POLARIZATION XAFS STUDY OF IONS AT SOLUTION SURFACE | SPring-8/SACLA Research Frontiers

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S o a p , w h e n d i s s o l v e d i n w a t e r , e a s i l y f o r m s b u b b l e s e v e n a t m i l l i m o l a r c o n c e n t r a t i o n s . S u c h b u b b l e s s u r r o u n d t h e i n c l u d e d a i r w i t h a monolayer of soap molecules, as seen for the large fatty acid sodium salt illustrated in Fig. 1 . Soap, a typical surfactant, has a strong tendency to come to th e su rf ac e of an aq ue ou s so lu ti on , re su lt in g in a h i g h l y c o n c e n t r a t e d s u r f a c e d e n s i t y . S u r f a c t a n t m o l e c u l e s n o r m a l l y p o s s e s s a l o n g h y d r o c a r b o n chain with an ionic head. The hydrophobicity of the long hydrocarbon makes the surfactant insoluble in w a t e r , s t i c k i n g t o o i l y s u b s t a n c e s i n s t e a d ; t h e h y d r o p h i l i c i t y o f t h e i o n i c g r o u p f o r c e s t h e surfactant molecule adhere tightly to the surface of the aqueous solution. The molecular packing of the surfactant at the s o l u t i o n s u r f a c e d e p e n d s o n t h e i d e n t i t y a n d c o n c e n t r a t i o n o f t h e c o u n t e r i o n ; f o r f a t t y a c i d s o a p s , u s e o f m e t a l i o n s , s u c h a s c a l c i u m ( I I ) , cadmium( II ) or zinc( II ), as the counter ion assist in POLARIZATION XAFS STUDY OF IONS AT SOLUTION SURFACE Fig. 1. A schematic model for stearate monolayer structure at the aqueous solution surface. t h e f o r m a t i o n o f a s o l i d m o n o m o l e c u l a r f i l m ( t h e L a n g m u i r m o n o l a y e r ) a t t h e s o l u t i o n s u r f a c e . Although the structure varies between the ions, well o r g a n i z e d s t r u c t u r e s i n c l u d i n g b o t h t h e f a t t y a c i d and the cations must form at the solution surface. As information concerning these surface structures ar e in te re st in g to co ll oi d an d so lu ti on ch em is ts in addition to being important in the d e t e r g e n t i n d u s t r y , a l a r g e n u m b e r o f t h e r m o d y n a m i c a n d spectroscopic studies have attempted to elucidate this structure. S u c h s t u d i e s h a v e u t i l i z e d l a s e r d r i v e n s u m - f r e q u e n c y - g e n e r a t i o n a n d s e c o n d - h a r m o n i c - gen era tio n spe ctr osc opy to add res s thi s que sti on, s i n c e t h e s e t e c h n i q u e s a r e s t r i c t l y s u r f a c e sensitive. None of these attempts, however, have yet been able to give the coordination structures of the metal ions at the Langmuir monolayer. Unlike the available methods to study solid surfaces, few te ch ni qu es ex is t th at ar e su it ab le fo r th e st ud y of liquid surfaces. Fig. 2. Total-reflection polarization-XAFS system for the study of solution surfaces at beamline BL39XU. W e a r e d e v e l o p i n g a m e t h o d o f X - r a y a b s o r p t i o n s p e c t r o s c o p y t o a l l o w t h e s t u d y o f solution surfaces. A monochromatized X-ray beam is in tr od uc ed on to th e so lu ti on su rf ac e at a sm al l gla nci ng inc ide nce ang le to all ow the det ect ion of s u r f a c e e l e m e n t s o n l y , u n d e r t o t a l - r e f l e c t i o n conditions. A high brilliance and small divergence X-ray beam deriving from the undulator at beamline BL39XU is most appropriate for this application. I n a d d i t i o n , t h e s o l u t i o n s u r f a c e i s a t w o - dimensional reaction field; the geometry of ions at the surface may preferentially orient with respect to t h e l i q u i d s u r f a c e . T o d e s c r i b e t h e c o o r d i n a t i o n s t r u c t u r e s o f i o n s a t t h e s o l u t i o n s u r f a c e b y t h e F i g . 3 . ( a ) M n K - e d g e X A N E S s p e c t r a f o r t h e M n ( I I ) - s t e a r a t e m o n o l a y e r a t t h e a q u e o u s s o l u t i o n surface. ( b ) T h e f i r s t d e r i v a t i v e X A N E S spectra for Mn K-edge. Photon Energy ( eV ) d μ μ t /dE ( arb. units) 6540 6550 6560 -0.4 -0.2 0 0.2 0.4 bulk vertical horizontal (b) Derivative XANES spectra Photon Energy ( eV ) 2 1 0 6545 6550 6555 Absorbance Mn K -edge XANES transmission PF / BL7C vertical polarization SPring-8 / BL39XU horizontal polarization SPring-8 / BL39XU horizontal polarization PF / BL7C (a) XAFS method, we require l i n e a r l y p o l a r i z e d X - r a y s b o t h p a r a l l e l a n d n o r m a l t o t h e s u r f a c e . A s t h e l i q u i d s u r f a c e c a n n o t b e turned upright, a beamline c a p a b l e o f s u p p l y i n g