Investigation of over-compressed spread l-dipalmitoyl phosphatidylcholine films and the influence of solvent vapour in the gas phase on Π/ A isotherms measured by using the captive bubble technique

Abstract

Π/ A isotherms of spread l-DPPC ( l-dipalmitoyl phosphatidylcholine) layers were determined up to the over-compressed state by using the captive bubble method. The influence of solvent vapour in the gas phase on the interfacial behaviour of monolayers was studied as traces of the spreading solution usually stay in the bubble. This was done for two cases; the pendant drop technique with spread 1-monoglyceride monolayer, where the partial pressure of solvents remains constant, and the captive bubble with spread l-DPPC layers, where the partial pressure increases during monolayer compression. Two cases of spreading solvents are distinguished: solvents, which do adsorb at the interface (chloroform), and those, which do not (heptane). Using a model, which takes into account only a small number of accessible parameters, the influence of none adsorbing solvents can be sufficiently described by an increasing cohesion pressure when the partial pressure remains constant. The influence of polar solvents may be quantified by a factor, which considers the apparent increase of the minimum molecular area demand. Spread DPPC monolayers determined by the captive bubble technique show a plateau in the range of about 50 mN m −1, which depends on the amount of spreading solvent in the atmosphere of the bubble. By monolayer compression, the solvent molecules are squeezed out of the monolayer. Over-compression of the DPPC monolayer occur when all solvent molecules are removed and the surface tension further decreases down to almost zero. In contrast to monolayer collapse the surface pressure do not decrease abruptly for DPPC-layer expansion. This was explained by a monolayer-folding. Both, the process of squeezing out solvent molecules and the over-compression are accompanied by an apparent loss of DPPC molecules, indicated by a shift of the Π/ A isotherms to higher surface coverage. Monolayer cycling leads to a stepwise decrease of solvent molecules in the bubble atmosphere. The solvent molecules are assumed to be irreversibly incorporated into collapsed and lamellar structures, which may be formed by monolayer folds. Over-compressed collapsed or folded structures may be packed together and removed from the interface by forming visible particles. Lamellar structures are discussed as preliminary forms of liposomes, which may also incorporate solvent molecules, remove them during monolayer expansion from the interface, and causing an apparent loss of lipid molecules.