Dhorda, Urmila J.

Joshi, K. M.

English

NOPR

Indian Journal of ChemistryVol. 14A, October 1976, pp. 749-753Studies on Adsorption of Organic Compounds at Mercury-SolutionInterface: Individual & Simultaneous Adsorption ofp-Aminobenzoic Acid & SulphanilamideK. M. JOSHI & URMILA J. DHORDAPhysical Chemistry Laboratory, Institute of Science, Bombay 400032Received 20 September 1975; accepted 17 February 1976Adsorption of sulphanilamide and of p-aminobenzoic acid from KN03 solutions at mercury-solution interface has been investigated. The dipole potential has been evaluated and is foundto obey the BDM theory at not too high positive charges on mercury. Surface excess values arereported over the entire range of potentials. Studies on simultaneous adsorption of the aboveorganics have also been made from solutions containing both the entities. It is concluded thatthough adsorption of one takes place by displacement of the other, adsorption of both is observedover the whole range of potentials.INour previous paper-, we reported our findings onthe simultaneous adsorption of hydroxybenzoicacid isomers at mercury-solution interface,where it was observed that the presence of twoadsorb able entities in solution reduced the extentof adsorption of either when present singlyand that the fraction of surface covered by twoadsorbates was not additive with respect to theindividual adsorptions. A quantitative approachhas been attempted for such adsorption by Tedo-radze et al.2 while Damaskin et al.3 suggested thatthe argument for two position isotherm (see alsoParry and Parsons-) be used to study adsorptionof two organics at the same time. The data forsuch interpretation is limited in that the individualcoverage in simultaneous adsorption is very seldomreported. In this paper, we report complete dataregarding the individual and simultaneous adsorptionof p-aminobenzoic acid (PABA) and sulphanilamide(SA) at mercury-solution interface.The choice of these compounds was made becauseof the antimetabolic activity of SA which is astructural analogue to PABA, a metabolic factor inthe growth of bacterial.Materials and MethodsThe Lipmann electrometer described elsewherefwas used to obtain interfacial tension at variousapplied potentials. The measured potentials arecorrect up to ± 1mY. A saturated calomel electrodewas used as reference in all the experiments and thepotentials (E) reported are with respect to SeE.The base electrolyte was O'lM KNOB' the ionicstrength being maintained constant in all theexperiments. Since the reference electrode wasnot changed, liquid junction potentials are assumedto be approximately the same in all experiments.The potentials (E) will differ from the potentials (E-)obtained with reference to an electrode reversibleto an anion in solution as required, if the generalelectro capillary equation? is to be used. However,this difference will be a constant for all valuesof potentials reported involving no error in thecomputations from the general equation. Solutionswere made in triply distilled water and deoxy-genated using purified nitrogen. Triply distilledmercury was used in the electrometer. KNOB (BDHAnalar) was purified by twice recrystallization fromwater and dried in a vacuum desiccator. PABAand SA (E. Merck, Analar) were purified by recrys-tallization from aqueous solutions. Their puritywas checked by observing their melting points.The cell was thermostated at a temperature of34°±O·2°.Results and DiscussionElectrocapillary curves and surface excess - Theelectro capillary curves for PABA and SA forconcentrations ranging from 2 X l()-4M to lo-2Mare shown in Figs. 1 and 2 together with the baseline curve for O·lM KNOB' The maximum con-centration used in the case of SA is the saturationconcentration. For these compounds, it is seen thatthe potential of electrocapillary maximum (e.c.m.)shifts to more negative potentials with respect tothat for base solution in conformity with theobserved behaviour for aromatic compounds. Forequivalent concentrations, PABA is found to lowerthe interfacial tension to a greater extent