Monitoring of molecular collective behavior at a liquid/liquid interface by a time-resolved quasi-elastic laser scattering method

We monitored the change in the number density of celyltrimethylammonium bromide (CTAB) molecules at a water/nitrobenzene (W/NB) liquid/liquid interface by a newly developed time-resolved quasi-elastic laser scattering (QELS) method. The results are used to discuss the molecular collective behavior there. From the time-courses after the injection of a CTAB solution beyond its critical micelle concentration (cmc), we found an anomalous temporary increase of the number density of CTAB molecules at the interface, which cannot be explained by a simple diffusion model. This suggests that the transfer of CTAB micelles across the interface occurs in the following process: the collapse of micelles at the interface region; the oriented adsorption of CTAB molecules onto the interface, forming a monolayer; and the desorption from the interface. Thermodynamic evaluation results also support this model; that is, the equilibrium number density of CTAB molecules at the interface follows the Langmuir adsorption isotherm obtained from our measurement, and the adsorption energy calculated from the isotherm agrees well with the theoretical value of the micelles.