Vibrational study of the salicylate interaction with metallic ions and surfaces

Article abstract of DOI:10.1016/S1386-1425(00)00328-0

Infrared and Raman spectroscopy are used in this work to study the metallic complexes of salicylic acid with silver and copper, comparing the interaction between salicylate and the cations (Ag⁺ and Cu²⁺) in the metal complexes with the SERS spectra when adsorbed on colloidal metal surfaces of the same metals. The salicylate complexes with the above metals were compared to those of Na⁺, Fe³⁺ and Al³⁺ cations. A different interaction mechanism is deduced for salicylate in the metal complex and when adsorbed on the metal surface.

Full text of DOI:10.1016/S1386-1425(00)00328-0

Spectrochimica Acta Part A 56 (2000) 2471–2477
www.elsevier.nl/locate/saa
Vibrational study of the salicylate interaction with metallic
ions and surfaces
M.C. Alvarez-Ros, S. Sa´nchez-Corte´s *, J.V. Garc´ıa-Ramos
Instituto de Estructura de la Materia, CSIC, Serrano, 121, 28006 Madrid, Spain
Received 8 May 2000; accepted 8 June 2000
Abstract
Infrared and Raman spectroscopy are used in this work to study the metallic complexes of salicylic acid with silver
and copper, comparing the interaction between salicylate and the cations (Ag+ and Cu2+) in the metal complexes
with the SERS spectra when adsorbed on colloidal metal surfaces of the same metals. The salicylate complexes with
the above metals were compared to those of Na+, Fe3+ and Al3+ cations. A different interaction mechanism is
deduced for salicylate in the metal complex and when adsorbed on the metal surface. © 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Salicylic acid; Infrared; Raman; Metallic complexes; SERS; Adsorption
soils in relation to the plant nutrition [1,2]. One of
the most relevant metal binding sites existing in
humic substances is the salicylate moiety (Fig. 1).
The interaction of salicylate with metallic ions
and mineral interfaces has been already studied in
relation to environmental aspects [3–5]. Another
point of interest regarding the salicylate molecule
is its presence in many commercial drugs [6,7].
The study of the interaction of this molecule with
ions is, thus, an interesting topic which deserves a
deep research.
1. Introduction
The ability of soil organic matter to bind metals
of different nature and inorganic matrices is of a
great importance regarding the metal richness of
In this respect we believe that a comparative
study of the interaction of salicylate with metal
cations and metal surfaces could also afford a
valuable information concerning the ability of this
molecule to bind metals under different states.
This could also serve to understand the different
Fig. 1. H-bond in salicylic acid.
* Corresponding author. Tel.: +34-91-5616800; fax: +34-
91-5645557.
E-mail address: imts158@iem.cfmac.csic.es (S. Sa´nchez-
Corte´s).
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00328-0

