Adsorption of Dyes and Their Surface Spectra

14 Adsorption of Dyes and Their Surface Spectra

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A . H . H E R Z , R. P.

DANNER,

and G . A . J A N U S O N I S

Research Laboratories, Eastman Kodak Company, Rochester, N e w York, 14650

The adsorption halide,

silver

by changes

of dyes and particularly or mica

substrates

in the dye

spectra.

Depending

these changes have been quantitatively reflectance

measurements

Munk relation centrations surface

areas and average

independently

Langmuir energies

adsorption of

adsorbed

and

by

Kubelka-

Surface

generally

measurements

coefficients,

system,

either

of the

dimensions

These values

adsorption,

silver

con-

of the dyes as well

particle

determined

on the

spectroscopy.

coverages

at

accompanied

evaluated

application

or by transmission

strates were obtained. with

with

and saturation

of cyanines

is generally

apparent

probable

as

of the

sub-

agreed

well

and

yielded

standard

orientations

free of

the

dyes.

T t is the p u r p o s e of this p a p e r to describe m e t h o d s for d e t e r m i n i n g a n d A

i n t e r p r e t i n g d y e spectra i n aqueous dispersions of s i l v e r h a l i d e s a n d

other substrates. S u c h spectra c a n b e u t i l i z e d for the d i r e c t m e a s u r e m e n t of surface concentrations of dyes f r o m w h i c h , i n t u r n , the surface area of the substrate c a n be d e r i v e d .

T h e techniques i n v o l v e d are not l i m i t e d

to a specific d y e class b u t w i l l b e i l l u s t r a t e d i n this p a p e r b y the b e h a v i o r of c y a n i n e dyes. T h e k n o w n factors w h i c h influence the s o l u t i o n spectra of cyanines h a v e b e e n r e c e n t l y r e v i e w e d (14,

46, 73).

It w i l l be sufficient to s u m

m a r i z e here some of the c o n c e n t r a t i o n - d e p e n d e n t

spectral properties for

the specific case of l , l ' - d i e t h y l - 2 , 2 ' - c y a n i n e ; this c y a n i n e w a s e m p l o y e d i n m a n y of the present experiments. I n a l c o h o l as i n d i l u t e w a t e r solutions this d y e , w h i c h w i l l b e r e f e r r e d to as P s e u d o c y a n i n e , a l t h o u g h i t has also b e e n c a l l e d P s e u d o i s o c y a n i n e , appears to exist o n l y i n a n e x t e n d e d c o n figuration

a n d has its m a x i m u m a b s o r p t i o n near 523 n . m . T h i s t r a n s i t i o n 173

In Adsorption From Aqueous Solution; Weber, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

174

ADSORPTION F R O M

AQUEOUS SOLUTION

is associated w i t h i s o l a t e d molecules a n d is r e f e r r e d to as the m o l e c u l a r or M - b a n d a n d is a c c o m p a n i e d b y a s u b s i d i a r y v i b r a t i o n a l s h o u l d e r near 490 n . m . O n i n c r e a s i n g the d y e c o n c e n t r a t i o n b e y o n d about 5 X

10^ M, 5

the i n t e n s i t y of the m o l e c u l a r b a n d decreases a n d a n e w m a x i m u m a p pears near 480 n . m . ; this b a n d is associated w i t h f o r m a t i o n of a d i m e r a n d is a c c o r d i n g l y c a l l e d the D - b a n d . A f u r t h e r increase i n d y e c o n c e n t r a t i o n causes b r o a d e n i n g of the d i m e r b a n d a n d e n h a n c e d

absorption

i n the shorter or h y p s o c h r o m i c w a v e l e n g t h r e g i o n . W i t h some cyanines,


a l t h o u g h not w i t h P s e u d o c y a n i n e , this shift to h y p s o c h r o m i c

absorption

is a c c o m p a n i e d b y f o r m a t i o n of definable m a x i m a or H - b a n d s . T h e s e are b e l i e v e d to arise f r o m the f o r m a t i o n of p o l y m e r s o t h e r t h a n the d i m e r . T h e s e c o n c e n t r a t i o n - d e p e n d e n t s p e c t r a l changes m a y also b e b r o u g h t a b o u t b y i n e r t electrolytes, surfactants, or some o r g a n i c solvents

and

h a v e b e e n r e l a t e d to d y e - d y e interactions. A n alternate v i e w (42, 43,

44),

w h i c h a s c r i b e d these m e t a c h r o m a t i c changes to d y e - c o u n t e r i o n i n t e r a c t i o n , has b e e n r e f u t e d o n the basis of c o u n t e r i o n - a c t i v i t y d e t e r m i n a t i o n s (51).

C o n v e r s e l y , there seems to b e n o disagreement a b o u t the p o l y m e r i c

association that occurs i n aqueous P s e u d o c y a n i n e at concentrations a b o v e 5 X 10" M ( 2 5 ° C ) . 3

I n s u c h r e l a t i v e l y c o n c e n t r a t e d solutions, a n a b n o r

m a l increase of viscosity is observed, together w i t h the a p p e a r a n c e of a n a r r o w b a n d of h i g h a b s o r p t i v i t y w h i c h is l o c a t e d at w a v e l e n g t h s longer t h a n the m o l e c u l a r b a n d , at a p p r o x i m a t e l y 570 n . m . T h i s n e w a n d

fluo

rescent t r a n s i t i o n , r e f e r r e d to as the / - b a n d , w a s first i d e n t i f i e d b y J e l l e y (26, 27)

a n d b y S c h e i b e (56, 57, 58, 59)

a n d is t h o u g h t to b e c a u s e d b y

aggregates of stacked c y a n i n e molecules h e l d b y v a n der W a a l s forces i n a c l o s e - p a c k e d a n d p o s s i b l y h e l i c a l c o n f i g u r a t i o n (39, 40, 53).

Absorp

t i o n w i t h i n this b a n d is c o n s i d e r e d to arise f r o m a n electronic t r a n s i t i o n p e r p e n d i c u l a r to the c h r o m o p h o r e

of i n d i v i d u a l molecules a n d p a r a l l e l

to the axis of the m u l t i m o l e c u l a r a r r a y (25, 41, 56, 57, 58,

59).

F r o m this s u m m a r y it is a p p a r e n t that w a t e r solutions of

Pseudo

c y a n i n e a b o v e ca. 5 X 1 0 " M c o n t a i n a m u l t i c o m p o n e n t m i x t u r e of differ 5

e n t l y a b s o r b i n g species.

H e n c e , the r e s u l t i n g s o l u t i o n spectra are n e i t h e r

e x p e c t e d n o r f o u n d to e x h i b i t isosbestic points (12). be

H o w e v e r , as w i l l

s h o w n , i n the presence of a p p r o p r i a t e adsorbents

the spectra

of

P s e u d o c y a n i n e c a n b e d r a s t i c a l l y m o d i f i e d b y i m p o s i t i o n of a n e q u i l i b r i u m between

monomeric

d y e i n s o l u t i o n a n d its / - a g g r e g a t e at the

substrate surface. Experimental M a t e r i a l s . D y e structures are g i v e n i n the figures. P s e u d o c y a n i n e (7,21) (l,l'-diethyl-2,2'-cyanine chloride, C 3 H N C 1 X H 0 ) h a d a m o l a r a b s o r p t i v i t y of 7.0 ± 0.2 X 1 0 M c m . " at 523 n . m . i n w a t e r 2

4

- 1

2 3

2

2

1


14.

HERZ E T A L .

Adsorption

of

175

Dyes

( 2 3 ° C . ) at concentrations b e l o w 5 X 1 0 " M . T h e p-toluenesulfonate ( p t s ) salt of this c y a n i n e was also u s e d ; it gave i d e n t i c a l surface spectra a n d a d s o r p t i o n d a t a i n silver a n d silver h a l i d e dispersions (23). Astraphloxin (55) ( l j l ' - d i e t h y l - S ^ j S ^ S ' - t e t r a m e t h y l i n d o c a r b o c y a n i n e pts, C34H40O3N2S X 0.5 H 0 ) h a d a m o l a r a b s o r p t i v i t y of 13.8 ± 0.4 X 1 0 M c m . " at 542 n . m . at concentrations b e l o w 5 X 1 0 " M i n w a t e r . A t h i g h e r concentrations b o t h P s e u d o c y a n i n e a n d A s t r a p h l o x i n f a i l e d to o b e y Beer's l a w , o w i n g to d y e - d y e i n t e r a c t i o n l e a d i n g to f o r m a t i o n of n e w absorption bands (12). S y n t h e t i c m i c a w a s o b t a i n e d f r o m the M i n n e s o t a M i n i n g a n d M a n u f a c t u r i n g C o . , St. P a u l , M i n n . , as B u r n i l M i c r o Plates H X - 6 0 0 . T h i s colorless p o w d e r is d e s c r i b e d b y the m a n u f a c t u r e r as c o n s i s t i n g of p l a t e lets 20-100 A . t h i c k a n d ten times as l o n g . C o l l o i d a l silver w i t h a n average p a r t i c l e d i a m e t e r of a b o u t 150 A . w a s p r e p a r e d b y the d e x t r i n - r e d u c t i o n of h y d r a t e d s i l v e r oxide, a c c o r d i n g to the general p r o c e d u r e of C a r e y L e a (67). A f t e r a d d i t i o n of g e l a t i n , the d i a l y z e d p a r e n t d i s p e r s i o n w a s a d j u s t e d w i t h b r o m i d e to p B r 3 a n d w i t h a c i d to p H 6.5; i t c o n t a i n e d 5 % g e l a t i n b y w e i g h t a n d was 2.5 X 1 0 M i n respect to silver. O n d i l u t i o n w i t h w a t e r , a clear y e l l o w sol r e s u l t e d ; it o b e y e d Beer's l a w at least u p to 3 X 1 0 " M a n d e x h i b i t e d a m o l a r a b s o r p t i v i t y of 1.45 X 1 0 M c m . ' at its 405 n . m . a b s o r p t i o n m a x i m u m . O n e of the silver b r o m i d e d i s p e r sions w a s a l r e a d y u s e d i n p r e v i o u s a d s o r p t i o n experiments (see R e f e r ences 22 a n d 23, D i s p e r s i o n D ) . It c o n t a i n e d 6 m o l e - p e r c e n t i o d i d e a n d h a d a specific surface area of 1.1 m e t e r / g r a m A g X , as d e t e r m i n e d f r o m the s a t u r a t i o n coverage w i t h P s e u d o c y a n i n e . A m o l e c u l a r area of 57 A . h a d b e e n assigned to this d y e . ( A d s o r p t i o n d e t e r m i n a t i o n s w i t h P s e u d o cyanines i n A g B r dispersions w h o s e surface areas w e r e m e a s u r e d b y three i n d e p e n d e n t methods h a v e p r e v i o u s l y l e d to a l i m i t i n g area of 54 ± 4 A . p e r d y e m o l e c u l e (23). F u r t h e r w o r k has s u p p o r t e d the u p p e r l i m i t as the m o r e accurate v a l u e ; a c c o r d i n g l y , w e h a v e a d o p t e d a m o l e c u l a r area of 5 7 A . t h r o u g h o u t o u r c a l c u l a t i o n s . ) T h i s r e l a t i v e l y coarse A g B r d i s p e r s i o n w a s g e n e r a l l y u s e d at a c o n c e n t r a t i o n of 7 X 1 0 " M i n 0 . 2 % g e l a t i n at p H 6.5 a n d p B r 3. T h e other silver b r o m i d e d i s p e r s i o n w a s a L i p p m a n n t y p e (47) a n d consisted of s m a l l , n e a r l y s p h e r i c a l p a r t i c l e s w i t h a n average d i a m e t e r of 585 ± 3 5 A . to 700 ± 4 0 A . , as e s t i m a t e d f r o m e l e c t r o n m i c r o g r a p h s a n d l i g h t scatter, respectively. T h e source of this c o n s i d e r a b l e d i s c r e p a n c y was not f u r t h e r i n v e s t i g a t e d a n d the L i p p m a n n A g B r d i s p e r s i o n was u s e d i n a d i l u t e state at 5 X 1 0 " M or less, at the a l r e a d y c i t e d Br~, H , a n d g e l a t i n concentrations. I n most of the experiments d e s c r i b e d , g e l a t i n was present as a convenience. S i n c e g e l a t i n as w e l l as other o r g a n i c p o l y m e r s m a y i n d u c e m e t a c h r o m a c y i n c y a n i n e dyes (1, 2, 6, 11, 28, 29), it w a s necessary to establish i f the g e l a t i n u s e d i n the dispersions i n t e r f e r e d w i t h the measurements. T h i s w a s d o n e b y c a r r y i n g out a d s o r p t i o n d e t e r m i n a t i o n s w i t h P s e u d o c y a n i n e i n silver a n d silver b r o m i d e dispersions b o t h i n the absence a n d presence of g e l a t i n u n d e r o t h e r w i s e i d e n t i c a l c o n d i t i o n s . I n agreement w i t h expectations ( 9 ) , no e v i d e n c e w a s f o u n d that g e l a t i n i n f l u e n c e d surface s a t u r a t i o n b y the d y e at a d s o r p t i o n e q u i l i b r i u m (62, 70). H o w e v e r , w i t h one p a r t i c u l a r silver d i s p e r s i o n it w a s n o t e d that, after s a t u r a t i o n coverage of the surface b y P s e u d o c y a n i n e h a d b e e n a t t a i n e d w i t h c o n c o m i t a n t f o r m a t i o n of a / - b a n d at 580 n . m . , excess d y e p r o d u c e d a s e c o n d / - b a n d at 570 n . m . 5

