Predicting the size of salt-containing aqueous Na-AOT reverse micellar water-in-oil microemulsions with consideration...

Reverse micellar solutions are thermodynamically stable systems in which surfactant molecules surround water droplets within a continuous organic phase. Among their many applications, they can be used for the synthesis of nanoparticles of controlled agglomeration. Here, we consider the role specific ion effects play in reverse micelle size reduction.

Dynamic light scattering measurements and the Gouy-Chapman electrical double layer model were combined to study water/AOT/isooctane reverse micellar systems (w_o = 10). Linear relationships between the solvodynamic diameter (D) of reverse micelles containing various concentrations of FeSO₄, Mg(NO₃)₂, CuCl₂, Al(NO₃)₃, Fe(NO₃)₃, Y(NO₃)₃, NaBH₄, ZrOCl₂, and NH₄OH, and their calculated Debye screening lengths, κ^-1, were observed with decreasing D and increasing salt concentration (c).

By comparing the linear fits for reverse micelle size as a function of c^-1/2, we determined the size can be described as a function of the Debye screening length, cation valency (z), and specific anion hydrated radius (r_an), where D = 3.1z κ^-1 + b_i, and b_i is linearly related to r_an. Our model accurately predicts reverse micelle sizes with the addition of monovalent, divalent, and trivalent salts for which the primary hydrolyzed cation species has a charge that is equal to the cation valency.