1,750,859 research outputs found
An ellipsometric study of protein adsorption at the saliva-air interface
At the liquid-air interface of human saliva a protein layer is adsorbed. From ellipsometric measurements it was found that the thickness of the surface layer ranged from 400 to 3600 Å and the amount of protein material adsorbed was 9–340 mg/m2. Based on the concentration of protein in the layer the samples could be classified into two groups: a low concentration (ca. 0.15 g/ml) and a high concentration (0.7–1.1 g/ml). In the low concentration group the surface layers appeared to be thin (500–600 Å) while those in the high concentration group appeared to be much thicker (1000–3500 Å). A correlation between the bulk pH and the thickness of the surface layer could be established
Highly Enhanced Concentration and Stability of Reactive Ce^3+ on Doped CeO_2 Surface Revealed In Operando
Trivalent cerium ions in CeO_2 are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce^3+ species on the surface and in the bulk of Sm-doped CeO_2(100) in hundreds of millitorr of H2–H2O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce^3+ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface’s 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce^3+ over wide ranges of temperature and oxygen partial pressure may be responsible for the high activity of doped ceria in many pollution-control and energy-conversion reactions, under conditions at which Ce^3+ is not abundant in the bulk
Concentration polarization, surface currents, and bulk advection in a microchannel
We present a comprehensive analysis of salt transport and overlimiting
currents in a microchannel during concentration polarization. We have carried
out full numerical simulations of the coupled Poisson-Nernst-Planck-Stokes
problem governing the transport and rationalized the behaviour of the system. A
remarkable outcome of the investigations is the discovery of strong couplings
between bulk advection and the surface current; without a surface current, bulk
advection is strongly suppressed. The numerical simulations are supplemented by
analytical models valid in the long channel limit as well as in the limit of
negligible surface charge. By including the effects of diffusion and advection
in the diffuse part of the electric double layers, we extend a recently
published analytical model of overlimiting current due to surface conduction.Comment: 15 pages, 11 figures, Revtex 4.
Compact Layer of Alkali Ions at the Surface of Colloidal Silica
The forces of electrical imaging strongly polarize the surface of colloidal
silica. I used X-ray scattering to study the adsorbed 2-nm-thick compact layer
of alkali ions at the surface of concentrated solutions of 5-nm, 7-nm, and
22-nm particles, stabilized either by NaOH or a mixture of NaOH and CsOH, with
the total bulk concentration of alkali ions ranging from 0.1- to 0.7-mol/L. The
observed structure of the compact layer is almost independent of the size of
the particles and concentration of alkali base in the sol; it can be described
by a two-layer model, i.e., an ~ 8 Angstrom thick layer of directly adsorbed
hydrated alkali ions with a surface concentration 3x10(18) m(-2), and a ~ 13
Angstrom thick layer with a surface concentration of sodium ions 8x10(18)
m(-2). In cesium-enriched sols, Cs+ ions preferentially adsorb in the first
layer replacing Na+; their density in the second layer does not depend on the
presence of cesium in the sol. The difference in the adsorption of Cs+ and Na+
ions can be explained by the ion-size-dependent term in the electrostatic Gibbs
energy equation derived earlier by others. I also discuss the surface charge
density and the value of surface tension at the sol's surface.Comment: 32 pages 10 figure
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