Optical Properties of Liquid Crystalline Alkoxy Benzoic Acids with Dispersed Citrate-Capped Gold Nanoparticles

Citrate-capped gold nanoparticles (Gnps) have multiple applications in all optical devices and influence the properties of birefringence in liquid crystalline (LC) molecules. In the present paper, systematic studies have been carried out on LC compounds p-n-undecyloxy benzoic acid (11oba) and p-n-dodecyloxy benzoic acid (12oba) with dispersed citrate-capped Gnps in different low molar concentrations. The phase transition temperatures of the LC nanocomposites obtained from polarizing thermal microscopy (POM), differential scanning calorimetry (DSC) and modified spectrometry are found to be reduced with the increase in concentration of dispersed citrate-capped Gnps. The information was obtained from textural data utilizing the simple technique gradient measurement of standard deviation (GMSD) using the MATLAB tool, which is analogous to DSC. Further, the existence of citrate-capped Gnps in 11oba and 12oba was determined by spectroscopic techniques like ultraviolet-visible spectrometry and scanning electron microscopy. The birefringence measurements were carried out at different wavelengths (460, 500, 570 and 635 nm) using a wedge-shaped cell with a modified spectrometer. The birefringence (on) increases with increase in weight concentration of citrate-capped Gnps in the nematic thermal range. Further, the order parameter S values obtained from the Kuczynski internal field model, the Vuks model, the Haller extrapolation model and the effective geometry parameter methods are in good agreement. It is found that the order parameter S increases with increase in dispersion of concentration of citrate-capped Gnps. This is due to strong Van der Waal's interaction between the LC molecules and citrate-capped Gnps, which increases the alignment of the LC molecules.