Steady-state and time-resolved fluorescence studies of bis(2-ethylhexyl) sodium succinate (AOT) reverse micelles in supercritical ethane

The properties of bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles in supercritical ethane (T_c = 32.3°C, P_c = 48.8 bar) have been investigated by fluorescence spectroscopy. Two different fluorophores (1-anilino-8-naphthalenesulfonic acid (1,8-ANS) and 6-propionyl-2-(dimethylamino)naphthalene (PRODAN)) were used to probe the microenvironment of this system. Probe and surfactant concentrations, temperature, and fluid density were found to have significant influence on the reverse micelle dynamics. The change in fluorescence intensity with density is rationalized by changes in the partition coefficient between the probe and the AOT micelle. This is also consistent with density-dependent changes of the emission spectral contours. The 1,8-ANS decay kinetics were fit best by two discrete components, indicating that the probe is located simultaneously in two different AOT domains. The PRODAN decay too is best fit with a two-component model; however, one of the decay terms is Lorentzian distributed and the other is discrete. The distributed component represents PRODAN located simultaneously in an ensemble of AOT domains. The shorter-lived, discrete component correlates well with PRODAN free in the ethane continuous phase. By using the lifetime information we determine the density dependent ΔH° and ΔS° for the probe-AOT-ethane equilibrium. In turn, we find that the equilibrium constant (K_eq) decreases with increased continuous phase density. However, the ΔH° for this process becomes more favorable with increasing density. The reason for the decrease in K_eq lies in the ΔS° term. Initially, ΔS° is positive (favorable), but eventually it becomes quite negative at elevated densities. Thus, entropy governs (mostly) this particular equilibrium process.