Cationic Liposomes in Mixed Didodecyldimethylammonium Bromide and Dioctadecyldimethylammonium Bromide Aqueous Dispersions Studied by Differential Scanning Calorimetry, Nile Red Fluorescence, and Turbidity Eloi Feitosa,* ,† Fernanda Rosa Alves, † Anna Niemiec, ‡ M. Elisabete C. D. Real Oliveira, § Elisabete M. S. Castanheira, § and Adelina L. F. Baptista § Physics Department, Sa ˜ o Paulo State UniVersity, Sa ˜ o Jose ´ do Rio Preto, SP, and Instituto de Quı ´mica, UniVersidade Estadual de Campinas, Campinas, SP, Brazil, and Physics Department, UniVersity of Minho, Campus de Gualtar, 4710-057 Braga, Portugal ReceiVed NoVember 30, 2005. In Final Form: February 14, 2006 The thermotropic phase behavior of cationic liposomes in mixtures of two of the most investigated liposome- forming double-chain lipids, dioctadecyldimethylammonium bromide (DODAB) and didodecyldimethylammonium bromide (DDAB), was investigated by differential scanning calorimetry (DSC), turbidity, and Nile Red fluorescence. The dispersions were investigated at 1.0 mM total surfactant concentration and varying DODAB and DDAB concentrations. The gel to liquid-crystalline phase transition temperatures (T m ) of neat DDAB and DODAB in aqueous dispersions are around 16 and 43 °C, respectively, and we aim to investigate the T m behavior for mixtures of these cationic lipids. Overall, DDAB reduces the T m of DODAB, the transition temperature depending on the DDAB content, but the T m of DDAB is roughly independent of the DODAB concentration. Both DSC and fluorescence measurements show that, within the mixture, at room temperature (ca. 22 °C), the DDAB-rich liposomes are in the liquid-crystalline state, whereas the DODAB-rich liposomes are in the gel state. DSC results point to a higher affinity of DDAB for DODAB liposomes than the reverse, resulting in two populations of mixed DDAB/DODAB liposomes with distinctive phase behavior. Fluorescence measurements also show that the presence of a small amount of DODAB in DDAB-rich liposomes causes a pronounced effect in Nile Red emission, due to the increase in liposome size, as inferred from turbidity results. Introduction The mixture of lipids in solution is a promising field of research, since it can be used to monitor the structure and phase behavior of lipid mixtures suitable for specific applications in science and technology; such applications may require sample preparation with well-controlled properties. 1-5 The mixture of lipids in solution can thus be used to modify the phase behavior of the individual lipids by varying the lipid composition in the mixture and monitoring some physical properties of these mixtures. Mixtures of lipids can also be used to control specific properties of the aggregates by investigating the interaction with other systems. For example, to overcome the precipitation problems related to the strong interaction of polyelectrolytes with oppositely charged lipids, a colipid with neutral or opposite charge is often added to the main lipid, to reduce electrostatic effects. Homologue lipids can also be mixed to change properties such as the aggregate size and architecture and chain conformation in the lipid aggregates. The homologue double-chain liposome-forming cationic lipids dioctadecyldimethylammonium bromide (DODAB) and didodec- yldimethylammonium bromide (DDAB) have been some of the most investigated cationic lipids, 6 and liposome formation in aqueous solutions of these lipids is extensively reported, 7-9 but the literature is scarce on the liposome formation in aqueous mixtures of these surfactants. The difference in chain length of these lipids (C 18 and C 12 ) yields interesting characteristics and behavior for mixed solutions of these lipids, as shown in this paper. It has been reported that micelle-forming nonionic surfactants reduce 10 whereas cationic surfactants tend to increase 11 the T m of DODAB. 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