In this thesis we have studied the influence of transport mechanisms on the adsorption kinetics of colloidal suspensions, as well as in the distribution of colloidal particles at the adsorbed layer. Adsorption of colloidal particles is controlled by the geometric exclusion effects at the surface originated by the finite size of the particles and by the irreversible character of the adsorption in these systems, and is also influenced by the specific transport mechanisms which control the arrival of the colloids from the bulk to the interface. Our main objective has been to elucidate the effects of each contribution, as well as their relative importance and mutual influence in some specific situations.After a first chapter in which we have introduced the basic kinetic models which have been proposed in the literature to take into account surface exclusion effects (random sequential adsorption (RSA) and ballistic (BM) models), in the second chapter we have studied the adsorption in the presence of an external field parallel to the substrate. Our objective has been to elucidate how surface exclusion effects are sensitive to the imposed external conditions upon which adsorption takes place, showing that they do not arise from purely geometric constrains. Instead of carefully describing the transport process, we have conveniently modified the usual kinetic models. The basic feature now is that a new minimum length at which two particles can approach on the substrate appears, and the kinetics becomes asymmetric in the sense that this minimum distance depends on the side of the preadsorbed disk at which the incoming particle adsorbs. We have observed a decrease of the jamming limit with the strength of this imposed force, and also a tendency to form more locally ordered aggregates when this force increases. New adsorption mechanisms are induced by the external field: a particle may roll over a number of preadsorbed spheres before being either adsorbed or rejected, implying that adsorption becomes non-local in the sense that an incoming particle may interact with many preadsorbed disks. The pair distribution function exhibits a richer structure indicating that the clusters have internal structure since their relative separation is not univocally fixed. If the exclusion distance is a multiple of the diameter, then a resonance is observed when using BM rules, and a highly locally ordered substrate is formed.In chapter 3 we have studied the effect of the transport on the adsorption of colloidal particles at high Peclet number in the presence of a gravity field, when their diffusion can be neglected. As a new mechanism with respect to previous studies, we have incorporated hydrodynamic interactions (HI) existing between the incoming particle and the adsorbed ones due to the fact that the particles are suspended in a fluid. We have shown that the basic effect of HI is to introduce an effective repulsive interaction between the incoming particle and the preadsorbed ones. Some analytic results obtained in very simplified conditions have helped us to understand the effects of HI, although un general, computer simulations have been carried out to study the adsorption process. We have seen that, although the global quantities obtained with HI do not differ quantitatively from the ones obtained for BM, as for example the coverage as a function of time, the jamming limit, or the available fraction of the line, the local structure differs significantly from the one obtained with BM. The pair correlation function is characterized by having a series of peaks due to the rolling of incoming spheres over preadsorbed ones. The behaviour behind the peaks is different, showing a slower decay with HI, indicating that because of the effective repulsion, larger gaps between the spheres are preferred, which implies that HI induce the formation of looser local structures on the substrate. We have shown that the effective repulsion introduced by HI favours the formation of elongated triplets on the surface where BM would predict more isotropic clusters, meaning that HI changes the structure of the clusters formed at the interface. We have compared with experimental results on the adsorption of mellamine particles. The curves obtained with HI agree better with the experimental ones than those obtained with BM, explaining therefore some of the discrepancies observed when comparing with BM. In particular, the slower decay behind the first peak can be thought of as being due to the effect of HI. This has served to show that BM, which was thought to be a good model to describe the adsorption of heavy colloidal particles, is restricted to situations where inertial effects dominate the transport to the wall.Finally, in chapter 4 we have developed a thermodynamic theory for the adsorption process. We have focused our analysis on the situation in which adsorption is controlled by a surface energy barrier, which is more realistic for the adsorption of small particles. In this case, the transport to the interface is controlled by the diffusion through the energy barrier. In order to describe this process properly, we have introduced an additional internal variable for the fields at the surface in the thermodynamic description.The surface exclusion effects at this level are introduced considering that the system at the surface is not ideal. In this way we have derived a local generalized Langmuir equation for the evolution of the surface concentration. If the adsorption is not controlled by an energy barrier, then the local thermodynamic description is different. We have shown how it is possible to obtain global generalized Langmuir equation which describes the evolution of the global surface concentration, using the fact that entropical barriers appear for the incoming particles. We have also studied the fluctuations around steady states in a systematic way. We have shown how to deduce the corresponding fluctuation-dissipation theorem when an internal degree of freedom is introduced, and we have applied the results to analyze the density correlation function of a simple adsorption model with diffusion.

