Fourier transform infrared investigation on the state of water in reverse micelles of quaternary ammonium gemini surfactants C 12- s-C 12⋅2Br in n-heptane

Abstract

The state of water in the reverse micelles of C 12- s-C 12⋅2Br homologues has been investigated by Fourier transform infrared spectroscopy. The results showed that the solubilized water had four states: the quaternary ammonium head-group-bound, the Br −-bound, the bulklike, and the free water. With increasing W 0 , the number of bulklike water per surfactant ( n b ) rapidly increased, which indicated swelling of the reverse micelle. The number of the head-bound water per surfactant ( n N + ) gradually increased. This was attributed to a reduction of the interfacial curvature, which permitted more water molecules to associate with the ionic heads of surfactants and also led to a part of n-hexanol being expelled from the interface and thus water filled up. Owing to the existence of n-hexanol in the interface, the head-bound water of the present system was smaller than that of AOT system at the same W 0 . The number of counterion-bound water per surfactant ( n Br − ) remained unchanged with W 0 . This was due to much smaller dissociation of the head of C 12-2-C 12⋅2Br than that of AOT. With increasing s, unchanged n N + is attributed to the comprehensive effects of enlarged head, which promotes the hydration, increased ionization degree, and reduced size of the water pool. Owing to increased ionization degree, n Br − increases with s.