  In this work, we present the first example of the self-assembly of
phospholipid monolayers at the interface between air and an ionic solvent. Deep
eutectic solvents are a novel class of environmentally friendly non-aqueous
room temperature liquids with tunable properties, that have wide ranging
potential applications and are capable of promoting the self-assembly of
surfactant molecules. We use a chemically-consistent Bayesian modelling of
X-ray and neutron reflectometry measurements to show that these monolayers
broadly behave as they do on water. This method allows for the monolayer
structure to be determined, alongside the molecular volumes of the individual
monolayer components without the need for water-specific constraints to be
introduced. Furthermore, using this method we are able to better understand the
correlations present between parameters in the analytical model. This example
of a non-aqueous phospholipid monolayer has important implications for the
potential uses of these solvents and for our understanding of how biomolecules
behave in the absence of water.
