Effect of alkyl chain length of surfactant molecules on the formation of lyotropic biaxial nematic phases via polarising light microscopy, laser conoscopy and small-angle x-ray scattering

• Grant number: 15/01391-9
• Support Opportunities: Research Grants - Visiting Researcher Grant - International
• Start date: June 14, 2016
• End date: September 09, 2016
• Field of knowledge: Physical Sciences and Mathematics - Physics - Condensed Matter Physics
• Principal Investigator: Antonio Martins Figueiredo Neto
• Grantee: Antonio Martins Figueiredo Neto
• Visiting researcher: Erol Akpinar
• Visiting researcher institution: Abant Izzet Baysal University (AIBU), Turkey
• Host Institution: Instituto de Física (IF). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 11/13616-4 - Optical and structural properties of elastomers and complex fluids of biological interest, AP.TEM

Abstract

Since the first lyotropic liquid crystal mixture, potassium laurate (KL)/decanol (DeOH)/water exhibiting biaxial nematic phase (NB) was reported in the literature in 1980, various hypotheses or models were put forward on the existence of the NB phase. Due to the fact that NB phase is an intermediate phase between both two uniaxials discotic nematic (ND) and calamitic nematic (NC) phases, it was previously asserted that NB phase arises from the co-existence of both ND and NC phases. However, from both subsequent theoretical modeling and experimental studies, it was proved that NB phase could not be a mixture of these two phases and then it was a distinct stable phase. An important point for biaxial nematic phases is the determination of which factors affecting these phases and also finding of reasons related to these factors. For instance, in our previous study in the literature, it was found that the carbon chain length of alcohols used for the formation of the biaxial nematic phases is one of the most important parameter. In addition, the interactions between head groups of surfactant molecules and the counterions or salts at the micelle surfaces play a role on the formation of three nematic phases. At this point, it is important to be found what other factors could be. In the frame of this project, we mainly aim to determine the effect of the alkyl chain length of the surface active molecules (surfactants) being main component of lyotropic liquid crystal mixtures. For this purpose, (a) polarizing light microscopy, (b) laser conoscopy and (c) small-angle x-ray scattering techniques will be used. Polarizing light microscope will be used to determine the textures of new lyotropic nematic phases and construct their phase diagrams. The micelle dimensions in both uniaxial nematic and biaxial nematic phases will be evaluated by small-angle x-ray scattering. The birefringences of the nematic phases and the phase transition temperatures from uniaxial-to-biaxial phases will be determined by laser conoscopy.We believe that this novel parameter, surfactant alkyl chain length, will provide important output to the literature and help to understand the formation of biaxial nematic phases, supporting to some models, especially "intrinsically biaxial micelles model, IBM". This model was proposed by Prof. Neto and our collaboration will lead to the proceeding on the studies of biaxial phases. (AU)