result of different dye solubility, fluorescence quantum yield, or molecular density of coexisting phases[@Fluor]. A complementary and recently developed technique is Brewster-angle microscopy, which allows imaging of a monolayer without the addition of fluorescent probes[@Henon].

After injection of a water-soluble protein into the aqueous subphase, the surface tension typically decreases, indicating that the protein is at least partially incorporated into the monolayer[@Mohwald; @Wiedmann; @Schwinn; @Vogel]. This is due to the protein affinity to the water/air interface. The specific type of this attraction is not well understood and probably is due in part to structural changes (denaturation) of the protein in the monolayer or at the water surface, associated with the unfolding of hydrophobic groups.

One of the striking experimental observations[@Schwinn; @Haas] was that some proteins adsorb preferentially along the [*boundary line*]{} between the LE and LC domains when the monolayer is in the LE/LC coexistence region. These observations were made for fluorescently labeled small proteins, such as [*concanavalin*]{} A[@Haas] or [*streptavidin*]{}[@Schwinn], interacting with phospholipid monolayers. These experimental findings motivated our present theoretical study.

In the following, we describe a simple model, which (i) assumes the LE/LC transition to be a simple first-order condensation transition, yielding coexisting domains for temperatures below 