The formation and reductive desorption of self-assembled monolayers of 6-mercaptohexanol on mercury has been studied by using cathodic stripping voltammetry and capacitative transients, including the possibility of expanding or contracting the electrode area at the end of the preconcentration step. Experimental evidence shows the existence of three sequential stages during the formation of a thiol self-assembled monolayer. Each of these stages can be associated to the presence of (i) a low surface density state of oxidized thiol molecules, characterized by a single electrodimerization wave, (ii) a high surface density state, characterized by the emergence of a second voltammetric wave, and (iii) an ordered monolayer, which gives rise to a voltammetric spike. On the basis of electrode expansion experiments, a method is described to determine the surface concentrations of oxidized products, which does not require a baseline subtraction of the voltammograms to account for the nonfaradaic current. Quantitative voltammetric fits are consistent with the initial formation of a mixture of noninteracting monomers and dimers of oxidized thiol. The value of the maximum surface concentration and the ability to block the Ru(NH 3 ) 6 3+ electron transfer reveal that oxidized thiol molecules adopt a nearly perpendicular orientation in the high surface density state, which hampers ionic permeation. A theoretical model is proposed to account for the observed voltammetric behavior. The transition from the lower to the higher surface density states is modeled as a chemical step involving the exchange of metal free sites. Capacitative transients are also interpreted in terms of the three-stages model.

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.

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.

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The extent of DNA decompaction induced by m-s-m gemini surfactants depend on the surfactant's tail length and on spacer's length.