J. Chem. SOC., Faraday Trans. I , 1987, 83, 1685-1701 Metachromasy in Clay Minerals Sorption of Pyronin Y by Montmorillonite and Laponite Zvi Grauer,? Goldye L. Grauer, David Avnir" and Shmuel Yariv*f Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel The adsorption of pyronin Y by montmorillonite and laponite has been studied by visible, infrared and X-ray diffraction spectroscopies. The saturation point is much higher in montmorilionite, being 100 and 41 mmol pyronin per 100 g montmorillonite and laponite, respectively. The adsorbed cationic dye is located in the interlayer space. In montmorillonite adsorption leads to metachromasy of the dye and the appearance of a new band at shorter wavelengths than the original band (480 and 545 nm, respectively) even at very small coverages.In laponite, on the other hand, no meta- chromasy is observed with small amounts of dye. It is observed only when the degree of saturation approaches the saturation point. In montmorillonite the organic cation is oriented with the plain of the rings parallel to the silicate layer. In this parallel orientation n interactions between the oxygen plane of the aiuminosilicate and the aromatic dye give rise to metachromasy of the dye. In laponite the plain of the aromatic ring is tilted relative to the silicate layer and n interactions between the oxygen plane and the aromatic dye do not occur. Metachromasy is observed when dimers or aggregates of dye cations are formed in the interlayer space or in the interparticle space of flocs of laponite.Adsorption of dyes by clay minerals often results in significant spectral changes, especially in the electronic spectrum. Little is known, however, about the adsorption interactions which cause these alterations. There is also a practical point to this problem: smectite group minerals are often used as fillers in various dyeing and painting processes and often the use of this additive leads to colour changes of the dye molecule. In previous publications from our laboratory the adsorption of three cationic dyes by montmorillonite was described :1-3 methylene blue, acridine orange and rhodamine 6G. The adsorption of these dyes takes place by the mechanism of cation exchange. The adsorption of the first two dyes is accompanied by metachromasy, i.e. the appearance of a hypsochromically shifted band.From X-ray measurements and i.r. spectroscopic studies it was concluded that metachromasy resulted from n interactions between the oxygen planes of montmorillonite and the aromatic rings of the organic dye, and not due to the aggregation of the organic cation in the interlayer space, as was previously suggested by Bergmann and O'Konski.* The idea of n interactions between the aluminosilicate layer and the aromatic entity was later supported by comparing the adsorption of dibenzotropone and dibenzosuberone by rnontmoril1onite.j Only in the first molecule is aromaticity induced, and its adsorption by montmorillonite results in n interactions between the organic molecule and the clay mineral. In rhodamine 6G the phenyl ring is sterically constrained to be roughly perpendicular to the planar xanthene group.6 Owing to this steric effect, n interactions between the organic dye cation and the aluminosilicate layer cannot occur.Thus, no metachromasy is observed when rhodamine 7 Present address: Department of Chemistry, Columbia University, New York, N.Y. 10027, U.S.A. J On sabbatical leave at the Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T'6G 2G2. 16851686 Metachromasy in Clay Minerals 6G penetrates into the interlayer space of montmorillonite. It may therefore be concluded that metachromasy of cationic dyes in montmorillonite suspensions is expected only if there is no steric hindrance and when 7t interactions may occur between the aromatic rings and the oxygen plane of the aluminosilicate layer.Laponite is a synthetic hectorite. This clay is of industrial importance as it often replaces montmorillonite as a filler in various painting and dyeing processes. In spite of the observed differences in the staining between these two minerals7* * very little work has been done on the interactions between cationic dyes and laponite. In our previous study on the adsorption of rhodamine 6G by both minerals we showed that their behaviour towards this dye is similar, namely both adsorb the dye by the mechanism of cation exchange and no 71 interactions occur between the clay oxygen plane and the aromatic rings. In the present study we show that in the absence of steric hindrance, 7t interactions between the dye cation and aluminosilicate layer do occur.Differences in spectroscopic behaviour between montmorilloni te and laponite dye suspensions were observed. The cationic dye which is examined in the present study is the xanthene dye, pyronin Y (PY). Unlike rhodamine 