Formation and Properties of Lamellar Phases in Systems of Cationic Surfactants and Hydroxy-Naphthoate

Abstract

We investigated the phase behavior and the phase transitions in aqueous solutions of 100 mMcetyltrimethylammonium hydroxide (CTAOH) with 3-hydroxy-2-naphthoic acid (HNC) and of 100 mMcetyltrimethylammonium bromide (CTAB) with sodium-3-hydroxy-2-naphthoate (SHNC). The naphthoate/surfactant ratio has been varied. As previously observed by the groups of C. Manohar and J. Candau we observed for the second system two viscoelastic gel-like regions, two liquid crystalline regions, and a precipitate region. For the CTAOH/HNC system one finds with increasing concentration of HNC a low viscous solution, a viscoelastic gel, and a viscoelastic liquid crystalline Lα-phase. In both surfactant systems the lamellar phase is formed around an equimolar ratio of cationic surfactant and naphthoate. The lamellar phases have been examined by polarization microscopy and freeze-fracture electron microscopy. The Lα-phase in the system CTAOH/HNC consists of densely packed multilamellar vesicles while the lamellar phase in the system CTAB/SHNC contains vesicles, as well as stacked bilayers and tubuli. Corresponding to their different microstructures the lamellar phases in the system, CTAOH/HNC and CTAB/SHNC have different rheological properties. The vesicular phase is highly viscoelastic and has a yield stress value while the bilayer phase has a much lower viscosity and no yield stress value. The transition from the micellar to the vesicle phase occurs for CTAOH/HNC over a two-phase region, where micelles and vesicles coexist. In the case of CTAB/SHNC the transition from the micellar to the lamellar phase occurs over a three-phase region, where a surfactant-poor phase coexists with a lamellar and a coacervate phase. In mixtures of CTAB and SHNC a thick precipitate is formed at an equimolar ratio of CTAB and SHNC. This precipitate consists of condensed multilamellar vesicles that contain little water and stick together, as the vesicles collapse due to the shielding of the repulsive forces by NaBr from an unbinding to a binding state. The precipitate can be retransformed to a swollen lamellar phase by charging the vesicles with an excess of ionic surfactant, by adding electrolyte in high concentrations, or by increasing the temperature. As predicted by C. Manoharet al.the vesicle phases show a phase transition at a critical temperatureTcof 46°C. This transition was detected by us for the first time by DSC and by conductivity measurements. It occurs within a narrow temperature range of 2–3 degrees with an enthalpy change of 0.5 kJ/mol. The transition is observed both in the swollen and in the precipitated vesicle phase. It is well separated from the vesicle/rod transition at higher temperatures (>70°C) and the liquid crystalline/crystalline transition at lower temperatures (25–30°C) that has a melting enthalpy of 55 kJ/mol. It is conceivable that the observed transition at 46°C is due to the melting of a two-dimensional solid-like lattice of the HNC-counterions on the vesicle interface.