Physical science of the dioctadecyldimethylammonium chloride-water system. 2. Kinetic and mechanistic aspects

Hydration reactions in the dioctadecyldimethylammonium chloride (DODMAC)-water system display an extremely wide dynamic range (about 10⁸ over a 15°C temperature span) and occur by a one-step process. The rate of hydration is usually determined by the kinetics of diffusion of water through a surface layer of the hydration product phase, but hydration of the dry crystal X is anomalously fast. Hydration of the monohydrate crystal X·W is accelerated by heating this phase for a short time to just above its peritectoid temperature, but the hydration product is a metastable crystal phase X·2Wm. Dehydration and disproportionation phase reactions are both highly variable kinetically and complex mechanistically. Crystal mono- and dihydrates decompose at peritectoid discontinuities by first congruently "melting" to form a metastable lamellar liquid crystal phase. The equilibrium reaction products are formed by slower nucleation and crystal growth processes. Cooling the lamellar liquid crystal phase below the Krafft discontinuity yields, at equilibrium, the liquid and X·2W crystal phases. Equilibration during this reaction occurs in at least three (probably four) discrete stages. The fast initial process is believed to be disproportionation, to form intermediate phase states having approximately the equilibrium compositions. This is followed by much slower reorganization toward the equilibrium phase structure, via initial formation of X·W. The last step is the stacking of restructured bilayers to form the bulk phases. With nucleation, stacking occurs extensively in hours; in the absence of nucleation, stacking may be deferred for more than 4 years.