Interfacial Dynamics of Water-in-Oil Microemulsion Droplets: Determination of the Bending Modulus Using Iodine Laser Temperature Jump

The dynamic response of water-in-oil AOT microemulsion droplets to perturbations caused by rapid energy transfer has been interpreted in terms of the surfactant interface bending rigidity. The iodine laser temperature jump technique was used to increase the microemulsion temperature rapidly (within 1 µs) and thus disturb the system equilibrium. The dynamic relaxation to the new equilibrium position was monitored by measuring the scattering intensity at 180° of the microemulsion over a 1-ms time interval immediately after the temperature jump. The rate and direction of the scattering intensity change depended on the initial temperature, size, and salt content of the water droplets; experiments were conducted for S₀ (ratio of isooctane/AOT molar concentrations) values of 5 and 13, W₀ (ratio of water/AOT molar concentrations) 5 to 50, and at the 10–50°C temperature range. The observed characteristic relaxation times of 2°10 µs were attributed to the relaxation of the surfactant interface. A bending modulus of approximately 0.4kT was derived for the AOT monolayer, based on published models for the shape perturbation of a spherical interface. Addition of NaCl to the aqueous pools resulted in a decrease of the bending modulus, in agreement with published theoretical predictions.