EFFECT OF SURFACE ACTIVE COMPOUNDS ON THE REDOX REACTIONS OF SOME ELEdRO-ACTIVE SPECIES

Abstract

Surfactants or surfaceective compounds have been investigated in detail from the basic and applied view point. The influence of surfactants on electrochemical processes of industrial and metallurgical importance have been studied in detail over a number of years. Basic research on electro-oxidation and electro-reduction in the presence of surfactants or in micellar systems has been of recent origin. Enough scope exists for studies in this field, particularly with respect to the effect of surfactants on the electrode processes at the solution mercury interface. Several factors, viz., blocking of the electrode surface, electrostatic interaction between the depolarizer and the adsorbed surfactant,change in the structure of the double layer etc., singly or collectively affect electrode processes. The present investigation, were carried out with the specific aim to elucidate the role of non-ionic surfactants on (i) adsorption-desorption phenomenon at the d.m.e. (ii) electro-reduction of pyrazolin-5-ones and cobalt chloride in MS. Besides the conventional d.c. polarography, other techniques such as tensammetry, electrocapillary curves, i-t measurements have been used in these investigations. -11- The investigations have been incorporated in four chapters. The first chapte* gives a critical account of surfactant micelles and their effects on electrochemical and electrode reactions. Chapter II describes the results of the systematic study of the polarographic reduction of 4-arylhydrazono- , N|-benzylsulphonyl.3-methyl-2-pyrazolin-5-ones in the absence and presence of various non-ionic surfactants. The different aspects^ the problem investigated are: substituent effect/of solvent composition, ionic strength and surfactants on the electro-reduction and kinetics of electrode process. An attempt has been made to identify the reduction path from the logk and *C values , ^yy app ? The main findings are :- (A) In the absence of surfactants. (i) All the compounds gave asingle four electron transfer diffusion controlled irreversible wave, id of which in pH independent and E1/2 PH dependent upto PH 10.5 (ii) The substituent effect on Ey2 is in accordance with the known reduction mechanism. (iii) E1/2 was found to be independent of ionic strength, which is explained on the basis of "f {iv) Various catioInlsb aaififepc^-c+ t+vh.eq %w ^ o~-fp tmhese waves..The shift in E1/2 was found to be in the order (C^) N+> -Ill- Rb+> E*> jJa*> 3fci+| whioh .s the samG ^ the order of their decreasing ability to salt out hydrophilic substances from the colloidal systems. (v) On the progressive increment of the organic solvent in the mixture, the E1/2 shifted towards more negative potential followed by adecrease in ifl ,which was explained on the basis of parameters like increase in dissociation constant of the protonated species, change in pH,decrease in surface concentration of the depolarizer due to displacement by solvent molecule* and increase in viscocitv- (B)ln the presence of Surfactants. Addition of non-ionic surfactants beyond the maximum suppression point, the wave got split into two waves. The following facts were observed with respect to the splitting Of waves. (i) The id of the first wave went on decreasing while that of second wave gradually increase. (ii) The E1/2 of the first and second wave shifted towards more negative potential. The relative efficiences of the surfactants for the effective splitting of the reduction wave was found to be - IV - in the order Triton x-100 > Brij-35> Tween-20> Tween-40> Tw©ea-60> P.E.G.6000* The splitting was explained on the basis of the inhibition of one of the stePs involved in the reduction of the depolarizer as well as due to the electrostatic interaction between the surfactant dipole and the intermediate reduction product. Further studies on the effect of surfactants on the kinetics of irreversible electrode process of these hydrazono compounds revealed that the irreversible reduction becomes more irreversible as the surfactant concentration increases. The variation in the transfer coefficient indicates that the transfer of electron is made increasingly difficult in the presence of surfactants. She variation in logkapp and^^^ values indicate that the reduction takes place at the inner sphere of the electrode. Micelle formation in presence of electrolytes and organic solvent is often met with, in processes of industrial and biological importance. The conventional methods are not suitable for determining the c.m.c. values under these conditions. The electrochemical methods can be put to good -Vuse for this purpose. Amongst these polarographic methods,the maximum suppression method has so far beer, employed to determine the c.m.c. *he possibility cf usine other related tecchnnni^cqmuees. nhaasa hb«e«e„n i•nvestigated and. forms the subject matter of Chapter III. The main findings are :- (a) Effect of non-ionic surfactants on A.C.polarograms (tensammetric waves) of different supporting electrolytes. In the presence of non-ionic surfactants the base current suppressed and a peak was observed at a.ore negative potential. The potential of the peak E-and height of the peak V varied w±th ^ ^^ concentration,all reaching alimiting value. The plots of concentration of surfactant vs (1) depression in base current (ii) variation in peak potential E liiij peak height I and (iv) F te P- v^v; V- Emax were employed for the determination of c.m.c. of surfactants. of (b) Effect/surfactants on electrocapillary curves. In the presence of surfactants the drop time decreased and the electrocapillary maxima shifted towards more positive potential. The change in dr0p time with surfactant was used for the determination of c.m.c. values. — VI- (•J Effect of surfactants on current-time (i-t) curves The i-t curves during the formation of asingle drop are .highly sensitive to the presence of surfactants. The decrease in current and drop time were employed for the determination of the c.m.c of surfactants. (d) Effect of surfactants on the reduction waves of Co(ll) and Ni(ll), Surfactant shifts the E1/2 of these metal ions to more -ve potential , due to the effect of the adsorbed layer of surfactant. The plot of E1/2 vs. cone, of surfactants was used for c.m.c. determination. (•) Effect of surfactants on the maxima of hydrazonopyrazolin- -5-ones, The relative efficiences of different surfactants in suppressing the polarographic maxima of 4-aryhydrazono- N^-benzylsulphonyl-3-methyl-2-pyrazolin-5-ones was found to be in the order Triton X-100> Brij-35 > Tween-20> Tween-40> Tween-60 > P.E.G-.6000. M.S.P. andP.M.P. values has been determined. All these methods except maxima suppression method gave almost concordant values. The relative merits of each method as well as the industrial, technical and analytical importance of these methods has been discussed. - Vll- The polarographic reduction of metals in presence potassium thiocyanate has been a subject of study of the last many years. Conflicting reports based on the existance of a minimum current due to adsorption and maxima due to catalytic effect have been reported by several workers. The problem needed further elucidation. For this purpose polarographic reduction of Co(ll) in potassium thiocyanate was studied with and without the presence of surfactants, employing the conventional d.c. polarography, A.C.polarography and i-t curves. This forms the subject matter of Chapter IV. The main results are :- 1) D.Cpolarograms gives a plateau in the potential range -1.0 to -1.25V. ii) Addition of non-ionic surfactants causes disappearence of the plateau and the splitting of the original wave. Further addition .does not change the id and E1/2 of the first wave, but increase the i of the second wave as well as shifts the Ey2 to more negative potential. in Besides the above changes/the polarographic characteristics of the wave , the appearence of the minimum was also observed on the addition of surfactants. - Vlll- (iii) The A.C.polarograms of Co(ll) in EONS gives a small adsorption-desorption peak at a potential -1.145V. Addition of the surfactant shifts the peak to a little more negative potential (-1.15V) and makes it more sharper. Besides another peak of much more larger height in realised at a more negative potential (-1.75V). The first peak is due to the adsorptiondesorption of the depolarizer, (Co Cr!S)+, and the surfactant, while the second peak is purely the tensammetrie wave of the surfactant. The hump in the A.C.polarogram is attributed to the adsorption of the depolarizer. (iv) The electrocapillary maximum of Co(ll)- KC1 got shifted to more negative potential along with a change in shape towards the positive part of the curve electrocapillary/in presence of ECNS. Addition of the non-ionic surfactant, however, shifts the electrooapillary maximum to less negative potential accompanied by a change in shape towards the negative part of the curve, (v) The i-t curves were studied at potentials of -1.0V (adsorption region), -1.2V (desorption region of the - IX - depolarizer and surfactants) and -1.6V (desorption of the surfactant alone). The i-t curves at -1.0V as expected due to strong adsorption are deformed. Large depression in current is also observed. The i-t curves at the other two potentials are longmuir type. The following mechanism of reduction has been proposed + Go(ll) + OffS" —- -> ( CoCNS) **( CoCNS}* ads. (GoGWS)ads + 2e 5> Oo(o)ncs- (I) CoNCS~ ——™_^ Co(ll) + CN~ + S"2 (II) On the basis of the polarographic studies it is proposed that the reduction of Co-KCNS system involves both inhibitory effect as well as catalytic effects. The first effect is observed in step I while second effect is operative in the reduction of CMS" ion into cyanide and sulphide ion in the presence of regenerated Co(ll). The critical micelle concentration of the non-ionic surfactants has also been determined from the plots of depth of minima and S1/2 of second wave against concentration of the surfactant.