2


4

_ 1

1

5

- 1

4

4

_ 1

1

2

2

2

2

2

4

+


176

ADSORPTION F R O M

AQUEOUS SOLUTION


T h i s s e c o n d b a n d w a s r e a d i l y differentiated f r o m the f o r m e r a n d c o n t r o l experiments c a r r i e d out i n the absence of silver d e m o n s t r a t e d t h a t t h e 5 7 0 - b a n d w a s c a u s e d b y i n t e r a c t i o n of the d y e w i t h the specific g e l a t i n d e r i v a t i v e u s e d i n that system. P r o c e d u r e s . U n l e s s otherwise specified, spectra or a d s o r p t i o n deter m i n a t i o n s w e r e m a d e after e q u i l i b r a t i o n of d y e a n d substrate for at least 1 h r . at 23° ± 1 ° C . T h e coarse A g B r t e n d e d to settle a n d w a s a g i t a t e d e v e n i n the s p e c t r o p h o t o m e t r i c c e l l . T h e silver halides w e r e a l w a y s h a n d l e d u n d e r p h o t o g r a p h i c safelights. C o n v e n t i o n a l a d s o r p t i o n isotherms w e r e o b t a i n e d b y u s u a l c e n t r i f u g a t i o n a n d phase-separation p r o c e d u r e s ( 2 3 ) . D i f f e r e n t i a l d y e spectra w e r e d e t e r m i n e d w i t h a d o u b l e - b e a m spectrophotometer, u s u a l l y a B e c k m a n D K - 2 Spectroreflectometer, b y p l a c i n g the d y e d d i s p e r s i o n i n the s a m p l e c e l l (0.1 to 40 m m . ) a n d b y u s i n g the i d e n t i c a l b u t u n d y e d d i s p e r s i o n i n the c o m p a r i s o n c e l l as the s p e c t r a l reference. B e c a u s e c o l l o i d a l silver absorbs s t r o n g l y i n the b l u e s p e c t r a l r e g i o n , i t w a s not feasible to measure d i f f e r e n t i a l spectra w i t h this d i s p e r s i o n at w a v e l e n g t h s b e l o w ca. 480 n . m . W i t h the w e a k l y scat t e r i n g a n d s m a l l - p a r t i c l e dispersions of s i l v e r , m i c a , a n d silver b r o m i d e of t h e L i p p m a n n t y p e , t r a n s m i s s i o n s p e c t r o p h o t o m e t r y p r o v e d t o b e p r a c t i c a l . H o w e v e r , w i t h coarse a n d h i g h l y t u r b i d silver h a l i d e d i s p e r sions, the diffuse reflectance, R, w a s d e t e r m i n e d u n d e r c o n d i t i o n s s u c h that R p r e v a i l e d — i . e . , a n increase i n c e l l l e n g t h h a d n o m e a s u r a b l e influence o n reflectivity. I n the reflectance measurements the u n d y e d d i s p e r s i o n w a s a g a i n u s e d as the s p e c t r a l reference a n d experiments w i t h p o l a r i z i n g screens s h o w e d that the l i g h t reflected f r o m the s i l v e r h a l i d e dispersions c o n t a i n e d no a p p r e c i a b l e F r e s n e l components. x

Results T h e t r a n s m i s s i o n spectra of F i g u r e 1 A s h o w that, w i t h progressive a d d i t i o n s of synthetic m i c a to a constant c o n c e n t r a t i o n of P s e u d o c y a n i n e s o l u t i o n , the i n t e n s i t y of the M - b a n d d i m i n i s h e d , w i t h the c o n c o m i t a n t a p p e a r a n c e of n e w m a x i m a . T h e first n e w p e a k a p p e a r e d near 460 n . m . a n d w a s f o l l o w e d b y another b a n d near 480 n . m .

T h e appearance

these

by

hypsochromic

transitions w a s

accompanied

f o r m a t i o n of

of a

b a t h o c h r o m i c / - b a n d w i t h a p r i n c i p a l a b s o r p t i o n at 568 n . m . a n d a s h o u l d e r near 577 n . m . A t r e l a t i v e l y l o w m i c a / d y e ratios ( C u r v e s a - c ) , t h e spectra pass t h r o u g h isosbestic points, a n o b s e r v a t i o n w h i c h suggests that the free d y e is i n e q u i l i b r i u m w i t h d y e a d s o r b e d i n its d i m e r i c a n d its /-state. H o w e v e r , the isosbestic points are not m a i n t a i n e d at the h i g h est m i c a concentrations w h e r e the h y p s o c h r o m i c b a n d s lose a b s o r b a n c e a n d d e f i n i t i o n a n d g i v e w a y to a single intense / - b a n d at 568 n . m . (c

=

2.3 X l O W " c m ; ) . T h i s b a n d has a h a l f - w i d t h of a b o u t 13 n . m . a n d is 1

1

associated w i t h a w e a k s u b s i d i a r y m a x i m u m near 520 n . m . T h e c h a r a c t e r of b o t h transitions strongly resembles t h a t o b t a i n e d o n n a t u r a l , f r e s h l y cleaved mica

(63).



Adsorption

of

177

Dyes

Wavelength

(n.m)

Figure 1. Transmission spectra at 23°C. of a fixed concentration of Pseudocyanine and varying amounts of aqueous dispersions. Dispersions without dye served as spectral references A. 10' M dye in water: (a) 0.0 gram, (b) 0.008 gram, (c) 0.016 gram, (d) 0.064 gram, (e) 0.4 gram, (f) 2.0 gram, (g) 2.8 gram synthetic mica per liter B. 4X 10~ M dye in 1 % gelatin at pH 6, pBr 5.3: (a) 0.0 gram, (b) 0.1 gram, (c) 0.2 gram, (d) 0.3 gram, (e) 0.4 gram, (f) 0.6 gram, (g) 0.7 gram colloidal silver per liter C. 5 X 10 M dye in 0.24% gelatin at pH 6.5, pBr 3: (a) 0.0 gram, (b) 0.02 gram, (c) 0.12 gram, (d) 0.22 gram, (e) 0.87 gram AgBr (Lippmann) per liter 5

5

e

I n c o n t r a d i s t i n c t i o n to the spectra o b t a i n e d i n the m i c a d i s p e r s i o n , P s e u d o c y a n i n e , w h e n a d d e d to the c o l l o i d a l silver a n d silver h a l i d e systems of F i g u r e s I B a n d 1 C , y i e l d e d no H - b a n d s b u t f o r m e d

well

defined / - b a n d s . T h e latter also e x h i b i t e d a w e a k secondary p e a k l o c a t e d near the a b s o r p t i o n m a x i m u m of d i s s o l v e d , u n p e r t u r b e d d y e . A l t h o u g h the p o s i t i o n a n d i n t e n s i t y of the / - b a n d v a r i e d w i t h the substrate (4, 38, 61),

17,

the d i f f e r e n t i a l spectra o b t a i n e d i n these silver systems e x h i b i t e d

m a r k e d similarities.

A n increase i n the c o n c e n t r a t i o n of the substrate

p r o d u c e d i n b o t h cases a m o n o t o n i c change i n the absorbance of the


M-

178

ADSORPTION F R O M

AQUEOUS SOLUTION

a n d / - b a n d s ; whereas t h e f o r m e r decreased, t h e / - b a n d s i n c r e a s e d i n intensity u n t i l a m a x i m u m v a l u e w a s r e a c h e d .

A d d i t i o n of f u r t h e r s u b

strate h a d t h e n n o n o t i c e a b l e effect o n t h e a b s o r p t i v i t y of this b a n d . T h e s e s p e c t r a l changes, w h i c h w e r e a c c o m p a n i e d b y t h e a p p e a r a n c e of isosbestic regions near 555 n . m . w i t h t h e silver, a n d near 545 n . m . w i t h the silver b r o m i d e d i s p e r s i o n , suggested the existence of a substrate d e p e n d e n t e q u i l i b r i u m b e t w e e n t w o states: U n p e r t u r b e d d y e i n s o l u t i o n a n d d y e a d s o r b e d i n its /-state.


