Estudio de procesos de transferencia de carga en interfases convencionales y micrométricas acopladas con reacciones químicas homogéneas

Resumo

The present Doctoral Thesis is devoted to the theoretical and experimental study of different heterogeneous charge transfer processes by means of electrochemical methods with electrode|solution and liquid|liquid interfaces of conventional and micrometric size. The electrochemical response is defined by the interplay of a number of physiochemical phenomena (interfacial charge transfer, mass transport by diffusion, homogeneous chemical reactions,¿), that must be modelled adequately for the correct interpretation of the electrochemical signal and for the extraction of kinetic, thermodynamic and mechanistic information. The main goal of this Thesis is the deduction of analytical equations that describe the electrochemical response of complex charge transfer processes (both electronic and ionic) in different techniques. Also, numerical methods are employed in the case of systems of high complexity. Special attention is paid to study the effect of the chemical reactivity of the electroactive species and to the use of micrometric interfaces that offer relevant advantages for quantitative analysis. Moreover, the theoretical solutions are implemented in computer programs for the study of the influence of the chief variables of the technique and the system and the experimental study of the latter. With respect to ion transfer reactions, the use of two-polarizable interfaces cells is also investigated under non-conventional conditions (for example, with transfer of the target ion across the two interfaces) that can offer some advantages. The methodology employed includes the mathematical resolution of electrochemical problems by means of analytical mathematical methods (such as the dimensionless parameter method) and the use of finite-difference and finite-element numerical methods. Also, calculation programs are designed based on the theoretical solutions obtained, mainly by using the mathematical software Mathcad and the programming language C++. In the experimental studies, the use of different techniques is considered, including cyclic voltammetry, square wave voltammetry and chronoamperometry. In some cases, complementary instrumental techniques are also employed such as UV-Vis spectroscopy and conductimetry. Considering that coupled chemical reactions are at equilibrium, the response of the so-called ladder mechanism is modelled for any electrochemical technique and microelectrodes of different geometries (micro(hemi)spheres, microdics, microbands) under transient and stationary conditions. The results are applied to the experimental characterization of the formation of ion pairs between the reduced anthraquinone monoanion radical and the tetrabutylammonium cation in acetonitrile. The values of the association constant and the formal potential of reduction of anthraquinone are obtained. Similar experimental studies are performed in ion-transfer mode where the association between the protonated form of 2-phenyl-ethylamine and the ionophore DB18C is quantified, as well as the encapsulation of two cations (1-octyl-methyl-imidazolium and the protonated for of clomipramine) by hydrophilic cyclodextrins. Moreover, explicit analytical equations are obtained for the study of ion transfer reactions at asymmetric liquid|liquid micro-interfaces. These solutions enable the accurate determination of formal transfer potentials taking into account the influence of the interface geometry, and the study of the conditions under which a true steady state response can be obtained. A new ion transfer mechanism (the ACDT scheme) with application in speciation studies is defined and modelled theoretically. Finally, a new analytical theory is developed for ion transfer processes in two-polarizable interface systems of macrometric and micrometric size. This new theory is very general, being applicable to complex systems where several ionic species can be transferred across each interface, regardless of their number, charge and lipophilic nature. Experimental situations of interest that can lead to unusual behaviours are reported, discussed and verified experimentally.