EVALUATION OF SURFACTANT MONOMER CONCENTRATIONS IN EQUILIBRIUM WITH MICELLES AND THEIR TEMPERATURE-DEPENDENCE FOR OXYETHYLENE OXYPROPYLENE/OXYETHYLENE TRIBLOCK COPOLYMERS USING DSC/

A method is presented that may be used to calculate the concentration of surfactant (C-mon) in thermodynamic equilibrium with micellar aggre gates using high sensitivity differential scanning calorimetry (HSDSC) output. The temperature dependence of this equilibrium process has be en evaluated for a number of oxyethylene/oxypropylene/oxyethylene trib lock copolymers and the results are compared with previously published CMC data that were obtained using a dye solubilization technique. The polydispersity of the block copolymers preclude identifying C-mon wit h the CMC. However, the C-mon values obtained using the HSDSC techniqu e are of the same order of magnitude and show the same functional rela tionship with temperature as the reported CMC data. It is therefore pr oposed that HSDSC provides a suitable method for the rapid evaluation of C-mon for the surfactants examined over a fairly wide range of temp eratures. Analysis of the derived C-mon data points to the importance of the oxypropylene block in determining C-mon values. At any particul ar temperature the greater the size of the oxypropylene block the lowe r the C-mon value. On the other hand, increasing the oxyethylene block size increases the value of C-mon.