Kinetic evidence for temperature-induced demixing of a long chain dioxolane in aqueous micellar solutions of sodium dodecyl sulfate: a new application of the pseudophase ion exchange model

Abstract

The effect of anionic micelles of sodium dodecyl sulfate (SDS) on the rates of acid-catalyzed hydrolyses of 2-(p-alkoxyphenyl)-1,3-dioxolanes with alkoxy groups of different chain lengths, methoxy (p-MPD), nonoxy (p-NPD), and tetradecoxy (p-TPD), were determined as a function of SDS and NaCl concentrations and temperature. First-order rate constants, k obs , were obtained from plots of ln(A∞-A) vs time that were linear for at least three half-lives. The k obs -[SDS] profiles were fitted by using the pseudophase ionexchange (PPIE) model and substrate binding constants, K s , and second-order rate constants for reaction in the micellar pseudophase, k 2m , were estimated from the simulations. Values of K s and k 2m for p-MPD and p-NPD are completely consistent with micellar effects on other acid-catalyzed hydrolyses in SDS and the assumptions of the PPIE model ; i.e. hydrolysis of monomeric substrate occurs in either the aqueous or the micellar pseudophases. However, kinetic profiles for p-TPD in SDS are more consistent with this substrate acting as a nonionic surfactant that does not mix ideally with SDS. The K s for p-TPD decreases about 800-fold as the temperature is increased from 25 to 50 °C. In 0.012 M SDS, k obs for p-TPD passes through a sharp maximum at 32 °C with increasing temperature whereas the plot for p-NPD is linear. At constant SDS concentration, added Nacl initially speeds the hydrolysis of p-TPD instead of inhibiting the reaction as observed for p-NPD and salt effects on other micellar-catalyzed reactions. All these results can be interpreted by assuming that increasing the temperature induces demixing of p-TPD and SDS to give two populations of mixed micelles each enriched in one of the surfactants and in dynamic equilibrium. Thus the PPIE model can be used to identify nonrandom distributions of reactive and nonreactive surfactants in aggregated systems.