Seminário do INCT/NAP/GFCx
"Effect of the presence of strong and weak electrolytes on the existence of the uniaxial and biaxial nematic phases in lyotropic mixtures"
Palestrante: Antonio Martins Figueiredo Neto
Professor Titular do Instituto de Física da USP
6ª. FEIRA 19/08/2016, às 15h00
Local: Auditório Adma Jafet
Abstract:
The lyotropic mixture of potassium laurate/decanol/water presenting only the uniaxial nematic calamitic phase was doped with one strong (KCl) and 13 weak electrolytes with phenyl-rings (DL-mandelic acid, benzoic acid, DL-phenyllactic acid, phenylacetic acid, phenol, phenylmethanol, benzene) and with cyclohexyl-ring (RS-hexahydromandelic acid, cyclohexanecarboxylic acid, cyclohexaneacetic acid, cyclohexanol, cyclohexylmethanol and cyclohexane), separately. The nematic phase sequences, in particular the presence of the biaxial nematic phase, were investigated as a function of the dopant molar concentration and temperature. The laser conoscopy technique was used to characterize the different nematic phases. Weak electrolytes having COOH group as polar part were shown to be very effective in stabilizing the three nematic phases (two uniaxial and the biaxial). Guest molecules with only the OH group did not show any effect on the stabilization of other nematic phases. Our result!
s indicated that there is no direct relation between the solubility of the guest molecule in water and its effectiveness to stabilize the different nematic phases. However, we observed that the guest molecule acidity constant Ka or pKa was shown to be an important parameter: dopants with pKa in the acidic region (pKa < 7) give rise to the three nematic phases. These dopants were shown to be more effective in the stabilization of the three nematic phases when comparing to strong electrolytes (as inorganic alkali salts) since smaller molar concentrations of them are needed to induce the presence of the three nematic phases. We interpreted the results in terms of the location of dopant molecules at the micelle surfaces and their effectiveness of screening the polar head (of the main amphiphile) repulsion. This situation favors the relaxing of more curved surfaces in the micelle, increasing the more flat micellar surfaces. These flat surfaces are perpendicular to the main amphi!
philic bilayer. The increase of the micellar dimensions in thi!
s plane favors the orientational fluctuations characteristic of the nematic discotic phase, as observed in our experimental results. Finally, the use of weak electrolytes may be accepted as a new control parameter to obtain different nematic phases, especially biaxial one [1].
Financial support: Brazil - CNPq, FAPESP, INCT and NAP-FCx; Turkey TÜBİTAK and BAP.
References
[1] E. Akpinar et al., accepted Liquid Crystals.
