Infrared and XAFS study on structure and transition behavior of zinc stearate.

Article abstract of DOI:10.1016/S1386-1425(00)00225-0

Structure and transition behavior of zinc(II) stearate crystal were investigated by infrared and XAFS spectroscopies. Structure of zinc stearate at room temperature was estimated as follows. From XAFS analysis, the coordination number of the carboxylate groups around the zinc atom was evaluated as 4 and the Zn-O distance as 1.95 A. Based on the infrared spectrum and a normal mode analysis, the conformation of the alkyl chain was confirmed as all-trans and the sub-cell packing was considered as parallel type, and also the coordination form of the carboxylate groups was determined as bridging bidentate type. As increasing temperature, zinc stearate has a solid liquid phase transition at 130 degrees C. At the transition, the alkyl chains goes into liquid like state as reported by Mesubi but the coordination structure was confirmed to be maintained.

Full text of DOI:10.1016/S1386-1425(00)00225-0

Spectrochimica Acta Part A 56 (2000) 1731–1737
www.elsevier.nl/locate/saa
Infrared and XAFS study on structure and transition
behavior of zinc stearate
Tsutomu Ishioka a,*, Kaori Maeda a, Iwao Watanabe b, Shigehiro Kawauchi b,
Makoto Harada b
a Department of Chemistry, Faculty of Science, Toyama Uni6ersity, Gofuku, Toyama 930-8555, Japan
b Department of Chemistry, Graduate School of Science, Osaka Uni6ersity, Toyonaka, Osaka 560-0043, Japan
Received 4 January 2000; accepted 18 January 2000
Abstract
Structure and transition behavior of zinc(II) stearate crystal were investigated by infrared and XAFS spectro-
scopies. Structure of zinc stearate at room temperature was estimated as follows. From XAFS analysis, the
coordination number of the carboxylate groups around the zinc atom was evaluated as 4 and the ZnꢀO distance as
,
1.95 A. Based on the infrared spectrum and a normal mode analysis, the conformation of the alkyl chain was
confirmed as all-trans and the sub-cell packing was considered as parallel type, and also the coordination form of the
carboxylate groups was determined as bridging bidentate type. As increasing temperature, zinc stearate has a
solid–liquid phase transition at 130°C. At the transition, the alkyl chains goes into liquid like state as reported by
Mesubi but the coordination structure was confirmed to be maintained. © 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Zinc stearate; Infrared spectra; XAFS spectra
was reported by Konkoly-Thege et al. [10] Ac-
cording to them, zinc stearate has one solid–liq-
uid phase transition at 130°C. The entropy change
at the transition is sufficiently larger than cad-
mium soaps, indicating that zinc stearate does not
form liquid crystalline phase above the transition
point and goes into liquid state. Infrared spectra
of zinc stearate have been observed by Mesubi
[11] at room temperature and around the phase
transition point. Based on the observations, he
interpreted that the alkyl chains are in an ex-
tended state and the carboxylate groups have
chelating bidentate coordination structure around
1. Introduction
Zinc(II) stearate is one of typical ionic surfac-
tant of metal soaps. Up to today, detailed struc-
tural and vibrational information about metal
soaps are very few [1,2]. Systematic crystal struc-
tural investigations [3–5] and normal mode analy-
ses [6–9] have been made only for a series of
potassium soaps. For zinc soaps, a thermal study
* Corresponding author. Tel.: +81-76-4456610; fax: +81-
76-4456549.
E-mail address: ishioka@sci.toyama-u.ac.jp (T. Ishioka)
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00225-0

1732
T. Ishioka et al. / Spectrochimica Acta Part A 56 (2000) 1731–1737
the zinc atom at room temperature. At the transi-
tion, the chains goes into liquid-like state and the
coordination structure changes into a mixture of
chelating bidentate and monodentate structures.
Although these studies reported some information
about the structure and transition behavior of
zinc stearate, more precise studies about spectral
assignments, and molecular and coordination
structure are needed.
