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A kinetic and thermodynamic study on hydrolysis of sodium laurate in aqueous phase accompanied by transfer into oil phases containing different organic additives (I).

Yamasaki M, Kurioka E, Nakamura S, Sugihara G, Isoda-Yamashita T

Daiichi College of Pharmaceutical Sciences, 22-1 Tamagawa-cho Minami-ku, Fukuoka 815-8511, Japan.

The kinetic and thermodynamic behavior at the interface between an aqueous solution of sodium laurate (NaLA) and various oil phases comprised primarily of benzene (Bz) and/or different organic compounds including amphiphiles has been investigated in regard to the hydrolysis of NaLA accelerated at the interface, transfer of lauric acid (LA) into oil phase and reverse transfer of Bz into aqueous phase in addition to interface tension. The contact of aqueous NaLA solution with the oil phase was found to accompany the mass transfer of LA and simultaneously promote the hydrolysis of NaLA in water phase. Analysis of the change of OH- ion concentration ([OH-]) over time allowed us to treat the events as a first order reaction. From the rate constant data the activation parameters such as the activation enthalpy and entropy, both of which control the transfer of LA molecules, were determined. The parameters were found to depend greatly on varied situations of the oil phase, being clearly able to explain the physicochemical behavior of the interface. Comparing the cases where the oil phase is one of the respective single systems such as Bz, dodecane (C12) and dodecylbenzene (C12Bz), C12Bz resulted in the lowest rate constant. The transfer (or hydrolysis) rate was measured for the amphiphile-added oil systems as a function of amphiphile concentration. When 0.206 M C16OH-Bz came in contact with aqueous phase, emulsion formation at the interface layer was brought about with approximately zero activation enthalpy, leading to facile or spontaneous transfer of LA. In addition, UV absorbance representing the transfer of Bz from the oil phase to the aqueous phase also demonstrated the effects of added amphiphiles on the action of the interface.

Published 10 October 2005 in Colloids Surf B Biointerfaces, 45(3): 189-99.
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