*2.1. Surface Potential Measurements*

DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine; 16:0 PC), DSPC (1,2-distearoylsn-glycero-3-phosphocholine; 18:0 PC), DAPC (1,2-diarachidoyl-sn-glycero-3-phosphocholi ne; 20:0 PC); DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine; 18:1 PC), egg SM (containing 86% of N-(hexadecanoyl)-sphing-4-enine-1-phosphocholine; 16:0 SM) and DPPE (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine; 16:0 PE) in purities > 99% were supplied by Avanti Polar Lipids. All the investigated phospholipids (except for DOPC) were investigated below their chain melting temperatures (specified in Figure 1). Ethanol (98%) and spectral grade chloroform (stabilized with ethanol) were delivered by Sigma-Aldrich. Spreading solutions for Langmuir experiments were prepared by dissolving each compound in chloroform or chloroform:ethanol (9:1) with a typical concentration of 0.2–0.3 mg·mL−<sup>1</sup> . In a standard experiment, 50–100 µL of the investigated solution was spread with a microsyringe (precise to ±2.5 µL). After spreading, the monolayers were left for 10 min for solvent evaporation before starting the compression at a barrier speed of 20 cm2/min. Deionized ultrapure water from a Millipore system with a resistivity of 18.2 MΩ·cm, pH 7, and a surface tension of 72.8 mN/m was used as a subphase. The subphase temperature (20 ◦C) was controlled to within 0.1 ◦C using a thermostat from Julabo. Experiments were carried out with a two-barrier Langmuir 612D NIMA trough (total area 600 cm<sup>2</sup> ) placed on an antivibration table. Surface pressure was measured with an accuracy of 0.1 mN/m using a Wilhelmy plate made of chromatography paper (Whatman Chr1) as the pressure sensor. Electric surface potential measurements were performed using a Kelvin probe (model KP2, NFT) mounted on a 612D NIMA trough. The vibrating plate was located ca. 2 mm above the water surface while the reference electrode, made from platinum foil, was placed in the water subphase. The surface potential measurements were reproducible to <sup>±</sup>15 mV and <sup>±</sup>2 Å<sup>2</sup> per molecule. Experimental results of surface pressure–area and electric surface potential–area isotherms presented here are representative curves selected from at least three overlapping experiments.
