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: firstname.lastname@example.org
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.
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