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-alkyldimethylbenzylami-nonium chloride (C(n)BACl) for n-alkylpyridinium chloride (CnPyCl) and n-alkyltrymethyl ammonium 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 CnPyCl (n = 12, 14, 16) and n-alkylpyridinium bromides (CnPyBr, n = 12, 14, 15, 16) presented in this paper. (c) 2005 Elsevier Inc. All rights reserved.