6G, this dye has no side rings which may prevent metachromasy due to steric hindrance, as is the case in adsorption of acridine orange.2' pyronin Y (PY) Experimental Pyronin Y was supplied by Aldrich and was recrystallized from ethanol. Laponite XLG (a synthetic Na-hectorite) was from Laporte Industries Inc. Wyoming bentonite (Na-montmorillonite) was supplied by Wards Natural Establishment Inc. and was ground to 80 mesh. Quartz was separated by sedimentation. The preparation of the clay suspensions, the recording of the spectra of the clay-dye suspensions, the preparation of samples for X-ray diffraction and the recording of the X-ray diffractograms have been described previously.2y The specimens which were examined by X-ray were also examined by i.r.spectroscopy. Infrared spectra were recorded on Perkin-Elmer 457 and 597 grating instruments. The specimens were examined in the form of KBr discs and as oriented deposits on polythene windows. Oriented deposits were tilted by 45" or 90" with respect to the direction of the i.r. beam. Clay-dye suspensions and supernatants were examined in tubes transparent to U.V. radiation. Results Electronic Spectra Absorption Spectrum of PY in Aqueous Solution The effect of concentration on the visible absorption spectrum of PY in aqueous solution is shown in fig.1 . The very dilute solution (2.1 x mol dmP3) gives a single absorption band with a maximum at 545 nm, assigned as band a. More concentrated dye solutions show metachromasy, i.e. the appearance of a new band with a maximum at 512 nm, assigned as band /I. Further increase in the dye concentration results in the increase in the intsnsity ratio p/a.2. Grauer et al. 1687 ‘.-.I 500 550 600 650 wavelength/nm Fig. 1. Absorption spectrum of aqueous solution of PY: (a) 4.2 x mol dm-3 and (b) 2.1 x mol dmP3. The Eflect of Montmorillonite on the Absorption Spectrum of PY A series of clay-dye suspensions (montmorillonite or laponite) with varying clay concentrations were prepared, keeping the concentration of the dye in each sample at a constant value of 1.57 x mol dm-3.Thus, the formal degree of saturation (expressed in mol dye per 100 g clay) increased with the decreasing clay concentration. When the solid phase was separated from the solution by centrifugation it showed an intense coloration. The supernatant was either colourless (in the presence of high clay concentrations) or showed weaker absorption intensity than the clay-free dye solution. This clearly indicates adsorption of PY by montmorillonite and by laponite. Some representative spectra of montmorillonite-PY suspensions at different clay concentra- tions are shown in fig. 2. Comparison of fig. 1 and 2 shows that the adsorption of PY by montmorillonite results in metachromasy, namely a pronounced B band appears. In the presence of small amounts of clay (or a high formal degree of saturation) soluble PY contributes to the adsorption spectrum [fig.2(d)] and the spectrum shows a high intensity of band a at 545 nm. With increasing clay concentrations (or decreasing formal degree of saturation) no PY remains in the aqueous phase and the /3 band increases [fig. 2(c)]. Adsorbed PY shows a bathochromic shift of a to 550 nm. The intensity of this shifted a band increases with increase of clay concentration. Fig. 3(a) shows the effect of clay concentration in the suspension on the locations of bands a and in the absorption spectrum of the dye. As a result of adsorption, bandI688 Metachromasy in Clay Minerals I I I I 1 500 550 600 650 wavelength/nm Fig. 2. Absorption spectrum of PY (1.57 x lop5 mol dm-3) in the presence of montmorillonite.(a) 27.5 x lop3 wt % montmorillonite (formal degree of saturation 3 mmol PY per 100 g clay), (b) 2.75 x wt % (60 mmol PY per 100 g clay) and ( d ) 0.625 x 1 0-3 wt % (1 30 mmol PY per 100 g clay. wt % (30 mmol PY per 100 g clay), (c) 1.375 x a shifted to a longer wavelength (from 545 nm in the aqueous solution up to 550 nm), whereas band b shifted to a shorter wavelength (from 512 nm in the aqueous solution up to 48 1 nm). A bathochromic shift of the a band due to adsorption by montmorillonite was observed previously with acridine orange and rhodamine 6G29 and was interpreted as reflecting a surface polarity which is not as high as that of water. Similarly, as long as the added PY is completely adsorbed by the clay, the band maximum appears at ca.550 nm. As soon as free PY is present in the suspension the maximum of the a band is shifted towards 545 nm. Fig. 3 shows that the PY adsorption capacity of montmorillonite (' saturation point ') was 100 & 5 mmol dye per 100 g clay. The spectroscopic behaviour of the /3 band is different. The location of its maximum depends on the formal degree of saturation. As this degree increases, the p band shifts2. Grauer et al. 560 t 1689 50 100 150 200 250 50 I00 I50 200 250 degree of saturationlmmo1 per 100 g clay Fig. 3. (a) The effect of the formal degree of saturation of montmorillonite by PY on the locations of bands a ( j and /? (A j and (b) the effect on the absorption intensities of these bands. All spectra were recorded at a fixed PY concentration of 1.57 x mol dmP3.The clay concentration was between 0.3 x and 27.5 x wt % . to shorter wavelengths, reaching a minimum at 481 with 45 mmol PY per 100 g montmorillonite. Further increase in the degree of saturation shifts band /l to longer wavelengths. X-Ray studies (see below) indicate that this minimum is associated with the occurrence of two types of PY-H,O-montmorillonite complexes. At 100 mmol PY per 100 g clay, the /3 maximum is located at 487 nm. With higher formal degrees of saturation this band shifts to longer wavelengths owing to the presence of unadsorbed PY molecules in the suspension. Similar gradual hypsochromic shifts of the /l band with increasing adsorption of the dye, followed by bathochromic shifts, were observed during the adsorption of methylene blue and acridine orange by montmorillonite.l* If we assume that metachromasy in montmorillonite is the consequence of n interactions between the oxygen plane and the aromatic dye, then the hypsochromic shift is an indication of stronger n interactions.Such a strengthening trend is expected in the system owing to the increase in hydrophobic character of the interlayer space. This occurs because of the exchange of the inorganic hydrophilic cation with the organic cationic dye.1° The bathochromic shift of the /l band, detected at the higher saturations, was previously attributed to a transition of the organo-clay complex from one type into a second type. This will be discussed again in connection with the X-ray measurements.1690 Metachromasy in Clay Minerals Since the dye concentration was constant in all clay suspensions of the present series, it was expected that the absorbance of band B would increase as a had decreased and vice versa.Fig. 3 shows the absorbance of both bands as a function of the formal degree of clay saturation and shows that the adsorption of the dye by the clay resulted in a considerable decrease in the intensities of both bands. It was previously shown3 that absorbance depends on the particle size of the solid fraction, decreasing with clay flocculation. The shapes of the absorbance intensity curves (fig. 3) can be interpreted similarly, as reflecting flocculation and peptization. Flocculation of the clay starts with a small degree of saturation; it becomes most effective at the stage in which the amount of adsorbed dye is slightly below the saturation point of montmorillonite.At higher formal degrees of saturation the clay was peptized. Two important conclusions may be deduced from the present series of spectroscopic measurements: (1) metachromasy in montmorillonite takes place even at very dilute solutions of the dye, which in the absence of the clay do not show this effect; (2) metachromasy in montmorillonite is already observed at a very low formal degree of saturation, namely its occurrence does not depend on the surface concentration of the dye. Eflect of Laponite on the Absorption Spectrum of PY Some representative spectra of laponite-PY suspensions with different clay concentra- tions are shown in fig. 4.Comparison of fig. 2 and 4 shows that the effect of laponite on the metachromasy of the dye is much smaller than that of montmorillonite. Band a is almost always the principal absorption band, whereas two metachromic bands (B and y at 510-515 and 480 nm, respectively) became significant at a formal degree of saturation of 33-50 mmol PY per 100 g laponite [fig. 4(b)-(d)]. In the presence of small amounts of clay (or a high formal degree of saturation), soluble PY is the principal contributor to the absorption spectrum and the spectra show a high intensity of band a [fig. 4(e) and (f)]. Owing to the fact that in most spectra bands p and y appear as weak shoulders, it was difficult to follow the effect of the degree of saturation on their locations and on their absorbance values, as was done for montmorillonite. Fig.5 shows the effect of the clay concentration in the suspension (expressed by the formal degree of saturation) on the location and absorbance of band a. As with montmorillonite, the adsorption of PY by laponite results in a bathochromic shift of band a to 550 nm. As soon as the clay is saturated and free PY is present in the suspension, the maximum of the a band is shifted towards 545 nm. Fig. 5 shows that the PY adsorption capacity of laponite (‘saturation point’) was 41 f 5 mmol dye per 100 g clay. X-Ray measurements (see below) show that adsorption of PY by laponite continues to some extent at a formal degree of saturation above the saturation point. At this stage only part of the dye is adsorbed by the clay.The absorbance intensity of the band a depends on the formal degree of saturation. Up to 15 mmol PY per 100 g clay the intensity changes only very slightly. Increasing the amount of dye per clay results in flocculation of the particles, accompanied by a decrease in intensity of absorption. A minimum is obtained at 41 mmol PY per 100 g clay. At higher degrees of saturation the clay is peptized and the intensity increases. From this series of laponite spectroscopic measurements it is concluded that meta- chromasy on this clay depends on the formal degree of saturation. No metachromasy occurs when the amount of adsorbed dye is either small or high (i.e. peptization below or above the ‘saturation point’). Metachromasy is significant when the amount of adsorbed dye is equal to the saturation point of laponite and the clay is flocculated.The significant differences between metachromasies in laponite and in montmorillonite lead us to conclude that the weak metachromasy in laponite does not result from n interactions between the aromatic rings and oxygen plane, but is due to n interactions1691 w avelength/nm Fig. 4. Absorption spectrum of PY (1.57 x (a) 25 x 100 gclay); (c) 2.0 x lop3 wt % (41 mmol PY per 100 gclay); ( d ) 1.65 x 100 g clay); (e) 1.4 x mol dm-3) in the presence of laponite: wt % laponite (3.3 mmol PY per 100 g clay); (b) 2.5 x lop3 wt % (33 mmol PY per wt % (49 mmol PY per wt% (66 mmol wt% (58 mmol PY per 100 g clay); (f) 1.25 x PY per 100 g clay).1692 0.2 0.1 Metachrornasy in Clay Minerals - - I I I I I 520 "'9 I I I I I I 0 50 I00 I50 200 2 50 Fig.5. The effect of the formal degree of saturation of laponite by PY on the location of band a (a) and on the absorption intensity (b). All spectra were recorded in a fixed PY concentration of 1.57 x mol dm-3. The clay concentration was between 0.3 x and 27.5 x wt % . between neighbouring dye cations either adsorbed on the clay surface or trapped in the intertactoid space of a floc. Eflect of Ultrasound on the Absorption Spectrum of PY A series of clay-dye suspensions (montmorillonite or laponite) were prepared in which the clay concentration was kept constant (0.036 wt % ) but the dye concentration varied between 1.4 x mol dm-3. For samples with a degree of saturation above 150 mmol PY per 100 g clay, the clay concentration was only 0.018 wt % and the recorded absorbance values were normalized to 0.036 wt % .Spectra of the suspensions were recorded immediately after their preparation and also after an ultrasonic treatment of 6min. The supernatants were examined for the presence of free PY. With small amounts of dye, below the saturation points of the clays, the dye was totally adsorbed by both clays. Further addition of PY resulted in coloration of the supernatants. Fig. 6 and 7 show the absorbance values of bands a and p in the spectra of montmorillonite-PY and laponite-PY suspensions, respectively, as a function of the formal degree of saturation, before and after the ultrasound treatment. Both systems showed significant deviation from Beer's law, before and after the ultrasound treatment.Each absorbance curve can be divided into three distinct regions. In the first region there is a linear increase in absorbance as a function of the dye concentration; namely, at a low formal degree of saturation the system obeyed Beer's law. In the second region and 41 .O x2. Grauer et al. 10 9 - 8 - 8 f - p 6 - 9 5 - 2 1693 - (a) - 3 - 2 - / I I I I 1 5 0 100 IS0 200 250 50 1 0 0 I50 200 2 so degree of saturation/mmol per 100 g clay Fig. 6. Absorption intensity of bands a and p [(a) and (b), respectively] as a function of the formal degree of saturation of montmorillonite. Spectra were recorded before (a, A) and after (0, A) an ultrasound treatment of 6min. The suspensions contained a fixed clay concentration of 0.036 wt % . The dye concentration was between 1.4 x and 41.0 x mol dm-3.absorbance values either remained constant, or decreased with increasing dye concen- tration. In the third region absorbance values increased with increasing dye concentra- tion. As one could observe with the naked eye, flocculation occurred in the samples in the second region, whereas samples in the first and third regions were well peptized. Flocculated systems should be affected by ultrasound treatment, resulting in a better dispersed system. Indeed, fig. 6 and 7 show that the ultrasound treatment had a tremendous effect on the absorbance of both bands in the spectra of samples from the first region. Comparison of fig. 6 and 7 reveals a significant difference in the adsorption mechanism between montmorillonite and laponite.At a low degree of saturation on montmorillonite, band p is more intense than band a, owing to 71 interactions between the dye cation and the clay layer. Band a becomes significant only above the saturation point when part of the dye molecules are not adsorbed. Band a, which in the third region of the absorbance curve represents soluble PY, is not affected at this stage by the ultrasound treatment, whereas band p, which represents mainly adsorbed PY, was affected by the same treatment. With laponite, band a was more intense than band /3 during most stages of this series of experiments owing to the absence of 71 interactions between the dye and the oxygen1694 Metachromasy in Clay Minerals 50 100 I50 degree of saturation/mmol per 100 g clay 200 Fig.7. Absorption intensities of bands a and D [(a) and (b), respectively] as a function of the formal degree of saturation of laponite. Spectra were recorded before (@, A) and after (0, A) an ultrasound treatment of 6 min. The suspensions contained a fixed clay concentration of 0.036 wt % . The dye concentration was between 1.4 x and 41 .O x mol dm-3. plane of the mineral. When the formal degree of saturation is above 140 mmol PY per 100 g clay, dimerization of dye cations in the soluble state occurs to a high extent and band becomes more intense than band a. At lower dye concentrations some dimerization and polymerization of dye cations occurs in the solid-liquid interphase, giving rise to the appearance of band P. With a low degree of saturation this kind of surface dimerization and polymerization is very low, but, as one would expect, it increases with dye concentration.There is a significant difference in ultrasound effect on band a between the two clays. The intensity of this band increases in laponite, mainly because of peptization of the flocs; in montmorillonite this increase is cancelled by transfer of adsorbed species responsible for a absorption into P-absorbing species. This supports the assumption that monomeric PY is the principal species adsorbed by laponite. Determination of Saturation Point from Fluorescence Intensity As already observed for rhodamine 6G, adsorption quenches the fluorescence markedly.3* l1 The same effect is observed with PY: adsorption on montmorillonite quenches the fluorescence completely; adsorption on laponite significantly reduces the fluorescence intensity.Consequently, even traces of unadsorbed PY cause the whole suspension to fluoresce intensively (when illuminated with an ordinary He lamp). This phenomenon was used for determination of saturation point.3 At this point the fluorescence of the suspension is quenched. Furthermore, because of peptization, a conventional centrifuge is sometimes not sufficient to separate between the solid clay and2. Grauer et al. I695 Table 1. Basal spacings (nm) of montmorillonite and laponite treated with various amounts of PY : (A) equilibrated for one week at room temperature under an atmosphere of 40% humidity; (B) heated for one week in U ~ C U O at 215 "C formal degree of montmorillonite laponite saturation/mmol PY per 100 g clay A B A B 0 12 24 36 48 60 72 96 120 144 1.28 s 1.40 s 1.39 s 1.47 m, 2.01 s 1.58 w, 2.01 s 2.01 s 2.01 s 1.96 s 2.01 s 2.01 s 0.98 1.34 s 1.23 1.30 s (1.92 wsh) 1.29 1.32 s (1.96 sh) 1.32 1.36 s, 1.96 w 1.32 1.36 m, 2.10 m 1.28 1.38 m, 2.10 s 1.30 1.43 w, 2.10 s 1.34 (1.47 sh), 2.10 s 1.35 (1.49 sh), 2.10 s 1.34 (1.47 sh), 2.10 s 1.08 1.18 1.23 1.28 1.28 1.31 1.36 b 1.39 b 1.40 b 1.38 b s, strong; m, medium; w, weak; sh, shoulder; wsh, weak shoulder; b, a very broad and asymmetric peak.the aqueous solution in clay-dye systems, and the supernatant contains peptized clay samples. The fluorescence measurement is therefore very useful to identify non-adsorbed dye in the presence of adsorbed dye. There was a very good correlation between this method and the visible spectroscopy method, in which the saturation point had been determined from the location of the band a (fig.3 and 5): the suspension started to fluoresce when the degree of saturation was 100 and 41 mmol PY per 100 g montmo- rillonite and laponite, respectively. It should be noted that for laponite the saturation point was determined from the fluorescence measurements of the supernatants (which contained small amounts of peptized clay), whereas for montmorillonite the suspensions as well as the supernatants could be used for this purpose. X-Ray Study Oriented specimens of the clay samples treated with various amounts of PY were examined by X-ray diffraction under ambient conditions after being equilibrated at 40% humidity and after drying at 215 "C in a vacuum oven for 7 days.The results are summarized in table 1. All samples gave non-integral higher orders of reflections, indicating random interstratification of layers with different c spacings. By comparing the c spacings obtained before and after the thermal treatment it is clear that the interlayer space of samples equilibrated at 40% humidity contained water. The