Theory for the Equation of State of Phospholipid Monolayers

We present a simple theory for the interactions among the phospholipids in bilayers and monolayers over a wide range of surface densities, temperatures, and chain lengths. This work draws from three recent efforts: (i) lattice-based interphase theory which explicitly treats the chain conformations in three-dimensional interfacial phases of short-chain molecules, (ii) a recent “equation of state” treatment of van der Waals interactions which predicts volume changes, enthalpies, and temperatures of melting of the “rotator” phases of n-alkanes, and (iii) detailed study of the extensive measurements of Mingins et al. (Mingins, J.; Taylor, J. A. G.; Pethica, B.; Jackson, C. M.; Yue, B. Y. T. J. Chem. Soc., Faraday Trans. 1 1982, 78, 323) of pressure-area isotherms of phosphatidylcholines at the heptane/water interface which we have used to assess empirical characteristics of the head group interactions and of the “solid state”. This approach rationalizes pressure-area isotherms and phase transitions of monolayers of lecithins of chain lengths from 16 to 22 and predicts temperatures, areas, volume changes, and enthalpies of melting of the corresponding bilayers.