Emulsion formation and stability during reversed micelle extraction

The formation of emulsions in the reversed micelle extraction of proteins f rom filtered broth is a major problem which can significantly increase the cast of downstream separation. In this work the effect of some aqueous phas e constituents such as buffer salts, other added cations, phospholipids and five molecular-weight fractions (fractionated using ultrafiltration membra nes) of a fermentation broth of Bacillus stearothermophilus on emulsion for mation and stability of a water-AOT-isooctane reversed micelle extraction s ystem were examined. The parameters used for the determination of emulsion stability were the settling time and interfacial tension, and this varied f rom 33mNm(-1) for pure water-isooctane to around 1mNm(-1) for high AOT conc entrations (>100mmoldm(-3)). K+ ions of the buffer solution, as well as Kions and Na+ ions from added salts had a destabilising effect on a water-AO T-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 50mmol dm(-3) AOT from 500min to less than 5min. The phospholipids phosphatidylcholine and phosphatidylin ositol were also found to be effective demulsifiers, with phosphatidylcholi ne having a slightly greater effect. Albumin and beta-galactosidase both ex hibited a strong emulsifying effect on a surfactant-free water-isooctane sy stem, but were capable of strongly reducing emulsification in the presence of AOT. High molecular weight fractions (>30M) of the B stearothermophilus fermentation broth had a much stronger tendency to form emulsions than MW f ractions <30kDa, while the 30-50kDa MW fraction had the best demulsifying e ffect, reducing the separation time to 751min. Demulsifying properties of t he components tested were attributed to the competition of these compounds with AOT for control of the aqueous-organic interface leading to a less sta ble mixed interfacial layer. (C) 2000 Society of Chemical industry.