title: The nature of the sodium dodecylsulfate micellar pseudophase as studied by reaction kinetics.
abstract: The nature of the rate-retarding effects of anionic micelles of sodium dodecyl sulfate (SDS) on the water-catalyzed hydrolysis of a series of substituted 1-benzoyl-1,2,4-triazoles (1a-f) has been studied. We show that medium effects in the micellar Stern region of SDS can be reproduced by simple aqueous model solutions containing small-molecule mimics for the surfactant headgroups and tails, namely sodium methyl sulfate (NMS) and 1-propanol, in line with our previous kinetic studies for cationic surfactants (     Buurma      et al.   J. Org. Chem.     2004  ,   69  ,   3899 - 3906  ). We have improved our mathematical description leading to the model solution, which has made the identification of appropriate model solutions more efficient. For the Stern region of SDS, the model solution consists of a mixture of 35.3 mol dm(-3) H(2)O, corresponding to an effective water concentration of 37.0 mol dm(-3), 3.5 mol dm(-3) sodium methylsulfate (NMS) mimicking the SDS headgroups, and 1.8 mol dm(-3) 1-propanol mimicking the backfolding hydrophobic tails. This model solution quantitatively reproduces the rate-retarding effects of SDS micelles found for the hydrolytic probes 1a-f. In addition, the model solution accurately predicts the micropolarity of the micellar Stern region as reported by the E(T)(30) solvatochromic probe. The model solution also allows the separation of the individual contributions of local water concentration (water activity), polarity and hydrophobic interactions, ionic strength and ionic interactions, and local charge to the observed local medium effects. For all of our hydrolytic probes, the dominant rate-retarding effect is caused by interactions with the surfactant headgroups, whereas the local polarity as reported by the solvatochromic E(T)(30) probe and the Hammett ρ value for hydrolysis of 1a-f in the Stern region of SDS micelles is mainly the result of interactions with the hydrophobic surfactant tails. Our results indicate that both a mimic for the surfactant tails (NMS) and a mimic for the surfactant headgroups (1-propanol) are required in a model solution for the micellar pseudophase to reproduce all medium effects experienced by a variety of different probes.
authors.affiliation: Physical Organic Chemistry Centre, School of Chemistry, Cardiff University, Cardiff, United Kingdom.
authors.forename: Lavinia
authors.initials: L
authors.lastname: Onel
authors.forename: Niklaas J
authors.initials: NJ
authors.lastname: Buurma