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binding possibilities that this molecule can
demonstrate in soils or physiological systems. Sur-
face-enhanced Raman spectroscopy (SERS) is a
technique which has been largely used to study
the interaction of an adsorbate on metal surfaces.
The high sensitivity of this technique allows for a
selective study of only those molecules which are
directly adsorbed onto the surface.
case of salicylate/Cu2+, and 3:1 in the case of
salicylate/Fe3+ and salicylate/Al3+ complexes.
The salicylate complex with Ag+ was obtained by
mixing a 0.1 M aqueous solutions of Na salicylate
and a 0.1 M aqueous solution of AgNO3. After
mixing the above solutions an spontaneous pre-
cipitation of the corresponding metal complex
was observed. The suspensions were centrifuged
and the precipitate obtained was washed three
times with triply distilled water. Then, these were
dried at 60–90°C and, afterwards, they were kept
in a desiccator. Salicylic acid in the solid state was
prepared by adding hydrochloric acid to a 0.5 M
Na salicylate aqueous solution until pH=3. At
this pH the salicylic acid precipitated due to its
insolubility in water. Then the precipitate was
washed several times and dried as indicated
above.
The aim of this work is to develop a vibrational
study (infrared and Raman) of metallic complexes
of salicylic acid with Ag+ and Cu2+ and their
comparison with the SERS spectra of this
molecule when adsorbed on colloidal metal sur-
faces of the same metals: Ag and Cu. Although in
the literature one can find SERS studies regarding
the adsorption of metallic complexes [8,9], few
studies has been undertaken dealing with a com-
parison between the interaction of a ligand with
the same metal under different states: cation, in
the metallic complex, and surface, when this lig-
and is adsorbed on it. The salicylate complexes
with the above metals were compared with those
of Na+, Fe3+ and Al3+ cations, corresponding to
cations that interact through an electrostatic inter-
action (Na+) and cations that are known to
strongly interact with salicylate through a coordi-
nate bond, in the case of Fe3+ and Al3+ [10,11].
The assignments of the vibrational bands corre-
sponding to salicylic acid and salicylate were
made on the basis of data found in the literature
[12–15].
2.3. Preparation of colloids
The silver colloid was prepared by the
Creighton method [16]: 10 ml of an aqueous
AgNO3 solution (10−3 M) were added drop by
drop to 30 ml of an ice-cold aqueous NaBH4
solution (2×10−3 M) and vigorously stirred; the
resulting initial colloid showed a yellow color.
Afterwards, the Ag colloid becomes brown, but
on stirring the resulting colloid, the final colour
becomes again light yellow.
The Cu-coated Ag colloid was prepared by
dissolving the AgNO3 in a previously obtained Cu
colloid up to a final concentration of 10−3 M,
and subsequent reduction of the mixture by
means of an ice-cold aqueous sodium borohydride
solution (2×10−3 M), following the same proce-
dure described above. The Cu colloid was pre-
pared by using the procedure of Creighton et al.
[17] with some modifications: A 1 ml aqueous
solution of CuSO4 (10−2 M) was added to 16 ml
of a trisodium citrate solution (2.8×10−3 M),
and 6 ml of a freshly prepared solution of NaBH4
(2×10−2 M) and sodium hydroxide (2×10−2
M) was added drop by drop with vigorous stir-
ring. The resulting brown colloid must be aged for
at least 1.5 h to be employed in SERS measure-
ments. After this time the colloid becomes dark
red due to its partial aggregation.
2. Experimental
2.1. Materials
Sodium salicylate was purchased from Sigma.
FeCl3, Al(NO3)3, AgNO3, CuCl2, NaBH4 and
trisodium citrate were purchased from Merck.
2.2. Preparation of metal complexes
The salicylate complexes with Cu2+, Fe3+ and
Al3+ in solid state were obtained by mixing a 1 M
aqueous solution of sodium salicylate and an 1 M
solution of CuCl2, FeCl3 or AlCl3 aqueous solu-
tions, with the following volume ratios: 2:1 in the

