STUDIES WITH SOME PYRIDINOL AZO DYES AND THEIR INTERACTIONS WITH CHLORITE MINERAL

Abstract

The reactions of clay minerals and organic mole cules can be used as the basis for analytical techniques for the identification of the clay minerals, for the deter mination of certain properties of the clay minerals them selves, and for the determination of geometry and proper ties of the organic molecules on clay surface. Clays possess aluminosilicate surface layers rich in oxygen atoms and hydroxyl groups. These surfaces are basically highly polar in character and possess an intense residual force. Generally these forces are responsible for holding other ions or molecules at the surface temporarily or permanently depending on the nature of the forces operating at it, and lead to adsorption. In the present work anionic dyes have been chosen as adsorbates to see the effect of adsorbed molecules on the edge surface of clay particles. The application of dyes are increasing in every field of life. Azo-compounds have become extra ordinarily interesting class for investigators both from theoretical as well as practical point of view. Pyridinol azo-dyes have received much attention as analytical reagents, in the last three decades. These compounds have a wide range of applications, because these are easy to synthesize, have high stability, solubile in water, give spectra in visible region etc They are sensitive and selective towards metal ions and their reactions are highly coloured. These dyes are good metal indicators in complexometric VI titrations and can also be used as chromatographic spray reagents. These are also polarographically reducible. Most of the work reported in literature deals with montmorillonite, Kaolinite and illite minerals, because these minerals are abundantly found in nature. Very few investigations are available on chlorite and it's reaction with organic molecules. In the last few years numerous investigations have been reported on the interaction of organic compounds like cationic and non-ionic surfactants, proteins, amino acids and cationic dyes etc, with clays. The adsorption of dyestuffs on clays has been exploited to determine the cation exchange capacity, surface area etc and in the identi fication of various minerals. The interaction of anionic dyes with clays is an equally important aspect of clay-organic systems and nothing significant has so far been reported on this topic The adsorption of acidic dyes, taking place by anion exchange, may provide information about the edge surface chemistry of clays which is one of the most interesting area of further investigations. Keeping the above mentioned view point in mind systematic investigations on the two pyridinol azo-dyes, AHP-4S and DHP-4S, and their interaction with chlorite mineral have been undertaken. The investigations included in this dissertation are : vii 1. Electrochemical studies of two days and determination of their aggregation number Section I : Electrochemical investigations of the two pyridylazo dyes, viz., l-(2'-amino~3»-hydroxy-4'-pyridyl-azo) benzene-4~sulphonic acid (AHP-4S) and l-(2»-3'-dihydroxy- 4'-pyridylazo) benzene-4-sulphonic acid (DHP-4S), have been made at dropping mercury electrode (d.m.e.) and at rough pyralytic graphite electrode (RPGE) using polarography, linear and cyclic sweep voltammetry and coulometry, with a view to use the electrochemically observed mechanisms and products of electrode reactions to provide some infor mation on the redox-reactions possible with these dyes. Method for the preparation of RPGE have also been discussed. Dyes were prepared by coupling of diazotized sulphanilic acid with substituted pyridines, viz., AHP and DHP. Purity of these dyes have been checked by TLC and elemental analysis. Both the dyes get reduced in a single, pH-dependent, well defined two electron step in the pH range 2.8 to 11.3. The linear plots of id vs fh and low values of temperature coefficient (below 1.8/ dig" ) indicate the diffusion controlled nature of the limiting current. The plots of Sl/2 vs pH arG linear for both the dyes and showed a break around pH 8.0. In the concentration range 0.1 mM to 1.0 mM the E-,/2 remains constant and the limiting current shows a linear increase with concentration. The reversible Vlll nature of the wave was confirmed by log plots analysis and a value of 30 mV for E-^-E-,/, for both the dyes also confirm a reversible wave. Linear and cyclic sweep voltammetric studies of AHP-4S and DHP-4S at rough pyrolytic graphite electrode give a well defined reduction peak (I ) in the pH range C 2.8 to 7.0. After pH 7.0, the peak I becomes broad in nature. The peak potential for peak I is dependent on pH and shifts towards more negative potentials with increase in pH. The plots of E_ vs pH show a break around pH 8.0. Besides this peak, I , II and II. peaks are also a a o observed in the voltammograms of these dyes. Peak II is a clearly observed in both the dyes if the sweep is initiated in an anodic direction. However, peak I rounds off and I almost vanishes in the entire pH range. The observation reflected the susceptibility of these dyes towards electrooxidation at RPGE. The potential for II peak is linearly a dependent on pH and shifts towards less positive potential with increase in pH. Controlled potential electrolysis of AHP-4S and DHP-4S on reduction at the plateau of the polarographic wave, using mercury pool or RPGE involve 2+0.2 electrons in the entire pH range. The values of n obtained for electro-oxidation of these dyes at RPGE indicated that 2 + 0.1 electrons are involved in entire pH range (2.8-11.3) ix Thin layer chromatography of reduced solution confirm the formation of only one product. The efforts for product characterization by gas chromatography proved futile due to its non-volatile nature. Section II : The aggregation of two dyes AHP-4S and DHP-4S have been studied by the polarographic method suggested by Hillson and McKay. From the plots of - alog D vs log C for the dyes AHP-4S and DHP-4S indicated that dyes are in monomeric state at concentration below 0.2 mM (for AHP-4S) and below 0.15 mM (for DHP-4S)• After this con centration there was a sudden change in the Alog i^ values, which continued upto 0.4 mM (for AHP-4S) and upto 0.45 mM (for DHP-4S)• At this point dye exists as dimer. Similarly at concentration greater than 0.85 mM, a trimer formation is indicated. 