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Counterion Effect on Krafft Temperature and Related Properties of Octadecyltrimethylammonium Bromide
Author(s) -
Sharker Komol Kanta,
Islam Md. Nazrul,
Das Shuvo
Publication year - 2017
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/s11743-017-1957-5
Subject(s) - chemistry , chaotropic agent , counterion , krafft temperature , pulmonary surfactant , micelle , inorganic chemistry , aqueous solution , ion , critical micelle concentration , molecule , organic chemistry , biochemistry
In the present work, we have investigated the effect of some counterions on the Krafft temperature ( T K ) and the micelle formation of octadecyltrimethylammonium bromide (OTAB) in aqueous solution. The results showed that the ions with more chaotropic nature increase the T K while those with a kosmotropic, hydrotropic and less chaotropic nature lower the T K of the surfactant. More chaotropic SCN − and I − , being weakly hydrated, form contact ion pairs with the octadecyltrimethylammonium ion and reduce the electrostatic repulsion between the surfactant molecules. As a result, these ions exhibit salting out behavior and raise the T K of the surfactant. On the other hand, less chaotropic Cl − and NO 3 − , kosmotropic SO 4 2− and F − and hydrotropic benzoate and salicylate ions increase the solubility of the surfactant, with a consequent decrease in the T K . SO 4 2− , F − , benzoate and salicylate cannot form contact ion pairs with the weakly hydrated cationic part of OTAB. Rather, being extensively hydrated and kosmotropic in nature, these ions do not show any tendency to shed their hydrated water molecules to form contact ion pairs with the weakly hydrated octadecyltrimethylammonium ion and therefore, stay apart. As a result, the T K of the surfactant decreases significantly in the presence of these ions. The critical micelle concentration (CMC) of the surfactant decreases significantly in the presence of these ions due to screening of the micelle surface charge by the added counterions. Consequently, the surfactant molecules attain better packing because of substantial reduction in the electrostatic repulsion between the charged head‐groups, showing a significant decrease in the CMC.