thermal treatment of natural montmorillonite and laponite resulted in a dehydration process with c spacings of 0.98 and 1.08 nm, respectively. Larger c spacings were recorded after the thermal dehydration of PY-treated montmorillonite or laponite (table I), indicating that PY was located in the interlayer space. Table 1 shows that for both minerals two types of PY-H,O-clay associations can be identified.The first (with c spacings of 14.0-15.8 and 13.0-14.3 nm in montmorillonite and laponite, respectively) predominated with small amounts of PY, whereas the second type (with c spacings of 20.1 and 19.6-21.0 nm in montmorillonite and laponite, respectively) predominated with larger amounts of PY. These observations are in agreement with the visible spectra observations. With montmorillonite, two types of associations could be deduced from the location of the band /I [fig. 3 (a)]. The first type1696 Metachrornasy in Clay Minerals with the low c spacing was gradually obtained mainly when the degree of saturation was below 45 mmol PY per 100 g clay. The second type of association, with the higher c spacing, was mainly formed with higher degrees of saturation.With laponite, the first type of association corresponded to samples with amounts of PY below the adsorption capacity of the mineral [fig. 5 (a)], whereas the second type corresponded to samples with amounts of PY above the adsorption capacity. As long as the c spacing is not above 1.4 nm, there is no doubt that the adsorbed PY forms a monolayer in the interlayer space with the aromatic rings parallel, or almost parallel, to the silicate layer. With such a c spacing there is no possibility for any kind of aggregation of the dye cation to take place inside the interlayer space. Spacings of 1.47-1.58 nm may account for the presence of a bilayer of water and/or of PY. The tilting of the cationic dye relative to the silicate layer is also possible at this stage.Spacings of 2.01-2.10 nm (second-type associations) may populate four water layers and/or aggregates of the cationic dye, but X-ray measurements cannot serve as conclusive evidence for this. All thermally dehydrated montmorillonite and laponite samples gave c spacings of ca. 1.3 nm, indicating that a monolayer of PY was formed in the interlayer space with the aromatic rings parallel, or almost parallel, to the aluminosilicate layer. We showed previously that polylayers of organic molecules in the interlayer space of montmorillonite persisted during thermal treatment similar to that given in the present study.5 It is therefore supposed that the monolayer of PY in the interlayer space of both clay minerals already existed before the thermal treatment.The first-order reflections in the diffractograms of thermal dehydrated laponites were always much broader than those of dehydrated montmorillonites. This broadening was significant when the formal degree of saturation was equal to or above the adsorption capacity of laponite. Such a broadening is characteristic for an inhomogeneous material. It is possible that the c spacings of a small number of interlayers were high enough to populate dimers and higher aggregates of PY. These metachromic aggregates could be responsible for the appearance of the weak p and y bands in the visible spectra of laponite-PY suspensions. In the case of montmorillonite, the presence of a monolayer of PY in the interlayer space was unequivocally proven from X-ray measurements for samples with a formal degree of saturation less than 30 mmol PY per 100 g clay.It is therefore concluded that metachromasy which was observed by visible spectroscopy of montmorillonite PY suspensions with low degrees of saturation resulted from the 7c interaction of the organic cation with the clay surface. The presence of a monolayer of PY in the interlayer space of laponite with low degrees of saturation was also unequivocally proven from the X-ray data. Nevertheless, there was almost no metachromasy in suspensions of laponite-PY with a low degree of saturation. It is therefore concluded that n interactions do not occur between PY and the oxygen plane of laponite. Infrared Study Fig. 8 shows the infrared spectrum of crystalline PY. The spectra of three samples of each of the clays saturated with increasing amounts of PY are also shown. All spectra were recorded as KBr discs.The locations of the different bands between 1100 and 1700 cm-l are summarized in table 2, together with their assignments. It is obvious that PY in the crystalline state is aggregated, forming n interactions between neighbouring cations. On the other hand, according to the visible and X-ray spectroscopic observations which were described in the previous sections, PY in laponite is in the monomeric state with no 71 interactions, for at least as long as the degree of saturation is below 30 