CONICET | Buscador de Institutos y Recursos Humanos

In this work we analyze the Brownian motion of ceramide-enriched condensed domains immersed in a fluid sphingomyelin-enriched monolayer at the air-water interface. The diffusion coefficient of the domains is determined under different molecular packings and domain arrays, and the monolayer viscosity is calculated. With this approach, the effect of domain crowding and of intrinsic monolayer viscosity can be split. We found that for mixed monolayers of palmitoylated sphingomyelin and ceramide, the monolayer viscosity depends on the lateral pressure and on the domain-domain distance. In a 9:1 proportion of sphingomyelin:ceramide, the viscosity is about below 15 mN m-1 (continuum sphingomyelin phase in a liquid expanded state) and increases at higher lateral pressures (continuum phase in a condensed state). The viscosity change with pressure is caused by both, an increase of intrinsic viscosity and an increase in domain crowding. At very high domain crowding, the monolayer viscosity increases because the domains are held in place by steric hindrance generated by the other condensed domains of the array. These are short range forces, effective when the domains are close together. As the domain array is relaxed and the domain-domain distance increases, these forces become negligible, and repulsive dipolar interactions appear to acquire importance. For the lipid mixture analyzed, the dipolar repulsion is more noticeable on subphases of NaCl 0.15M than on pure water. This unexpected result can be explained on the basis of the surface potential of each film, since on NaCl solutions the value is greater than on pure water, which implies that the domain-domain dipole repulsion would be higher on NaCl than on water. This conclusion is also supported by the observation that on pure water subphases, the average domain size is 1.5 times larger than on NaCl solutions. We also performed an estimation of the potential energy of a dipole in a dipolar confinement that further supports this hypothesis.