Seminars & Lectures
| * TITLE | The Influence of Molecular Coherence on Surface Viscosity | ||||||
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| * DATE / TIME | 2013-12-19, 4PM | ||||||
| * PLACE | 512 Seminar Room | ||||||
| * ABSTRACT | |||||||
| The surface viscosity of dipalmitoylphosphatidylcholine (DPPC) monolayers decreases by two orders of magnitude on addition of 3.7 mol% cholesterol, followed by a sharp increase in the monolayer elasticity above 3.7 mol%. We correlate these intriguing rheological properties with changes in the molecular organization as revealed by Grazing Incidence X-ray Diffraction (GIXD). Adding cholesterol at constant surface pressure decreases the tilt of the DPPC lattice in the same way as increasing the surface pressure at constant cholesterol. The correlation length of the DPPC lattice decreases with cholesterol fraction, suggesting that the increase in defect density leads to the decrease in surface viscosity. Above 4 mol%, the DPPC lattice parameter and correlation length saturates, showing that the observed increase in monolayer elasticity comes about from a percolated, emulsion-like cholesterol network. We propose that the free area in the classic Cohen and Turnbull model of viscosity is inversely proportional to the number of molecules in the coherence area, or product of the two coherence lengths. Using this free area collapses the surface viscosity data for all surface pressures and cholesterol fractions to a universal logarithmic relation. The extent of molecular coherence appears to be a fundamental factor in determining surface viscosity in monolayers. | |||||||
