Simultaneous observation of attractive interaction, depletion forces, and "sticky" encounters between AOT reverse micelles in isooctane using microelectrode voltammetry

Microelectrode voltammetry has been used to measure the long-time self-diffusion coefficients (D(s)) of AOT (AOT sodium bis-2-ethylhexylsulfosuccinate) reverse micelles in isooctane using K(3)Fe(CN)(6) as a micelle immobilized electroactive probe. D(s) values were found to be a function of both reverse micellar volume fraction (phi(mic)) and probe concentration. The results show that increasing the probe concentration results in a decrease in D(s), suggesting an increase in attractive intermicellar interaction upon addition of probe. This has been interpreted in terms of attenuated surfactant tail-group reorganization facilitating interpenetrating of the surfactant tails. Increasing the micellar volume fraction is seen to diminish attractive interaction which indicates the presence of entropy-driven solvent mediated depletion forces. Self-diffusion coefficients at the limit of zero pribe concntration (D(s)') were found to be micellar volume fraction dependent, and the behavior conformed to the linear interaction theory for interacting micellar systems giving an attractive intermicellar interaction parameter of -2.33. Extrapolation to infinite dilution (i.e., phi(mic) --> 0, the limit of zero interaction) reveals a reverse micellar long-time self-diffusion coefficient (D(s)(0)) of 6.13 +/- 0.07 x 10(-7) cm(-2) s(-1), which gives a corresponding micellar hydrodynamic radius of 7.5 +/- 0.02 nm. This value is approximately a factor of 2 greater than the know micellar size, and the behavior is interpreted in terms of adhesive "sticky" intermicellar interactions indicating the measurements of micellar cluster diffusion.