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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 Comparison between lyotropic cholesteric phase behavior with partly fluorinated surfactants and their exact hydrogenated counterparts(Elsevier Science Bv, 2018) Akpınar, Erol; Yurdakul, Sinan; Figueiredo Neto, Antonio MartinsIn this study, we examined lyotropic intrinsic cholesteric phase properties of some amino acid-based chiral surfactants, with partly fluorinated and hydrogenated chains, to investigate the effect of the twist structure of fluorocarbon chain on the helical twisting power of chiral surfactants. We synthesized some chiral and achiral surfactants, and their fluorinated/hydrogenated counterpart surfactants, taking into account the rule that [1 CF2 = 1.5 CH2]. We prepared lyotropic mixtures exhibiting discotic cholesteric phases by dissolution of chiral surfactants L-alaninehydrochloride undecylester (L-AUnDE), L-serinehydrochloride undecylester (L-SUnDE) and their partly fluorinated counterparts (L-APFOE and L-SPFOE, respectively) into sodium chloride (NaCl)/water mixtures, separately. The pitch measurements were used to evaluate the helical twisting powers of each fluorinated/hydrogenated chiral surfactants. The results indicated that the twist structure of fluorocarbon chain provides higher helical twisting power with respect to the hydrocarbon chain. (C) 2018 Elsevier B.V. All rights reserved.Öğe Dynamic light scattering and viscosity measurements in a ternary and quaternary discotic lyotropic nematic liquid crystal : tuning the backflow with salt(Amer Physical Soc, 2013) Risi, C. L. S.; Figueiredo Neto, Antonio Martins; Akpınar, Erol; Lacerda Santos, M. B.Using a dynamic light scattering technique, we measure the damping rate of thermal fluctuations of the nematic director for the so-called disklike nematic N-D phase of both the ternary lyotropic K-laurate-1-decanol-H2O system and the quaternary one of similar composition except for the addition of salt (K2SO4). By varying the scattering angle in suitable geometries and polarizations, we are able to measure the orientational diffusivities associated with the pure deformations of splay and twist. A previous study made in the N-D phase of the same ternary system yielded a large deviation between the splay and twist diffusivities. The effect was then interpreted in terms of the anisotropy between their associated viscosities due to induced flows, or backflow. In the present work we observe a strong increase of the backflow as an effect of the added salt. In addition, we make auxiliary measurements of shear viscosity and magnetic instabilities, which help to characterize the effect of the salt in the orientational diffusivities as they are mixed quantities involving elastic constants and viscosity coefficients.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 head-group size of some tetradecylalkylammonium bromide surfactants on obtaining the lyotropic biaxial nematic phase(Springer, 2019) Akpınar, Erol; Güner, Emre; Ordu, Öznur Demir; Figueiredo Neto, Antonio MartinsLyotropic quaternary mixtures of some tetradecylalkylammonium bromide surfactants were prepared to examine the effect of the size of the surfactant head group on the stabilization of different lyotropic nematic phases. The lyotropic mixtures were prepared by the addition of the tetradecylalkylammonium bromides (TTAABr) in the mixture of NaBr/decanol (DeOH)/water. The uniaxial to biaxial nematic phase transitions were determined via laser conoscopy. Some micellization parameters such as critical micelle concentration, degree of counterion binding and micellization Gibbs energy were evaluated from the electrical conductivity measurements of diluted binary surfactants/water solutions. The results indicate that the head-group size of the surfactant molecules influences the amphiphilic molecular aggregate topology. Moreover, the effective area per surfactant head group is a key parameter on stabilizing the lyotropic biaxial nematic phase.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 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 Experimental conditions for the stabilization of the lyotropic biaxial nematic cesophase(Mdpi, 2019) Akpınar, Erol; Figueiredo Neto, Antonio MartinsNematic phases are some of the most common phases among the lyotropic liquid crystalline structures. They have been widely investigated during last decades. In early studies, two uniaxial nematic phases (discotic, N-D, and calamitic, N-C) were identified. After the discovery of the third one, named biaxial nematic phase (N-B) in 1980, however, some controversies in the stability of biaxial nematic phases began and still continue in the literature. From the theoretical point of view, the existence of a biaxial nematic phase is well established. This review aims to bring information about the historical development of those phases considering the early studies and then summarize the recent studies on how to stabilize different nematic phases from the experimental conditions, especially, choosing the suitable constituents of lyotropic mixtures.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 Role of kosmotrope-chaotrope interactions at micelle surfaces on the stabilization of lyotropic nematic phases(Springer, 2016) Akpınar, Erol; Türkmen, Meriç; Canıöz, Cihan; Figueiredo Neto, Antonio MartinsThree lyotropic quaternary systems of ionic surfactants were prepared to investigate the role of kosmotrope-chaotrope interactions at the micelle surfaces on stabilizing the different nematic phases. The ionic surfactants were potassium laurate (KL), sodium dodecylsulfate (SDS) and tetradecyltrimethylammonium bromide (TDTMABr), where KL is a kosmotrope surfactant, and others are chaotrope. The first system consisted of KL/decanol (DeOH)/water/alkali sulfate and the second of SDS/DeOH/water/alkali sulfate. The third system was prepared by adding sodium salts of chaotropic or kosmotropic anions to the primary mixture of TDTMABr/DeOH/water, separately. The characteristic textures of discotic nematic (N (D)), biaxial nematic (N (B)) and calamitic nematic (N (C)) phases were identified under polarizing light microscope. Laser conoscopy was employed to determine the uniaxial-to-biaxial phase transitions. The kosmotrope-kosmotrope or chaotrope-chaotrope interactions between the head groups of the surfactants and the ions of the electrolytes led to the stabilization of the N (D) phase. On the other hand, kosmotrope-chaotrope interactions stabilize the N (B) and/or N (C) 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).
