Dissertação de mestrado, Qualidade em Análises, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2015Cellulose, the most abundant organic polymer on earth, has numerous applications including the application in pharmacy as excipient in different kinds of pharmaceutical formulations where it comes in contact with metal compounds used in therapeutic purposes. Chemically, is composed of hundreds to thousands of β 1→4 linked D glucopyranose units. It is insoluble in water and most organic solvents and its structure cannot be loosened by heat or with solvents without causing irreversible chemical decomposition. However it can be broken down chemically into glucose units by treatment with concentrated acids at high temperatures. Aqueous solutions of several metals are also known to dissolve cellulose by deprotonating and coordinative binding the hydroxyl groups in the C2 and C3 position of the anxydroglucose. In this work, different metal complexes of copper, iron, vanadium and zinc, and native cellulose were used to a better understanding on the interactions established upon adsorption. Those metals are long used as components of dietary supplements, food additives and different kinds of drugs. It is also known from previous research that the metals complexes can react with cellulose, changing its structure or even cause its decomposition under certain conditions. In general, cellulose fibers have very few functional groups that are capable of bonding with metals and interaction between cellulose and metals and their compounds can occur in one of four ways: adsorption on the fibers, forming chemical bonds with the reactive groups of cellulose, incorporation into cellulose matrix, and formation of complexes with dissolved degradation products of cellulose. The objective of this work was to study the interaction of cellulose fibers with these four metals complexes, evaluating the changes induced by the matrix on the metal complex, specially by the identification of different metal adsorbed species, and assessing the changes in the cellulose structure namely in its crystallinity degree, upon the metal complex adsorption. For the analysis two spectroscopic techniques were used: diffuse reflectance infrared spectroscopy (DRIFTS) and diffuse reflectance ultraviolet-visible spectroscopy (DRUVS).
One of the effects that the adsorption of the metal can have in cellulose is by altering its chain spatial arrangement changing its crystallinity degree. The cellulose crystallinity of the samples estimated by DRIFTS, using the region between 1300 and 1180 cm-1 and bands at 898 and 1430 cm-1, showed that in the case of vanadium (V) and zinc (II) there has been increase in the degree of crystallinity while for the other metals crystallinity was not influenced by the presence of metal complexes. However, quantification of this change was only possible in the case of vanadium (V) where it was clear that in the acidic environment at pH 3 and 5 and concentrations higher than 1mM / g cellulose, crystallinity increased from approximately 44 to 76% with the increase of vanadium complex concentration from 1 to 10 mM / g cellulose.
The influence of the matrix on the metal complex structure, in the case of vanadium (IV) was analyzed by the alterations observed in the infrared bands, in particular in the regions 1000 -1200 cm-1 and 600 – 670 cm-1, showed that there has been a change in crystalline structure of the initial compound from water soluble α to insoluble β form. The shift of the bands at 1200 cm-1 in all three sets of samples (three different pH) indicated entrapment of the SO42- ion into cellulose and shift in the band at approximately 980 cm-1 indicates entrapment of different vanadium species in the cellulose chains but only in the high range of concentrations. The ultraviolet-visible absorption spectra (DRUVS) shown low intensity of d-d transition bands in the region after 500 nm for the samples with pH 3 and non-adjusted pH, which can be related with the oxidation from V(IV) to V(V). However, considering the blue color of these samples, characteristic for V(IV), the low intensity of the d-d bands in diluted samples appear weak only because they are so in comparison with the stronger CT bands at ~240nm. On the other hand, the high intensity of the CT relative to the d-d bands in samples with cellulose compared to the control may be indicative of binding of cellulose groups to the metal. In the set of samples with pH 5 there are indications of the formation of a new complex and also partial oxidation of V(IV) to V(V).
For vanadium (V) adsorbed onto cellulose, the analysis of the new appearing bands present at wavenumbers 960, 971 and 965 cm-1; 836, 829 and 829 cm-1 703, 750 and 747 cm-1 for the non-adjusted pH set of samples, pH 3 and pH 5 sets of samples, respectively, revealed deposition of different polymeric vanadium species onto cellulose when vanadium is present in higher concentrations. Since the equilibrium of vanadium species greatly depends on the pH value it was expected that different species deposited onto cellulose will be observed in the sets of samples with different pH values. At low pH values such as pH 3 it is found that decavanadates are dominant species. At pH 5 dominant species seemed to be mixture of tri and decavanadates and at pH 6-7 we can expect to have mixture of tri and tetravanadates deposited on cellulose. These conclusions are also confirmed after analysis of the DRUV spectra which showed that there hasn’t been any reduction from V (V) to V (IV) and that only different polyoxovanadates are formed in the samples. The DRIFT spectra of iron (II) indicated very weak interaction between the metal complex and the matrix, which means a deposition of the complex on the cellulose surface or on the amorphous phase. Analysis of DRUV spectra showed that it is possible that Fe (III) ions were formed in the samples during oxidation of Fe (II) under aerobic conditions.
A different behavior was proposed to iron (III), since in the DRIFT spectra of Fe(III) complex samples (at all preparation pH values) strong bands are appearing at 1384 cm-1 but they are probably just the result of the high complex concentration leading to deposition onto the cellulose exterior surface with low interaction. However, the shifted bands at 1360, 830 and 802 cm-1 indicate deposition of different species or more probably an adsorption onto the cellulose chains with stronger interactions.
In the case of zinc and copper metal complexes no significant effect is present in any of the spectra. In fact, neither the bands of the metal complex or those of cellulose are affected by the deposition of the metal complex onto cellulose. Moreover, no new bands are observed, pointing to a poor interaction between the complex and the matrix upon deposition. Since zinc has very low absorptivity in the UV/Vis region no additional information were obtained from the analysis of DRUV spectra. However, the absence of expected d-d bands at around 800 nm in the DRUV spectra of Cu (II) samples, confirmed that the deposition of octahedral copper species onto cellulose did not occurred.
Quantitative analysis is performed, for all the metals except zinc, using the Kubelka-Munk equation, or reemission function which gives the correlation between the intensity of the diffuse reflected radiation and concentration for solid samples, similarly to the lambert-Beer law for liquid samples. In all metal complex/cellulose pairs studied the increase of F(R) with increase of concentration was evident. In the full range of concentrations only regression models for V(V) set of samples with pH 5 and V(IV) set with pH 3 showed good fit for the data while in all the other cases only regression models in the lower range of the concentrations from 0.20 up to 1.00, 3.00 or 5.00 mM/g of cellulose (depending of the sample set) showed good fit. These results point to the necessity of optimizing the sample preparation, with a very low repeatability, reducing and replacing the steps involved that leads to a huge possibility of loss of analyte
