Phase Separation and Domain Interaction in Pheophytin Containing Monolayers

Surface pressure and fluorescence intensity of pheophytin containing lipid-monolayers are measured simultaneously at the air-water interface. A phase separation into two distinct phases is reported. Phase 1 exists of pheophytin domains whereas phase 2 is a lipid domain containing pheophytin in molar content between 2 % and 15 %, depending on the state of the lipid. The transition of the pheophytin domains is described within a model explaining correctly the observed concentration dependence of the transition pressure n,. The analysis shows that the pheophytin domains are two-dimensional and of limited size (smaller than 1 000 molecules). The domains are stabilized by their environment by steric forces. The change of the interactions during the phase transition is calculated. It amounts to 0.6 X 10-l3 erg per molecule in the bulk of the domain and to 1.2 X 10-13 erg per molecule at the domain wall.