Specific effects of organic and inorganic ions in soft interfaces

Abstract

Ionic specificity, also known as Hofmeister effects, is a widely studied phenomenon related with the ability of ions of the same valence to produce different effects over many interfaces. Anions and cations were arranged by Hofmeister in 1888 according to their capability of precipitating proteins. Ions are classified in these series as kosmotropes (strongly hydrated) or chaotropes (weakly hydrated). In addition, anions usually produce more intense effects than cations. Many authors have developed different models to explain ion specific phenomena considering ion characteristics as size, polarizability or ion hydration; and ion-surface interactions by dispersion forces, chemical bindings or water hydration layers. Nevertheless, in spite of the performed advances in this area, a detailed microscopic theory to completely understand ionic specificity is still absent. This thesis aims to elucidate the subjacent microscopic mechanisms in ionic specificity. For that, experiments, simulations and theoretical studies with different systems have been performed. We propose that ion accumulation or exclusion from interfaces is mediated by the interaction of the solvent with both, ions and surfaces. Special emphasis has been given to organic ions. The tetraphenyl ions are big hydrophobic ions recently added to the Hofmeister series as superchaotropes. These organic ions allow us to explore further in ion specific effects, since they have a deep impact on interfaces, in particular on hydrophobic soft systems as polymers or proteins. An attractive characteristic of the tetraphenyl ions is that the anion (Ph4B−) and the cation (Ph4As+) are nearly identical. We have demonstrated that the anion and the cation show different behaviors in solution and when they interact with other surfaces in spite of their similar chemical structure. Moreover, the anion always shows stronger specific effects than the cation. In addition, this difference is greater when ions interact with the thermoresponsive polymer PNIPAM (Poly (N-isopropylacrylamide)) instead of hard surfaces. This favorable interaction is mediated by the hydrophobic effect. PNIPAM undergoes a sharp and reversible transition from a soluble to an insoluble state in aqueous solution. The transition temperature can be modified by the ionic conditions of the medium. We have studied this transition temperature with inorganic and organic ions. We have observed that the transition depends on the capacity of the ions to compete with the water hydrating PNIPAM and also to the affinity of the poorly hydrated ions to accumulate on the hydrophobic moieties of the PNIPAM. The great ion accumulation in the insoluble state of PNIPAM produces important charge reversal of charged PNIPAM microparticles, when these last ions act as counterions. On the other hand, several proteins of biotechnological interest and different degree of hydrophobicity were studied by physical adsorption of the different proteins onto hydrophobic surfaces. We have observed that the accumulation of the tetraphenyl ions is greater on increasingly hydrophobic interfaces. In addition, these large ions interact so strongly with the soft biomolecules that proteins undergo conformational changes. Proteins films, in water, are swollen when ions enhance their electrostatic charge, while these layers are compressed when ions screen the electrostatic repulsion between molecules. Nevertheless, the ionic concentration needed to achieve similar states of structuring protein is one order of magnitude lower for Ph4B− than for Ph4As+ corroborating that the anion causes more intense effects than the cation. To carry out all this study we have used several experimental techniques, the most relevant of them have been: electrophoretic mobility, Atomic Force Microscopy (AFM), Quartz Crystal Microbalance (QCM), and Differential Scanning Calorimeter (DSC). In addition, Molecular Dynamics (MD) simulations were developed to analyze at microscopic level the interaction between ions and interfaces. La presente tesis tiene como objetivo dilucidar los mecanismos microscópicos que dan lugar a la especificidad iónica. Para ello, se han realizado experimentos, simulaciones y estudios teóricos con diferentes sistemas. Se propone que la acumulación o exclusión de los iones en las interfaces está mediada por la interacción del solvente tanto con iones como con superficies. Se ha hecho especial hincapié en los iones orgánicos. Los iones tetrafenil son iones hidrófobos de gran tamaño que han sido recientemente añadidos a las series de Hofmeister como súper caotrópicos. Estos iones orgánicos nos permiten explorar la especificidad iónica desde otro punto de vista. Los iones orgánicos interaccionan mucho más fuerte con las interfaces que los iones inorgánicos, sobre todo en sistemas hidrófobos de tipo soft como polímeros o proteínas. Una característica muy relevante de los iones tetrafenil es que el anión (Ph4B−) y el catión (Ph4As+) son prácticamente idénticos. A pesar de su similar estructura química, hemos demostrado que el anión y el catión muestran comportamientos diferentes en solución y cuando interactúan con otras superficies. De hecho, el anión siempre muestra efectos específicos más acusados que el catión.