Simulation Study to Model Extremely Slow Reaggregation Processes in Micelle Solutions

Abstract

Photon correlation measurements on the dodecylpyridinium iodide/water system as nuclei had shown that large micelles are formed after a temperature jump from 80 to 25 °C. The micellar radius exhibits relaxation characteristics with extremely long relaxation times, reaching values up to several hours. Proceeding from a thermodynamic approach of the size distribution of aggregates, model calculations have been performed in which the nonlinearized rate equations of the underlying isodesmic reaction scheme have been used to describe the micelle kinetics. The results of these calculations give valuable indications of the mechanism behind the extremely slow relaxation. In the solutions under consideration, the nucleation of a sufficient number of micellar aggregates obviously is hindered by the extremely unfavorable equilibrium concentration of intermediates of submicellar size. This leads to a strongly deviating nonequilibrium distribution with an excess of extra large micelles during thermal equilibrium. Establishing the final equilibrium is then a very slow process, due to the low monomer concentration, which is maintained by the presence of the extra large micelles. Our model is also capable to verify the slow and fast relaxation process as predicted by the linearized Aniansson−Wall model. It also shows up the ultrafast process of oligomer formation as studied recently by broadband ultrasonic spectrometry.