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spond to the real sequences of phases, and neither the thermotropic nor the
lyotropic behavior is interpreted meaningfully. This indicates that, while SCFT
is a robust method that should capture the structural complexity of this phases,
the physical model interpreting lipid-water as a block copolymer homopoly-
mer pair is much too simplistic to generate quantitative or even semiquantita-
tive agreement with experiments.
1.3.2
The SCFT of Water and Hydrogen-Bonded Lipids
The technical challenge of implementing SCFT for the latter types of systems
does not lie in either the theoretical framework or the numerical methods but
rather in accurate parameterization of the more complicated
interactions
and
short - scale molecular details
present in lipid-water mixtures. Specifi c compli-
cating factors in these systems are the presence of hydrogen bonding among
hydrophilic heads and water molecules, unsaturated bonds in the hydrocarbon
tails, and short tail length. Muller and Schick (1998) studied the equilibrium
self-assembly of a simple model of glycerolmonoolein in water using SCFT
techniques. The rotational isomeric state (RIS) model for representing a tail
structure was applied, and the head of glycerolmonoolein was treated as a rigid
rod. To obtain the single lipid chain partition function at fi xed fi eld Monte
Carlo simulations were applied. This stochastic method is computationally
very expensive (in comparison with the deterministic method of computing
partition functions described above), which made determining phase boundar-
ies diffi cult. Although the RIS model of Muller and Schick described the lipid
tail conformations quite realistically, including the presence of unsaturated
bonds, their results found limited agreement with the experimental phase
diagram (Muller and Schick, 1998; Qiu and Caffrey, 2000). It seems likely that
the neglect of specifi c interactions in the model related to head-group hydra-
tion and hydrogen bonding is responsible for this discrepancy. Nevertheless,
the Muller-Schick work is an impressive fi rst step toward extending SCFT
methods to food-grade systems.
While keeping the Gaussian chain approximation for the lipid tail, a step
toward a successful SCFT description of the self-assembly process in lipid-
water mixtures has been the recent work of Lee and co-workers, who have
been the fi rst to introduce the concept of reversible hydrogen bonds in the
energetic description of the lipid-water system (Lee et al., 2007, 2008). In that
work, a simplistic scenario is considered: The lipid and water can possibly
occupy two states: lipid and water unbound (state OFF) and lipid and water
bound (state ON). The two states differ in the sense that the volume of the
polar head, its enthalpy with the surrounding water (hydrophobic effect), and
that with the alkyl tail, all change depending on which of the two states is
occupied. The simultaneous statistical occurrence of ON and OFF states is
simply weighted by a Boltzmann term, with energy corresponding to the ener-
getic gain of one bound molecule of water. This physical approach has the
merit to describe in a very realistic way the self-assembly process but has also
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