*2.2. Surface Pressure-Area Isotherm*

The surface pressure (Π)-Area (*A*) isotherm of h-DPPC was measured using a Langmuir trough (Kibron, Finland) with a maximum area of 166.4 cm2. The trough was carefully cleaned with ethanol and Milli-Q water before filling it with 120 mL of HKM buffer. Subsequently, the solution of h-DPPC with a concentration of 0.1 mg·mL−<sup>1</sup> was spread over the subphase using a Hamilton microsyringe (Hamilton Co., Reno, NV, USA) with a precision of 1 μL. After the chloroform was evaporated for about 20 min, the variation of surface pressure during three different compression/expansion cycles was recorded using a Wilhelmy plate made of filter paper and applying a barrier speed of 5 cm2·min<sup>−</sup>1. The same cleaning and experiment preparation methods were followed for the NR and ellipsometry measurements. In all the experiments performed in this work, the temperature of the subphase was maintained at 21.0 ± 0.5 ◦C. Finally, it should be noted here that parameters such as the compression rate, the spreading solvent, the geometry and the dimensions of the trough, the temperature, or the composition of the subphase can affect the resulting isotherm [38]. In view of this, we compare our data with other isotherms performed in a similar way, including a similar compression ratio and spreading solvent.

#### *2.3. Neutron Reflectometry Data Acquisition*

Neutron reflectometry (NR) experiments were performed on FIGARO, a time-of-flight reflectometer [27,39,40] at the Institut Laue-Langevin, Grenoble (France). Two different angles of incidence (θ<sup>1</sup> = 0.6◦ and θ<sup>2</sup> = 3.7◦) and a wavelength resolution of 7% dλ/λ were used, yielding a momentum transfer of 7 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>Å</sup>−<sup>1</sup> <sup>&</sup>lt; *<sup>Q</sup>*<sup>Z</sup> <sup>&</sup>lt; 0.25 Å<sup>−</sup>1, normal to the interface, and defined as follows:

$$Q\_{\mathbf{z}} = \frac{4\pi}{\lambda} \sin(\theta) \tag{1}$$

where λ is the wavelength of the neutron beam. Usually, reflectivity (*R*) is defined as the ratio of the intensity of the neutrons scattered from the air/water interface over the incident intensity of the neutron beam. The measured *R*(*Q*Z) profile is linked to an in plane-averaged scattering length density (SLD) profile perpendicular to the interface, which is a measure of the coherent scattering cross-section of the molecular species that constitutes each interfacial layer. The data were reduced using COSMOS [41]. Data of the samples were normalized to a measurement of pure D2O.

NR experiments were performed using HKM buffer prepared with a mixture 8:92 *V*/*V* % of D2O:H2O as a solvent, generally known as air contrast matched water (ACMW) since its scattering length density is equal to the one of air, i.e., equals to zero. h-DPPC, cm-DPPC (with an SLD of the aliphatic tails equals to zero), and d62-DPPC monolayers were prepared using a Langmuir trough (NIMA, Coventry, UK) with a total area of 354 cm2. The volume of HKM buffer used to fill the trough was 200 mL. We compressed the monolayer with a barrier speed of 10 cm2·min−<sup>1</sup> until <sup>Π</sup> <sup>=</sup> 30 mN·m−<sup>1</sup> and we used the pressure control to keep the pressure constant along the reflectivity measurements.
