(524e) Relating Monolayer Structure to Interfacial Rheology
As improved micro-rheological techniques become available, it is now possible to accurately map out interfacial viscoelasticity over a much wider range than previously possible; this has been especially valuable for biologically relevant phospholipid and cholesterol films which have too low a viscosity for macroscopic rheometers. With micro-disc rheometry, we have been able to map out the viscosity of dipalmitoylphosphatidylcholine monolayers, and determine how the viscosity changes with additions of cholesterol and/or palmitic acid, which are common additives to clinical lung surfactants. We find that 1-3 wt% cholesterol (similar to that found in Infasurf, a clinical replacement surfactant) induces a two-three order of magnitude drop in the interfacial rheology. This drop correlates very well with the decrease in molecular coherence of the DPPC lattice as measure with synchrotron X-ray diffraction when used in a free-area model of the viscosity. However, palmitic acid induces an two order of magnitude increase in the viscosity of DPPC which is only partly correlated with the increase in molecular coherence of DPPC. Instead, a better explanation of the effects of PA on DPPC is the formation of a highly organized, low tilt phase that is about 1:1 DPPC:PA that appears at low surface pressure. For mixtures in which the PA fraction is below equimolar, we obtain a mixed solid phase of the 1:1 DPPC:PA cocrystal and a liquid phase of nearly pure DPPC. This phase separation is exacerbated by the presence of cholesterol, which segregates to the liquid phase. As the surface pressure increases, a second crystal forms of nearly pure DPPC. This is accompanied by a dramatic change in the rate of change of the surface viscosity vs surface pressure. At higher surface pressures, the viscosity resembles the DPPC-cholesterol mixtures, at lower surface pressure, the viscosity is that of the DPPC-PA. We attempt to prove this hypothesis using X-ray and electron diffraction of the monolayers along with fluorescence imaging that shows the lateral phase segration and change in crystal habit as a function of surface pressure. This solid-solid phase coexistence is likely common in many complex biological monolayers.