The critical reverse micelle concentrations of C-12-S-C-12 center dot 2Br (s = 2, 3, 4, 5, 6, 8, 12) in n-heptane/n-hexanol solutions, cmc(I) s, have been determined by absorption spectrum method using iodine as probe. The values of cmc(I) s are smaller than those of the critical micelle concentrations (cmc(aq) s) in aqueous solution and reach a maximum at s = 4, which is similar to the variation of cmc(aq) with s. The variation of cmc(I) with s is attributed to the alteration in the spacer chain conformation, i.e. when the spacer is short enough, it may adopt stretch configuration and when the spacer is long, it tends to bend towards the continuous oil phase to achieve a more densely packing of the quaternary ammonium heads on the surface of the core. (c) 2006 Elsevier Inc. All rights reserved.