(6cp) Orders of Magnitude of Sudden Increases of X-Ray Diffraction Intensity in Surfactant-Based Liquid Crystals Triggered By Co-Self-Assembly

Orders of Magnitude of Sudden Increases
of X-ray Diffraction Intensity in

Surfactant-Based Liquid Crystals
Triggered by Co-Self-Assembly

Yoon Seob Lee^*

Department of
Chemistry, The University of Dayton, Dayton, OH 45469

James F. Rathman

Department of Chemical and Biomolecular Engineering,
The Ohio State University,

Columbus, OH 43210

*
To whom correspondence should be addressed. E-mail: ylee01@udayton.edu

Orders of magnitude of sudden
increases of x-ray diffraction intensity have been observed in cationic
surfactant-based silicate-containing liquid crystals with different structures
and symmetries. These increases have been triggered by the co-self-assembly of one
particular co-surfactant. A direct comparison reveals that the x-ray intensity
and number of peaks that can be indexed are matched with those of solid single
crystals. To the best of our knowledge, this kind of observations has never
been reported before, and we here present its early results. The liquid
crystals have been prepared in cetyltrimethylammonium chloride (CTAC) micelle
solutions with the concentration ranges from 1.0 to 24.8 wt.% with decyl
alcohol (DeOH) as a co-surfactant. Tetraethylorthosilicate (TESO) has been used
as a silicate source. Once all of the components are mixed with a
pre-determined order, the liquid crystals begin to form within 1-7 minutes at
room temperature. They have been further aged for 20 hours ? 3 days, and
collected from the solutions through filtration and characterized. The mixing
order of the components is one of the critical factors to have the desired liquid
crystals, and aging in some cases further improves the crystallinity of the
liquid crystals. X-ray powder diffraction (XRD) patterns were taken on Scintag
PAD-V diffractometer with Cu Ka radiation of 1.54060 Å wavelength
at 20 mA, 45 kV at room temperature. To ensure the direct comparison, the same silicon
holder, the same scanning condition, and the same slit set-ups have been used
throughout the entire measurements. The changes in the x-ray intensity have
been compared with those liquid crystals obtained from the same or similar
self-assembly systems but without the co-surfactant, and those liquid crystals
that are obtained with other additives which have a potential to act as a
co-surfactant. The additives that have been tested in this study include simple
organic molecules (benzene, phenol, benzyl alcohol, cyclohexane, n-hexane,
cyclohexanol, 1-methyl-cyclohexanol, 3-methyl-cyclohexanol, 1-hexanol,
2-hexanol, and 3-hexanol) and surfactants (dodecyldimethylamine oxide and
perfluoro-1-octanesulfonyl fluoride). One such example that shows this dramatic
increase of the x-ray intensity is shown in the figure below. The one on the
right is the XRD pattern of the bicontinuous cubic liquid crystal obtained
without decyl alcohol, and the one on the left is the bicontinuous cubic liquid
crystal obtained with decyl alcohol. The x-ray intensity of the highest peak
has been increased over 50 times at the same condition and there are 18 peaks
that are clear enough to be indexed (shown is a part of them at low 2q
region and typical solid single crystals with cubic structure show 12 ? 20
peaks). We have identified a region where the most of the sharp-jumps like this
take place for this particular self-assembly system and the jump is often more
than 100 times. This not only drastically improves the quality of the existing
liquid crystals but creates the liquid crystals with new symmetries as well.

The kinetic studies that have been
performed along with the XRD measurements show that there is a clear
correlation between the increases of the x-ray intensity and the rate of the
initial reaction. As the degree of the co-self-assembly increases, this rate is
getting faster, and results in the higher increases of the x-ray intensity.
Decyl alcohol, once co-self-assembled with the cationic micelles, reduces the
distance between the adjacent cationic binding sites, which provides the
geometrically-matching environment for the silicate anions to be condensed
evenly and faster on the surface of the micelles, which increases the rate of
the inter-micellar condensation. This uniform condensation of the silicate
anions, each of which bound with each of the cationic head-group of the
surfactant molecule, reduces the defects and mismatches within the self-assembling
liquid crystals, thus making the final liquid crystals to have much higher
degree of the long-range order, possibly over the macroscopic scale. Nature
uses a very similar strategy for her biomineralization, in which the proteins
provide the geometrically-matching binding sites for inorganic precursor ions.
The difference is that those condensing inorganic ions on the surface of
proteins do not re-self-assemble the whole structures of the proteins.

These very unusual phenomena can logically be explained
using the concept of force balance for self-assembly (Lee, Y. S. Self-Assembly
and Nanotechnology: A Force Balance Approach, Wiley, 2008; Lee, Y.
S. Self-Assembly and Nanotechnology Systems: Design, Characterization, and
Applications, Wiley, 2012), and can be expanded to the viewpoint of the
materials discovery by self-assembly. The concept of force balance states that
all possible self-assembly building units including atomic, molecular,
polymeric, colloidal, bio-mimetic, and biological are composed of up to five
distinctive structural segments. What determines the process (critical and
hierarchical) and outcome (structural diversity and emergent properties) of
self-assembly is how these segments interact and balance each other through
intermolecular and colloidal forces by following five distinctive rules. Those
rules are: Rule 1. To have self-assembly, self-assembly building units
must have at least one segment that can generate an attractive force between
them, Rule 2. They also must have at least one segment that can generate
a repulsive force which can counterbalance the attractive force, Rule 3.
Certain segments such as hydrogen-bonding group make self-assembly to grow in
one direction, Rule 4. Asymmetric packing segments such as multiple hydrogen
bonding site and chiral carbon make self-assembly to assemble in an uneven
manner, and Rule 5. Functional segments such as ferrocenyl group and
azo-benzene group make self-assembly system to communicate with the
environment.

How far can we push chemical
self-assembly? was identified as one of the top 25 big
questions for the next quarter-century by Science (Science, 2005,
309, 95). Self-assembly offers a powerful way to fabricate nanoscale
materials and systems with controlled complexity and hierarchy. The problem
however has been how we use self-assembly which is seemingly chaotic in a
logical way while maintaining a good overview over its promises. Using the results
that are being reported here, a discussion will be made how to use self-assembly
to control nanoscale features at the level of individual building unit, which
is to show how to design the bottom-up building of nanoscale materials and
systems for a given purpose. It strongly suggests that there might be a family
of similar liquid crystal materials to be discovered from other self-assembly
systems, which of course could lead us to find new structures and new
properties. We anticipate that the properties of these highly-ordered liquid
crystals might be quite different from those of the known liquid crystals whose
structural order extends usually only up to the microscopic domains. These properties
include birefringence, viscoelasticity, the effect of chiral dopants, the
responses to light (with light harvesters) and temperature, and the effects of
electric and magnetic fields.