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Lower consolute boundaries up to 50 vol% surfactant have been determined for noctylpenta(oxyethy1ene glycol) (C,E,) in the presence of the monovalent salts NaF, LiC1, NaCl, KC1, CsCl, NaBr and NaI. We find that the lower consolute boundaries are shifted, with only small changes in shape, to lower or higher temperatures (salting-out or salting-in, depending on the salt used). The amount and sign of the miscibility shift is determined almost solely by the anion, and the shift is related to the surface charge density of the ion. The shifts in the lower consolute boundary cannot be explained by structural changes in the bulk water structure due to the addition of salts. We have applied the Flory-Huggins lattice theory to relate the shifts of the lower consolute boundaries to changes in micelle-solvent interactions. This analysis shows that small changes in the free energy of interaction between oxyethylene and water can explain the shifts in temperature of the lower consolute boundary. We discuss the use and limitations of the Flory-Huggins theory, as applied to micellar phase separation. The salt effects are explained in terms of salt-deficient or salt-rich regions around the oligo-oxyethylene chains.

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The effects of NaCl and polyethylene glycol (PEG) 4000 on the lower consolution boundary (LCB) of a nonionic surfactant (C,E,) were studied and compared. Micellar systems where NaCl and PEG 4000 are present are often used in membrane protein crystallization. While sodium chloride shifts the surfactant LCB to lower temperatures without a significant change in the shape of the boundary, PEG produces a large solubility change strongly depending on the surfactant concentration.The salt effect is explained by a reduced interaction of the micellar oligooxyethyiene chains with the water and the PEG effect by an unfavourable configurational interaction between the C,E, micelles and PEG molecules. Detergent Crystallization Membrane protein Polyethylene glycol

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