Addendum to: Exploring Hidden Local Ordering in Microemulsions with a Weak Directive Second Order Parameter
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ADDENDUM
Addendum to: Exploring Hidden Local Ordering in Microemulsions with a Weak Directive Second Order Parameter Henrich Frielinghaus1 Received: 23 January 2020 / Accepted: 17 April 2020 © The Author(s) 2020
Abstract In a recent publication, my group discussed a directive second order parameter that hypothetically could form micrometer large structures that influence the rheological behavior of a bicontinuous microemulsion. For this, the viscosities of two microemulsions with the non-ionic surfactants C 10E4 and C 8E3 were determined over the wide frequency and shear rate range. Contrarily to our previous publications there are no elevated viscosities towards slowest motions of the rheometer. Thus, no micrometer large structures form in microemulsions. However, we argue and confirm that there are compartments with the size of several correlation lengths. This finding supports the development of a directional order parameter in microemulsions. Keywords Microemulsion · Rheology · Dynamic light scattering · Order parameter
1 Introduction Microemulsions [1] consist of two immiscible liquids oil and water that are mediated by a surfactant. They are thermodynamically stable and form spontaneously. On the nanometer length scales the two liquids are still immiscible and form domains that are separated by the surfactant film. If mixing equal amounts of oil and water, then mostly bicontinuous microemulsions form [2]. Each of the liquids develops a sponge structure that hosts the other liquid. The surfactant film has the shape of minimal surfaces with a mean curvature zero and many stalks that connect the membrane network. The bicontinuous phase is very close to the phase inversion temperature T̃ (for non-ionic surfactants) where the spontaneous curvature changes sign. Furthermore, a minimum of surfactant 𝛾̃ is needed to warrant the thermodynamic stability of this bicontinuous one-phase system. The critical point ( 𝛾̃ , T̃ ) in this phase diagram (temperature versus surfactant amount) is called fish tail point, because the fish The original article can be found online at https://doi.org/10.1007/ s42250-020-00126-7. * Henrich Frielinghaus h.frielinghaus@fz‑juelich.de 1
Forschungszentrum Jülich GmbH, Jülich Center for Neutron Science at the Heinz Maier-Leibnitz Zentrum, Lichtenbergstrasse 1, 85747 Garching, Germany
body with a three phase coexistence is found towards lower surfactant amounts, and the fish tail with a one-phase region (predominantly with the bicontinuous structure) towards higher surfactant amounts. Below and above of these phase boundaries, two-phase coexistence is found with expelled oil and water, respectively. A review on microemulsions is found in Ref. 1, chapter 1. When bicontinuous microemulsions are exposed to external surfaces the near surface structure can change. One understandable example is the formation of lamellar structures [3] at hydrophilic surfaces that decay towards the bulk by an increasing number of stalks. But also cubic structures may be aligned by the external interf
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