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

The purpose of this study was to determine the influence of phenylazoaniline (PAA) on the shape of cetyltrimethylammonium bromide (CTAB) micelles. Change 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 (1996) 433), it was reported that the concentration of CTAB is related to the permeability coefficient of PAA in a triphasic system containing mineral oil, water and micellar phases. The diffusion coefficients of PAA in CTAB solutions were calculated from viscosity, dynamic light scattering and dialysis studies, (using U-tube viscometry, a Nicomp particle sizer and side-by-side diffusion cells, respectively) to determine any change in micellar shape with change in CTAB concentration. Diffusion coefficients were estimated from intrinsic viscosity using Simhaʹs relationship (Tanford, C., Physical Chemistry of Macromolecules. Wiley, New Yoirk, 1961). According to the viscosity and dynamic light scattering studies, CTAB micelles change from spheres to rods at a PAA: CTAB molar ratio of 0.468±0.02, whereas this transition occurred at 0.32±0.02, according to the dialysis data. The difference between these data can be explained by the effects of aqueous boundary layer present on either side of the permeating membrane and the membrane adsorption in the dialysis studies. Aqueous boundary layer effects were determined by changing the stirring rate and the membrane adsorption effect was determined by presaturating the membrane. These studies indicate that change in the permeability coefficient of PAA with increase in CTAB concentration in the triphasic system may be due to change in micellar shape.