Determination of surface potentials from the electrode potentials of a single-crystal electrode

Determination of surface potentials Psi0 at the metal oxide/aqueous solution interface from measured electrode potentials of a metal oxide single-crystal electrode (SCrE) is described. The proposed method is based on the surface complexation model and evaluates the surface potential at the isoelectric point, i.e., at pHiep. This value is used for calculation of Psi0 values from the measured electrode potentials. Both 1-pK and 2-pK models produced the same result so that the procedure does not depend on the assumed mechanism of the surface charging. It is proposed to determine the pristine point of zero charge pHeln and the isoelectric point pHiep, and use these data to set the scale of surface potentials. The value of pHeln can be obtained at a sufficiently low ionic strength where pHpzc coincides with pHiep. The method is demonstrated on the example of the anatase single-crystal electrode. From the shifts of pHiep and pHpzc with respect to the pristine point of zero charge pHeln it was concluded that Cl- ions exhibit higher affinity for association with positively charged surface groups than ClO-4 ions. Also, preferential surface association of Na+ cations compared to both anions was detected. The slopes of the Psi0(pH) functions were found to be significantly lower in magnitude with respect to the Nernst equation, which is due to the high degree of counterion association at the surface caused by their relatively high concentration.     

Influence of the pH on the Formation of the Catanionic n-Tetradecylammonium Cholate Surfactant Salt

The catanionic n-tetradecylammonium cholates, with the 1:1 and 1:2 cationic:anionic surfactant molar ratio, have been synthesized by reaction of tetradecylammonium chloride and sodium cholate. The stoichiometry of the compounds formed is strongly dependent on pH of the aqueous reaction solution. In the neutral and slightly acidic medium the isolated surfactant was detected as 1:1, at acidic medium (pH ≈ 3) as 1:2, while the cholic acid alone precipitated from the strong acidic solution (pH ≈ 2). The catanionic surfactants have been characterized by elemental and spectroscopic (IR, ¹H, and ¹³C NMR) analyses, and microscopic observations. The mole fractions of individual components in precipitate present in the heterogeneous aqueous system have been determined as a function of pH. The data interpretation in the pH regions where two different kinds of precipitates coexist pointed to the complex equilibrium in the examined systems. The evaluation of the relevant equilibrium constants was solved by the nonlinear regression analysis. In the acidic region where cholic acid and 1:2 precipitate coexisted the equilibrium constants of dissolution were K ₁ = 8.4 × 10^?10 and K ₄ = 3.7 × 10^?27, respectively. In the region where 1:2 and 1:1 precipitate coexisted the equilibrium constant of dissolution of 1:1 tetradecylammonium cholate was K ₂ = 4.2 × 10^?20.     

Evaluation of surface potential from single crystal electrode potential

The new method of evaluation of the point of zero potential for the metal oxide exhibiting a saddle-like surface potential function Ψ ₀(pH), as obtained by acid base potentiometric titration using Single Crystal Electrode, was proposed. The electrode potential of sapphire single crystal electrode (A crystal plane, $11\bar{2}0$ ) was measured, point of zero potential and surface potentials were evaluated, and the results were analyzed using the Surface Complexation Model. The electroneutrality point corresponding to the point of zero potential was found to be at pH_pzp=7.0. Thermodynamic protonation equilibrium constants for the first and the second step of protonation were obtained as $\lg K_{1}^{\circ} = 12.7$ ; $\lg K_{2}^{\circ} = 1.2$ .     

Modeling dimerizations of transmembrane proteins using Brownian dynamics simulations

The dimerizations of membrane proteins, Outer Membrane Phospholipase A (OMPLA) and glycophorin A (GPA), have been simulated by an adapted Brownian Dynamics program. To mimic the membrane protein environment, we introduced a hybrid electrostatic potential map of membrane and water for electrostatic interaction calculations. We added a van der Waals potential term to the force field of the current version of the BD program to simulate the short-range interactions of the two monomers. We reduced the BD sampling space from three dimensions to two dimensions to improve the efficiency of BD simulations for membrane proteins. The OMPLA and GPA dimers predicted by our 2D-BD simulation and structural refinement is in good agreement with the experimental structures. The adapted 2D-BD method could be used for prediction of dimerization of other membrane proteins, such as G protein-coupled receptors, to help better understanding of the structures and functions of membrane proteins.     

Influence of interfacial water layer on surface properties of silver halides: effect of pH on the isoelectric point

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2相似文献   

Synthesis of gold nanoparticles by microemulsion assisted photoreduction method

A new, clean, cost-effective and rapid method for the synthesis of stable spherical gold nanoparticles (AuNPs) is developed. This novel technique combines microemulsion as one of soft-nanotechnology techniques of wet chemistry, with photo-physics of UV-radiation in a unique versatile method to design and obtain controlled nanostructures for multifunctional materials. Based on a phase diagram in ternary water/Brij 30/n-heptane system pristine, and thiol functionalized, gold nanoparticles were obtained by a microemulsion assisted photoreduction technique, allowing increased flexibility during the synthesis and selection of materials. The spherical nanoparticles obtained by this route show a homogeneous size distribution, with an average diameter of 11 nm, for pristine gold nanoparticles and of 12 nm, for functionalized species. The evolution of the system at the nanoscale has been studied using, in tandem, UV-VIS and DLS measurements. The structure, size and shape of the final nanoparticles obtained have been evaluated by adequate instrumental techniques: FTIR, XRD and TEM image analysis. Kinetic studies have also been performed in order to follow the evolution of nanospecies during irradiation procedure.     

Syntheses of new conformationally constrained S‐[2‐[(1‐iminoethyl)amino] ethyl]homocysteine derivatives as potential nitric oxide synthase inhibitors

The efficient syntheses of two new types of conformationally constrained S‐[2‐[(1‐iminoethyl)amino]ethyl]homocysteine derivatives, 1‐amino‐3‐[2[(1‐iminoethyl)amino]ethylthio]cyclobutane carboxylic Acid ( 5 ) and (4S)‐4‐[[2‐[(1‐Iminoethyl)amino]ethyl]thio]‐L‐proline ( 6 ), are reported. These molecules represent the first attempts to probe conformational constraint near the α‐amino acid moiety of known homocysteine‐based inhibitors of nitric oxide synthase. Targets 5 and 6 were evaluated as potential inhibitors of the three human isoforms of nitric oxide synthase. © 2002 John Wiley & Sons, Inc. Heteroatom Chem 13:77–83, 2002; DOI 10.1002/hc.1109