In this study, the surface properties (in water and in the presence of varying concentrations of NaCl, CTAB, and TX-100) of four amphiphilic drugs are presented. The parameters evaluated are cmc (critical micelle concentration), Γmax (maximum surface excess concentration at the air/water interface), and A min (minimum area per surfactant molecule at the air/water interface). Γmax increases and cmc/A min decreases with increasing concentration of the additives. The cmc values calculated using a dye solubilization method for the systems also follow the same trend. The behavior is explained on the basis of counterion adsorption and mixed micelle formation.

Viscometry under Newtonian flow conditions has been used to study morphological changes in 0.3 M sodium dodecyl sulfate (SDS) micellar solutions in the presence of aliphatic amines (n-heptyl, C7NH2, or n-octyl, C8NH2) and different inorganic (MCl, M = Li, Na, K, or Rb; and NaX, X = Cl, Br, or SCN), organic salts (NaY, Y = salicylate (Sal), benzoate (Ben), anthranilate (An), or tosylate (Tos)), symmetrical quaternary bromides (R4NBr, R = H, CH3, C2H5, n-C3H7, n-C4H9, or n-C5H11) or tetrabutylphosphonium bromide, (n-C4H9)4PBr, at 30 °C. Addition of amines with or without organic/inorganic salts causes the usual viscosity increase until the phase boundary of the L1 phase is reached. However, in the presence of quaternary bromides, viscosity−[amine] profiles show an increase, a plateau, and slower decrease regions. The appearance of these regions has been found to be dependent on the chain length of the R-part and [quaternary bromide]. These effects have been discussed on the basis of solubilization of aliphatic amines in a micellar palisade layer or micellar exterior and their possible effects on the packing parameter and micellar morphology.

Here we report systematic viscosity measurements under Newtonian flow conditions for sodium dodecyl sulfate (SDS) micellar solutions in the presence of aliphatic hydrocarbons (n-hexane, n-heptane, n-octane), toluene, and 1-alkanols (1-heptanol, 1-octanol) at 30 °C. The influence of 0.1 M quaternary ammonium bromides (R4NBr; R ) H, CH3, C2H5, n-C3H7, n-C4H9, n-C5H11) was seen on relative viscosity (ηr) vs additive concentration profiles. Lower members of R4NBr (R ) H, CH3, C2H5) have marginal influence on viscosity profiles and hence on micellar morphology. This may be due to the less pronounced effect on association structures as well as on solubilization sites (and on solubilizate content at a particular site) of added organics. As the alkyl part of R4NBr was increased (R g n-C3H7), the effect, which depended upon the nature of the additive (i.e., its hydrophobic/polarity character), was significant. This is possibly due to salts affecting the partitioning content of organic additives at different micellar solubilization sites with concomitant changes in micellar morphology, as well as viscosity of the micellar solutions. The effect of tetra-n-butylammonium bromide (R ) n-C4H9) concentration was seen to substantiate the change in site of solubilization phenomenon of organic additives. These studies show that, with an appropriate R4NBr concentration present in combination with a surfactant, change of solubilization site even for nonpolar compounds (like hydrocarbons) is possible.