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 aggregates using high sensitivity differential scanning calorimetry (HSDSC) output. The temperature dependence of this equilibrium process has been evaluated for a number of oxyethylene/oxypropylene/oxyethylene triblock 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 with the CMC. However, the C-mon values obtained using the HSDSC technique are of the same order of magnitude and show the same functional relationship with temperature as the reported CMC data. It is therefore proposed that HSDSC provides a suitable method for the rapid evaluation of C-mon for the surfactants examined over a fairly wide range of temperatures. Analysis of the derived C-mon data points to the importance of the oxypropylene block in determining C-mon values. At any particular temperature the greater the size of the oxypropylene block the lower the C-mon value. On the other hand, increasing the oxyethylene block size increases the value of C-mon.