Q u i t e p a r a l l e l changes o c c u r w h e n a s o l u t i o n of t h e sulfate ester of poly ( v i n y l alcohol)

(52)

is a d d e d to d i l u t e aqueous P s e u d o c y a n i n e . A s

i l l u s t r a t e d i n F i g u r e 2, there is n o e v i d e n c e f o r t h e f o r m a t i o n of a d i n i e r ; one observes a g a i n the a p p e a r a n c e of a / - b a n d at 568 n . m . A t t h e highest polymer concentration a l l unperturbed dye h a d apparently disappeared ( C u r v e c ) a n d f u r t h e r p o l y m e r a d d i t i o n d i d n o t c h a n g e t h e shape of xio

4

IO.O

c

-

7.5

q

400

500

6 60 0

Wavelength (nmj Figure 2. Transmission spectra at 23°C. of aqueous Pseudocyanine with varying concentrations of poly(vinyl alcohol) sulfate ester: (a) 0.0 gram, (b) 0.004 gram, (c) 0.004 gram PVAsulfate per liter


14.

HERZ E T A L .

Adsorption

of

179

Dyes

this c u r v e so that this s p e c t r u m represents the a t t a i n a b l e c o n v e r s i o n of the M - to the /-state.

( I t cannot b e a s s u m e d that C u r v e c represents

c o m p l e t e c o n v e r s i o n of the d y e to its /-state.

C u r v e analysis b y

our

colleague, F . W e b s t e r , has d e m o n s t r a t e d that as m u c h as 5 4 % of the d y e m a y h a v e r e m a i n e d i n its u n p e r t u r b e d M - s t a t e . R e c o n s t r u c t i o n of C u r v e c o n that basis does not c h a n g e its p r i n c i p a l features b u t decreases the i n t e n s i t y of the m a x i m a near 500 a n d 535 n.m.)

T h i s state manifests

itself b y the w e a k b u t r e s o l v e d transitions near 500 a n d 535 n . m . a n d t h e


intense b a n d at 568 n . m . w i t h a n e x t i n c t i o n greater t h a n 1.2 X cm."

1

lO^M"

1

a n d a h a l f - w i d t h of a b o u t 15 n . m . E s s e n t i a l l y i d e n t i c a l results w e r e

r e p o r t e d for the spectra o b t a i n e d w i t h p o l y e t h y l e n e sulfonate a n d a n i o n i c polysaccharides b y A p p e l a n d Scheibe deductions,

(1).

I n agreement w i t h t h e i r

present results i n d i c a t e that salt f o r m a t i o n b e t w e e n

the

c a t i o n i c d y e a n d closely adjacent a n i o n i c sites i n the p o l y m e r is r e s p o n sible for the spectra i l l u s t r a t e d i n F i g u r e 2. T h u s , n o / - b a n d w a s o b t a i n e d w i t h poly (vinyl alcohol)

itself n o r w i t h a s u l f o n a t e d p o l y s t y r e n e .

In

the latter p o l y m e r the distance b e t w e e n a c i d sites a p p a r e n t l y w a s greater t h a n the ca. 5 A . , w h i c h appears to b e the m a x i m u m distance p e r m i t t i n g the / - s t a t e b y electronic c o u p l i n g b e t w e e n adjacent d y e m o l e c u l e s

(1).

T h e i m p o r t a n c e of salt f o r m a t i o n i n this system is also i l l u s t r a t e d b y the fact that P V A - s u l f a t e gave no / - b a n d w i t h a s u l f o a l k y l p s e u d o c y a n i n e w h i c h h a d a net charge of zero. It w a s n o t e d , h o w e v e r , that i n the silver and

silver h a l i d e dispersions this z w i t t e r i o n i c d y e e x h i b i t e d a d s o r p t i o n

and

s p e c t r a l properties that w e r e

essentially i d e n t i c a l w i t h those

of

P s e u d o c y a n i n e itself. T h e procedures u s e d to o b t a i n F i g u r e 1 c a n b e reversed. F i g u r e 3 illustrates the s p e c t r a l changes a c c o m p a n y i n g the a d d i t i o n of i n c r e a s i n g amounts of P s e u d o c y a n i n e to a fixed c o n c e n t r a t i o n of c o l l o i d a l silver. T h e r e s u l t i n g f a m i l y of d i f f e r e n t i a l spectra s h o w that, after the / - b a n d had

reached a critical intensity ( A

m a x

v a r i e d b e t w e e n 578-582 n . m . ) ,

a d d i t i o n a l d y e e x h i b i t e d o n l y the s p e c t r a l chracteristics of u n p e r t u r b e d d y e i n its s o l u t i o n state (cf.,

Figure I B , Curve a).

T h e salient features

of these spectra are m a d e m o r e o b v i o u s b y p l o t t i n g the c o n c e n t r a t i o n of a d d e d d y e against the a b s o r b a n c e of the c o r r e s p o n d i n g / - b a n d . T h i s w a s d o n e i n F i g u r e 4 A . F o l l o w i n g a n i n i t i a l l i n e a r increase i n the a b s o r b a n c e of the / - b a n d , a c r i t i c a l r e g i o n w a s r e a c h e d b e y o n d w h i c h f u r t h e r a d d i t i o n of d y e h a d little effect o n this t r a n s i t i o n a n d c a u s e d a m a r k e d c h a n g e i n the slope of the c u r v e .

( T h e m o n o m e l i c a b s o r p t i o n spectra of cyanines

often e x t e n d w e a k l y b u t m e a s u r a b l y i n t o the / - r e g i o n a n d e v e n u n p e r t u r b e d P s e u d o c y a n i n e i n s o l u t i o n w i l l m a k e a slight c o n t r i b u t i o n to / - b a n d absorbance.

M o r e o v e r , as a l r e a d y discussed, u n d e r some c o n d i t i o n s of

salt or surfactant concentrations, cyanines m a y e x h i b i t intense / - b a n d s i n solution.)

A c h a n g e i n slope at the same c r i t i c a l d y e c o n c e n t r a t i o n w a s



180

ADSORPTION F R O M

500

AQUEOUS SOLUTION

600

Wavelength

(am)

Figure 3. Transmission spectra of 4.65 X 10~ M colloidal silver in 1% gelatin at 23°C, pBr 5.3, pH 6.5 with varying concentrations of Pseudocyanine: (a) 0.8, (b) 1.6, (c) 2.4, (d) 4.0, (e) 5.6, (f) 7.6, (g) 9.0, (h) 11.0 X W M dye. Undyed colloidal silver served as reference, 2mm. cells were used 3

5

also o b s e r v e d w h e n the r a t i o of M to / absorbances w a s p l o t t e d against the c o n c e n t r a t i o n o f a d d e d d y e .

It s h o u l d b e n o t e d that absorbance i n

the M - b a n d r e g i o n involves not o n l y u n p e r t u r b e d d y e b u t also d y e i n its / - s t a t e w h i c h contains a c o m p o n e n t t h a t absorbs at shorter w a v e l e n g t h s . W e c o n s i d e r e d the c o n c e n t r a t i o n - d e p e n d e n t s p e c t r a l d a t a of F i g u r e 4 A to be a m a n i f e s t a t i o n of the a d s o r p t i o n e q u i l i b r i u m o f the dye.

Spe

cifically, w e a s s u m e d that the i n i t i a l l i n e a r increase of / - a b s o r b a n c e r e p r e sents b i n d i n g of a l l the a d d e d d y e at the substrate a n d that d e p a r t u r e f r o m this l i n e a r p o r t i o n corresponds to a d d e d b u t u n a d s o r b e d d y e ; the latter w o u l d t h e n a c c o u n t f o r the s u d d e n increase of the M/J

absorbance

ratio. It f o l l o w s f r o m this i n t e r p r e t a t i o n that the intercepts o b t a i n e d b y e x t r a p o l a t i n g t h e l i n e a r sections of the p l o t of d y e c o n c e n t r a t i o n vs. J. or M/J

absorbance, s h o u l d c o r r e s p o n d to the a m o u n t of d y e a d s o r b e d at

saturation of the surface.

If w e assume f u r t h e r that the absorbances

of

F i g u r e 3 a n d 4 A o b e y B e e r s l a w , t h e n the i n t e n s i t y of t h e / - b a n d w i l l be


14.

HERZ E T AL.

Adsorption

181

of Dyes

p r o p o r t i o n a l t o the surface c o n c e n t r a t i o n o f t h e d y e a n d the s p e c t r a l d a t a of F i g u r e 4 A c a n b e c o n v e r t e d i n t o a n a d s o r p t i o n i s o t h e r m . T h i s w a s d o n e i n F i g u r e 4 B , w h e r e the a m o u n t of d y e a d s o r b e d p e r m o l e o f c o l l o i d a l silver ( a ) is p l o t t e d against the e q u i l i b r i u m c o n c e n t r a t i o n o f d y e i n s o l u t i o n ( c ) . T h e r e s u l t i n g i s o t h e r m is expressed b y t h e L a n g m u i r adsorption l a w 1 aK

a*

a

w h e r e a i s the s a t u r a t i o n coverage a n d K is the L a n g m u i r a d s o r p t i o n Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch014

8

coefficient.

T h e slope o f t h e l i n e a r r e l a t i o n o f c/a against c y i e l d e d a

I

2

3

4

5

6

7 x IO"

6

7 x IO"

3

Millimoles dye added

I

2

3

4

5

5

Molarity of equilibrium concentration (c)

Figure 4A. Absorbance of the J-band and the M / J ratio at varying concentrations of Pseudocyanine in 50 ml. of 4.65 m M colloidal silver (cf. Figure 3) B. Adsorption isotherm of Pseudocyanine in colloidal silver calculated from the spectral data of Figure 4A. See text for details


182

ADSORPTION F R O M

AQUEOUS SOLUTION

s a t u r a t i o n coverage, a , of 10 m i l l i m o l e s of P s e u d o c y a n i n e p e r m o l e 8

of

silver. O n the a s s u m p t i o n that the p a c k i n g of the d y e at this s i l v e r surface is s i m i l a r to that o n silver h a l i d e substrates—i.e., 57A.

