The curious case of SDS self-assembly in glycerol: Formation of a lamellar gel

Lauren Matthews; Żaneta Przybyłowicz; Sarah E Rogers; Paul Bartlett; Andrew J Johnson; Robert Sochon; Wuge H Briscoe

doi:10.1016/j.jcis.2020.03.102

The curious case of SDS self-assembly in glycerol: Formation of a lamellar gel

J Colloid Interface Sci. 2020 Jul 15:572:384-395. doi: 10.1016/j.jcis.2020.03.102. Epub 2020 Mar 30.

Authors

Lauren Matthews¹, Żaneta Przybyłowicz², Sarah E Rogers³, Paul Bartlett², Andrew J Johnson⁴, Robert Sochon⁴, Wuge H Briscoe⁵

Affiliations

¹ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK; Bristol Centre for Functional Nanomaterials, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
² School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
³ ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX, UK.
⁴ GlaxoSmithKline, St George's Avenue, Weybridge KT13 0DE, UK.
⁵ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. Electronic address: wuge.briscoe@bristol.ac.uk.

PMID: 32272313
DOI: 10.1016/j.jcis.2020.03.102

Abstract

Hypothesis: Hydrogen-bonding capacities of polar nonaqueous media significantly affect self-assembly behaviours of surfactants in these media.

Introduction: Glycerol, a nonaqueous hydrogen-bonding solvent, is widely used in industrial formulations due to its desirable physical properties. Surfactants are ubiquitous in such applications; however, surfactant self-assembly in glycerol is not well understood.

Methods: The microscopic structure of the gel phase was studied using a series of imaging techniques: polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and environmental scanning electron microscopy (ESEM). The rheological properties of the gel were studied using viscometry and oscillation rheology measurements. Further nano-structural characterisation was carried out using small-angle neutron scattering (SANS).

Results: We have observed the unexpected formation of a microfibrillar gel in SDS and glycerol mixtures at a critical gelation concentration (CGC) as low as ~2 wt%; such SDS gelation has not been observed in aqueous systems. The microscopic structure of the gel consisted of microfibres some mm in length and with an average diameter of D ~ 0.5 μm. The fibres in the gel phase exhibited shear-induced alignment in the viscometry measurements, and oscillation tests showed that the gel was viscoelastic, with an elastic-dominated behaviour. Fitting to SANS profiles showed lamellar nano-structures in the gel microfibres at room temperature, transforming into cylindrical-micellar solutions above a critical gelation temperature, T_CG ~ 45 °C.

Conclusions: These unprecedented observations highlight the markedly different self-assembly behaviours in aqueous and nonaqueous H-bonding solvents, which is not currently well understood. Deciphering such self-assembly behaviour is key to furthering our understanding of self-assembly on a fundamental level.

Keywords: Gels; Glycerol; LMWGs; Lamellar gels; Low molecular-weight gels; Nonaqueous H-bonding solvents; SANS; Self-assembly; Small-angle neutron scattering; Surfactant mesophases.