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 40°C, in contrast to previous studies with salicylate counterions that showed a plateau modulus increasing with temperature over this range. The average micelle length, L, predicted by the model of Cates, depends experimentally on E _scisk- _BT, where 2 _scis is the scission energy of the chain and k _BT is the Boltzmann constant times the absolute temperature. With ethanol addition, the calculated average contour length, ̄L, 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.