Surfactant Adsorption at Liquid–Solid Interfaces

Abstract

Surfactant adsorption to the liquid–solid interface is of great importance to many industrial and consumer processes; from detergency to crop spraying and drilling for oil, the understanding of how these molecules behave is crucial to their design and further efficiency improvement.
This thesis describes how a Raman spectrometer for use in total-internal reflection (TIR)-Raman spectroscopy was built and commissioned to provide a new, open bench system. The prime aim was to improve on the time resolution of our existing commercial spectrometer (to <1 s) and allow for easy modification.
TIR-Raman spectroscopy allows us to be surface-selective by only measuring the Raman spectrum from very close to the interface, where the evanescent field excites molecules attached to, or very close to the surface. As this field decays exponentially with distance, only a small region at the interface is probed (≈100 nm).
Using an in-line mixer we were able to record adsorption and desorption isotherms on the surfaces. This technique utilised a continuously stirred tank to vary continually the concentration of solution entering the cell, hence a whole continuous range of concentrations (limited only by the time resolution) could be studied. The validity of our in-line mixer is tested with solutions of the Raman-active molecules acetonitrile, methanol, and sucrose.
The adsorption to silica of various anionic surfactants (sodium dodecyl sulphate (SDS) and sodium dodecylbenzenesulfonate (C12LAS)), nonionic surfactants (polyethylene oxide alkyl ethers (CnEms)) and cationic surfactants (single and double chained tertiary ammonium bromides (CnTABs) and DHDAB) were investigated. Varying mixtures of nonionic and anionic surfactant adsorption on hydrophobic silica are covered briefly.
With the individual surfactants, the investigation started with a plain (acid-washed) silica hemisphere, then moved to various coatings applied to silica, using the same TIR-Raman technique. The additional model substrates studied were hydrophobic silica (treated with hexamethyldisilazane), zeolite, kaolinite, polyester and (although largely unsuccessful) haematite.