Scaling of Structural and Rheological Response of L₃ Sponge Phases in the “Sweetened” Cetylpyridinium/Hexanol/Dextrose/Brine System

Abstract

We report a study of the shear response of sponge phases in cetylpyridinium chloride (CPCl)/hexanol/brine/dextrose systems by parallel measurements of rheology and structure by small angle neutron scattering (SANS). Our measurements show that this dextrose added to the extensively studied CPCl/hexanol/brine system is taken up exclusively by the brine solvent, resulting in an equivalent CPCl/hexanol membrane structure and phase behavior for this modified system. Adding dextrose to the brine in these systems to volume fractions up to 0.4 allows us to increase the solvent viscosity by more than an order of magnitude. This lowers the cooperative membrane diffusion coefficient in this system as measured by dynamic light scattering by the same factor, resulting in a corresponding slowing of the Helfrich fluctuation dominated membrane dynamics. Our results show clear and consistent evidence of shear-induced sponge to lamellar phase transformations in these systems. Further, both the rheological and microstructural responses of these systems follow universal master curves when plotted against a rescaled applied shear γ̇ηs/φ3, where φ is the membrane volume fraction and ηs is the viscosity of the brine/dextrose solvent. This well-defined shear response is characterized by three distinct regimes. At low shear rates the sponge phases exhibit Newtonian flow behavior and no structural change is observed. For intermediate shear rates, the systems shear thin and SANS measurements show that the sponge phases are progressively transformed into lamellar phases with the CPCl/hexanol membrane normals aligned parallel to the velocity gradient. This continuous process and the absence of a stress plateau in the rheological measurements both rule out the existence of a biphasic state in this region and thus of a first-order transition between sponge and lamellar phases as is observed in equilibrium phase diagrams. At higher shear rates, the systems are apparently again Newtonian, but the induced lamellar phase appears to collapse and a new (and as yet unidentified) large-scale structure forms. We have found these systems to be extremely sensitive to evaporation, and reproducibility of the scattering measurements required the use of a Couette shear cell with a specially designed vapor barrier.

Amplitude-Modulated Atomic Force Microscopy Reveals the Near Surface Nanostructure of Surfactant Sponge (L3) and Lamellar (Lα) Phases

“…In both phases, these characteristic distances scale inversely to the membrane volume fraction (ϕ) according to swelling laws. 8,24 For the cetylpyridinium chloride (CPCl)− hexanol−brine system, 15,27 the L 3 phase exists over a narrow range of hexanol/CPCl ratios but over a large range of membrane volume fractions with passage sizes ranging from nanometers to a few micrometers. Reducing the hexanol concentration moves the system though a narrow two-phase region (L 3 and L α phases) into the large L α phase, which is similarly stable over a wide range of membrane volume fractions.…”

Reversible shear-induced crystallization above equilibrium freezing temperature in a lyotropic surfactant system

“…So far, shear-induced phase transitions reported in bilayer-forming amphiphilic systems have shown that an isotropic sponge phase ðL 3 Þ of interconnecting bilayers can transform to a lamellar phase ðL α Þ consisting of a stack of bilayers with 1D quasi-longrange positional order (17) or that a dilute L α phase is sheartransformed to a collapsed surfactant-rich L α phase with very little water between the bilayers, coexisting with excess solvent (18). In this context, the studies have been restricted to the swollen L α or L 3 phase with highly flexible bilayers dominated by steric repulsion, where the transitions occur due to shearinduced suppression of thermal fluctuations above a critical shear rate of _ γ c (19).…”

“…In this context, the studies have been restricted to the swollen L α or L 3 phase with highly flexible bilayers dominated by steric repulsion, where the transitions occur due to shearinduced suppression of thermal fluctuations above a critical shear rate of _ γ c (19). Because _ γ c ∼ ϕ n , where ϕ is the surfactant volume fraction and the exponent n lies (18) in the range of 1.5-3, the transitions are absent for ϕ > 0.3 within the experimentally accessible range of shear rates (∼1,000 s −1 ) (20). Our present study is different from the earlier studies on sponge phases because it examines the role of steady shear in weakly swollen, concentrated (ϕ > 0.5) isotropic phases (15,21) …”

Topological Relaxation of a Shear-Induced Lamellar Phase to Sponge Equilibrium and the Energetics of Membrane Fusion

“…In light scattering measurements on ''hyperswollen'' C 12 E 5 sponges at membrane volume fractions below 2%, Yamamoto and Tanaka [3] were able to demonstrate that applied shear could induce an L 3 to L transition. Recently we showed that adding an inert thickener, dextrose, to the brine solvent in the widely studied cetylpyridinium(CPCl)-hexanol membrane system significantly slowed membrane dynamics resulting in much stronger responses to applied shear _ [4,5]. The response depends upon a rescaled shear rate parameter, _ s = 3 , where s is the viscosity of the solvent [6].…”