Simply complex: discerning specific ion effects for soft materials

This thesis investigates how specific ion effects impact the properties of several materials both fundamentally and technologically. Ions are used to probe fine modification in chemical interactions and associated to that, self-assembly, structural characteristics and ultimately performances of the materials investigated. Formed morphologies are characterized and their performance is linked to the molecular reality of ion specificity for various applications. More in detail, ions are used to tune the strength of H-bond interactions inducing the formation of organic ionic plastic crystalline (OIPC) phases and to control the ionic conductivity of solid polymer electrolytes (SPEs). Also, specific ion effects are used to modulate structure and performances of polymeric layer-by-layer (LbL) membranes. The findings obtained in this work provide a fundamental understanding on how specific ion effects can be used to tailor properties and performance of several materials. A summary of the contents of this thesis is provided below. Chapter 1 introduces the historical background on specific ion effects and provides a theoretical platform to evaluate their impact. The key points of related theories and the fundamental reasoning behind ion specificity are presented and connected. Practical implications of the use of specific ion effects in several fields are reported. With an eye to the expansion of the classical ion classification and to the evaluation of specific ion effects in non-aqueous systems, the motivation of the thesis is outlined. In Chapter 2 a new non-globular molecule containing 15-crown-5 ether moieties and amidic functionalities is synthesized and characterized. The formation of H-bonds between the amide groups is detected and the strength of these interactions is tuned by complexing the system with different sodium salts containing chaotropic anions. The type of chaotropic anion has an effect on the self-assembly of the non-globular molecules by weakening or disrupting of the H-bonds between the amidic functionalities. This leads to the formation of organic ionic plastic crystalline (OIPC) phases with rotational freedom and to the tuning of the thermal and morphological properties of the OIPC. In Chapter 3 a study of the effect of chaotropic anions on the properties of a solid polymer electrolyte is performed. Even though the mechanism of ion conduction in solid polymer electrolytes has been extensively studied in the past, the role of the counterion (i.e. anion) interacting with both the polymer and the cation has not been explored in detail. This study shows that the anion can severely impact the conduction as well. The effect of the anion (I-, SCN-, BF4- and PF6-) on the thermal, structural and electrical properties of a solid polymer electrolyte (15C5BA polymer) is investigated. The results clearly show that the nature of the anions can be used as a parameter to actually tune the properties of the polymer electrolyte. Chapter 4 describes the impact of the use of salts with a different chaotropic or kosmotropic nature on the self-assembly behaviour of oppositely charged strong polyelectrolytes to ultimately fabricate layer-by-layer membranes. The formation of a multilayer arrangement is detected and investigated in terms of mass adsorption and surface charge. The use of salts with a different chaotropic/kosmotropic nature in the coating solution leads to the formation of layers with various densities and charge based on the salt used during preparation. The impact of chaotropic or kosmotropic salts is also evident in the performances of the layer-by-layer membranes prepared. It is observed that water permeability and salt retentions are significantly influenced by the nature of the salts used in the coating solution. While the use of chaotropic salts results in membranes with a more positively charged surface, high retention of MgCl2 and relatively distinct odd-even effects, membranes prepared with kosmotropic salts exhibit a more negative surface with a higher retention for Na2SO4, plateauing of the odd-even effects and water permeabilities that reach up to 6 times those of the membranes prepared with chaotropic salts. These findings confirm that specific ion effects are a valuable tool to tailor membrane performances in technological applications. In Chapter 5 an expansion of the classification of ions into kosmotropic and chaotropic is addressed to include not just simple ions but also more complex polyions (polyelectrolytes). To do so, three polycations (poly(allylamine) hydrochloride (PAH), poly allylmultimethylammonium (PAMA) and poly diallyldimethylammonium chloride (PDADMAC) with different chaotropicity are selected and are used for the preparation of layer-by-layer membranes with the same polyanion (poly(sodium-4-styrenesulfonate or PSS). The chaotropicity of the monomer unit of the polycations used is studied in relation to their charge density. Results show that the polycation with the strongest chaotropic character is subject to higher extrinsic charge compensation and for this reason, delivers membranes with the highest water permeability. Moreover, the chaotropicity of the polycation has an effect on the surface charge of the membranes and on their retention behaviour. Finally, in Chapter 6 the main conclusions of the findings obtained in this thesis are reported. Furthermore, an outlook on the use of specific ion effects as tool to tailor materials properties for applications is provided with a careful look on the expansion of the concept to polyions and mixed salt systems