Stability and Conformational Studies of Reverse Micelles through Dynamic Laser Light Scattering

Reverse micelles of Water-AOT-Isooctane have been studied for the Water/AOT concentration ratio, W₀ varying from 1 to 50, by dynamic laser light scattering and capillary viscometry techniques in the AOT concentration range, 100, to 500 mM. The ratio of the rigidity of the interface, K-to that of effective surface tension, γ; (K/γ) defines a new length scale, ζ, (K/2γ) = ζ,² = R₀R 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 modelled as spheres purely from geometrical considerations and for W₀ > 20, these conformationally resemble deformed spheres. Correspondingly, the relative viscosity of the solution, n_r shows a micellar volume fraction dependence, , n_r(φ) = (1-φ_m-λφ²)^δ. For low AOT concentration (100 mM), δ = 2.52 5 ± 0.06 implying a near spherical shape for the micelles. This increased to δ = 2.91 ± 0.06 at the AOT concentration, 500 mM which we envisage as micelles having deformed spherical conformation. The micellar stability has been deduced from free energy calculations which yields an explicit expression for the pressure, P(R) inside the water core of a reverse micelle. The intermicellar interaction has been discussed within the frame work of short-range interaction potential and perturbation potential ∼r^-6.