Characteristic length and curvature of the AOT/brine/squalane “sponge” L3 phases

Abstract

The effect of salinity (NaCl concentration) was determined at 25 ​°C for systems composed of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and equal volumes of squalane and brine. At all the NaCl concentrations the samples are triphasic with an upper phase made of pure squalane while brine is present in two other phases. At NaCl concentration (in brine) below 1.6 %w/v there is the coexistence of lamellar and sponge (L₃) phases. Loading with NaCl above 1.6 %w/v one observes the coexistence of L₃ and pure brine. The amount of AOT dissolved in the AOT bilayer has been quantified by thermogravimetric analysis. At salinity above 2 %w/v of NaCl in brine, the AOT sponge phase shrinks expelling brine and the squalane whose presence in the bilayer becomes negligible. The L₃ phases have been further characterized by SAXS and diffusion-NMR. The measured self-diffusion coefficients of water and AOT are essentially coincident with those measured at the same salinity in the absence of squalane. The SAXS curves of sponge phases have been fitted to a single equation accounting for the contributions of the bilayer structure and of the large-scale interactions allowing the determination of the average interpore separation D∗ and the bilayer correlation length ξ. The correlation length has been interpreted as reflecting static randomness of connected bilayer instead of the dynamic thermal undulations. Considering the correlation length as the displacement along the bilayer surface subtending a critical angular change (θ∗) below which the surface appears as flat, permits to define the curvature (H) as |H| $=\frac{{\theta }^{\ast }}{\xi }$. The correlation length scales linearly with the square of the bilayer volume fraction as proposed by a previous theoretical model assuming the sponge phases are thermodynamically constrained to a curvature that equals the spontaneous curvature.