2) were lower than that of the pure SA monolayer, indicating that the mixed monolayers

were less condensed and more compressible than the pure monolayer. This is consistent with the plateau region existing in the π-A isotherm of every mixed see more system (Figure  1). Figure 3 Compressional modulus of SA/BSA monolayers vs surface pressure for discrete X BSA on pure water subphase at 26°C. The isotherm of pure SA showed two distinct regions: the first one corresponding to the monolayer in its liquid-condensed (LC) phase and the second one of a solid (S) film that was characterized selleck products by higher C s -1 values. The values of C s -1 obtained that were relatively high at X BSA = 0.1 are characteristic of a LC phase. The reason for this observation could be that at low concentrations of BSA, less lipid-protein interaction occurred in the mixed system. At surface pressure 30 and 35 mN m-1, C s -1 was observed to be below 50 mN m-1 for the entire range of BSA mole ratios, from X BSA ≥ 0.2 onwards, this being indicative of

the formation of the LE phase. This implied that the incorporation of BSA into the SA monolayers reduced their condensation. Molecular interactions can be expressed quantitatively in thermodynamic analysis. Total free energy of mixing ΔG mix is defined by the following equation: (4) where (5) and the excess free energy of mixing ΔG ex can be calculated from OICR-9429 π-A isotherms by [11, 17] (6) where A 12, A 1 and A 2 represent the area of the mixed system and

respective areas of components as 1 and 2, respectively, and π is the surface pressure of the monolayer. If the monolayer is ideally mixed, ΔG ex should be zero. Negative values of ΔG ex in the entire range of the monolayer composition indicated very strong attractions between molecules in the mixed system (Figure  4). The results showed that the binary SA/BSA mixed monolayers were thermodynamically stable. The most stable intermolecular interaction was observed at X BSA = 0.8, at discrete surface pressures, suggesting that SA interacted strongly with BSA molecules and were miscible in the system. This observation was supported by the A 12 and C s -1 measurements as discussed above. Figure 4 Free excess energy Δ G ex of SA/BSA monolayers vs X BSA on pure water subphase at 26°C. For discrete surface pressure of Urease 5 mN m -1 (diamond), 10 mN m -1 (circle), 15 mN m -1 (triangle), 20 mN m -1 (square) and 25 mN m -1 (right-pointing triangle). ΔG ex gradually decreased as the concentration of BSA rose. There was a slight recovery of ΔG ex at X BSA = 0.9. When the monolayer contained BSA only, ΔG ex was almost similar to X BSA = 0.9. This might be due to intermolecular repulsion occurring in the mixed monolayer system when the concentration of BSA was saturated in the system. The most compatible mixture of SA/BSA in a mixed monolayer was when X BSA = 0.8.