M. Almgren, Mixed micelles and other structures in the solubilization of bilayer lipidmembranes by surfactants, BBA-BIOMEMB, 1508(1-2), 2000, pp. 146-163
The solubilization of lipid bilayers by surfactants is accompanied by morph
ological changes of the bilayer and the emergence of mixed micelles. From a
phase equilibrium perspective, the lipid/surfactant/water system is in a t
wo-phase area during the solubilization: a phase containing mixed micelles
is in equilibrium with bilayer structures of the lamellar phase. In some ca
ses three phases are present, the single micelle phase replaced by a concen
trated and a dilute solution phase. In the case of non-ionic surfactants, t
he lipid bilayers reach saturation when mixed micelles, often flexible rod-
like or thread-like, start to form in the aqueous solution, at a constant c
hemical potential of the surfactant. The composition of the bilayers also r
emains fixed during the dissolution. The phase behavior encountered with ma
ny charged surfactants is different. The lamellar phase becomes destabilize
d at a certain content of surfactant in the membrane, and then disintegrate
s, forming mixed micelles, or a hexagonal phase, or an intermediate phase.
Defective bilayer intermediates, such as perforated vesicles, have been fou
nd in several systems, mainly with charged surfactants. The perforated memb
ranes, in some systems, go over into thread-like micelles via lace-like str
uctures, often without a clear two-phase region. Intermediates in the form
of disks, either micelles or bilayer fragments, have been observed in sever
al cases. Most noteworthy are: the planar and circular disks found in syste
ms containing a large fraction of cholesterol in the bilayer. Bile salts ar
e a special class of surfactants that seem to break down the bilayer at low
additions. Originally, disk-like mixed micelles were conjectured, with pol
ar membrane lipids building the disk, and the bile salts covering the hydro
phobic rim. Later work has shown that flexible cylinders are the dominant i
ntermediates also in these systems, even if the disk-like structures have b
een re-established as transients in the transformation from mixed micelles
to vesicles. (C) 2000 Elsevier Science B.V. All rights reserved.