Penetration behaviour of alkylbetainate chlorides into lipid monolayers

In this paper, the penetration behaviour of the alkylbetainate chloride surfactants (CnBC, n = 10–16) into lipid monolayers of dipalmitoylphosphatidylserine (DPPS), dipalmitoylphosphatidic acid (DPPA), dipalmitoylphosphatidylethanolamine (DPPE), palmitoyoleoylphosphatidylcholine (POPC) and cholesterol (CHOL) is investigated using the Langmuir trough technique. The penetration of CnBC is followed by measurement of the surface pressure increase (Δπ) at a constant surface area after the injection of CnBC into the aqueous phase, underneath the lipid monolayer previously spread at the air–water interface at 25 °C and at different initial surface pressures (πi). The influence of both the lipid head group and the surfactant hydrocarbon chain length on the effectiveness of CnBC penetration into these monolayers is discussed. The results have shown that CnBC adsorb at the air–water interface giving evidence of their surface-active properties. The adsorption kinetics of C16BC into different lipid monolayers are lipid head charge and lipid head volume-dependent. The magnitude of the surface pressure increase (Δπ) arises in the following order: DPPA > DPPS ≫ CHOL ≈ DPPE > POPC. CnBC penetration into negatively-charged (DPPS and DPPA) monolayers does not seem to depend on surfactant alkyl-chain length compared to uncharged (CHOL) and zwitterionic (DPPE and POPC) monolayers for which Δπ increases with a larger alkyl-chain length. Electrostatic interactions are mainly involved in the affinity of CnBC with monolayers but the hydrophobic effect plays also a role.