IJUP08 - Universidade do Porto

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.
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