The negative ξ implies that there is an interaction energy between two kinds of molecules

which is higher than the mean energy of interaction among the same molecules. The interaction energy Δɛ was obtained according to Eq. (7) and the higher this value the more intense the interaction is. equation(7) −Δε=−ξRT6where RT is the product of the gas constant and the Kelvin temperature. The first pseudo-binary mixture analyzed was EPC and DOTAP. Fig. 1A presents the π–A isotherms for EPC/DOTAP mixed monolayers. The profiles are characteristic of expanded liquids, without phase transitions. The higher the DOTAP molar fraction, the more expanded the isotherm is and no profile modification is observed. The collapse pressure, πcol, varies Staurosporine cell line from 47 mN m−1, for pure EPC, to 37 mN m−1, pure DOTAP (with increasing DOTAP molar fraction) ( Table 1). A slight negative deviation of the molecular surface area additivity rule is observed through the non-linear course of the A12 vs. the monolayer composition ( Fig. 1B), depending on the surface compaction of the monolayer. At surface pressures lower than 20 mN m−1

the deviations are evident for DOTAP mol fractions above 0.6, while for higher Metformin surface pressures (20–30 mN m−1), they occur in the whole range of DOTAP concentrations. These deviations together with negative values of excess free energy of mixing, ΔGExc ( Fig. 1C) were interpreted as attractive interactions, confirming the miscibility between the lipid molecules. The ΔGExc reaches a minimum (−1 kJ mol−1), for 0.5 < XDOTAP < 0.7. The interaction parameter (ξ) and interaction energy (Δɛ) for EPC/DOTAP mixed monolayers are presented in Table 2. The ξ values are negative for monolayers with excess of EPC and positive in the case of DOTAP molar excess. The influence of monolayer composition on the EPC monolayer

was also evaluated from Cs−1–π curves, for different XDOTAP, as presented in Fig. 1D. The maximum value of Cs−1 is presented in Table 1 as a function of the monolayer composition. Fig. 1D and Table 1 indicate that EPC and DOTAP pure monolayers Protein tyrosine phosphatase present maximum Cs−1 values of 82.5 and 60 mN m−1, respectively. The increase of XDOTAP promotes the decay of Cs−1, in accordance with the mean area per molecule behavior observed in the isotherms of Fig. 1A. This also means that the addition of DOTAP makes the monolayer more compressible or less elastic, in the sense of surface Young’s modulus. The π–A isotherms for EPC/DOPE mixtures are presented in Fig. 2A. Again, only an expanded liquid phase could be observed without defined phase transitions. However, any EPC/DOPE mixture produced more expanded monolayers than the individual components. The maximum expansion occurred for XDOPE = 0.