Christiaan Ridings

Surface structure of a “non-amphiphilic” protic ionic liquid

The nanostructure of the ethanolammonium nitrate (EtAN)-air surface has been investigated using X-ray reflectometry (XRR), vibrational sum frequency spectroscopy (VSFS) and neutral impact collision ion scattering spectroscopy (NICISS). The XRR data decays more rapidly than expected for a perfectly sharp interface, indicating a diffuse electron (scattering length) density profile. Modelling of the XRR data using three different fitting routines produced consistent interfacial profiles that suggest the formation of interfacial EtAN clusters. Consistent with this, VSFS reveals that the EtAN surface is predominantly covered by -CH(2)- moieties, with the -NH(3)(+) and -OH groups of the cation buried slightly deeper in the interface. The elemental profiles determined using NICISS also show enrichment of carbon relative to nitrogen and oxygen in the outermost surface layer, which is consistent with the surface cation orientation deduced from VSFS, and with the presence of EtAN aggregates at the liquid surface.

Significant changes of the charge distribution at the surface of an ionic liquid due to the presence of small amounts of water

The influence of small amounts of water dissolved in 1-hexyl-3-methylimidazolium chloride ([C(6)mim][Cl]) on the composition of the surface of the ionic liquid is investigated with the depth profiling technique neutral impact collision ion scattering spectroscopy. The concentration depth profiles of the elements in the sample were determined at three different water concentrations and show that small amounts of water affect the charge distribution in the ionic liquid along the surface normal. At low water concentrations (2500 ppm) the cation shows a strong presence at the surface with the alkyl chains oriented towards the gas phase, followed by a layer of anions below the alkyl chains of the cation. At higher water content (6000 to 10,000 ppm) the chloride anion shows an increased concentration at the ionic liquid surface while the alkyl chains move towards the bulk showing that the surface charge becomes more negative with increasing water content. The effect is attributed to the influence of water on the hydrogen bonding network in the ionic liquid.

Change of Surface Structure upon Foam Film Formation

The charge distribution and coverage with surfactant molecules at foam film surfaces plays an important role in determining foam film structure and stability. This work uses the concentration depth profiling technique neutral impact collision ion scattering spectroscopy to experimentally observe the charge distribution in a foam film for the first time. The charge distribution at the surface of a foam film and the surface of the corresponding bulk liquid were measured for a cationic surfactant solution and the surface excess as well as the electric potential were determined. Describing the internal pressure of foam films by using the electrochemical potential is introduced as a new concept. The foam film can be seen to have a more negative surface charge compared to the bulk liquid surface due to re-arranging of the surfactant molecules. It is discussed how the change in surface excess and electric potential change the electrochemical potential and the stability of the foam film.

Surface Ordering in Binary Mixtures of Protic Ionic Liquids

The surface composition of binary mixtures of the protic ionic liquids ethylammonium nitrate and propylammonium nitrate has been investigated using surface tension measurements and the perfectly surface sensitive method metastable induced electron spectroscopy. Given that the latter technique is sensitive only to the outermost layer, it allows for the determination of the surface fraction occupied by a given species. The piecewise linear relationship between surface fraction and surface tension found in this study can be described by a phase separation within the surface layer.