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

Abstract

Hypothesis -Hydrogen-bonding capacities of polar nonaqueous media significantly affect selfassembly 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 nanostructural 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 o C.Conclusions -These unprecedented observations highlight the markedly different selfassembly 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.

Unexpected observation of an intermediate hexagonal phase upon fluid-to-gel transition: SDS self-assembly in glycerol

“…The SANS results have confirmed the formation of an intermediate phase in the fluid-to-gel transition upon cooling of the SDS-in-glycerol system. The SANS profile before heating and after overnight (~8 h) cooling were identical, suggesting the thermodynamic equilibrium structure was the lamellar gel phase noted previously [31]. The hexagonal intermediate phase has not been previously reported, whilst the phase diagram of SDS in aqueous media shows that a lamellar-tohexagonal transition would occur upon cooling [32,33] but at a much higher c SDS ~ 80 wt%.…”

“…The formation of intermediate phases adds further details to the proposed mechanism for the gelation of SDS in glycerol at low concentrationsitself an unprecedented observation [31]. Here we have observed that the fluid phase consisting of cylindrical micelles cools via a hexagonal phase before forming the lamellar phase at thermodynamic equilibrium.…”

“…This points to the drastically different self-assembly driving forces in water and in glycerol, despite their similar physical properties such as H-bonding capacity and both being polar, as discussed in Ref. [31].…”

“…In a previous paper [31], we have reported the formation of a thermally reversible SDS-in-glycerol lamellar gel above a critical SDS gelation concentration (c CGC ~ 2 wt%) and below a critical gelation temperature (T GC ~ 45 • C; Curve 1 and 4, Fig. 1a).…”

“…Here, we report that an intermediate hexagonal phase formed in the fluid-to-gel transition upon cooling of 4.4 wt% sodium dodecyl sulfate (SDS)-in-glycerol gel. The supramolecular gel was thermally reversible [31], and below a critical gelation temperature (T CG ) ~ 45 • C, the gel phase comprised a fibrillar structure on the microscale and a lamellar structure on the nanoscale. Above T CG , the fluid phase consisted of cylindrical SDS micelles.…”