Erol Akpinar

Effect of alkyl chain length of alcohols on nematic uniaxial-to-biaxial phase transitions in a potassium laurate/alcohol/K2SO4/water lyotropic mixture

Lyotropic liquid crystalline quaternary mixtures of potassium laurate (KL), potassium sulphate (K2SO4)/alcohol (n-OH)/water, with the alcohols having different numbers of carbon atoms in the alkyl chain ( ), from 1-octanol to 1-hexadecanol, were investigated by optical techniques (optical microscopy and laser conoscopy). The biaxial nematic phase domain is present in a window of values of , where is the number of carbon atoms in the alkyl chain of KL. The biaxial phase domain became smaller and the uniaxial-to-biaxial phase transition temperatures shifted to relatively higher temperatures upon going from 1-nonanol to 1-tridecanol. Moreover, compared with other lyotropic mixtures these new mixtures present high birefringence values, which we expect to be related to the micellar shape anisotropy. Our results are interpreted assuming that alcohol molecules tend to segregate in the micelles in a way that depends on the relative value of n with respect to . The larger the value of n, the more alcohol molecules tend to be located in the curved parts of the micelle, favoring the uniaxial nematic calamitic phase with respect to the biaxial and uniaxial discotic nematic phases.

Lyotropic mixture made of potassium laurate/1-undecanol/K2SO4/water presenting high birefringences and large biaxial nematic phase domain: a laser conoscopy study

The lyotropic liquid crystalline quaternary mixture made of potassium laurate (KL), potassium sulphate, 1-undecanol and water was investigated by experimental optical methods (optical microscopy and laser conoscopy). In a particular temperature and relative concentrations range, the three nematic phases (two uniaxial and one biaxial) were identified. The biaxial domain in the temperature/KL concentration surface is larger when compared to other lyotropic mixtures. Moreover, this new mixture gives nematic phases with higher birefringence than similar systems. The behavior of the symmetric tensor order parameter invariants σ3 and σ2 calculated from the measured optical birefringences supports that the uniaxial-to-biaxial transitions are of second order, described by a mean-field theory.

Effect of alkyl chain length of alcohols on cholesteric uniaxial to cholesteric biaxial phase transitions in a potassium laurate/alcohol/potassium sulfate/water/brucine lyotropic mixture: evidence of a first-order phase transition.

Lyotropic cholesteric liquid crystalline phases were prepared by doping the quaternary mixture of potassium laurate (KL)/potassium sulfate (K(2)SO(4))/alcohol (n-OH)/water with the chiral agent brucine. Different long-chain alcohols whose alkyl chains (n) vary from 8 (1-octanol) to 16 (1-hexadecanol) were used. The cholesteric uniaxial to cholesteric biaxial phase transitions were investigated by measuring the birefringences via polarizing optical microscopy, and the phase diagram was constructed as a function of the alkyl chain length of the alcohols. Alcohols with 9 ≤ n ≤ 12 presented the three cholesteric phases (cholesteric discotic-Ch(D), cholesteric biaxial-Ch(B), and cholesteric calamitic-Ch(C)). The Ch(D)-to-Ch(B) transition was shown to be continuous, with a bare correlation length bigger than the typical micellar dimensions. Mixtures with n = 8 and n = 13 showed a first-order phase transition between the Ch(D) and the Ch(C) phases, without the presence of the Ch(B) phase in between. These results are interpreted in terms of the nanosegregation of the alcohol molecules in the micelles with respect to the main amphiphiles molecules.

