Phosphatidylcholine Monolayer Formation at a Liquid:Liquid Interface as Monitored by the Dynamic Surface Tension

摘要

Dynamic surface tension experiments have monitored the rate of phospholipid monolayer formation at a planar aqueous:CCl4 interface from a solution of phosphatidylcholine (PC) vesicles. The rates at which monolayers form show that monolayer formation is a barrier controlled rather than a diffusion controlled process. At sufficiently low bulk PC concentrations, monolayer formation kinetics are first order in aqueous PC concentration. The kinetics can be described by a rupture mechanism which postulates that vesicles disintegrate at the aqueous:CCl4 boundary allowing all of the monomers to adsorb to the interface. When coupled to an appropriate two dimensional equation of state this mechanism quantitatively models the observed dynamic surface tension data. Solutions of vesicles in their liquid crystalline phase form tightly packed monolayers at concentrations above approx. 2 micro Molar, while solutions of gel phase vesicles form expanded monolayers regardless of bulk PC concentration. From the Arrhenius behavior of the dynamic surface tension data, we find that the barrier to monolayer formation is larger for solutions of liquid crystalline vesicles than for solutions of gel state vesicles. These phenomena - the extent of monolayer formation and Ea for monolayer formation - support a model which describes monolayer formation as a thermodynamically driven transformation from monomers within a bilayer to monomers adsorbed at the aqueous:CCl4 interface.