DOPE is one phosphatidylethanolamine, with small polar head when compared to the hydrophobic tail. This configuration leads to a molecular structure in the form of a truncate cone and its dispersion in water promotes the aggregation in the inverted hexagonal phase (HII) [27]. DOTAP is one of the most popular lipids available for transfection purposes. Its polar head has a propyl ammonium group, which promotes the cationic characteristic (monocationic). The results from EPC/DOTAP indicate the miscibility of

DOTAP in EPC monolayers, as noted in the isotherms and collapse pressure profiles (Fig. 1A and Table 1). The slight non-ideal mixture behavior was confirmed by the mean area per molecule curves (Fig. 1B), indicating the presence of attractive selleck chemicals forces at higher surface pressures. The intuitive idea of mixing DOTAP in EPC is that the electrostatic repulsion among the cationic polar head groups induces a lateral expansion of the lipid monolayer and the higher the DOTAP content, the more intense this effect

is, which explains the maximum compression modulus Cs−1 profiles in Fig. 1D and Table 1. We could observe that for XDOTAP in the range of 0.4–0.6 the Cs−1 values are Onalespib similar ( Table 1). An interesting behavior occurs for the analysis of excess free energy (ΔGExc) ( Fig. 1C). When XDOTAP is in the same range (0.4–0.6), the ΔGExc reaches a minimum. This behavior indicates that at this mixed monolayer concentration there is an optimum balance between the induced dipoles from the zwitterionic and cationic charges

from the polar headgroups. Additionally, we can observe that when XDOTAP is increased the ξ value increases from −0.79 (XDOTAP = 0.2) to 0.89 (XDOTAP = 0.8), suggesting that there is a kind of transition (at XDOTAP = 0.6–0.4) from the viewpoint of interaction energy. Similar studies of monolayers composed of a zwitterionic phosphatidylcholine and a cationic lipid were evaluated by Zantl et al. [13]. DMPC/DMTAP (dimyristoylphosphatidyl-choline/dimyristoyl-trimethyl-ammonium AMP deaminase propane) monolayers presented a minimum in the area per headgroup at mol fraction of about 0.5, using simultaneous small and wide-angle X-ray scattering [13]. In a similar study, Matti et al. [28] characterized mixed monolayers composed of cationic gemini surfactant, 2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethyl-ammonium)butane dibromide (abbreviated as SS-1, a divalent cationic lipid) and its mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using Langmuir monolayer studies. The authors identified that the minimum area per lipid was reached when XSS-1 = 0.38. Similar results were also obtained by molecular simulation [11] and [12]. In all of the above cases, the authors investigated synthetic lipids. Zantl et al. [13] studied mixed monolayers formed by lipids with the same hydrophobic tails and the studied effects are exclusively a consequence of hydrophilic headgroups interactions. Matti et al.