2

per molecule

(23),

a specific area of 35 sq. meter per g r a m of silver w a s o b t a i n e d . A s i m i l a r analysis of the s p e c t r a l d a t a of F i g u r e I B gave the same results. I f the c o l l o i d a l silver particles are c o n s i d e r e d to b e s p h e r i c a l , this surface area is e q u i v a l e n t to a n average p a r t i c l e d i a m e t e r of 180 A . T h i s v a l u e agrees i n m a g n i t u d e w i t h d i a m e t e r estimates of 110, 140, a n d 190 A . o b t a i n e d


b y Stevens a n d B l o c k (64)

w i t h three i n d e p e n d e n t methods o n s i m i l a r l y

p r e p a r e d s i l v e r dispersions. M o r e o v e r , a p p l i c a t i o n of the d e s c r i b e d spec t r a l m e t h o d to a different silver d i s p e r s i o n y i e l d e d a n average p a r t i c l e d i a m e t e r of 45 A . , whereas a v a l u e of about 48 A . was o b t a i n e d f r o m electronmicrographs.

T h i s agreement

w i t h independent

size

estimates

makes i t p r o b a b l e that the assumptions a p p l i e d to the i n t e r p r e t a t i o n of the d y e spectra i n silver dispersions w e r e v a l i d . A s a l r e a d y i n d i c a t e d , the spectra of P s e u d o c y a n i n e i n the m i c a d i s p e r s i o n ( F i g u r e 1 A ) w o u l d r e q u i r e a m o r e d e t a i l e d analysis for t h e i r q u a n t i t a t i v e i n t e r p r e t a t i o n . I n c o m m o n w i t h silicates l i k e m o n t m o r i l l o nite ( 3 ) , the synthetic m i c a behaves as i f it possesses different types of b i n d i n g sites w h i c h c a u s e d a d s o r p t i o n of the d y e b o t h i n the d i m e r i c a n d i n the /-state.

I n contrast to the m i c a - d y e system, t r a n s m i s s i o n spectra

of P s e u d o c y a n i n e i n the c o l l o i d a l s i l v e r b r o m i d e i n d i c a t e d the p r e d o m i nance of o n l y one t y p e of surface i n t e r a c t i o n ( F i g u r e 1 C ) .

A n a l y s i s of

these spectra b y the m e t h o d d e s c r i b e d for the case of c o l l o i d a l silver, gave a specific area of 11 sq. m e t e r / g r a m A g B r w i t h a n average p a r t i c l e d i a m e t e r of 950 db 60 A . T h e a c c u r a c y of these results is u n c e r t a i n since the true p a r t i c l e d i m e n s i o n s w e r e not k n o w n (cf.,

section).

Experimental

T h e a p p l i c a b i l i t y of this in situ m e t h o d for the d e t e r m i n a t i o n of surface areas d e p e n d s not o n l y o n k n o w l e d g e of the dye's m o l e c u l a r area i n the a d s o r b e d state b u t also o n the a s s u m p t i o n t h a t the chosen s p e c t r a l p a r a m e t e r measures the surface c o n c e n t r a t i o n of the dye.

I n order to

test the r e l a t i o n b e t w e e n a d s o r p t i o n of d y e to s i l v e r h a l i d e a n d its spec t r a l characteristics i n the b o u n d state, the b e h a v i o r of P s e u d o c y a n i n e i n a coarse silver h a l i d e suspension ( D i s p e r s i o n D ) w a s s t u d i e d . T h i s p a r t i c u l a r d i s p e r s i o n w a s chosen because some of its r e l e v a n t a d s o r p t i o n characteristics h a d a l r e a d y b e e n e x a m i n e d (22, 23).

Moreover, observa

tions b y B o y e r a n d C a p p e l a e r e w i t h P s e u d o c y a n i n e a d s o r b e d o n A g B r powders (5)

i n d i c a t e d that / - b a n d i n t e n s i t y v a r i e d w i t h the a m o u n t of

a d s o r b e d d y e a n d w a s not sensitive to the c o n c e n t r a t i o n of A g

+

or B r "

ions i n the range p A g 3.3-8.7. T h e r e r e m a i n e d the q u e s t i o n of h o w to evaluate d y e

spectra i n

c o n c e n t r a t e d silver h a l i d e dispersions w h o s e p a r t i c l e sizes r a n g e d b e t w e e n 0.2-2.0 [i, a n d w h e r e m u l t i p l e i n t e r p a r t i c l e l i g h t scatter m a d e i t u n f e a s i b l e


14.

HEBZ E T AL.

Adsorption

of

183

Dyes

to a p p l y transmittance measurements of the k i n d u s e d i n F i g u r e s 1 a n d 3. I n other studies (46, 50, 70, 71)

the f o l l o w i n g r e l a t i o n ,

% Absorbance = 100 — % Transmittance — % Reflectance or one of its components, h a d b e e n used.

H o w e v e r , i n the absence of

c a l i b r a t i o n d a t a , the results d i d not y i e l d a verifiable d i r e c t r e l a t i o n b e t w e e n the p h o t o m e t r i c parameters a n d the surface c o n c e n t r a t i o n dye.

of

I n this w o r k w e a p p l i e d the K u b e l k a - M u n k reflectivity f u n c t i o n ,


w h e r e K a n d S are a b s o r p t i o n a n d scatter coefficients of the substrate, R

x

is the reflectivity of a n i n f i n i t e l y t h i c k layer, a n d C a n d c are the m o l a r

concentration

a n d m o l a r a b s o r p t i v i t y coefficients,

respectively.

This relation, w h i c h

has b e e n s t u d i e d i n m u c h d e t a i l b y K o r t u m , w h o adsorption phenomena

(31,

also a p p l i e d i t to

32, 3 3 ) , has a f o r m s i m i l a r to B e e r s l a w ,

except that i n reflectometry the c o n c e n t r a t i o n C is p r o p o r t i o n a l to

K/S

rather t h a n to absorbance. T h e K u b e l k a - M u n k f u n c t i o n is a p p l i c a b l e to silver h a l i d e dispersions, p r o v i d e d t h a t s p e c u l a r or F r e s n e l reflections

are n e g l i g i b l e a n d t h a t

infinite reflectivities, R , are a c t u a l l y m e a s u r e d (24). x

A f t e r it h a d b e e n

ascertained that these c o n d i t i o n s h a d b e e n met, a c o n c e n t r a t i o n series of P s e u d o c y a n i n e i n the coarse s i l v e r b r o m i d e suspension ( D i s p e r s i o n

D)

was p r e p a r e d a n d y i e l d e d the K/S spectra i l l u s t r a t e d i n F i g u r e 5 A . It is a p p a r e n t that these c o n c e n t r a t i o n - d e p e n d e n t

reflection spectra possess

characteristics s i m i l a r to the t r a n s m i s s i o n spectra o b t a i n e d w i t h the s i l v e r substrate i n F i g u r e 3. A g a i n one observes t h a t the / - b a n d near 572 n . m . tends t o w a r d a m a x i m u m v a l u e .

I n F i g u r e 5 B the m i l l i m o l e s of

dye

a d d e d p e r m o l e of s i l v e r b r o m i d e are p l o t t e d against / - b a n d i n t e n s i t y expressed i n K/S values. T h e character of the r e s u l t i n g c u r v e is s i m i l a r to the c o r r e s p o n d i n g similarly.

g r a p h i n F i g u r e 4 A a n d the d a t a w e r e

treated

E x t r a p o l a t i o n of the l i n e a r parts i n F i g u r e 5 B y i e l d e d a n

i n t e r c e p t w h i c h was a g a i n t a k e n to define the a m o u n t of d y e a d s o r b e d at saturation coverage a n d served as the reference p o i n t for the c o n v e r s i o n of the s p e c t r a l d a t a i n t o the a d s o r p t i o n i s o t h e r m i n F i g u r e 8 A . I n c o m p a r i s o n w i t h a n azo d y e a d s o r b e d o n a n h y d r o u s b a r i u m s u l fate ( 3 J ) , the K/S values of F i g u r e 5 are r e m a r k a b l y large. I n p a r t this is because of the h i g h e x t i n c t i o n coefficient of the Pseudocyanine's / - b a n d (cf.,

F i g u r e 1) a n d i n p a r t to the non-zero base l i n e i n F i g u r e 5 A . It w a s

e x p e r i m e n t a l l y c o n v e n i e n t to use a n 0.2 absorbance filter i n the s a m p l e b e a m of the spectrophotometer;

i n the absence of this

filter—it

h a d no

influence o n the final r e s u l t s — t h e K/S v a l u e at s a t u r a t i o n coverage w o u l d



184

ADSORPTION F R O M

400

500

AQUEOUS

SOLUTION

600

Wavelength (ryrO

Figure 5A. Reflection spectra of 6.43 X I 0 * M AgBr (Dispersion D) in 0.2% gelatin at 23°C, pBr 3, pH 6.5 with varying concentrations of Pseudocyanine chloride (a) 0.01, (b) 0.05, (c) 0.25. (d) 0.5, (e) 0.75, (f) 1.0, (g) 1.5, (h) 2.5, (i) 3.5, (k) 6.0, (I) 7.0 X 10~ M dye. Sample beam of spectrophotometer passed through a 0.2 absorbance filter, undyed AgBr served as spectral reference, 40 mm. cells were used. See text for explanation of reflection parameter K / S B. Dependence of ]-band reflectivity on concentration of Pseudocyanine. Millimoles of dye in 100 ml. of 6.43 X 10~ M AgBr 5

2


14.

HERZ E T AL.

Adsorption

b e 3.2 i n s t e a d of near 6.0.

of

185

Dyes

I n F i g u r e 5 B a slight toe is n o t i c e a b l e i n the

c u r v e at l o w d y e concentrations.

T o the extent t h a t this is c a u s e d b y a

systematic reflectivity error i t m a y b e c o r r e c t e d present purposes

(32, 34, 37),

there was n o n e e d to consider

b u t for

this d e p a r t u r e

from

l i n e a r i t y at l o w surface coverage b y the d y e . T h e v e r y same dispersions w h i c h h a d y i e l d e d the spectra of F i g u r e 5 A w e r e c e n t r i f u g e d to separate s o l i d a n d l i q u i d phases i n order

to

measure d y e a d s o r p t i o n b y c o n v e n t i o n a l m e t h o d s (23, 7 0 ) . T h e r e s u l t i n g Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch014

d a t a are also s h o w n i n F i g u r e 8 A w h e r e t h e y c a n b e c o m p a r e d w i t h the s p e c t r a l l y d e t e r m i n e d a d s o r p t i o n i s o t h e r m . T h e agreement b e t w e e n the t w o i n d e p e n d e n t m e t h o d s is g o o d a n d indicates that in situ spectra c a n b e u s e d to d e t e r m i n e d i r e c t l y the surface c o n c e n t r a t i o n of P s e u d o c y a n i n e . T h i s d y e is p a r t i c u l a r l y s u i t a b l e for that p u r p o s e because its spectra at e l e v a t e d surface coverages a l w a y s seem to b e associated w i t h the n a r r o w / - b a n d w h i c h has a h i g h e x t i n c t i o n coefficient. H o w e v e r , the f o r m a t i o n of a / - b a n d is not a p r e r e q u i s i t e for the spec t r a l d e t e r m i n a t i o n of surface concentrations of a d y e ; it is sufficient that the difference b e t w e e n s o l u t i o n a n d surface spectra c a n be e v a l u a t e d q u a n t i t a t i v e l y . A s t r a p h l o x i n is a n e x a m p l e of a d y e that meets these c o n d i t i o n s . I n d i l u t e aqueous s o l u t i o n its p r i n c i p a l a b s o r p t i o n at 542 n . m . is separated some 1250 c m .

