EQUATION OF STATE OF A CHARGED BILAYER SYSTEM - MEASURE OF THE ENTROPY OF THE LAMELLAR-LAMELLAR TRANSITION IN DDABR

The synthetic cationic double-chain surfactant didodecyldimethylammoni um bromide shows two distinct thermodynamically stable lamellar phases ; a dilute L-alpha, phase stabilized predominantly by electrostatic fo rces, and a condensed L-alpha' phase stabilized by ''hydration'' force s. Using six different experimental methods, applying osmotic stress f rom 10(2) to 10(9) Pa and varying temperature from 20 degrees C to 70 degrees C, we have measured the osmotic pressure vs interbilayer dista nce and thus mapped the phase diagram with an equation of state. In th is binary system, the area per headgroup as well as bilayer thickness vary with concentration and temperature. Hence, lateral compressibilit y has to be taken into account in the free energy balance. The osmotic stress needed to effect the swollen-to-collapsed lamellar phase trans ition is determined as a function of temperature. From these data the entropy of the L-alpha-L-alpha' transition is found to be a strong fun ction of temperature. Below 40 degrees C, condensation from the dilute L-alpha, phase, the change of entropy is negative as might be expecte d. Above 40 degrees C the entropy of condensation is positive, demonst rating that there is a release of degrees of freedom associated with t he bilayer condensation. These data allow us to think far more critica lly about the forces that stabilize bilayer systems. (C) 1998 American Institute of Physics.