In this study, we study detailed molecular con-
formation and coordination structure of zinc
stearate with infrared spectra and a normal mode
analysis. We also made XAFS analysis in order to
obtain direct structural information about the
coordination form. Based on these observations
and analysis, we obtained new information espe-
cially about the coordination structure of the
carboxylate groups around the zinc atom.
The purity was confirmed by the IR spectrum and
the atomic absorption analyses; Zn=10.5wt%
obs. and 10.34% calc., NaB0.1%. Infrared spec-
tra were recorded by a JASCO IR-810 spec-
trophotometer. The specimen for infrared
measurements was directly sandwiched between
KBr windows, since a usual KBr disk method
might influence the thermal behavior of the spec-
trum because of ionic exchange between RCO2Zn
and KBr. Temperature dependence of infrared
spectra (91°C accuracy) was recorded with a
JASCO HC-12 heating unit with a heating rate of
20 K h−1. Temperature was measured by a sealed
CA thermocouple with a ADVANTEST TR2114
digital multithermometer. The observed infrared
wavenumbers were calibrated with those of indene
and believed to be accurate within 92cm−1
.
The zinc K-edge absorption measurements were
performed at BL-10B of the photon factory of the
high energy accelerator organization in Tsukuba
in which the X-ray beam was monochromatized
by a Si(311) channel-cut-crystal monochromator.
X-ray transmission spectra were obtained for
powdered specimen mixed with BN powder by
using an aluminum holder. Temperature depen-
dence of the spectra was made by a home made
heating cell. The heating rate was the same as that
in the infrared spectra.
2. Experimental
Zinc stearate was synthesized from sodium
stearate (Sigma company 99%) in dry ethanol and
water (50:50% volume) solution by titrating
equimolar ZnCl2(aq) slowly. The solution was pre-
cipitated in diethylether. The product was filtered
and dried at 60°C under vacuum over night.
These procedures were repeated five times at least.
3. Results and discussion
3.1. Structure
3.1.1. XAFS spectra
In order to estimate the molecular structure of
zinc stearate, a knowledge of the coordination
structure of the carboxylate groups around the
zinc atom is needed. EXAFS or XANES spec-
trum of zinc stearate may give us information on
the coordination structure. Here, we discuss these
results at room temperature. The EXAFS analysis
procedures have been reported in detail [12], thus
will not be given here.
In Fig. 1 the X-ray absorption near edge spec-
tra (XANES) for zinc stearate at: (a) room tem-
perature; (b) −203; and (c) 170°C are compared
to those for: (d) zinc(II) ion in water; and (e)
Fig. 1. The Zn K-edge X-ray absorption near edge spectra
(XANES) for zinc stearate powdered crystal at: (a) room
temperature, (solid line); (b) −203°C, (narrow solid line); and
(c) 170°C, (dashed line). Those for: (d) 0.1M zinc(II) ion in
water, (dashed-double dotted line); and (e) anhydrous zinc
acetate, (dashed-dotted line) are included for comparison pur-
pose.

T. Ishioka et al. / Spectrochimica Acta Part A 56 (2000) 1731–1737
1733
of zinc ions in aqueous solution and zinc acetate
are octahedral and tetrahedral, respectively, and
since it is known that the white line intensity
increases with the coordination number of zinc
ions [13], the relative intensities of the spectra in
Fig. 1 indicates that zinc atoms in zinc stearate
have the coordination numbers of around four.
The coordination structures of zinc atoms can
be more precisely determined by the EXAFS anal-
ysis. The EXAFS oscillation spectra are extracted
for zinc stearate sample at various temperatures
as shown in Fig. 2. They are Fourier transformed
to show radial distribution functions in Fig. 3.
The FT calculations were performed using DFT
method and in the calculations the theoretical
parameters of phase shift and back-scattering am-
plitude for Zn (absorber) and oxygen (scatter)
were included, thus the first peaks in Fig. 3 appear
at the correct position. There clearly exist second
Fig. 2. The Zn K-edge EXAFS s3(s) spectra of zinc stearate
at: (a) room temperature, (solid line); (b) −203°C, (narrow
solid line); and (c) 170°C, (dashed line).