Anomalous behavior in the crossover between the negative and positive biaxial nematic mesophases in a lyotropic liquid crystal

A novel quaternary lyotropic liquid-crystalline mixture of dodecyltrimethylammonium bromide (DDTMABr)/sodium bromide/1-dodecanol/water, presenting the biaxial nematic phase (NB ) in addition to two uniaxial discotic (ND) and calamitic (NC) nematic ones, was synthesized. The partial phase diagram of this new mixture was constructed as a function of the DDTMABr molar-fraction concentration. The phase transitions from uniaxial to biaxial nematic phases were studied by means of the temperature dependence of the optical birefringence. In a particular region of the phase diagram, anomalous behavior was observed in the crossover from N-B to N+b: the contrast of the conoscopic fringes, which allows the birefringence measurements, almost vanishes, and the sample loses its alignment. This behavior, which was not observed before in lyotropics, was interpreted as a decrease in the mean diamagnetic susceptibility anisotropy (Δχ) of the sample, which was related to the shape anisotropy of the micelles. Small-angle X-ray scattering measurements were performed to evaluate the micellar shape anisotropy; these revealed that this mixture presented a smaller shape anisotropy than those of other lyotropic micellar systems presenting the NB phase.

Effect of Hofmeister anions on the existence of the biaxial nematic phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water

Lyotropic mixtures including different sodium salts of Hofmeister anions were studied in order to investigate the effect of these anions on the existence of biaxial nematic phase and on the uniaxial to biaxial phase transitions. For this purpose, these sodium salts were added singly into different mixtures of dodecyltrimethylammonium bromide (DDTMABr)/1-dodecanol/water, keeping the relative molar concentration of all the constituents constant. The uniaxial to biaxial phase transitions were determined from the temperature dependence of the birefringences by laser conoscopy. Micellar shape anisometry and average micellar volume were evaluated from small-angle X-ray scattering measurements. The results indicated that the Hofmeister anions were bounded to the head groups of DDTMABr molecules at the micelles’ surfaces, which significantly affect the different orientational fluctuations responsible for the formation of different nematic phases, biaxial phase domains and uniaxial to biaxial phase transition temperatures.

Study of the Cholesteric-to-Cholesteric Phase Transitions on the Lyotropic Mixture of KL/K2SO4/1-Undecanol/Water/Brucine Presenting the Cholesteric Biaxial Phase

Lyotropic cholesteric liquid crystalline phases were prepared by doping the quaternary mixture of potassium laurate (KL)/potassium sulfate (K2SO4)/1-undecanol (UndeOH)/water with brucine. The phase diagram was constructed as a function of the brucine concentration. Three cholesteric phases were identified: cholesteric biaxial, calamitic uniaxial, and discotic uniaxial. It was observed that there is a critical brucine concentration (X* b ) to cholesterize the nematic host phase when the mixture is confined in a thin sample holder, below which the cholesteric helical arrangement is not achieved. The helical twisting power of brucine was calculated as 12.12 ± 0.40 μm−1.

Effect of the surfactant alkyl chain length on the stabilisation of lyotropic nematic phases

ABSTRACT Lyotropic quaternary mixtures of potassium alkanoates (KCx) and sodium alkyl sulphates (NaCxS), where x is the number of carbon atoms in their alkyl chains, were prepared to investigate the effect of the surfactant alkyl chain length on the stabilisation of lyotropic nematic phases. The lyotropic mixtures investigated were formed by the dissolution of KCx (NaCxS) surfactants in the mixture of Rb2SO4/1-decanol/water (Na2SO4/1-decanol/water), separately. The uniaxial-to-biaxial nematic phase transitions were identified from the temperature dependence of the birefringences of the nematic phases by means of laser conoscopy. The micelle dimensions were obtained from small-angle X-ray scattering measurements. It was observed that the increase in the surfactant alkyl chain length causes the micellar growth in the plane perpendicular to the main amphiphile bilayer. The surfactant alkyl chain length plays a key role on the shape anisotropy of micelles, which triggers the orientational fluctuations that are responsible for the stabilisation of the different lyotropic nematic phases. Graphical Abstract

Effect of micelle size and intermicellar distance on the chirality transfer in the intrinsic lyotropic cholesteric phases

