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Öğe Anomalous behavior in the crossover between the negative and positive biaxial nematic mesophases in a lyotropic liquid crystal(Wiley-V C H Verlag Gmbh, 2014) Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Antonio M.A novel quaternary lyotropic liquid-crystalline mixture of dodecyltrimethylammonium bromide (DDTMABr)/sodium bromide/1-dodecanol/water, presenting the biaxial nematic phase (N-B) in addition to two uniaxial discotic (N-D) and calamitic (N-C) 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 (Delta chi) 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 N-B phase.Öğe Comments on the stabilization of the biaxial nematic phase in lyotropic liquid crystals(Taylor & Francis Ltd, 2017) Reis, Dennys; Akpınar, Erol; Figueiredo Neto, Antonio MartinsWe have discussed the stabilization of the biaxial nematic phase in lyotropic liquid crystals, based on previously published articles. New mixtures were reported, as well as a detailed investigation of the role of each component of the mixtures to the stabilization of the biaxial nematic phase. The roles of the kosmotropic or chaotropic characters of the ions and amphiphiles. head groups were also highlighted. [GRAPHICS] .Öğe 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(Amer Chemical Soc, 2013) Reis, Dennys; Akpınar, Erol; Figueiredo Neto, Antonio MartinsLyotropic cholesteric liquid crystalline phases were prepared by doping the quaternary mixture of potassium laurate (KL)/potassium sulfate (K2SO4)/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.Öğe Effect of alkyl chain length of alcohols on nematic uniaxial-to-biaxial phase transitions in a potassium laurate/alcohol/K2SO4/water lyotropic mixture(Taylor & Francis Ltd, 2012) Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Antonio MartinsLyotropic 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 (n), 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 n = n(KL) +/- 2, where n(KL) = 11 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 nKL. 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.Öğe Effect of Hofmeister anions on the existence of the biaxial nematic phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water(Taylor & Francis Ltd, 2015) Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Antonio MartinsLyotropic 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.Öğe Effect of the anionic azo dye sunset yellow in lyotropic mixtures with uniaxial and biaxial nematic phases(Elsevier, 2020) Akpınar, Erol; Topçu, Gökhan; Reis, Dennys; Figueiredo Neto, Antonio MartinsThe effects of the anionic azo dye Sunset Yellow on the stabilization of lyotropic uniaxial and biaxial nematic phases and on the uniaxial to biaxial phase transitions were studied. The dye was added in different concentrations to the host ternary mixture of cationic surfactant dodecyltrimethylammonium bromide, 1-dodecanol, and water. Furthermore, an investigation examined the role of the dye in the lyotropic host mixture in comparison to some inorganic salts containing anions of the Hofmeister series. The following inorganic salts were added to the same host mixture: NaBr, NaNO3, NaClO3, NaI, NaSCN, and NaClO4. All the samples were investigated by polarizing optical microscopy, laser conoscopy, and small-angle X-ray scattering. The results indicated that (a) the anionic dye Sunset Yellow has a chaotropic character between that of the I- and SCN- ions, being similar to the last one, (b) it gives a larger biaxial nematic phase temperature range in the partial phase diagram in comparison with the SCN- ion and other inorganic salts, and (c) it shifts the temperatures of both uniaxial to the biaxial nematic phase transitions. The small-angle X-ray scattering results also indicated that, at least in the investigated concentration range, the dye is mainly located on the micelles' surfaces, just like a conventional ion, and that on increasing concentration the dye molecules induces higher aggregation number of the micelles. (C) 2020 Elsevier B.V. All rights reserved.Öğe Effect of the presence of strong and weak electrolytes on the existence of uniaxial and biaxial nematic phases in lyotropic mixtures(Taylor & Francis Ltd, 2016) Akpınar, Erol; Otluoğlu, Kemal; Türkmen, Meriç; Canıöz, Cihan; Reis, DennysThe 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. [GRAPHICS] .Öğe Effect of the surfactant alkyl chain length on the stabilisation of lyotropic nematic phases(Taylor & Francis Ltd, 2018) Akpınar, Erol; Canıöz, Cihan; Türkmen, Meriç; Reis, Dennys; Figueiredo Neto, Antonio MartinsLyotropic 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. [GRAPHICS] .Öğe Effect of the surfactant head-group size dependence of the dye-surfactant interactions on the lyotropic uniaxial to biaxial nematic phase transitions(Elsevier, 2021) Akpınar, Erol; Uygur, Nazlı; Ordu, Öznur Demir; Reis, Dennys; Neto, A. M. FigueiredoIn the present study, the interactions of the anionic azo dye Sunset Yellow with dodecylalkylammonium bromide (DAABr) surfactants in lyotropic mixtures have been investigated for the stabilization of the different lyotropic nematic phases. Sunset Yellow doped mixtures of DAABr surfactants, with different head-group size, were studied by polarizing optical microscopy, laser conoscopy, and small-angle X-ray scattering. It was found that the extent of the interaction of Sunset Yellow with DAABr surfactant head groups on micelle surfaces affects the stabilization of different nematic phases, by causing the change in the micellar structural parameters. The results also indicated that the micelle surface curvature should be different in the three nematic phases. (C) 2021 Elsevier B.V. All rights reserved.