The adsorption of glycyl-glycine from 0.1 mol 1-1 aqueous
KPF6 solutions has been studied by electrocapillary and capacity
methods. In contrast to glycine, glycyl-glycine behaves as a typical
organic adsorbate. Its adsorption isotherm is congruent when potential
is used as the electrical variable and not when charge is
used. The large repulsive effects between adsorbed \u27Species are
ascribed to solvation rather than electrostatic effects

L. M. Baugh

Roger Parsons

English

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CR 0 AT IC A CHEMIC A A CT A 45 (1973) 127 CCA-750 541.183 :547.466 Conference Paper The Adsorption of Amino Acids at the Mercury-Water Interphase. II: Glycyl-glycine* L. M. Baugh and Roger Parsons SchooL of Chemistry, University of BristoL, BristoL, EngLand Received November 2, 1972 The adsorption of glycyl-glycine from 0.1 mol 1-1 aqueous KPF6 solutions has been studied by electrocapillary and capacity methods. In contrast to glycine, glycyl-glycine behaves as a typical organic adsorbate. Its adsorption isotherm is congruent when po-tential is used as the electrical variable and not when charge is used. The large repulsive effects between adsorbed 'Species are ascribed to solvation rather than electrostatic effects. INTRODUCTION In the previous paper1 the adsorption of glycine was studied in acid, neutral, and basic solutions. The glycinium ion and the glycinate ion are adsorbed weakly but behave in a way to be expected for a cation and an anion. The adsorption of the zwitterion is in some ways characteristic of a highly polar molecule carrying no net charge and yet the very small potential shifts ac-companying adsorption at constant electrode charge lead to the supposition that the main component of the large dipole moment2 5.2 X 10-29 Cm (15.5 D) is oriented parallel to the electrode surface. It was therefore of interest to study a similar molecule with an even higher dipole moment. Glycyl-gly cine is also a zwitterion and has a dipole moment2 of 9.2 X 10-29 Cm (27.6 D). EXPERIMENTAL Double layer capacity and electrocapillary measurements were made at 25 °c as described previously3 using the same methods for preparing the base electrolytes1. Technical grade glycyl-glycine was twice recrystallized from water at 50-60 °C and dried at 110 °c. RESULTS Differential capacity curves for glycyl-glycine in 0.1 mol 1-1 KPF0 were measured at 25 °c at eleven concentrations. Some of the results are shown in Fig. 1. It is apparent from these curves that glycyl-glycine adsorbs prefer-entially near the point of zero charge and therefore behaves like a typical organic adsorbent. Some form of desorption process must occur in the region around - 1.15 V where there is an inversion in the concentration dependence of the capacity. However, no frequency dispersion of the capacity was observed here. Frequency effects were observed in dilute solution (< 0.04 mol 1-1) on the positive side of the point of zero charge. The capacity was plotted against the square root of the frequency and the capacity extrapolated linearly to zero * Based on a lecture presented at the III InternationaL Conference on the Chemistry at Interfaces, Rovinj, Yugoslavia, June 27-30, 1972. 128 L. M. BAUGH AND ROGER PA-RSONS 38 36fi 24 22 20 l'i u u: ~ u 18 16 14 i \ \ \ '- / Em /v +02 o -0.2 -0. 4 -o.6 -a.a -1.0 -1 .2 Fig. 1. Capacity potential curves for a mercury electrode in solutions of g!ycyl-glycine in 0.1 mo! 1-1 aqueous KPF, . - · - base solution; o - 0.01; x - 0.10; v - 0.25 ; o - 0.40; /::; - 0.65, m o! 1-1 glycy!-g lycine. Potential with respect to a 0.1 m ol 1-1 KC! calomel electrode. frequency was used in the further analysis4• The error on these was rather large owing to the poor low frequency response of the bridge as a result of its transformer input and output. Electrocapillary curves were measured for four concentrations. They agreed well with the twice integrated capacity curves on the negative branch except at concentrations above 0.4 mol 1-1. These high concentrations were therefore not used in the analysis. Discrepancies on the positive branches appeared at the low concentrations as with glycine and were ascribed to contact angle changes5•6 . The integrated capacity curves were used in the subsequent analysis. Coordinates of the point of zero charge are given in Table I. The potential of zero charge was determined using the streaming electrode method. Analysis of Results Surface excesses of glycyl-glycine were determined using a third order polynomial