Effect of phenylazoaniline to cetyltrimethyl-ammonium bromide ratio on micellar shape

The purpose of this study was to determine the influence of phenylazoanilin e (PAA) on the shape of cetyltrimethylammonium bromide (CTAB) micelles. Cha nge in micellar shape may have an impact on PAA transport characteristics. In a previous publication (Yoon, K.A. and Burgess, D.J., Pharm. Res. 13 (19 96) 433), it was reported that the concentration of CTAB is related to the permeability coefficient of PAA in a triphasic system containing mineral oi l, water and micellar phases. The diffusion coefficients of PAA in CTAB sol utions were calculated from viscosity, dynamic light scattering and dialysi s studies, (using U-tube viscometry, a Nicomp particle sizer and side-by-si de diffusion cells, respectively) to determine any change in micellar shape with change in CTAB concentration. Diffusion coefficients were estimated f rom intrinsic viscosity using Simha's relationship (Tanford, C., Physical C hemistry of Macromolecules. Wiley, New Yoirk, 1961). According to the visco sity and dynamic light scattering studies, CTAB micelles change from sphere s to rods at a PAA: CTAB molar ratio of 0.468 +/- 0.02, whereas this transi tion occurred at 0.32 +/- 0.02, according to the dialysis data. The differe nce between these data can be explained by the effects of aqueous boundary layer present on either side of the permeating membrane and the membrane ad sorption in the dialysis studies. Aqueous boundary layer effects were deter mined by changing the stirring rate and the membrane adsorption effect was determined by presaturating the membrane. These studies indicate that chang e in the permeability coefficient of PAA with increase in CTAB concentratio n in the triphasic system may be due to change in micellar shape. (C) 2001 Published by Elsevier Science B.V. All rights reserved.