Ternary Phase Equilibria of the 1-Dodecylpyridinium Bromide–Dodecanol–Water System

Abstract

The isothermal ternary phase diagram for the 1-dodecylpyridinium bromide (1-DPB)–water–dodecanol system was determined at 40°C, using2H NMR, polarizing microscopy, and SAXS methods. All of the phases were characterized, and their ranges of existence were determined. The surfactant is easy to dissolve in water, yielding a normal micellar solution phase. After the normal micellar phase, on the binary surfactant–water axis, a normal hexagonal liquid crystalline phase is found at higher surfactant concentrations. On addition of dodecanol, four more phases are formed, i.e. a cubic, a lamellar, and a reverse hexagonal phase, followed by a reverse micellar solution phase. The lamellar liquid crystalline phase dominates the ternary phase diagram. The structures of the liquid crystalline phases were further examined using SAXS measurements, and the results are discussed in terms of the critical packing parameter, cpp, and electrostatic forces. The SAXS experiments show a pronounced swelling of the rods in the hexagonal phase, from 28.5 to 33 Å on addition of dodecanol, whereas the cylindrical aqueous core of the reverse hexagonal phase has a diameter of 18–21 Å, depending on sample composition. The average bilayer thickness of the lamellar phase is about 24 Å.