Influence of molecular coherence on surface viscosity

Siyoung Q. Choi, Kyuhan Kim, Colin M. Fellows, Kathleen D. Cao, Binhua Lin, Ka Yee C. Lee, Todd M. Squires, Joseph A. Zasadzinski

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

Adding small fractions of cholesterol decreases the interfacial viscosity of dipalmitoylphosphatidylcholine (DPPC) monolayers by an order of magnitude per wt %. Grazing incidence X-ray diffraction shows that cholesterol at these small fractions does not mix ideally with DPPC but rather induces nanophase separated structures of an ordered, primarily DPPC phase bordered by a line-active, disordered, mixed DPPC-cholesterol phase. 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. Cholesterol significantly reduces the coherence area of the crystals as well as the interfacial viscosity. 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 ordered monolayers.

Original languageEnglish (US)
Pages (from-to)8829-8838
Number of pages10
JournalLangmuir
Volume30
Issue number29
DOIs
StatePublished - Jul 29 2014

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