chapter  2
38 Pages

2

I. Introduction .................................................................. 38 II. Chemical Relaxation Methods..................................... 39

A. Principle ................................................................. 39 B. Relaxation Times................................................... 41

1. Single-Step Processes...................................... 41 2. Multistep Processes......................................... 43

C. Relaxation Amplitudes.......................................... 44 D. Chemical Relaxation Techniques ......................... 45

1. T-jump............................................................... 46 2. p-jump............................................................... 48 3. Shock Tube....................................................... 51 4. E-jump .............................................................. 52 5. Ultrasonic Absorption ..................................... 53 6. Stopped-Flow.................................................... 56

III. Time-Resolved Luminescence Quenching .................. 58

IV. Nuclear Magnetic Resonance (NMR)........................... 62 V. Electron Spin Resonance (ESR) ................................... 64 VI. Rheology......................................................................... 66 VII. Miscellaneous Methods ................................................. 68

A. Capillary Wave Propagation .................................. 68 B. Fluctuation Spectroscopy....................................... 68

VIII. Conclusions .................................................................... 69 References............................................................................... 69

I. INTRODUCTION

Some processes that occur in systems containing surfactant self-assemblies — most particularly the exchange process involving surfactant, cosurfactant, or solubilizates (see the preface and Chapter 3) — can be extremely rapid. They cannot be investigated by methods used in conventional kinetic studies that involve mixing reactants and monitoring the formation of products. Indeed, fast mixing apparatuses usually have a dead time of 1-10 ms and, therefore, do not permit studies of kinetics of reactions with half-time of reaction shorter than this limit. This fact led to the use of chemical relaxation methods in the early studies of micellar kinetics. These methods were developed in the 1950s.1 In chemical relaxation methods one starts from a mixture of reactants and products in a state of thermodynamic equilibrium and perturbs this equilibrium by very rapidly changing one of the external parameters: pressure p or temperature T, for instance. The system responds to the perturbation by shifting toward a new state of equilibrium imposed by the new value of the modified external parameter. The evolution of the system with time is referred to as chemical relaxation. It can be described in terms of one (or several) time constant(s), the chemical relaxation time(s), which characterizes the ability of the system to follow the perturbation. The relaxation time(s) depends on the rate constants of the investigated reaction(s).