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Temperature and composition effects on the micelle formation by a catanionic surfactant: a surface tension and conductivity study R. F. Fernandes, B. F. B. Silva and E. F. Marques Centro de Investigação em Química, Department of Chemistry, University of Porto, Portugal. Surfactants are molecules with a polar headgroup, which favors contact with water (hydrophilic part), and an apolar tail, which avoids contact with the solvent (hydrophobic part). As a consequence, they have two important properties: they adsorb at polar/apolar interfaces, reducing the interfacial tension; they self-assemble in water into different kinds of aggregates, such as micelles and liquid crystals. The formation of micelles is a spontaneous and cooperative process that occurs at a given concentration, the critical micelle concentration (CMC), for constant temperature. For an ionic surfactant, the CMC is influenced by factors such as temperature, ionic strength and pH. For anionic/cationic surfactant mixtures, the mixing ratio is also a determinant parameter. Typically, there is a significant decrease of the mixture CMC relative to the CMC of the individual surfactants [1], a relevant property for practical applications (e.g. in detergent formulation). In this work, we have used surface tension and electrical conductivity techniques in order to investigate the micellization behavior of a newly synthesized surfactant, hexadecyltrimethylammonium octylsulfonate (TASo). This surfactant belongs to the class of catanionics and is obtained by the equimolar pairing of a long chain cationic molecule and a short chain anionic one, followed by removal of the remaining counterions (sodium bromide) [2]. TASo has several unusual and interesting properties, such as being watersoluble at room temperature, to yield micelles as the first aggregate, followed by spontaneous liposomes [3], a type of colloidal aggregates extremely useful in biomedical applications, such as drug delivery and gene therapy. We have investigated the effect of temperature, salt and composition (addition of excess ionic surfactant) on the CMC and the degree of counterion dissociation of the micelles of TASo. This allows the determination of the enthalpy, entropy and Gibbs function of micellization. A comparative study of the ionic surfactant sodium octylsulfonate was also carried out. It is observed that the CMC of TASo increases with temperature in a similar way to non-ionic surfactants. However, in all other properties the surfactant behaves like an ionic surfactant, e.g. the CMC decreases significantly with salt addition. This is rationalized by the fact that TASo acts as a weakly dissociated molecule [3], due to the much higher solubility of the shorter chain anionic component, leaving the micelles with a residual electrostatic charge. References: [1] Khan, A., Marques, E. F. (1997), Catanionic Surfactants, in Specialist Surfactants, Blackie Academic and Professional, London. [2] Silva, B.F.B. and Marques, E.F. (2005) Thermotropic behavior of an asymmetric chain length catanionic surfactant, J. Colloid Interface Sci., 290, 267-274. [3] Silva, B.F.B, Marques, E.F. and Olsson U. (2007) Lamellar Miscibility Gap in a Binary Catanionic Surfactant-Water System, J Phys. Chem B, 111, 13520-13526. 61
PARALLEL ORAL SESSIONS III – POS.III.1 THURSDAY, FEB. 21 III.1 - LIFE & HEALTH II Maria José Oliveira & Luís Miguel Madeira 26 Phenolic compounds, organic acids and antioxidant properties of Rumex induratus L. Guerra, C. Pereira, R. Gonçalves, P. B. Andrade, R. M. Seabra, P. Valentão MUC1 overexpression is associated with distant metastases development in canine mammary 37 carcinomas de Oliveira JT, Pinho S, Matos AJ, Lopes C, Barros R, Hespanhol V, Reis C, Gärtner F 42 Analysis of the efficiency of the mitotic checkpoints in glioblastoma cell lines A. Nascimento, E. Logarinho, O. Martins, R. M. Reis, H. Bousbaa Prenylated and bromoalkylated xanthones as potential antitumor agents: synthesis and biological 104 activities A. Paiva, E. Sousa, M. Pinto, A. Camões, N. Nazareth, M. S. J. Nascimento 157 P-cadherin: role in breast cancer cell migration and invasion Ribeiro AS, Paredes J, Correia AL, Schmitt F Impairment of retrograde signalling via adenosine in toxin-induced Myasthenia gravis: cross-talk 162 with muscarinic autoreceptors Diogo Trigo, Ana Sá-e-Sousa, Tiago Morais, Maria Alexandrina Timóteo, Teresa Magalhães- Cardoso, Laura Oliveira, Paulo Correia-de-Sá
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