- 1

f r o m a s u b s i d i a r y a n d p r o b a b l y v i b r a t i o n a l m a x i m u m at

521 n . m . A t concentrations i n excess of 2 X

1 0 " M , this d y e exhibits a 4

n e w h y p o s o c h r o m i c t r a n s i t i o n at 505 n . m . c o n s i d e r e d to be c a u s e d b y a d i m e r (12);

no / - b a n d has b e e n f o u n d i n A s t r a p h l o x i n solutions u n d e r

c o n d i t i o n s w h e r e P s e u d o c y a n i n e does f o r m one.

F i g u r e 6 illustrates the

spectra o b t a i n e d b y a c o n c e n t r a t i o n series of A s t r a p h l o x i n i n the same silver b r o m i d e suspension ( D i s p e r s i o n D ) a n d u n d e r the same c o n d i t i o n s as h a d b e e n u s e d i n c o n n e c t i o n w i t h F i g u r e 5.

H o w e v e r , i n s t e a d of

p l o t t i n g the d a t a as K/S values, w e h a v e r e c o r d e d t h e m as t h e m o r e accessible reflection absorbances ( — L o g R ) f r o m w h i c h % R x

c o u l d b e c o m p u t e d for a n y d e s i r e d w a v e l e n g t h .

a n d K/S

x

I n order to f a c i l i t a t e

comparisons, F i g u r e 6 also contains a t r a n s m i t t a n c e s p e c t r u m of A s t r a phloxin (dashed curve) w i t h the difference

i n the absence of the silver h a l i d e . I n a c c o r d

i n refractive i n d e x of w a t e r a n d A g B r

(72),

the

s p e c t r u m of the d y e at l o w surface coverages is d i s p l a c e d a p p r o x i m a t e l y 25 n . m . t o w a r d s l o n g e r w a v e l e n g t h s ( C u r v e a, b ) .

H e n c e , the n e w p r i n

c i p a l t r a n s i t i o n near 565 n . m . is d e s i g n a t e d the M - b a n d because i t is a

associated w i t h i s o l a t e d , u n p e r t u r b e d b u t a d s o r b e d molecules.

A s the

d y e c o n c e n t r a t i o n is increased, t w o other transitions b e c o m e defined i n F i g u r e 6, a h y p s o c h r o m i c p e a k (H) b a n d (B)

near 580 n . m .

near 530 n . m . a n d a b a t h o c h r o m i c

A t the most e l e v a t e d d y e levels, the B - b a n d

reaches a l i m i t i n g v a l u e whereas the H - b a n d shifts t o w a r d s the m a x i m u m associated w i t h the free d y e i n s o l u t i o n (542 n . m . ) . A t first i t w a s t h o u g h t



186

ADSORPTION F R O M

400

500

Wavelength

AQUEOUS SOLUTION

600

(ryu)

Figure 6. Reflection spectra of 6.26 X 10*M AgBr (Dispersion D) in 0.2% gelatin at 23°C, pBr 3, pH 6.5 with varying concentration of Astraphloxin: (a) 0.01, (b) 0.10, (c) 1.0, (d) 2.0, (e) 3.0,(f) 4.0, (g) 5.0 X 10~ M dye. Undyed AgBr served as spectral reference, 40 mm. cells were used. Dashed curve: Transmission absorbance of 7 X lO^M dye (1 cm. ceU) in same solvent system after removal of AgBr by centrifugation 5

that the H- a n d B - b a n d s m i g h t represent t w o different states of a d s o r b e d Astraphloxin.

H o w e v e r , experiments analogous

to those i n F i g u r e 1,

w h e r e d y e c o n c e n t r a t i o n w a s k e p t constant b u t A g B r w a s a d d e d i n v a r i ous amounts, y i e l d e d spectra w h i c h suggested a different i n t e r p r e t a t i o n . O v e r a l i m i t e d r a n g e of A g B r , a f a m i l y of spectra r e s u l t e d w h i c h , i n a d d i t i o n to the c h a r a c t e r i s t i c transitions associated w i t h free d y e ,

ex

h i b i t e d H-, a n d B - b a n d s . A l l these spectra passed t h r o u g h a n isosbestic p o i n t (558 n . m . ) ; thus t h e y not o n l y i n d i c a t e d a n e q u i l i b r i u m b e t w e e n free a n d a d s o r b e d d y e b u t also suggested that the c i t e d b a n d s are a m a n i f e s t a t i o n of a single state of the a d s o r b e d d y e .

( W h e n Astraphloxin

is a d s o r b e d o n 100-faces of c u b i c A g B r r a t h e r t h a n o n its o c t a h e d r a l I l l - f a c e s , the transitions of the B-state are d i s p l a c e d b a t h o c h r o m i c a l l y b y a b o u t 5 n . m . M o r e o v e r , i n some s i l v e r h a l i d e systems, shoulders n e a r 490 a n d 610 n . m . c a n b e n o t e d ; the latter has b e e n / - b a n d (54).)

F o l l o w i n g W e s t et al. (46),

d e s c r i b e d as a

this c o n d i t i o n w i l l be r e f e r r e d


14.

Adsorption

HERZ E T AL.

187

of Dyes

to as t h e B-state a n d is assumed to b e a c o n s e q u e n c e of p e r t u r b a t i o n between

n e i g h b o r i n g d y e molecules

adsorbed

a n d p a c k e d closely o n

t h e i r l o n g axis. U n l i k e the s i m i l a r l y a r r a y e d / - s t a t e , it is t h o u g h t that the p r i n c i p a l t r a n s i t i o n o f the B-state involves a n e l e c t r o n i c m o m e n t p a r a l l e l to the c o n j u g a t e d c h a i n o f the m o l e c u l e .

E i t h e r the B - o r the M - b a n d a

c a n b e u s e d t o c a l c u l a t e the surface s a t u r a t i o n b y A s t r a p h l o x i n . D e s p i t e o v e r l a p p i n g a b s o r p t i o n f r o m free d y e , t h e M - b a n d has t h e a d v a n t a g e a

that i t measures c o n t r i b u t i o n s f r o m a d s o r b e d , i s o l a t e d molecules at l o w


coverage as w e l l as c o n t r i b u t i o n s f r o m t h e p e r t u r b e d B-state w h i c h is o b s e r v e d at h i g h e r surface concentrations.

Hence, b y using differential

reflection spectra, some of w h i c h are s h o w n i n F i g u r e 6, the c o n c e n t r a t i o n d e p e n d e n c e of the M a - b a n d w a s o b t a i n e d a n d is expressed i n F i g u r e 7 i n terms o f various p h o t o m e t r i c parameters. %

R

x

nor — L o g R

x

I t c a n b e seen t h a t n e i t h e r

(reflection a b s o r b a n c e ) p r o d u c e d a d i r e c t r e l a t i o n

f r o m w h i c h the dye's surface c o n c e n t r a t i o n c o u l d b e estimated. O n the other h a n d , the K/S reflectivity p a r a m e t e r a g a i n y i e l d e d t w o l i n e a r plots w h o s e e x t r a p o l a t e d intercept f u r n i s h e d the a m o u n t of d y e a d s o r b e d a t saturation coverage. A s before, this m a d e i t possible t o convert the K/S d a t a into a n a d s o r p t i o n i s o t h e r m w h i c h w a s a g a i n f o u n d t o b e i n a c c o r d w i t h independently obtained adsorption determinations ( F i g u r e 8 A ) . T h e a d s o r p t i o n isotherms

of A s t r a p h l o x i n a n d P s e u d o c y a n i n e i n

F i g u r e 8 A w e r e also expresed b y the L a n g m u i r a d s o r p t i o n e q u a t i o n . T h e r e s u l t i n g l i n e a r r e l a t i o n of F i g u r e 8 B demonstrates excellent

agreement

b e t w e n the d a t a o b t a i n e d f r o m phase separation a n d s p e c t r a l techniques.

K S

o 20 8 40

rr

60

- 80

.1

.2

.3

.4

.5

.6

.7

.8

.9

100

i.o

Millimoles dye added/mole Agx Figure 7. Dependence of M -band reflectivity of Astraphloxin in Figure 6 expressed in terms of different reflection parameters. See text for significance of K / S values a


188

ADSORPTION F R O M

AQUEOUS SOLUTION

F r o m the slope a n d i n t e r c e p t of these plots the s a t u r a t i o n coverage a n d a d s o r p t i o n coefficient given equation.

w e r e o b t a i n e d b y a p p l i c a t i o n of the p r e v i o u s l y

T h e results are l i s t e d i n T a b l e I, w h i c h also i n c l u d e s

the area p e r d y e m o l e c u l e at s a t u r a t i o n coverage a n d the s t a n d a r d free energy of a d s o r p t i o n , A G ° . A l t h o u g h the latter p a r a m e t e r w a s c a l c u l a t e d as b e f o r e ( 2 3 ) , its t h e r m o d y n a m i c v a l i d i t y is q u e s t i o n a b l e since r e v e r s i


b i l i t y of a d s o r p t i o n of these dyes w a s not d e m o n s t r a t e d . T h e m o l e c u l a r

1

1

1

-

-

-

/*

1

1.0

1

2.0

Molarity of equilibrium concentration

1

3.0

(c)

Figure 8A. Adsorption isotherms of Pseudocyanine (No. 1, Circles) and of Astraphloxin (No. 2, Squares) in AgBr (Dispersion D) containing 0.2% gelatin at 23°C., pBr 3, pH 6.5. The data are expressed as the concentration of free dye (c) in equilibrium with dye adsorbed per mole of AgBr (a). Open data points and solid lines: Results calculated from surface spectra. Solid data points and dashed lines: Results obtained from phase-separation procedure B. Adsorption isotherms of Figure 8A expressed in terms of the Langmuir equation. See text


14.

HERZ E T AL.