En esta tesis se reformulan de manera detallada la primera aproximación de Born relativista con ondas planas (PWBA), para colisiones inelásticas de electrones, positrones y partículas pesadas con átomos o iones, suponiendo que la función de onda atómica puede describirse mediante un modelo de partículas independientes. Además, se deducen expresiones cerradas de las intensidades de oscilador generalizadas longitudinal (GOS) y transversal (TGOS), y para la intensidad de oscilador óptica (OOS). Un aspecto básico de nuestra formulación es que hemos evitado la aproximación de pequeñas transferencias de momento utilizada por Fano (1963), que altera significativamente las secciones eficaces integradas para energías cercanas al umbral de ionización. Liberada de esta aproximación, la PWBA es idéntica a la DWBA (la aproximación de Born con ondas distorsionadas) en el límite de potencial distorsionador nulo.En la tesis también se describen métodos numéricos robustos para el cálculo de las GOSs longitudinal y transversal, así como esquemas de interpolación y de extrapolación para obtener la la sección eficaz doblemente diferencial (SEDD) a partir de las GOSs tabuladas, que hemos implementado en un conjunto de programas de cálculo. Utilizando el potencial autoconsistente de DHFS, hemos generado una base de datos completa de GOSs, TGOSs, OOSs, y perfiles --Born-Compton' para todas las capas de la configuración del estado fundamental de átomos neutros de los elementos desde el hidrógeno hasta el einstenio. Las GOSs calculadas presentan desviaciones sistemáticas de la regla de suma de Bethe, que son consecuencia de efectos relativistas. A partir de nuestra base de datos de GOSs, hemos calculado secciones eficaces totales, secciones eficaces de frenado, y secciones eficaces de dispersión de energía para colisiones inelásticas de electrones, positrones y protones con átomos libres, integrando numéricamente la SEDD.Teniendo en cuenta las desviaciones de la regla de suma, en esta memoria también se deducen fórmulas asintóticas para las secciones eficaces integradas. Además, a partir de ellas hemos obtenido correcciones de capas directamente, como las diferencias entre estas fórmulas y los valores numéricos de las secciones eficaces. Tanto las desviaciones de la regla de suma como las correcciones de capas se originan principalmente en las capas internas, que son poco sensibles al estado de agregación, por lo que los valores obtenidos son también aproximadamente válidos para colisiones con moléculas y con medios densos.Combinando la PWBA y la DWBA, finalmente en esta tesis se detalla un procedimiento robusto para el cálculo de secciones eficaces de ionización de capas internas por impacto de electrones y positrones, para energías arbitrariamente grandes. Para energías por debajo de 16 veces la energía de ionización, utilizamos la DWBA para determinar las correcciones de distorsión e intercambio a la PWBA. Por encima de esta energía, la PWBA escalada con el factor empírico E/(E+bEa) reproduce fielmente los valores DWBA. Utilizando este esquema, hemos generado una base de datos de secciones eficaces de ionización para las capas K, L y M de todos los elementos desde Z=1 hasta Z=99, por impacto de electrones y positrones con energías entre 50 eV y un 1 GeV. Esta base de datos es útil para el análisis cuantitativo en EPMA y AES, y para la simulación MC del transporte acoplado de electrones y fotones. Hemos implementado esta base de datos en el código PENELOPE, y simulado la generación de rayos x en muestras irradiadas por haces de electrones. La comparación con medidas experimentales confirma que al utilizar DWBA en vez de la PWBA mejora la fiabilidad de las simulaciones. Para facilitar el uso de nuestras secciones eficaces de ionización, hemos parametrizado los valores numéricos mediante expresiones analíticas simples. / We present a detailed formulation of the relativistic plane-wave Born approximation (PWBA) for inelastic collisions of charged particles with free atoms and positive ions. The wave functions of the target atom or ion are calculated from a central-¯eld independent-electron model with the Dirac-Hartree-Fock-Slater self-consistent potential. The double-differential cross section, depending on the energy loss and the recoil energy, is expressed as a sum of two terms which are products of purely kinematical factors and the generalized oscillator strengths (GOSs). Transitions induced by the instantaneous Coulomb interaction between the projectile and the active target electron are described by the longitudinal GOS. Transitions caused by the transverse interaction (exchange of virtual photons) are accounted for by a transverse GOS. We derive closed expressions for the longitudinal and transverse GOSs in terms of vector coupling coefficients and radial integrals. A set of Fortran programs have been written to compute the GOSs, the energy-loss differential cross section and integrals of the latter. We also present systematic derivations of asymptotic formulas for the total cross section, the stopping cross section and the energy-straggling cross section. Shell corrections for these quantities are obtained from the differences between computed numerical values and the predictions of the asymptotic formulas.A method is also described for computing total cross sections for the ionization of inner shells of atoms and positive ions by impact of electrons and positrons with arbitrary energies. The method combines the relativistic PWBA with a semirelativistic version of the distorted-wave Born approximation (DWBA). The difference between the total ionization cross sections that result from the DWBA and the PWBA (considering the longitudinal interaction only) has been calculated numerically for projectiles with kinetic energies up to 16 times the ionization energy of the active shell. In this energy range, ionization cross sections with the accuracy of a distorted-wave calculation are obtained by simply adding this difference to the cross section resulting from the conventional PWBA. For higher energies, the cross section is obtained by multiplying the PWBA cross section by an energy-dependent scaling factor that is determined by a single fitted parameter. Numerical results are shown to agree with experimental data, when these are available.