Ion Specificity at Non-Aqueous Solvent Surfaces: Concentration Depth Profiles of Monovalent Inorganic Ions

Author: Anand Kumar

Kumar, Anand, 2023 Ion Specificity at Non-Aqueous Solvent Surfaces: Concentration Depth Profiles of Monovalent Inorganic Ions, Flinders University, College of Science and Engineering

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Against the continuum electrostatic description of the image charge interaction, it is known that inorganic ions are present at the vapour-water interface and show ion specificity in their presence/absence at the interface. Ion specificity for the presence/absence of inorganic ions at the vapour-water interfaces has been investigated using various advanced surface-sensitive techniques as discussed in this thesis. However, it is still largely unexplored at other vapour-solvent interfaces. Here, the specific ion effects (SIE) for the distribution of monovalent inorganic ions along the depth scale at the vapour-solvent interfaces of 4 non-aqueous solvents are investigated. Neutral impact collision ion scattering spectroscopy (NICISS) is employed to elucidate the ion specificity within the distribution of ions at these vapour-solvent interfaces. NICISS provides individual ion concentration depth profiles (CDPs) formed from the inorganic electrolytes dissolved in the solvents. This information is obtained from the energy loss of helium projectiles during their trajectory into the sample and then backscattering from the samples’ constituent elements. Employing NICISS, in this thesis, the complexity of ion specificity due to separate ion contributions and solvent perturbations at the vapour-solvent interface is presented.

Thirteen different glycerol solutions are studied to compare CDPs of inorganic monovalent ions. Results show that smaller chloride ions (Cl-) are comparatively more abundant at the vapour-glycerol interface than larger bromide ions (Br ) and show equal enhancement to larger iodide ions (I-). This is in contrast with aqueous electrolyte solutions but consistent with the expected influence of the change in the solvent. NICISS measurements reported here reveal the anions’ strong influence towards the CDPs of the cations, but cations only influence the CDP of Br , whereas Cl and I- distribution are mostly independent of the counterion. Following, a reverse Hofmeister series for monoatomic inorganic ions at the vapour-formamide interface in comparison to the vapour-water interface is reported. The CDPs of ions are found to be independent of the counterion at the vapour-formamide interface. The latter observation suggests that the ions in formamide follow a “Hofmeister paradigm” where the counterion has no impact on the order of the series. This is also observed in the various electrolyte solutions through X-ray absorption near-edge structure (XANES) measurements. Even though a negligible counter-ion effect is observed, cations seem to drive the overall depletion of electrolytes which is measured using NICISS and suggested through surface tension measurements.

In the last experimental chapter, ionic CDPs of selected monovalent inorganic electrolytes were investigated at the vapour-solvent interfaces of propylene carbonate (aprotic) and benzyl alcohol (protic). Here, the SIE at both solvent interfaces is alike suggesting that the interfacial hydrogen bonding does not influence the ion-specific CDPs at these vapour-solvent interfaces. The dominant role of cations in overall electrolyte depletion is also observed. This work further emphasizes the importance of anion-cation interactions towards ion specificity at the vapour-solvent interface. These overall findings display the variation in ionic behaviour at various vapour-solvent interfaces thus displaying the role of solvent perturbation towards the ion specificity. These findings were then tested against various existing concepts and hypotheses for the adsorption/desorption of ions at the vapour-water interface to elucidate the underlying mechanism of ion specificity at the vapour-solvent interfaces.  

Keywords: Specific Ion Effects, Concentration Depth Profiles, Neutral Impact Collision Ion Scattering Spectroscopy, Vapor-solvent Interface, Non-aqueous Solvents, Monovalent Inorganic Ions, Ion Specifcity, Solvent Surfaces, Protic Solvent, Aprotic Solvent

Subject: Physics thesis

Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Prof. Gunther Andersson