Interactions and Reactivity at Liquid-Liquid Interfaces

The interface between two immiscible electrolyte solutions (ITIES) provides a well-defined platform for fundamental studies of adsorption processes and molecular reactivity in general. Two bio-inspired systems are considered in the present work: adsorbed phospholipid monolayers at the classical water-1,2-dichloroethane interface and photosensitized reduction of carbon dioxide at the novel water-supercritical CO2 interface. In first instance, theoretical modeling of adsorption processes at an ideally polarizable ITIES was carried out. In this part of the work, extension of previous models was performed in order to account for the ionic partition of the surface-active molecule. For the sake of simplicity, only a potential-dependent Langmuir isotherm was considered. A general model was then obtained in which prediction of triangular-shaped adsorptive peaks in voltammetric measurements arises naturally. Additionally, the interplay between the Gibbs energies of adsorption and ion-transfer was shown to be especially important for intermediate values between -20 to -40 kJ·mol-1. Adsorbed phospholipid monolayers at the ITIES were employed as a model for mimicking half of a cell membrane. Interactions between peptides and L-α-dipalmitoylphosphatidylcholine (DPPC), the most abundant phospholipid in cell membranes, were proven to occur by voltammetry. Further confirmation of the existence of non-covalent complexes was obtained by the complementary use of biphasic electrospray ionization mass spectrometry (BESI-MS); a technique recently developed in the Laboratoire d'Electrochimie Physique et Analytique (LEPA). Cationization of neutral peptides was equally observed to induce the complex formation with DPPC observed as a net current across the ITIES. Antimicrobial peptides were also considered given their high affinity to cell membranes. Therefore, melittin, selected as model antimicrobial peptide, exhibited a well-defined ion-transfer process accompanied by adsorptive signals. Contact angle estimation of supported interfaces over a platinum electrode indicated that these signals are reversible and induce deformation of deposited aqueous droplets. Earlier desorption of phospholipids was equally observed in presence of the antimicrobial peptide, confirming its membrane-disrupting ability by cyclic voltammetry. Finally, reduction of carbon dioxide was studied at the water-supercritical interface, in an attempt to mimic natural photosynthetic systems. As a first approach, the reduction of CO2 to formate was obtained in presence of decamethylferrocene, an organometallic compound which is capable of reducing protons under anaerobic conditions in 1,2-dichloroethane. However, the reaction follows a different pathway in presence of supercritical CO2 and formate is detected as the most abundant product. Taking one step further the application of water-supercritical CO2 interfaces, the photosensitized reduction of CO2 was conducted. In this approach, nickel(II)-1,4,8,11-tetraazacyclotetradecane (Ni(II)cyclam) was employed as catalyst, ruthenium(II)tris(2,2'-bipyridyl) ([Ru(bpy)3]2+) as photosensitizer and sodium ascorbate as sacrificial electron donor. All the components of the catalytic cycle are dissolved in water. Hence, the presence of supercritical CO2 provides with adsorption sites in which intermediates are hypothesized to be adsorbed, leading to increased efficiency and selectivity simultaneously. Adsorption of the catalyst was observed at the air-water interface and further corroborated by pendant drop shape analysis at the water-supercritical CO2 interface. In what is considered as a very promising approach, modification of the supercritical phase with co-solvents opens the route for the study of many different systems in presence of large amounts of CO2 with the long-term goal of conceiving an efficient artificial leave.