INTERCALATION CHARACTERISTICS OF 1,1 '-DIETHYL-2,2

(CN-1), and 1,1'-diethyl-2,2'-dicarbocyanine iodide (CN-2) were prepared by Hayashibara Biochemical 392...
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Clays and Clay Minerals, Vol. 48~ No. 3, 392-399. 2(X)0.

INTERCALATION CHARACTERISTICS OF 1,1 '-DIETHYL-2,2'-CYANINE A N D OTHER CATIONIC DYES IN SYNTHETIC SAPONITE: ORIENTATION IN THE INTERLAYER MASASH1 IWASAKI,1 MASAKI KITA, 1 KENGO ITO,2 ATSUYA KOHNO,l AND KOUSHI FUKUNISHI1 ~Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan 2Sony Corporation, Atsugi Tec. No. 2, Atsugi-shi, Kanagawa 243-0021, Japan Abstract--The basal spacings of complexes of saponite with five cationic dyes, 1,1 '-diethyl-2,2'-cyanine, crystal violet, methylene blue, l, 1'-diethyl-2,2'-carbocyanine, and 1,1 '-diethyl-2,2'-dicarbocyanine, varied with degree of saturation of each dye. At low loading of dye to saponite, each cationic dye showed nearly the same absorption spectrum in the UV-visible region as that of its dilute aqueous solution, whereas the spectrum changed distinctly at high loading. With increasing degree of dye loading, the absorption band shifted to longer wavelength for 1,1'-diethyl-2,2'-cyanine (J band) and to shorter wavelength for the others (D, H bands). On the basis of the basal spacing of each respective dye-clay complex, the orientation of the intercalated dye molecules is proposed as follows: the major plane of the cationic dye lies horizontal to the 2:1 layer surface at low loading. With increasing loading, the dye molecules interact with adjacent dye molecules and orient vertically to the 2:1 layer at high loading near the cation-exchange capacity. Key Words--Clay-Dye Complex, Crystal Violet, Cyanine Dyes, Methylene Blue, Orientation of Dye Molecules, Saponite, Visible Spectroscopy. INTRODUCTION Many classes of dyes in aqueous solution often show changes in their absorption spectra when the concentration of the dye is varied. Such spectral changes are attributed to the interaction of the dye molecules that form dimers and/or higher aggregates (Duxbury, 1995). The aggregates are caused mainly by the interaction between the ~r-electrons of the neighboring dyes. This spectral change is called "metachromasy". Many cationic dyes, such as methylene blue, acrydine orange, and crystal violet, show a shift of absorption bands to shorter wavelength (Hypsochromic shift) upon the interaction of dye molecules to form H-aggregates, whereas some cyanine dyes, such as 1,1'-diethyl-2,2'-cyanine, show a shift to longer wavelength (Bathochromic shift) upon the specific interaction of dye molecules to form J-aggregates (Jelley, 1936) in aqueous solution. J-aggregation of cyanine dyes is intensively studied for photographic technology, because the interaction of dyes plays an important role in the spectral sensitization of silver halide (Carroll e t al., 1980). Addition of a small amount of adsorbent to a dilute dye solution causes similar effects to those observed by increasing the concentration of dye. Presumably, the adsorbed dyes form dimers and higher aggregates at adsorption sites because the dyes are highly concentrated on the surfaces of adsorbents relative to the bulk solution. When cationic dyes are adsorbed to clays, the monomer band becomes weaker and new bands appear at shorter and/or longer wavelength in ultraviolet (UV)-visible spectra. For example, substiCopyright 9 2000, The Clay MineralsSociety