Öğe Investigation of the interaction of alkali ions with surfactant head groups for the formation of lyotropic biaxial nematic phase via optical birefringence measurements(Spie-Int Soc Optical Engineering, 2013) Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Antonio MartinsLyotropic liquid crystals exhibiting nematic phases were obtained from the mixtures potassium laurate/alkali sulfate salts (M2SO4)/1-undecanol (UndeOH)/water and sodium dodecyl sulfate (SDS)/M2SO4/1-dodecanol (DDeOH)/water, where M2SO4 represents the alkali sulfate salts being Li2SO4, Na2SO4, K2SO4, Rb2SO4 or Cs2SO4. The birefringences measurements were performed via laser conoscopy. Our results indicated that cosmotropic and chaotropic behaviors of both ions and head groups are very important to obtain lyotropic biaxial nematic phase. To obtain the biaxial nematic phase, surfactant head group and ion present in lyotropic mixture have relatively opposite behavior, e.g. one more cosmotropic (more chaotropic) other less cosmotropic (less chaotropic) or vice versa.Öğe 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(Mdpi Ag, 2014) Akpınar, Erol; Reis, Dennys; Yıldıırm, Muhammet; Figueiredo Neto, Antonio MartinsAmino acid-based surfactants were used as the main surfactants to prepare new lyotropic mixtures presenting three nematic phases. One of them is biaxial (N-B), and the two others are uniaxial, discotic (N-D) and calamitic (N-C). 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 (similar to 9 degrees 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.Öğe On the coexistence of uniaxial nematic phases in the lyotropic mixture sodium dodecyl sulphate/1-decanol/D2O(Taylor & Francis Ltd, 2017) Oliveira-Filho, Arnaldo G.; Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Arnaldo MartinsPreviously examined lyotropicmixtures sodium dodecyl sulphate (SDS) 1-decanol (DeOH) - D2O were prepared in four different molar ratios, M = [D2O]/[SDS], where M = 30, 31, 32 and 33, to study the coexistence of uniaxial discotic (N D) and calamitic (N C) nematic phases as was claimed in recent articles. Those mixtures were investigated in a specific temperature range by laser conoscopy, polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) to construct partial phase diagram. By combining the results obtained from all these techniques, we could not observe the N-D - N-C phase coexistence in the phase diagram of SDS/DeOH/D2O after the detailed investigation, as it was formerly reported. The results obtained in this study are in a good agreement with symmetry arguments and theoretical predictions but not with literature claims. The present results are also consistent with experimental investigations of other lyotropic mixtures whose phase diagrams were previously identified.Öğe Properties of the uniaxial and biaxial nematic phases of the dodecyltrimethylammonium bromide/1-dodecanol/water lyotropic mixture including azo dye and drug molecules(Taylor & Francis Ltd, 2024) Akpınar, Erol; Tabak, Merve; Reis, Dennys; Neto, Antonio Martins FigueiredoLyotropic nematic phase properties of novel mixtures of dodecyltrimethylammonium bromide (DTMABr)/1-dodecanol (DDeOH)/water, doping with anionic azo dyes (amaranth and tartrazine) and a drug molecule disodium cromoglycate (cromolyn, DSCG), were investigated. The textures of the uniaxial and biaxial nematic phases were observed by polarizing optical microscopy (POM). Uniaxial-to-biaxial nematic phase transitions were determined from the temperature dependence of the birefringences via laser conoscopy. Partial phase diagrams of the mixtures were constructed as a function of concentrations of dyes and drug molecules by combining the POM and laser conoscopy results. Small-angle X-ray scattering (SAXS) was performed to evaluate micellar structure parameters. The results indicate that those dyes and drug molecules have a greater effect on (a) the nematic-nematic phase transition temperatures, (b) the biaxial-nematic phase-domain range in the partial phase diagrams, and (c) micelle-shape anisotropy, comparing to the addition of other conventional inorganic electrolyte ions. Furthermore, dye and drug molecules may be sequenced in the Hofmeister series of ions, considering the number of ionic groups in their molecular structures and also the chaotropic and/or kosmotropic degrees of the ionic groups. Since these molecules may have a resonance structure (e.g. DSCG), as a result of the existence of the aromatic parts in their structures, this resonance structure should be considered to investigate their effects on the stabilization of the lyotropic nematic phases.Öğe Study of the cholesteric-to-cholesteric phase transitions on the lyotropic mixture of KL/K2SO4/1-undecanol/water/brucine presenting the cholesteric biaxial phase(Taylor & Francis Ltd, 2013) Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Antonio MartinsLyotropic 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 +/- 0.40m(-1).Öğe A survey on the phase diagram of the system sodium dodecyl sulphate/1-decanol/D2O concerning an eventual coexistence of uniaxial nematic phases(Taylor & Francis Ltd, 2018) Oliveira-Filho, Arnaldo G.; Akpınar, Erol; Reis, Dennys; Figueiredo Neto, A. M.We examine an eventual coexistence of discotic () and calamitic () nematic uniaxial phases in the sodium dodecyl sulphate (SDS)/1-decanol (DeOH)/D2O mixture phase diagram. This coexistence is claimed in some previous works. Four different molar ratios [D2O]/[SDS] were investigated, where the square brackets represents the molar concentration of the compound, and 33, with temperatures ranging from 20 degrees C up to about 52 degrees C. The experimental techniques employed for phase identification were laser conoscopy, polarised optical microscopy and small-angle X-ray scattering. In the investigated surface of the phase diagram, no such nematic phases coexistence region was found. The discotic () and biaxial () nematic phases were identified in some regions of the phase diagram. Coexistence domains of the nematic ND/lamellar, and lamellar/micellar isotropic were identified. Our results agree with the theoretical predictions and symmetry arguments that consider the nematic/nematic phase transitions of second order, well described by a mean-field Landau-type model, preventing the nematic/nematic coexistence domains. Moreover, these results are consistent with all the experimental results we encountered in more than 30 different lyotropic mixtures investigated by using complementary experimental techniques to univocally identify the mesophases and phase-coexistence regions. [Graphics] .
