FORMATION AND PROPERTIES OF LAMELLAR PHASES IN SYSTEMS OF CATIONIC SURFACTANTS AND HYDROXY-NAPHTHOATE

We investigated the phase behavior and the phase transitions in aqueou s solutions of 100 mM cetyltrimethylammonium hydroxide (CTAOH) with 3- hydroxy-2-naphthoic acid (HNC) and of 100 mM cetyltrimethylammonium br omide (CTAB) with sodium-3-hydroxy-2-naphthoate (SHNC). The naphthoate /surfactant ratio has been varied. As previously observed by the group s of C. Manohar and J. Candau we observed for the second system two vi scoelastic gel-like regions, two liquid crystalline regions, and a pre cipitate region. For the CTAOH/HNC system one finds with increasing co ncentration of HNC a low viscous solution, a viscoelastic gel, and a v iscoelastic liquid crystalline L-alpha-phase. In both surfactant syste ms the lamellar phase is formed around an equimolar ratio of cationic surfactant and naphthoate. The lamellar phases have been examined by p olarization microscopy and freeze-fracture electron microscopy. The L- alpha-phase in the system CTAOH/HNC consists of densely packed multila mellar vesicles while the lamellar phase in the system CTAB/SHNC conta ins vesicles, as well as stacked bilayers and tubuli. Corresponding to their different microstructures the lamellar phases in the system, CT AOH/HNC and CTAB/SHNC have different rheological properties. The vesic ular phase is highly viscoelastic and has a yield stress value while t he bilayer phase has a much lower viscosity and no yield stress value. The transition from the micellar to the vesicle phase occurs for CTAO H/HNC over a two-phase region, where micelles and vesicles coexist. In the case of CTAB/SHNC the transition from the micellar to the lamella r phase occurs over a three-phase region, where a surfactant-poor phas e 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 vesicle s that contain little water and stick together, as the vesicles collap se due to the shielding of the repulsive forces by NaBr from an unbind ing to a binding state. The precipitate can be retransformed to a swol len lamellar phase by charging the vesicles with an excess of ionic su rfactant, by adding electrolyte in high concentrations, or by increasi ng the temperature. As predicted by C. Manohar et al. the vesicle phas es show a phase transition at a critical temperature T-c of 46 degrees C. This transition was detected by us for the first time by DSC and b y conductivity measurements. It occurs within a narrow temperature ran ge of 2-3 degrees with an enthalpy change of 0.5 kJ/mol. The transitio n is observed both in the swollen and in the precipitated vesicle phas e. It is well separated from the vesicle/rod transition at higher temp eratures (>70 degrees C) and the liquid crystalline/crystalline transi tion at lower temperatures (25-30 degrees C) that has a melting enthal py of 55 kJ/mol. It is conceivable that the observed transition at 46 degrees C is due to the melting of a two-dimensional solid-like lattic e of the HNC-counterions on the vesicle interface. (C) 1998 Academic P ress.