tuted porphyrins, which are trapped in interlamellar galleries of Laponite and flattened by constraint within those galleries, show a red shift in their absorption bands (Chernia and Gill, 1999). This spectral change, however, is different from the metachromasy caused by dye aggregation. Ogawa and Kuroda (1995) reviewed the intercalation of cationic dyes in clays to discuss the effect on physical and chemical properties of the dye-clay complexes. However, the orientation of dyes in the interlayer remains unclear. Only a few studies on the intercalation of cyanine dyes in clay, for example, by Ogawa e t al. (1996), have been reported. We investigated the intercalation of cationic dyes in synthetic saponite to develop color-image fixation in thermal-transfer printing, where the dense color of dyes markedly changes the hue (Ito e t al., 1994, 1996). Thus, the cause of the hue change on dye-clay fixation is being investigated for the application of full-color printing. In this study, the orientation of a series of intercalated l,l'-diethyl-2,2'-cyanine dyes in saponite is investigated. Related cationic dyes, planar methylene blue (Hang and Brindley, 1970; Saehr e t al., 1978; Cenens and Schoonheydt, 1988; Schoonheydt and Heughebaert, 1992; Breen and Rock, 1994), and propeller-shaped crystal violet (Yariv e t al,, 1990) are examined also. Models are proposed from UV-visible spectral analysis and powder X-ray diffraction data. EXPERIMENTAL Three cyanine dyes, 1,1'-diethyl-2,2'-cyanine chloride (CN-0), 1,1'-diethyl-2,2'-carbocyanine iodide (CN-1), and 1,1'-diethyl-2,2'-dicarbocyanine iodide (CN-2) were prepared by Hayashibara Biochemical

392

Vol. 48, No. 3, 2000

Intercalation characteristics of cationic dyes

N

~

I

CI

393 I

I

0.8 (HBC)2N~ S"~ " N(CHj)2 Methylene Blue (MB)

M

N(CH3)2

o x to 0.4

Crystal Violet (CV) 400

500

600

Wavelength (nm) I

I

C2H5

X-

C2H5

n=0, X=Cl; 1,1'-diethyl-2,2'-cyanine (CN-0) n = l , X=I; 1,1'-diethyl-2,2'-carbocyanine (CN-1) n=2, X=I; 1,1'-diethyl-2,2'-dicarbocyanine (CN-2) Figure 1.

Structure of the 0studied dyes.

Laboratories, Inc. (Okayama, Japan). Two other dyes, methylene blue (MB) and crystal violet (CV), were purchased in "'special-grade" quality from Nakalai Tesque Inc. (Kyoto, Japan). Figure 1 shows the structures of the five dyes. The clay used here is a commercially available synthetic saponite ( " S U M E C T O N S A " ) with a formula of [(SiT.20Alo.80)(Mgs.97Alo.03) O20(OH)a]-~ "-0-77 from Kunimine Industry Co. Ltd. (Tochigi, Japan). The cation-exchange capacity (CEC) is calculated at 99.7 meq/100 g-clay and MB adsorption is 132 mmol/100 g-clay on the basis of data from Kunimine. Absorption spectra of aqueous solutions and dyeclay suspensions were measured in the visible region using a spectrophotometer, Hitachi U-3410. Aqueous solutions of CN-0, MB, and CV were used to prepare dye-clay suspensions. A solution of ethanol and water (50/50 v/v) was used for CN-1 and CN-2 because of their poor solubilities in pure water. The dye-clay suspensions were maintained at 25~ for 2 d with frequent agitation. The aqueous suspension of fine particles (< 1 p~m) of saponite was transparent in the visible region. After spectroscopic study of the suspensions, the supernatants were isolated by centrifugation at 10,000 rpm for 1 h, and then filtered through a 0.4~m membrane film. No absorption in the spectra of the filtrates indicated that the total amount of added dye had adsorbed to the clay.

Figure 2. Absorption spectra of CN-0 in aqueous solutions. Curve (a) shows the spectrum for a concentration of 1.0 X 10-5 tool dm -3 and (b) for 2.0 >( 10-4 mol dm 3. ~ is the molar absorption coefficient (mo1-1 drn3 cm-t).

Samples for powder X-ray diffraction (XRD) were separated from the suspensions by centrifugation and, after washing with pure water, they were dried at 60~ for 2 d under vacuum. The X RD data of the samples were collected at 20 = 1.5~ ~ at a scan speed of 1.0 cm/min using a Rigaku RINT-2000 powder diffractometer, with Ni-filtered CuK radiation. The amount of dyes in each sample was determined by the elemental analysis for carbon, hydrogen, and/or nitrogen. RESULTS A N D D I S C U S S I O N

