(195d) Relating Rheology with Morphology: Cholesterol in a Model Lung Surfactant Monolayer

The addition of cholesterol to lung surfactant (LS) model monolayers decreases viscosity and affects domain morphology. Recent viscosity measurements of the DPPC:HD:cholesterol system, often used as a model for LS studies, showed this trend with cholesterol. For cholesterol concentrations from zero to four percent, increased packing density corresponds to an increase in viscosity. However, samples with eight mole percent cholesterol were shown to decrease in viscosity as packing density increases, a surprising result. We present a morphological study to complement recent viscosity measurements of the DPPC:HD:cholesterol system. We performed confocal imaging on DPPC:HD:cholesterol monolayers in a Langmuir trough to identify morphological changes that correspond with the observed changes in viscosity. Characteristic features at various cholesterol concentrations are identified, and we observe a possible three-phase system in six percent and eight percent cholesterol samples. We also consider the role of experimental parameters such as camera gain, saturation, and dye quenching in the interpretation of these phase-separated systems. The resulting images shed new light on a potential mechanism for viscosity regulation in LS.