Biomedical Engineering Reference
In-Depth Information
Chromonic mesophases, which are considered to be the lyotropic coun-
terparts of discotic phases, represent a broad but not well-understood
class of sot matter in which the reversible self-aggregates formed by non-
amphiphilic molecules show LC phases [15-17]. h ey are formed by a
range of polyaromatic compounds, including drugs [18-22], dyes [23-26],
and nucleic acids [27, 28], and their structure and properties are distinct
from those of the mesophases formed by conventional amphiphiles that
usually form micellar or lamellar aggregates. h e name chromonic was
i rst suggested by Lydon and his coworkers, as it has the connotations of
both color and chromosomes. One of the i rst extensively studied chro-
monic mesogens reported was disodium cromoglycate, which is known
as
anti
-asthmatic drug, marketed under the trade name “INTAL” [15, 22].
Chromonic molecules have a plank-shaped or disc-like polyaromatic
central core that is functionalized at the periphery with ionic or hydro-
philic groups for solubility in aqueous media. Chromonic molecules gen-
erally stack one on top of another through face-to-face orientation forming
H aggregates (untilted stacks) and J aggregates (tilted stacks). h is is to
minimize the unfavorable contact of the hydrophobic central core with
water, and thus maximize the favorable interaction with the ionic solubi-
lizing groups at aggregates-water interface. Small angle X-ray dif raction
analysis and spectroscopic (NMR and polarized UV-vis and IR) measure-
ments have been used to prove the structures of the aggregates [20, 26,
29-33]. In H-aggregates the typical spacing between the adjacent mole-
cules is approximately 3.4 Å along the stacking direction, which does not
depend on the chromonic concentration and the phase of the solution [20,
26, 31]. Here the line density of electric charge along the aggregate can be
very high depending on the ionization of polar groups. Unlike other aggre-
gates, there are no chemical bonds to i x the aggregates length in chro-
monic systems.
h e two commonly observed phases in chromonic systems are the uni-
axial nematic phase (N) and hexagonal columnar (M) phase (as shown in
Figure 9.1). In the N phase, molecules stack to form columnar aggregates
in which the long aggregates align parallel to each other having no posi-
tional order among the columns. In the M phase, molecules stack into col-
umns in a hexagonal array with aggregates forming a hexagonal lattice in
the plane perpendicular/parallel to the common direction (director, n) [31,
34]. It should be noted that upon increasing concentration of chromonic
materials the aggregates multiply, elongate and align parallel to the direc-
tor. Both one- and two-phase regions were observed for chromonic LCs.
h e phase properties and length of aggregates of chromonic LCs are
inl uenced by many factors including structure and concentration of