Adsorption

of

189

Dyes

area of A s t r a p h l o x i n w a s o b t a i n e d f r o m its s a t u r a t i o n coverage r e l a t i v e to t h a t of P s e u d o c y a n i n e to w h i c h a n area of 57 A . h a d b e e n assigned. 2

A s discussed elsewhere (23),

a v a l u e of this m a g n i t u d e is n o t o n l y c o n

sistent w i t h t h e t h e o r e t i c a l d i m e n s i o n s of the d y e b o u n d o n its l o n g edge i n a closely p a c k e d a r r a y , b u t it is also i n reasonable agreement

with

v a r i o u s c i t e d measurements o b t a i n e d i n other laboratories. Adsorption determinations w i t h Astraphloxin i n homodisperse

cubic

a n d o c t a h e d r a l s i l v e r b r o m i d e dispersions of the t y p e w h i c h w e r e p r e v i Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch014

ously d e s c r i b e d (20, 23), 76 ±

7 A.

2

i n d i c a t e d a m e a n l i m i t i n g area for this d y e of

w h i c h is i n f a i r agreement w i t h the 69 A .

2

obtained

from

F i g u r e 8. Since a m o n o l a y e r of d y e i n its flat o r i e n t a t i o n w o u l d r e q u i r e a n area i n excess of 130 A .

2

p e r m o l e c u l e , the results i n d i c a t e t h a t at

saturation A s t r a p h l o x i n w a s a d s o r b e d at its l o n g edge (ca.

19 A . ) w i t h

a n i n t e r m o l e c u l a r s p a c i n g of a b o u t 4.0 A . A s seen i n the spectra, a d s o r p t i o n o c c u r r e d w i t h o u t f o r m a t i o n of a / - s t a t e a n d its absence i n these experiments m a y account for its r e l a t i v e l y l o w free energy of a d s o r p t i o n (Table I).

It m u s t b e stressed that, a l t h o u g h the g e m - d i m e t h y l groups

of A s t r a p h l o x i n are l o c a t e d p e r p e n d i c u l a r to the conjugated c h a i n , close s p a c i n g of molecules is s t i l l possible i f t h e y are o r i e n t e d o n t h e i r l o n g edge i n a staggered array. P r e s u m a b l y , it is this o r i e n t a t i o n w h i c h , c o n t r a r y to Sheppard's e a r l y s u p p o s i t i o n (61),

is responsible for the f o r m a

t i o n of / - b a n d s i n the N , N ' - d i m e t h y l a n a l o g of this d y e (54)

a n d i n its

d e r i v a t i v e s h a v i n g substituents i n the b e n z e n e r i n g . T h e s t u d y of surface-dye i n t e r a c t i o n b y the in situ o p t i c a l p r o c e d u r e has

also

been

applied

to

5,5'-dichloro-3,3',9-triethylthiacarbocyanine,

w h i c h was k n o w n to b e a d s o r b e d i n m u l t i l a y e r s ( 7 0 ) .

W h e n this d y e

w a s a d d e d to a suspension of o c t a h e d r a l A g B r at p B r 3, a / - b a n d w a s f o r m e d near 640 n . m . ( F i g u r e 9 A ) .

I n c r e a s i n g d y e concentrations

en

h a n c e d the r e f l e c t i v i t y of this b a n d u n t i l i t r e a c h e d a l i m i t i n g i n t e n s i t y ; once r e a c h e d , a d d i t i o n a l d y e c o n t r i b u t e d i n s i g n i f i c a n t l y to reflectivity values i n t h a t s p e c t r a l r e g i o n .

(However, overlapping absorption from

d y e i n other states m a y , at h i g h d y e concentrations, cause a n a p p a r e n t c o n t r i b u t i o n to a b s o r p t i o n . level shown i n Figure 9A.) flection

T h i s effect is i l l u s t r a t e d b y the highest d y e A s before, the c o n c e n t r a t i o n - d e p e n d e n t

re

densities of the / - b a n d w e r e c o n v e r t e d to K / S f u n c t i o n s ( F i g u r e

9 B ) f r o m w h i c h the a d s o r p t i o n i s o t h e r m m a r k e d " O p t i c a l M e t h o d " w a s then derived (Figure 9 C ) .

T h e same

figure

also shows a n a d s o r p t i o n

i s o t h e r m o b t a i n e d f r o m the same system b y the c l a s s i c a l phase separation technique.

T h e latter m e a s u r e m e n t exhibits the slight h o r i z o n t a l step

w h i c h W e s t et al. h a d i n t e r p r e t e d as s i g n i f y i n g c o m p l e t i o n of a m o n o l a y e r before the onset of p o l y l a y e r a d s o r p t i o n S t r i k i n g differences

(70).

are a p p a r e n t b e t w e e n

the t w o

independently

d e t e r m i n e d a d s o r p t i o n isotherms w h i c h are i l l u s t r a t e d i n F i g u r e 9 C .


ADSORPTION F R O M AQUEOUS

SOLUTION


Mmoles dye added / mole Ag

Wavelength (m/x)

o.i 0.2 Millimoles dye / liter

Figure 9A. Reflection spectra of 0.1M octahedral AgBr (Dispersion C of Reference 23) in 0.3% gelatin at 23°C, pBr 3, pH 6.5 with added 5,5'-dichloro-3,3'-9-triethu1r thiacarhocyanine bromide in millimoles per mole AgBr B. Dependence of J-band reflectivity at 640 n.m. on concentration of the thiacarbocyanine in the AgBr dispersion. See text for significance of K / S values C. Adsorption isotherms of the thiacarbocyanine in the octahedral AgBr dispersion. Open data points: Results calculated from surface spectra of Parts A and B. Solid data points: Results obtained from phase-separation procedure


14.

Adsorption

HERZ E T AL.

191

of Dyes

W h e r e a s the o p t i c a l m e t h o d i n d i c a t e s a t t a i n m e n t of s a t u r a t i o n coverage b y the d y e i n its /-state, results o b t a i n e d w i t h the phase-separation t e c h n i q u e s h o w that, after r e a c h i n g a p p a r e n t saturation, f u r t h e r a d s o r p t i o n occurs as the d y e c o n c e n t r a t i o n i n s o l u t i o n is increased. T h e h o r i z o n t a l step o b s e r v e d w i t h the phase separation m e a s u r e m e n t is i n a p p r o x i m a t e agreement w i t h the m a x i m u m surface c o n c e n t r a t i o n of d y e i n its /-state as d e t e r m i n e d b y the o p t i c a l m e t h o d . t r a r y to earlier suppositions

(70),

H e n c e , i t is c o n c l u d e d that, c o n

o n l y the first l a y e r of this d y e is


a d s o r b e d i n its / - s t a t e ; subsequent d y e layers m u s t b e a d s o r b e d i n d i f ferent states. Table I.

Adsorption Data for Cyanines on Silver Bromide

7 X 10~ M AgBr (Dispersion D) in 0.2% gelatin at pH = 6.5, pBr = 3 2

23°C,

Pseudocyanine Saturation Coverage ( m M Dye/mole AgX) Area per molecule ( A . ) Langmuir Coefficient K ( m M " ) A G ° (kcal./mole)

0.57 0.56* 57 3 X 10 -9.8

1

6

0.46° 0.47* 69 7 X 10 -8.8

s

2

a

Astraphloxin

3

2

Values obtained from phase-separation procedure. Values calculated from spectra of Figures 5 and 6.

Discussion O n c o m p a r i n g the P s e u d o c y a n i n e spectra of F i g u r e s 1, 2, 3, a n d 5 w i t h those o b t a i n e d i n c o n c e n t r a t e d aqueous d y e solutions (12, 58, 59),

56,

57,

obvious similarities of the r e s u l t i n g /-states w i l l be n o t e d . Present

a n d earlier d a t a m a k e it a p p a r e n t that electronic c o u p l i n g

between

adjacent d y e molecules c a n p r o d u c e s i m i l a r /-states, regardless of w h e t h e r a p p r o p r i a t e o r i e n t a t i o n a n d p r o x i m i t y of the molecules Coulombic

a t t r a c t i o n , as

i n salt f o r m a t i o n w i t h

w h e t h e r i t is i n d u c e d b y v a n der W a a l s forces.

is caused

polymeric

ions,

by or

T h e latter are p r i m a r i l y

i n v o l v e d i n the f o r m a t i o n of d y e aggregates i n w a t e r as w e l l as i n the c h a r g e - i n d e p e n d e n t a d s o r p t i o n of d y e m o n o l a y e r s at silver h a l i d e sur faces

(23,46).

T h e p r i n c i p a l s p e c t r a l feature of the /-state of P s e u d o c y a n i n e is the b a t h o c h r o m i c b a n d w h i c h , d e p e n d i n g o n the substrate, absorbs b e t w e e n 568-582 n . m . w i t h a n e x t i n c t i o n coefficient of 0.5-2.3 X 1 0 M 5

_ 1

cm.

- 1

and

a h a l f - w i d t h of less t h a n 50 c m . " . T h e s e values are i n r o u g h agreement 1

w i t h those o b t a i n e d i n aqueous gels of P s e u d o c y a n i n e at concentrations a b o v e 0 . 1 M (12).

I n a d d i t i o n to this intense a n d n a r r o w b a n d , the

/ - s t a t e also contains components w h i c h absorb w e a k l y at shorter w a v e -


192

ADSORPTION F R O M

lengths.

AQUEOUS SOLUTION

T h i s spectral s p l i t t i n g of the /-state has b e e n discussed

with

reference to c o u p l e d oscillators a n d to q u a n t u m t h e o r y b y F o r s t e r ; m o r e recently, M c R a e a n d K a s h a h a v e t r e a t e d this subject i n terms of m o l e c u l a r e x c i t o n m o d e l (16,

45).

the

T h e l o w - i n t e n s i t y transitions of the

/-state o v e r l a p the a b s o r p t i o n of u n p e r t u r b e d d y e i n s o l u t i o n ; t h e y are p o o r l y defined i n F i g u r e 1, w h e r e they a p p e a r as shoulders f r o m about 480-540 n . m .

extending

H o w e v e r , they are c l e a r l y r e s o l v e d i n the

spectra r e s u l t i n g f r o m i n t e r a c t i o n w i t h

appropriate anionic

polymers


a n d e x h i b i t d i s t i n c t s u b s i d i a r y m a x i m a of 500 a n d 535 n . m . ( F i g u r e 2, a n d Reference 1 ) .

T h e s e short-wave c o m p o n e n t s c a n also b e

observed

o n e x a m i n i n g the a n i s o t r o p i c b e h a v i o r of fibrous /-aggregates i n w a t e r , w h e r e t h e y a p p e a r near 500 a n d 540 n . m . w i t h the electric vector p e r p e n d i c u l a r to the fiber axis a n d p a r a l l e l to the longest d i m e n s i o n of the m o l e c u l e (25, 41, 56, 57, 58, 59).

