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Lech Longa - One of the best experts on this subject based on the ideXlab platform.

  • Biaxial Nematic Phase in model bent-core systems.
    Physical review letters, 2011
    Co-Authors: Piotr Grzybowski, Lech Longa
    Abstract:

    We study a class of models for bent-core molecules using low density version of Local Density Functional Theory. Arms of the molecules are modeled using two- and three Gay-Berne (GB) interacting units of uniaxial and Biaxial symmetry. Dipole-dipole interactions are taken into account by placing a dipole moment along the C2 symmetry axis of the molecule. The main aim of the study is to identify molecular factors that can help stabilizing the Biaxial Nematic Phase. The Phase diagrams involving isotropic (I), uniaxial (NU) and Biaxial (NB) Nematic Phases are determined at given density and dipole strength as function of bent angle. For molecules composed of two uniaxial arms a direct I − NB Phase transition is found at a single Landau point, which moves towards lower bent angles with increasing dipole magnitude. For the three-segment model strengthening of the dipole-dipole interaction results in appearance of a line of Landau points. There exists an optimal dipole strength for which this line covers the maximal range of opening angles. Interestingly, the inclusion of Biaxial GB ellipsoids as building blocks reveals the direct I − NB transitions line even in a non-polar, two-arms model. The line is shifted towards higher opening angles as compared to the uniaxial case.

  • Landau-De Gennes Theory of Biaxial Nematics Reexamined
    Physical Review E, 2008
    Co-Authors: David W Allender, Lech Longa
    Abstract:

    Recent experiments report that the long-looked-for thermotropic Biaxial Nematic Phase has been finally detected in some thermotropic liquid crystalline systems. Inspired by these experimental observations, we concentrate on some elementary theoretical issues concerned with the classical sixth-order Landau\char21{}de Gennes free energy expansion in terms of the symmetric and traceless tensor order parameter ${Q}_{\ensuremath{\alpha}\ensuremath{\beta}}$. In particular, we fully explore the stability of the Biaxial Nematic Phase giving analytical solutions for all distinct classes of the Phase diagrams that theory allows. This includes diagrams with triple, critical, and tricritical points and with multiple (reentrant) Biaxial and uniaxial Phase transitions. A brief comparison with predictions of existing molecular theories is also given.

  • Landau-de Gennes theory of Biaxial Nematics reexamined.
    Physical review. E Statistical nonlinear and soft matter physics, 2008
    Co-Authors: David W Allender, Lech Longa
    Abstract:

    Recent experiments report that the long-looked-for thermotropic Biaxial Nematic Phase has been finally detected in some thermotropic liquid crystalline systems. Inspired by these experimental observations, we concentrate on some elementary theoretical issues concerned with the classical sixth-order Landau-de Gennes free energy expansion in terms of the symmetric and traceless tensor order parameter Q alpha beta. In particular, we fully explore the stability of the Biaxial Nematic Phase giving analytical solutions for all distinct classes of the Phase diagrams that theory allows. This includes diagrams with triple, critical, and tricritical points and with multiple (reentrant) Biaxial and uniaxial Phase transitions. A brief comparison with predictions of existing molecular theories is also given.

  • Stability of Biaxial Nematic Phase for systems with variable molecular shape anisotropy.
    Physical Review E, 2007
    Co-Authors: Lech Longa, Grzegorz Pajak, Thomas Wydro
    Abstract:

    We study the influence of fluctuations in molecular shape on the stability of the Biaxial Nematic Phase by generalizing the mean field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems - a mixture of potassium laurate, 1-decanol, and D2O, where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the Biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimeric- or tetrapod-like molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the Biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to Biaxial Phase is observed.

  • Stability of Biaxial Nematic Phase for systems with variable molecular shape anisotropy.
    Physical review. E Statistical nonlinear and soft matter physics, 2007
    Co-Authors: Lech Longa, Grzegorz Pajak, Thomas Wydro
    Abstract:

    We study the influence of fluctuations in molecular shape on the stability of the Biaxial Nematic Phase by generalizing the mean-field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems--a mixture of potassium laurate, 1-decanol, and D2O , where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the Biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimericlike or tetrapodlike molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the Biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to Biaxial Phase is observed.

A. M. Figueiredo Neto - One of the best experts on this subject based on the ideXlab platform.

  • Effect of the anionic azo dye Sunset Yellow in lyotropic mixtures with uniaxial and Biaxial Nematic Phases
    Journal of Molecular Liquids, 2020
    Co-Authors: Erol Akpinar, Dennys Reis, Gokhan Topcu, A. M. Figueiredo Neto
    Abstract:

    Abstract The 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.