Absorption spectra of cationic dyes in aqueous solution Metachromasy of cationic dyes in the aqueous solutions is commonly observed. The monomer band of CN-0 occurs at 520 nm with a secondary peak at 490 nm, which is caused by the vibrational actions of the dye molecule (Figure 2). In the dilute-concentration region, < 1 0 s mol dm 3, the absorption spectra of CN0 did not show significant differences, However, the spectrum of a 2.0 X 10 4 tool dm -3 solution showed a slight absorption band of J-aggregates at 570 nm and the shift of the peak at ~ 4 9 0 nm to shorter wavelength by the appearance of the dimer band at 480 nm. On addition of KBr into the aqueous solution of 10 -4 mol dm 3, where KBr induces an apparent increase of the dye concentration, spectral differences (Figure 3) were observed by the decrease of the M band at 520 nm and the increase of the J band at 570 nm. The characteristic J band of CN-0 occurred in concentrated aqueous solutions and in aqueous solutions with salts.

Iwasaki et aL

394

i

i

Clays and Clay Minerals

I

0.8

0.8

M

/q a

M

%

c-

D 0.4

to 0.4 ~ ! / ~

X

a

t~

b

400

J

|

500

600

W a v e l e n g t h (nm) Figure 3,

b

Absorption spectra of CN-0 in aqueous solutions

for a concentration of 1.0 • 10 4 tool dm -3 and KBr concentrations of (a) 0.04 tool dm 3 and (b) 0.4 mol dm -3, Optical path length: 1 mm. A t e x t r e m e l y h i g h c o n c e n t r a t i o n s , the intensity o f the M b a n d at 520 n m d e c r e a s e d a n d the J b a n d o c c u r r e d at 5 7 2 n m as a s h a r p peak. D a l t r o z z o e t al. (1974) p r o p o s e d four p o s s i b l e structures for J - a g g r e g a t e s a n d d i s c u s s e d the close r e l a t i o n s h i p b e t w e e n crystallization a n d J - a g g r e g a t i o n . T a n a k a e t al. (1980) studied the e l e c t r o n i c spectra o f s i n g l e c r y s t a l s o f C N - 0 to d e t e r m i n e the origin o f the J band. Y o s h i o k a a n d N a k atsu (1971) s h o w e d b y X - r a y structural studies that t w o q u i n o l i n e r i n g s are twisted relative to e a c h other, a n d e a c h ring stacks w i t h o n e q u i n o l i n e ring o f an a d j a c e n t m o l e c u l e . T h e p r o b a b l e g e o m e t r y o f the Ja g g r e g a t e s is e x p e c t e d to b e similar to that o f the crystal structure. In c o n t r a s t to the structure study, h o w ever, J - a g g r e g a t i o n occurs in solution only w h e r e the slipping a n g l e o f a d j a c e n t m o n o m e r units is large a n d the d y e d i s t r i b u t i o n r e m a i n s h o m o g e n e o u s in the solution ( S t u r m e r a n d Heseltine, 1977). M B in dilute solutions, < 1 • 10 -4 tool d m 3, s h o w e d a m a x i m u m at 665 n m w i t h a s h o u l d e r at 615 n m , l a b e l e d as M a n d D b a n d s , r e s p e c t i v e l y (Figure 4). M o r e c o n c e n t r a t e d solutions, 2.5 • 10 -4 tool d m -3, s h o w e d a c h a n g e in the relative intensity o f the t w o b a n d s w i t h the D b a n d g r e a t e r t h a n the M b a n d . A further i n c r e a s e in c o n c e n t r a t i o n r e s u l t e d i n a b r o a d e r b a n d n e a r 6 0 0 rim. T h e m e t a c h r o m a s y o f a q u e o u s M B solutions m a y b e related to the f o r m a t i o n o f d i m e r s a n d H - a g g r e g a t e s . T h e m o l e c u l e s f o r m i n g a d i m e r unit are p r o b a b l y b o n d e d b y ~r-~r i n t e r a c t i o n s a n d h y d r o p h o b i c i n t e r a c t i o n s (Tanford, 1980) w h i c h o v e r c o m e the electrostatic r e p u l s i o n b e t w e e n the p o s i t i v e c h a r g e s o f e a c h M B m o l e c u l e ( B e r g m a n n and O ' K o n s k i ,

0

500

600 700 Wavelength(nm) !