S u c h h i g h - f r e q u e n c y bands w e r e f o u n d

to b e associated as a n i n t e g r a l p a r t of the /-state w i t h a l l cyanines w h i c h were examined. A p p a r e n t l y , it w a s l a r g e l y l a c k of r e c o g n i t i o n of these

short-wave

c o m p o n e n t s of the /-state w h i c h r e c e n t l y l e d P a d d a y a n d W i c k h a m to the c o n c l u s i o n that i n silver h a l i d e dispersions P s e u d o c y a n i n e is a d s o r b e d at s a t u r a t i o n coverage i n t w o d y e layers. It was s u p p o s e d t h a t a b s o r p t i o n i n the M - b a n d r e g i o n (540 n.m.) w a s c a u s e d o n l y b y a flat m o n o l a y e r of d y e c o v e r i n g the surface a n d that a second d y e l a y e r w a s b o u n d o n top of the first i n a n edge-on o r i e n t a t i o n w h i c h t h e n gave rise to t h e / - b a n d at 573 n . m . (50). figuration

T h e s e v i e w s c o n c e r n i n g a n a priori

unlikely dye con

h a v e r e c e i v e d no e x p e r i m e n t a l s u p p o r t f r o m earlier or present

evidence. roll (8),

H o w e v e r , i n agreement w i t h conclusions s u m m a r i z e d b y C a r present experiments have s h o w n that a d s o r p t i o n of

isolated

P s e u d o c y a n i n e c o u l d b e detected b y the appearance of a n M - b a n d o n l y a

u n d e r c o n d i t i o n s of v e r y l o w surface coverage. T h e M - b a n d at 543 n . m . a

i n F i g u r e 5 ( C u r v e a ) is b a r e l y e v i d e n t before the onset of d i s t i n c t / - b a n d f o r m a t i o n at h i g h e r d y e

concentrations

r e o r i e n t a t i o n of the d y e molecules

w h i c h presumably involves

(JO, 70).

a

T h e extent to w h i c h M a -

b a n d s f o r m appears to d e p e n d also o n the c r y s t a l h a b i t of t h e silver halide.

T h u s , c o n t i n u i n g the p r e v i o u s l y r e p o r t e d a d s o r p t i o n

ments of

Pseudocyanine

dispersions (23),

i n c r y s t a l l o g r a p h i c a l l y defined

measure

AgBr

gelatin

i t w a s f o u n d that, i n the c u b i c A g B r system, t h e d y e

was a d s o r b e d i n its M - s t a t e at coverages a p p r o a c h i n g 5 % of the surface a

before the /-state b e c a m e prevalent. O n the other h a n d , i n the o c t a h e d r a l A g B r d i s p e r s i o n n o M - b a n d c o u l d b e detected a n d o n l y the / - b a n d was a

observed.

T h i s d e p e n d e n c e of M - s t a t e c o n c e n t r a t i o n o n the c r y s t a l h a b i t a

of the substrate is b e l i e v e d to i n d i c a t e differences

i n their adsorption

forces. It is l i k e l y that, w h e r e substrate-dye i n t e r a c t i o n is strongest, the d y e w i l l be a d s o r b e d as isolated a n d p o s s i b l y flat molecules

(10,


70),

14.

HERZ E T AL.

Adsorption

of

Dyes

193

whereas l a t e r a l d y e - d y e i n t e r a c t i o n l e a d i n g to the /-state w i l l be f a v o r e d w h e n c o m p e t i n g forces f r o m the substrate are r e l a t i v e l y w e a k . A c c o r d i n g to this i n t e r p r e t a t i o n , the energy of a d s o r p t i o n of P s e u d o c y a n i n e at l o w coverages of A g B r s h o u l d be greater for the c u b i c t h a n for the o c t a h e d r a l substrate. T h e silver d i s p e r s i o n gave no i n d i c a t i o n that P s e u d o c y a n i n e

was

b o u n d i n the M - s t a t e at a n y surface coverage ( F i g u r e s I B a n d 3 )

and

a

i n this respect it b e h a v e d l i k e the o c t a h e d r a l A g B r substrate.

Although


c o l l o i d a l silver h a v i n g r e p r o d u c i b l e spectral properties w a s r e a d i l y p r e p a r e d b y the i n d i c a t e d p r o c e d u r e , these silver preparations are n e i t h e r homodisperse, n o r do they possess a w e l l - d e f i n e d surface. A m o r e s u i t a b l e silver substrate w a s p r e p a r e d b y d e p o s i t i n g silver o n g o l d n u c l e i

(48).

T h i s sol was p u r i f i e d b y d i a l y s i s ; i t h a d a n a r r o w size d i s t r i b u t i o n w i t h a n average

p a r t i c l e d i a m e t e r of 320 A . a n d o n c e n t r i f u g i n g gave a n

o p t i c a l l y clear supernatant l i q u i d .

U n d e r the same c o n d i t i o n s as those

d e s c r i b e d i n F i g u r e I B , P s e u d o c y a n i n e a g a i n gave a / - b a n d near 580 n . m . ; h o w e v e r , its m o l a r e x t i n c t i o n w a s t w i c e as h i g h as that s h o w n i n F i g u r e IB.

M o r e d e t a i l e d e x a m i n a t i o n of the i n t e r a c t i o n of this silver sol w i t h

the p-toluenesulfonate

salt of P s e u d o c y a n i n e b y b o t h the c e n t r i f u g a t i o n

and spectral procedure,

d e m o n s t r a t e d that a d s o r p t i o n of t h e d y e

and

/ - b a n d f o r m a t i o n o c c u r r e d i n the presence b u t not i n the absence of c h l o ride, bromide,

or

iodide.

Furthermore, if reducing

compounds

h y d r a z i n e or s i l v e r - c o m p l e x i n g agents s u c h as sulfite or c y a n i d e

like were

a d d e d to the system before the d y e , neither / - b a n d f o r m a t i o n n o r a d s o r p t i o n w a s detected, e v e n i n the presence of 1 0 " M or m o r e h a l i d e . 4

w e tentatively conclude

Hence,

that the a d s o r p t i o n sites for P s e u d o c y a n i n e

in

these dispersions are not silver b u t consist of silver h a l i d e . I n this c o n n e c t i o n i t is significant that p r e v i o u s l y r e p o r t e d a d s o r p t i o n of this d y e o n silver substrates w a s o b s e r v e d o n a d d i t i o n of h a l i d e salts of the d y e (19, 4 9 ) . Present d a t a i l l u s t r a t e the t e c h n i q u e for a n in situ d e t e r m i n a t i o n of surface areas. R e l a t e d methods h a d b e e n a p p l i e d p r i m a r i l y to the s t u d y of site d i s t r i b u t i o n s i n c l a y m i n e r a l s , p a r t i c u l a r l y b y R u s s i a n w o r k e r s (66), a n d they were used b y Bergmann a n d O ' K o n s k i i n a detailed inves t i g a t i o n of

the m e t h y l e n e

blue-montmorillonite

system

(3).

changes i n electronic spectra a r i s i n g f r o m surface interactions

In

fact,

received

sufficient a t t e n t i o n i n the past to w a r r a n t t h e i r r e v i e w b y A . T e r e n i n (65).

M o s t of these investigations i n v o l v e d t r a n s m i t t a n c e spectra

n e w techniques i n reflection s p e c t r o p h o t o m e t r y

but

a n d a p p l i c a t i o n s of the

K u b e l k a - M u n k r e l a t i o n h a v e f a c i l i t a t e d the q u a n t i t a t i v e e v a l u a t i o n of spectra i n h i g h l y t u r b i d m e d i a (35, 69, 7 7 ) .

T h u s , i n agreement

with

the w o r k of K o r t i i m o n p o w d e r s a n d a n h y d r o u s dispersions ( 3 1 , 32, 3 3 ) , o u r results demonstrate the a p p l i c a b i l i t y of the K u b e l k a - M u n k f u n c t i o n


194

ADSORPTION F R O M

AQUEOUS SOLUTION

to s t r o n g l y scattering aqueous silver h a l i d e s . T h e y also s h o w t h a t the p e r t u r b a t i o n of a dye's electronic transitions i n d u c e d b y its a d s o r p t i o n c a n y i e l d d i r e c t i n f o r m a t i o n o n surface coverages (24)

and allows con

clusions to b e m a d e about m o l e c u l a r o r i e n t a t i o n i n the d y e

monolayer.

F o r t h e p u r p o s e of m e a s u r i n g the surface area of a substrate b y s u c h s p e c t r a l changes, the use of P s e u d o c y a n i n e c o m m e n d s

itself.

N o t only

is the m o l e c u l a r area of this d y e reasonably w e l l k n o w n b u t the n a r r o w a n d intense t r a n s i t i o n associated w i t h its c l o s e - p a c k e d

adsorbed

state


does not d e p e n d strongly o n the c o m p o s i t i o n of the silver h a l i d e or its crystal habit.

M o r e o v e r , present results a n d earlier experiments

demonstrated

t h a t d y e a d s o r p t i o n measurements

with

have

Pseudocyanine,

either i n the absence or presence of p r o t e c t i v e c o l l o i d s , c a n y i e l d silver h a l i d e surface areas w h i c h are i n g o o d agreement p e n d e n t estimates

w i t h various inde

(9,23).

It r e m a i n s to be d e t e r m i n e d to w h a t extent the d y e

adsorption

t e c h n i q u e is a p p l i c a b l e to other substrates.

N o evidence was obtained

for P s e u d o c y a n i n e a d s o r p t i o n to M n 0 , F e 0

3

2

2

or to p u r e silver surfaces,

a l t h o u g h this d y e c a n b e b o u n d to m i c a , l e a d h a l i d e s , a n d m e r c u r y salts w i t h f o r m a t i o n of a / - b a n d (61).

N o t o n l y cyanines b u t other d y e classes

c a n y i e l d surface spectra w h i c h m a y b e s i m i l a r l y a n a l y z e d . T h i s is spe cifically the case w i t h the p h t h a l e i n a n d a z i n e dyes w h i c h w e r e

recom

m e n d e d b y F a j a n s a n d b y K o l t h o f f as a d s o r p t i o n i n d i c a t o r s i n p o t e n t i o m e t r i c titrations (15, 30).

T h e t e c h n i q u e s d e s c r i b e d are also c o n v e n i e n t

for d e t e r m i n i n g rates a n d heats of a d s o r p t i o n a n d surface concentrations of dyes; t h e y h a v e a l r e a d y f o u n d a p p l i c a t i o n i n studies of (18)

a n d electrophoresis

(68)

luminescence

of silver h a l i d e s as a f u n c t i o n of

dye

coverage. Sorption of

Meso-alkylcarbocyanines

A l t h o u g h w i t h P s e u d o c y a n i n e the existence of a /-state does not d e p e n d o n the silver halide's c r y s t a l structure, m e s o - a l k y l c a r b o c y a n i n e s c a n e x h i b i t r e m a r k a b l y different surface spectra o n c u b i c a n d o c t a h e d r a l silver b r o m i d e s (13,17).

