A diffusion study on the ternary system, sodium cholate–sodium deoxycholate–water V. Vitagliano a , R. Sartorio a, ⁎, O. Ortona a , L. Paduano a , G. D'Errico a , F. Capuano b , G. Mangiapia c a Dipartimento di Chimica, Università degli Studi di Napoli “Federico II”, Complesso, Universitario di Monte Sant'Angelo, Via Cinthia, 80126 Napoli (NA), Italy b Istituto Tecnico Commerciale “Alberto Pitentino”, Via Torquato Tasso n. 5, 46100 Mantova, Italy c CSGI. “Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase” Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy abstract article info Available online 27 April 2010 Keywords: Mutual diffusion coefficients Ternary system Bile salts Mixed micelles The mutual diffusion coefficients, D ij , of the ternary system sodium cholate (NaC, component 1) + sodium deoxycholate (NadC, component 2) + water have been determined at five average compositions (c 1 , c 2 ) keeping c 1 constant and varying c 2 . The possibility to obtain expressions for the four diffusion coefficients in term of the micellization parameters and of the diffusivity of the species in solution is discussed. Data have been qualitatively interpreted. Results for the main term diffusion coefficients have been used in the much more difficult analysis of the cross term diffusion coefficients. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The bile salts are present in significant amounts in the bile fluid and have essentially the function to solubilize and transport lipophilic molecules as fatty acids and mono and diglycerides [1]. They derive from oxidation of cholesterol [2] and, containing well separated hydrophilic and hydrophobic moieties, behave as “special” surfac- tants. In fact, because of their steroid structure and of the spatial distribution of the hydrophilic groups, they have a hydrophobic face and the other hydrophilic. This is the reason why they can form aggregates quite different from the traditional micellar structures. Several studies, performed using a wide range of experimental technique, are present in literature on binary aqueous solutions of bile salts, and on ternary aqueous solutions of a bile salt and a hydrophobic molecule. The aim of these papers was essentially to clarify the self-aggregation of the bile salts and how their aggregates can solubilize guest hydrophobic molecules [3–24]. Regarding self-aggregation, the most accepted model [25–29] can be summarized as follows: in a first step two bile salt molecules form a back to back dimers, by stacking of their hydrophobic faces, then some dimers form, through hydrophilic “lateral” interaction (H-bonding), a “primary” disk-shaped micelle. For successive over- lapping of primary micelles driven by hydrophilic interaction, the secondary micelles are formed. According to this mechanism, the growth of the micelle is along one direction while the other micelle dimension is constant. Surprisingly only few papers are dedicated to the study of the properties of aqueous solutions containing two different bile salts even if mixed micelles should be, in respect to pure bile salt micelles, much more abundant in bile [30–33]. In this paper we present a diffusion study on a ternary system containing two different bile salts sodium cholate (NaC) and sodium deoxycholate (NadC) with the aim to get information on the transport properties of mixed micelles and to gain information on the micellization process. 2. Experimental section In the following all the quantities relative to sodium cholate will be indicated with the suffix “1”, those relative to sodium deoxyxcholate with the suffix “2”. 2.1. Materials Sodium cholate (NaC, stated purity N 98%, molecular weight 430.57) and sodium deoxyxcholate, (NadC, stated purity N 99%, molecular weight 414.58) were purchased from Aldrich (St. Louis, MO, USA) and Acros Organics (Geel, Belgium) respectively, and used without any further purification. Doubly distilled and degassed water was used for preparing all solutions; its molecular weight has been chosen as 18.016 g mol -1 . 2.2. Solution preparation All solutions were prepared by weight starting by concentrated stock binary solutions, NaC–H 2 O and NadC–H 2 O. 2.3. Density measurements To convert composition of solutions prepared by weight from molality to molarity scale, the density of all the solutions has been Journal of Molecular Liquids 156 (2010) 70–75 ⁎ Corresponding author. E-mail address: roberto.sartorio@unina.it (R. Sartorio). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.04.017 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq