The infrared spectra of glycine adsorbed on various cation-substituted montmorillonites

J m o r ~ n u ( I. ( ' h e m . .

THE

1972, Vol. 34, pp. I503-1509.

Pergamon Press.

Printed in Great Britain

INFRARED SPECTRA OF GLYCINE ADSORBED ON VARIOUS CATION-SUBSTITUTED MONTMOR! LLONITES*

S U N G DO J A N G and R. A. C O N D R A T E . St. Division of ('eramic Engineering and Science, State University of New York College of Ceramics, Alfred University, Alfred, N.Y. 14802

(R eceit, ed 2 August 1971 ; in revised form 14 September 1971 ) A l m t r a e t - I h e i.r. spectra 14000-1200 cm -~) are obtained for several cation-substituted-montmorilIonite-glycine complexes. Interpretation of the spectra for Cu-montmorillonite containing less than 20 meq intercalated glycine/100g, clay and prepared at a pH near the isoelectric point of glycine indicates the presence of both a bidentate chelate complex and a monodentate complex in which a Cu )" ion is coordinated to the N-atom of the amino acid. The monodentate complex is the predominant adsorbed species on clay films prepared at a pH below the isoelectric point while the bidentate complex predominates on films prepared at higher pH's. Glycinium cations are the adsorbed species on H-montmorillonite. These cations become a major adsorbed species on Cu-montmorillonite as one increases the concentration of glycine adsorbed on the clay film. Similar observations have been made for glycine adsorbed on Ni- and Zn-montmorillonite. INTROI)UCTION

THE STRUCTURES of many compounds adsorbed on clays have been effectively determined by the use of i.r. spectroscopy[1,2]. Recently, Cloos et al. [3] have

investigated the i.r. spectra of several amino acids adsorbed on Na-, Ca- and Hmontmorillonites, and determined that the protonated amino acid cations were the dominant adsorbed species. In this investigation, analysis has been made of the i.r. spectra of glycine adsorbed on several different transition metal ion substituted montmorillonites which have been prepared at different pH's and with different concentrations of amino acid in order to determine whether this type of complex is the only type of amino acid complex that can be adsorbed in the interlamellar spaces of montmorillonites. The i.r. spectra of related complexes in crystals and solutions have been used to identify the adsorbed species, and to make band assignments [4-12]. "Presented in part at the 161st National Meeting of the American Chemical Society, Los Angeles, California, March, 197 I. Submitted by Sung Do Jang to the Faculty of the College of Ceramics at Alfred University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in (eramics. I. M. I.. Hair. Infrared Spectroscopy in Surface Chemistry, p. 191, Marcel Dekker, New York (1967). 2. 1.. H. Little, Infi'ared Spectra ofA bsorbed Specie,s, Academic Press, New York (1966). 3. P. Cloos, J. J. Fripiat, B. Calicis and K. MaKay, Proc. Intern. Clay Conj. (IsraelJ i, 223 (1966). 4. G. F. Svatos. C. Curran and J. V. Quagliano, J. Ant. chem. Soc. 77. 6159 (I 955). 5. R. A. Condrate and K. Nakamoto. J. chem. Phys. 42.2590 (1965). 6. S. Suzuki and T. Shimanouchi, Spectrochim. A cta. 19, I 195 (1963). 7. K. Nakamoto, Y. Morimoto and A. E. MartelI,J. Am. chem. Soc. 83, 4528 ( 1961 ). 8. M. Tsuboi, T. l-akenishi and A. Nakamura, Spectrochhn. A cta. 19, 271 (1963). 9. E. R. I,ippincott and R. K. Khanna, Colloq. Spectrosc. Intern., 12th, p. 513. Exeter, England (1965). It). J. A. Kieft and K. N akamoto, J. inorg, nucl. Chem. 29. 2561 (I 967). 1503

