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 (φ 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 probe concentration ( ) 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., φ mic → 0, the limit of zero interaction) reveals a reverse micellar long-time self-diffusion coefficient ( ) of 6.13 ± 0.07 × 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.