Characterization of reverse micelles by dynamic light scattering

Reverse micelles of water–AOT–isooctane have been studied for the [Water]/[AOT]=W0 range varying from 1 to 50 by dynamic laser light scattering and capillary viscometry techniques with AOT concentrations chosen between 100 and 500 mM. The ratio of the rigidity of the liquid–liquid interface K to that of effective surface tension γ, (K/γ) defines a length scale ξ, (K/2γ)=ξ2=RR0 where a reverse micelle of radius R is formed of surfactants having a spontaneous curvature 1/R0. We show that for W0≤20, the reverse micelles can be modeled as spheres purely from geometrical considerations and for W0>20, these conformationaly resemble deformed spheres. Correspondingly, the relative viscosity (to water) of the solution ηr shows a micellar volume fraction dependence ηr(φ)=(1−φ−λφ2)δ, λ being a fitting parameter. For low AOT concentration (100 mM), δ=2.52±0.06 implying a near spherical shape (theoretical value=2.5) for the micelles. This increased to δ=2.9±0.06 for AOT concentration 500 mM which we construe as micelles having deformed spherical conformation. The micellar stability has been deduced from free energy calculations and this yields an explicit expression for the pressure P(R) experienced inside the water core of a reverse micelle. The inter micellar interaction has been discussed within the framework of short-range interaction potential and perturbation potential ∼r−6.