Surfactants at the air-water interface
Philip Reynolds, Mark Henderson and John White
The air-water interface is a hydrophilic-hydrophobic interface interesting in its own right, but related to many others such as oil-water. One can produce surface layers at this interface whose properties may differ from bulk materials. For soluble amphiphiles this may be by means of a surface excess, but for insoluble amphiphiles simply spreading at the surface may be enough. The resulting films can be manipulated by means of a Langmuir trough. The films can be examined by many techniques, such as ellipsometry or Brewster angle microscopy but the most powerful techniques are neutron and X-ray reflectivity. By using different deuterations of the water and amphiphiles we can measure quantitatively film thicknesses down to molecular monolayers or less, as well as film compositions. We have been able to measure how a polystyrene monolayer film collapses to a trilayer when it is compressed by the Langmuir trough [1,3,5]. We have been able to see how a large globular dendrimer molecules, illustrated above, can change shape on compression, and how if we chemically modify their surfaces to be more hydrophilic then it hunkers down onto the water surface [2,4]. We can follow how a mixture of polyisobutylene-based surfactants [6] segregate at the surface laterally, and how the more hydrophobic component is enriched at the surface. However care must be taken - we have needed up to seven different deuteration mixtures to properly define a single chemical film. This is because the films are not perfect - apart from the aforementioned possible occurrences of trilayers- we have also seen much larger aggregations, and the converse when the films are so rigid that areas of water empty of surfactant between the 'floes' persist. These molecularly simple films are excellent models for biological systems such as occur in biomineralisation, where the required deuterations are much more difficult
These experiments have been supported by Linkage grants from the Australian Research Council including cash contributions from Orica Ltd. and "in kind" contributions from the ANU.