Given the potential of ionic liquids (ILs) for batteries, supercapacitors and advanced lubricants, it is crucial to understand how electric fields affect the interfacial behaviour in IL-solvent systems and the intricate relationship between nanostructure and tribotronic properties. This thesis investigates the structural and compositional changes of ILs with different solvents at electrified interfaces.

The four papers constituting this thesis can be broadly divided into two studies. The first study outlines the electro-interfacial behaviour of various monocationic (MILs) and dicationic ILs (DILs) dispersed in propylene carbonate. Combining electrochemical quartz crystal microbalance, neutron reflectivity (EC-NR), and atomic force microscopy, a voltage-induced interphase transition from a self-assembled cation bilayer to a conventional electrical double-layer structure has been revealed in bis(oxalato)borate anion MILs. This interphase transition has not been observed in DILs, attributed to the dual charge centres reducing the segregation between polar and apolar domains of dications for self-assembly interaction. 

The second study explores three MILs sharing the same phosphonium cation with varying orthoborate-based anions dissolved in 2-ethylhexyl laurate (2-EHL). EC-NR measurements reveal a solvent-rich interfacial corona layer and the subtlety of anion architecture in tuning electro-interfacial properties. Meanwhile, EC-NR has been used as a complementary probe to elucidate the nano-scale structural and compositional changes in the boundary films of IL/2-EHL systems with varying potentials, providing a direct link between the molecular controllability and macroscopic tribotronic performance studies.

This thesis contributes to the fundamental understanding of electro-interfacial behaviour and controllability of IL-solvent systems and offers valuable molecular insights for deploying these novel ILs as additives in advanced tribological and electrochemical contexts.

Med tanke på potentialen hos jonvätskor (IL) för batterier, superkondensatorer och avancerade smörjmedel är det viktigt att förstå hur elektriska fält påverkar gränsytans beteende i IL-lösningsmedelssystem och det intrikata förhållandet mellan nanostrukturer och tribotroniska egenskaper. Denna avhandling undersöks strukturella och kompositionella förändringar av ILs med olika lösningsmedel vid elektrifierade gränssnitt.

Avhandlingen baseras på fyra artiklar och kan grovt delas i två bredare studier. Den första studien beskriver det elektro-interfaciala beteendet hos olika monokatjoniska (MILs) och dikatjoniska ILs (DILs) dispergerade i propylenkarbonat. Kombinationen av elektrokemisk kvartskristallmikro-balans, neutronreflektivitet (EC-NR) och atomkraftsmikroskopi har lett till avslöjandet av en spänningsinducerad fasövergång från ett självassocierad katjonbilager till en konventionell elektrisk dubbelskiktsstruktur i MIL med bis(oxalato)boratanjon. Denna interfasövergång har inte observerats i DILs, vilket tillskrivs de dubbla laddningscentra som minskar segregeringen mellan polära och apolära domäner av dikationer för självassoceringsinteraktion. 

Den andra studien undersöker tre MIL som delar samma fosfoniumkatjon med olika ortoboratbaserade anjoner löst i 2-etylhexyllaurat (2-EHL). EC-NR-mätningar avslöjar ett lösningsmedelsrikt korona-skikt i gränsytan och hur anjonarkitekturen påverkar de elektro-interfaciala egenskaperna. Samtidigt har EC-NR belyst de nanoskaliga strukturella och kompositionella förändringarna i gränsskikten i IL/2-EHL-system med varierande potentialer, vilket ger en direkt koppling av den molekylära styrbarheten och makroskopiska tribotroniska prestandan.

Denna avhandling bidrar till den grundläggande förståelsen av elektro-interfacial beteende och styrbarhet hos IL-lösningsmedelssystem och erbjuder värdefulla molekylära insikter för att använda dessa nya IL som tillsatser i avancerade tribologiska och elektrokemiska sammanhang.

Commercial (protiated) samples of the "green" and biodegradable bioester 2-ethylhexyl laurate (2-EHL) were mixed with D-2-EHL synthesized by hydrothermal deuteration, with the mixtures demonstrating bulk structuring in small-angle neutron scattering measurements. Analysis in a polymer scattering framework yielded a radius of gyration (R (g)) of 6.5 angstrom and a Kuhn length (alternatively described as the persistence length or average segment length) of 11.2 angstrom. Samples of 2-EHL dispersed in acetonitrile formed self-assembled structures exceeding the molecular dimensions of the 2-EHL, with a mean aggregation number (N-agg) of 3.5 +/- 0.2 molecules across the tested concentrations. We therefore present structural evidence that this ester can function as a nonionic (co)-surfactant. The available surfactant-like conformations appear to enable performance beyond the low calculated hydrophilic-lipophilic balance value of 2.9. Overall, our data offer an explanation for 2-EHL's interfacial adsorption properties via self-assembly, resulting in strong emolliency and lubricity for this sustainable ester-based bio-oil.

A series of 19 ionic liquids (ILs) based on phosphonium and imidazolium cations of varying alkyl-chain lengths with the orthoborate anions bis(oxalato)borate [BOB]−, bis(mandelato)borate, [BMB]− and bis(salicylato)borate, [BScB]−, are synthesized and studied using small-angle neutron scattering (SANS). All measured systems display nanostructuring, with 1-methyl-3-n-alkyl imidazolium-orthoborates forming clearly bicontinuous L3 spongelike phases when the alkyl chains are longer than C6 (hexyl). L3 phases are fitted using the Teubner and Strey model, and diffusely-nanostructured systems are primarily fitted using the Ornstein-Zernicke correlation length model. Strongly-nanostructured systems have a strong dependence on the cation, with molecular architecture variation explored to determine the driving forces for self-assembly. The ability to form well-defined complex phases is effectively extinguished in several ways: methylation of the most acidic imidazolium ring proton, replacing the imidazolium 3-methyl group with a longer hydrocarbon chain, substitution of [BOB]− by [BMB]−, or exchanging the imidazolium for phosphonium systems, irrespective of phosphonium architecture. The results suggest there is only a small window of opportunity, in terms of molecular amphiphilicity and cation:anion volume matching, for the formation of stable extensive bicontinuous domains in pure bulk orthoborate-based ILs. Particularly important for self-assembly processes appear to be the ability to form H-bonding networks, which offer additional versatility in imidazolium systems.

The structure and interaction of ionic liquids (ILs) influence their interfacial composition, and their arrangement (i.e., electric double-layer (EDL) structure), can be controlled by an electric field. Here, we employed a quartz crystal microbalance (QCM) to study the electrical response of two non-halogenated phosphonium orthoborate ILs, dissolved in a polar solvent at the interface. The response is influenced by the applied voltage, the structure of the ions, and the solvent polarizability. One IL showed anomalous electro-responsivity, suggesting a self-assembly bilayer structure of the IL cation at the gold interface, which transitions to a typical EDL structure at higher positive potential. Neutron reflectivity (NR) confirmed this interfacial structuring and compositional changes at the electrified gold surface. A cation-dominated self-assembly structure is observed for negative and neutral voltages, which abruptly transitions to an anion-rich interfacial layer at positive voltages. An interphase transition explains the electro-responsive behaviour of self-assembling IL/carrier systems, pertinent for ILs in advanced tribological and electrochemical contexts.