The hydrolysis of C12 primary alkyl sulfates in concentrated aqueous solutions. Part 1. General features, kinetic form and mode of catalysis in sodium dodecyl sulfate hydrolysis
As part of an investigation into the observed rapid hydrolysis of sodium primary alkyl sulfates in commercial concentrated aqueous mixtures (typically 70% by weight of surfactant) at 80 °C, the rate of hydrolysis of sodium dodecyl sulfate (SDS) in water has been followed acidimetrically over a wide range of initial SDS concentrations in aqueous buffers, in the presence of added sulfuric acid (0.20 mol kg−1) and also in unbuffered, initially neutral solution. First order rate coefficients derived from the initial rates of sulfuric acid-catalysed reactions showed unexpected, non-monotonic variations with increasing initial [SDS] at constant [H2SO4] and with [H2SO4] at 70% SDS. Reactions in initially neutral solutions were found to have an autocatalytic form arising from the existence of both an uncatalysed and an acid catalysed pathway from reactants to products. Both pathways are characterised by rate coefficients (derived by computer simulation of the initial phase of the hydrolysis) that vary with the initial [SDS]; those for the acid-catalysed pathway show similar but less dramatic variation than observed in reactions in the presence of sulfuric acid. Possible reasons for the difference in behaviour are discussed. The autocatalysis observed in initially neutral solutions is shown to arise from the production of hydrogen sulfate ions during the hydrolysis, but experiments in buffer solutions at both high and low SDS concentrations show the characteristics of specific hydrogen ion catalysis. Solvent kinetic deuterium isotope effects on both pathways are, however, small. Examination of the dodecanol produced by SDS hydrolysis at low and high initial concentrations in 18O-enriched water showed no incorporation of the label, signifying exclusive S–O cleavage in the acid-catalysed pathway. It is argued that the results, taken in conjunction with literature data, are consistent with an SN2 displacement of sulfate ion by water in the uncatalysed hydrolysis pathway. While, for the hydrogen ion catalysed pathway, a previously suggested unimolecular cleavage of SO3 from dodecyl hydrogen sulfate, with concerted intramolecular proton transfer, appears more consistent with the observations, proton transfer concerted with direct transfer of SO3 to a preassociated water molecule is a plausible alternative.