SURFACE-ENHANCED RAMAN STUDIES OF THE
LIQUID/SOLID INTERFACE
S. Garoff, C. Sandroff
To cite this version:
S. Garoff, C. Sandroff.
SURFACE-ENHANCED RAMAN STUDIES OF THE LIQUID/SOLID INTERFACE. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-483-C10486. <10.1051/jphyscol:19831097>. <jpa-00223555>
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JOURNAL DE PHYSIQUE
Colloque C10, supplkment au n012, Tome 44, dkcembre 1983
page Cl0-483
S . Garoff and C . 3 . Sandroff
Exxon Research and Engineering Company, P.O. BOX 45, Linden,
New Jersey 07036, U.S.A.
-.Resume
La d i f f u s i o n Raman e x a l t e e de s u r f a c e (SERS) pouvant d e t e c t e r des monocouches adsorbees sans i n t e r f 6 r e n c e avec l e s m o l & c u l e s des phases a d j a centes, e l l e c o n s t i t u e un o u t i l s p e c t r o s c o p i q u e i d e a l p o u r e t u d i e r l a s t r u c t u r e des adsorbats d l ' i n t e r f a c e l i q u i d e / s o l i d e e t pour r e l i e r c e t t e s t r u c t u r e aux p r o p r i @ t e s macroscopiques a l ' i n t e r f a c e .
Vous avons u t i l i s e l e SERS p o u r examiner l a c o n f o r m a t i o n m o l e c u l a i r e d ' u n
sdf-factant
un a l k y l t h i o l
longue chaine
s u r une couche d ' A r g e n t granul a i r e . Dans l e cas oG l e s s u b s t r a t s r e c o u v e r t s de s u r f a c t a n t s o n t en c o n t a c t
avec l ' a i r ou avec un f l u i d e non m o u i l l a n t , t o u t e l a c h a i n e a l k y l du s u r f a c t a n t prend une c o n f o r m a t i o n t r a n s , seinblable a l a c o n f o r m a t i o n du t h i o l dans
l e s o l i d e p u r . Cependant, q u z n f l u i d e m o u i l l a n t e n t r e en c o n t a c t avec l e
s u b s t r a t r e c o u v e r t du s u r f a c t a n t , l e s carbones proches de l a s u r f a c e g a r d e n t
l e u r c o n f o r m a t i o n t r a n s t a n d i s que l e s carbones p l u s e l o i g n e s m o d i f i e n t l e u r
une l i a i s o n p l u s gauche. Ces m o d i f i c a t i o n s de
c o n f o r m a t i o n e t pr-ent
c o n f o r m a t i o n suggPrent que l e s f l u i d e s mouil-penPtrent
dans l e s couches
de s u r f a c t a n t adsorbees,contrairement aux f l u i d e s non m o u i l l a n t s q u i e n
s o n t e x c l u s p a r hydrophobie.
-
-
A b s t r a c t S i n c e surfaced-enhanced Raman s c a t t e r i n g (SERS) can d e t e c t adsorbed monolayers w i t h o u t i n t e r f e r e n c e f r o m molecules i n a d j a c e n t b u l k
phases, i t i s an i d e a l s p e c t r o s c o p i c t o o l f o r s t u d y i n g a d s o r b a t e s t r u c t u r e
a t t h e l i q u i d / s o l i d i n t e r f a c e and f o r r e l a t i n g adsorbate s t r u c t u r e t o
macroscopic p r o p e r t i e s a t t h e i n t e r f a c e .
-
We have used SERS t o examine t h e m o l e c u l a r c o n f o r m a t i o n o f a s u r f a c t a n t
a long chain alkyl thiol-on a silver-island film.
For s u r f a c t a n t coated
substrates i n contact w i t h a i r o r a nonwetting f l u i d , t h e e n t i r e a l k y l
c h a i n o f t h e s u r f a c t a n t t a k e s on an a l l t r a n s conformation, s i m i l a r t o t h e
However, when a w e t t i n g
conformation o f t h e t h i o l i n t h e n e a t m d .
f l u i d c o n t a c t s t h e s u r f a c t a n t c o a t e d s u b s t r a t e , t h e carbons near t h e s u r f a c e m a i n t a i n t h e i r t r a n s c o n f o r m a t i o n w h i l e carbons f a r t h e r f r o m t h e
s u r f a c e a l t e r t h e i r c o n f o r m a t i o n and e x h i b i t more gauche bonding.
