Effect of Electrolytes on the Krafft Temperature of Cetylpyridinium Chloride in Aqueous Solution
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This paper presents the effect of some electrolytes on the Krafft temperature (T K) of cetylpyridinium chloride in aqueous solution. The results show that more chaotropic anions raise while less chaotropic anions lower the T K of the surfactant. More chaotropic Br−, SCN− and I− form contact ion pairs with the cetylpyridinium ion and reduce the electrostatic repulsion between the surfactant molecules. As a result, these ions exhibit salting-out behavior, showing an increase in the T K of the surfactant. On the other hand, less chaotropic NO3 − increase the solubility of the surfactant, with a consequent decrease in the T K. Surface tension data of the salt solutions reveal that more chaotropic ions show a relatively less molar increase in surface tension compared to less chaotropic ions. This indicates that less chaotropic ions have a preferential tendency to be negatively adsorbed at the air–water interface as well as hydrocarbon–water interface and thereby disturb the hydration of the surfactant. SO4 2− being a strong kosmotrope cannot form contact ion pairs with the cationic part of the surfactant. Rather this ion preferentially remains in the bulk because of its strong tendency for hydration and thereby stays apart. As a result, SO4 2− also causes a significant lowering of the T K of the surfactant. Thus it appears that contrary to the usual trend SO4 2− behave like a chaotrope showing salting-in effect of the surfactant.
KeywordsKrafft temperature Hofmeister effect Kosmotropes Chaotropes Cationic surfactant
Authors thank the members of the Board of Postgraduate Studies for helpful discussion during the preparation of the research proposal. The financial assistance (CASR-236/32) approved by the Committee for Advanced Studies and Research (CASR), BUET for carrying out the present work is highly appreciated.
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