Effect of titanium oxide nanoparticles on the dielectric properties and ionic conductivity of a new smectic bis-imidazolium salt with dodecyl sulfate anion and cyanobiphenyl mesogenic groups

https://doi.org/10.1016/j.molliq.2020.113939Get rights and content

Highlights

  • A new ionic liquid crystal was synthesized and doped with TiO2 nanoparticles.

  • LC's ionic component has a dominant influence in the composite's conductivity.

  • Conductivity increased with temperature & dopant concentration, at constant frequency.

  • Electrode polarization phenomena are responsible for high increase in permittivity.

  • Relaxation time increases with sample thickness & decreases with TiO2 concentration.

Abstract

A new bis-imidazolium salt with two cyanobiphenyl groups and dodecyl sulfate counterion (BIC) was prepared by the metathesis reaction of the bromide salt with sodium dodecyl sulfate. The ILC shows at room-temperature a stable smectic A phase, confirmed by differential scanning calorimetry, polarized optical microscopy, and powder X-ray diffraction investigations. The ILC was doped with TiO2 nanoparticles, having a high specific area, in concentrations of 0.1%, 0.2%, 1% and 2%.

Broadband Dielectric Spectroscopy spectra have been registered in the (10−1–107) Hz frequency range, and (250–330) K temperature domain.

The study shows that at constant frequency, the conductivity increases with the temperature and dopant concentration. The characteristic times of the observed relaxation processes showed a temperature variation according to the Vogel-Fulcher-Tammann law. Higher values of characteristic times were calculated at the increase of the thickness of the sample. On the other hand, the increase of the concentration of TiO2 nanoparticles leads to a decrease of the characteristic relaxation times.

In order to detect whether the obtained high permittivity increase is due to the Maxwell-Wagner or Electrode Polarization (EP) type phenomena, studies were made on samples of the same concentration, but different thicknesses, and EP was assigned as the main cause of this dielectric constant variation.

Introduction

Ionic liquid crystals (ILCs) are a class of materials integrating the special conductive properties of ionic liquids (ILs) and the self-organization characteristics of the liquid crystals (LCs) [[1], [2], [3], [4]].

While being formed of cation/anion typical to the ILs, the attaching of long alkyl chains helps the setting up of mesophases characteristic to the LCs, such as the smectic ones. These types of materials are currently being studied extensively, and their field of utilization is constantly expanding due to their recent applications: solar cells [5], membranes, battery materials, electrochemical sensors, electroluminescent switches, etc.

On the other hand, in the last decade, attempts have been done to improve the LCs characteristics by doping with nanoparticles (NPs), in an attempt to combine some beneficial features of the LCs and NPs [6,7].

Due to the interaction with the host matrix, NPs can be aligned or reoriented by the LC, producing notable changes in their electrical and optical properties [[8], [9], [10], [11]], accompanied by thermodynamic changes in phase transitions [[12], [13], [14]]. A wide range of NPs has been used as dopants: carbon nanotubes [[15], [16], [17], [18]], graphenes [19], aerosils [10,11,20,21], gold, magnetic NPs [[22], [23], [24]], diamond [25] etc. The TiO2 particles were used as dopant in LCs, obtaining increases in ionic conduction and decreases in switching voltage [[26], [27], [28]].

In this work we present the design and synthesis of a new ILC, a dimeric bis-imidazolium salt with cyanobiphenyl groups and dodecyl sulfate counterion. The ILC was characterized by 1H and 13C NMR spectroscopy. Its crystalline properties have been analysed by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) studies. The dielectric spectra of the ILC doped with different amounts of TiO2 NPs have been registered in the frequency range from 10−1 to 107 Hz and in the temperature range from 250 to 330 K corresponding to the different phases of the ILC (mesophase and isotropic state). We discuss the influence of the dopant in the obtained permittivity, dielectric loss and conductivity. The activation energy was calculated by employing the Vogel–Fulcher–Tammann law [29] and the characteristic time was obtained by fitting the spectra of the dielectric loss with the Havriliak–Negami functions [22,30]. Since electrode polarization [31] could shield the dielectric response of the studied material, we have also analyzed samples of difference thicknesses at constant dopant concentration.

Section snippets

Experimental

All the chemicals were from commercial sources and used as supplied (Sigma-Aldrich and Merck). TLC was performed on commercial coated aluminium plates with Silica Gel matrix with fluorescent indicator 254 nm (Sigma Aldrich), and the detection was done by an UV lamp. C, H, N analyses were carried out with an EuroVector EA 3300 instrument.

1H and 13C NMR spectra were recorded on a Bruker Fourier 300 spectrometer operating at 300 MHz, using CDCl3 as solvent. 1H and 13C chemical shifts were

Synthesis

The synthesis of the new ionic liquid crystal (2-BIC) with two dodecyl sulfate groups as counterions is described in Scheme 1. The new bis-imidazolium salt was prepared starting from 1,1′-(1,6-hexanediyl) bisimidazole (3) [33] and 4′-(6-bromohexyloxy)biphenyl-4‑carbonitrile (4) in acetonitrile under reflux. The resulting bromide salt (1) was purified on silica using dichloromethane - methanol mixture as eluant and recrystallized from a mixture of dichloromethane and ethyl ether. In the next

Conclusions

A new dimeric ILC based on bis-imidazolium salt with two cyanobiphenyl groups and dodecyl sulfate counterion (BIC) was prepared by the metathesis reaction of the corresponding bromide salt with sodium dodecyl sulfate and its structure was confirmed by 1H and 13C NMR spectroscopy and elemental analysis. The bis-imidazolium salt shows a room-temperature smectic A phase stable in the 278–309 K temperature range as confirmed by a combination of the DSC, POM and powder X-ray diffraction

Author statement

All authors have equally contributed to the elaboration of this paper and have equal relevant credit roles.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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