T h e s e dyes are p a r t i c u l a r l y sensitive to v a r i a t i o n

i n s p a c i n g of lattice sites at ( 1 0 0 ) a n d ( 1 1 1 ) faces a n d to the different field forces e m a n a t i n g f r o m t h e m . T h u s , w i t h v a r i o u s l y c h a r g e d 9 - m e t h y l t h i a c a r b o c y a n i n e s (22, 23, 36),

f u r t h e r experiments at p B r 3-4 i n c u b i c A g B r

dispersions s h o w e d H- a n d B - b a n d s of essentially e q u a l i n t e n s i t y near 520 a n d 590 n . m . , respectively.

T h e s e b a n d s w e r e n e a r l y absent o n the

o c t a h e d r a l substrate w h i c h , i n s t e a d , c a u s e d f o r m a t i o n of a / - b a n d near 620 n . m . O f the v a r i o u s possibilities to account for these s p e c t r a l differ ences, there are at least three w h i c h w a r r a n t close c o n s i d e r a t i o n : ( a )

The

t h i a c a r b o c y a n i n e m a y b e a d s o r b e d either flat or o n its l o n g edge, d e p e n d -


14.

Adsorption

HERZ E T AL.

i n g o n the substrate,

(b)

of

195

Dyes

S t a c k i n g of d y e molecules o n t h e i r l o n g e d g e

m a y v a r y i n o r i e n t a t i o n s u c h that the n i t r o g e n atoms of the d y e c a n b e near the surface i n one case, whereas i n the other, s u l f u r w o u l d b e i n that p o s i t i o n . A l t e r n a t i n g orientations of this t y p e o c c u r r e d w i t h a t h i a c a r b o c y a n i n e (75, 76)

i n its c r y s t a l l i n e state, ( c )

D i f f e r e n t stereoisomers

of the d y e are a d s o r b e d o n the t w o substrates.

T h e existence of i n t e r

c o n v e r t i b l e stereoisomers strated

(60,

of 9 - a l k y l t h i a c a r b o c y a n i n e s has b e e n

a n d Zechmeister

74)

has r e p o r t e d

the


separation of a t h i a t r i c a r b o c y a n i n e i n t o s u c h isomers

demon

chromatographic (78).

O f these alternatives o n l y the first c a n n o w b e e l i m i n a t e d since the c a t i o n i c 9 - m e t h y l t h i a c o r b o c y a n i n e w a s f o u n d to o c c u p y the same l i m i t i n g area of ca. 60 A . p e r m o l e c u l e i n b o t h c u b i c a n d o c t a h e d r a l A g B r 2

(23).

F o r a c l o s e - p a c k e d m o n o l a y e r , this area r e q u i r e m e n t is consistent w i t h essentially v e r t i c a l a l i g n m e n t b u t is too s m a l l , b y a factor greater t h a n t w o , to a l l o w for h o r i z o n t a l o r i e n t a t i o n of the a d s o r b e d d y e .

However,

r e l a t i v e l y m i n o r v a r i a t i o n s i n t i l t angle of the m o l e c u l a r planes, e.g.,

50°

vs. 60° w o u l d p r o b a b l y not b e detectable b y these area d e t e r m i n a t i o n s . Y e t s u c h a n g u l a r d i s p l a c e m e n t s (14,45)

are e x p e c t e d to exert p r o n o u n c e d

effects o n surface spectra. H e n c e , i t is clear that sufficient d a t a are not yet a v a i l a b l e to f u l l y s u p p o r t a n e x p l a n a t i o n of s p e c t r a l properties

of

m e s o - s u b s t i t u t e d c a r b o c y a n i n e s w h i c h are b o u n d to s i l v e r h a l i d e s h a v i n g different c r y s t a l h a b i t s . Acknowledgments It is a pleasure to t h a n k H . E l i n s , for h a v i n g b r o u g h t to o u r a t t e n t i o n the i n t e r a c t i o n of P s e u d o c y a n i n e w i t h P V E - s u l f a t e ; F . G r u m , for assist ance i n the d e t e r m i n a t i o n of spectra; a n d J . H e l l i n g , for m u c h h e l p i n e a r l y a d s o r p t i o n measurements. Note T h e area d e t e r m i n a t i o n s b y d y e a d s o r p t i o n f r o m s o l u t i o n d i s c u s s e d here are a p p l i c a b l e to aqueous dispersions. A l t h o u g h s a t u r a t i o n c o v e r a g e of s i l v e r h a l i d e s b y P s e u d o c y a n i n e r e m a i n e d u n c h a n g e d i n 4 0 % m e t h a n o l b y v o l u m e , it is k n o w n that i n o r g a n i c solvents w h e r e i o n - p a i r s m a y b e a d s o r b e d , the m o l e c u l a r cross section of the c y a n i n e c a n v a r y w i t h the dye's anion—cf. Reference

23 for

d i s c u s s i o n a n d l i t e r a t u r e citations.

R e c e n t d e t e r m i n a t i o n s of A g l areas b y a d s o r p t i o n of P s e u d o c y a n i n e w e r e r e p o r t e d to h a v e b e e n u n r e a l i s t i c a n d salt-dependent ( v a n d e n H u l , H . J . , L y k l e m a , J . , / . Phys.

Chem.

9 0 , 3010 ( 1 9 6 8 ) ) .

A l i k e l y reason f o r this

result is the c i r c u m s t a n c e that these investigators c a r r i e d out t h e i r m e a surements i n a l c o h o l dispersions of the substrate w h e r e the c i t e d solventdependent limitations w o u l d apply.


196

ADSORPTION F R O M


Literature

AQUEOUS

SOLUTION

Cited

(1) A p p e l , W . , Scheibe, G . , Z . Naturforschg. 13b, 359 (1958). (2) Bean, R. C., Shepherd, W . C., K a y , R. E., W a l w i c k , E. R., J. Phys. Chem. 6 9 , 4 3 6 8 (1965). (3) Bergmann, K . , O'Konski, C., J. Phys. Chem. 67, 2169 (1963). (4) Borginon, H., Danckaert, V., Phot. Korr. 98, 74 (1962). (5) Boyer, S., Cappelaere, J . , J. Chim. Phys. 60, 1123 (1963). (6) Boyer, S., Pinchon, L., Degove, O., J. Chim. Phys. 60, 301 (1965). (7) Brooker, L. G . S., Keyes, G . H., J. Am. Chem. Soc. 57, 2488 (1935). (8) Carroll, B . H., Phot. Sci. Eng. 5, 65 (1961). (9) Curme, H. G . , Natale, C . C., J. Phys. Chem. 68, 3009 (1964). (10) D a v e y , E. P . , Trans. Faraday Soc. 36, 323 (1940). (11) Dickinson, H. O., Nature 163, 485 (1949). (12) Ecker, H., Koll. Z. 92, 35 (1940). (13) Eggers, J . , Gunther, E., Moisar, E . , Phot. Korr. 102, 144 (1966). (14) Emerson, E., C o n l i n , M., Rosenoff, A . , Norland, K . , Rodriguez, H., C h i n , D . , B i r d , G . , J. Phys. Chem. 7 1 , 2396 (1967). (15) Fajans, K., " N e w e r Methods of Volumetric Chemical Analysis," trans, by R. Oesper, D . V a n Nostrand C o . , N e w York, 1938. (16) Förster, T h . , "Fluoreszenz Organischer Verbindungen," p. 256 ff, V a n denhoeck and Ruprecht, Goettingen, 1951. (17) Frieser, H., Graf, A., Eschrich, D . , Z . Electrochem. 65, 870 (1961). (18) G i l m a n , P. C., Phot. Sci. Eng. 11, 222 (1967). (19) Groszek, A., W o o d , H., J. Phot. Sci. 13, 133 (1965). (20) Gunther, E., Moisar, E., J. Phot. Sci. 13, 280 (1965). (21) Hamer, F. M., J. Chem. Soc. 1928, 206. (22) H e r z , A . H., H e l l i n g , J . O., J. Colloid Sci. 17, 293 (1962). (23) H e r z , A. H., H e l l i n g , J . O., J. Colloid Interface Sci. 22, 391 (1966). (24) H e r z , A . H., H e l l i n g , J . O., Koll. Z. and Z. Polymere 218, 157 ( 1 9 6 7 ) . (25) H o p p e , W., Koll. Z. 109, 27 (1944). (26) Jelley, E. E., Nature 138, 1009 (1936). (27) Ibid., 139, 631 (1937). (28) K a y , R. E., W a l w i c k , E . R., Gifford, C . K., J. Phys. Chem. 68, 1896 (1964). (29) Ibid., 68, 1907 (1964). (30) Kolthoff, I. M., Chem. Rev. 16, 87 (1935). (31) Kortüm, G . , Vogel, J . , Chem. Ber. 93, 706 (1960). (32) Kortüm, G . , Oelkrug, D . , Z . Physik. Chem. (Frankfurt) 34, 58 (1962). (33) Kortüm, G . , Braun, W . , Z . Physik. Chem. (Frankfurt) 48, 282 (1966). (34) Kortüm, G . , Oelkrug, D., Naturwiss. 53, 600 (1966). (35) Kottler, F . , "Progress i n Optics," V o l . III, p. 1, E . W o l f , ed., North Holland Publishing C o . , Amsterdam, 1964. (36) K r a g h , A . , Peacock, R., Reddy, G . , J. Phot. Sci. 14, 185 (1966). (37) L i e u , V., F r o d y m a , M., Talanta 13, 1319 (1966). (38) M a r k o c k i , W . , J. Phot. Sci. 13, 85 (1965). (39) Mason, S. F., Proc. Chem. Soc. 1964, 119. (40) Mason, S. F., "Optische Anregung Organischer Systeme," p. 143, 2nd. Intern. Farbensymposium, Verlag Chemie, W e i n h e i m , Germany, 1966. (41) Mattoon, R. W . , J. Chem. Phys. 12, 263 (1944). (42) M c K a y , R. B . , Hillson, P. J., Trans. Faraday Soc. 6 1 , 1800 (1965). (43) Ibid., 62, 1439 (1966). (44) Ibid., 63, 777 (1967). (45) M c R a e , E., Kasha, M., "Physical Processes i n Radiation Biology," p. 23 ff., Academic Press, N e w York, 1964. (46) Mees, C . E . K., James, T . H., " T h e Theory of the Photographic Process," 3rd. ed., Chapt. 12, The M a c m i l l a n C o . , N e w York, 1966.


14.


(47) (48) (49) (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63) (64) (65) (66) (67) (68) (69) (70) (71) (72) (73) (74) (75) (76) (77) (78)

HERZ E T AL.

Adsorption

197

of Dyes

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9, 16 (1953).

1967.


Adsorption of Dyes and Their Surface Spectra

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