Role of quaternary bromides in changing the solubilization site of n‐heptylamine in cationic micellar solutions

Newtonian flow conditions were adopted to take viscosity measurements at 30°C of solutions of cationic surfactants (cetyltrimethylammonium bromide, C16TAB, and tetradecyltrimethylammonium bromide, C14TAB) with the addition of n‐heptylamine (C7NH2) and quaternary salts (tetra‐n‐propylammonium bromide, Pr4NBr; tetra‐n‐butylammonium bromide, Bu4NBr; tetraamylammonium bromide, Am4NBr; tetra‐n‐octylammonium bromide, Oc4NBr; tetra‐n‐butylphosphonium bromide, Bu4PBr; tetraphenylphosphonium bromide, Φ4PBr; n‐propyltriphenylphosphonium bromide, PrΦ3PBr3) by having either a fixed C7NH2 or salt concentration and varying the other. Systems containing a comparatively higher Bu4NBr content showed a dramatic decrease in viscosity at higher C7NH2 concentration. Further, the viscosity‐salt concentration profiles of 0.1 M C16TAB solutions containing fixed C7NH2 concentrations showed a peaked behavior. The peak positions and salt contents at the peaks were dependent on the length of the alkyl part of the particular salt and the connecting atom (N or P) but independent of the nature of the surfactant and C7NH2 concentration. The behavior is discussed in the light of a change in the solubilization site of C7NH2 caused by the presence of quaternary salts. Cavities in the quaternary salts present in the bulk solvent are proposed to be the new sites of solubilization.