Kinetics of the acid-catalysed hydrolysis of dodecylsulphate and dodecyldiethoxysulphate surfactants in concentrated micellar solutions. Part 2.—Effects of added electrolytes on the hydrolysis of sodium dodecylsulphate and sodium dodecyldiethoxysulphate

Abstract

The effects of electrolyte additives upon the rates of acid-catalysed hydrolysis of sodium dodecylsulphate (SDS) and sodium dodecyldiethoxysulphate (SDE₂S) in relatively dilute micellar solutions (0.035 mol dm^–3) and for SDS also in concentrated micellar solutions (0.35 mol dm^–3) have been studied. In dilute micellar solutions all effects are inhibitory, with salts containing doubly charged cations (Mg and Ca) being more effective than those containing singly charged alkali and ammonium ions, for which the inhibition order is N(CH₃)₄ < Li < NH₄ < Na < K < Cs. Theoretical treatment in terms of the pseudo-phase ion-exchange theory has led to estimates of ion-exchange equilibrium constants for displacement of hydrogen ions from the micellar surface by other cations. There are very good linear correlations of the logarithms of these constants for each surfactant with the hydrated ion radii (Stokes' radii) for alkali ions, as expected from the electrostatic theory of ion binding to the highly charged micellar surface. Unexpectedly, the effect of electrolytes upon the micellar-catalysed acid hydrolysis of SDS in concentrated solutions (0.35 mol dm^–3) is not uniformly inhibitory. Potassium and caesium chlorides show a small inhibitory effect at low concentrations, but enhance rates at higher concentratrations. The order of inhibitory effects of alkali salts at low concentrations in these concentrated micellar solutions is the reverse of that observed in dilute micellar solutions. These effects can be explained if deviations from ideality of electrolytes in concentrated aqueous pseudo-phase are taken into account. The effects of salts containing doubly charged cations remain inhibitory, the sequence being Ni > Ca > Mg.