  • Effect of head-group size of some tetradecylalkylammonium bromide surfactants on obtaining the lyotropic Biaxial Nematic Phase
    The European Physical Journal E, 2019
    Co-Authors: Erol Akpinar, Emre Guner, Oznur Demir-ordu, A. M. Figueiredo Neto
    Abstract:

    Lyotropic 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. Graphical abstract

  • Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic MesoPhase
    Crystals, 2019
    Co-Authors: Erol Akpinar, A. M. Figueiredo Neto
    Abstract:

    Nematic 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, ND, and calamitic, NC) were identified. After the discovery of the third one, named Biaxial Nematic Phase (NB) 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.

  • Comments on the stabilization of the Biaxial Nematic Phase in lyotropic liquid crystals
    Molecular Crystals and Liquid Crystals, 2017
    Co-Authors: Dennys Reis, Erol Akpinar, A. M. Figueiredo Neto
    Abstract:

    We 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 investigatio...

  • Effect of Hofmeister anions on the existence of the Biaxial Nematic Phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water
    Liquid Crystals, 2015
    Co-Authors: Erol Akpinar, Dennys Reis, A. M. Figueiredo Neto
    Abstract:

    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 temp...

Erol Akpinar - One of the best experts on this subject based on the ideXlab platform.

  • Effect of the anionic azo dye Sunset Yellow in lyotropic mixtures with uniaxial and Biaxial Nematic Phases
    Journal of Molecular Liquids, 2020
    Co-Authors: Erol Akpinar, Dennys Reis, Gokhan Topcu, A. M. Figueiredo Neto
    Abstract:

    Abstract The 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.

  • Effect of head-group size of some tetradecylalkylammonium bromide surfactants on obtaining the lyotropic Biaxial Nematic Phase
    The European Physical Journal E, 2019
    Co-Authors: Erol Akpinar, Emre Guner, Oznur Demir-ordu, A. M. Figueiredo Neto
    Abstract:

    Lyotropic 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. Graphical abstract

  • Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic MesoPhase
    Crystals, 2019
    Co-Authors: Erol Akpinar, A. M. Figueiredo Neto
    Abstract:

    Nematic 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, ND, and calamitic, NC) were identified. After the discovery of the third one, named Biaxial Nematic Phase (NB) 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.

  • Comments on the stabilization of the Biaxial Nematic Phase in lyotropic liquid crystals
    Molecular Crystals and Liquid Crystals, 2017
    Co-Authors: Dennys Reis, Erol Akpinar, A. M. Figueiredo Neto
    Abstract:

    We 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 investigatio...

  • Effect of Hofmeister anions on the existence of the Biaxial Nematic Phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water
    Liquid Crystals, 2015
    Co-Authors: Erol Akpinar, Dennys Reis, A. M. Figueiredo Neto
    Abstract:

    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 temp...

Thomas Wydro - One of the best experts on this subject based on the ideXlab platform.

  • Stability of Biaxial Nematic Phase for systems with variable molecular shape anisotropy.
    Physical Review E, 2007
    Co-Authors: Lech Longa, Grzegorz Pajak, Thomas Wydro
    Abstract:

    We study the influence of fluctuations in molecular shape on the stability of the Biaxial Nematic Phase by generalizing the mean field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems - a mixture of potassium laurate, 1-decanol, and D2O, where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the Biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimeric- or tetrapod-like molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the Biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to Biaxial Phase is observed.

  • Stability of Biaxial Nematic Phase for systems with variable molecular shape anisotropy.
    Physical review. E Statistical nonlinear and soft matter physics, 2007
    Co-Authors: Lech Longa, Grzegorz Pajak, Thomas Wydro
    Abstract:

    We study the influence of fluctuations in molecular shape on the stability of the Biaxial Nematic Phase by generalizing the mean-field model of Mulder and Ruijgrok [Physica A 113, 145 (1982)]. We limit ourselves to the case when the molecular shape anisotropy, represented by the alignment tensor, is a random variable of an annealed type. A prototype of such behavior can be found in lyotropic systems--a mixture of potassium laurate, 1-decanol, and D2O , where distribution of the micellar shape adjusts to actual equilibrium conditions. Further examples of materials with the Biaxial Nematic Phase, where molecular shape is subject to fluctuations, are thermotropic materials composed of flexible trimericlike or tetrapodlike molecular units. Our calculations show that the Gaussian equilibrium distribution of the variables describing molecular shape (dispersion force) anisotropy gives rise to new classes of the Phase diagrams, absent in the original model. Depending on properties of the shape fluctuations, the stability of the Biaxial Nematic Phase can be either enhanced or depressed, relative to the uniaxial Nematic Phases. In the former case the splitting of the Landau point into two triple points with a direct Phase transition line from isotropic to Biaxial Phase is observed.