fit of the ~ - log c data. The results are shown in Fig. 2. They confirm the conclusion derived from the capacity curves that the adsorption behaviour is typical of an organic compound. Maximum adsorption occurs when the charge on the electrode is zero. Desorption occurs rather more rapidly with negative charges than with positive. Apart from this slightly stronger adsorption ADSORPTIOIN OF AiM[iNOACIDS 129 TABLE I Coordinates of the Point of Zero Charge of Mercury in Contact with 0.10 mol z-1 KPF6 Containing Glycyl-glycine at 25 °c c/mmol 1-1 -E"/mV y2 /mN m-1 C"htF cm- 2 10.0 539.5 424.1 22.96 16.0 539.0 22.85 25.0 538.5 22.92 40.0 537.5 423.5 22.89 63.0 537.0 22.63 100 536.0 22.07 160 535.0 422.0 21.13 250 534.0 19.92 400 532.5 18.20 630 530.0 418.0 15.59 1000 523.5 12.16 Potentials with respect to a 0.1 mol 1-1 KC! calomel electrode in contact with a 0.1 mol 1-1 solution of KPF• (not containing glycyl-glycine). on positive charges the form of these curves is quite different from that of the glycine curves. The adsorption isotherms shown in Fig. 3 are logarithmic with slopes strongly dependent upon charge. At the adsorption maximum the 0 -2 -4 -· Fig. 2. Surface excess of glycyl-glycine as a function of charge on the electrode . . _ 0.025 ; • - 0.040 ; v - 0.063: !::. - 0.10; o - 0.16 ; x - 0.25 ; 0 - 0.40 mo! 1-1 gly-cyl-glycine LOG CONC . 0 -1 .s -1 .6 -1-~ -1.2 -1.0 -o .s -o .6 -o .4 3. Surface excess of glycyl-glycine as a function of logarithm of its concentration in solution. The charge on the electrode in µC cm-• is indicated by each line. 130 L. M. BAiUGH ANiD ROGiER PARSONS slope corresponds to a two-dimensional second virial coefficient of 2.7 nm2 per molecule (270 A2) with correspondingly larger values at larger charges. The projection of a molecular model lying flat on a plane has an area of 0.45 nm2 so that the value of the second virial coefficient indicates the existence of strong repulsive forces. The apparent change in these with charge means that the adsorption isotherms are not congruent in the charge. This is con-firmed from surface pressure plots although the surface pressures are very small, the highest value for the 0.4 mol 1-1 solution being 4.3 mN m-1. Consequently this conclusion relies on the assumption that very small dif-ferences are not artefacts. A similar conclusion is reached from the plot of the potential drop across the inner layer against the amount adsorbed (Fig. 4) which is not linear except at the point of maximum adsorption. The fact that this occurs at zero charge on the electrode suggests that the dipole is essentially parallel to the electrode surface. -o.s -o.e - 13 - 2 10 r / MOLS. CM. 0 -· -0-9~ '::-----c'-::----":---..J.:--_,,L-....L.--'--"-__J Q.5 1· 0 1-5 2-0 2-5 J .Q 3.5 4 -0 A -5 Fig. 4. Potential drop across the inner layer ( + a constant) as a function of surface excess of g!ycyl-glycine. The charge on the electrode in µC cm·2 is indicated by each !me. The form of the isotherms in Fig. 3 with lower slopes at lower amounts adsorbed suggested that these might be segments of an isotherm approaching ideal behaviour at the lower amounts adsorbed. It therefore seemed worth investigating whether a more reasonable behaviour occurred with the isotherms at constant potentiaF,8. These are plotted in Fig. 5 and it is at once evident that these are more nearly congruent. In fact they superpose quite closely except at the smallest amounts adsorbed where experimental inaccuracies become ADSORPTION OF AM:LNOACIDS 131 - 0·6V I I 6 0 10'1/molec cm1 / / 07V l/~:: of;/ -4 l!/o / 03V ;}/I Ir /)0/ o /0 0 . ;/lo 0 I .... 2 (fo/0/ 0 0/ 02V o/ / /o ,~:/ / / ~o _.,.....o o --o o_....... -;;jo=a-o-log(c/moll'J -1 ·~ -1.,0 - 0 ·5 I F ig. 5. surface excess of gly cy l-glycine as a function of logarithm of i ts concentrat-ion in solution. The p ot en tia l of the e lectrod e in indicat ed b y each line. 6 1Cf r/molec cm-1 •' .; 4 •" 2 " log c + f(E) -2 -1 F ig. 6. The da t a of Fig . 