Phase Coexistence in Model Lung Surfactant Monolayers

Bret N. Flanders, Sarah A. Vickery and Robert C. Dunn

Department of Chemistry, The University of Kansas, 2007 Malott Hall, Lawrence, KS 66044

Respiratory distress syndrome (RDS) is due to an insufficiently developed lung surfactant, the mixture of lipids and proteins that coat the alveolar surfaces. The effort to improve RDS therapies would benefit from deeper understanding of the physical principles that underlie the properties of healthy surfactant (i.e. high compressibility and reversible spreading). To this end, confocal, atomic force and near field microscopies have been used to study the phase coexistence of palmitic acid monolayers containing a truncated form of lung surfactant protein B (*SP-B_1-25). Analysis of the phase coexistence yielded estimates for the *SP-B_1-25 concentration (C_*SP-B) dependent molar area, a. A discontinuity in the second order derivative of the molar Gibbs free energy, �a/�C_*SP-B, and a divergence in the molar area fluctuations are evident at 11.3wt.% *SP-B_1-25. These results suggest that a second order phase transition occurs at 11.3wt.% *SP-B_1-25 and that at least three phases coexist below this critical concentration.