DR-UV/vis spectroscopy for studying the interaction of vanadium compounds with cellulose

Cakaj, Fatmir

http://hdl.handle.net/10400.1/4526

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Authors

Cakaj, Fatmir

Advisor(s)

Cavaco, Isabel Maria Palma Antunes

Garcia, Ana Rosa

Abstract(s)

Cellulose is considered the most abundant natural polymer in our environment. It is assumed an almost infinite source of raw material for the increasing demand for environmentally friendly and biocompatible products. In spite of its very low solubility in water, cellulose is used in a wide range of applications including composites, netting, coatings, packing, paper, etc. The morphology of cellulose has a thoughtful effect on its reactivity, the hydroxyl groups located in the amorphous regions are highly accessible and highly reactive, whereas those in crystalline regions with close packing and strong interchain and intermolecular bonding can be completely inert. Cellulose nanofibers are one of the most fascinating nanoscale biomacromolecular building blocks that exist in nature. Their characteristic widths are in the range of 5-20 nm and their lengths may exceed 5 μm. This high aspect-ratio nanostructure is valuable and contributes to high mechanical performance of nanofiber networks and composite materials. In pharmaceutical industry cellulose derivatives are often used to adapt the release of drugs in tablet and capsule formulations and also as tablet binding, thickening and rheology control agents, for film formation, water retention, improving adhesive strength, for suspending and emulsifying. On the other hand microcrystaline cellulose (MIC) is used as diluent and disintegrating agent for release oral solid dosage. There is strong evidence for the catalytic role of transition metals during the oxidative degradation of cellulose. In the surface layer of Earth, vanadium is 22nd in abundance (0.013% w/w) and more plentiful than copper and zinc. Vanadium is very a versatile metal acting as catalyzer and the use of such earth-abundant metal as an oxidant would be a significant advance in ability to break carbon-carbon bonds in 1.2-hydroxyether compounds such as lignin. It was the purpose of the present work to analyse the chemical behavior of a vanadium complex (VOSO4) adsorbed onto two types of cellulose: microcrystalline (MIC) and native (NC). This was achieved by diffuse reflectance Ultraviolet Visible (DRUV) spectroscopy. The diffuse Reflectance (DR) is considered as special method for IR and UV-VIS spectroscopy used for powders and samples with irregular surface. The “Kubelka-Munk” (KM) function (F(R)) plays a significant role for the quantitative evaluation of DR spectra, which integrates the degree of reflection on the sample surface at infinite thickness and is expressed with following equation: F(R)=K/S or F(R)=((1-R)2)/2R, where R is the total amount of radiation reflected diffusely by the sample, K represents the absorption modulus and S the scattering modulus. The DRUV spectra, obtained after depositing the vanadium complex onto cellulose, show that the initial V(IV) complex can be involved in several equilibria, leading to the formation of different V(IV) and V(V) complexes. The main absorption band of V(IV) species has a maximum at 750 nm and a clear shoulder near 640 nm. In the region 200 to 500 nm is a high band overlapping for V(V) and V(IV) species is possible and the band assignment in this region is not straightforward. Nevertheless, the maximum at 240 nm can be related with oligovanadates forming from the oxidation and hydrolysis of V(IV) to V(V), in the case of the samples with a higher initial concentration of VOSO4. The behavior of the vanadium complex spectra with the initial concentration of VOSO4 was assessed comparing two different mathematical treatments for data analysis: the Kubelka-Munk function F(R) and log(1/R). It was concluded that F(R) explains more accurately the vanadium species evolution deposited or adsorbed onto cellulose when comparing to the log(1/R) function. The precision of the sample preparation method is represented by repeatability and intermediate precision. Precision was measured as units of F(R) measured at several wavelengths. It was obtained from the spectra of samples prepared within a limited period of time (~2 months) and replicates of samples prepared simultaneously. In this study repeatability represents variance between samples (duplicates and quadriplicates) prepared in the same day, and intermediate precision represents variance between samples (different series) prepared in the different days. In order to evaluate these parameters, data at the wavelength of 240, 380, 640 and 750nm were chosen. The data were analyzed using Microsoft Office 2003 ANOVA software. The ANOVA data analysis showed that there is a difference in variance between Repeatability and Intermediate precision (p-values 1.36x10-6 and 0.0013 at 750 nm for MIC and NC, 3.7x10-8 and 0.0013 at 240 nm for MIC and NC, respectively) significant at a 5% level, this means that the samples prepared within one day have much smaller variance comparing with the samples prepared in different days. The V(IV) species deposited onto cellulose, with maxima absorption at 640 and 760 nm, shown an almost linear behavior of F(R) with the initial concentration of VOSO4, which might suggest that at these wavelengths the dominant band generating species are stable. The bands at 240 nm, probably due to V(V) formed by slow oxidation of V(IV), and at 260 nm was observed that in MIC there is higher linearity behavior of absorbing species comparing to NC where it can be observed a slight decrease and then increase of absorbing species. At 320 and 400 nm, where both V(IV) and V(V) complexes can absorb, the F(R) value is decreasing and then increasing with initial vanadium concentration, which is possible due to existence of two different absorbing species which are in equilibrium and one of them is decreasing and other one is increasing in concentration. Regarding the differences observed when comparing the results obtain with both celluloses they are mainly related with the values of F(R) than with the spectra profile. Therefore, the chemical behavior of the vanadium complexes is not much affected by the cellulose structure, ie the higher amount of amorphous regions (with more accessible reacting groups) do not alter significantly the equilibria of the vanadium species. It can be observed that the signal from vanadium species in MIC is much higher than in NC, this can be explained by the lower surface area of MIC, which results in a higher surface concentration of vanadium species.