,
peaks at around 3.2 A in the FT spectra. In the
present curve-fitting analysis, two-shell model cal-
culations were performed on the back-trans-
formed -spectra from the r-space data over the
,
first and the second peaks, i.e. 1.49–3.86 A.
The variable parameters in the curve-fitting cal-
culation are r, the Debye–Waller-like factor |,
the amplitude factor B (B=SN), where S is the
amplitude reduction factor, and the absorption-
edge shift DE0. The back-scattering amplitude
F(k) and the phase shift ƒ(k) are from the tables
by Teo and Lee [14,15] and McKale et al. [16] The
coordination numbers N were determined by us-
ing the data for zinc acetate as a standard for
Fig. 3. The Zn K-edge EXAFS FT spectra of zinc stearate at:
(a) room temperature, (solid line); (b) −203°C, (narrow solid
line); and (c) 170°C, (dashed line).
anhydrous zinc acetate. Here, we discuss (a), (d),
and (e); (b) and (c) will be dealt with in Section
3.2 described below. The coordination structures
Table 1
Fitting parameters of XAFS analysis for zinc stearate at various temperatures
,
,
Sample
Shell
r/A
N
|/A
DE0/e
Zinc stearate
At room temperature
Zinc stearate
At −203°C
1(O)
2(O)
1(O)
2(O)
1(O)
2(O)
1(O)
1.95
3.25
1.95
3.25
1.95
3.26
2.07
4.2
4.3
4.2
4.4
4.0
3.2
6.4
0.050
0.059
0.035
−0.029
0.058
0.082
7.13
7.13
7.57
7.57
7.47
7.47
4.66
Zinc stearate
At 170°C
0.1 M Zn2+
In water
0.083
Zn(CH3COO)2
1(O)
1.96
4.0*
0.053
7.02
* The data from Zn(CH3COO)2 were used to determine the coordination numbers N for other samples.

1734
T. Ishioka et al. / Spectrochimica Acta Part A 56 (2000) 1731–1737
Fig. 4. Infrared spectrum of zinc stearate at room temperature in the 400–1800 and 2500–3500 cm−1 regions. Calculated
frequencies of the methylene progressive bands for all-trans conformation are shown by vertical lines.
which N=4.0. The results by the curve-fitting
calculations are listed in Table 1.
From the curve-fitting analysis, zinc atoms in
and HCH angles with the CꢀC and CꢀH bond
,
lengths equal to 1.54 and 1.093A, respectively.
The CCO and OCO angles were postulated to be
zinc stearate at room temperature are concluded
,
120° with a CꢀO length equals to 1.27 A. The
,
to have the same ZnꢀO distance of 1.95 A and the
valence force constants were adjusted from those
in our study of potassium palmitate and potas-
sium caprate [8,9]. The constants consist of three
parts: the alkyl chain, the CaH2 group, and the
carboxylate group. Those of the alkyl chain were
well established and we used calculation V of
Snyder et al. as listed in [17]. The a-methylene
group should be considered for metal soaps as in
the case of normal fatty acids. We transferred the
constants of Umemura [18] as initial values and
adjusted only a-methylene bending Hl(h) from
coordination number of 4. The short distances of
,
1.95A and the small white line intensities in the
XANES in Fig. 1 also are in excellent accord with
the tetrahedral coordination structure of zinc
atoms.
We have attempted to identify the elements
which produce the second peaks in the FT spec-
tra. The curve-fitting calculations, however, did
not give meaningful difference in the fit when the
parameters for carbon or zinc as the back-scatter
were used instead of those for oxygen. Therefore
it is possible that either carbon or oxygen of COO
0.505 to 0.491 mdyn A rad−2. The constants of
,
the carboxylate group were evaluated for potas-
sium soaps by us [8,9]. With referring these, we
changed the CO stretch constant Kr(COO) from
,
group or even zinc atoms locate at around 3.2 A
from the zinc atoms in zinc stearate.