mmol PY per 100 g clay. By comparison between the spectrum of the KBr disc of PY and that of laponite with small amounts of PY, the differences2.Grauer et al. 1697 \ v c H wavelength/ nm Fig. 8. Infrared spectra of PY: (a) in KBr disc; (b)-(d) adsorbed on montmorillonite; (e)-(g) adsorbed on laponite: 20, 50 and 120 mmol PY per 100 g clay, respectively.1698 Metachromasy in Clay Minerals Table 2. Absorption maxima (cm-l) and assignments of bands recorded in the infrared spectra of PY in KBr discs and adsorbed by montmorillonite and laponite [formal degrees of saturation for a and b are 20 and 100 mmol PY per 100 g clay, respectively] montmorillonite laponite band assignmentb KBr disc a b a b A B C D E F G H I 1650 - ring (i) - ring (0) - ring (i) 1525 ring (i) 1497 CZH5 1430 1404 Ar-N 1356 R-N 1166 1 588-1 600 - 1660 1606 1593 1532 1502 1435 1410 1357 1168 1657 1596-1 606 1527 1502 1435 1406 1175 - - 1357-1368 1660 1610 1595 1530 1502 I435 1410 1358 1170 1658 1607 1595 1530 1502 1435 1410 1360 1170 a The assignment which is suggested here follows the treatment of Rao concerning group frequencies.13 i, in plane; 0, out of plane (see paragraph on i.r. spectra in polarized light).between the spectrum of aggregated PY and that of the monomeric species can be envisaged. The locations of all the bands slightly changed with the aggregation of the organic dye, always shifting to lower wavenumbers (table 2). Most of the bands were sharper in the monomeric variety compared to the aggregated one [compare curves (a) and (e) in fig. 81. Bands B and C were very sensitive to aggregation. Both bands were nicely seen in the spectrum of the monomer. They became broad in the aggregated variety.Owing to overlapping, they appeared in the recorded spectrum of the dye in KBr as a single broad band. Band H also became broad as a result of aggregation. The effect of the degree of saturation of PY on the infrared spectrum was very small in the case of laponite, but was very significant in the case of montmorillonite. Most of the bands remained sharp in the spectra of laponite even when the formal degrees of saturation were high. Only bands B, C and H became broader, indicating that some aggregation of the organic cation was taking place on the surface of laponite. This aggregation seems to be responsible for the small amount of metachromasy which was observed during the study of the electronic spectrum of the aqueous suspensions of laponite (fig.4). Metachromasy was observed in the electronic spectrum of montmorillonite-PY with very small degrees of saturation (fig. 2). Nevertheless, the infrared spectrum differed from that of PY in KBr discs (table 2). This is an indication that in the present system metachromasy does not result from aggregation of the cationic dye, but stems from a different process. Significant spectroscopic changes resulted from increasing the forrnal degree of saturation. Bands A, D and I shifted gradually from 1660, 1532 and 1168 cm-l to 1655, 1527 and 1 175 cm-l, respectively. Band B became relatively more intense than the rest of the bands. At 60 mmol per 100 g clay this band and band C became broad and overlapped, Band H was sharp at small dye concentrations but became broad at 60 mmol per 100 g clay, extending between 1357 and 1368 cm-l.These observations support the assumption that during the adsorption of PY different types of bonding occur in montmorillonite and laponite which lead to metachromasy. Infrared Spectra in Polarized Light Oriented specimens of layer silicates are commonly used to distinguish between possible adsorption orientations relative to the basal plane. It is expected that if the adsorbed2. Grauer et al. 1699 A 1 I I I 1600 1400 1200 wavelength/nin Fig. 9. Infrared spectra of PY: (a) and (b) adsorbed on montmorillonite; (c) and ( d ) adsorbed on laponite (20 mmol PY per 100 g clay), oriented deposits on polyethylene, (a) and (c) normal incidence; (b) and ( d ) 45" incidence (polyethylene reference band is between E and F).organic cationic dye is parallel to the clay surface, the infrared spectra in polarized light can distinguish between in-plane and out-of-plane ring vibrations. If such a distinction is not observed, one may conclude that the adsorbed cationic dye is not parallel to the clay surface. Oriented specimens of montmorillonite and laponite treated with various amounts of PY were obtained by sedimentation of the organo-clay complexes from dilute aqueous suspensions on polyethylene. Spectra were recorded at normal and 45" incidence without the separation of the film from the supporter (fig. 9). When PY-saturated montmorillonite1700 Metachromasy in Clay Minerals films were tilted by 45" with respect to the i.r. beam, changes occurred in the relative intensities of some of the skeletal vibrations. The intensity of band C increased