1504

S.D. JANG and R. A. CONDRATE, Sr

EXPERIMENTAL Preparation of clay films The montmorillonite used in this study originated from Wyoming. Clay films were prepared from clay particles whose equiv',dent diameters were less than two microns. The fraction containing these clay particle sizes was collected by sedimentation from a 5 per cent clay suspension in distilled water [ 13]. Transition metal substituted montmorillonite was prepared by saturating natural clay with a 1 N aqueous solution of the appropriate metal chloride. The clay had to be washed several times with distilled water to eliminate free metal ions. H-montmorillonite was prepared immediately prior to use from a 1 per cent suspension of natural clay by passing it through a column of Amerite I.R.-120 (H-form). The concentrations of the resulting clay suspensions were adjusted so that the clay made up approximately 2-5 per cent of the weight of the aqueous suspensions. Appropriate amounts of the clay suspension and an aqueous solution of glycine (J. T. Baker Chemical Company) were mixed together in order to form the desired complex. The pH of the mixture was measured and adjusted to the appropriate value by adding 0.1 N hydrochloric acid or sodium hydroxide solution. Self-supporting thin films (approximately I in. dia., 2-4 mg/cmz) were prepared by evaporating 2 ml of the clay complexdistilled water suspensions on an aluminum foil supported by a flat glass plate. Since most exchangeable cations react with aluminum foil, the foil had to be lined with a thin film of collodion to protect the clay from chemical reactions. Distilled water was used to wash away unadsorbed glycine from the clays[13]. The specimens were dried over P205 in a slightly evacuated desiccator. The resulting dried film specimens were easily stripped from the foil by drawing the foil over a sharp edge. X-ray diffraction analysis indicated that glycine formed single-layer complexes in the clays. Infrared studies The i.r. adsorption spectra (4000-1200 cm-1) were obtained for each clay specimen by placing their corresponding self-supporting thin films in the sample beam ofa Perkin-Elmer Model 621 double beam grating spectrophotometer. The clay films were mounted in an evacuated cell with KCI windows. An air purging unit was employed to eliminate adsorption bands due to atmospheric water and carbon dioxide. Calibration of the spectrophotometer was carried out using polystyrene bands and a wavenumber accuracy of__+3cm-lwas obtained.

RESULTS AND DISCUSSION T h e i.r. s p e c t r a o f C u - m o n t m o r i l l o n i t e c o m p l e x e s c o n t a i n i n g less t h a n 20 m e q i n t e r c a l a t e d g l y c i n e / 1 0 0 g clay p r e p a r e d at several different p H ' s are s h o w n in Fig. I. T h e s t r o n g b a n d at ca. 3 6 3 0 c m -1 is d u e to the lattice h y d r o x y l s t r e t c h i n g m o d e for m o n t m o r i l l o n i t e . T h e o t h e r b a n d s are a s s i g n e d in T a b l e l using p r e v i o u s b a n d a s s i g n m e n t s m a d e for similar c o m p l e x e s in c r y s t a l s a n d solutions. I n the N H s t r e t c h i n g region, the o b s e r v e d w a v e n u m b e r s for all C u - c l a y - g l y c i n e c o m plexes are l o w e r t h a n t h o s e in the s p e c t r u m o f the g l y c i n a t e a n i o n (i.e., 3 3 8 0 a n d 3 3 6 0 c m -1 for p o t a s s i u m g l y c i n a t e ) , but are v e r y close to t h o s e o b s e r v e d for the c h e l a t e d c o p p e r c o m p l e x [4, 5]. T h i s o b s e r v a t i o n suggests that n i t r o g e n a t o m s o f g l y c i n e are c o o r d i n a t e d to C u 2÷ ions in the p r e p a r e d c o p p e r - c l a y c o m p l e x e s . In the l o w e r w a v e n u m b e r region the a s y m m e t r i c a n d s y m m e t r i c s t r e t c h i n g m o d e s o f the c a r b o x y l g r o u p c a n be a s s i g n e d to two b a n d s f o u n d at ca. 1600 a n d 1 4 0 0 c m -~ in each s p e c t r u m . T h e N H z s c i s s o r i n g m o d e also o c c u r s at ca. 1600 c m -1 a n d is p r o b a b l y r e s p o n s i b l e for the b r o a d n e s s o f b a n d at this w a v e n u m b e r location. T h e i.r. s p e c t r a o f the C u - m o n t m o r i l l o n i t e - g l y c i n e c o m p l e x e s 11. M. Takeda, R. E. S. lavazzo, D. Garfinkel, I. H. Scheiberg and J. T. Edsall, J. ,4m. chem. Soc. 80, 3813 (1958). 12. A. Rosenberg, Acta. chem. scand. 10, 840 (1956). 13. S. D. J ang, Ph. D. Thesis, State University of New York College of Ceramics at Alfred U niversity, Alfred. New York, 1971.

