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Content archived on 2024-04-16



The phase diagram of the water/surfactant (C12E5) system has been reinvestigated. An isotropic liquid phase has been examined by neutron scattering and conductivity measurements, which demonstrate that the phase is made up of randomly connected membranes, resembling the structure of a sponge. This phase was predicted theoretically for quaternary and quinary systems, but has not been previously shown for a binary system.

Direct imaging of the structure has been achieved by freeze fracture and electron microscopy. The light scattering behaviour has been explained using a 2-order parameter theory, confirming the theory that the sponge phase has a special symmetry related to the way the space is divided by the surfactant membrane. A variable contrast experiment has been performed which demonstrated the similarities between the sponge structure and bicontinuous microemulsions. Effectively, when the water and oil scattering is matched a regular bicontinuous microemulsion reveals the same scattering behaviour as a sponge phase. Some theoretical predictions have been made concerning the dynamics of the sponge phase.

The work done on the behaviour of multicomponent water oil amphiphile systems has been extended to other polar liquids like formamide, with the recognition that general patterns in these systems govern the essential features. Investigation of fluid microstructures of microemulsions has allowed the clarification of some issues, for example the actual length scales involved. A comparison between the microemulsion and sponge phases has demonstrated the similarities and differences between the 2 structures.

The system of water and nonionic surfactant (C12E5) and water has been reinvestigated. The phase diagram has been studied as a function of temperature and concentration. Qualitative and quantitative differences have been found with the previously published phase diagram. A very dilute lamellar phase has been discovered, whose repeat distance could increase up to 3000 angstroms, corresponding to 1.5% of surfactant. The system has been studied using neutron scattering. Oriented (single crystal) samples have been prepared and a 2-dimensional detector (PAXY and PAXE) has been used to measure the anisotropic scattering.

The Bragg peak was accessible in the range of light scattering (iridescence) due to the extreme dilution of this phase. A specially designed experimental layout allowed the measurement of peak positions as a function of the surfactant concentration.

Analysis of the data from neutron and light scattering experiments showed that the surfactant layers are strongly undulating due to thermal fluctuations. This leads to an excess area which can be measured experimentally. It has been shown for the first time that this excess area is a logarithmic function of concentration, as predicted by theory.

The small angle scattering due to concentration fluctuations is very anisotropic. Previously the dilute phase was only stable at above 50 C which limitedexperimental possibilities, but by the addition of a long chain alcohol the very dilute lamellar phase has been prepared at room temperature, allowing more sophisticated experiments to be performed. These include quantitative measurements of the smectic A elastic constants by dynamic light scattering on single crystals.

Samples in which the lamellar phase is polymerised have been prepared and analysed using X-ray scattering and direct electron microscopy.

It has been demonstrated that shearing allows the production of spherical microcapsules of controlled size. Electron microscopy has been performed dire ctly on the sample in a purpose built Couette cell. Freeze fracture images have been obtained.
Recent developments in the field of surfactants science have many fundamental and applied implications (including biology). The Max Planck Institut fur biophysikalishe chemie at Gottigen and the centre de recherche Paul Pascal at Bordeaux have been two very active laboratories in this field, with many connections with american Laboratories (MIT, Exxon, UCLA). They have in the last few years worked on closely related problems (see publications list). A suitable collaboration will allow not only a more rapid progration in new discoveries for each laboratory but also to start new programs of research together (see detailed projects). These projects include on one hand experimental studies on phase diagrams and their theoretical interpretation. On the other hand, experiments on dilute lamellar phases will be used to model biological membranes in connection with the synthesis of new double tailed surfactant molecules.


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