APPARENT VOLUME AND HEAT-CAPACITY OF OXYETHYLENE(12)-OXYPROPYLENE(67)OXYETHYLENE(12), PLURONIC L-122, IN AQUEOUS-SOLUTIONS AND DISPERSIONS

The phase diagram of aqueous Pluronic L-122, oxyethylene<12>-oxypropylene<67>-oxyethylene<12>, was determined from -10 to +40-degrees-C and up to 50% (w/w). Isotropic solution, liquid crystal, and gel phases are observed as well as an anomalous region consisting of slightly turbid blueish solutions. Two endothermic transitions border a region of high viscosity that leads to the gel phase. The apparent volume and heat capacity of Pluronic L-122 and of 2-butoxyethanol in the aqueous mixtures of Pluronic L-122 were determined at 10, 22, and 35-degrees-C. At 10-degrees-C, the isotropic solutions of Pluronic L-122 go through phase separation with a continuous increase in the apparent volume and a sharp increase in the apparent heat capacity. At 22-degrees-C, a transition occurs at low concentrations into the biphasic region, with a sharp increase in the apparent volume and a large maximum in the apparent heat capacity. It is suggested that, from 22 to 35-degrees-C, the transition is displaced to a concentration lower than that of the range covered in this investigation so that the trends of the thermodynamic properties correspond to the tail of the transition. This is consistent with an extrapolated volume higher than the one calculated by additivity of the homopolymers at infinite dilution at 35-degrees-C.