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 Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

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 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

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 Return to "Recent Talks" Page

Phases of Matter Return to "Recent Talks" Page

Thermotropic Liquid Crystals L = 300 J/gmL = 30 J/gm T solidliquid crystalliquid Return to "Recent Talks" Page

Orientational Order Order Parameter Return to "Recent Talks" Page

Liquid Crystal Phases smectic A smectic C Return to "Recent Talks" Page

Lyotropic Liquid Crystals micelle vesicle Return to "Recent Talks" Page

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 Return to "Recent Talks" Page

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% 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 Liquid Crystalline above 6 wt% Return to "Recent Talks" Page

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, (2005) Return to "Recent Talks" Page

X-ray Diffraction Sunset Yellow (1)Peak at q = 18.5 nm -1 (d = 0.34 nm): concentration independent (2)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 nm 2 molecular area ~ 1.0 nm 2 Return to "Recent Talks" Page

Aggregation Theory (0 th 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. Return to "Recent Talks" Page

Aggregation Theory (0 th Order) To see what size aggregates contribute the most to the free energy, let’s 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. Return to "Recent Talks" Page

Aggregation Theory (1 th Order) Goal: find the distribution of sizes that minimizes the free energy. But this means minimizing a function of an infinite number of variables (N n )! Fortunately, there is a constraint: Use a Lagrange multiplier : and solve for N n in terms of  Substitute N n back into the constraint equation, yielding and thereby also yielding N n. Return to "Recent Talks" Page

Results of 1 st Order Aggregation Theory Return to "Recent Talks" Page

Absorption Experiments Return to "Recent Talks" Page

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: Return to "Recent Talks" Page

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, |1>, |2>, |3>, etc: The transition dipole moment of a coupled state is given by its superposition of single molecule excited states. Return to "Recent Talks" Page

Exciton Theory Graphs of |  | 2 /n for different values of n: Prediction Aggregation causes a shift in wavelength and broadening! Return to "Recent Talks" Page

Sunset Yellow FCF Exciton Theory Absorption coefficient: Fitting Results  = 22.6 ± 0.1 Return to "Recent Talks" Page

Bordeaux Ink X-ray Results Cylinder area = 3.24 ± 0.04 nm 2 Molecular area ~ 1.2 nm 2 Absorption Results  = 24.5 ± 0.1 Return to "Recent Talks" Page

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

Order Parameter Measure: (1) indices of refraction (2) absorption of polarized light 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