'

-

-

Figure 4. Absorption spectra of MB in aqueous solutions. Curve (a) shows the spectrum for a concentration of 1.0 • 10 -~ mol dm -3,(b) for 1.0 • 10 -4 m o l d m 3 and(c) for 2.5 • 10 4 mol dm 3 9 is the molar absorption coefficient (mol -~ dm 3 cm-~).

1963). T h e i n t e r a c t i o n b e t w e e n m o l e c u l e s s h o u l d b e greatest w h e n the m o n o m e r units are in a " s a n d w i c h l i k e " structure w i t h the p r i n c i p a l m o l e c u l a r axes b e i n g parallel. T h e c o u l o m b i c r e p u l s i o n is m u c h l o w e r e d w h e r e the c h a r g e d a m i n o g r o u p s lie a l o n g the opposite e d g e s o f t h e s a n d w i c h . T h e stacked s a n d w i c h structure is the m o s t stable for the h i g h e r aggregates. Similarly, the m a i n b a n d in the a b s o r p t i o n spectra of p r o p e l l e r - s h a p e d C V w a s split into t w o parts, the M b a n d at 590 n m a n d D b a n d at 5 5 0 nm. T h e relative intensity o f the M a n d D b a n d s v a r i e d with t h e dye c o n c e n t r a t i o n ( S c h u b e r t a n d L e v i n e , 1955; S t o r k e t al., 1972; Takatsuki, 1980). A t m o d e r a t e l y h i g h c o n c e n tration, the spectral c h a n g e s h o w e d a n e n h a n c e m e n t o f the D b a n d a n d a d i m i n u t i o n o f the M band. W h e n the dye c o n c e n t r a t i o n w a s i n c r e a s e d further, b o t h M a n d D b a n d s were d e p r e s s e d a n d a g g r e g a t e b a n d s (H-aggregates) occurred, w h i c h a p p e a r e d o n the shorter w a v e l e n g t h side of the D b a n d . A " s a n d w i c h - l i k e " structure for C V d i m e r s is also preferable as well as for M B to m i n i m i z e the e n e r g y o f association. T h e a b s o r p t i o n spectra o f the t w o c y a n i n e dyes in the m i x t u r e o f e t h a n o l a n d w a t e r s h o w e d a p e a k at 605 n m for CN-1 a n d a 7 0 9 - n m p e a k for CN-2. T h e effect o f the d y e c o n c e n t r a t i o n o n the a b s o r p t i o n spectra o f CN-1 a n d C N - 2 did n o t s h o w a p p r e c i a b l e c h a n g es in the c o n c e n t r a t i o n r a n g e o f < 1 0 _5 m o l d m -3. In aqueous solution, however, CN-1 s h o w e d a n o t a b l e D b a n d at 5 6 0 n m in this c o n c e n t r a t i o n r a n g e (Iwasaki e t al., 1992).

Vol. 48, No. 3, 2000

Intercalation characteristics of cationic dyes I

I

CN-O0~

395 I

I

1.6

-~ "~ 2o o

M

? ,/

p/ g% O

/

/

/

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X

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jc 0

,

0

I

20

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40

Dye concentration in aqueous solution [Dye] x 105 (mol dm"3) Figure 5. Adsorption isotherms of MB (0) and CN-0 (O) for saponite clay at 25~

Effect of synthetic saponite on absorption spectra of cationic dyes The adsorption of cationic dyes in synthetic saponite occurs by a cation-exchange reaction. Exchangeable metal cations (Na +) are simultaneously released into aqueous solution when dyes are intercalated into the clay. The adsorption isotherms for MB and CN-0 are shown in Figure 5, where the dye content adsorbed in the clay is plotted against equilibrium concentration of the dye in solution. Whereas the adsorption of MB was saturated at 113 retool/100 g-clay, corresponding to the CEC value of saponite, the amount of CN-0 adsorbed increased beyond the CEC value following Freundlich-type adsorption. This result suggests the contribution of physical adsorption in addition to ionic adsorption for intercalation of CN-0 in the clay. The effect of an increment of the clay on the absorption spectra of the dye was studied. The dye solution with a small amount of the clay showed a change in color. In such suspensions, the interlayer of the clay is saturated with some dye molecules and other molecules remain in the solution. After filtration through the membrane film, the filtrate showed the characteristic spectrum of the free dye. When the amount of clay in suspension was increased, free-dye molecules were adsorbed in the clay and floating floccules were observed. The filtrates of these suspensions were transparent and showed no absorption in the visible range. On further increase of the clay amount, the dye molecules uniformly adsorbed in the clay and the dye-clay complexes were distributed homogeneously in suspensions. The absorption spectra of such dyeclay suspensions resembled that of the dilute dye so-

0

I

500

,

I

,

600

700

Wavelength (nm) Figure 6. Absorption spectra of MB in clay suspensions for a concentration of 2.0 • 10 5 mol dm -3. Clay concentrations: (a) 40 mg dm 3 (b) 70 mg dm -3, (c) 120 mg dm 3. Optical path length: 1 cm.

lution. Figure 6 shows the visible spectra of MB in aqueous suspensions with saponite. The supernatants of these suspensions were transparent. The broad peak at --570 nm (Figure 6, curve a) appeared at high loading of MB and corresponded to H-aggregates, whereas the spectrum at low loading resembled that of the MB monomer in dilute aqueous solution (Figure 6, curve c). This result suggests that most MB molecules can exist as monomers on the clay in suspensions of low loading. Similar effects were also reported for planar acridine orange (Cohen and Yariv, 1984) and pyronin Y (Grauer et al., 1987), both of which resemble MB in molecular shape. The adsorption of CV in saponite produced a slight metachromasy similar to that observed for MB. CV in suspensions of low loading showed an absorption band with an intensity maximum at 590 nm, whereas at higher loadings of CV, the absorption band shifted to shorter wavelength with the decrease of the M band, indicating the formation of dimers and higher aggregates of CV. These results correspond to the spectral changes of adsorption of CV to Laponite and montmorillonite studied by Yariv et al. (1990). Similarly, the absorption bands of CN-1 and CN-2 in the dyeclay suspensions were wider and shifted to shorter wavelength, i.e., to 470 and 520 nm, respectively, with the decrease of the respective M bands, when compared to those in aqueous solutions. Figure 7 shows the absorption spectra of suspensions of CN-0 and saponite with different clay concentrations. Note that the spectra differ from those of MB-saponite suspensions. Small amounts of saponite

Iwasaki et al.

396 ~

t

Table 2. Calculated size of dye molecules.

0.8 d

M

Clays and Clay Minerals

a

Dye

Width (W)

Height (H)

Length (L)

MB CV CN-0 CN- 1 CN-2

4.7 6.1 5.3 5.1 5.1

7.9 15.0 9.7 9.6 9.5

[ 6.4 16.5 16.9 19.3 22.3

(H - W)

3.2 8.9 (10.4) x 4.4 4.5 4.4

' The value in parentheses is (L - W), see text.

D 0.4

.s

<

c b

b

to f o r m J - a g g r e g a t e s in the interlayer o f saponite. T h e absorption b a n d s o f d y e - c l a y s u s p e n s i o n s are s u m m a r i z e d in Table 1.

Orientation o f cationic dyes intercalated in saponite 0 400

500

600

Wavelength (nm) Figure 7. Absorption spectra of CN-0 in clay suspensions for a concentration of 1.0 X 10 -5 tool dm 3. Clay concentrations: (a) 5.0 mg dm -3, (b) 7.5 mg dm 3, (c) 11.5 mg dm 3. Optical path length: 1 c m

in the C N - 0 solution p r o d u c e a significant b a n d (J) at 573 n m (Figure 7, c u r v e a). Further increases in the a m o u n t o f the clay gradually increases the intensity o f the M band, b e c a u s e the interaction o f the dye m o l e cules in the interlayer d e c r e a s e s (Figure 7, c u r v e c) o w i n g to f e w e r m o l e c u l e s p e r unit area in the interlayer. T h e s e results s u g g e s t that C N - 0 has a t e n d e n c y

The m o s t stable m o l e c u l a r structure o f e a c h cationic d y e was d e t e r m i n e d b y m e a n s o f M O P A C ( M o l e c u l a r Orbital P a c k a g e ) c o m p u t a t i o n and A M 1 (Austin M o d el 1) t h e o r y (Dewar, 1985) to m i n i m i z e the e n e r g y o f the d y e molecule. The c o m p u t a t i o n was p e r f o r m e d with a m o l e c u l a r m o d e l i n g application, CS c h e m 3 D , f r o m C a m b r i d g e S o f t Corp. ( C a m b r i d g e , M a s s a c h u s setts, U S A ) , and applied only for a free m o l e c u l e and not for d i m e r s and aggregates. The three planar dyes, M B , C N - 1 , and CN-2, are fiat c h r o m o p h o r e s , and the n o n - p l a n a r dyes, C V and CN-0, have t w i s t e d conformations, e.g., the twist angle b e t w e e n the t w o quinoline planes o f C N - 0 is 47 ~ The calculated sizes o f e a c h d y e m o l e c u l e are g i v e n in Table 2 and the space-filling m o d e l s for M B and C N - 0 are illustrated in Figure 8, T h e p r e f e r a b l e m o l e c u l a r structure o f M B has the size o f 4.7 (width, W), 7.9 (height, H), and 16.4 _A (length,

Table 1. Basal spacing [d(001), A] of dye-saponite complexes with adsorbed amount of dye, and absorption bands of their respective suspensions. Dye

Low Loading

High Loading

MB

d(001) Adsorbed amount (mmol/100 g-clay) Absorption band (nm)

13.2 13.4 19.1 29.2 M at 665 >> D at 615

13.6 65.2

14.7 15.5 98.5 135.1 broad H (D) at 570 > M at 670

CV

d(001) Adsorbed amount (retool/100 g-clay) Absorption band (nm)

14.0 14.7 10.0 19.0 M at 590 >> D at 550

16.7 39.5

21.0 24.5 58.0 75.8 broad H (D) at 540 > M at 590

CN-0

d(001) Adsorbed amount (mmol/100 g-clay) Absorption band (nm)

14.3 14.1 M at 520

16.4 44.6

17.0 64.1

17.3 18.0 90.1 114.8 D a t 4 8 0 , J a t 570 > M at 520

CN-I

d(001) Adsorbed amount (retool/100 g-clay) Absorption band (nm)

13.8 14.8 M at 610

14.2 25.9

16.7 52.4

17.7 18.4 71.3 95.8 broad H (D) at 500 > M at 610

CN-2

d(001) Adsorbed amount (mmol/100 g-clay) Absorption band (nm)

13.8 16.7 M at 710

14.2 20.5

16.7 35.0

17.0 18.4 53.9 69.9 broad H (D) at 560 > M at 710

>> and > show the relative intensities between the absorption bands, they mean "much higher" and "higher", respectively.

Vol. 48, No. 3, 2000

Intercalation characteristics of cationic dyes

0~ r

397

i

:.,., v

.......... : .... :,..,. ....

..___

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Figure 8. The space-filling models for MB and CN-0 determined by MOPAC computation. Width, height, and length are abbreviated as W, H, and L, respectively. L). Non-planar C V and C N - 0 were also estimated and, because of the deviation from planarity, gave a large width. The untreated saponite showed a basal spacing [d(001)] of 12.6 .~ by p o w d e r X R D . In this case, the o b s e r v e d basal spacing is the sum o f the thickness (Al) o f a 2:1 layer and the diameter of the exchangeable hydrated-metal cation. P o w d e r X R D patterns of dyesaponite c o m p l e x e s for C N - 0 at various amounts of adsorbed dye are illustrated in Figure 9. The d(001) value for the dye-clay c o m p l e x varies depending upon the amount of cationic dyes absorbed, as shown in Table 1 and Figure 10. The X R D data of the dye-clay c o m p l e x e s showed that the basal spacings expanded to > 12.6 A. The larger values of d(001) were obtained f r o m samples with a higher degree of saturation of dyes, indicating that dyes were located in the interlayer. The interlayer gradually expanded with increasing dye content and approached a m a x i m u m value at near the CEC. At low loadings ( < 2 0 m m o l dye/100 g-clay), the d(001) value was 13-14 ,~ for each o f the five dyes. At high loadings near the C E C value, d(001) values for M B - c l a y and CV-clay c o m p l e x e s were - 1 6 and - 2 5 ,~, respectively. For the c o m p l e x e s of CN-0, CN-1, and O1"4-2, the d(001) values were --18 ,~. T h e d(001) value varied with an increase in the absorbed amount and with the shape and the size of the dye used. The difference between the basal spacings for clays with high and low loading reflects the apparent variations in the orientation of dye molecules in the interlayer. These differences for each dye-clay c o m plex were about 3, 11, and 4 ,~ for M B , CV, and three cyanine dyes, respectively. The estimated values of 3 ,~ and 4 A for M B and three cyanine dye c o m p l e x e s

