Aggregation Behavior and Liquid Crystal Properties of Water-Soluble Dyes

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Aggregation Behavior and Liquid Crystal
Properties of Water-Soluble Dyes

• Peter J. Collings
• Department of Physics Astronomy, Swarthmore
  College
• Department of Physics Astronomy, University of
  Pennsylvania
• 21st ILCC
• July 4, 2006

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Acknowledgements

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

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Outline

• Introduction
• Chromonic Liquid Crystals
• Materials Sunset Yellow FCF, Bordeaux Ink
• Theoretical Considerations
• Simple Theory of Aggregation
• More Rigorous Theory of Aggregation and Liquid
  Crystal Phases
• Experimental Results
• Absorption Measurements in Dilute Solutions
• X-ray Diffraction Measurements Over a Wide
  Concentration Range
• Birefringence Measurements
• 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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Lyotropic Liquid Crystals

• Amphiphilic 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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Chromonic Phases
N phase (orientationally ordered columns)
M phase (positionally and orientationally ordered
columns)
J. Lydon, in Handbook of Liquid Crystals,
edited by J. Goodby, G. W. Gray, H.-W. Spiess,
and V. Vill (Wiley-VCH, New York, 1998), Vol.
2B, Chap. XVIII, p. 981.
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Disodium Cromoglycate

• Drug developed for the treatment of asthma.
• Liquid crystal phases at room temperature for
  concentrations greater than about 10 wt.
• X-ray measurements 0.34 nm spacing between
  rings, column diameter of 2-3 nm, column spacing
  about 4 nm.
• NMR points to a high value of the order
  parameter.
• Light scattering and viscosity measurements
  suggest a column diameter of about 2 nm and an
  average length of about 20 nm at the
  nematic-isotropic transition.
• Cross-sections of one and four molecules have
  been suggested.
• Birefringence of the nematic phase is small and
  negative.

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Chromonic Structures
J. Lydon, in Handbook of Liquid Crystals, edited
by J. Goodby, G. W. Gray, H.-W. Spiess, and V.
Vill (Wiley-VCH, New York, 1998), Vol. 2B,Chap.
XVIII, p. 981.
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Sunset Yellow FCF

• Disodium salt of 6-hydroxy-5-(4-sulfophenyl)azo-
  2-napthalenesulfonic acid
• Anionic Monoazo Dye
• Food Color (Yellow 6)

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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

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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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Simple Theory

• The partition function Q for a collection of
  non-interacting aggregates is
• where n is the number of molecules in an
  aggregate, qn is the partition function of a
  single aggregate with n molecules, and Nn is the
  number of aggregates with n molecules.
• The chemical potential per molecule ?n for an
  aggregate with n molecules is then
• At equilibrium, all chemical potentials per
  molecule are equal.

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Simple Theory (continued)

• Including translational degrees of freedom and a
  decrease in energy of ?kT for each pair of
  neighboring molecules in an aggregate,
• where V is the sample volume, ?n is the thermal
  wavelength of an aggregate with n molecules
  (assumed to be constant), and ?n is the internal
  energy of an aggregate with n molecules.
• Equating chemical potentials and denoting the
  volume fraction of aggregates with n molecules as
  xn, one obtains

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Simple Theory (continued)

• But the total volume fraction for all molecules ?
  is
• The volume fraction of single molecules is
  therefore

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Results of Simple Theory
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More Rigorous Theory

• M. P. Taylor and J. Herzfeld, Langmuir 6, 911
  (1990) Phys. Rev. A 43, 1892 (1991)
• Linear aggregates
• hard-core potentials
• short-range repulsions
• pair-wise attraction
• For ? 0.26
• S 0.65
• ltngt 6

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Absorption Experiments
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Exciton Model

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

For n aggregated molecules
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Theory-Experiment Comparison
Assumption Absorption coefficient
Fitting Results
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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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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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Bordeaux Ink (Absorption)
Assumption Absorption coefficient
Fitting Results
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Bordeaux Ink (X-ray)

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Analysis of Aggregate Size
Fitting Result area of cylinder 3.24
0.04 nm2 molecular area 1.2 nm2
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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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