Assembly Processes in Chromonic Liquid Crystals: IR-806 and Pinacyanol Acetate
Regan, Margaret H.
IR-806 is a near infrared dye that self-assembles in aqueous solution and forms
a chromonic liquid crystal at concentrations of 0.5 wt% and higher. In the liquid
crystal phase, the assemblies tend to orient in the same direction as they
diffuse throughout the volume. Usually, chromonic liquid crystals have one type
of assembly structure. However, IR-806 shows a two-step assembly process: a
non-threshold process at lower concentrations followed by a threshold process at
higher concentrations. By numerically solving for the equilibrium conditions when
the many reactions that form intermediate-size assemblies occur together with reactions
in which intermediate-size assemblies form large assemblies, a model that
agrees with the experimental data on IR-806 is possible.
Pinacyanol acetate is another dye that may have a two-step assembly process. For
example, pinacyanol acetate forms assemblies in solutions with water that in turn
form a chromonic liquid crystal phase at concentrations as low as 0. 75 wt%. Many
absorption spectra of pinacyanol acetate were analyzed and decomposed into six
Gaussian peaks. The behavior of the amplitude of the peaks as the concentration
increases gives evidence for a two-step process, but there is no clear indication
of a threshold concentration to mark the formation of the large assemblies as is
true for IR-806. Measurements of the phase diagram and absorption (at varying
temperature) reveal different concentration-temperature dependences, indicating
that the assemblies that form the liquid crystal phase do not form in any numbers
until a concentration higher than 0.1 %. Finally, the kinetics of assembly formation
in pinacyanol acetate is much faster than for IR-806.
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