Exploring Hidden Local Ordering in Microemulsions with a Weak Directive Second Order Parameter
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ORIGINAL ARTICLE
Exploring Hidden Local Ordering in Microemulsions with a Weak Directive Second Order Parameter Michael Kerscher1,2 · Frederik Lipfert1,3 · Henrich Frielinghaus1 Received: 9 January 2020 / Accepted: 15 February 2020 © The Author(s) 2020
Abstract So far, the near-surface ordering of microemulsions was focused on lamellar ordering while the bulk microemulsion was bicontinuous. In a series of different non-ionic surfactants the near-surface ordering of microemulsions at a hydrophilic silicon surface was studied using grazing incidence small angle neutron scattering. For the surfactant C8E3, most likely a gyroid structure was found at the solid–liquid interface, while the more efficient surfactants find lamellar ordering up to lamellar capillary condensation. The ranges for near-surface ordering are deeper than the bulk correlation lengths. These findings point towards theories that use directional order parameters that would lead to deeper near-surface ordering than simple theories with a single scalar order parameter would predict. Rheology experiments display high viscosities at very low shear rates and, therefore, support the existence of a directional order parameter. Keywords Microemulsion · Near surface ordering · Grazing incidence small angle scattering
1 Introduction In theories of liquid crystals [1–4] and of granular media [5–8], a directive field is often needed to describe the various liquid crystalline ordered phases. From this standpoint it often seems mandatory to have an additional directive (i.e. vectorial or tensorial) order parameter in order to describe the thermodynamics of complicated ordering phenomena and responses to directed external fields of soft matter systems. Contrarily, for diblock copolymers [9] (Grain boundaries of diblock copolymers have been analyzed experimentally [10, 11]) and microemulsions [12–14], a wide range of phenomena can be explained on the basis of a single order parameter describing the local concentration of one species versus the other, i.e. monomer A vs. B or water vs. oil. This is a result of a high degree of symmetry. It is still possible to describe a huge variety of ordered phases and responses to * Henrich Frielinghaus h.frielinghaus@fz‑juelich.de 1
Jülich Centre for Neutron Science At MLZ, Forschungszentrum Jülich GmbH, Lichtenbergstrasse 1, 85747 Garching, Germany
2
Wacker Chemistry, Johannes‑Hess‑Str. 24, 84489 Burghausen, Germany
3
Etrality GmbH, Zillenweg 9, 83115 Neubeuren, Germany
directed external fields. So the number of model parameters is kept at a minimum, while the richness of phenomena is still strikingly high. For instance bicontinuous microemulsions develop lamellar order adjacent to planar hydrophilic (hydrophobic) surfaces (in experiment and theory) [15–17]. Thus, the question arises how many order parameters are needed to describe the structure of microemulsions adjacent to a planar hydrophilic solid surface. Simple extensions to two order parameters have been undertaken [18]. However, local ordering of dom
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