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Thermodynamics of self-assembly of sodium octanoate: comparison with a fully fluorinated counterpart

2003; Taylor & Francis; Volume: 101; Issue: 21 Linguagem: Inglês

10.1080/00268970310001620177

1362-3028

Alfredo González‐Pérez, Gerardo Prieto, Juan M. Ruso, Félix Sarmiento,

Thermodynamic properties of mixtures

The isotherms of conductivity of sodium octanoate were measured and the critical micelle concentration (cmc) and degree of ionization of the micelles, β, determined in a range of temperatures (273–343 K) above the Krafft point. The thermodynamic parameters, Gibbs free energy ΔG m 0, enthalpy ΔH m 0, and entropy ΔS m 0 of micelle formation, were determined from polynomial adjustments of the temperature dependence of cmc and from a proposed thermodynamic model based on the works of Muller [1993, Langmuir, 9, 96] and Rodríguez et al. [2002, J. Colloid Interface Sci., 250, 438]. The increase in heat capacity upon micellization, ΔC pm 0, was estimated from the parameters of the model and the enthalpy—entropy compensation phenomena discussed. Finally, for information on their structural differences, hence to understand their different behaviours, thermodynamic parameters are discussed, comparing the corresponding fluorocarbon compound. A remarkable shift in minimum temperature in the U-shaped curve of cmc versus temperature was found when hydrogen was substituted by fluorine in the hydrophobic chain of the surfactant. This behaviour is a consequence of the special characteristics of the fluorine substituent in the hydrophobic tail and was reflected in the thermodynamic parameters and in the enthalpy—entropy compensation parameters, presenting different intercepts at the same compensation temperature.