Dielectric Spectroscopy Studies of Biopolymer Produced by Rhizobium Tropici

Some bacteria, such as Rhizobium tropici benefit plant growth. Extracellular polymeric substances (EPS) have been extracted from Rhizobium tropici (RT-EPS). RT-EPS has been shown to enhance soil moisture retention, nutrient content, and adhesion (Larson et al. 2016). It is therefore very useful to study the dielectric properties of biopolymer produced by Rhizobium Tropici. In this research broad band dielectric relaxation spectroscopy is used. RT-EPS was prepared according to a procedure previously developed (Staudt et al., 2012). The dielectric and conductivity measurements were taken using a broadband dielectric spectrometer (BDS) Novo control technology (Germany) concept 80 at 1 V AC signal. The frequency range was from 0.1 Hz to 1 MHz The bio polymer was placed in between two brass discs. The thickness of the RT-EPS biopolymer is 0.18 mm. Various parameters such as dielectric constant, dielectric loss, loss tangent, electric modulus, conductivity etc were measured. The dielectric constant is a measure of the charge storing capacity of the polymer. At room temperature the dielectric constant is ~ 3 in the frequency range 0.1 Hz to 10⁴ Hz. It then decreases to 1 at 10⁶ Hz. Two relaxations are observed in the dielectric loss spectra namely at ~ 1 Hz and ~50000 Hz. The ratio of the dielectric loss to the dielectric constant is the loss tangent. The relaxations are observed at ~1000 and 10⁵ Hz. This indicates the presence of relaxations in the polymer. The electric modulus is the reciprocal of permittivity and is expressed as M* = 1/e*= M’+ j M” where M’ is the real part of the modulus and M” is the imaginary part of the modulus. M’ and M” are related to e’ and e” by the following expression M’ = e’/ [ e^,2+e”²] and M” = e"/ [ e^,2+e”²]. Peaks are observed at higher frequencies for the modulus. The real part of the electric modulus (M’) shows onset of peak at high frequency (10⁶ Hz). The formation of proper semicircle in the complex modulus plot (M” vs M’) shows a conductive relaxation with a single relaxation time. The formation of a perfect semicircle implies the presence of single Debye type of relaxation, and the radius of the arc represents the electrical conductivity of the material. Since the complex modulus part is a semicircle, there is a Debye type relaxation. In general, the frequency-dependent AC conductivity at constant temperature is described by the Jonscher Universal power relation σ(ω) = σ_dc + Aωⁿ where σdc is the dc conductivity at low frequency, A is the pre-exponential factor, ω is the angular frequency (2πf), and n is the fractional exponent between 0 and 1. At low frequency, the applied electric field drifts the charge carriers over a large distance, and a constant plateau region (independent of frequency, σ_dc) is observed. With increasing frequency, the mean displacement of charges is reduced, and after reaching a certain critical frequency (fc), the law follows σ(ω) ≈ ωⁿ characterizing the hopping mechanism. The conductivity of the Biopolymer produced by Rhizobium Tropici is dependent upon the frequency and therefore the conductivity is due to hoping mechanism. The conductivity is ~10^-13 S/Cm lat 0.1 Hz.

Reference
[1] Larson Steven L. Evaluation of Rhizobium tropici–derived biopolymer for erosion control of protective berms. Field study. 2016.
[2] Staudt, A. K., Wolfe, L. G., & Shrout, J. D. (2012). Variations in exopolysaccharide production by Rhizobium tropici. Archives of Microbiology, 194(3), 197–206.