than SA.The curves however, appear to coincide with thebase curve at extreme negative and positive polariza-tions showing desorption of adsorbed entity at theextreme ends of the potential scale. This becomesmore obvious when the surface excess which isequal to-(RT~~na)Ewhere Y is interfacial tension and 'a' is the activityof organic (here assumed to be equal to concen-tration since low concentrations are employed)is calculated by differentiating the interfacialtension versus log C curves at constant potential.A graphical differentiation was carried out to obtain749INDIAN J. CHEM., VOL. 14A, OCTOBER 1976:l, V\,/ 41O 410'"'" ""III ZZ >~ 00 .. ZZ 00 :;;on zZ 400 "" 400III ~~...I ...I~<I(VV <I(<I( ...•.. II:II: ""III ~•..390 ~! 390380o - 200 - 400 - 600 - 600 -1000 ·1200POTENTIAL I E-', IIIVFig. 1 - Electrocapillary curves for O·IM KNO,+ (A)O;(B) 2 X 10-'; (C) 5 X 10-'; (D) 1 X 10"'; (E) 3 X 10-'; (F) 6 X 10-';(G) 1 xl0-IM p-aminobenzoic acid solutionthe surface excess which involved a personal errorof ± 5% at extreme ends of the potential scale.The electrode charge (q) was obtained as afunction of potential by differentiating the electro-capillary curves at various values of potentials.From these values, the dependence of surface excess(r) on charge q was plotted (Figs. 3 and 4). It isseen that the curves are symmetrical about the pointof maximum adsorption which is +3 !J. Coul. cm-2for SA and l·g !J. Coul. crrr? for PABA. It is to benoted that this value is concentration dependentin the case of PABA becoming slightly negative athigher concentrations. The concentration dependencefor SA is not however noticeable. Such concen-tration dependence of qmax has also been noticed pre-viouslys, From these curves, it is apparent thatboth SA· and PABA adsorb in single orientationsince at extreme polarization, no tendency to reorientis observed and desorption takes place. From theshift of e.c.m. with concentration to more negativepotentials, it is obvious that the molecules preferan orientation with the negative end of the dipoletowards the mercury.Shift in the potential at constant charge q dfte tointerfacial adsorption - The observation that thepotential at constant q changes with adsorption ofthe organic is to be correlated with the dipolar poten-tial due to adsorption at the interface. Thedisplacement of water molecules by the organic750420-200 -400 -600 -800 -1000 -1200POTENTIAL IE-' , IIIVFig. 2 - Electrocapillary curves for O·lM KNO,+ (A)O:(B) 2 X to-a; (C) 5 X H,'; (D) 1 X 10"1; (E) 3 X 10-'; (F) 6 X 10"1and (G) 1 X to-1M sulphanilamide solution•..,v:; 2·0...Io,o-2x...3·01-0OL-.-L- __ ~ ~ __ ~ ~ __ ~ ~~-5 -4 -2 0 +2 +4 +6 +8CHARGE DENSITY, MICRO COULOMBS I CM'Fig. 3 - Surface excess vs charge density for PABA con-centration of 2X10-' (0); 5 X10-&(8); 1 X10-1(~); 3X10-1(0); 6X10-1 (®): 1 x 10" (W): [Without SA (full lines) and, with SA (dotted lines))JOSHI ct DHORDA: ADSORPTION OF ORGANICS AT MERCURY-SOLUTION INTERFACEJiIjI;Ii1~OL-~ __~ ~ __~ ~ __~ ~ __-J-5-4 -2 0 -t-2 +.. +b +8 +10CHARGE DENSITY, MICRO COULOMBS ICM2Fig. 4 - Surface excess vs charge density for SA concen-tration'of 5 X 10-'(.); 1 X 10-1(8); 3 X 10-1 (.); 6 X 10-1 (6); +/001 xlQ-1 (.) [Without PABA (full lines) and with PABAdotted lines)-100contributes to the change in the 1. potential whichis a sum of the contribution due to water in down +200(i-) and up (t) pos itionss. Further the organic '--------"-----~-------'molecules contribute their own dipolar potential Fig. 5 _ Dependence of ~E on rPABA in the case of PABAproportionate to amount adsorbed. Thus, we can at 6 (0); 4 (6); 2 (8); 0 (.); -2 (&); -4 (~) qMwritew 0 ~---------===~==~--~----------~q,where the factor in the bracket (called R) representsthe net fractional orientation of water molecule,*=. is the dielectric constant and IL's are the dipolemoments, x is the size factor equal to the ratio ofcross sectional areas of organic to water molecule.It has been shown that if'. the case of organic mole-cules with (L = 0, the ~ depends solely on the firstterm-". b.Ejrorg• plots at constant q for PABA andSA are shown in Figs. 5 and 6. Rearranging Eq.