Emulsion formation and stability during reversed micelle extraction

Abstract

The formation of emulsions in the reversed micelle extraction of proteins from ®ltered broth is a major problem which can signi®cantly increase the cost of downstream separation. In this work the effect of some aqueous phase constituents such as buffer salts, other added cations, phospholipids and ®ve molecular-weight fractions (fractionated using ultra®ltration membranes) of a fermentation broth of Bacillus stearothermophilus on emulsion formation and stability of a water±AOT±isooctane reversed micelle extraction system were examined. The parameters used for the determination of emulsion stability were the settling time and interfacial tension, and this varied from 33 mN m À1 for pure water±isooctane to around 1 mN m À1 for high AOT concentrations (b100 mmol dm À3 ). K ions of the buffer solution, as well as K ions and Na ions from added salts had a destabilising effect on a water±AOT±isooctane emulsion. This effect was found to be greatest for the K ions, possibly due to their smaller hydrated radius compared with the other ions, and reduced the phase separation time with 50 mmol dm À3 AOT from 500 min to less than 5 min. The phospholipids phosphatidylcholine and phosphatidylinositol were also found to be effective demulsi®ers, with phosphatidylcholine having a slightly greater effect. Albumin and b-galactosidase both exhibited a strong emulsifying effect on a surfactant-free water±isooctane system, but were capable of strongly reducing emulsi®cation in the presence of AOT. High molecular weight fractions (b30 kDa) of the B stearothermophilus fermentation broth had a much stronger tendency to form emulsions than MW fractions`30 kDa, while the 30±50 kDa MW fraction had the best demulsifying effect, reducing the separation time to 75 min.Demulsifying properties of the components tested were attributed to the competition of these compounds with AOT for control of the aqueous±organic interface leading to a less stable mixed interfacial layer.