Myelin membranes spread as Langmuir monolayers at the air/water interface. They show coexisting domains whose morphology depends on compression state and ionic character of the subphase. The equilibrium domain radius was estimated from the transition from circular to elliptical shapes (1) to be ~70 µm (2). It depends on the competition between line tension (λ) and dipolar repulsion inside the domains which is proportional to the dipolar density difference (µ2) between the phases. In this work we focus on the determination of both parameters.
Line tension was indirectly inferred for myelin lipid domains, rendering extremely low values (fN range) (3). We directly measured λ by shape relaxation methods (4) and found it within 0.65-0.95 pN depending on subphase conditions. Some authors ascribe ë to the difference in height between the lipid phases. In order to estimate the difference in height we measured the reflectivity by Brewster angle microscopy.
The other force that competes with line tension is dipolar repulsion. We measured the restricted Brownian motion of trapped domains and, from the distribution of positions; we calculated µ2 between the phases and found it ranging between 10-40 fN. Having both, λ and µ2, an equilibrium radius may be approximated.