems to be directly correlated with the amount of clay surface charge. In the nanocomposites, polymer intercalation is also observed, but partial exfoliation is present, modifying significantly the morphology of the material. The degree of dispersion of the clay platelets, as well as the resulting properties of the nanocomposites, were found again to be systematically, and almost linearly, correlated with the intrinsic surface charge of the clays, which varied between 44 and 120 meq/100 g. Increased dispersion was seen in the nanocomposites made from clays with low surface charge, here Hectorite and Laponite, suggesting that these can be suitable alternatives to the more employed Montmorillonite for enhancement of thermal properties. The thermal stability was found to be better for the nanocomposites than for the pure polystyrene.
Transition metals added to sodium aluminum hydride by high-energy ball milling have been shown to significantly enhance its absorption and desorption properties. In the present study, we have used small-angle X-ray scattering to elucidate how TiCl3 affects the nanostructure of NaAlH4 particles. Scattering data from as-purchased and ball milled NaAlH4 for 6 and 15 min are compared with NaAlH4 ball milled for the same time with 4 mol % TiCl3. Drastic differences were noticed in the two systems which cast a new light on the decomposition of NaAlH4, in particular on the effect of ball milling and of TiCl3 on the morphology, grain size, and distribution of the phases. The particle morphology of pure NaAlH4 showed significant evolution/changes during heating from room temperature to 290 °C, as evidenced by the variations in the power-low scattering parameter, α. Drastic changes were noticed in the particle surface structure during the phase transformation from NaAlH4 to Na3AlH6 + Al, when the system becomes less compact and the particle surface rougher. The addition of TiCl3 induces a different effect on both surface and mass structure, at least in the nanometer length scale considered in this study: the particles retain their surface morphology at all temperatures. Furthermore, even after short ball milling times the addition of TiCl3 increases the system compactness with reduction of internal voids.
Turbidity, structure, and rheological features during gelation via the Ugi multicomponent condensation reaction of semidilute solutions of alginate have been investigated at different polymer and cross-linker concentrations and reaction temperatures. The gelation time of the system decreased with increasing polymer and cross-linker concentrations, and a temperature rise resulted in a faster gelation. At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional, variant omega(n)(')) and loss modulus (G' ' proportional, variant omega(n)(' ')) was observed for all gelling systems with n' = n' ' = n. By varying the cross-linker density at a fixed polymer concentration (2.2 wt %), the power law exponent is consistent with that predicted (0.7) from the percolation model. The value of n decreases with increasing polymer concentration, whereas higher temperatures give rise to higher values of n. The elastic properties of the gels continue to grow over a long time in the postgel region, and at later stages in the gelation process, a solidlike response is observed. The turbidity of the gelling system increases as the gel evolves, and this effect is more pronounced at higher cross-linker concentration. The small-angle neutron scattering results reveal large-scale inhomogeneities of the gels, and this effect is enhanced as the cross-linker density increases. The structural, turbidity, and rheological features were found to change over an extended time after the formation of the incipient gel. It was demonstrated that temperature, polymer, and cross-linker concentrations could be utilized to tune the physical properties of the Ugi gels such as structure, transparency, and viscoelasticity.
This combined study using small-angle neutron scattering (SANS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and adsorption isotherm techniques demonstrates radical changes in the microstructure of porous hematite (α-Fe 2 O 3 ) nanoparticles upon calcination in air. TEM images of the as-synthesized hematite sample show that it consists of subrounded nanoparticles [50 (8)–61 (11) nm in average minimum and maximum diameters] with an apparent porous structure of nanosized pores/channels or cracks. SANS data confirm the presence of two characteristic sizes, one originating from the particle size and the other from the pore/void structure. Furthermore, the TEM images show that the particle sizes are nearly unaffected by calcination at 623 K, whereas their pore/void structure changes radically to an apparently pitted or spongy microstructure with cavities or/and voids. The change in microstructure also causes a reduction in the surface area as calculated by gaseous adsorption. The XRPD and SANS data show that the crystallite and SANS particle sizes are virtually unchanged by calcination at 623 K. Calcination at 973 K induces a significant alteration of the sample. The XRPD data reveal that the crystallite size increases significantly, and the SANS and adsorption isotherm studies suggest that the specific surface area decreases by a factor of ∼5–6. The TEM images show that the particles are sintered into larger agglomerates, but they also show that parts of the porous microstructure observed in the sample calcined at 623 K are retained in the sample calcined at 973 K.
