"Cap-Pair" Effect as the Reason of the Catalytic Properties of Nicotinic Acid: Experimental and Theoretical Studies.

The influence of nicotinic acid (NC) on the kinetics and the mechanism of electroreduction of Zn2+ ions in the acetate buffer (pH=6.0) was investigated using electrochemical methods (EIS, CV, SWV and DC). It was shown that the anions of NC catalyze the electrode reaction (cap-pair effect) by adsorbing on the surface of the mercury electrode. The catalytic activity of NC is due to its ability to form active NC-Zn2+ complexes on the electrode surface, facilitating the electron transfer process. However, no evidence of the formation of such complexes in the solution was found using classical molecular dynamics. Moreover, it was proved that the electroreduction of Zn2+ ions in the presence of NC is a two-stage process. The first stage involves the transfer of the first electron, preceded by the partial loss of the hydration shell by the Zn2+ ions and formation of the active complex. Moreover, it was shown that in the range of lower concentrations, c≤1.10-2  mol.dm-3 , the nicotinic acid shows weaker catalytic abilities than another form of vitamin B3 - nicotinamide. In the range of its higher concentrations, the nicotinic acid is a more effective catalyst for the electroreduction of Zn2+ ions.

Molecular Dynamics Simulations of Interactions between Human Telomeric i-Motif Deoxyribonucleic Acid and Functionalized Graphene.

The work deals with molecular dynamics (MD) simulations of protonated, human telomeric i-motif deoxyribonucleic acid (DNA) with functionalized graphene. We studied three different graphene sheets: unmodified graphene with hydrogen atoms attached to their edges and two functionalized ones. The functionalization of graphene edge consists in attaching partially protonated or dissociated amine and carboxyl groups. We found that in all cases the protonated i-motif adsorbs strongly on the graphene surface. The biased MD simulations showed that the work necessary to drag the i-motif out from amine-doped graphene is about twice larger than that in other cases. In general, the system i-motif/amine-doped graphene stands out from the rest, e.g., in this case, the i-motif adsorbs its side with 3' and 5' ends oriented in the opposite to surface direction. In other cases, the DNA fragment is adsorbed to graphene by 3' and 5' ends. In all cases, the adsorption on graphene influences the i-motif internal structure by changing the distances between i-motif strands as well as stretching or shortening the DNA chain, but only in the case of amine-doped graphene the adsorption affects internal H-bonds formed between nucleotides inside the i-motif structure.

The inhibition effect of water on the purification of natural gas with nanoporous graphene membranes.

Molecular dynamics simulations are used to investigate the inhibiting effect of water on the natural gas separation with nanoporous graphene. The membrane separation process involves CH4 + N2 mixtures with and without the addition of water. The results show that water is able to form hydrogen bonds with nitrogen atoms located in a nanopore rim. This effect causes a decrease of separation selectivity as well as a reduction of gas permeation. In the extreme case, when the nanopore rim contains only nitrogen atoms, water agglomerates at the center of the nanopore and effectively closes down the permeation path. The conclusions are confirmed by the analysis of stability and kinetics of hydrogen bonds.

The Octyltrimethylamonium Bromide Adsorption at the Mercury Electrode/NaClO4 Solution Interface.

The behaviour of octyltrimethylammonium bromide electrosorption on the mercury electrode in 1 mol/L NaClO4 was determined by means of double layer differential capacity measurements. The adsorption constants were derived from surface pressure data as a function of electrode charge density and cationic surfactant bulk concentration. Adsorption of octyltrimethylammonium bromide was analyzed using constants obtained from Frumkin, corrected Flory-Huggins and virial isotherms. It was found that the repulsive interactions for electrode charges close to 0 were the weakest between the adsorbed cations C11H26N+. In these conditions the surface concentration of the studied surfactant was the greatest.

Transitional hydrogen bonds in aqueous perchlorate solution.

Different mechanisms of H-bond formation between perchlorate anions and water are presented using the molecular dynamics simulations. The detailed methods in searching for multi-centered hydrogen bonds are proposed. The time evolution of H-bond geometric parameters for classical, bifurcated and trifurcated hydrogen bonds in the aqueous perchlorate solution indicates the transitional character of hydrogen bridges as well as the rigid nature of the solvation structure formed by the ion and its first solvation shell. This is supported by the values of free energy binding of water to perchlorate ions determined for particular types of hydrogen bridges.

Adsorption of anionic surfactant at the electrode-NaClO4 solution interface.

ABSTRACT: Adsorption of 1-decanesulfonic acid at the electrode-NaClO4 solution interface was determined by double-layer differential capacity measurements. At potentials less than -1,200 mV, the adsorption of the anionic surfactant on the electrode does not occur. Low concentrations of the anionic surfactant (below cmc) causes slight changes in the zero charge potential, Ez, and the surface tension at this potential, γz. The adsorption of the anionic surfactant was analyzed using the constants obtained from the following isotherms: Frumkin, corrected Flory-Huggins, and virial.

The Adsorption of Tetramethylthiourea at the Interface Electrode - NaClO4 in the Presence of Sodium 1-decanesulfonate.

Mixed adsorption layers properties made by two organic substances in which 1-decanesulfonic acid sodium salt undergoes physical adsorption and tetramethylthoiurea (TMTU) chemical adsorption in 1M NaClO4 on the mercury electrode are described. Based on differential capacity data - potential curves obtained at mercury electrode the double layers TMTU adsorption parameters were calculated whereas the adsorption constants were derived from pressure data as a function of electrode charge density and TMTU's bulk concentration. The interaction parameters point to weak repulsive interaction between the adsorbed TMTU molecules. The free adsorption energy was clearly smaller than in the detergent's absence. The potential drop rectilinear segments across the inner layer dependence on the relative surface excess were analyzed in order to calculate the electrostatic parameters of the inner layer. Zn2+ ions reduction in kinetic studies provided information concerning adsorption equilibrium at potentials distant from adsorption maximum.

Mixed adsorption layers of 0.1 M tert-butanol-tetramethylthiourea at the interface of Hg/aqueous perchlorate solutions.

The electrosorption behavior of tetramethylthiourea (TMTU) on a mercury electrode from 1.0, 0.5, and 0.1 M NaClO(4) solutions containing 0.1 M tert-butanol (TB) is described by means of adsorption isotherm constants. The adsorption parameters for the double layer were calculated from a double-layer differential capacity measurement extrapolated to zero frequency. The values of the relative surface excess increase along with the concentration of NaClO(4). It seems that the adsorption of TMTU is determined by an interaction between adsorbed molecules. The presence of TB in the adsorption layer distinctly influences the parameters of the inner layer.