Lyotropic nematic and cholesteric liquid crystalline phases were produced using the racemic amphiphiles, potassium N-dodecanoyl-DL-alaninate (DL-KDDA) and DL-serinate (DL-KDDS) and their l-enantiomers, potassium N-dodecanoyl- L-alaninate (L-KDDA) and L-serinate (l-KDDS), respectively. The different racemic nematic and cholesteric phases were characterised by a polarising light microscope. In order to calculate the helical twisting powers (htp) of the cholesteric phases, their pitches were determined as a function of concentrations of the L-enantiomers. The results showed that the L-KDDA system had a higher helical twisting power than the L-KDDS system. Small-angle X-ray scattering (saxs) techniques were performed for determination of the structural differences between the racemic micelles and the chiral micelles of the racemic and intrinsic cholesteric phases, respectively. Saxs results showed that the intrinsic cholesteric micelles are more compact than their corresponding racemic micelles. These results were interpreted in terms of the different molecular arrangements of the chiral amphiphiles in micelles compared to the amphiphiles in racemic micelles. According to our experimental results and the results on induced cholesteric phases in the literature, we proposed that there must be (1) a crucial distance and (2) a critical micelle size in order to obtain chiral induction, supporting some theoretical studies reported in the literature. In addition, we also showed that chiral induction does not take place completely as was suggested by pairwise interaction or sterical interaction models in the literature, but through anisotropic chiral interactions via distorted micelles for which a crucial distance between the micelles and a critical micelle size are required.

Effect of the presence of strong and weak electrolytes on the existence of uniaxial and biaxial nematic phases in lyotropic mixtures

ABSTRACT The lyotropic mixture of potassium laurate/decanol/water presenting only the uniaxial nematic calamitic phase was doped with one strong (potassium chloride, KCl) and 11 weak electrolytes with phenyl-rings (DL-mandelic acid, benzoic acid, DL-phenyllactic acid, phenylacetic acid, phenol and phenylmethanol) and with cyclohexyl-ring (RS-hexahydromandelic acid, cyclohexanecarboxylic acid, cyclohexaneacetic acid, cyclohexanol and cyclohexylmethanol), separately. We also chose two nonpolar dopant molecules, benzene and cyclohexane, for the comparison of them with weak electrolytes, since they are located in the hydrocarbon core of the micelle. 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 and small-angle X-ray scattering techniques were used to characterise the different nematic phases. Weak electrolytes having –COOH group as polar part were found to be very effective in stabilising the three nematic phases (two uniaxial and a biaxial). Guest molecules with only the –OH group did not show any effect on the stabilisation of other nematic phases. The experimental results are interpreted considering the screening effect of the hydrophilic parts of the dopants on the repulsion between the polar heads of the main amphiphilic molecules at micelle surfaces. This process favours the increase of the more flat micellar surfaces of micelles, which triggers the orientational fluctuations responsible for the biaxial and discotic nematic phases. Graphical Abstract

New Lyotropic Mixtures with Non-Chiral N-Acylamino Acid Surfactants Presenting the Biaxial Nematic Phase Investigated by Laser Conoscopy, Polarized Optical Microscopy and X-ray Diffraction

Amino acid-based surfactants were used as the main surfactants to prepare new lyotropic mixtures presenting three nematic phases. One of them is biaxial (NB), and the two others are uniaxial, discotic (ND) and calamitic (NC). These surfactants were the non-chiral molecules, potassium N-dodecanoyl-dl-alaninate (dl-KDDA), potassium N-dodecanoyl-dl-serinate (dl-KDDS), disodium N-dodecanoyl-dl-aspartate (dl-NaDDAs) and potassium N-dodecanoyl-glycinate (KDDGly). Measurements of the optical birefringences and X-ray diffraction analysis were used to characterize the nematic phases and phase transitions. Mixtures with dl-KDDS exhibited the largest biaxial phase domain (~9 °C) with respect to the other mixtures in this study. The results obtained with the KDDGly mixture showed that the existence of hydrogen bonding between the head groups of the surfactant molecules seems to hinder the orientation of the micelles under the action of an external magnetic field.