5 super posed by adding a potentia l-dependent quantity to the log concentration scale. significant (Fig. 6) . The amount by which the isotherm must be shifted along the log c axis for coincidence is closely proportional to (E - E m)2 where .Em is the potential corresponding to maximum adsorption, Fig. 7. Thus the · free I I I t; l o g c Fig. 7. The sh ift of th e isotherm alon g the concen tration axis r eq uired for superposition in F ig. ti . plotted against t h e squ are of the poten tia l w ith respect to the potential of maximum adsorption . energy of adsorption is a parabolic function of the potential. The congruence of the isotherm with respect to potential is confirmed by plotting according to the 'virial' equation (Fig. 8). Although there are some deviations which may be ascribed to experimental error, the lines at the different potentials are essentially parallel and their slope corresponds to a two-dimensional second virial coefficient of 2.3 nm2 per molecule. This is in fai r ly close agreement with 132 L. M. BAiUGIH ANID ROGER PAHSOINS the value derived from the line at q = 0 in Fig. 3 since this is also a line at constant potential. Further confirmation of these conclusions is obtained by plotting the electrode charge against the amount adsorbed at constant potential (Fig. 9). -6 q If.JC crri' 10 log (1o·'hc) Fig. 8. Tpe dat'il of Fig . 5 plotted as :;. 'virial' adsorption isotherm. Th e potentinl of the electrode is indicated b y each line . -2 0 -0-4V 4 6 8 7 / .. , / 10 r/molec cm·> / 1 2 3 4 Fig. 9. The charge on the electrode as a function of the surface excess of glycyl--glycine. The potential of the e lectrode is indicated by each line . · Straight lines are obtained in agreement with Frumkin's postulate'. For an isotherm which is congruent in the potential thR.se lines are described by the equationn q - q" = kT I' (o ln (J/d E) (1) where qh is the value of q in the base solution and fJ is the adsorption coefficient. When t he standard free energy of adsorption is a quadratic function of the potential, this may be expreseed and where ln (J = ln (J"' - (b/2) Ll2 o In (1/o E = - b LI LI = E-E"' (2) (3) and the superscript m indicates the potential at which the adsorption is a maximum. Thus equation (1) may be written as q - q" = - b . I kT I' (4) and the slopes of the lines in Fig. 9 should -be given by ADSOIR'PTIOIN -OF AiMINOACIDS 133 (iJ q/iJ I') = - b kT LI (5) The slopes of the tines in Fig. 9 are plotted against the potential in Fig. 10 and these points are· compared with the line calculated from the value of b obtained from Fig. 7 acco'rding to equation (2). The satisfactory agreement shows that this system of equations describes the adsorption of glycyl-glycine adequately. I I 1c)'(aq/ol)/ µc molec-· 1 0 -1 03 0·5 I 07 0·9 Fig. 10. The coefficient (iJq/iJf)E as a function of the potential of the electrode. Points are calculated from the slopes of the lines in_ Fig. 9. The line is calculated from the slope of the line in Fig. 7. DISCUSSION Adsorption of glycyl-glycine on mercury contrasts strongly with that of glycine in that the larger molecule behaves as a typical organic adsorbate although still being adsorbed rather weakly. This difference of behaviour is unlikely to be due to a difference in the energy of the two molecules in solution. The solubility of glycine10 in water at 25 °C is 2.886 mol i-1 while that of glycyl-glycine11 is 1.512 mol 1-1• This corresponds to a difference in salvation energy of less than 2 kJ mo1-1• If the adsorbed layer is assumed to be ideal, the difference of the free energy of adsorption at the potential of zero charge is about twice this value. Since both molecules are apparently oriented with their dipoles parallel to the interface the effect of the image force must be very similar for both. The effect of the larger number of water molecules displaced by glycyl-glycine must therefore be outweighed by some other factor in favour of its adsorption. This can be tentatively suggested to be that the larger molecule causes a smaller disruption of the structure of the interfacial water layer. The most striking difference b'etween glycine and glycyl-glycine is of course the charge-dependence of the adsorption. It is suggested that the reason for this is the greater difficulty in re-orienting the larger molecule. Thus at all accessible charges glycyl-glycine adsorbs parallel to the interface and consequently its large dipole moment has no effect on the adsorption characte-ristics, which are consequently in this respect closely similar to those of much less polar organic adsorbates. If this is correct then the behaviour of glycine can be ascribed to some reorientation at extreme charges and is consistent with theoretical expectations7, 12,13• Some support for the relative degrees of interaction with interfacial solvent can be obtained from the values of two-dimensional· second virial coefficient which at uncharged mercury is larger for glycine than for glycyl-glycine. It seems unlikely that these large repulsive interactions are due