I.R. spectra of glycine-montmorillonite complexes i

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prepared at low equilibrium pH values (i.e., at pH 4) show those two bands at 1593 and 1410 cm-'. These two carboxyl stretching wavenumbers are very close to those observed for the glycine zwitterion (1594 and 1415 cm-') in which the carboxyl group is ionized[6]. However, the i.r. spectra of complexes prepared at pH values greater than 6 showed the appearance of new bands at 1632 and 1378cm -1 while the bands at 1595 and 1410cm-' decrease in intensity. The spectrum of the complex prepared at a pH value of 8-0 showed a weak shoulder at 1600 cm-', the complete disappearance of the 1410 cm-' band and an increase in the intensities of bands observed at 1632 and 1378cm-L In this case, the shoulder at 1600 cm -1 may be attributed to the NH2 bending mode. The observation that the carboxyl asymmetric stretching mode shifts to a higher wavenumber while the carboxyl symmetric stretching mode shifts to a lower wavenumber indicates that the carboxyl group of intercalated glycine molecules are coordinated to the Cu 2+ ion in the clay mineral when the Cu-montmorilionite-glycine complexes are prepared at equilibrium pH values greater than 6. Nakamoto et al. [7] have investigated the asymmetric and the symmetric carboxyl stretching modes of a large series of metal complexes formed with amino acids. They noted thal

1506

S. D. J A N G and R. A. C O N D R A T E , Sr Table 1. Band assignments of glycine-montmorillonite complexes prepared at several different pH values

pH

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Assignments

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the asymmetric mode increased its wavenumber while the symmetric mode decreased its wavenumber when the metal-oxygen bond between the metal and the carboxyl oxygen increased its covalent character. The possibility of a coordinated carboxyl group is further supported by the shift of the NH stretching modes toward higher wavenumbers for the complex prepared at high pH values. The wavenumbers of these stretching modes should depend upon the ligands that

I.R. spectra of glycine-montmorillonitecomplexes

15{)7

are located in the 6is-position relative to the amino group in the copper square planar complexes. The replacement of a bond between a Cu 2÷ ion, and hydrated water molecule by a bond between the Cu ~+ ion and the glycine's carboxyl group would cause a shift of the NH stretching wavenumbers. The observed band at 1450 cm -I in each spectrum for the Cu-montmorilioniteglycine complex prepared at a pH < 6.9 is assigned to the CH,, scissoring mode for intercalated glycine. This mode appears at 1432 cm-' for the complex prepared at a pH 8-0. The complex prepared at pH 6.0 showed both bands at 1450 and 1432 cm -~ as well as the C O 0 - symmetric stretching mode at 1410 and 1378 cm-'. Consequently, these two bands at 1450 and 1432 cm -~ were assigned to the CH2 scissoring mode of the monodentated and bidentated glycine. respectively. The CH._, wagging mode ca. 1330cm-' for each Cu-clay complex prepared at various pH values changes very little from that of the zwitterion or Cu-glycine complex. The interpretation of the spectra for ('u-clay-glycine complexes prepared at various pH values using low concentrations of glycine leads to the following conclusions. At pH values near the isoelectric point of glycine {6.0}, the clay may contain two different species, the monodentate complex tA), and the bidentate complex (B). -H.,O and

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Below the isoelectric point, the monodentated complex is predominant, while the bidentate complex may predominate above the isoelectric point. The i.r. spectra of an H-montmorillonite-glycine complex is shown in Fig. 2. The NH stretching modes appear at 3220 and 3160 cm -1. These bands are significantly lower than those observed for NH2 group. They are slightly higher than those observed for glycine cations and zwitterion in crystals, which possess NH3" groups[6, 8]. Mortland[14, 15] investigated the NH stretching modes for ammonia and several organic cations containing NH3 ~ groups that were intercalated into montmorillonite and compared them to the bands observed for the same cations in either crystals or solutions, in all cases, he noted that the observed bands were higher in wavenumber for the intercalated cationic molecules. His interpretation is that N H C ions and +NH3 groups undergo less intermolecular and environmental interactions in clays and, therefore, their stretching modes appear at higher wavenumbers. A similar mechanism can explain our observations for the NH stretching modes of intercalated glycine cations. The broad band at 3390 cm -1 in the spectra of H-clay complex and the Cu-clay complex prepared at very low pH can be attributed to the OH stretching mode of hydrated 14. M. M. Mortland,J. J. Fripiat,J. Chaussidonand J. Uytterhoeven,J. phys. Chem. 67. 248 (1963). 15. M. M. Mortland.ClayMin. 6, 143 (1966).

