抄録
The antimicrobial activity of preservatives incorprated in oil-in-water emulsion systems has been stated to be controlled by the concentration of the undissociated free species in the aqueous phase. An ultrafiltration technique using Diaflo membrane was investigated in order to measure the free methyl-p-hydroxybenzoate (MP) in the aqueous phase of the oil-in-water emulsion. Free MP in the aqueous phase were separated from emulsions containing various amounts of MP and analyzed quantiatively. Free MP concentration in the aqueous phase from the ultrafiltration method coincided well with the results from the antimicrobial activity in the miorobiological method.
But, the ratio between free MP concentration in the aqueous phase and total MP concentration did not hold constant but depended upon total MP concentration.
Thus, the interaction of MP with polyoxyethylene lauryl ethers having a various number of oxyethylene unit was investigated by ultrafiltration and also nuclear magnetic resonance spectrometry. The results obtained suggested the existence of two kinds of interaction mechanisms between MP and the nonionic emulsifier micelle. The binding parameters in the each class were calculated using a multiple regression technique. In the primary class of sites, the interaction obeyed the form of the Langmuir type adsorption and the bound molecules of MP were probably located at the interface of the hydrocarbon core and the polyoxyethylene mantle of the micelle. Moreover, in the second class of sites, the interaction could be described as a simple partition between the micelles and the aquous phase and the preservative molecules seemed to be situated in the polyoxyethylene mantle.
Using these results, the theoretical equation including the interaction of MP and nonionic emulsifier micelles in oil-in-water emulsion. Free MP concentration in the aquous phase of emulsions containing various amounts of MP were calculated from this equation. The results calculated were in good agreement with those obtained from the ultrafiltration method.
