APPROCHE THERMODYNAMIQUE ET CINETIQUE DE L EXTRACTION A DEUX PHASES AQUEUSES À L AIDE DE TENSIOACTIFS NON IONIQUES / A THERMODYNAMIC AND KINETIC APPROACH OF TWO-AQUEOUS PHASE EXTRACTION WITH NON-IONIC SURFACTANTS

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

The present work forms a part of the study of a two-aqueous phase extraction process using the occurrence of a cloud point (Tc) in the phase diagrams of water/polyethoxylated nonionic surfactant systems. Its main goals were: modelling and predicting the corresponding cloud point curves, investigating the mechanism controlling the mass transfer between phases as well as the kinetic phenomena related to phase separation and clarification. Cloud point curves were analyzed by applying the Flory-Huggins theory for polymer solutions, adapted to alcohol ethoxylates by Rupert. In most cases, the thermodynamic model fits the experimental values of Tc obtained for commercial surfactants rather well. The variations of interaction enthalpy and entropy as a function of carbon chain length and number of ethylene oxide units are also modelled. The kinetic study of the evolution of turbid H2O/Triton X114 mixtures towards equilibrium was carried out by means of an optical technique: following the transmission signal given by the Turbiscan Lab Expert as a function of time. Above the cloud point, two distinct phenomena are observed: phase separation with interface formation and stabilization, and phase clarification, during which tiny drops of one phase join the other after the interface has appeared. The effect of several parameters (temperature, surfactant and solute concentration) on the kinetics of phase separation was investigated. Phases separate faster with surfactant concentration reduction, temperature rise and increasing benzyl alcohol (solute) concentrations. As regards clarification, the Turbiscan diagrams reveal different behaviours according to the proximity of the cloud point. About 5C above Tc, the lighter (dilute) phase begins to clarify as the interface appears, the coacervate remaining turbid. The time required for complete phase clarification, i.e. corresponding to maximum (equilibrium) transmitted intensity, is 10 to 18 hrs. 10C above the cloud point, the two phases do not reach equilibrium after 2 days and the coacervate shows an oscillatory behaviour with a characteristic period of ca. 25 hrs.

ASSUNTO(S)

tensioactifs non ioniques polyéthoxylés point de trouble turbiscan lab expert cinétique de séparation et de clarification des phases phase separation and clarification kinetics extraction par coacervat non ionic polyethoxylated surfactant théorie de flory-huggins engenharia quimica coacervate extraction cloud point

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