The minimum surface tension and respreadability of a surfactant monolayer is limited by a two to three dimensional instability called collapse. Liquid-condensed or solid phase monolayers collapse via fracture followed by loss of material. Liquid-expanded phase monolayers collapse by solubilization into the subphase. Monolayers that retain a continuous liquid-expanded phase network surrounding islands of liquid-condensed or solid phase collapse at low surface tensions via a localized, large amplitude buckling. The buckled regions coexist with the flat monolayer, remain attached to the interface, and reversibly reincorporate into the monolayer upon expansion. [S0031-9007(98)06943-9]

Both human lung surfactant protein, SP-B, and its amino-terminal peptide, SP-B1-25, inhibit the formation of condensed phases in monolayers of palmitic acid, resulting in a new fluid phase. This fluid phase forms a network, separating condensed-phase domains at coexistence. The network persists to high surface pressures, altering the nucleation, growth, and morphology of monolayer collapse structures, leading to lower surface tensions on compression and more reversible respreading on expansion. The network is stabilized by the low line tension between the fluid phase and the condensed phase as confirmed by the formation of "stripe" phases.

Human lung surfactant protein SP-B and its amino terminus (SP-Bl_25) alter the phase behavior of palmitic acid (PA) monolayers by inhibiting the formation of condensed phases and creating a new fluid PAprotein phase. This fluid phase increases the compressibility of the monolayers by forming a network that separates condensed phase domains at coexistence and persists to high surface pressures. The network changes the monolayer collapse nucleation from a heterogeneous to a more homogeneous process through isolating individual condensed phase domains. This results in higher surface pressures at collapse, and monolayers easier to respread on expansion, factors essential to the in vivo function of lung surfactant. The network is stabilized by low line tension between the coexisting phases as confirmed by the formation of extended linear domains or "stripe" phases. Similar stripes are found in monolayers of fluorescein-labeled SP-B1 25, suggesting that the reduction in line tension is due to the protein. Comparison of isotherm data and observed morphologies of monolayers containing SP-Bl_25 with those containing the full SP-B protein shows that the peptide retains most of the native activity of the protein, which may lead to cheaper and more effective synthetic replacement formulations.