Composition and temperature effects on the solution structure of SDS/octanol/brine by SANS, NMR and microscopy.
basic_science · Level V
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- Also identified by DOI 10.1039/d3sm01098h.
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Abstract
We investigate the solution structures of model sodium dodecyl sulfate/octanol/brine ternary mixtures across the lamellar (L<sub><i>α</i></sub>), vesicle (L<sub>4</sub>) and micellar (L<sub>1</sub>) phases employing small angle neutron scattering (SANS), optical microscopy and nuclear magnetic resonance (NMR). Specifically, we examine the effect of co-surfactant octanol (0.2-9.48 w/v%) and temperature (25-65 °C) along dilution lines at fixed octanol : SDS ratios (0.08-1.21). A transition from L<sub><i>α</i></sub> to sponge phase (L<sub>3</sub>) above 35 °C is found along the octanol : SDS = 1.21 isopleth, with phase coexistence above <i>ϕ</i> ≈ 0.14 weight fraction of surfactant and co-surfactant. The lamellar bilayers swell upon dilution, with an approximately linear increase of <i>d</i>-spacing, accompanied by a decrease of the Caillé parameter, indicative of greater membrane rigidity. At a lower octanol : SDS ratio of 0.62, coexistence of oblate micelles and vesicles is observed with preferential formation of vesicles at low concentrations. Dilution of the L<sub>1</sub> phase, along octanol : SDS = 0.08, results in elongated micelles, as the NaCl : SDS ratio increases, while higher temperatures favour the formation of less elongated micelles. Our results provide a detailed map of the equilibrium structures found in the L<sub><i>α</i></sub> vicinity of this extensively investigated flow-responsive surfactant system.