Chromonic Liquid Crystals: A New Form of Soft Matter

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Chromonic Liquid Crystals A New Form of Soft
Matter

• Peter J. Collings
• Department of Physics Astronomy
• Swarthmore College
• Department of Physics, Williams College
• April 6, 2007

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Acknowledgements

• Chemists and Physicists
• Robert Pasternack, Swarthmore College
• Robert Meyer Seth Fraden, Brandeis University
• Andrea Liu Paul Heiney, University of
  Pennsylvania
• Oleg Lavrentovich, Kent State University
• Michael Paukshto, Optiva, Inc.
• Swarthmore Students
• Viva Horowitz, Lauren Janowitz, Aaron Modic,
  Michelle Tomasik, Nat Erb-Satullo
• Funding
• National Science Foundation
• American Chemical Society (Petroleum Research
  Fund)
• Howard Hughes Medical Institute

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Outline

• Introduction
• Soft Matter
• Liquid Crystals
• X-ray Diffraction
• Theory for Fluid Systems
• Experimental Results
• Simple Theory of Aggregating Systems
• Electronic States of Aggregates
• Exciton Theory
• Absorption Measurements
• Birefringence and Order Parameter Measurements
• Conclusions

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Motivation

• Spontaneous aggregation is important in many
  different realms (soft condensed matter,
  supramolecular chemistry, biology, medicine).
• Chromonic liquid crystals represent a system
  different from colloids, amphiphiles, polymer
  solutions, rigid rod viruses, nanorods, etc.
• Understanding chromonic systems requires
  knowledge of both molecular and aggregate
  interactions.
• Chromonic liquid crystals represent an aqueous
  based, highly absorbing, ordered phase, opening
  the possibility for new applications.

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Soft Matter

• Condensed Matter (Fluids and Solids)
• Soft Matter (Fluids but not Simple Liquids)
• Polymers
• Emulsions
• Colloidal Suspensions
• Foams
• Gels
• Elastomers
• Liquid Crystals
• Thermotropic Liquid Crystals
• Lyotropic Liquid Crystals

Chromonic Liquid Crystals
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Phases of Matter
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Thermotropic Liquid Crystals
solid
liquid crystal
liquid
L 300 J/gm
L 30 J/gm
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Orientational Order
Order Parameter
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Liquid Crystal Phases
smectic C
smectic A
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Lyotropic Liquid Crystals
micelle
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vesicle
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Chromonic Liquid Crystals

• Lyotropic Systems
• Behavior is dominated by solvent interactions
• Critical micelle concentration
• Bi-modal distribution of sizes (one molecule vs.
  many molecules)
• Chromonic Systems
• Intermolecular and solvent interactions important
• Aggregation occurs at the lowest concentrations
  (isodesmic)
• Uni-modal size distribution

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Sunset Yellow FCF (Yellow 6)

• Disodium salt of 6-hydroxy-5-(4-sulfophenyl)azo-
  2-napthalenesulfonic acid
• Anionic Monoazo Dye
• Liquid Crystalline above 25 wt

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Bordeaux Ink (Optiva, Inc.)

• Results from the sulfonation of the cis
  dibenzimidazole derivative of 1,4,5,8-
  naphthalenetetracarboxylic acid
• Anionic dye
• Oriented thin films on glass act as polarizing
  filters
• Liquid Crystalline above 6 wt

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Sunset Yellow FCF
Crossed Polarizers
V. R. Horowitz, L. A. Janowitz, A. L. Modic, P.
A. Heiney, and P.J. Collings, Phys. Rev. E 72,
041710 (2005)
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X-ray Diffraction

• Sunset Yellow
• Peak at q 18.5 nm-1 (d 0.34 nm)
  concentration independent
• Peak at q 2.0 nm-1 (d 3.0 nm) concentration
  dependent

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X-ray Diffraction Results
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Aggregate Shape?
Large Planes Long Cylinders
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Analysis of Aggregate Shape
Fitting Result area of cylinder 1.21
0.12 nm2 molecular area 1.0 nm2
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Aggregation Theory (0th Order)

• System is held at at constant temperature volume
  changes can be ignored .. use Helmholtz Free
  Energy.
• Assume energy is lowered by an amount ?kT for
  each face-to-face arrangement of two molecules in
  an aggregate.
• Assume for entropy considerations that aggregates
  act like ideal gas molecules.

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Aggregation Theory (0th Order)

• To see what size aggregates contribute the most
  to the free energy, lets imagine all the
  aggregates have the same number of molecules n.
• This competition between the two terms means
  there is a distribution of aggregate sizes that
  minimizes the free energy.

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Aggregation Theory (1th Order)

• Goal find the distribution of sizes that
  minimizes the free energy. But this means
  minimizing a function of an infinite number of
  variables (Nn)!
• Fortunately, there is a constraint
• Use a Lagrange multiplier ?
• and solve for Nn in terms of ??
• Substitute Nn back into the constraint equation,
  yielding ? and thereby also yielding Nn.

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Results of 1st Order Aggregation Theory
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Absorption Experiments
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Exciton Theory

• Strong molecular absorption is due to a
  collective excitation with some charge separation
  (two state system)
• Aggregation results in a coupling between the
  excited states of identical nearest neighbor two
  state systems

For n aggregated molecules
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Exciton Theory

• The transition probability for absorption is
  proportional to the intensity of the light and
  the square of the transition dipole moment. For
  single excited molecule states, 1gt, 2gt, 3gt,
  etc
• The transition dipole moment of a coupled state
  is given by its superposition of single molecule
  excited states.

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Exciton Theory
Graphs of ?2/n for different values of n
Prediction Aggregation causes a shift in
wavelength and broadening!
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Sunset Yellow FCF
Exciton Theory Absorption coefficient
Fitting Results ? 22.6 0.1
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Bordeaux Ink
X-ray Results Cylinder area 3.24 0.04
nm2 Molecular area 1.2 nm2
Absorption Results ? 24.5 0.1
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Birefringence
Birefringence
Notice (1) Birefringence decreases with
increasing temperature (2) Birefringence is
negative
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Order Parameter
Measure (1) indices of refraction (2) absorption
of polarized light
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Conclusions

• Sunset Yellow FCF forms linear aggregates with a
  cross-sectional area about equal to the area of
  one molecule.
• The energy of interaction between molecules in an
  aggregate is fairly large (22 kT).
• The aggregates probably contain on the order of
  15 molecules on average.
• Bordeaux Ink appears to behave similarly, except
  the cross-sectional area is about equal to two or
  three molecules.

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