Effects of Curvature on Monolayer Morphology and Dynamics

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

CBET - 1706378 PI: Zasadzinski, Joseph A. The behavior of emulsions, foams, and other multiphase systems, including biological materials, is strongly affected by surface-active molecules, such as phospholipids and surfactants, at interfaces. For more than a century, the behavior of molecules at flat interfaces has been studied using a Langmuir trough. However, most interfaces in multiphase systems are curved, which affects the distribution of the molecules on the interface and the properties of the interface. This award will support the use of new tools to examine the morphology and dynamics of molecular monolayers on bubbles with radii from 30 to 500 microns. A confocal microscope will be used to determine how the chemical composition of the monolayer and molecular arrangements in the monolayer are affected by the curved surface of the bubble. Atomic force microscopy and X-ray diffraction measurements will help reveal how molecular packing is affected by curvature. Variations in surface tension will be measured for a bubble whose size is changed at different rates. Results from this research will help explain how effects of interfacial curvature on monolayers influence the stability of curved liquid surfaces in the lung, which is especially important in lung inflammation resulting from disease or trauma. Results will also be useful in understanding the stability and rheology of commercially important emulsions and foams and the dynamics of tear films in the eye and wetting behavior of contact lenses. The project will engage students at all academic levels, as well as local high-school students and teachers. Instructional modules containing images from the research will be prepared to illustrate the importance of material properties on lung health. Monolayers of phospholipids, cholesterol, and fatty acids or alcohols can form coexisting, ordered, solid-like domains in a continuous, disordered, liquid-like matrix, or two immiscible liquid-like phases. On the flat interface of a Langmuir trough, the solid domains resist coalescence due to a long-range electrostatic dipole-dipole repulsion even though a measurable line tension acts to minimize the domain perimeter. Recent results suggest that for bubbles smaller than 200 µm, the connectivity of the solid phase domains changes to a mesh-like network with the fluid phase domains disconnected and isolated from each other. This change in morphology and connectivity of the solid phase alters the dynamic dilatational modulus, which describes how the surface tension changes with interfacial area. Very little is known about experimental values of the dilatational modulus of mixed phospholipid, protein, fatty acid and cholesterol monolayers. Confocal microscopy will be used to image highly curved bubble surfaces to determine how interfacial curvature and monolayer composition alter the domain morphology, and a new capillary pressure microtensiometer will be used to measure how this morphology influences the dynamic dilatational modulus. Lateral phase separation in monolayers is typical for clinical and native lung surfactants. The Laplace pressure suggests that interconnected bubbles or alveoli of different radii are, at best, metastable if the surface tension is constant. However, if the dilatational modulus is sufficiently large, resistance to interfacial compression can overcome the Laplace pressure and stabilize interconnected alveoli. Hence, the dilatational modulus, and how it depends on monolayer composition, morphology, interfacial curvature and changes in interfacial area are essential to lung stability.