9.457 to 9.100 mdyn A−1, the OCO bend
,
−2
,
Ha(COO) from 1.872 to 2.000 mdyn A rad
,
3.1.2. Infrared spectrum
and the CCO bend Hb(COO) from 0.877 to 1.000
A normal mode analysis was made for zinc
stearate by the Wilson GF matrix method using
an IBM RS/6000-580 computer. In this study, we
assumed the alkyl-carboxylate molecule having an
all-trans conformation and belongs to the Cs
point group with the A% and A¦ species. A tetra-
hedral geometry was assumed for the CCC, CCH,
mdyn A rad−2, and the COO out-of-plane bend y
,
,
from 0.497 to 0.450 mdyn A. The observed in-
frared spectrum at room temperature in the 400–
1800 and 2500–3500 cm−1 regions is shown in
Fig. 4. The calculated frequencies of the
methylene progressive bands are also shown by

T. Ishioka et al. / Spectrochimica Acta Part A 56 (2000) 1731–1737
1735
vertical lines. The agreements between the ob-
served and calculated frequencies were fairly
good. The detail of the assignments has been
described in [19].
the vibrational frequencies of the carboxylate
modes of the antisymmetric and symmetric
stretches, the bending and the out-of-plane bend-
ing closely coincided each other within 10 cm−1
between the two zinc acetates [22,19]. However,
the frequencies of the COO rocking mode were
different over 50 cm−1, i.e. the chelating bidentate
form in the zinc acetate dihydrate gave the fre-
quency at 473 cm−1 (Raman), while the bridging
bidentate form in anhydrous zinc acetate gave it
at 522 (IR) and 526 cm−1 (Raman). Hence, the
COO rocking mode is the key band to distinguish
the two coordination forms. Additionally, the
ZnꢀO (carboxylate) distance is also the key point,
since the chelating bidentate has the distance of
The methylene progressive bands in the 700–
1400 cm−1 region reflect the conformational or-
derliness of the alkyl chain. The bands of zinc
stearate (Fig. 4) were satisfactorily assigned by the
normal mode analysis where the all-trans confor-
mation was assumed, indicating that the molecule
has all-trans conformation. In the long-chain
compounds, the alkyl chains usually packed in a
small periodic structure within the real unit cell.
This small periodic structure is called as subcell.
There are typically two types of subcell which
have parallel and perpendicular forms of lateral
chain packing. In the perpendicular case, the cor-
relation split in the subcell is generally known to
appear for the CH2 bending and CH2 rocking
regions, but in the parallel case no split appears.
In this case of zinc stearate, the lateral packing
form (sub-cell) of the alkyl chains was concluded
as a parallel type, since we did not observe the
correlation splits in the CH2 rocking and CH2
bending regions.
,
2.184 A in average, while the bridging bidentate
,
has the distance of 1.957 A. In the case of zinc
stearate, the COO rocking mode was observed at
548 cm−1 and the ZnꢀO distance was evaluated
,
by the XAFS analysis as 1.95 A. These indicate
that the COO groups in zinc stearate have the
bridging bidentate structure. The coordination
number was evaluated as 4. It is well-known that
the frequency difference between the COO anti-
symmetric and symmetric stretches is a criterion
of the coordination structures [23]. Zinc stearate
From the normal mode analysis, we made de-
tailed assignments of the spectrum ascribed to the
COO group. Based on the assignments, we discuss
the coordination structure around the zinc atom
in zinc stearate. In order to estimate the structure,
we have made precise studies on the relation
between the vibrational spectra and the coordina-
tion forms for zinc acetate dihydrate and anhy-
drous zinc acetate as model compounds. The zinc
acetate dihydrate has the monoclinic crystal struc-
ture [20]. The molecule has the 6-coordination
structure and two-fold symmetry axis. The
ZnꢀO’s (carboxylate) distances are 2.179(4) and
has a somewhat large difference of 139 cm−1
.
This may also indicate that the coordination form
is the bridging bidentate.