Such
c o n f o r m a t i o n a l changes suggest t h a t w e t t i n g f l u i d s p e n e t r a t e adsorbed s u r f a c t a n t l a y e r s as opposed t o n o n w e t t i n g f l u i d s which a r e h y d r o p h o b i c a l l y
excluded.
The w e t t a b i l i t y o f a s o l i d i s j u s t one example o f a macroscopic p r o p e r t y t h a t can be
Amphiphil i c molecules, p o s s e s s i n g a p o l a r
g r e a t l y a f f e c t e d by m o l e c u l a r adsorbates.
head group a t t a c h e d t o a long, n o n p o l a r t a i l , a r e p r o b a b l y t h e most common s u r f a c e a c t i v e agents used t o a l t e r w e t t a b i l i t y and s u r f a c e t e n s i o n s a t t h e l i q u i d / s o l i d
interface.
To what e x t e n t i s a d s o r b a t e s t r u c t u r e and c o n f o r m a t i o n a f f e c t e d by t h i s
f l u i d c o n t a c t and how might such m i c r o s c o p i c i n f o r m a t i o n p r o v i d e i n s i g h t i n t o ' t h e
l a r g e - s c a l e processes o c c u r r i n g a t t h e 1 i q u i d / s o l i d i n t e r f a c e ?
We e x p l o r e such
q u e s t i o n i n t h e p r e s e n t surface-enhanced Raman s c a t t e r i n g (SERS) study1.
The a m p h i p h i l e used h e r e was 1-hexadecane t h i o l ( C H SH, h e r e a f t e r HXDSH), w h i c h
was f o u n d t o s t r o n g l y chemisorb t o s i l v e r surfaces. l6~E?F?Ss p e c t r a were o b t a i n e d f r o m
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831097
C 10-484
JOURNAL DE PHYSIQUE
HXDSH adsorbed on s i l v e r - i s l and f i l m s whos9 production, morphology and o p t i c a l
HXnSH was deposited onto t h e f i l m s
p r o p e r t i e s have been t h o r o ghly c h t r a c t e r i z e d
f rom d i l u t e s o l u t i o n s (lo-' t o 10- M ) o f t h e t h i o l i n chloroform o r o t h e r v o l a t i l e
solvents.
The HXDSH-containing s o l u t i o n remained i n c o n t a c t w i t h t h e i s l a n d f i l m
f o r -15s a f t e r which time t h e excess s o l u t i o n was removed by spinning t h e s u b s t r a t e
f o r 30s a t 2000 RPM, l e a v i n g behind a t h i n , even c o a t i n g o f adsorbed HXDSH.
The
e x c i t i n g 488.0nm l i g h t was l i n e - f o c u s e d o n t o a s p i n n i n g sample and an o p t i c a l m u l t i channel analyzer was used f o r f a s t a c q u i s i t i o n o f t h e SERS spectra.
.
A SERS spectrum o f t h e C-S s t r e t c h i n g r e g i
t h e neat l i q u i d and s o l i d t h i o l .
The HXDSH
i s shown i n F i g u r e 1 and compared t o
almost c e r t a i n l y bound t o t h e s i l v e r
L
."A
I
Solid
I
I
-4
I
-2
Log (concentration)
I
I
Bare ' *
1
I
-4
I
I
I
-2
Log (concentration)
F i g u r e 1. Carbon-sulfur s t r e t c h i n g
r e g i o n i n HXDSH (a) l i q u i d , (b)
s o l i d , ( c ) adsorbed.
( 6 ) and ( T )
are gauche and t r a n s conformations.
F i g u r e 2.
(a) Frequency o f C-S v i b r a t i o n as a f u n c t i o n o f coverage.
E r r o r s bars represent v a r i a t i o n i n
frequency found i n d i f f e r e n t samples. (b) Contact angle (measured i n
a i r ) f o r water on HXDSH coated f i l m s
as a f u n c t i o n o f coverage.
surface v i a i t s s u l f u r atom, as has been observed f o r many o t h e r organo-sulfur
compounds3. The ab ence o f t h e d i s t i n c t i v e S-H s t r e t c h and t h e appearanc o f a weak
s i g n a l a t -260 c i ' suggestive o f t h e Ag-S s t r e t c h i n bulk a l k y l t h i o l s g l e n d supp o r t t o t h i s claim.
