We monitored the change in the number density of cetyltrimethylammonium 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.
The relationship between the concentration of a phase-transfer catalyst, tetrabutylammonium bromide (TBAB), and the rate of its cyclic reaction was investigated using the quasi-elastic laser scattering (QELS) method. TBAB forms an ion pair (TBA + C6H5O -) with sodium phenoxide (C6H5ONa) at the interface. This ion pair moves from the interface to the organic phase where it reacts with diphenylphosphoryl chloride and produces triphenyl phosphate. During this reaction, tetrabutylammonium chloride, which is generated simultaneously with the product, returns to the water phase. An increase in the concentration of the phase-transfer catalyst (TBAB) dose not promote the cyclic reaction. The QELS study indicates that this is because some of the TBAB catalyst adsorbs on the interface and disturbs mass transfer of the other chemical species (TBA + C6H5O -). Interfacial coverage of the catalyst should be taken into account to find the optimum concentration of the catalyst. This paper shows for the first time that the interfacial catalyst promotes the reaction and at the same time disturbs mass transfer of the other chemical species.
A spectroscopic technique of time-resolved quasi-elastic laser-scattering measurement was applied to monitor the dynamic and collective behavior of anionic surfactant sodium dodecyl sulfate (SDS) and neutral surfactant Triton X-100 molecules around their critical micelle concentrations (cmc's) at a water/nitrobenzene liquid/ liquid interface. The time courses of the capillary-wave frequencies after the injection of the surfactant solutions into the water phase were investigated. We found that the dynamic and collective behavior of these two kinds of surfactant molecules at the interface differ from that of the cationic surfactant cetyltrimethylammonium bromide we reported previously. Our results suggested that for SDS the monomolecules at the interface coexist with some molecular aggregates such as micelles at the interface; for Triton X-100, the monolayer at the interface was disrupted and some molecular aggregates, such as micelles, were formed at the interface.
We developed a quasi-elastic laser scattering method and applied it to the density estimation of liquid/liquid (alcohol/water, carboxylic acid/water) interfacial regions. The observed densities are smaller than the expected values in most of the cases, indicating that the interfacial densities are smaller than the bulk phase densities. Moreover, there is a marked tendency for the observed density to become smaller as the hydrophobic group in the alcohol and carboxylic acid molecules becomes larger. The results are consistent with theoretical predictions using the molecular dynamics method. The interfacial structure is discussed in connection with these results.
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