3.2. Transition beha6ior
Zinc stearate of the carbon number 18 has a
solid–liquid phase transition at 130°C. At the
transition, DH=103 kJ mol−1 and DS=256 J (K
mol)−1 [10]. In the case of normal alkane having
the carbon number 18, melting point Tm=
28.180°C, DH=61.99 kJ mol−1, and DS=205.7
J (K mol)−1 [1]. Normal fatty acid having the
same carbon number, Tm=69.6°C, DH=63.2 kJ
mol−1, DS=184 J (K mol)−1 [1]. Comparing
these compounds, zinc stearate has high transition
temperature. This may be caused by the ionic
groups which play a role of anchor and stabilize
the alkyl chains. Sufficiently large DH and DS of
zinc stearate indicate that the transition includes
not only the chain melting but also some struc-
tural deformation of the ionic groups. Above the
,
2.189(5) A and the Zn–OH2 distance is 1.987(4)
,
A. The acetate groups have the chelating biden-
tate coordination form. The anhydrous zinc ac-
etate also has the monoclinic crystal structure
[21], but the molecule has the 4-coordination
structure. The Zn-O’s (carboxylate) distance is
,
1.957 A in average. The acetate groups have
bridging bidentate coordination forms in a syn-
anti arrangement. We have made normal mode
analyses for these two compounds and found that

1736
T. Ishioka et al. / Spectrochimica Acta Part A 56 (2000) 1731–1737
transition, the chains have liquid-like conforma-
tions and the IR carboxylate antisymmetric
stretch band splits into a few bands. Mesubi
observed this spectral change and considered
that the change was due to the change in the
coordination structure [11]. Here, we examine
the XAFS and infrared spectral changes of zinc
stearate at the transition and consider whether
the change is due to the change in the coordina-
tion structure.
4.0, respectively, as listed in Table 1. Hence,
there is no coordination change at the transition
and the infrared spectral change reported by
Mesubi is not due to the change in the coordi-
nation structure.
3.2.2. Infrared spectra
Fig. 5 (a) shows temperature dependence of
infrared spectra of zinc stearate in the 400–1500
cm−1 region. The bands in this region are called
as methylene progressive bands and characteris-
tic of all-trans conformation of the alkyl chain.
Above the transition, these bands smeared, indi-
cating the chain melt. The band at 1398 cm−1
is ascribed to the CaH2 bend and the carboxy-
late symmetric stretch. The band at 744 cm−1 is
the carboxylate bend. As increasing temperature,
these bands spread broadly at the transition, in-
dicating that some structural disorder occurred
at the carboxylate end. At the same time, the
carboxylate antisymmetric stretch in the 1500–
1600 cm−1 split from the band at 1538 cm−1 to
the four bands at 1630, 1596, 1558 and 1545
cm−1 above the transition point as shown in
Fig. 5 (b). From our XAFS analysis, it was
concluded that the coordination structural
change did not occur at the transition as de-
scribed above.
3.2.1. XAFS spectra
In order to confirm whether the coordination
structural change occurred or not for zinc
stearate, we observed temperature dependence of
XAFS spectra. Fig. 1 shows the XAFS spectra
of zinc stearate at: (a) room temperature; (b)
−203; and (c) 170°C. Three lines almost coin-
cide each other. By least square fittings of
k3(k) plots, we estimated the ZnꢀO distances
,
at three temperatures are 1.95, 1.95 and 1.95 A
and the coordination numbers are 4.2, 4.2 and
We consider that the spectral change in the
carboxylate antisymmetric stretch region is as-
cribed to geometrical distortion in the tetrahe-
dral coordination structure of the carboxylate
groups around the zinc atom where the ZnꢀO
distance and the coordination number are re-
tained. This kind of split has been also reported
for potassium soaps [24,25] in the high tempera-
ture phase where the orientational disorder of
the carboxylate groups occur. The large fre-
quency split above the transition point may be
also due to the transition dipole-dipole coupling
among the carboxylate groups. The split in the
frequency due to the coupling between the car-
bonyl CꢁO groups in carboxylic acids has been
observed as over 50 cm−1 [26]. In the case of
the carboxylate group, the coupling is somewhat
large because of the large transition dipole mo-
ment [8], may cause the large split.