2. Characterization of clay sample The clay mineral obtained from Bureao of mines, Nepal was analysed by well known techniques and results are discussed in Chapter 3. Cation exchange capacity for the sample is 31.0 meq and anion exchange capacity is 12.5 mec per 100 g of clay. Sample exhibits little swelling in water. Organic contents are 1.5 percent. The chemical analysis data for the sample indicates the presence of chlorite mineral which is also supported by it's C-E'C. value. A higher MgO percentage also indi cates the presence of chlorite mineral. Further verifica tion of the nature of mineral is achieved by X-ray data. The prominent basal reflections observed at 14.1, 7.05, 4.69, 3.52, 2.54 and 1.538 A° confirm the presence of chlorite mineral. The characteristic 001 reflection and C-spacing of the mineral also remains unchanged if the sample is heated upto 200 C» The findings mentioned above have also been confirm ed by differential thermal analysis. The two endothermic peaks at 650°C (broad), 800°C (sharp) and an exothermic peak at 860°C indicate the presence of chlorite. Findings are further verified by infrared spectra getting bands at 3560, 3440, 1640, 1050, 990, 760, 655 cm"1. 3. Adsorption of dyes on clay mineral The studies of spectrophotometry behaviour of two dyes (Section I), their adsorption on the chlorite mineral under different physical conditions and thermodynamic para meters of the process (Section II) are given in the fourth Chapter. Section I : Before carrying out adsorption studies, the absorp tion spectra of the dyes AHP-4S and DHP-4S with changing pH and concentration have been studied. Both the dyes indicate a change in colour with pH viz., yellow at pH 2.5 xi orange at pH 7.0 and red at pH 10.0. Three chromopheric species have been suggested to account for this colour change. Acidic species giving band at 425 nm (AHP-4S) and 440 nm (DHP-4S) is due to the protonation of nitrogen atom of pyridine ring. Consequently the non-bending elec trons of azo-nitrogens get shifted towards protonated side, which is responsible for the absorption band observed at lower wavelength. However, small fraction of unprotonated molecules still present at this pH (2.5) produce a shoulder around 490 nm (AHP-4S) and 480 nm (DHP-4S). This band is observed due to the n—;> r, transition of nonbondin^ electrons of unprotonated species. The same species (neutral) exist at pH 7*0, and same transition as mentioned above is responsible for sharp peaks observed at this pH at their respective ^ ax* In alkaline range one more pro ton is eliminated from the hydroxyl group of the pyridine ring due to ionization and the anion thus obtained is stabi- lized by a tautomeric keto-enol equilibria. Both keto-enol forms are less stable than the original neutral molecule but are better stabilized than the protonated form at pH 2.5. Hence it shows a sharp band almost at the same wave number as observed at pH 7.0, but with less intensity. The effect of ionic strength on the absorption spectra of two dyes have been studied at different concen trations of dyes in 0.5 to 2.0 M sodium chloride. This reflects a positive effect of ionic strength on the dye stability and colour. XI1 The effect of urea on absorption spectra of the two dyes, AHP-4S and DHP-4S shows that the peak intensity increases in the case of AHP-4S and decreases in the case of DHP-4S with increasing urea concentration. This behaviour indicates a tendency towards aggregation for the dye DHP-4S as in this case the surface tension and dielectric effects of urea clearly outweigh it1s disaggregating effect, while in case of AHP-4S, a normal behaviour of tendency towards disaggregation is observed. In general urea is quite well known for it's disaggregation effect. The effect of dielectric constant on two dyes have been studied for 40 yi and 80 '/. solutions (by volume) of dioxan and acetone. In case of AHP-4S no band shift is observed, either with acetone or with dioxan, while a sharp shifting of absorption band (about 50 nM) towards lower wavelength side is observed for DHP-4S in dioxan and not in acetone. This hypsochromic shift may be ascribed to the nonpolar nature of the solvent (dioxan) and the absence of hydrogen bonding of dye with solvent molecules. Here the electron density of oxygen atom of hydroxyl groups tends to go towards azo-linkage ,and so the n—* n transi tion involves higher energy. But in presence of polar solvents hydrogen bonding can take place between solvent and dye molecules and no band shift is observed. The AHP-4S molecule due to intramolecular polarity, is not affected by solvent molecules. Xlll Section II : The adsorption isotherms with chlorite mineral were run at 0.005 mM to 0.25 mM concentrations of the two dyes at pH 4.0 and 9*0 and temperature 30 and 