Colloids and Surfaces B: Biointerfaces 75 (2010) 34–41 Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb Thermodynamic and elastic fluctuation analysis of langmuir mixed monolayers composed by dehydrocholic acid (HDHC) and didodecyldimethylammonium bromide (DDAB) Paula V. Messina a,∗ , Gerardo Prieto b , Juan Manuel Ruso c , Marcos D. Fernández-Leyes a , Pablo C. Schulz a , Félix Sarmiento b a Departamento de Química, Universidad Nacional del Sur, CONICET-INQUISUR, Av Alem 1253, (8000) Bahía Blanca, Argentina b Biophysics & Interfaces Group, Departamento de Física Aplicada, Facultade de Física, Universidad de Santiago de Compostela, (15782) Santiago de Compostela, Spain c Soft Matter and Molecular Biophysics Group, Departamento de Física Aplicada, Facultade de Física, Universidad de Santiago de Compostela, (15782) Santiago de Compostela, Spain article info Article history: Received 22 October 2008 Received in revised form 5 August 2009 Accepted 5 August 2009 Available online 13 August 2009 Keywords: Dehydrocholic acid DDAB Langmuir-monolayers ADSA Thermodynamic parameters Static elasticity modulus abstract The physicochemical and elastic properties of Langmuir mixed monolayers composed by dehydrocholic acid (HDHC) and didodecyldimethylammonium bromide (DDAB) were evaluated. The experiments were performed with a constant surface pressure penetration Langmuir balance based on Axisymmetric Drop Shape Analysis (ADSA). The behavior of such amphiphiles in monolayer was clearly non-ideal and would be seriously influenced by the amount of HDHC molecules present. The presence of bile acid type molecules caused the monolayer be more condensed (A c diminution) and the intermolecular attractive interactions be stronger (high ε 0 values). This fact would be related to H-bond formation between water and carboxilate and carbonile groups in the cholesteric ring and agreed with the existence of laterally structured microdomains at the monolayer (determined by the analysis of the first virial coefficient, b 0 < 1, of the state equation). The miscibility of both surfactants in the monolayer, their high bulk hydrophobicity ( c > 35 mJ m -2 ) just with the obtained negative values of the free energy of mixing G mix , and the excess second virial coefficient (b 1 ) E obtained allows us to infer that net attractive interaction existed between HDHC and DDAB molecules at the monolayer and that mixed systems would be able to be used in the formulation of supramolecular assemblies. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Bile salts with their rigid ring structures are in many respects unusual surfactants, and are usually described as having both a hydrophilic and a hydrophobic faces. The physical chemistry of bile salts micellization has been, and still is, an active area of research. The high specificity and capacity for bile salts trans- port systems during their enterohepatic circulation might form the basis of a current research on drug–bile acids delivery for specific drug targeting to the liver and for improving the intestinal adsorp- tion of poorly adsorbed or non-adsorbed drugs, such as peptides [1,2]. A variety of techniques have been employed to gain more insight into the structure/size/shape of bile salts aggregates. Sev- eral studies which involved the interactions of such surfactants in mixtures with cationic amphiphiles have been performed and ∗ Corresponding author. Tel.: +54 291 4551447; fax: +54 291 4551447. E-mail address: pau423ve@yahoo.com.ar (P.V. Messina). different types of supramolecular assemblies have been obtained [3]. Swanson-Vethamuthu et al. [4] have studied the behavior of sodium cholate (NaC, with three hydroxyl groups on the hydropho- bic face) and sodium desoxycholate (NaDOC, with two hydroxyl groups) in combination with C 16 TAB and C 16 TAC. In such mixed sys- tems NaC gave mixed micelles that decrease monotonously in size, whereas NaDOC induced a transition to rod-like micelles, at close to equimolar concentration a separation into two phases occurred [4]. According to that, the aim of this article is the evaluation of the physicochemical and elastic properties of Langmuir mixed mono- layers composed by a bile salt type (dehydrocholic acid, HDHC) and a double tailed cationic surfactant (didodecyldimethylammonium bromide, DDAB) as a previous step in their use as structural units for the construction of new delivery systems. The selection of both amphiphiles was because of their biocompatibility [5,6] and of their packing parameters [7]. Although a bilayer cannot simply be considered to consist of two independent monolayers, the head group–water interface in the two systems may be sufficiently similar to make monolayers 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2009.08.008