M.C. Al6arez-Ros et al. / Spectrochimica Acta Part A 56 (2000) 2471–2477
2473
man spectra were recorded by using a 180° ge-
ometry and a 4 cm−1 resolution. The final spectra
were the result of 1000 scans accumulation.
SERS spectra in the visible were recorded in a
U-1000 Jobin-Yvon Raman spectrophotometer,
using the 514.5 nm radiation line of a Spectra-
Physics 165 Model Ar+ laser. The laser power at
the sample was about 40 mW, and the SERS
spectra were the result of five scans average
recorded at a 1 cm−1 per second rate. Samples for
SERS measurements were placed in a 2 mm di-
ameter capilar. A 90° geometry was employed in
this case with a 4 cm−1 resolution.
3. Results and discussion
3.1. FTIR spectra of salicylate complexes
FTIR spectra of Na+, Ag+, Cu2+ and Fe3+
,
salicylates and salicylic acid in KBr are displayed
in Fig. 2. The broad band appearing in the
1700−1650 cm−1 region in the FTIR spectrum
of salicylic acid, which is assigned to C=O
stretching of aromatic carboxylic acids, is absent
in the spectra complexes spectra, while the bands
attributed to the carboxylate moiety, appearing at
about 1590 cm−1 (was(COO−)) and about 1380
cm−1 (ws(COO−)) are evident in these complexes,
thus indicating that the acid is dissociated in the
presence of the metal. However, the above car-
boxylate bands undergo different shifts depending
on the interacting metal. The strongest changes
are seen for the was(COO−), which is one of the
most intense in the IR spectra. In the case of
sodium salicylate, this band is split in two compo-
nents appearing at 1597 and 1583 cm−1. But in
the case of Fe3+ and Cu2+ complexes the
was(COO−) mode is markedly weakened. This
variation may indicate a significant change in the
symmetry of these modes induced by the interac-
tion with the metal.
Fig. 2. FTIR spectra in KBr of salicylic acid (a), Na+ salicy-
late (b) and salicylate complexes with (c) Ag+; (d) Fe3+ and
(e) Cu2+
.
Samples for SERS measurements were prepared
by adding aliquots of an initial 0.1 M sodium
salicylate aqueous solution to 1 ml of the corre-
sponding colloid (Ag or Ag on Cu colloids) until
reaching a 1.7×10−2 M final concentration. This
relatively high concentration was necessary for
aggregating the colloid without adding any aggre-
gation agent that could influence the interaction
of salicylate with the metal.
2.4. Instrumentation
IR spectra were recorded with a Perkin-Elmer
FTIR 1725 spectrophotometer. FT-Raman spec-
tra were recorded with a Bruker RFS 100/S by
using a Nd:YAG laser source at 1064 nm, the
output power was 50 mW. The samples in the
solid state were prepared by placing a small
amount of the compound in a brass metallic
cylinder with a small hole in the centre. FT-Ra-
The bands attributed to aromatic ring also un-
dergo remarkable changes. In general, we ob-
served more changes again in the case of Fe3+
and Cu2+ complexes. The bands appearing at
1613 and 1582 cm−1 are displaced to opposite
directions in the presence of the cation: to 1621

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(Fe3+) and 1625 cm−1 (Cu2+) the first; and to
1570 (Ag+), 1564 (Fe3+) and 1559 cm−1 (Cu2+),
the second. In the case of sodium salicylate no
significant displacement is observed for the lower
component, while the upper one is down shifted
to 1597 cm−1. The ring stretching bands appear-
ing in the 1400–1500 cm−1 region also undergo
evident changes. But the stronger variations are
seen in the case of the ring bands of the 1300–
1350 cm−1 region which are remarkably weak-
ened in the Fe3+ and Cu2+ complexes. All these
results corroborate the strong influence of these
two metals in the salicylate ring vibrations [10,11].
Finally, the metal has also an influence on the
w(C-OH) bonds of salicylate. This effect is differ-
ent in Ag+ and Cu2+ in relation to Fe3+. The
1249 cm−1 band is up shifted in the complexes of
of the intramolecular H-bond existing in the
molecule (Fig. 1), as a consequence of the interac-
tion with the metal.
3.2. Raman spectra of salicylate and its
complexes with Fe3+ and Al3+
Fig. 3 displays the FT-Raman spectra of sali-
cylic acid, sodium salicylate and the complexes
with Fe3+ and Al3+ in solid state. In the sodium
salycilate, the ionization of the carboxyl group in
the complex is suggested by: (1) the marked weak-
ening of the 1635 cm−1 band, which may contain
contributions from ring stretching motions as well
as w(C=O) vibrations of carboxylic acid group;
(2) the intensity decrease of the 1324 cm−1 band,
assigned to a w(C−O) mode in the carboxylic
acid group coupled to ring stretching vibrations;
(3) the enhancement of the bands appearing at
1583 cm−1, attributed to was(COO−) in combina-
tion with ring stretching motions; and (4) the
appearance of a new bands at 1386 cm−1, due to
ws(COO−); at 1209 cm−1 attributed to was(COO−)
coupled to phenolic w(C−O) motion (15); and at
671 cm−1, that may correspond to d(COO−). All
these facts corroborates the assignments found in
the literature for salicylate. On the other hand, the
observation of the w(C=O) band of carboxylic
acid group at a lower frequency in the FT-Raman
when compared with the FTIR spectrum (Fig. 2a)
can be explained on the basis of the different state
of salicylic acid in solid and in KBr.
the two first cations to 1255 and 1254 cm−1
,
respectively, while in the Fe3+ is strongly down
shifted to 1239 cm−1. The marked down shift
undergone with Fe3+ may indicate a breakdown
The FT-Raman of the Fe3+ shows many differ-
ences in relation to the sodium salt that corrobo-
rates the existence of a strong interaction of this
ion with salicylate. The spectrum is dominated by
two intense bands appearing at 1599 and 1321
cm−1. The ring stretching motions of the 1500−
1400 region, corresponding to w8 and w9 of the
Green nomenclature [13] are markedly reduced,
indicating a strong influence of this metal in the
aromatic system. Moreover, the intensity decrease
of the ws(COO−) mode at 1385 cm−1 and the
down shift of the w(C−O) mode at 1243 cm−1
also indicates a strong interaction of Fe3+ with
both the carboxylate and the phenolic groups, as
it was also deduced from the IR spectrum. This
simultaneous interaction lead to a subsequent
Fig. 3. FT-Raman spectra of salycilic acid (a), Na+ salicylate
(b) and salicylate complexes with (c) Fe3+ and (d) Al3+
.

