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Influence of molecular coherence on surface viscosity.

Choi SQ, Kim K, Fellows CM, Cao KD, Lin B, Lee KY, Squires TM, Zasadzinski JA - Langmuir (2014)

Bottom Line: Adding small fractions of cholesterol decreases the interfacial viscosity of dipalmitoylphosphatidylcholine (DPPC) monolayers by an order of magnitude per wt %.Cholesterol significantly reduces the coherence area of the crystals as well as the interfacial viscosity.The extent of molecular coherence appears to be a fundamental factor in determining surface viscosity in ordered monolayers.

View Article: PubMed Central - PubMed

Affiliation: Chemical Engineering and Materials Science, University of Minnesota , Minneapolis, Minnesota 55455, United States.

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.

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(A) Coherence lengths, L11, in the(11) or tilt direction, and L02, in the(02) or untilted direction, normalized by their respective latticeconstants, d11 and d02. L02 decreases significantlywith cholesterol fraction, but is less affected by surface pressure. L02 for 40 mN/m has more scatter and is nonmonotonicwith cholesterol fraction, likely the result of film instabilitiesdue to trough leakage and monolayer collapse at higher surface pressure.(B) The surface viscosity of DPPC/Chol monolayers measured with amicrobutton magnetic rheometer decreases exponentially with cholesterolfraction for a given surface pressure for small mole fractions ofcholesterol, then plateaus in the same fashion as L02. (Surface viscosity for pure DPPC at 40 mN/m was toohigh for the viscometer to measure.) (C) Free area model for DPPC/Cholmonolayers (eqs 6-8) providesan excellent correlation between the surface viscosity and the numberof correlated molecules, (L02·L11)/ab, over the entire rangeof cholesterol fraction. The lines are linear regression fits of eq 7 to the data (p < 0.01). (D)Normalizing to a reference state (taken to be that of pure DPPC for20 and 30 mN/m and 0.4% Chol for 40 mN/m surface pressures), collapsesthe data onto a single universal curve relating surface viscosityto the molecular organization. The line is a linear regression fitof eq 8 to the data with p <.001 showing that the data is well described by the free area model.
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fig7: (A) Coherence lengths, L11, in the(11) or tilt direction, and L02, in the(02) or untilted direction, normalized by their respective latticeconstants, d11 and d02. L02 decreases significantlywith cholesterol fraction, but is less affected by surface pressure. L02 for 40 mN/m has more scatter and is nonmonotonicwith cholesterol fraction, likely the result of film instabilitiesdue to trough leakage and monolayer collapse at higher surface pressure.(B) The surface viscosity of DPPC/Chol monolayers measured with amicrobutton magnetic rheometer decreases exponentially with cholesterolfraction for a given surface pressure for small mole fractions ofcholesterol, then plateaus in the same fashion as L02. (Surface viscosity for pure DPPC at 40 mN/m was toohigh for the viscometer to measure.) (C) Free area model for DPPC/Cholmonolayers (eqs 6-8) providesan excellent correlation between the surface viscosity and the numberof correlated molecules, (L02·L11)/ab, over the entire rangeof cholesterol fraction. The lines are linear regression fits of eq 7 to the data (p < 0.01). (D)Normalizing to a reference state (taken to be that of pure DPPC for20 and 30 mN/m and 0.4% Chol for 40 mN/m surface pressures), collapsesthe data onto a single universal curve relating surface viscosityto the molecular organization. The line is a linear regression fitof eq 8 to the data with p <.001 showing that the data is well described by the free area model.

Mentions: Figure 7Ashows that the coherence length, L02,in the untilted direction for pure DPPCis about 70 lattice repeats, or >300 Å, more than five timesthat in the tilted direction, L11 ∼60 Å or about 12 lattice repeats (Table 2). For both 20 and 30 mN/m, L02 decreasesmonotonically with increasing cholesterol fraction to ∼20 latticerepeats, but L11 only decreases to ∼10lattice repeats. At 40 mN/m, L02 doesnot monotonically decrease with cholesterol fraction, the scatterin L02 is much greater than at lower surfacepressures, and L02 is always less thanexpected from the results for the lower surface pressures. This islikely due to a decrease in film stability caused by a combinationof leakage under the trough barriers and slow monolayer collapse duringthe 3-5 h required for GIXD. The AFM images in Figure 5 show that the average DPPC (light gray) domain size decreasesfrom microns to 100–200 nm with cholesterol. Even with thedecreasing domain size, the positional ordering given by the coherencelengths are orders of magnitude smaller than the domain size for agiven cholesterol fraction. However, the orientational order extendsfor tens of microns as shown by the spiral domain textures in Figure 6.10 DPPC/Chol monolayershave nanometer-range positional order and micron-range orientationalorder,20,42 similar to tilted Smectic C liquid crystals43 and other Langmuir films that are classifiedas hexatics.44


Influence of molecular coherence on surface viscosity.

