STUDIES ON THE INTERACTION OF SOME COORDINATION COMPOUNDS WITH CLAYS

Abstract

It has been shown that some monovalent large organic cations (strychnine, quinine, morphine etc)coagulate a negatively chargedlyophobic colloid at concentrations considerably lower than simple inorganic ions of the same charge. Even on clays, the uptake of these organic cations is more in comparison to simple inorganic cations. It has also been shown that such cations affect the double layer and sometimes a charge reversal of colloidal particles takes place at sufficient concentrations. It is generally accepted that their enhanced activity and their ability to reverse the charge are due to stronger adsorption. Besides this the size and configuration of the interacting cation is also important. A survey of literature shows that no systematic effort has so far been made to correlate the charge, size and configuration of a complex cation with coagulation or stabilization. In view of the above consideration systematic inves tigations on the interaction of metal complexes with clays were undertaken. Metal complexes were chosen for these inves tigations because with the use of the same central atom and different ligands it is possible to prepare complex ions of same charge but of various size and geometry. The investiga tions reported in this thesis can be divided into the following sub-parts: (i) Adsorption, X-ray and electrokinetic studies of clay cobalt ammine complexes. (ii) Polarographic reduction of the cobalt ammine complexes in presence of clay. 124 (iii) Effect of these cobalt ammine complexes on the capill ary characteristics of clays, (iv) Plow properties of clay-cobalt ammine suspensions. Bentonite clay was chosen for these investigations because of its high exchange capacity. The three cobalt ammines: cobalt hexammine, cobalt ethylenediamine and cobalt pentammine were used in the investigations as adsorbate. 1. Adsorption, electrokinetic jand_ X-ray .studies: The adsorption isotherms indicate a positive adsorption. They are regular and concave to complex concen tration axis. The data fits well in the Langmuir equation in each case. The maximum uptake of each complex on H-bentonite is less than its cation exchange capacity. The order in which these are adsorbed on H-bentonite is hexammine> ethylenediamine^* pentammine. The uptake of exchanging complex ions depends on the charge and size of the ions. The sorption of pentammine complex is lowest. Although hexammine and ethylenediamine complex have same charge but the uptake is less in the case of ethylenediamine- due to its bigger size. The lesser uptake of complex cations in compa rison to exchange capacity of the mineral is either due to cover up effect of the exchanging ion on the clay surface or due to their inaccessibility in the inter layer region of the aggregate. 125 Adsorption studies were also carried out on sodium and cesium bentonite. Sodium clay adsorbs, cobalt ions much more than the cesium clay. The adsorption of cobalt ammines is also more on sodium clay. The uptake of cobalt hexammine exceeds the exchange capacity of the mineral while in the case of cobalt ethylenediamine the adsorption is almost equivalent to the exchange capacity. On the other hand the binding of pentammine complex is much less on the two clays. The greater adsorption of the chelates by sodium bentonite is due to the inaccessibility of much of the surface area of cesium bentonite as a result of face to face aggregation of unit layers. In the case of hexammine complex a part is taken up by ion exchange and the additional adsorption occurs via ion-dipole interactions. On the basis of saturation coverage of cobalt ammines on the two clays it is observed that the ratio of the accessible surface areas of these clays is about 1.5 to 1. Consequently the average number of unit layers per cesium bentonite aggregate is 1.5. This value is slightly lower than the one reported by Banin and Lahav (A. Banin and N.Lahav, Israel. J. Chem. 6, 235, 1968). This deviation may be due to the fact that partial overlap of unit layers has not been considered. The increase in the net negative charge on the particles at high pH accounts for the increase in adsorption with increasing pH because of change in polarity from positive to negative at the edges. 126 The X-ray diffraction patterns show an expansion »^ e-^ac,1li of 1.33A°and 1.99 A0 with hexammine complex, 1A° and 1.53A° with ethylenediamine complex at pH 2.5 and 8.5 respectively. Expansion in C-spacing observed with pentammine complex is 0.81A at pH 2.5. These results are in consistency with the adsorption data. With increasing cobalt ammine concentration the electrophoretic mobility of the clay particle decreases sharply in the beginning and there after the fall is slow. A larger amount of cobalt pentammine is required to decrease the electrophoretic velocity of the clay particles to the same extent as observed with cobalt hexammine and cobalt ethylenediamine complex. Sorption isotherms at a higher temperature reveal that the process is exothermic in the case of cobalt hexammine and cobalt ethylenediamine whereas in the case of cobalt pentammine it is endothermie The heat of adsorption passes through a minimum point when plotted against surface coverage of clay particles. The minimum point is observed when the monolayer is almost complete. However, at higher pH the minima is observed at high surface coverage. The magnitude of the heat of adsorption of the three complexes are in the order of their molecular size. The thermodynamic parameters, AG0, equilibrium constant K,and, E the energy of interaction between the adjacent ions on the surface, have been calculated from the 127 plots of log K' (K1 being selectivity