The C-S s t r e t c h i ~region of t h e adsorbed HXDSH i s dominated by a s i n g l e i n t e n s e
band near 700 cm- , and comparison o f t h e SERS spectrum t o t h e Raman spectra o f t h e
neaf t h i o l r e v e a l s a c l o s e k i n s h i p between adsorbed and s o l i d HXDSH where t h e 700
cm- band dominates over t h e o t h e r C-S f e a t u r e near 650 cm-l.
S t r e t c h i n g frequenc i e s of t h e C-S bond are a c u t e l y s e n s i t i v e t o molecular conformatin near t h e s u l f u
head group: t h e 650 cm-I s r e t c h o r i g i n a t e s from gauche rotame s and t h e 700 cms t r e t c h from t r a n s rotamers
Hence, t h e preponderant 700 c i E band i n t h e SERS
spectrum o f f e r s evidence t h a t t h e conformation of t h a t p a r t o f t h e HXnSH molecule
S.
F
F i g u r e 4. E f f e c t s o f f l u i d over1ayers on t h e carbon-carbon s t r e t c h i n g region. HXnSH i n c o n t a c t w i t h
(a) a i r , (b) water, ( c ) chloroform.
( 6 ) and (T) as i n Figure 2.
F i g u r e 3. Carbon-carbon s t r e t c h i n g r e g i o n i n HXDSH (a) 1i q u i d ,
(b) s o l i d , (c) adsorbed.
(G)
and (T) as i n F i g u r e 2.
n e a r e s t t h e s u l f u r atom, and t h e r e f o r e nearest t h e surface, i s l a r g e l y t r a n s , remiWhile t h e SERS spectrum always shows a s i n g l e
n i s c e n t of HXDSH i n t h e s o l i d phase.
i n t e n s e band i n t h e C-S s t r e t c h i n g region, t h e p o s i t i o n o f t h i s band s h i f t s t o lower
The 30 cP-' f r e q u y c y decrease
frequency as coverage i s decreased ( F i g u r e 2a).
observed when going f r o m HXDSH s o l u t i o n c o n c e n t r a t i o n s o f 10- M t o 10- M i s due t o
a decreasing i n t e r a c t i o n between adjacent adsorbed HXDSH molecules, and i s s i m i l a r
t o v i r a t i o n a l s h i f t s observed f o r CO on n i c k e l surfaces i n u l t r a vacuum conditions
The macroscopic w e t t i n g behavior o f t h e HXDSH coated i s l a n d f i l m a l s o
As shown i n F i g u r e 2b, as t h e s o l u t i o n concentrachanges w i t h adsorbate coverage.
t i o n decreases, t h e adsorbate coverage decreases and t h e r e f o r e t h e surface changes
f r o m t h e very hydrophobic s t a t e a t h i g h coverages tcward t h e w a t e r wet s t a t e o f t h e
uncoated i s l a n d f i l m .
8.
I n c o n t r a s t t o t h e C-S s t r e t c h i n g r e g i o n which conveys s t r u c t u r a l i n f o r m a t i o n from
t h a t p o r t i o n of t h e HXDSH molecule very near t h e s i l v e r surface, t h e C-C s t r e t c h i n g
r e g i o n ( s h w n i n F i g u r e 3) conveys conformational d e t a i l s from t h e more d i s t a n t
JOURNAL DE PHYSIQUE
ClO-486
The SERS spectrum w i t h i t s i n t e n s e 1130 cm-l band and weaker 1090
c a r on skeleton7.
c m neighbor - i n d i c a t i v e o f a l a r g e number of t r a n s bonds w i t h gauche bonds
suggests t h a t t h e adsorbed HXDSH
appearing a t t h e ends o f l o n g t r a n s segments
l a y e r has a " s o l i d - l i k e " s t r u c t u r e , extending beyond t h e r e g i o n near t h e surfacebound s u l f u r and i n t o t h e hydrocarbon t a i l .
-
To e x p l o r e hcw adsorbed molecules were a f f e c t e d by c o n t a c t w i t h o v e r l y i n g bulk
f l u i d s , we recorded t h e SERS spectrum o f HXDSH-coated i s l a n d f i l m s i n c o n t a c t w i t h
various neat l i q u i d s .
The e f f e c t s of some o v e r l y i n g neat l i q u i d s as r e f l e c t e d i n
Contact
t h e C-C s t r e t c h i n g r e g i o n o f t h e SERS spectrum a r e d i played i n F i g u r e
w i t h b u l k water caused no change i n t h e t r a n s (1130 cm- )/gauge (1090 cm ) r a t i o o f
adsorbed HXDSH.
