Determination of the hydrolysis rate of AOT in AOT–isooctane–water microemulsions using sodiumnitroprusside as chemical probe

The alkaline hydrolysis of sodium nitroprusside, [Fe(CN) 5 NO] 2− , to yield [Fe(CN) 5 NO 2 ] 4− was studied in water in oil (w/o) AOT–isooctane–water microemulsions [AOT = sodium bis(2‐ethylhexyl) sulphosuccinate]. Kinetic data show an initial increase of absorbance at 415 nm, due to the formation of [Fe(CN) 5 NO 2 ] 4− . Following this initial increase, the absorbance started to decrease after a period of time which depends on the particular experimental conditions and returned to the initial value. This anomalous behaviour was interpreted as a result of competitive AOT hydrolysis consuming HO − ions and regenerating [Fe(CN) 5 NO] 2− . The influence of the microemulsion composition on the kinetic results was interpreted in terms of pseudophase formalism, assuming a uniform distribution of HO − and [Fe(CN) 5 NO] 2− ions in the aqueous pseudophase. Analysis of the absorbance–time profiles enables us to obtain the second‐order rate constants for hydrolysis of [Fe(CN) 5 NO] 2− in the water droplet, $\font\smc=cmr10 scaled 800\def\sc#1{\hbox{\smc #1}} = k_{1}^{\rm NP} = 0.108\,\sc{M}^{-1}\hbox{ s}^{-1}$ (half that reported in pure water) and the second‐order rate constant for hydrolysis of the surfactant (AOT), $\font\smc=cmr10 scaled 800\def\sc#1{\hbox{\smc #1}} = k_{2}^{\rm AOT}=4.4\times 10^{-4} \, \sc{M}^{-1} \hbox{ s}^{-1}$ , in the microemulsion. Copyright © 2002 John Wiley & Sons, Ltd.