coefficient) with the saturation of clay particles. The linear plots confirm the Barrer and Falconer equation (R.M.Barrer and J.D,Falconer, Proc. Roy. Soc(London), 236, 227,1956.). log K' = log K + G (1-2 X) where X is the fraction of occupied sites and 2.303 kl N where N, and Ng are the number of ions of the type A and B in clay and N is the total number of available sites. The higher magnitude of Ew in the case of hexammine and ethylenediamine complex can only be reconciled if the exchange mechanism in these two cases is assumed to approach very closely to a chemical reaction between the exchanging species with sodium or cesium ions. The desorption of the three complex cations from clay surface has also been carried out by a number of mono and bivalent cations. The desorbing effect of various ions follows the order Rb > NH„ / K J> Na > Li for monovalent, Ba2+\ Sr2*/ Ca2_f\> Mg2+ for bivalent and CTa\ TMA for quaternary ammonium ions. 2» Polarographic reduction of cobalt ammine complexes, in presence of clay: The reduction of the cobalt ammines in the presence of clays has been investigated at the d.m.e. Each 128 cobalt ammine complex gives a single well defined wave in the pH range 2.5, 4.5 and 8.5 in the case of cobalt ethylenediamine and cobalt hexammine complex and pH 2.5, 4.5 and 7.0 in the case of pentammine complex. The Sk/p of ethylenediamine complex is -0.48 volts and -0.51 and -1.35 volts respectively in the case of hexammine and pentammine complex. The E, /? of the cobalt ammines does not change by the addition of clay but causes large decrease in the diffusion current. A linear relationship is found to exist between the decrease in v/ave height and the amount of clay mineral added. The polarographie data could also provide a method to determine the uptake of complex cation by the clay particle. It was found that in no case the binding exceeds the cation exchange capacity of the mineral in the concentration range studied. In the case of cobalt hexammine and ethylenediamine complexes the uptake is equivalent to the exchange capacity of the mineral. 5 • Plow properties _of clay-cobalt ammine suspensions: The viscosity variations of H-bentonite suspension has been observed on adding increasing amounts of the cobalt ammine complexes at pH 3.0. Viscometric constants, such as intrinsic viscosity and interaction index have been calculated using the Schulz Blaschke equation. Wo = KC,1] °sp/c + 1*} where QSX)/C is specific viscosity, [n] the intrinsic viscosity •K' the interaction index and c is the concentration of the suspended particles. 129 In the initial stages with the addition of cobalt ammines viscosity of the suspension remains almost constant. Viscosity of clay suspension increases only when a certain - threshold value of the complex compound is reached. Above this value, viscosity increases rapidly in each case. The threshold value is found to be 4.99 millimoies, 6,66 milli moies and 9.99 millimoies per 100 g of clay in the case of cobalt ethylenediamine, hexammine and pentammine complex respectively. Furthermore a very small decrease in viscosity is observed in the beginning. The decrease in viscosity in initial stages may be due to the adsorption of chloride ions on the edge surface of clay particles. Nash (Proc National Conference on clays I960) while investigating the effect of NaCl on the viscosity of clay suspensions attributed the initial decrease to the adsorption of chloride ions and measured the activity of chloride ions with Ag:AgCl electrode. Beyond the threshold value the binding of cationic complexes on the planar surface of the clay particle becomes very large and this consequently results in the compression of double layer and increasing the intermicellar attractive forces with the result that the viscosity increases after the threshold concentration. Intrinsic viscosity and interaction index have been calculated from the linear plots of viscosity number vs specific viscosity. The values of intrinsic viscosity[ rj] increases while the interaction index K decreases as 130 increasing amounts of cobalt ammines are added to clay. This indicates that the increased adsorption of the complexes must be accompanied by a decrease in either or both the hydrodynami and electrical interactions between the suspended units. With increasing amount of cobalt ammine the edge to face attraction of the suspended unit is made possible. Apart from this there exists a probability of edge to edge association at high cobalt ammine concentrations. A face to face stacking would form blocky aggregates with enhanced c-spacing. Since the increase in basal spacing observed in these studies is very less (chapter I), it can be concluded that edge to face and edge to edge association should certainly be more as compared to face to face associa tion in cobalt ammine-bentonite complexes. 4. The__effect of eojaalt ammine complexes, on the capillary characteris_ti_cs_ _of _cl ay_: The effect of the cationic complexes has also been observed on the rate and magnitude of rise of water in clay columns. The time of capillary rise of water is greatly affected even by small concentrations of the ammine complexes The time of rise of water has also been found to depend on -pH and the particle size. The maximum effect is observed at pH 6.5 and 150 mesh particle size. This has been explained in terms of increased surface area, per g and increased cohesive forces between particles. The effect of the three 131 cationic complexes in inhibiting the rise of water follows the order: (if ^, ethylenediamine complex > hexammineU^mpTe^ pentammine. It is further observed that the higher^s^the complex concen tration of the metal chelate added to clay, the lesser is the magnitude or rise of water.