Here is a typical isotherm of a phospholipid film. This diagram is based on measurements of L-DPPC at 23° C published by Weidemann and Vollhardt in 1996.

The isotherm is based on the film's response to a single compression. The plateaus in the isotherm correspond to changes in phase, that is, changes in the ordering and compressibility of the phospholipid molecules. (Weidemann and Vollhardt, 1996, binder)

Phases of Phospholipid Surfactants

Gaseous (G)

In this phase, the molecules are widely spread and disordered. Their chains make extensive contact with the water. (Lipp et al., 1996, binder) This phase is probably not found in the lung. (c.f. phase diagrams, Ingenito et al., 1999, Theory binder)

(Kaganer et al., 1999, online)

Liquid-Expanded (LE)

As the film is compressed, the phospholipid chains lift from the surface of the water. However, the phase remains disordered and fluid. (Lipp et al., 1996, binder)

(Kaganer et al., 1999, online)

Liquid-Condensed (LC)

This phase is also known as tilted-condensed. Molecules in this state are ordered, often in a hexatic pattern which exhibits sixfold symmetry. The molecules also have a consistent tilt.

(Kaganer et al., 1999, online)

Solid (S)

On further compression, molecules enter the solid phase, also known as the untilted-condensed phase. Solid-phase phospholipid molecules retain the liquid-condensed ordering, but their tilt is nearly perpendicular to the surface. This makes the surface film more rigid and more prone to collapse. (Kaganer et al., 1999, online) Since proteins affect phospholipid ordering and phase behavior, the solid phase may not be found in the lung. (c.f. phase diagrams, Ingenito et al., 1999, Theory binder)

(Kaganer et al., 1999, online)

Because it has a large headgroup, DPPC remains tilted at very high surface pressures. Therefore, a DPPC film is less likely to collapse than films of other phospholipids. (Weidemann and Vollhardt, 1996, binder)

Phase Transitions

Phase transitions do not occur evenly or instantaneously. Instead, domains of different phases coexist. The following diagram shows islands of a liquid-condensed phase surrounded by the more fluid liquid-expanded phase:

(Möhwald, 1990, binder)

The competition between the line tension at the boundary between phases and the repulsion of more condensed domains produces different domain shapes. Here are some possible domain shapes, visible on the scale of micrometers:

(McConnell, 1991, binder; Seul and Andelman, 1995, binder)

Chiral molecules can produce chiral domain shapes. In particular, L-DPPC and R-DPPC display triskelion shapes:

(McConnell, 1991, binder)

Even if a monolayer is entirely in a condensed phase, it may have regions with different molecular tilt. The monolayer is likely to fracture along these lines of competing tilt. (Schief et al., 2000, binder)

Adding Proteins

Both SP-B and SP-C collect in the fluid areas of phospholipid monolayers. Adding SP-B fluidizes a monolayer so that the liquid-expanded phase survives at higher degrees of compression, sometimes even to the point of monolayer collapse. SP-B also makes condensed domains smaller and more numerous. This prevents fractures from spreading throughout a monolayer. (Krol et al., 2000, SP-B and C binder) SP-B and SP-C both promote a reversible collapse which occurs at the boundaries of LC domains or even the interiors of LE domains, rather than from the center of an overcompressed condensed domain. (Kramer et al., 2000, binder; Krol et al., 2000, binder)

Monolayer collapse produces a horizontal isotherm segment similar to a phase transition. Indeed, since collapse changes molecular arrangement and film compressibility, it may be considered a different sort of phase. SP-B and SP-C produce differing collapse structures. Adding SP-B to a monolayer of appropriate phospholipids leads to disc-shaped protrusions or buckling upon collapse:

(Lipp et al., 1996, binder; Krol et al., 2000, SP-B and C binder)

Adding SP-C leads to larger structures of stacked bilayers:

(Galla et al., 1998, SP-B and C online)

Whole lung surfactant collapses into relatively wide, flat plateaus.

(Tchoreloff et al., 1991, binder)