Phase transition processes displayed by coumarin monolayers at the water-mercury interphase

Adsorption and Reduction of Berberine at a Mercury Electrode

“…Tensametric curves are shown in Figure 8. If berberine concentration is smaller than 3610 À6 M neither berberine nor canadine are strongly adsorbed, but signi®cant adsorption occurs for 5610 À6 M. It is marked by the desorption of canadine at À1.46 V. In the concentration range between 10 À5 and 10 À4 M condensed ®lms of berberine and canadine exist between À0.6 V and À1.7 V. A transition from the ®rst to the second berberine surface structure is accompanied by a capacitance peak with the potential of maximum which changes from À0.58 V for 5610 À6 M to À0.43 V for 7610 À5 M. Similar peaks were observed in experiments with coumarin [21], thiourea [22], sodium dodecylsulfate [24], isochroman [28], adenine [29] and adamantane [30]. They were ascribed to the reorientation of the molecules into the vertical position [21, 28±30].…”

“…This phenomenon is well documented in the literature [17±26]. It was observed with many organic molecules, such as quinoline [17], 3-methyl-isoquinoline [18], thymine [19], cytidine [20], coumarin [21], thiourea [22], uracil [23], sodium dodecylsulfate [24], a-and g-cyclodextrins [25] and hypoxanthine [26]. These are mostly electroinactive molecules, while berberine can be reduced at À1.1 V and electrogenerated canadine may form a new condensed ®lm at the electrode surface.…”

Condensation of Nucleobases at Mercury/Aqueous Solution Interface–A Structural Perspective Using Hydrogen Bonding Considerations

“…Second, the magnitude of electrostatic effects are altered due to changes in pH. While the electrostatic forces influence the appearance of transition potential pit-width, the (strength of) adsorbate-adsorbate interactions dictates the critical temperatures (15,16,28,29,40,(56)(57)(58). In the present context, the existence of quadruplex forms is valid for neutral pH ranges (5-7).…”

“…Further, the predominant influence of H bonds in dictating the interfacial condensation at mercury electrode surfaces has been repeatedly emphasized (11, 21-26, 43-46, 54, 55). It should, however, be pointed out that adsorbates such as coumarin, which are not capable of forming H bonds, also undergo condensation (40). In such cases, the presence of other types of structural stabilization needs to be investigated (62).…”

“…4 alone has the least strain and other structural modifications will involve a larger steric hinderance (1) and hence are discarded. range attraction energy term appearing in various adsorption isotherms (36,37) and Ising models of electrosorption (12,15,16,27,30,40,41). On the other hand, the structure depicted in (f) Uracil.…”