to electrostatic effects since dipoles rotating 134 L. M. B.AJUGH ANID iRiOGiER PiARSOiNS in a plane should attract each other. They are more probably due to orientation of neighboring solvent molecules into a two-dimensional salvation shell. The specific nature of the. interaction between the solvent and the ad-sorbate is emphasized by comparing · the behaviour of glycyl:-glycine with that of butan-1 :4-diol or butyne-1 :4-diol. The projected area of these molecules is quite similar being 0.45 nm2 for glycyl-glycine and 0.38 nm2 for the dials. However, as shown previously14 the adsorption of the dials conforms closely to a Langmuir isotherm in contrast with . the present system where strong repulsive forces are evident. Similarly in the potential dependence of the standard free energy of adsorption, the value of b in equation (3) is found to be approximately three times as large for glycyl-glycine as for the dials (the value for the dials is not quite constant; as pointed out previously14 a more nearly constant value is obtained by considering the free energy depen-dence on charge). This difference also indicates that a simple water-replacement model is inadequate to describe the behaviour of real systems with respect to the nature of the adsorbing species. No detailed study was made of the adsorption of glycyl-glycine in acidic and basic solutions. The curves shown in Figs. 11 and 12 suggest that the 'organic' nature of glycyl-glycine remains quite strongly expressed in acid soiut'ions but that in alkaline solutions it behaves more like an anion. 4 0 38 36 3 4 ' 32 I 30 28 26 2 4 "· ':> u "' " 22 20 18 16 f,,, / V + 0.2 0 -0-2 -().4 -0.6 -o .s Fig. 11. Capacity-potential curves for a mercury electrode in 0.25 mo! 1·1 HPFs - - - , o - containing 0.1 mo! 1- 1 gly-cine ; x - containing 0.1 mo! 1-1 glycyl--glycine. 38 34 32 2 8 r:'i u · "' ~26 u 24 22 20 18 16 I ' I I I I I I I I I I \\ l'-" \ x, \ , \ \ \ ' x ',,,  0 -0.2 · -o.4 -0-6 -o.s -1 .0 - 1.2 -I·"' - 1.c Fig. 12. Capacity-potential curves for a mercury electrode in 0.1 m o! 1-1 KOH --; o - containing 0.1 mo! 1-1 g lycin e; x - con taining 0.1 mo! 1- 1 glycyl-g!ycine. ADSORPTtION OF AMJ:NOACIDS 135 AcknowLedgements. We are pleased to acknowledge the receipt of financial assistance from the Thorton Research Centre of Shell during this work (1969-70) . REFERENCES 1. L. M. B au g h and R. Pa rs on s, J . Eiectroanai. Chem. 41 (1973) 311. 2. J . T. Eds a 11, Fortschr. Chem. Forsch. 1 (1949-50) 156. 3. L. M. Baugh and R. Parsons, J. Eiectroanai. Chem. 40 (1972) 407. 4. R. Parsons and P. C. Symons, Trans. Faraday Soc. 64 (1968) 1077. 5. J . L aw re n c e, R. Pars on s, and R. Payne, J . ELectroanaL Chem. 16 (1968) 193. 6. S. Tr as at ti, J. ELectroanaL Chem. 31 (1971) 17. 7. A. N. Frumkin, Z. Physik. 35 (1926) 792. 8. B. B. Damaskin, 0. A. Petri, and V. V. Batrakov, Adsorption of Organic Compounds on ELectrodes, Plenum, New York, 1971. 9. R. P a r s o n s, Trans. Faraday Soc. 55 (1959) 999. 10. T . L. McMeekin, E. J. Cohn, and J. H. Weare, J . Am. Chem. Soc. 58 (1936) 2173. 11. T. L. Mc Meek in, E. J . Cohn, and J . H. We are, J. Am. Chem. Soc. 57 (1935) 626. 12. J . A. V. But 1 er, Proc. Roy. Soc. Al22 (1929) 399. 13. J . O'M Bock r is, M. A. V. Dev an at ha n , and K. M ii 11 er, Proc. Roy. Soc. A274 (1963) 55. 14. E. Du t k i e w i c z, J. D. Garnish, and R. Parsons, J . ELectroanaL Chem. 16 (1968) 505. IZVOD Adsorpcija amino kiselina na medufazi ziva-voda L. M . Baugh i Roger Parsons Mjerenjem elektrokapilarnih krivulja i diferencijalnog kapaciteta na zivinim elektrodama u elektrolitu KPF6 proueavana je adsorpcija glicil-glicina. Za razliku od glicina, glicil-glicin se ponasanja poput organskog adsorbata, tj. biva preferen-cijalno adsorbiran blizu tocke nula naboja. Adsorpciona izoterma glicil-glicina kon-gruentna je samo ako se kao elektricna varijabla uzima potencijal, a ne naboj elektrode. Veliki repulzivni efekti izmedu adsorbiranih molekula pripisuju se efek-tima solvatacije, a ne elektrostatskim silama. Ipak je pokazano da jednostavni model zamjene vode ne objasnjava dovoljno dobro opazene fenomene. Izlaz iz nejasne situacije predlaze se prihvacanjem modela u kojem susjedne molekule otapala tvore dvodimenzionalni solvatacioni sloj oko molekula glicil-glicina. SCHOOL OF CHEMISTitY U NIVERSITY OF BRISTOL BRISTOL, ENGLAN D Primljeno 2. studen oga 1972. 

The Adsorption of Amino Acids at the Mercury-Water Interphase. II: Glycyl-glycine

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