M.C. Al6arez-Ros et al. / Spectrochimica Acta Part A 56 (2000) 2471–2477
2475
complex is shown in Fig. 4b compared with the
SERS on Ag colloid (Fig. 4c) and that of the
sodium salt (Fig. 4a). The Raman of the complex
in solid state shows strong features at 1553 and
1262 cm−1, along with the intense bands of C−
H ring deformation appearing at 1032 and 807
cm−1. The up shift of salicylate w(C−O) band at
1250 cm−1 towards 1262 cm−1 in the Ag+ com-
plex (Fig. 4b) indicates that the hydroxyl group
may not participate in the interaction with the
metal as in the case of Fe3+. However, the inter-
action with the carboxylate group may cause a
great distortion of the aromatic system as indi-
cated by the appearance of a strong band at 1553
cm−1. The SERS spectrum (Fig. 4c) shows a
different behavior in relation to the solid complex.
The salicylate band at 1250 cm−1 is down shift to
1244 cm−1, thus suggesting the participation of
the phenol group in the interaction with the sur-
face, as has been also referred by other authors
studying the adsorption of salicylate on silica
[18,19]. This different interaction is also suggested
by the enhancement of the 1305 cm−1 band in the
SERS spectrum and the lower intensity of that at
1332 cm−1, which undergoes an up shift to 1340
cm−1 in the SERS. The involvement of both the
phenol and carboxylate groups in this vibrations
is also indicative of an interaction of salicylate
through these two groups.
Fig. 4. (a) FT-Raman spectrum of Na+ salicylate; (b) FT-Ra-
man spectrum of Ag+ salicylate complex; and (c) SERS of
salicylate on Ag colloid (1.7×10−2 M, lex=514.5 nm).
electronic modification of the aromatic p system
[10,11].
The spectrum of sodium salicylate shows a
weak band at about 2600 cm−1 that may corre-
spond to the w(O−H) motion of an alcohol bond
involved in a H-bond [7], which is not seen in the
SERS of salicylate (result not shown), thus cor-
roborating the possible breakdown of the in-
tramolecular H-bond due to the simultaneous
interaction of salicylate through the carboxylate
and the phenolic group. Nevertheless, the exis-
tence of a close hydroxyl group may induce a
possible reorientation on the molecule that is not
completely perpendicular in relation to the sur-
face, as deduced from the absence of any w(C−
H) band in the high wavenumbers region.
In the case of Al3+ salicylate complex we also
observed changes concerning the ring stretching
bands and the carboxylate, but the 1250 cm−1
band of phenol group is not down shift and the
Raman spectrum in general does not undergo a so
deep modification as in the case of Fe3+, suggest-
ing that the interaction of Al3+ with salicylate is
weaker. It is worthy noting in this complex the up
shift and enhancement of the 1633 cm−1 band,
and the strong up shift of the ws(COO−) band to
1418 cm−1. All these facts suggest a strong charge
transfer from the cation to the salicylate through
a back donation mechanism.
The FT-Raman spectrum of Cu2+ salicylate
complex is shown in Fig. 5b compared with the
SERS spectrum on mixed Cu/Ag colloid (Fig. 5c)
and that of sodium salicylate (Fig. 5a). The high
3.3. Raman of salicylate complexes with Ag+ and
Cu2+: comparison with SERS spectra
The FT-Raman spectrum of Ag+ salicylate

