Electrochemical Deposition of Polyelectrolytes Is Maximum at the Potential of Zero Charge

Relative Strength of Polycation Adsorption on Oxide Surfaces

Polyelectrolyte adsorption onto surfaces is widely employed in water treatment and mining. However, little is known of the relative interaction strengths between surfaces and polymer. This fundamental property is assumed to be dominated by electrostatics, i.e., attractive interactions between opposite charges, which are set by the overall ionic strength ("salt concentration") of the solution, and charge densities of the surface and the polymer. A common, counterintuitive finding is a range of salt concentrations over which the amount of adsorbed polyelectrolyte increases as electrostatic interactions are tempered by the addition of salt. After an adsorption maximum, higher salt concentrations then produce the expected gradual desorption of polyelectrolyte. In this work, the salt response of the adsorption of the same narrow molecular weight distribution polycation, poly(N-methyl-4-vinylpyridinium), PM4VP, to a variety of surfaces was explored. Oxide powders for adsorption included Al2O3, SiO2, Fe2O3, Fe3O4, TiO2, ZnO, and CuO. Planar surfaces included silicon wafers, mica, calcium carbonate, and CaF2 single crystals. The PM4VP was radiolabeled with 14C so that sensitive, submonolayer amounts could be detected. The position of the peak maximum, or the lack of a peak, in response to added salt was used to rank the electrostatic component of the interaction. The importance of charge regulation, a shift in the surface pKa in response to solution species, was highlighted as a mechanism for adsorption on the "wrong" side of the isoelectric point and also as a factor contributing to the difficulty of reaching the totally desorbed state even at the highest salt concentrations.

Thin Protective Coatings on Metals Formed by Organic Corrosion Inhibitors in Neutral Media

Protection of metals in neutral media with pH 5.0–9.0 (in humid atmospheres and various aqueous solutions) can be achieved by formation of thin coatings (up to several tens of nm) on their surfaces due to adsorption and more complex chemical interactions of organic corrosion inhibitors (OCIs) with the metal to be protected. The review contains three sections. The first section deals with coatings formed in aqueous solutions, while the second one, with those formed in organic and water-organic solvents. Here we consider metal protection by coatings mainly formed by the best-known classes of OCI (carboxylates, organophosphates and phosphonates) and estimation of its efficiency. The third section discusses the peculiarities of protection of metals in the vapor-gas phase, i.e., by volatile OCIs, and a relatively new type of metal protection against atmospheric corrosion by the so-called chamber inhibitors. OCIs with relatively low volatility under normal conditions can be used as chamber OCIs. To obtain a protective coating on the surfaces of metal items, they are placed in a chamber inside which an increased concentration of vapors of a chamber OCI is maintained by increasing the temperature. This review mainly focuses on the protection of iron, steels, copper and zinc.

Electrochemical Beacon Method to Quantify 10 Attomolar Nucleic Acids with a Semilog Dynamic Range of 7 Orders of Magnitude

Change in the dynamics of single-stranded DNA or RNA probes tethered to an Au electrode on immunospecific binding to the analyte is a versatile approach to quantify a variety of molecules, such as heavy metal ions, pesticides, proteins, and nucleic acids (NAs). A widely studied approach is the electrochemical beacon method where the redox of a dye attached to the probe decreases as its proximity to the underlying electrode changes on binding. The limit of quantification (LOQ) defined by the semilog dependence of the signal on target concentration is in the picomolar range. Here, a method was studied where, by differential reflectivity, multiple reactions were measured on a monolith electrode. An alternative contrast mechanism was discovered, which led to an approach to enhance the LOQ to 10 aM and increase the dynamic range to 7 orders of magnitude using similar probes and binding conditions. Quantitative analysis on sequences with the G-C fraction ranging from 37 to 72% was performed. The approach will allow for the development of a label-free, enzyme-free microarray to detect biomolecules including NAs and proteins on a single electrode at quantification from 10 aM to 0.1 nM with high specificity.