Model 1 consists of a single layer describing the whole phospholipid molecule, with a fixed thickness that is determined from the all-trans length of the DSPC molecule with a tail chain tilt of 30 • . The maximum tail group length is found as 24.3 Å from the Tanford equation , which results in a thickness of 21.0 Å when the tilt is considered and the head group is 10 Å from the work of Campbell et al., giving a total thickness of 31 Å. No interfacial roughness was considered in this model, and a single variable parameter was used, which is the volume fraction of the layer, ϕ. This parameter was given a uniform prior probability from [0.5, 1) , where the remainder of the volume fraction is made up of air and therefore offer a negligible scattering. The scattering length density, ρ was therefore calculated as,