XRR measurements were taken on I07 at Diamond Light Source, at 12.5 keV photon energy using the double-crystal-deflector. 4 The reflected intensity was measured in a momentum transfer range from 0.018 Å=1 to 0.7 Å=1. The data were normalised with respect to the incident beam and the background was measured from off-specular reflection and subsequently subtracted. Samples were equilibrated for at least one hour and preserved under an argon atmosphere to minimise the adsorption of water by the subphase.

XRR data were collected for each of the lipids, DLPC, DMPC, DPPC and DMPG at four surface pressures (DLPC: 20, 25, 30, and 35 mNm=1, DMPC: 20, 25, 30, and 40 mNm=1 , DPPC: 15, 20, 25, and 30 mNm=1, DMPG: 15, 20, 25, and 30 mNm=1), as measured with an aluminium Wilhelmy plate; all measurements were conducted at 22 ◦C. The aluminium Wilhelmy plate was used over a traditional paper plate due to the low wetability of paper by the DES.

The NR experiments were performed on FIGARO at the Institut Laue-Langevin using the time-of-flight method. 5 Data at two incident angles of 0.62◦ and 3.8◦ were measured to provide a momentum transfer range from 0.005 Å=1 to 0.18 Å=1 . Two surface pressures for each system and contrast was measured (DMPC: 20 and 25 mNm=1 , DPPC: 15 and 20 mNm=1 ). Similar to the X-ray procedure, samples were given enough time to equilibrate (at least two hours), kept under an inert atmosphere, and all measurements were conducted at 22 ◦C.