I 4

I 8

I 12

I 16

I 20

I 24

I 28

2 8 (degree) Figure 9. XRD patterns of CN-0-saponite complexes. Adsorbed amounts of CN-0 in 100 g clay: (a) 14.1 retool, (b) 44.6 mmol, (c) 64.1 mmol, (d) 90.1 mmol, (e) 114.8 mmol. Basal spacings of respective complexes are listed in Table 1. correspond to the respective differences (H - W ) o f the height and the width o f dye molecules, as listed in Table 2. The value of 1 t ,~ for the CV-clay c o m p l e x , was m u c h closer to that (L - W ) of the length and the width of a C V molecule. C V molecules are likely to interact with each other vertically with a 60 ~ rotation to reduce steric hindrances. To estimate the thickness o f a 2:1 layer (Al), an appropriate size of a dye m o l e c u l e was subtracted

.-.---,

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150

Adsorbed amount of dye (mmol/100g-clay)

Figure 10. Basal spacing of dye-saponite complexes varied with the adsorbed amount of dyes.

398

Iwasaki et al.

Clays and Clay Minerals

Figure 12. Probable intercalated alignment of CN-0 in the dye-clay complex, a) at low loading, b) at high loading. Al represents the 2:1 layer thickness. The 2:1 layer is drawn schematically.

Figure I 1. Orientation of MB intercalated in the dye-clay complex, a) at low loading, b) at medium loading, c) at high loading. A1 represents the 2:1 layer thickness. The 2:1 layer is drawn schematically.

from the measured basal spacing, d(001). The estimated value was 8.6 --. 0.4 A by subtracting each width of five dye molecules from the corresponding d(001) at low loading, and assuming that the dye molecules are lying fiat between adjacent layers. At high loading, near the C E C value, a similar calculation by subtracting each height g a v e 8.6 +_ 0.9 A, where the orientation of dye molecules is assumed to occur vertically to the layers. Regardless of the different shape and size of the dye molecules and the variable loads up to the C E C , a consistent value was obtained for A1. The d(001) values suggest that the orientations change f r o m horizontal at low loadings to vertical at high loadings of dye molecules between the 2:1 layers. Note that the 2:1 layer thickness of 8.6 ,~ differs from the unit-cell spacing of 9.6 A c o m m o n to talc or pyrophyllite because it does not include the interlayer region between adjacent layers. In summary, 1) these dyes are oriented fiat, i.e., lying with their -rr planes parallel to the clay surface at low loading, 2) tilt towards the vertical (i.e., c ~ axis) with an increase in dye saturation, and 3) stand vertically to the clay surface at high loading, as illustrated in Figure 11, In Figures 11 and 12, two 2:1 layers and an interlayer with dye molecules are illustrated for each condition. Although such structures for the dimer and the aggregates could

not be determined by the computations, as noted above, the arrangements of the planar dye molecules with aromatic ring(s) in the interlayer are reasonably modeled. These models use the thickness of a 2:1 layer, A/, and simple assumptions that the dye molecules are plate-like and may be represented by width, height, and length values, and that they have m a x i m u m contact with neighboring molecules o w i n g to ~r-Tr interactions. The g e o m e t r y of aggregates in the interlayer is also described based on the sandwich structure of the M B dimer (Bergmann and O ' K o n s k i , 1963). The favorable 7r-~r interactions and balanced electrostatic interactions of the cationic dye molecules are produced by the alternating vertical orientation as shown in Figure 1 l c instead o f any combination of stacking of horizontal molecules. Similar results for protonated tetram e t h y l b e n z i d i n e on h e c t o r i t e w e r e r e p o r t e d by M c B r i d e (1985). Such a topology permits the adsorption of colored benzidine as m o n o m e r i c cations with an orientation that is horizontal to the surface, followed by intercalation of the cations as paired species oriented vertically to the surface. The specific cationic cyanine dye, CN-0, has a twisted c h r o m o p h o r e with a dihedral angle of 47 ~ owing to the steric hindrance between the hydrogen atoms at the 3,3'-position. The spectrum at the highestloading conditions shows a marked J band (e.g., Figure 7, c u r v e a). The relative amount o f C N - 0 at which the marked J band occurred, was higher than the C E C value, which differs from these cases of the H-aggregates of the other dyes. The basal spacings in Table 1 varied from 14 to 18 A showing the spectral change from the M band to the J band. Daltrozzo et al. (1974) proposed the models for J-aggregates of CN-0. Be-

