  In order to precisely quantify the fundamental interactions between
heterogeneous lipid membranes with coexisting liquid-ordered (Lo) and
liquid-disordered (Ld) domains, we performed detailed osmotic stress SAXS
experiments by exploiting the domain alignment in raft-mimicking lipid
multibilayers. Performing a Monte Carlo (MC) based analysis allowed us to
determine with high reliability the magnitude and functional dependence of
interdomain forces concurrently with the bending elasticity moduli. In contrast
to previous methodologies, this approach enabled us to consider the entropic
undulation repulsions on a fundamental level, without having to take recourse
to crudely justified mean-field like additivity assumptions. Our detailed
Hamaker coefficient calculations indicated only small differences in the van
der Waals attractions of coexisting Lo and Ld phases. In contrast, the
repulsive hydration and undulation interactions differed significantly, with
the latter dominating the overall repulsions in the Ld phase. Therefore,
alignment of like domains in multibilayers appears to originate from both,
hydration and undulation repulsions.
