Emilio Roldán

Programmable microcomputer-controlled ramp generator for use in electrochemical experiments

Abstract In this article a microcomputer-controlled circuit for generation of linear analog waveforms for use in electrochemical experiments is proposed. The circuit permits the generation of complex waveforms, uses an inexpensive 8-bit D/A converter, and is adaptable to microcomputers with parallel I/O interface.

Kinetic Study of the Cetyltrimethylammonium/DNA Interaction

A kinetic study of the interaction of the surfactant cetyltrimethylammonium (CTA(+)) with DNA was carried out in water and in salt (NaCl) solutions. The results can be explained in terms of a reaction mechanism involving two consecutive reversible steps. The first step corresponds to the union/separation of the surfactant with/from the DNA. The second step corresponds to a conformational change of the surfactant/DNA complex. The equilibrium constant, calculated from the forward and reverse rate constants of these steps, agrees with the results of a previous thermodynamic study.

Kinetic study of the reaction *[Ru(bpy)3]2++S2O82− in solutions of Brij-35 at premicellar and micellar concentrations

The title reaction was studied in solutions of polyoxyethylene(23) lauryl ether (Brij-35) at premicellar and micellar concentrations. The reaction rate goes through a maximum, which appears at a concentration close to the critical micellar concentration (cmc). The results can be rationalized by an extension of previous models on premicellar and micellar effects on the kinetics of reactions. This model can explain similar results from other groups.

DNA conformational changes induced by cationic gemini surfactants: the key to switching DNA compact structures into elongated forms

The DNA conformational changes induced by different members of the N,N′-bis(dimethyldodecyl)-α-ω-alkanediammonium dibromide series (m-s-m, m = 12, s = 3 and 6) and the analogous series of hexadecyl gemini surfactants (m = 16, s = 3 and 6) were investigated in aqueous media by means of circular dichroism (CD), zeta potential, dynamic light scattering (DLS), viscometry, and atomic force microscopy (AFM) methods. The measurements were carried out by varying the gemini surfactant–DNA molar ratio, R = Cm-s-m/CDNA. For the conditions investigated two significantly different conformational changes were observed, the second of them being worth noting. At low molar ratios, all methods concurred by showing that gemini surfactants were able to form ordered aggregates which precedes DNA compaction. The second effect observed, at high molar ratios, corresponds to the transition from the compact state to a new more extended conformation. The degree of decompaction and the morphologies of the visualized structures are different not only depending on the surfactant tails length, but also on the spacers length. The results obtained for the 16-3-16/DNA and 16-6-16/DNA systems point out that the compaction/decompaction processes are somewhat different to those previously visualized for the analogous monoquaternary chain surfactant CTAB.

Improving the understanding of DNA–propanediyl-1,3-bis(dodecyldimethylammonium) dibromide interaction using thermodynamic, structural and kinetic approaches

A kinetic, thermodynamic and structural study of the interaction of the gemini surfactant propanediyl-1,3-bis(dimethyldodecylammonium dibromide) (12-3-12.2Br) with calf thymus DNA was carried out at several ionic strengths (NaCl) in aqueous solutions. A new 12-3-12(2+)-selective membrane was prepared in order to gain insight into the factors that control the binding of 12-3-12.2Br to DNA. We used ethidium bromide (EB) as a fluorescence probe to follow the kinetics of the interaction by using the stopped-flow fluorescence technique. The results can be explained in terms of a reaction mechanism involving two consecutive reversible (fast and slow) steps. The fast step was attributed to the union/separation of the surfactant with/from the DNA polynucleotide. Changes in the kinetic constants in the forward and backward directions were discussed in terms of the Brönsted-Pitzer equation and of the increase in hydrophobic interactions of the surfactant tails as a consequence of salting-out effects, respectively. The slow step corresponds to a conformational change of the surfactant-DNA complex to a more compacted form. The equilibrium constant, calculated from the forward and reverse rate constants of these steps, agrees with the results obtained from potentiometric titration using a 12-3-12-(2+) selective electrode.

Reorientation of Thiols during 2D Self-Assembly: Interplay between Steric and Energetic Factors

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.

On the simultaneous evaluation of charge transfer kinetics and adsorption: reduction of parabanic acid in low acidity xM HCl + (2 − x)M LiCl mixtures

Abstract Parabanic acid reduction has been studied in low acidity xM HCl + (2 − x)M LiCl mixtures by the faradaic impedance method. The overall reaction obeys an electrochemical scheme with a quasi-reversible electron transfer, a coupled homogeneous following reaction and weakly adsorbed reactant and products; the reaction is further complicated by the presence of a polarographic maximum. Analysis of the electrode admittance over an extended potential range (about 1 V) allowed the determination of the electron transfer and adsorption parameters.

Electrochemical reduction of tricarbonyl compounds: Reduction mechanism of alloxan on a mercury electrode

Abstract A study has been carried out on the reduction mechanism of alloxan over a DME corresponding to the first two-electron transfer over the pH range 0–12. Polarographic and voltammetric results show the reduction process to be kinetically controlled. Rate and equilibrium constant data for the prior chemical reaction have been evaluated and several reaction mechanisms for different pH zones have been proposed.

Keto-enol tautomerism in the electrochemical reduction of parabanic acid

Abstract Convolution potential sweep voltammetry has been applied to the electrochemical reduction of parabanic acid in the 10–100 V/s scan rate range. The overall reduction corresponds to a quasireversible two-electron, two-proton transfer to give an enediol intermediate compound. This compound undergoes an enol-keto transformation in a following homogeneous chemical reaction. This chemical reaction appears to be acid-base catalyzed in the 2 M-4×10 −3 M H + concentration range. The catalytic and electrochemical constants were determined.

Influence of solution acidity on the adsorption and charge transfer kinetics of parabanic acid reduction

Abstract Parabanic acid reduction has been studied in the − 1.4 ⩽ H 0 ⩽ 0.9 acidity range by the ac impedance technique. Changes in the adsorption of reactant and products with solution acidity lead to protonation equilibrium constants that differ from their bulk values. The observed p K a shifts were consistent with interfacial dielectric saturation. Variation of the charge transfer parameters with potential and proton activity allows a detailed analysis of the reduction mechanism to be performed in terms of the scheme of squares.