Vol. 48, No. 3, 2000

Intercalation characteristics of cationic dyes

c a u s e o f the i n h e r e n t a s y m m e t r y of the twisted chrom o p h o r e , C N - 0 f o r m s J - a g g r e g a t e s in at least three different ways: a) aggregates w i t h alternating antipodes (non-helical), b) clockwise, a n d c) c o u n t e r c l o c k w i s e helical c o n f i g u r a t i o n s c o n s i s t i n g o f o n l y o n e type of antipode. P r o p o s e d interlayer a l i g n m e n t s o f C N - 0 in d y e - c l a y c o m p l e x e s at l o w a n d h i g h loading are illustrated in F i g u r e 12. A n o n - h e l i c a l m o d e l is g i v e n for a g g r e g a t e s (Figure 12b) b e c a u s e o f the achiral env i r o n m e n t o f the s a p o n i t e layer. T h i s m o d e l is preferred in that the o r i e n t a t i o n is m o r e stable w h e n the ar planes o f a d j a c e n t m o l e c u l e s are parallel to e a c h o t h e r a n d the p o s i t i v e l y c h a r g e d n i t r o g e n a t o m s are on the opposite edges. ACKNOWLEDGMENT The authors are grateful to the reviewers for helpful comments, and wish to thank S. Guggenheim and L. Ukrainczyk for important suggestions and remarks. REFERENCES Bergmann, K. and O'Konski, C.T. (1963) A spectroscopic study of methylene blue monomer, dimer, and complexes with montmorillonite. Journal of Physical Chemistry, 67, 2169-2177. Breen, C. and Rock, B. (1994) The competitive adsorption of methylene blue on to montmorillonite from binary solution with thioflavin T, proflavine and acrydine yellow, steadystates and dynamic studies. Clay Minerals, 29, 179-189. Carroll, B.H., Higgins, G.C., and James, T.H. (1980) Introduction to Photographic Theory, The Silver Halide Process. John Wiley & Sons, New York, 160-195. Cenens, J. and Schoonheydt, R.A. (1988) Visible spectroscopy of methylene blue on hectorite, Laponite B, and barasyn in aqueous suspension. Clays and Clay Minerals, 36, 214-224. Chernia, Z. and Gill, D. (1999) Flattening of TMPyP adsorbed on Laponite. Evidence in observed and calculated UV-vis spectra. Langmuir, 15, 1625-1633. Cohen, R. and Yariv, S. (1984) Metachromasy in clay minerals, Acridine orange by montmorillonite. Journal of the Chemical Society, Faraday Transaction 1, 80, 1705-1715. Daltrozzo, E., Scheibe, G., Gschwind, K., and Haimerl, E (1974) On the structure of the J-aggregates of pseudoisocyanine. Photographic Science and Engineering, 18, 441450. Dewar, M.J.S., Zoebisch, E.Z., Healy, E.E, and Stewart, J.J.P. (1985) AMI: A new general purpose quantum mechanical molecular model. Journal of the Chemical Society, 107, 3902-3909. Duxbury, D.E (1995) Photochemistry and photophisics of triphenylmethane dyes in solid and liquid media. Chemical Review, 93, 381-433. Grauer, Z., Grauer, G.L., Avnir, A., and Yariv, S. (1987) Metachromasy in clay minerals, sorption of pyronin Y by montmorillonite and Laponite. Journal o f the Chemistry Society, Faraday Transaction t, 83, 1685-1701.

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