Determination of Inter-Phase Line Tension in DMPC/D-Cholesterol mixed Langmuir Films

Abstract

The hydrodynamic response of a thin fluid film, whether a Langmuir monolayer at the air/water interface or a cell membrane, is difficult to model, since it involves the coupling of both bulk and surfaces phases. However, such hydrodynamic response is not only intrinsically critical for transport within the layer, it also provides a major available means to evaluate an important parameter for phase-separated layers such as rafts, the line tension. We have developed a line-integral formulation of the hydrodynamic response of phase-separated layers with short-ranged forces, and tested it by comparisons between numerical simulations based on this model and experiment. These experiments both validate the model and demonstrate that the line tension can be determined with unprecedented accuracy and precision. Long-range dipole-dipole interactions are introduced into the model. The method is applied to coexistence between phases in binary phospholipid/cholesterol mixed layers. Data is evaluated for both Brewster and microscopy and fluorescence microscopy and implications of the use of fluorescent probes are discussed.