Adsorption of Nonionic Surfactants with Ethylene Oxide Headgroup Chemistry at the Titania–Water Interface

Adsorption isotherms and structure of cationic surfactants adsorbed on mineral oxide surfaces prepared by atomic layer deposition

The adsorption isotherms and aggregate structures of adsorbed surfactants on smooth thin-film surfaces of mineral oxides have been studied by optical reflectometry and atomic force microscopy (AFM). Films of the mineral oxides of titania, alumina, hafnia, and zirconia were produced by atomic layer deposition (ALD) with low roughness. We find that the surface strongly influences the admicelle organization on the surface. At high concentrations (2 × cmc) of cetyltrimethylammonium bromide (CTAB), the surfactant aggregates on a titania surface exhibit a flattened admicelle structure with an average repeat distance of 8.0 ± 1.0 nm whereas aggregates on alumina substrates exhibit a larger admicelle with an average separation distance of 10.5 ± 1.0 nm. A wormlike admicelle structure with an average separation distance of 7.0 ± 1.0 nm can be observed on zirconia substrates whereas a bilayered aggregate structure on hafnia substrates was observed. The change in the surface aggregate structure can be related to an increase in the critical packing parameter through a reduction in the effective headgroup area of the surfactant. The templating strength of the surfaces are found to be hafnia > alumina > zirconia > titania. Weakly templating surfaces are expected to have superior biocompatibility.

Mesoporous metallic cells: design of uniformly sized hollow mesoporous Pt-Ru particles with tunable shell thicknesses

A new class of hollow mesoporous Pt-Ru and Pt particles with uniform size, named 'mesoporous metallic cells', are synthesized through a dual-templating approach using colloidal silica particles and non-ionic surfactants. To realize the full potential of mesoporous metals as electrocatalysts, the shell thicknesses, compositions, and hollow cavity sizes are precisely controlled.

X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 5. Halide ion (F-, Cl-, Br-, I-) adsorption

Halide anion (F(-), Cl(-), Br(-), and I(-)) adsorption and its impact on sodium adsorption at the hematite/water interface were studied by cryogenic X-ray photoelectron spectroscopy (XPS). Measurements were carried out on frozen, centrifuged wet hematite pastes that were previously equilibrated in 50 mM electrolytic solutions in the pH 2-11 range. XPS-derived halide ion surface loadings decreased in the order F(-) > I(-) ≈ Cl(-) > Br(-), whereas sodium loadings were in the order Na(F) > Na(I) > Na(Br) > Na(Cl). The greater sodium loadings in NaF and in NaI resulted from larger anion loadings in these systems. Bromide ion had the lowest loading among all halide ions despite having a charge-to-size ratio that is intermediate between those of Cl(-) and I(-). This unexpected result may have arisen from specific properties of the hematite/water interface, such as water structure and electric double layer thickness. Fluoride ion adsorption proceeded via the formation of hydrogen bonds with the surface hydroxo groups (e.g., ≡Fe-OH(2)···F(-) or ≡Fe-OH···F(-)). Surface-bound fluoride ions exert a greater charge-screening effect than the other halide anions, as demonstrated by considerably small zeta potential values. Fe-F bond formation was excluded as a possible interfacial process as the F 1s peak binding energy (684.2 eV) was more comparable to that of NaF (684.6 eV) than FeF(3) (685.4 eV). Overall, these findings motivate further refinements of existing thermodynamic adsorption models for predicting the ionic composition of hematite particle surfaces contacted with sodium halide aqueous solutions.