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 (〈τs〉) of 710 ps. In the water pool, 〈τs〉 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), 〈τs〉 is ∼830 ps. This is 4 times shorter than 〈τs〉 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.