Nanoparticle's size, surfactant and concentration effects on stability and isotropic-nematic transition in ferronematic liquid crystal

Isotropic-nematic phase transition in composites based on 4-cyano-4-hexylbiphenyl (6CB) liquid crystal and spherical magnetic iron oxide nanoparticles with various nanoparticle sizes and various concentrations were investigated. The iron oxide nanoparticles were covered by oleic acid surfactant that prevents their aggregation. Dependences of the phase transition temperature shift on the nanoparticle size and the nanoparticle volume fraction were acquired by optical measurements, differential scanning calorimetry measurements and capacitance measurements. TEM and SEM examination showed a nonuniform thickness of oleic acid layers for different nanoparticle sizes. At the same nanoparticle volume fraction, therefore, the volume fraction of oleic acid varies with nanoparticle size. Although oleic acid surfactant prevents the magnetic aggregation, it modifies isotropic-nematic phase transition temperature significantly. The theoretical considerations of oleic acid provided in the present paper give the interpretation of obtained experimental results.