Solvation dynamics in the water pool of an aerosol-OT microemulsion. Effect of sodium salicylate and sodium cholate

Abstract

The effect of sodium salicylate (Na-sal) and sodium cholate (Na-cholate) on the solvation dynamics in the water pool of aerosol-OT (AOT, sodium dioctylsulfosuccinate) microemulsion in n-heptane is reported. In the absence of any additive, the solvation dynamics of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in AOT microemulsions is found to be biexponential with an average solvation time (τ_s) of 710 ps. In the water pool, τ_s decreases to 330 ps on addition of 1 mM Na-sal and increases to 3050 ps in the presence of 100 mM Na-cholate. The spectral width (Γ) of the time-resolved emission spectra of DCM in the water pool is found to be time dependent. This is ascribed to diffusion of the probe (DCM). The Na-sal-induced acceleration of the solvation dynamics is ascribed to the increase in the size of the water pool. In bulk water, in the presence of 100 mM Na-cholate (a bile salt), τ_s is ~830 ps. This is 4 times shorter than τ_s in the presence of Na-cholate in the water pool. This is ascribed to extreme crowding in the water pool because of the presence of Na-cholate aggregates. In bulk water, the emission spectral width displays a very small time dependence in the presence of Na-cholate aggregates. This suggests that in this case self-diffusion is unimportant and the slow solvation arises entirely from the dynamic exchange.