Abstract
The capillary condensation in bicontinuous microemulsions takes place when two parallel surfaces are narrowed that result in a completely lamellar microemulsion. We expected that this phase transition is also observable when the amount of hydrophilic surfaces from clay particles is raised, because hydrophilic surfaces induce lamellar ordering locally. Using small angle neutron scattering, the structure of microemulsions was observed as a function of clay content. The critical concentration is indicated by discontinuous structural changes and depends on the platelet diameter and is explained by the free energy of the platelets competing with the fluctuating medium. The gel phase transition is observed in the spectroscopic measurements where the diffusion motion is widely suppressed in the gel phase, but otherwise superimposes with the membrane undulations.
Highlights
Microemulsions are thermodynamically stable mixtures of oil and water [1,2,3] that are mediated by the surfactant
An attempt of explaining this finding is that the Laponite RD (LRD) acts against a well-defined lamellar phase, and so the correlation length suddenly decreases at the phase boundary, while MMT promotes the well-defined lamellar structure and the fluctuation spectrum is more suppressed allowing for a denser lamellar phase
The MMT system would possibly lead to smaller crystalline regions with stronger grain boundary scattering, while the LRD system might even have facilitated grain alignment and ordering that leads to such large crystallites that they are not observable in the actual Q-window
Summary
Microemulsions are thermodynamically stable mixtures of oil and water [1,2,3] that are mediated by the surfactant. There are oil and water domains that are observable by scattering experiments [4] that are separated by the surfactant film. Liquid crystalline order between the domains can be observed. In the following we restrict ourselves to the bicontinuous microemulsion. The explanation for this choice of the system is a relatively simple handling (equilibrium means reproducibility) and very well documented characterization. A long history is written on bicontinuous microemulsions with polymeric additives [5,6,7,8,9]. Microemulsions are used as cleaning agents [10] and are discussed as fuels with much less pollutant emissions from the combustion [11, 12]
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