The hydrolysis of C12 primary alkyl sulfates in concentrated aqueous solutions. Part 2. Influence of alkyl structure on hydrolytic reactivity in concentrated aqueous mixtures of sodium primary alkyl sulfates: 1-benzoyl-3-phenyl-1,2,4-triazole as a probe of water activity

摘要

The kinetics of the hydrolysis of aqueous solutions of three sodium C12-alkyl sulfates (SXS), sodium 2-methylundecylnsulfate (SMS), sodium cycloundecylmethyl sulfate (SCS) and sodium 2-pentylheptyl sulfate (SPS), hasnbeen investigated at concentrations up to 70% and compared with the behaviour of sodium dodecyl sulfate (SDS).nThe same kinetic form as previously described for SDS was observed, namely, autocatalysis by protons generatednvia hydrogen sulfate ion, but there were substantial variations in the reactivity as the alkyl structure changed;nβ-branching reduced the reactivity, particularly for SMS which was the least reactive of the surfactants studied.nThe patterns of reactivity by the uncatalysed and hydrogen-ion catalysed pathways for the different SXS were rathernsimilar, but it is argued that the results are consistent with an SN2 mechanism for uncatalysed hydrolysis and thenconcerted SO3 cleavage (or transfer to a pre-associated water molecule)/proton transfer mechanism for the catalyticnroute, as previously proposed for SDS. Changes in the microenvironment of the sulfate group in aggregates formednfrom the different SXS are seen as being responsible for much of the rate variation. Attempts have been made tonestablish the dependence of observed rate constants in dilute solutions of SXS above the c.m.c. on the water activitynas indicated empirically by the rate of pH-independent hydrolysis of 1-benzoyl-3-phenyl-1,2,4-triazole (BPT) in thensame solutions. It appears, however, that BPT hydrolysis is not a useful guide to water activity in SXS solutions andnvalues of d(ln k)/d(ln [H2O]) are generally much larger than expected on the basis of simple ideas of transition statencomposition. The effects of surfactant aggregation on the microenvironment in which chemical reactions take placenare suggested to be the dominant kinetic influence both on SXS and BPT hydrolysis.