Understanding interactions between nanoparticles and model membranes is relevant to functional nano-composites and the fundamentals of nanotoxicity. In this study, the effect of polyamidoamine (PAMAM) dendrimers as model nanoparticles (NP) on the mesophase behaviour of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) has been investigated using high-pressure small-angle X-ray scattering (HP-SAXS). The pressuretemperature () diagrams for POPE mesophases in excess water were obtained in the absence and presence of G2 and G4 polyamidoamine (PAMAM) dendrimers (29 Å and 45 Å in diameter, respectively) at varying NP-lipid number ratio 0.0002-0.02) over the pressure range p = 1-3000 bar and temperature range T = 20-80 °C. The phase diagram of POPE exhibited the L β , L α and H II phases. Complete analysis of the phase diagrams, including the relative area pervaded by different phases, phase transition temperatures (T t ) and pressures (p t ), the lattice parameters (d-spacing), the pressure-dependence of d-spacing (d/p), and the structural ordering in the mesophase as gauged by the Scherrer coherence length (L) permitted insights into the size-and concentration-dependent interactions between the dendrimers and the model membrane system. The addition of dendrimers changed the phase transition pressure and temperature and resulted in the emergence of highly swollen lamellar phases, dubbed L β-den and L α-den . G4 PAMAM dendrimers at the highest concentration  = 0.02 suppressed the formation of the H II phase within the temperature range studied, whereas the addition of G2 PAMAM dendrimers at the same concentration promoted an extended mixed lamellar region in which L α and L β phases coexisted.

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