Determination of the partition coefficients and absorption kinetic parameters of chemicals in a lipophilic membrane/water system by using a membrane-coated fiber technique.

The absorption kinetics of chemicals in a lipophilic membrane/water system was studied with a membrane-coated fiber (MCF) technique, in which the partition coefficient, membrane diffusivity and boundary layer adjacent to the membrane were taken into account. The cumulative amount permeated into the membrane was expressed as a function of absorption time in an exponential equation. Two constants were introduced into the model. Both of them were clearly defined by the physiochemical parameters of the system and were obtained by regression of the experimental data sampled over a limited time. The partition and diffusion coefficients, as well as the thickness of the boundary layer, were calculated from the two constants. The kinetic model adequately described the absorption kinetics of the MCF technique. All of the theoretical predictions were supported by the experimental results. The measured partition coefficients correlated well with the published octanol/water partition coefficient (R(2)=0.91). The thickness of the boundary layer was 5.2 microm in a solution stirred at 400 rpm. An inference of the kinetic model revealed that the contribution of the boundary layer to the absorption kinetics is significant for lipophilic chemicals by a lipophilic membrane. It suggested that the absorption rate of a very lipophilic compound could be controlled by the boundary layer even though the diffusivity of the compound in the membrane is lower than that in the solution. It was demonstrated that the MCF technique could be used to determine the partition, diffusion and permeation coefficients, as well as the thickness of the boundary layer in a lipophilic membrane/water system.