A. J. Palangana - One of the best experts on this subject based on the ideXlab platform.

  • refractive index measurements in a reentrant discotic Nematic Biaxial Nematic Phase transition
    Phase Transitions, 2018
    Co-Authors: W.s. Braga, O.r. Santos, D.d. Luders, A.r. Sampaio, N.m. Kimura, M. Simões, A. J. Palangana
    Abstract:

    The optical characterization of uniaxial (reentrant discotic – NDr and discotic – ND) and Biaxial Nematic (NB) Phases requires the measurements of two and three refractive indices, respectively. Th...

  • Conoscopic image of a Biaxial Nematic Phase in a sodium decyl sulfate – decanol – D2O mixture
    Molecular Crystals and Liquid Crystals, 2016
    Co-Authors: O.r. Santos, W.s. Braga, D.d. Luders, A.r. Sampaio, N.m. Kimura, M. Simões, A. J. Palangana
    Abstract:

    ABSTRACTThe Nematic Phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The Phase diagram obtained from these lyotropic materials predicts the occurrence of a direct Phase transition, which does not present the Biaxial Nematic Phase, between the discotic (ND) and calamitic (NC) Nematic Phases. A Biaxial Nematic (NB) Phase is optically characterized and confirmed through conoscopic image, inside the Biaxial range, between the two uniaxial Nematic Phases. Also, their respective transition points are determined by means of image processing. The NB Phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial Nematic Phases.

  • conoscopic image of a Biaxial Nematic Phase in a sodium decyl sulfate decanol d2o mixture
    Molecular Crystals and Liquid Crystals, 2016
    Co-Authors: O.r. Santos, W.s. Braga, D.d. Luders, A.r. Sampaio, N.m. Kimura, M. Simões, A. J. Palangana
    Abstract:

    ABSTRACTThe Nematic Phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The Phase diagram obtained from these lyotropic materials predicts the occurrence of a direct Phase transition, which does not present the Biaxial Nematic Phase, between the discotic (ND) and calamitic (NC) Nematic Phases. A Biaxial Nematic (NB) Phase is optically characterized and confirmed through conoscopic image, inside the Biaxial range, between the two uniaxial Nematic Phases. Also, their respective transition points are determined by means of image processing. The NB Phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial Nematic Phases.

  • temperature dependence of refractive indices near uniaxial Biaxial Nematic Phase transition
    Physics Letters A, 2006
    Co-Authors: P.a. Santoro, A.r. Sampaio, H. L. F. Da Luz, A. J. Palangana
    Abstract:

    Abstract The uniaxial discotic ( N D ) and calamitic ( N C ) Nematic Phases are characterized by an order parameter which is a second-rank, symmetric, traceless tensor with two different eigenvalues and three in the Biaxial Nematic ( N B ) Phase. The optical dielectric tensor may be chosen as the microscopic order parameter. The optical characterization of these uniaxial and Biaxial Nematic Phases requires the measurements of two and three refractive indices, respectively. These optical parameters were determined near the reentrant isotropic ( I RE ) – N D – N B – N C –isotropic ( I ) Phase transitions in a lyotropic mixture of potassium laurate, decanol and D 2 O. The dynamic of micellar configuration of the uniaxial and Biaxial Nematic Phases is discussed as a consequence of our refractive index data.

  • Temperature dependence of refractive indices near uniaxial–Biaxial Nematic Phase transition
    Physics Letters A, 2006
    Co-Authors: P.a. Santoro, A.r. Sampaio, H. L. F. Da Luz, A. J. Palangana
    Abstract:

    Abstract The uniaxial discotic ( N D ) and calamitic ( N C ) Nematic Phases are characterized by an order parameter which is a second-rank, symmetric, traceless tensor with two different eigenvalues and three in the Biaxial Nematic ( N B ) Phase. The optical dielectric tensor may be chosen as the microscopic order parameter. The optical characterization of these uniaxial and Biaxial Nematic Phases requires the measurements of two and three refractive indices, respectively. These optical parameters were determined near the reentrant isotropic ( I RE ) – N D – N B – N C –isotropic ( I ) Phase transitions in a lyotropic mixture of potassium laurate, decanol and D 2 O. The dynamic of micellar configuration of the uniaxial and Biaxial Nematic Phases is discussed as a consequence of our refractive index data.