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 Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

Outline Introduction Theoretical Considerations Experimental Results 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 Return to "Recent Talks" Page

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. Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

Chromonic Phases N phase M phase (orientationally ordered columns) (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. Return to "Recent Talks" Page

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. Return to "Recent Talks" Page

Chromonic Structures Return to "Recent Talks" Page 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. Return to "Recent Talks" Page

Sunset Yellow FCF Disodium salt of 6-hydroxy-5-[(4-sulfophenyl)azo]-2-napthalenesulfonic acid Anionic Monoazo Dye Food Color (Yellow 6) Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

Sunset Yellow FCF Return to "Recent Talks" Page Crossed Polarizers V. R. Horowitz, L. A. Janowitz, A. L. Modic, P. A. Heiney, and P.J. Collings, Phys. Rev. E 72, 041710 (2005) Return to "Recent Talks" Page

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. Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

Simple Theory (continued) But the total volume fraction for all molecules  is The volume fraction of single molecules is therefore: Return to "Recent Talks" Page

Results of Simple Theory Return to "Recent Talks" Page

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 <n> = 6 Return to "Recent Talks" Page

Absorption Experiments Return to "Recent Talks" Page

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: Return to "Recent Talks" Page

Theory-Experiment Comparison Assumption Absorption coefficient = Fitting Results Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

X-ray Diffraction Results Return to "Recent Talks" Page

Aggregate Shape? Large Planes Long Cylinders Return to "Recent Talks" Page

Analysis of Aggregate Shape Fitting Result area of cylinder = 1.21 ± 0.12 nm2 molecular area ~ 1.0 nm2 Return to "Recent Talks" Page

Birefringence Birefringence Notice: (1) Birefringence decreases with increasing temperature (2) Birefringence is negative Return to "Recent Talks" Page

Order Parameter Measure: (1) indices of refraction (2) absorption of polarized light Return to "Recent Talks" Page

Bordeaux Ink (Absorption) Assumption Absorption coefficient = Fitting Results Return to "Recent Talks" Page

Bordeaux Ink (X-ray) Return to "Recent Talks" Page

Analysis of Aggregate Size Fitting Result area of cylinder = 3.24 ± 0.04 nm2 molecular area ~ 1.2 nm2 Return to "Recent Talks" Page

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. Return to "Recent Talks" Page