Thermodynamic and Kinetic Study of the Sphere-to-Rod Transition in Nonionic Micelles: Aggregation and Stress Relaxation in C14E8 and C16E8/H2O Systems

Abstract

The energetics and dynamics of the growth of nonionic n-alkyl poly(ethylene glycol) ether surfactant micelles in the isotropic L 1 phase have been analyzed on the basis of a recently described, concentration dependent thermal transition. 1 This highly cooperative endothermic transition is assigned as a sphere-to-rod transition. We analyze the thermodynamic data in terms of aggregation-fusion of complete micelles and also discuss the kinetics in the frame of this random micelle model. The analysis of the calorimetric data leads to a cooperative unit of about 160 detergent molecules undergoing simultaneously the transition from spheroidal to cylindrical structure. We investigate the aggregation kinetics in the microsecond to millisecond range using the temperature-jump (TJ) method with observation of scattered and transmitted light and the stress relaxation dynamics monitoring transient electric birefringence (TEB). As expected from the model, the aggregation dynamics gets faster with increasing temperature, while the stress relaxation dynamics slows down with temperature, leading to apparent negative activation energies. At the transition temperatures, the dynamics of the structural transition for C 1 4 E 8 is characterized by an association and dissociation rate constant of 4.4 x 10 6 M - 1 s - 1 and 1.8 x 10 3 s - 1 , respectively, compared to the much slower dynamics of C 1 6 E 8 with the corresponding rate constants 8.5 x 10 4 M - 1 s - 1 and 29 s - 1 , respectively. The rate constants of micelle fusion increase, while those of micelle scission decrease with increasing temperature. The TEB results are discussed using the relations derived in the literature for the competition between rotational motion and chain scission. The fast dynamics of C 1 4 E 8 compared to the much slower dynamics of C 1 6 E 8 provides an understanding of the differences in stress compliance and viscosity for these systems. As a major result of the study, we present a consistent mechanism of micellar growth which involves the aggregation-fusion of two micelles in a single, rate-limiting reaction step and not the stepwise incorporation of single surfactant molecules into pre-existing micelles. The speeding up of the TJ kinetics in C 1 6 E 8 at higher temperatures indicates a further structural transition, possibly network formation.