Self-Consistent Field Modeling of Three-Dimensional Morphologies of Branched Lipid Surfactant at Air-Water Interface Kolattukudy Poulose Santo, Andriy Kovalenko, Maria Stepanova* Introduction Copolymers and amphiphilic surfactants form an impor- tant class of organic compounds. They consist of multiple blocks of different homopolymer sequences, which are directly responsible for their ability to form so-called mesoscopic phases (mesophases) such as, for example, monolayers, bilayers, and micelles. [1] In particular, under- standing the mesophase behavior of amphiphilic lipid systems is of key importance in the contexts of both biophysics and polymer science and engineering. [2,3] Phospholipid bilayers form the cell membranes, whose mesophase stability is believed to determine membrane functions such as endocytosis and exocytosis. [4,5] An important factor is the complex composition of the membranes, which may include several kinds of lipids, as well as protein molecules, whose presence leads to dramatic changes in the biophysical properties of the membrane. [21] In lungs, the amphiphilic pulmonary surfactant is responsible for stabilizing the alveolar cells during the respiratory cycle. The lung surfactant, which is a complex system involving phospholipids, proteins, and cholesterol, stabilizes the lung by undergoing a number of morphologic transitions during inhalation/exhalation reducing the surface tension and preventing the lung from collapse. [6–11] The major phospholipid component of the Full Paper K. P. Santo, A. Kovalenko, M. Stepanova National Institute for Nanotechnology NRC, Canada E-mail: maria.stepanova@nrc-cnrc.gc.ca K. P. Santo, M. Stepanova Department of Electrical and Computer Engineering, University of Alberta, Canada A. Kovalenko Department of Mechanical Engineering, University of Alberta; Alberta, Edmonton, Canada Three-dimensional mesoscopic morphologies and the thermodynamics of structural phase transitions of amphiphilic lipids at air-water interfaces are studied using self-consistent field theory. Changing the relative amount of lipids in the system led to a series of 3D morphologic phases with varying average interfacial area per molecule, mimicking a compression of the model membranes. Membranes of both saturated and unsaturated lipids undergo a transition from cylindrical micelle to lamella when the lipid con- tent in the system increases from 2% to about 19– 20%. With further increase in the lipid content, saturated lipids first develop non-uniform quasi- 2D distributions in the lamella and then gradually transform into a hybrid morphology containing quasi-planar lamellae. In contrast, unsaturated lipids develop reverse-micellar morphologies. 228 Macromol. Theory Simul. 2010, 19, 228–239 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mats.200900076