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Enthalpies of micellization of quaternary tetradecyl- and cetyltrimethylammonium

par Paredes, Sergio ;Tribout, Marianne;Sepulveda, Luis
Référence Journal of Physical Chemistry (1952), 88, 9, page (1871-1875)
Publication Publié, 1984

Article révisé par les pairs

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Résumé :

Direct enthalpies of micellization (ΔHm°) of cetyltrimethylammonium salts (CTAX, X = OH-, F-, Cl-, Br-, and NO3-) and tetradecyltrimethylammonium hydroxide (TTAOH) have been measured by using a microcalorimeter method. The dilution of the detergent solution was carried out in the calorimeter chamber in where heats of dilution were detected. The chamber was fed with two peristaltic pumps, one pump delivered the solvent and the other the detergent solution whose concentration varied continuously. The concentration gradient was obtained by inserting a flask containing solvent in the detergent pump path followed by a flask containing the detergent solution at a concentration about ten times its critical micelle concentration (cmc). When the dilution in the calorimeter chamber is done with water and the resulting solution is above the cmc of the given detergent, the heat detected arises from the heat of demicellization and the heat of micellar dilution. On the other hand, when the dilution is done with a solution just above the cmc, the heat detected would only correspond to the heat of micellar dilution. The standard enthalpies of micellization (ΔHm°) obtained in this manner are discussed in terms of hydrophobic and electrostatic contributions. The ΔHm° values and the standard free energies of micellization obtained from the cmcs were used to calculate the standard entropies of micellization (ΔSm°). A linear trend between ΔSm° and ΔHm° was obtained which was interpreted in terms of dehydration of ions when they are transfered from the water to the micellar phase. It is concluded that the binding of small ions to micelles would be promoted mainly by entropic factors. It is also concluded that the thermodynamic behavior of micellar solutions is esentially determined by the nature and properties of the counterions. © 1984 American Chemical Society.