Study of surface waves at the air-water interface covered by a monolayer

Abstract

We have studied the domain structures and the viscoelastic properties of charged lipid monolayers at the air-water interface with an adsorbed polyelectrolytes. Coupling of flexible (PAAS:polyacrylic acid sodium salt) and semiflexible ($\lambda$ DNA) polyelectrolytes onto an oppositely charged lipid monolayer (DOTAP:dioleoyl trimethylammonium propane chloride) at the air-liquid interface leads distinctive collapse behavior with lateral compression. In both cases, trilayer domains form by nucleation growth mechanism followed by multilayer formation with further compression. The flexible chain induces circular, fluid domains while the semiflexible chain induces elongated, solid-like domains. These morphological differences show that the flexibility of adsorbed polymers strongly affects the rigidity of the lipid membrane. Next, we focused on the effects of the chain length. We have obtained informations about domain structure, comprssibility, dilational and shear surface viscosities. Within a low frequency range $(200Hz \sim 400Hz)$,using externally exited surface wave spectroscopy, the change of chain length of PSS(poly styrene sulfonic acid with sodium salt) in the subphase affects damping constant in surface wave. However at high frequency $(2kHz \sim 7kHz)$,using heterodyne surface wave spectroscopy, this tendency is dissapeared. Kramer`s dispersion relation which is good agreement with low molecular weight surfactant molecules can not explain our data. In this picture, we obtained negative dilational viscosities. But Harden`s general dispersion relation for viscoelastic polymer film can explain. This means that the surface waves are highly affected by adsorbed polymer.