X - r a y s p o l a r i z e d b o t h h o r i z o n t a l l y a n d v e r t i c a l l y to the surface is essential f o r t h e s e m e a s u r e m e n t s . undulator monochromator phase retarder mirror solution surface Fig. 5. The Fourier-transformed EXAFS spectra for the Zn ( II ) -stearate monolayer at the aqueous solution surface. Fig. 4. The prop osed coor dinat ion stru cture for t h e M n ( I I ) - s t e a r a t e m o n o l a y e r a t t h e a q u e o u s s o l u t i o n s u r f a c e . T h e p o l a r i z a t i o n d i r e c t i o n s correspond to the spectra in Fig. 3, respectively. Horizontal Vertical r (Å) FT amplitude 0.6 0.4 0.2 0 0 2 4 6 8 The phase retarder at BL39XU, originally installed f o r X - r a y m a g n e t i c c i r c u l a r d i c h r o i s m ( X M C D ) studies, is also an ideal component for the present study, capable of quickly converting the horizontally polarized X-rays into vertically polarized beams. T h e X - r a y s f r o m t h e m o n o c h r o m a t o r p a s s t h r o u g h a d i a m o n d c r y s t a l p h a s e r e t a r d e r t o b e reflected by a silicon mirror ( Fig. 2 ). The mirror is t i l t e d a t a n a n g l e d e s i g n e d t o d e f l e c t t h e b e a m h o r i z o n t a l l y a t a n a n g l e l a r g e e n o u g h t o r e m o v e h i g h e r h a r m o n i c s f r o m t h e d o u b l e - c r y s t a l m o n o c h r o m a t o r ; t h e b e a m i s a l s o d e f l e c t e d v e r t i c a l l y a t 1 m r a d , t h e t o t a l - r e f l e c t i o n i n c i d e n c e angle at the solution surface. The solution cell, described in a previous report [ 1 ] , c o n t a i n s a s a m p l e s o l u t i o n w i t h 1 m m o l / d m 3 M n ( I I ) i o n . T h e s o l u t i o n s u r f a c e i s c o v e r e d b y a monolayer of stearic acid ( CH 3 ( CH 2 ) 16 COOH ). We obt ain ed the Mn K -ed ge X-r ay abs orp tio n spe ctr a using two linearly polarized X-rays ( Fig. 3(a) ). The vertical polarization spectrum possesses a shoulder a t t h e e d g e ; t h e h o r i z o n t a l s p e c t r u m d o e s n o t , clearly demonstrated in the first derivative spectra ( Fi g . 3 (b ) ). In o u r e va l u a ti o n o f th e tr a n sm i ss i o n s p e c t r a f o r b u l k M n ( I I ) s o l u t i o n s , t h e s h o u l d e r structure or double peaks in the first derivative are c h a r a c t e r i s t i c o f s u c h b u l k s o l u t i o n s . A s t h e coordination structure of the Mn ( II ) ion in the bulk s o l u t i o n i s o c t a h e d r a l c o n t a i n i n g s i x c o o r d i n a t i n g water molecules, the coordination structure on the v e r t i c a l a x i s m u s t b e s i m i l a r t o t h a t o f b u l k o n e . S t r u c t u r e s i n t h e h o r i z o n t a l p l a n e m u s t d i f f e r f r o m t h e b u l k s t r u c t u r e . W e p r e s e n t a candidate model for such a structure ( Fig. 4 ). For a Zn ( II ) -containing solution covered by a m o n o l a y e r o f s t e a r i c a c i d , t h e t w o polarization XANES spectra at Zn K -edge are s i m i l a r , y e t d i f f e r f r o m t h e b u l k o n e . T h e s p e c t r a c l e a r l y i n d i c a t e t h a t Z n ( I I ) a d o p t s t e t r a h e d r a l s y m m e t r y a t t h e L a n g m u i r m o n o l a y e r , i n c o n t r a s t t o t h e o c t a h e d r a l s t r u c t u r e o b s e r v e d i n t h e b u l k [ 2 ] . C a r e f u l inspection of the EXAFS spectra indicated that a diff eren ce betw een two pola riza tion EXAF S at around 5 Å ( Fig. 5 ); a small peak appears in the horizontal spectrum only. This peak may c o r r e s p o n d t o t h e Z n - Z n d i s t a n c e i n t h e Fig. 6. The proposed coordination structure for the Zn ( II ) -stearate monolayer at the aqueous solution surface. Iwao Watanabe Osaka University E-mail: watanabe @ chem.sci.osaka-u.ac.jp References [ 1 ] I . W a t a n a b e , H . T a n i d a , S . K a w a u c h i , M . H a r a d a a n d M . N o m u r a , R e v . S c i . I n s t r u m . 6 8 (1997) 3307. [ 2 ] I . W a t a n a b e e t a l . , J . A m . C h e m . S o c . 1 1 9 (1997) 12018. Langmuir monolayer. A distance of approximately 5 Å correlates with that found for zinc acetate in an unh ydr ate d cry sta l, a str uct ure ado pti ng a lay ere d conformation. Each layer accommodates zinc ions on the same plane, connected by acetate bridges of a 4.8 Å distance. We outline our proposed model for the Zn-stearate Langmuir monolayer in F i g . 6 . This new spectroscopic method is useful for the a n a l y s i s o f l i q u i d s u r f a c e s , p r o v i d i n g i n f o r m a t i o n previously thought to be impossible. 5 Å solution air