Abstract
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.
Highlights
One of the most interesting mesophases exhibited by liquid crystals is the biaxial nematic one.Yu and Saupe [1] reported the first experimental evidence of this phase in 1980, exploring the phase diagram of a ternary lyotropic mixture
We present five new lyotropic mixtures based on N-acylamino acid surfactants, which present the nematic biaxial phase (NB) phase
With the birefringences measured in the three nematic phases, the symmetric invariants of the tensor order parameter, σ2 and σ3 (σ1 = 0), can be calculated [18]: where is the mean index of refraction of the mixture
Summary
One of the most interesting mesophases exhibited by liquid crystals is the biaxial nematic one. To the best of our knowledge, all the lyotropic mixtures presenting the ChB phase were obtained by the doping of nematic mixtures with brucine [13] or brucine sulfate heptahydrate [12], i.e., non-amphiphilic chiral molecules. Transitions between these cholesteric phases were investigated theoretically and experimentally [13,14], showing fundamental differences with respect to the transitions in nematics. The main experimental techniques used to characterize the nematic phases are X-ray diffraction (XRD), polarized optical microscopy (POM) and laser conoscopy
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