Wettability of Binary Solid-Supported Films of Zwitterionic/Anionic Phospholipids

Abstract

The effect of different amounts of anionic phosphatidylglycerol in the zwitterionic phosphatidylcholine monolayers on their properties was quantitatively described in terms of the interactions between these two compounds as well as the changes of mixed monolayers' apparent surface free energy and its components. For all Langmuir films, surface pressure–area per molecule (π–A) isotherms were used to monitor the surface thermodynamics (i.e. the excess area and excess free enthalpy of mixing). This allowed to evaluate the kind and magnitude of interactions that have an influence on the phase behaviour and structural properties of the monolayers. Then, assuming that the molecular properties of Langmuir films are preserved upon transfer to a solid support, further characterizations were accomplished with advancing and receding contact angle measurements on the Langmuir–Blodgett films deposited at 35 mN/m on mica. These enabled the possibility to track changes in the wettability of the tightly packed or phase-separated phospholipid films and their surface free energy as a function of the monolayer composition. It is believed that these results throw a light on better understanding of the organizational structure of lipid films and their interactions with the surrounding environment. Moreover, the obtained results can be also helpful to identify the optimal lipid composition in the production of more effective therapeutic surfactants.