considerably relative to that of the band at 1475 cm-l (polyethylene reference band in fig.9), whereas bands B, D and E became weak. Similar observations were obtained with various formal degrees of saturation. It may be concluded that most PY cations are oriented with their aromatic rings parallel to the clay surface.' Laponite treated with PY did not show this phenomenon. The infrared spectra of the oriented films showed the same intensities whether the film was normal or at an angle of 45" to the direction of the incident beam. It may be concluded that the cationic dye is tilted at a definite angle or randomly relative to the clay surface. These observations support our assumption that n interactions occur between the oxygen planes of the silicate in montmorillonite but not in laponite.Conclusions All spectroscopic studies show that the adsorption of PY by montmorillonite differ from adsorption by laponite. (1) The features of the visible spectra of montmorillonite-PY suspensions differed from those of laponite-PY suspensions. Metachromasy in montmorillonite takes place from very dilute dye solutions and does not depend on the surface concentration of the dye. In laponite it takes place only when the degree of saturation equals the saturation point (or slightly above and below this point) and the clay is highly flocculated. (2) Ultrasound treatment of the flocculated laponite-PY system has a strong effect on the intensity of band a.On the other hand, the same treatment has a strong effect on the intensity of band p in the spectrum of montmorillonite-PY. This observation indicates that a metachromic PY species governs the montmorillonite-PY system, whereas a monomeric, non-metachromic species occurs in the laponite-PY system. (3) The intensity of the fluorescence of PY decreases with the adsorption of PY by the clay mineral. The quenching in montmorillonite is much stronger than that in laponite. According to Schoonhedt et al.12 the quenching of the fluorescence upon adsorption by clays is due to iron impurities in the clays. Indeed for laponite, which contains less iron, the quenching was less severe, It seems to us that the metachromasy in montmorillonite should also contribute to the quenching of the fluorescence of PY in this mineral, (4) X-Ray measurements showed that a monolayer of PY was formed in the interlayer space of montmorillonite and of laponite.From these data, together with the information gained from visible spectroscopy, it is clear that interactions occur between the organic cation and the oxygen plane in montmorillonite, but not in laponite. (5) Infrared spectroscopy showed that the organic cation was parallel to the silicate layer in montmorillonite, but not in laponite. Such an orientation should facilitate x interactions between the organic dye and the oxygen plane of montmorillonite. At present there is no convincing explanation for the differences in behaviour between the two smectite minerals. The different behaviours should be associated with structural differences.Montmorillonite is dioctahedral, whereas laponite is trioctahedral. The negative charge of laponite results from octahedral substitution, whereas in Wyoming bentonite, the montmorillonite used in the present study, a very small amount of tetrahedral substitution occurs as well. Such a substitution leads to increasing basic strength of the oxygen plane in montmorillonite compared to laponite.1° It is possible that the tetrahedral substitution is responsible for the n interactions occurring in montmorillonite. Further investigation is needed with other expanding clay minerals. This work was completed while S. Y. was spending a sabbatical year at the Department of Chemistry, The University of Alberta, Edmonton, Canada. His stay in Canada wasZ . Grauer et al. 1701 possible owing to the support of the Natural Sciences and Research Council of Canada (N.S.E.R.C.), The Alberta Oil Sands Technology and Research Authority (AOSTRA) and the Hebrew University of Jerusalem. These financial supports are gratefully acknowledged. D.A. is a member of the F. Haber Research Center for Molecular Dynamics, Jerusalem. References 1 S. Yariv and D. Lurie, Isr. J. Chem., 1971, 9, 537; S. Yariv and D. Lurie, Zsr. J. Chem., 1971, 9, 553. 2 R. Cohen and S. Yariv, J. Chem. SOC., Faraday Trans. I , 1984,80, 1705. 3 2. Grauer, D. Avnir and S. Yariv, Can. J. Chem., 1974, 62, 1889. 4 K. Bergmann and C. T. OKonski, J. Phys. Chem., 1963, 67, 2169. 5 Z. Grauer, S. Yariv, L. Heller-Kallai and D. Avnir, J. Thermal Anal., 1983, 26, 49; Z. Grauer, 6 F. L. Arbeloa, I. L. Gonzales, P. R. Ojeda and I. L. Arbeloa, J. Chem. SOC., Faraday Trans. 2, 1982, 7 T. Furukawa and G. W. Brindley, Clays Clay Miner., 1973, 21, 279. 8 E. F. Vansant and S. Yariv, J. Chem. 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