Effect of the nature of the counterion on the properties of anionic surfactants. 3. Self-association behavior of tetrabutylammonium dodecyl sulfate and tetradecyl sulfate: Clouding and micellar growth

The surfactants tetrabutylammonium dodecyl sulfate (TBADS) and tetradecyl sulfate (TBATS) have been synthesized by ion-exchange, and their self-association behavior has been investigated. The solutions of these surfactants show clouding and phase separation as the temperature is increased. The clouding temperature T-c of TBADS solutions has been found to be about 4-5 degreesC higher than for TBATS solutions in the whole range of composition up to a surfactant content of 64 wt %. The critical micellization concentration (cmc) of TBATS has been determined using the electrical conductivity method. The micelle aggregation numbers (N) have been determined using the time-resolved fluorescence quenching (TRFQ) method, with pyrene/dodecyl-pyridinium chloride as fluorescent probe/quencher pair. For the two surfactants, N increases with the surfactant concentration and above a threshold concentration with the temperature. This latter increase as well as the phase separation observed for TBADS and TBATS are very unusual features for ionic surfactants. At temperatures approaching Tc from below, the TRFQ data show evidence of quencher/probe migration between micelles. This process is shown to occur via collisions between micelles. Approximate calculations indicate that the maximum number of TBA(+) ions that can be packed at the TBADS micelle surface is smaller than the experimentally determined value of the number of bound TBA(+) ions. This result suggests that some bound TBA(+) ions must be located in an outer and probably incomplete second layer of bound TBA(+) ions. The origin of the attractive intemicellar interaction responsible for the observed phase separation is discussed on the basis of the existence of the second layer of bound TBA(+) ions and the capacity of TBA(+) ions to self-associate in water.