N. N. Vlasova

Adsorption of the Herbicide 4-Chloro-2-methylphenoxyacetic Acid (MCPA) by Goethite

Interaction between the goethite surface and 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide was studied using density functional theory (DFT) calculations combined with molecular dynamics (MD). The important step made here lies in the use of a periodic DFT method enabling the study of a mineral surface of different protonation states, in strong contrast with previous molecular modeling studies limited to single protonation state corresponding to the point of zero charge. Different surface OH groups and MCPA proton states were used to mimic the strong effects of pH on the outer- and inner-sphere surface complexes that are theoretically possible, together with their binding energies, and their bond lengths. Modeling both a solvated and a protonated (110) goethite surface provided a major breakthrough in the acidic adsorption regime. An outer-sphere complex and a monodentate inner-sphere complex with the neutral MCPA molecule were found to be the most energetically stable adsorbate forms. MD modeling predicted that the latter forms via the sharing of the carbonyl oxygen between the MCPA carboxylate group and a singly coordinated surface hydroxyl group, releasing an H2O molecule. All the other complexes, including the bidentate inner-sphere complex, had higher relative energies and were therefore less likely. The two most likely DFT-optimized structures were used to constrain a surface complexation model applying the charge distribution multisite complexation (CD-MUSIC) approach. The adsorption constants for the complexes were successfully fitted to experimental batch equilibrium data.

Interaction of proteins and substituted aromatic drugs with highly disperse oxides in aqueous suspension

Abstract Adsorption of bovine (BSA) and human serum albumin (HSA), egg albumin, gelatin and some substituted aromatic drug from aqueous solution onto the surface of fumed silica, alumina, titania, silica/titania, and silica/alumina was studied at different concentrations of the components. A maximal adsorption of albumin or drugs (in mg per m 2 of the oxide surface area) was observed for titania or titania/silica having the smallest specific surface area ( S ). A maximal adsorption of drugs in per cent of the initial concentration in solution was found for silica possessing the highest S value among all the studied oxides. Adsorption of Verapamil hydrochloride and HSA from a mixed aqueous solution onto the silica surface showed a weak influence of the protein on the drug adsorption. At a small concentration of oxides in the aqueous suspension, BSA adsorption (in mg per g of oxide) increased exponentially with decreasing oxide concentration accompanied by decreasing the efficiency of albumin adsorption (in per cent of the initial concentration) from solution. Theoretical calculations of solvation effects for the drugs showed a dramatic increase in the solvation energy upon proton attachment to N atoms that could cause a relatively small adsorption of drug cations (as their hydrochlorides were used) onto the hydrophilic surface of oxides.

Adsorption of components of nucleic acids on a titanium dioxide surface

The adsorption of heterocyclic nucleic acid bases and nucleosides at titanium dioxide-aqueous electrolyte solution interface has been studied. Equilibrium constants have been calculated for the acid-base reactions of the surface hydroxyl groups of titanium dioxide and the formation of surface complexes with biomolecules.

Adsorption of organic acids on a cerium dioxide surface

Adsorption of organic acids from aqueous solutions on a cerium dioxide surface has been quantitatively studied in terms of the theory of complexation at interfaces. Potentiometric titration data have been used to calculate the equilibrium constants of acid-base reactions of surface hydroxyl groups of cerium dioxide. The study of adsorption of organic acids on a cerium dioxide surface from aqueous solutions as depending on pH has allowed us to calculate surface complexation constants complexation constants.

Adsorption of biogenic amines on albumin-modified silica surface

The adsorption of biogenic amines, tryptamine and tyramine, from aqueous solutions on the surface of finely dispersed silica modified by the adsorption of bovine serum albumin is studied depending on pH and adsorbate concentration. Silica surface modification with albumin is shown to widen the pH range of amine adsorption from aqueous solutions.

Spectroscopic study of biogenic amine complexes formed at fumed silica surface.

The structure and stability of biogenic amine complexes formed at the fumed silica surface were studied by UV-, IR-spectroscopy and TPD MS techniques. It was found that surface complexes are formed due to electrostatic interactions between amine cations and ionized silanol groups. The mechanism of thermal transformations of surface complexes is proposed. Kinetic parameters of thermal reactions at the fumed silica surface have been calculated.

Adsorption complexes of acridine diaminoderivatives on silica surface

The adsorption of acridine diaminoderivatives on the surface of finely dispersed silica from aqueous solutions is studied in relation to pH, ionic strength, and adsorbate concentration. The stability constants of the surface complexes of acridine derivatives with unionized and dissociated silanol groups of silica are calculated within the framework of the electrostatic model of surface complexation (the basic Stern model). Dimerization of all examined compounds in aqueous solutions is taken into account. It is established that, in the case of acridine dyes (orange and yellow), complexes containing both monomeric and dimeric forms of these compounds are formed on silica surface. The selection in favor of one of the possible sets of the constants, which quantitatively characterize the equilibria of the reactions, results from the analysis of the adsorption isotherms of examined diaminoacridines.

Adsorption of 2,2'-dipyridyl onto silica modified by Cu2+ ions

Abstract The dependences of dipyridyl adsorption from solutions onto highly dispersed silica modified by Cu 2+ ions on pH and dipyridyl concentration were studied. The data obtained were used for the determination of the compositions and stabilities of the ternary surface complexes which are formed in this system. Two methods were used for the calculations: one is based on material balance equations and the other is analogous to Bjerrums method. The equilibrium constant of the reaction of the first dipyridyl molecule binding with Cu 2+ ions on the surface ( K 1 ) is greater than that of the second dipyridyl molecule ( K 2 ), as is usually observed for complex formation in solutions. The difference between K 1 and K 2 depends on the concentration of Cu 2+ ions on the surface.

Adsorption of Pyrimidine Nucleotides on a Titanium Dioxide Surface

The adsorption of pyrimidine mononucleotides from aqueous solutions on the surface of nanocrystalline titanium dioxide is studied. The interaction of the nucleotides with hydroxyl groups of titanium dioxide is interpreted in terms of the surface complexation theory. The results of an experimental study of the dependence of adsorption on the pH and ionic strength of solutions are used to calculate the stability constants of the outer-sphere adsorption complexes, which result from the electrostatic interaction of protonated groups with anionic forms of nucleotides.

Adsorption of amino acids on a cerium dioxide surface

Adsorption of glutamic, aspartic, and pteroylglutamic (folic) acids from aqueous solutions on the surface of nanocrystalline cerium dioxide has been studied as depending on the pH and ionic strength of the solutions. Stability constants have been calculated for surface complexes that result from the interaction of anionic forms of the amino acids with protonated surface groups of cerium dioxide. The structure of the surface complexes has been confirmed by IR spectroscopy.