Interfacial tension of double-chain cationic surfactants
The paper deals with surface and interfacial tension measurements for the cationic, double-chain N-alkyl-N-alkyl’-N,N-dimethyl-ammonium bromides. The surfactants CxCyDMABr, with the variable alkylchains Cx and Cy, are synthesized by a quaternization reaction of the alkyldimethylamine CxDMA with the alkylbromide CyBr in acetonitrile and purified by recrystallization in ether/methanol. The CMC for these surfactants was determined by surface tension measurements with a ring-tensiometer and by interfacial tension measurements with a spinning drop tensiometer.
The surface tension measurements show that the CMC of the surfactants decreases with increasing chain length. For surfactant concentrations c s>CMC the surface tension decreases further and shows a second break (CMCII). The CMCII is an indication of a change in micelle shape from globular to rodlike micelles.
The interfacial tension measurements show that the values of the interfacial tension at the CMC decreases with the increase of the chain length. The same effect as for the CMC could be found for the values of the interfacial tension of a 10 mM surfactant solution.
The plots of the interfacial tension versus the logarithm of surfactant concentration showed more than two breaks for some of these surfactants.
Key wordsDouble-chain cationic surfactants surface tension interfacial tension
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