(1), we obtainb.E = [x(N "IN;t ) iiH,O+ [Joorg.]t rorl. . .. (2)which shows that the shift (~) is linearly dependenton T.arg. at constant q, since R is solely dependen ton q as per BDM theory. From Figs. 5 and 6, it isobserved that linear plots arc realized only whenq is less than .-4 (L Coul. em-i. The change in slopeof the linear plots with charge on electrode can beattributed to change in R, i.e. net orientation ofsolvent molecules in the interface. At large positiveq the curves deviate enormously from linearity.This is probably due to the 1t-electron interactionof the organic with the positively charged elect rodein addition to the fact that at high positive q,specific adsorption of NO; will be taking place. Animplicit explanation of the nature of ~/r curvesthus is not feasible due to complexities of the pheno-menon.-200r---------~r---------~----------~~+200~ ~ ~ ~~oFig. 6 - Dependence of l!.E on rSA in the case of SA at8 (0); 6 (8); 4 (6); 2 (.); 0 (®); -2 (A); -4 (.) qM751t\.-100+100v : .;; INDIAN' J. CHEM.; VOL.14A, OCTOBER 1976Evaluation of surface coverage (6) - The surfacecoverage (6) is defined as rITmaxwhere T.ma;' is theconcentration in moles cm-s of surface to obtain aunimolecular layer. The evaluation of rmax is usuallycarried out on the basis of (i) cross sectional areaof the molecule which can be obtained from bondangles, covalent radii of atoms etc. and (ii) on theorientation of the adsorbed molecule. Although(i) can be evaluated satisfactorily from space fillingmodels. the knowledge of orientation is a majorindecisive factor.For PABA. rmax calculated from bond angles andcovalent radii of atoms on the basis of perpendicularand planar orientations are 7·53 and 2·4x 10-141 molescm-li respectively. The maximum observed experi-mentally wa~ (3·0 ± 0·09) X 10-18 moles crrr+, Hence,the possibility of planar orientation is ruled out forPABA unless a multilayer type of adsorption isassumed. For SA. the maximum surface excessfound was (3,0 ± 0·09) X 10-10 moles cm-2 while cones-pending calculated values for planar and perpen-dicular orientations for maximum coverage are 2·9and 8·3 X 10-18 moles cm-J respectively. It isapparent that SA is also adsorbed in a perpendicularorientation. Surface coverages were calculated usingfue above values.Simultaneous adsorption of PABA and SA - Fordetermining the dependence of adsorption of PABAI. .,;420410~v.,...II>..,Z>-0 400£0;;;z..,~~~U 390~"-IX..,to-~380370~ __-L ~ __~~ __-L ~~~~o -zoc -400 -600 -aoo -1000 -1200POTENflA~ (E-) , mVFig. 7 - Electrocapillary curves for (A) O·lM KNO.;+(B)1 xl~ SA; and+(C) 2 xlO-4;+(L 5xl0-4;+(E} 1 xl04;+ (F}3xl0-';+(G) 6xl04; +(H) 8 >-16-1;+(I) 1 xl0-';+(J)1·5 xl0-';+(K) 2,5 xlQ-'; and +(L)4xlO-'M PABA752420-;;;410•••Z>-oZo;;;z~ 400...•«v~Il:\AII-Z 300o -200 -400 -600 -800 -1000 -1200POTENTIAL t E-) , mVIIiFig. 8 - Electrocapillary curves for (A) O'IM KNO.; +(B)1 xl04 PABA; and +(C) 1 xl04; +(D) 3 xt04; + (E)6xl04; +(F) 8 X 10-11; + (G)1 X 10-'; +(H) 1·7 X 10-'; +(1)2'S x 10-'; + (J) 3·5 X 10-'; and + (K) 5 X lO-IM SAin the presence of SA on charge of the metalelectrode, electrocapillary curves for the followingsolutions were obtained. and are shown in Figs. 7and 8.Set-A: O·IM KNOs + 1X lO-sM SA+Y PABAwhere Y varies from 2 X 10""M to 4 X l()-lM.Set-B: O·lM KNOs +1 X 10-3M PABA+X SAwhere X varies from t X to-aM to 5X to-1M .The electrocapillary equation for such systems atconstant temperature and presure is .-ar = qMaE-+rK+a(.LKNo,+rsAa(.LSA+rPABA3(.LPABA... (3)where E- is the potential of the reference electrodereversible to NOi. The reference electrode usedbeing SeE throughout, dE- corresponds to dEcatc,the small error due to change in liquid junctionpotentials is neglected. At constant (LKNO,and(.LSA(set-A).rpABA=-(~)3(LPABA(.LKNO"(.LSA 'd."from which surface excess due to PABA fromKNOs+SA solutions was evaluated .. Similarly forSet-B, . ,, ,l.,rSA=-(~)~(.LSA (LKNO.,(LPABA... (4)... (5)JOSHI & DHORDA: ADSORPTION OF ORGANICS AT MERCURY-SOLUTION INTERFACETABLE1- SIMULTANEOUSADSORPTIONOF PABA ANDSA 'r, '.'qM (j) O. Os (j. Ototal OtotalI.tCoul./ =0.