Effect of γ-Oryzanol on the LE–LC Phase Coexistence Region of DPPC Langmuir Monolayer

We have studied the effect of relative composition of γ-Oryzanol (γ-Or) on the liquid expanded–liquid condensed phase coexistence region in the mixed Langmuir monolayer of γ-Or and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) molecules at air–water interface. The surface manometry studies at a fixed temperature show that the mixture of γ-Or and DPPC forms a stable monolayer at air–water interface. As the relative composition of γ-Or increases the range of area per molecule over which the coexistence of liquid expanded (LE)–liquid condensed (LC) phases exists reduces. Although the LE–LC phase coexistence corresponds to the first-order phase transition, the slope of the surface pressure–area per molecule isotherm is non-zero. Earlier studies have attributed the non-zero slope in LE–LC phase coexistence region to the influence of the strain between the ordered LC phase and disordered LE phase. The effect of strain on the coexistence of LE–LC phases can be studied in terms of molecular density–strain coupling. Our analysis of the liquid condensed–liquid expanded coexistence region in the isotherms of mixed monolayers of DPPC and γ-Or shows that with the increase in the mole fraction of sterol in the mixed monolayer the molecular lateral density–strain coupling increases. However, at 0.6 mole fraction of γ-Or in the mixed monolayer the coupling decreases. This is corroborated by the observation of minimum Gibb’s free energy of the mixed monolayer at this relative composition of γ-Or indicating better packing of molecules. Graphical Abstract