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M.C. Al6arez-Ros et al. / Spectrochimica Acta Part A 56 (2000) 2471–2477
fluorescence background of Cu2+ complex ob-
served exciting at 1064 nm avoids obtaining a
good Raman spectrum (Fig. 5b). The excitation
with a visible line induces a so high fluorescence
that no band can be seen in the Raman spectrum.
As in the case of Fe3+ complex, we observed a
of these complexes (Fig. 2c and e). This disparity
may be a consequence of the higher contribution
in Raman of ring modes, which may be differently
changed in each complex, depending on the inter-
action strength between the cation and the ligand.
According to this, the down shift observed for the
w(C–O) band to 1240 cm−1 could not be at-
tributed to a direct interaction of Cu2+ with the
phenol group, but rather to a strong distortion of
the ring vibrations which are also reflected in a
change of the above phenol band.
down shift of the w(C−O) band to 1240 cm−1
,
and an intense band at 1535 cm−1, which may
correspond to that observed for the Ag+ and
Al3+ complexes at 1553 and 1544 cm−1, respec-
tively. Moreover, the carboxylate ws(COO−) band
is up shifted to 1392 cm−1. This may indicate a
simultaneous interaction with both the phenol
and carboxylate groups as in the case of the Fe3+
complex. However, the differences observed be-
tween the FT-Raman spectra of Cu2+ and Fe3+
complexes (Fig. 5b and Fig. 3d, respectively) con-
trasts with the high similarity of the FTIR spectra
The SERS of salicylate could not be obtained
on Cu colloid. For this reason we have tried to
obtain the SERS spectrum of salicylate on a
mixed colloid composed by Cu and Ag with the
goal of increasing the affinity of the Cu surface
with respect to this ligand. The SERS spectrum of
salicylate adsorbed on this mixed colloid is very
weak (Fig. 5c). The spectral profile is in general
different in relation to the SERS on Ag or the
Ag+ complex, thus indicating that the observed
SERS spectrum may correspond to the salicylate
interacting with metallic Cu surface. Moreover,
this spectrum shows many differences in relation
to that of Cu2+ complex: the w(O–H) band is not
down shifted, while the carboxylate band does not
undergo the up shift observed for the Cu2+ com-
plex, thus revealing a weaker interaction of salicy-
late with the metallic Cu surface through these
groups. This can be the cause of the weaker SERS
intensity. However, strong changes can be ob-
served in the bands corresponding to the aromatic
ring, since a marked weakening is observed for
the bands appearing in the 1650−1400 cm−1
region and a prominent band is seen at 1316
cm−1
.
4. Conclusions
The interaction of salicylate with metallic
cations has a distinct influence on the different
molecular moieties that integrate this molecule:
carboxylate, hydroxyl and aromatic ring. In solid
state Fe3+ strongly interacts with the carboxylate
and hydroxyl groups simultaneously, while Ag+,
Cu2+, and Al3+ only interact with the carboxy-
late, although these cations induce strong changes
Fig. 5. (a) FT-Raman spectrum of sodium salicylate; (b)
FT-Raman spectrum of Cu2+ salicylate complex; and (c)
SERS of salicylate on Cu/Ag colloid (1.7×10−2 M, lex
=
514.5 nm).

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2477
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This work has been supported by Direccio´n
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