Many biologically important monolayers show coexistence of discrete domains of long-range ordered condensed phase dispersed in a continuous, disordered, liquid-expanded phase. In this work, we studied the viscous and elastic compressibility modulus and the shear viscosity of monolayers exhibiting phase coexistence with the aim of elucidating the contribution of each phase properties on the observed monolayer properties. With this purpose, mixed monolayers of different proportions of distearoilphosphatidylcholine (dspc) and dimiristoilphosphatidylcholine (dmpc) were prepared and their rheological properties were analyzed.
We find that the isothermal compressibility modulus shows a non-ideal behavior with the mole fraction. At 10 mN m-1, the compressibility of the mixed monolayer is independent of the dspc amount up to about 60% of dspc (40% of the monolayer area occupied by the condensed phase). For higher dspc content, the modulus increases linearly.
The diffusion coefficient of the domains inserted in the monolayer, which depends on the shear viscosity, decreases to zero as the content of dspc is increased in the mixture. Both independent measurements are consistent because when the domain crowding is high, domains are unable to exchange nearest neighbors, becoming trapped within a cage formed by the neighbors. In these conditions, domain-domain repulsion prevails over the thermal entropy and the monolayer becomes less compressible.
On the other hand, we found that the increase in the compressibility modulus at high dspc content is related to an increase in the loss component, which is also in agreement with the decrease in the domain Brownian movement and the increase in domain-domain interaction.
