Journal of Physical Chemistry B, Vol.104, No.33, 8061-8067, 2000
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 K3Fe(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.