Fig. 5. Temperature dependence of infrared spectra for zinc
stearate at: (a) 24 and 134°C in the 400–1500 cm−1 region;
and (b) 31 and 131°C in the 1300–1700 cm−1 region.

T. Ishioka et al. / Spectrochimica Acta Part A 56 (2000) 1731–1737
1737
[12] H. Sakane, T. Miyanaga, I. Watanabe, N. Matsubayashi,
S. Ikeda, Y. Yokoyama, Jpn. J. Appl. Phys. 32 (1993) 4641.
[13] H.K. Pan, G.S. Knapp, S.L. Cooper, Coll. Polym. Sci. 262
(1984) 734.
[14] B.K. Teo, EXAFS: Basic Principles and Data Analysis,
Springer, Berlin, 1985.
Acknowledgements
This work has been performed under the ap-
proval of the Photon Factory Advisory Commit-
tee (proposal NO. 95G203).
[15] B.K. Teo, P.A. Lee, J. Am. Chem. Soc. 101 (1979) 2815.
[16] A.G. McKale, B.W. Veal, A.P. Paulikas, S.K. Chan, G.S.
Knapp, J. Am. Chem. Soc. 110 (1988) 3763.
[17] J.H. Schachtschneider, R.G. Snyder, Spectrochim. Acta 19
(1963) 117.
References
[18] J. Umemura, J. Chem. Phys. 68 (1978) 42.
[19] T. Ishioka, Y. Shibata, M. Takahashi, I. Kanesaka, Spec-
trochim. Acta A54 (1998) 1811.
[1] D.M. Small, Handbook of Lipid Research, vol. 4, Plenum
Press, New York, 1986.
[2] M.S. Akanni, E.K. Okoh, H.D. Burrows, H.A. Ellis,
Thermochim. Acta 208 (1992) 1.
[20] T. Ishioka, A. Muratam, Y. Kitagawa, K.T. Nakamura,
Acta Crystallogr. C53 (1997) 1029.
[3] J.H. Dumbleton, T.R. Lomer, Acta Crystallogr. 19 (1965)
301.
[21] W. Clegg, I.R. Little, B.P. Straughan, Acta Crystallogr.
C42 (1986) 1701.
[4] E.L.V. Lewis, T.R. Lomer, Acta Crystallogr. B25 (1969)
702.
[22] T. Ishioka, Y. Shibata, M. Takahashi, I. Kanesaka, Y.
Kitagawa, K.T. Nakamura, Spectrochim. Acta A54 (1998)
1827.
[23] K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds Part B, Fifth ed, Wiley,
New York, 1997.
[24] T. Ishioka, Rep. Progr. Polym. Phys. Jpn. 32 (1989) 195.
[25] T. Ishioka, S. Murotani, I. Kanesaka, S. Hayashi. Proceed-
ings of the SPSJ International Polymer Conference, Osaka,
Japan, The Society of Polymer Science Japan, 1995, pp.
2–33.
[5] D.M. Glover, Acta Crystallogr. A37 (1981) 251.
[6] T. Ishioka, Bull. Chem. Soc. Jpn. 64 (1991) 2174.
[7] T. Ishioka, I. Kanesaka, Polym. J. 26 (1994) 643.
[8] T. Ishioka, S. Murotani, I. Kanesaka, S. Hayashi, J. Chem.
Phys. 103 (1995) 1999.
[9] T. Ishioka, H. Wakisaka, T. Saito, I. Kanesaka, J. Phys.
Chem. 102 (1998) 5239.
[10] I. Konkoly-Thege, I. Ruff, S.O. Adeosun, S.J. Sime,
Thermochim. Acta 24 (1978) 89.
[11] M.A. Mesubi, J. Mol. Struct. 81 (1982) 61.
[26] P. Bosi, G. Zerbi, Chem. Phys. Lett. 38 (1976) 571.
.