  A great variety of biologically relevant monolayers present phase coexistence
characterized by domains formed by lipids in an ordered phase state dispersed
in a continuous, disordered phase. The difference in surface densities between
these phases originates inter-domain dipolar interactions, which are relevant
for the determination of the spacial distribution of domains, as well as their
dynamics. In this work, we propose a novel manner of estimating the dipolar
repulsion using a passive method that involves the analysis of images of the
monolayer with phase coexistence. The method is based on the comparison of the
pair correlation function obtained from experiments with that obtained from
Brownian dynamics simulations of a model system. As an example, we determined
the difference in dipolar density of a binary monolayer of DSPC/DMPC at the
air-water interface from the analysis of the radial distribution of domains,
and the results are compared with those obtained by surface potential
determinations. A systematic analysis for experimentally relevant parameter
range is given, which may be used as a working curve for obtaining the dipolar
repulsion in different systems.
