The structure of mixed monolayer films of DPPC and hexadecanol

Although dipalmitoyl phosphatidycholine (DPPC) is the primary surface active component of natural lung surfactant (NLS), under physiological conditions it does not adsorb rapidly to the interface or respread efficiently or rapidly after compression beyond film collapse. Other materials must be present to enhance these functions and one synthetic exogenous surfactant used in replacement therapy mixes DPPC with hexadecanol and tyloxapol. In the present study the interactions of DPPC and hexadecanol in spread monolayers have been investigated by film balance measurements, and Brewster angle microscopy. Two phases can be observed when the surface pressure is low and the mole fraction of DPPC is high: a condensed phase of hexadecanol with some DPPC and an expanded phase. Increasing surface pressure generates a second condensed phase that is DPPC rich. On further compression the expanded phase disappears and the two condensed phases merge, but miscibility rules indicate that the expanded phases do not mix even though there is no evidence for two phases in the experimental data.