+0. (calc.)cm2 ~I , , "-.- " ~. .'.6 0·211 0·175 0·146 0·060 0·206 0·3244 0·235 0·189 <r172 0-105 0·277 0·3492 0·247 0·195 0·184 0·111 0·295 0·3630 0·226 0-175 0·182 0·104 0·286 0'335-2 0·193 0·125 0·154 0·055 0·209 0·275-4 0·154 0·098 0·122' 0'042 0·164 0·22501=0 due to PABA in 1 X lO-SM PABA'+O·lM KND~ solution.0.=0 due to SA in 1 X10-3M SA+O'lM KN03 solution,03=Odpe to PABA in lxl0-3M SA+1 x 10M-3M PABA+O'lM KN03,solution.0.=6 due to SA in 1 X 10~3M SA+ 1 X10-3M PABA+0'1M RN03 solution, 'Ototai=O due to PABA andBA in the mixed solution.' ,the adsorption.due to' SA was evaluated. The chargeon the metal beingfimply (-~Y/~Ecal)'tLPABA' 'tLSAatconstant concentration of PABA and SA wasevaluatedfor each of the above curves in sets A and'B. The'dependence of adsorption on charge q ofSA (or PABA) from KNOs solution containingPABA (or SA) is brought out in Figs, 3 and 4.Comparing the number of moles of PABA (or SA)adsorbed to the adsorption of PABA (or SA)in the' mixed solution' of both. it is apparentthat on the addition of SA (or PABA) to a solutioncontaining PABA (or SA) alone, additional adsorp-tion due to the added en ti ty takes place to .a con-siderable extent over the range of potentials. How-ever, this adsorption is seen to take place by partialr-eplacement of the already adsorbed orgar:~cspecies--. This is brought out in Table 1. It 15evident that the total coveraze due to both in themixed solution is less than the sum of respectivecoverages in solutions of only one organic adsorbatebut is nearly equal to that due to the more adsorb-able entity. It has been stated by Butler et al.12that such a phenomenon will be obtained in the cas?of not very strongly adsorbed compounds. It ispossible to calculate 68 and 6, (Table 1) on thebasis of Langmuir-Frumkin isotherm:001B01C01= -1 6 .exp[ -2a01601]- 01where 601 is the surface coverage due to the adsor-bate. Hence, for a mixture of two adsorbates, thesurface coverage 63 due to (1) in the presence of (2)can be correlated as68BtCl = 1~6s-6, .exp(-2a16a)... (6)•.. (7)when B is given as (e-6.G/RT) where AG is' the freeen ergy of adsorption an d a',s are the lateral interaction,coefficients. Writing similar equation for componentC~)and assuming B1 ::;= B01 anda1 = aOI for, sake ofsimplicity, we obtain-! "6 - 601-601'602 d 6 _ 60Z-00l'60!8-- an ,-, 1-601'602, , 1 .• 601'602The calculated values are shown in the last columnof Table 1. The calculated values are much higherthan observed experimentally. This points out tothe fact that simplification of the Langmuir-Frumkinequation -by assuming that the free energies 'ofadsorption are the same and that the interactionparameters are identical is not justified.References. .. (8)1. JOSHI, K. M. & Rf>JAGOPALAN,S" Electrochim, Acta, ~7(1972),479 '2. TEDORADZE,G, A., ARAK~LYAN, R. A. & BELOKOLOS,, E. D., Electrokhimiya, 2(5) (1966), 563.3. DAMASKIN, B. B., MISHUTUSHKINA, I.P., GEROVICH,V. M. &' KAGANOVICH,R. I., Zhttr. Fiz., Khim., 38(1964), 1797. " '4. PA~RY, J. M. & PARSONS, R., J.electroch'4tl. Soc., 113(1966), 992.5. DOYLE, F. P. & NAYLER, J. H. C., Adv. drug Res., 1, (1964), 4J. ' , , , '6. JOSHI, K. M.,BAPAT, M. R. & DHAWALE,S. W., J. Indianchem. Soc., 47 (1970), 949. ' ,7. PARSONS,R. & DEVANATHAN,M. A. Y.;' Trans. Far!JdaySoc., 49 (1953), 404.8. BLOMGREN,E., BOCKRIS, J. O'M,. &' JESCH, C., J. phys.Ckem., 65 (1961), 2000." .9. BOCKRIS,J. O'M., DEVANATHAN,M. A. V. & MULLER,K.,Proc. R. Soc. (London), 274A (1963), 55.10. CONWAY,B. E. & DHAR, H. P., Croatica. Chemica. 4cta,45 (1973), 109. " " . ,11. JOSHI, K. M. & RAJAGOPALA.li,S., IndianJ, Chen;.• 12(1974), 1294.753

Studies on Adsorption of Organic Compounds at Mercury-Solution Interface: Individual & Simultaneous Adsorption of p-Aminobenzoic Acid & Sulphanilamide

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Studies on Adsorption of Organic Compounds at Mercury-Solution Interface: Individual & Simultaneous Adsorption of p-Aminobenzoic Acid & Sulphanilamide

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