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Influence of Some Hofmeister Anions on the Krafft Temperature and Micelle Formation of Cetylpyridinium Bromide in Aqueous Solution
Author(s) -
Islam Md. Nazrul,
Sarker Khokan Chandra,
Sharker Komol Kanta
Publication year - 2015
Publication title -
journal of surfactants and detergents
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.349
H-Index - 48
eISSN - 1558-9293
pISSN - 1097-3958
DOI - 10.1007/s11743-014-1642-x
Subject(s) - chaotropic agent , chemistry , krafft temperature , hofmeister series , micelle , cetylpyridinium chloride , critical micelle concentration , pulmonary surfactant , counterion , bromide , inorganic chemistry , aqueous solution , ion , molecule , thermodynamics of micellization , organic chemistry , biochemistry
In this work, the effect of some Hofmeister anions on the Krafft temperature ( T K ) and micelle formation of cetylpyridinium bromide (CPB) have been studied. The results show that more chaotropic anions increase, while the less chaotropic ones lower the T K of the surfactant. More chaotropic I − and SCN − form contact ion pairs with the cetylpyridinium ion and reduce the electrostatic repulsion between the CPB molecules. As a result, these ions show salting‐out behavior, with a consequent increase in the T K . In contrast, less chaotropic Cl − and NO 3 − increase the activity of free water molecules and enhance hydration of CPB molecules, showing a decrease in the T K . A rather unusual behavior was observed in the case of SO 4 2− and F − . These strong kosmotropes shift from their usual position in the Hofmeister series and behave like moderate chaotropes, lowering the T K of the surfactant. Because of the high charge density and the strong tendency for hydration these ions preferentially remain in the bulk. Rather than forming contact ion pairs, these ions stay away from the CPB molecules, decreasing the T K of the surfactant. In term of decreasing the T K , the ions follow the order NO 3 − > SO 4 2− > Cl − > F − > Br − > SCN − > I − . The critical micelle concentration (CMC) of the surfactant decreases significantly in the presence of these ions due to the screening of the micelle surface charge by the excess counterions. The decreasing trend of the CMC in the presence of the salts follows the order SCN − > I − > SO 4 2− > NO 3 − > Br − > Cl − > F − .