Choi SQ, Kim K, Fellows CM, Cao KD, Lin B, Lee KY, Squires TM, Zasadzinski JA - Langmuir (2014)

(A) Coherence lengths, L11, in the(11) or tilt direction, and L02, in the(02) or untilted direction, normalized by their respective latticeconstants, d11 and d02. L02 decreases significantlywith cholesterol fraction, but is less affected by surface pressure. L02 for 40 mN/m has more scatter and is nonmonotonicwith cholesterol fraction, likely the result of film instabilitiesdue to trough leakage and monolayer collapse at higher surface pressure.(B) The surface viscosity of DPPC/Chol monolayers measured with amicrobutton magnetic rheometer decreases exponentially with cholesterolfraction for a given surface pressure for small mole fractions ofcholesterol, then plateaus in the same fashion as L02. (Surface viscosity for pure DPPC at 40 mN/m was toohigh for the viscometer to measure.) (C) Free area model for DPPC/Cholmonolayers (eqs 6-8) providesan excellent correlation between the surface viscosity and the numberof correlated molecules, (L02·L11)/ab, over the entire rangeof cholesterol fraction. The lines are linear regression fits of eq 7 to the data (p < 0.01). (D)Normalizing to a reference state (taken to be that of pure DPPC for20 and 30 mN/m and 0.4% Chol for 40 mN/m surface pressures), collapsesthe data onto a single universal curve relating surface viscosityto the molecular organization. The line is a linear regression fitof eq 8 to the data with p <.001 showing that the data is well described by the free area model.
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4334248&req=5

fig7: (A) Coherence lengths, L11, in the(11) or tilt direction, and L02, in the(02) or untilted direction, normalized by their respective latticeconstants, d11 and d02. L02 decreases significantlywith cholesterol fraction, but is less affected by surface pressure. L02 for 40 mN/m has more scatter and is nonmonotonicwith cholesterol fraction, likely the result of film instabilitiesdue to trough leakage and monolayer collapse at higher surface pressure.(B) The surface viscosity of DPPC/Chol monolayers measured with amicrobutton magnetic rheometer decreases exponentially with cholesterolfraction for a given surface pressure for small mole fractions ofcholesterol, then plateaus in the same fashion as L02. (Surface viscosity for pure DPPC at 40 mN/m was toohigh for the viscometer to measure.) (C) Free area model for DPPC/Cholmonolayers (eqs 6-8) providesan excellent correlation between the surface viscosity and the numberof correlated molecules, (L02·L11)/ab, over the entire rangeof cholesterol fraction. The lines are linear regression fits of eq 7 to the data (p < 0.01). (D)Normalizing to a reference state (taken to be that of pure DPPC for20 and 30 mN/m and 0.4% Chol for 40 mN/m surface pressures), collapsesthe data onto a single universal curve relating surface viscosityto the molecular organization. The line is a linear regression fitof eq 8 to the data with p <.001 showing that the data is well described by the free area model.
Mentions: Figure 7Ashows that the coherence length, L02,in the untilted direction for pure DPPCis about 70 lattice repeats, or >300 Å, more than five timesthat in the tilted direction, L11 ∼60 Å or about 12 lattice repeats (Table 2). For both 20 and 30 mN/m, L02 decreasesmonotonically with increasing cholesterol fraction to ∼20 latticerepeats, but L11 only decreases to ∼10lattice repeats. At 40 mN/m, L02 doesnot monotonically decrease with cholesterol fraction, the scatterin L02 is much greater than at lower surfacepressures, and L02 is always less thanexpected from the results for the lower surface pressures. This islikely due to a decrease in film stability caused by a combinationof leakage under the trough barriers and slow monolayer collapse duringthe 3-5 h required for GIXD. The AFM images in Figure 5 show that the average DPPC (light gray) domain size decreasesfrom microns to 100–200 nm with cholesterol. Even with thedecreasing domain size, the positional ordering given by the coherencelengths are orders of magnitude smaller than the domain size for agiven cholesterol fraction. However, the orientational order extendsfor tens of microns as shown by the spiral domain textures in Figure 6.10 DPPC/Chol monolayershave nanometer-range positional order and micron-range orientationalorder,20,42 similar to tilted Smectic C liquid crystals43 and other Langmuir films that are classifiedas hexatics.44

Bottom Line: Adding small fractions of cholesterol decreases the interfacial viscosity of dipalmitoylphosphatidylcholine (DPPC) monolayers by an order of magnitude per wt %.Cholesterol significantly reduces the coherence area of the crystals as well as the interfacial viscosity.The extent of molecular coherence appears to be a fundamental factor in determining surface viscosity in ordered monolayers.

View Article: PubMed Central - PubMed

Affiliation: Chemical Engineering and Materials Science, University of Minnesota , Minneapolis, Minnesota 55455, United States.

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.

Show MeSH
Related in: MedlinePlus