Alkaline hydrolyses of ester-linked detergents in the solution phase and at the air–water interface as studied by surface-tension measurements

Abstract

Alkaline hydrolyses of the anionic (Aerosol OT, AOT) and cationic (3-lauroxy-N-trimethylanilinium iodide, LANTI) detergent esters in the solution phase, and of the monolayer-forming ester N,N-dimethyl-m-aminophenyl-laurate (DAPL) at the air–water interface, have been studied by a new technique using surface-tension measurements. Second-order rate constants (k₂) for alkaline hydrolysis are 0.018 dm³ mol^–1 s^–1 at 22 °C and 4.6 dm³ mol^–1 s^–1 at 25 °C for AOT and LANTI, respectively. The value k₂ for LANTI obtained from independent spectrophotometric measurements, 4.0 dm³ mol^–1 s^–1, agrees excellently with that given by the surface-tension technique. The surface pressure (depression of surface tension at the monolayer phase) of DAPL decreases in the course of the reaction and reaches zero when the reaction is complete. First- and second-order rate constants for the hydrolysis of DAPL in the presence of salts at a concentration of 1 mol dm^–3 increase in the order LiCl < KCl < NaCl < NaI. The salt penetrates into the monolayers and enhances the reactivity. This new and convenient method should aid future studies on the kinetic analyses of detergent-type esters, which are difficult to carry out by conventional spectrophotometric methods.