A dilute nematic gel produced by intramicellar segregation of two polyoxyethylene alkyl ether carboxylic acids

Patrick Denk; Lauren Matthews; Sylvain Prévost; Thomas Zemb; Werner Kunz

doi:10.1016/j.jcis.2024.01.014

A dilute nematic gel produced by intramicellar segregation of two polyoxyethylene alkyl ether carboxylic acids

J Colloid Interface Sci. 2024 Apr:659:833-848. doi: 10.1016/j.jcis.2024.01.014. Epub 2024 Jan 8.

Authors

Patrick Denk¹, Lauren Matthews², Sylvain Prévost³, Thomas Zemb⁴, Werner Kunz⁵

Affiliations

¹ Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93053 Regensburg, Germany.
² ESRF - The European Synchrotron, 71 avenue des Martyrs, F-38043 Grenoble, France.
³ ESRF - The European Synchrotron, 71 avenue des Martyrs, F-38043 Grenoble, France; Institut Laue-Langevin - The European Neutron Source, 71 avenue des Martyrs, F-38042 Grenoble, France.
⁴ Institut de Chimie Séparative de Marcoule, BP 17171, F-30207 Bagnols sur Cèze, France.
⁵ Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93053 Regensburg, Germany. Electronic address: werner.kunz@ur.de.

PMID: 38218087
DOI: 10.1016/j.jcis.2024.01.014

Abstract

Motivation: Surfactants like C₈E₈CH₂COOH have such bulky headgroups that they cannot show the common sphere-to-cylinder transition, while surfactants like C_18:1E₂CH₂COOH are mimicking lipids and form only bilayers. Mixing these two types of surfactants allows one to investigate the competition between intramicellar segregation leading to disc-like bicelles and the temperature dependent curvature constraints imposed by the mismatch between heads and tails.

Experiments: We establish phase diagrams as a function of temperature, surfactant mole ratio, and active matter content. We locate the isotropic liquid-isotropic liquid phase separation common to all nonionic surfactant systems, as well as nematic and lamellar phases. The stability and rheology of the nematic phase is investigated. Texture determination by polarizing microscopy allows us to distinguish between the different phases. Finally, SANS and SAXS give intermicellar distances as well as micellar sizes and shapes present for different compositions in the phase diagrams.

Findings: In a defined mole ratio between the two components, intramicellar segregation wins and a viscoelastic discotic nematic phase is present at low temperature. Partial intramicellar mixing upon heating leads to disc growth and eventually to a pseudo-lamellar phase. Further heating leads to complete random mixing and an isotropic phase, showing the common liquid-liquid miscibility gap. This uncommon phase sequence, bicelles, lamellar phase, micelles, and water-poor packed micelles, is due to temperature induced mixing combined with dehydration of the headgroups. This general molecular mechanism explains also why a metastable water-poor lamellar phase quenched by cooling can be easily and reproducibly transformed into a nematic phase by gentle hand shaking at room temperature, as well as the entrapment of air bubbles of any size without encapsulation by bilayers or polymers.

Keywords: Akypo LF2; Akypo RO 20 VG; Bicelle; Discotic nematic phase; Lyotropic liquid crystal; Molecular segregation; Nematic gel; Polyoxyethylene alkyl ether carboxylic acid; Small-angle scattering.