40°C« All these isotherms indicate a positive adsorption. These are regu lar and concave to dye concentration axis. Data obtained fits in the Langmuir adsorption model, almost over the whole range of the adsorbate concentration at both tempera tures and pH. Values obtained for Lang.muir' s parameters show that tendency to achieve saturation is higher in case of DHP-4S at pH 4.0 in comparison to pH 9*0, while for AHP-4S it is more at pH 9.0. The values of equilibrium parameter 'R' for clay anionic dye systems have also been calculated at both temperatures and pH. These values also indicate a favourable adsorption for this anionic clay dye systems. Besides this, adsorption process under considera tion also follows the Freundlich adsorption model in lower concentration range. The edge surface area of the mineral, has been calculated on the basis of the increase in adsorption of methylene blue observed when the suspension is changed from acidic to alkaline medium (from pH 3.0 to 10«0). This increase is presumably taken to be the same as the per centage of the total surface area of crystals as contribut ed by edge surfaces. The value thus obtained for the xiv edge surface of chlorite mineral is 10.8 m /g. The area pe?" exchange site of the mineral and the area associated with dye molecules on clay surface have also been calculat ed. The area associated with the dye molecule on edge surface is more than the area per anion exchange site. It shows a covering up effect of the dye molecules on chlorite surface and would consequently lead to a lesser uptake of the dye in comparison to anion exchange capacity of this mineral. The adsorption of the anionic dye decreases with increasing pH. This is due to the fact that the positive double layer existing at the edge surface changes polarity in alkaline medium. Desorption of the two dyes has also been tried with mono, di, and polyvalent anions. It was observed that adsorption as well as desorption is more in the case of AHP-4S • The X-ray diffraction patterns have been obtained for pure clay sample and its complexes with two dyes at pH 4.0 and 9«0 (at a loading of 0.5 moles per litre of dye per 100 g of clay). An increase in edge spacing with a simultaneous contraction in basal spacing is observed when dyes are adsorbed. The changes are greater in case of AHP-4S as compared to DHP-4S. Adsorption isotherms were also run at different temperatures and heat of adsorption have been calculated. XV It was observed that the uptake of dyes decreases with temperature, and heat of adsorption increases with higher amount of dye adsorbed. The isothermal differential heat of adsorption and the free energy change of the process indicate a weak interaction with clay surface. 4. Flow Properties of Clay-Dye Suspension Viscosity variations of clay-dye suspension have been measured with a view to have an idea of particle in teraction forces, their dissymmetry and the degree of assocation. Particular emphasis has been placed on Schulz-Blaschke equation for the calculation of various viscometeric constants. There is a considerable decrease in the viscosity of the suspension with increasing addition of the dye. This shows that the stiff suspension turns into a more fluid system by breaking the particle links. Here it is suggested that, very small quantity of the dye deflocculates the suspension showing that the anion is the important factor rather than the cation and dye is used to neutralize the positive edge surface. The break in the plots is observed at the same point at which the monolay er formation is complete at the edge surface. At pH 9-0, even smaller amounts of the dye added produce large changes in viscosity. Due to smaller number of positive XVI adsorption sites at edges at this pH, the dye added is quite sufficient to neutralize the charge, thus creating conditions of deflocculation and reducing the viscosity of the suspension to a large extent. The Schulz-Blaschke equation has been employed to study the particle-particle interaction forces and the modes of particle association. The equation is expressed as : nsp/c =k[n] nsp + M where rj , Tje_/C and [n] are the specific viscosity, viscosity number and intrinsic viscosity respectively. The coefficient k, known as interaction index, is consi dered to be a measure of the amount of particle-particle interaction. The values of [ti] and k have been obtained from the slope and intercept of the plot between visco sity number and specific viscosity. A decrease in intrinsic viscosity with increasing amounts of the dyes shows that the dissymmetry of the suspended particles decreases with increase in dye adsorption. The increasing values of interaction index, k, indicate an increase in either or both, the hydrodynamic and electrical interactions between the suspended units* Thus in the presence of dyes, both the double layers are affected in a way that the positive edge to negative face attraction is eliminated and a strong edge xvii to edge and edge to face repulsion is created. The repul sive forces dominate the forces of attraction and the card house structure breaks down to smaller units of reduced dissymmetry. This also reduces the yield stress and a decrease in viscosity is observed. A simultaneous increase in the electrical and hydrodynamic interactions between the suspended units of lower dissymmetry is also evident. The axial ratio of the particles in presence of dyes have been calculated by Kuhn's equation. A decrease in axial ratio with increasing dye concentration shows a decrease in anisometry of the particles or an approach to sphericity.