Journal of Colloid and Interface Science 287 (2005) 685–693 www.elsevier.com/locate/jcis Effects of micelle-to-vesicle transitions on the degree of counterion binding Yousuke Ono, Hideya Kawasaki, Masahiko Annaka, Hiroshi Maeda ∗ Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received 17 June 2004; accepted 10 February 2005 Available online 31 March 2005 Abstract Effects of micelle-to-vesicle transitions on the degree of counterion binding (β ) were investigated on three systems. For the concentration- dependent micelle-to-vesicle transition in the didodecyldimethylammonium bromide (DDAB)/water system, in the region of coexistent micelles and vesicles, less than 3 mM, the β values increased significantly with DDAB concentration: β (0.07 mM) = 0.35 and β (3 mM) = 0.93. In the coexistent region, activities of the bromide ion, a Br , were almost independent of the DDAB concentration, suggesting the pseudo-phase nature of both micelles and vesicles. In the concentration-dependent vesicle-to-lamellar transition region above 5 mM, where multilamellar vesicles were prevailing, on the other hand, the β values were only little affected by this transition. This suggests that the increase in the layer number of DDAB multilamellar vesicles scarcely affects the β values. This was also supported by the fact that the destruction of multilamellar vesicles by ultrasonication did not change the β values. These results strongly suggest that the inner and outer monolayers of DDAB multilamellar vesicles are characterized by similar β values. The second system, cetyltrimethylammonium bromide (CTAB)/DDAB mixtures, showed composition-dependent transitions depending on the mole fraction of DDAB X DDAB : spherical micelles (0 <X DDAB < 0.2) → rodlike micelles (0.2 <X DDAB < 0.4) → vesicles (0.6 <X DDAB < 1). Values of β increased with both the sphere- to-rodlike micelle transition and the micelle-to-vesicle transition. The increase in the β value was larger for the latter than for the former transition. Little effects of ultrasonication on β was also observed for the multilamellar vesicles of CTAB/DDAB mixtures. In the third system of the mixtures of two single chain surfactants, protonated and non-protonated of oleyldimethylamine oxide, the dependence of β on the degree of ionization (α) can be well described with a theoretical curve, derived on the basis of the linear relation between the surface charge density and the α value, despite the occurrence of the micelle-to-vesicle transition.  2005 Elsevier Inc. All rights reserved. Keywords: Micelles; Vesicles; Counterion binding; Micelle–vesicle transition; Didodecyldimethylammonium bromide; Amine oxides 1. Introduction Surfactant molecules self-assemble into a variety of ag- gregate structures such as spherical micelles, wormlike mi- celles, vesicles, and bilayers. Shapes of surfactant aggregates are determined by geometrical structures of surfactant mole- cules, as discussed by Israelachvili et al. [1]. Conical mole- cules with a large head group attached to a single-chain tail form spherical micelles, while cylindrical molecules with a small head group attached to a double-chain tail form bilay- * Corresponding author. Fax: +81 92 681 8080. E-mail address: h.maescc@mbox.nc.kyushu-u.ac.jp (H. Maeda). ers. A simple dimensionless molecular parameter that con- nects the shape of the aggregates to the molecular geomet- rical structure is given as the surfactant packing parameter p (= v/al ) [1]; p< 1/3 (spherical micelles), p = 1/3–1/2 (cylindrical micelles), p = 1/2–1 (flexible bilayers and vesi- cle), and p = 1 (planar bilayers). Here v is the volume occu- pied by the alkyl chain, l is the length of the chain, and a is the area on the surface of a given aggregate. In ionic surfactant systems, the head groups of ionic ag- gregates repel each other and they occupy a large effective area a on the aggregate surface. As the ionic strength is in- creased, a decreases. As a result, p increases, and changes from spherical to cylindrical micelles can occur. The elec- 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.02.021