Static and kinetic studies of the phase transition of bilayer membrane of dioctadecyldimethylammonium bromide

Abstract

The gel-liquid crystal phase transition of the bilayer membrane of dioctadecyldimethylammonium bromide was studied statically by monitoring the turbidity, the volume, the conductivity, and the sound velocity of the aqueous solution. With decrease of the temperature, these physical properties changed very steeply at 40-39°C and the phase transition temperature T m was determined to be 39.5°C. When the temperature was raised, a hysterisis was observed and the value of T m was about 3°K higher. A decrease of the volume and increases of the conductivity and the compressibility observed at the phase transition from the liquid crystal to the gel indicated that a phase transition accompanies the dissociation of the counterion from the bilayer membrane. The kinetic studies by the pressure-jump method and the ultrasonic absorption method showed that the phase transition is composed by at least two processes, one is a nucleation and annihilation process with the time constant of the order of 10 −2 sec and the other is a growth and shrinking process on the order of 10 −9 sec. Both relaxation phenomena have maximum relaxation time and amplitude at around T m, which indicates that both processes are cooperative. Static and kinetic studies were also performed for the bilayer membranes of dimethyloctadecyldodecylammonium bromide and didodecyldimethylammonium bromide, but no evidence of the phase transition was observed. The pressure-jump relaxation observed in the time ranges of 10 −2 and 10 −3 sec, respectively, was ascribed to the monomer association and dissociation to and from the bilayer membrane.