1508

S . D . . I A N G and R. A. C O N D R A T E , Sr.

or coordinated water molecules. This same band was also noted for H-montmorillonite with no glycine added. The band at 1745 cm -I of the H-clay complex may be assigned to the carbonyl stretching mode of the unionized carboxyl group. It is interesting to note that a weak shoulder c a . 1720 cm -j appears in the spectrum of the Cu-montmoriiloniteglycine complex prepared at a low pH value (Fig. 1). This weak peak seems to indicate that a small amount of glycine cation may be present in the interlamellar space at low pH's. The spectrum observed for the H-montmorilionite-glycine complex in the lower wavenumber region was very similar to that of glycinium ion in a KBr pellet. The NH3 ÷ symmetric deformation occurred at 1507 cm -1 for the glycineH-montmorillonite complex. The OH in-plane deformation mode appears at 1430 cm -~ similarly to crystalline compounds. The CHz wagging mode occurred at 1340 cm -~ for the clay complex which is consistent with observing a band at 1338 cm -~ for the glycinium ion in the crystalline state. The spectra obtained for Cu-clay complexes containing larger amounts of glycine (i.e., 75 meq/100g clay), in Fig. 3, show many bands in addition to those

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I.R. spectra of glycine-montrnorillonite complexes

1509

observed previously for Cu-montmorillonite complexes containing less than 20 meq glycine/100g clay. The observed bands in spectrum(I), Fig. 3, at 3338, 3284, 3220, 3160 and 3040 cm -1 are assigned to the NH stretching modes. The first two bands are attributed to the coordinated NH,, group. In the lower region of spectrum, the observed bands at 1745, 1585, 1508, 1440, 1408 and 1235 cm-' are assigned to the COOH. COO- asymmetric stretching, NH:~~ deformation, CH, scissoring, C O 0 - symmetric stretching and COOH deformation modes. Therefore, it can be seen that the Cu-clay complexes prepared at pH below the isoelectric point with higher equilibrium concentrations of glycine contain two different adsorbed species, the cationic and monodentated glycine. When Cttmontmorillonite is suspended in a solution containing a large amount of glycine, a large amount of Cu 2~ ion was liberated from the clay even at the natural pH value. Consequently, the clay adsorbed cationic glycine to replace Cu"- ions. The contribution of glycinium ion to be adsorbed species is negligible for the clay complexes containing less than 35meq glycine/100g, clay at their natural preparation pH values. Spectral observations noted for glycine adsorbed on Ni- and Zn-montmorilIonite are similar to those observed for the Cu-clay films. For example, the spectra obtained for the Z~a- and Ni-montmorillonite-glycine complexes prepared by mixing aqueous solutions of glycine with the appropriate clay at their natural pH's are very similar to that of the Cu-clay-glycine complex prepared at pH ca. 4 (see Fig. 2). The observed wavenumbers at 1615, 1420 and 1455 cm -~ in Ihe spectrum of the Zn-clay complex are assigned to the asymmetric COO- stretching. symmetric COO- stretching and CH., scissoring modes. The same modes are observed at 1610, 1410 and 1450cm-' in the spectrum of the Ni-clay-glycine complex. The bands ca. 1330cm -j in both spectra are assigned to the CH., wagging mode. Similarly to the Cu-clay complexes prepared with glycine at lower pH's. the Ni ''+ and Zn'-'" ions form monodentate complexes with the glycine anion. A,'ktu~wledgement- Robert A. Condrate, Sr., would like to thank the College ("enter of the Finger I .akes for a grant-in-aid supporting this stud)'.