The " s o l i d - 1 i k e " s t r u c t u r e of t h e adsorbed l a y e r i n f e r r e d from
spectroscopic a n a l y s i s o f t h e C-S and C-C s t r e t c h i n g regions i m p l i e s s i z e a b l e
adsorbate-adsorbate i n t e r a c t i o n and t h e r e f o r e c l o s e p h y s i c a l p r o x i m i t y of adsorbed
HXDSH molecules.
We b e l i e v e t h e u n a l t e r e d t r a n s / auche r a t i o upon c o n t a c t w i t h
o f adsorbed HXOSH and
water can be a s c r i b e d t o t h e f o r m a t i o n o f a we&ayer
t h e hydrophobic e x c l u s i o n o f water by t h e a l k y l chain.
-4.
f
I n c o n t r a s t t o t h e p o l a r water overlayer, nonpolar chloroform i n contact w i t h
adsorbed HXDSH c l e a r l y reduces t h e number o f t r a n s segme t s as i n f e r r e d from t h e
decrease i n t h e i n t e n s i t y o f t h e " t r a n s marker" a t 1130 cm-I r e l a t i v e t o t h e band a t
1090 cm-I s e n s i t i v e t o gauche c o n f x i o n s .
The e f f e c t o f t h e c h l o r o f o r m o v e r l a y e r
was found t o be r e v e r s i b l e ; and when t h e o v e r l a y e r was removed, t h e SERS spectrum o f
t h e HXDSH r e v e r t e d t o i t s o r i g i n a l , i n - a i r appearance.
Interestingly, while t h e
appearance of t h e SERS spectrum i n t h e C-C r e g i o n was d r a m a t i c a l l y altered, t h e C-S
r e g i o n remained unchanged, s t i l l dominated by a s i n g l e i n t e n s e t r a n s s t r e t c h near
Given t h e adsorbate c o n f i g u r a t i o n discussed e a r l i e r , t h i s i m p l i e s t h a t
700 cmt h e e f f e c t of t h e l i q u i d chloroform o v e r l a y e r i s c o n f i n e d t o t h a t region o f t h e
HXDSH molecule r e l a t i v e l y d i s t a n t from t h e metal surface.
Thus, w h i l e t h e long,
carbon t a i l of HXOSH becomes more " l i q u i d - l i k e " when i n c o n t a c t w i t h chloroform, t h e
carbons nearer t h e surface-bound s u l f u r r e t a i n t h e i r " s o l i d - l i k e " character.
.
A s i m i l a r decrease i n t h e number o f t r a n s conformers i s observed i n Paman s p e c t r a o f
d i p a l m i t o y l phosphatidycholine d i s p e r s i o n s i n water as t h e dispersed amphiphile
undergoes phase t a n s i t i o n s between l a m e l l a r phases w i t h decreasing order i n t h e
hydrocarbon t a i l s
We conclude, therefore, t h a t t h e e f f e c t o f t h e c h l o r o f o r m
o v e r l a y e r i s t o f a c i l i t a t e d i s o r d e r i n g o f t h e l o n g a l k y l chain, perhaps through
impregnation by t h e nonpolar s o l v e n t molecul es.
f.
Visual observation o f small drops of chloroform and water on HXDSH-coated i s l a n d
It
f i l m s makes c l e a r t h a t chloroform more f u l l y wets t h e surface than does water.
i s tempting t h e r e f o r e , t o a s s o c i a t e t h e a b i l i t y of a l i q u i d t o wet a s u r f a c t a n t f i l m
w i t h conformation changes induced i n t h e amphiphile by t h e w e t t i n g l i q u i d .
Indeed,
a canvassing o f d i f f e r e n t o v e r l y i n g l i q u i d s (hexane, cyclohexane and carbon
t e t r a c h l o r i d e ) i s c o n s i s t e n t w i t h t h i s idea; each o f these l i q u i d s wetted HXDSHcoated i s l a n d f i l m s and a l s o caused a decrease i n t h e SERS i n t e n s i t y o f t h e " t r a n s
marker".
In t h e mating o f novel microscopic probes ( l i k e SERS) w i t h t r a d i t i o n a l
probes o f l a r g e - s c a l e s u r f ace p r o p e r t i e s (1 ike c o n t a c t angle measurements), we hope
t o g a i n i n s i g h t i n t o fundamental processes a t t h e 1 i q u i d l s o l i d i n t e r f a c e .
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