hide
Free keywords:
LASER-TEMPERATURE-JUMP; SODIUM DODECYL-SULFATE; PLURONIC F127 EO97PO69EO97; ANGLE NEUTRON-SCATTERING; OXIDE) BLOCK-COPOLYMERS; DROP SHAPE-ANALYSIS; AQUEOUS-SOLUTION; CHEMICAL RELAXATION; ELECTROCHEMICAL MEASUREMENTS; CONCENTRATION FLUCTUATIONS
Abstract:
During the last 6 years, chemical relaxation studies of the kinetics of micelle formation in triblock copolymers such as L64 (EO13PO30EO13) and B40 (EO13BO10EO13) Which behave as nonionic surfactants have been studied. The micellar system is characterized by two well-defined relaxation times as predicted by Aniansson and Wall for the isodesmic model A(1) + A(n-1) reversible arrow A(n) in which monomeric A(1) and micellar, A(n), are the principal species. A closer examination of the published work, however, reveals that some issues have not been satisfactorily resolved. For example it is claimed that the fast relaxation time associated with the single step monomer/micelle exchange exists in two time domains, namely, 10(-4)-10(-5) s (T-jump) and 10(-7) s (ultrasonic) for the same triblock copolymers. In an attempt to resolve this predicament, we report here some ultrasonic relaxation measurements on a number of triblock copolymers as micellar systems and also as mixed micellar systems in the presence of hexadecyltrimethylammonium bromide using both H2O and D2O as solvents. We carried out a careful and systematic experimental study including light scattering and surface tension. Following the analysis of the ultrasonic absorption data, the resulting relaxation parameters were incorporated into well-documented relaxation equations to evaluate rate data and test thermodynamic predictions for the monomer/micelle exchange. After careful considerations we concluded that the ultrasonic relaxation observed in micellar solutions of water soluble triblock copolymers is not associated with the monomer/micelle exchange as defined in the Aniansson and Wall treatment. This conclusion is based on (a) the unreasonable values above the diffusion controlled limit of the resulting rate constants and (b) the amplitude of the relaxation process being incompatible with the known thermodynamic parameters for the micellization process. The most likely explanation for the origin of the ultrasonic relaxation in Pluronics is associated with critical phenomena involving concentration fluctuations in the micellar aggregates. We believe that this process is active in ultrasonic relaxation through a coupling with monomer concentration changes which accompany the fluctuations. The fast relaxation times measured in triblock copolymer systems with the iodine laser temperature jump by Holzwarth et al.(12-15) therefore provide the correct rate constants for a monomer micelle/exchange process.
