Effects of organic solvents on the scission energy of rodlike micelles

The linear viscoelastic response of erucyl bis(hydroxyethyl) methylammonium chloride with added KCl has been studied as a function of temperature and nonpolar solvent addition. The plateau modulus is independent of temperature from 25 to 40degreesC, in contrast to previous studies with salicylate counterions that showed a plateau modulus increasing with temperature over this range. The average micelle length, (L) over bar, predicted by the model of Cates, depends experimentally on E-scis/k(B)T, where E-scis is the scission energy of the chain and k(B)T is the Boltzmann constant times the absolute temperature. With ethanol addition, the calculated average contour length, (L) over bar, decreases by a factor of 4 as ethanol concentration varies from 0 to 1.3 M. This corresponds to an apparent energy for scission, E-scis, decrease from 81+/-8 to 74+/-7 kJ/mol. On the other hand, only 80 mM of hexane is required to cause a decrease in E-scis to the same level, and for hexane addition levels above 70 mM a disruption in the plateau modulus indicates the disruption of the rodlike structure. The correspondence between the effect of temperature and the effect of solvent addition allows the development of solvent/temperature superposition rules to predict the rheology of these viscoelastic fluids at elevated temperatures.