Spectroscopic methods

Infrared spectroscopy and ultraviolet-visible spectroscopy are two spectroscopic methods that were used for sample characterisation in this research project. The working principles and general sample preparation methods are presented.

4.2.2.1 Fourier Transform Infrared Spectroscopy (FTIR)

The different pre-treatments and surface modification result in changes in the functional groups on cellulose surface. Infrared spectroscopy is a vibrational spectrophotometric method that is used in the structural and compositional elucidation of polymeric materials (Mitchell Jr, 1987).

This analytical tool can identify the attachment of surface groups, such as carboxylic acid and amine groups on the cellulose (Foster et al., 2018).

The general working procedure for spectrophotometers is schematically represented in Figure 4.6. It involves the application of specific infrared radiation from the electromagnetic spectrum onto the sample under test. The sample absorbs energy from the radiation and becomes excited to a higher energy level. Upon removing the radiation source, the sample again returns to the lower energy state by transmitting the absorbed energy. A detector is used to record the absorbed or transmitted energy which is transformed into a spectrum (Campbell et al., 2000).

CHAPTER4. EXPERIMENTAL METHODS AND SAMPLE CHARACTERISATION

Figure 4.6: Schematic representation of the FTIR

The absorption of infrared radiation by a molecule brings about vibrational stretching and bending, occurring at different frequencies that are unique to the nature of bonds and atoms that are present in the sample (Campbell et al., 2000). The instrument collects and plots the absorbance or transmittance through the sample against the wavenumber (number of wavelengths per unit distance) in an FTIR spectrum. Dispersive infrared spectrometers, which sequentially collect spectral data were previously used. These have now been replaced by Fourier transform spectrophotometers (Campbell et al., 2000; Foster et al., 2018). The latter grade of the instrument uses the Michelson interferometer to gather spectral data simultaneously before performing a Fourier mathematical transformation of the data, thereby offering quick and efficient measurement (Campbell et al., 2000).

Different sample measurement modes such as the KBr and the attenuated total reflectance (ATR) modes are used to present the sample to the instrument. While the KBr requires extensive sample preparation, which involve pellet formation and mostly suitable for liquid samples, the ATR requires minimal or no sample preparation. Samples can also be measured in the solid and in the liquid forms (Campbell et al., 2000). The ATR is composed of a crystal tip, typically diamond, onto which an IR beam is directed, creating an evanescent wave. The changes in the total internally reflected infrared beam upon contact with a sample is then detected

CHAPTER4. EXPERIMENTAL METHODS AND SAMPLE CHARACTERISATION

(PerkinElmer, 2005). Samples must have a good contact with the ATR tip in order to generate an accurate result. A schematic representation of the ATR is shown in Figure 4.7.

Figure 4.7: Schematic representation of the ATR tip (PerkinElmer, 2005)

Analysis of an unknown compound can then be made by comparing the wavenumber of each bond stretch with that of an already documented wavenumber for the functional group (Foster et al., 2018).

Herein, a Frontier FTIR unit from Perkin Elmer, was used for two purposes; to monitor the changes in the cellulose surface upon pre-treatments and to study the interactions between the alginate matrix and the different loadings of CNF materials. An image of the instrument and corresponding specifications are provided in Table 4.5.

Table 4.5: Photograph and technical details of Frontier FTIR

FTIR

Manufacturer: Perkin Elmer (Shelton, USA)

Model: Frontier

Wavelength range: 8300 – 350 cm⁻¹

Interferometer: Michelson

Sampling mode used: ATR

Standard detector: deuterated triglycine sulphate

CHAPTER4. EXPERIMENTAL METHODS AND SAMPLE CHARACTERISATION

4.2.2.2 Ultraviolet-Visible Light Spectroscopy (UV-Vis)

The UV-Vis is a spectroscopic method used in the characterisation of changes in the absorb-ance, transmittance and reflectance of materials within the ultraviolet, visible and for some instruments near infrared regions (Stuart, 2008). It is the main instrument used in the detection and quantification of colouring materials in water. Hence, it is applied in many dye adsorption studies (Voisin et al., 2017; Putro et al., 2017). Therefore it is used in this thesis for the detection and quantification methylene blue (MB) dye concentration in aqueous solutions.

The general principle of spectroscopic analysis as described for FTIR applies to UV-Vis spectroscopy. The difference lies in the energetic range of electromagnetic radiation used. A UV-Vis spectrometer measures the energy absorbed by electrons when irradiated with ultraviolet radiation and visible radiation between 200-800nm in wavelength (Campbell et al., 2000). The UV-Vis spectrum is generated as a plot of absorbance/transmittance vs wavelength, where the peaks provide information on the type of bond present. Covalent bonds withπ-electrons absorb energy at longer wavelengths (visible region) while those ofσ-electrons absorb energy at lower wavelengths (UV region) (Campbell et al., 2000). Coloured molecules such as dyes and pigments absorb in the visible region at longer wavelengths because of the large number of delocalised electrons within their aromatic ring(s) (Campbell et al., 2000; Stuart, 2008).

The UV-Vis used in this research project comes with an extended radiation source in the near infrared (NIR) region, collectively called UV-Vis-NIR spectrometers. A photographic image and the specifications of the Lambda 750 UV-Vis-NIR from Perkin Elmer are provided in Table 4.6.

The spectrometer is designed with two cell holders; one for the reference solution and the other for the sample under test. The instrument was used to monitor the changes in concentration of MB dye, using the Beer Lambert’s law (Equation 4.9), which directly relates absorbance to concentration (Stuart, 2008).

A = log₁₀(I₀

I ) = ²l c (4.9)

Here A is the absorbance, I0is the incident radiation intensity, I is the transmitted radiation intensity,² is the coefficient of molar adsorption, l is the path length of the radiation within the solution and c is the concentration.

CHAPTER4. EXPERIMENTAL METHODS AND SAMPLE CHARACTERISATION

Table 4.6: Photograph and technical details of Lambda 750 UV-Vis-NIR

UV-Vis-NIR

Cell: 200-2500 nm radiation range, 10 mm path length and 3.5 mL sample capacity Suprasil Quartz cuvettes