Adsorption of Surfactants at the Solid-Liquid Interface: A Quartz Crystal Microbalance study

This thesis is concerned with the interfacial behaviour of surfactants at solid-liquid interfaces. The main technique used for the adsorption measurements during this thesis work was the Quartz Crystal Microbalance-Dissipation (QCM-D). This technique allows both the adsorbed amount, as evaluated as a change in frequency (∆f), and the change in the dissipation factor (∆D) that is a measure of the energy dissipated in the system, to be determined simultaneously. Methods like null ellipsometry already exist, and they measure the amount adsorbed to a planar, reflecting surface accurately, and the thickness of the adsorbed layer may also be determined. The QCM-D technique has, however, some advantages. For instance, the quartz crystal can be coated (physical/chemical) in a large number of ways. In addition, simultaneous measurement of the dissipation factor allows another parameter to be determined, this parameter is a measure of the interaction between the adsorbed surfactant layers and the bulk solution. Further, opaque or even non-transparent solutions can be studied with the QCM-D, which is not possible with the ellipsometer. When the project began, an aim was to investigate the visco-elastic properties of polyelectrolytes at different surfaces. This turned out to be more complex than we expected so I decided to use a less complex systems in order to more fully understand the results. Hence, the choice became to study surfactant adsorption, a topic which is well documented before by several other techniques. The choice was based on the surfactants low molecular weight, and the relatively simple distribution of a polar (hydrophilic) and non-polar (hydrophobic) part, and a significant general knowledge about their interfacial behaviour. The methodology for adsorption studies in liquid for the QCM-D was only in its infancy, so parameters like temperature dependence, surface roughness, surface modification and cleaning had to be kept under control or developed at the same time. Systematic surfactant adsorption studies from liquids with the QCM technique do not exist. Hence, the aim of this thesis was to achieve an understanding of the information provided by measured shifts in frequency and dissipation factor for such systems, and from this draw conclusions about the interfacial behaviour of both non-ionic and cationic surfactants. Further I aimed to learn how valuable the QCM-D technique was for these systems and what pitfalls there are in evaluating the results observed with this technique.