Investigating the Surface Structure of Liquids Containing Ionic Species

Author: Christiaan Ridings

Ridings, Christiaan, 2015 Investigating the Surface Structure of Liquids Containing Ionic Species, Flinders University, School of Chemical and Physical Sciences

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In this work liquid systems containing ionic species are investigated in order to further understand the forces that govern the surface structure of liquids. The distribution of ions along the surface normal, or charge distribution, is especially important in foam films (such as soap bubbles) where the electrostatic forces generated from the separation of charges at the liquid/air interfaces play a pivotal role in stopping the film from collapsing. Many powerful techniques for investigating surfaces require the samples to be measured under ultra-high vacuum. The volatile nature of liquids makes their use in these instruments difficult, especially so in the case of foam films which are already in a fragile, metastable state. Specialised equipment and experimental techniques are developed for the investigation of foam films under vacuum. A glass film holder is used to generate and hold the film. This film holder is held inside an enclosed cell designed to minimise the evaporation of solvent from the film and aid film stability. Variations to the setup are designed and tested which allow for greater stability of the foam film, along with the means to measure the films under altering conditions. Foam films of a cationic surfactant, hexadecyltrimethylphosphonium bromide, were investigated. A range of measurements were performed that demonstrated the thinning, and consequential surface rearrangement, of the foam film over time. These results also indicated a decrease in the surface potential upon foam film formation, partially owing to the reorientation of surfactant molecules at the surface. Foam films containing a non-ionic surfactant, dodecyldimethyl phosphine oxide, were also investigated using the above technique. These films were studied with no added electrolyte as well as added salts, where the anion was varied. Comparing the concentration depth profiles of the foam films to the corresponding bulk liquid surfaces for the various systems studied allowed for the determination as to how the liquid surface changes upon foam film formation. It was found that the addition of salt increased the surfactant adsorption at the surface of both the foam film and bulk liquid. Additionally, it was seen that while iodide was detected as a surface excess at the bulk liquid, chloride was not. Both are detected as a surface excess at the foam film surface. Surfaces of ionic liquids were also investigated, as they represent a unique situation of a liquid being comprised totally of charged species. Thus, the charge distribution is not mediated by an additional species, as is the case with aqueous solutions. The effect of small amounts of water as a surface impurity in (C6mim)(Cl) was investigated. Increasing the water content lead to an increase in the amount of anion adsorbed at the surface, indicating how forces other than the electrostatic interaction between ions governs surface structure. It was seen that for both protic and aprotic ionic liquids that the cation aliphatic chain length had a significant influence on the surface structure. Increasing the chain length caused increased adsorption of the cation, with subsequent cation reorientation at the surface.

Keywords: liquids,surface science,ion scattering,charge distribution,specific ion effects,foam films,surfactants,ionic liquids

Subject: Chemistry thesis, Physics thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Chemical and Physical Sciences
Supervisor: Gunther Andersson