Sphere to rod transitions in homologous alkylpyridinium salts: a Stauff-Klevens-type equation for the second critical micelle concentration

Abstract

In the present paper, we analyze the dependence of the second critical micelle concentration (second cmc) of ionic amphiphiles on the number of atoms in the hydrocarbon molecular chain, n(c). A molecular thermodynamic model for the interaction energy between the end caps of rodlike micelles, g(N), is introduced and the linear dependence of this object on n(c) analyzed, thus leading to a Stauff-Klevens-like behavior of the second cmc. The predictions agree with previously reported data for n-alkyldimethylbenzylammonium chloride (C(n)BACl) for n-alkylpyridinium chloride (C(n)PyCl) and n-alkyltrymethylammonium bromide (C(n)TABr) at 35 degrees C. These conclusions are reinforced by the conductivity, density, and ultrasound velocity measurements of the second cmc of several C(n)PyCl (n=12, 14, 16) and n-alkylpyridinium bromides (C(n)PyBr, n=12, 14, 15, 16) presented in this paper.