Infrared study on annealing effect on conformation of zinc stearate

Article abstract of DOI:10.1016/j.saa.2006.05.025

The molecular conformation and thermal transition behavior of two zinc stearate specimens, unannealed one and annealed one, were compared. The unannealed specimen has one thermal transition at 134 °C. Annealing was made by increasing temperature to 150 °C and cooling to room temperature slowly. This annealed specimen has an exothermic peak at 103 °C, and endothermic shoulders and a peak at 118, 124 and 131 °C, respectively. The observed frequencies of all bands of the unannealed specimen at room temperature are assigned to the all-trans conformation. We found new bands at 858, 823, 793, 766, 688, and 604 cm^-1 for the annealed specimen. Based on the normal mode analyses, these bands are assigned to the TGT conformation at the COO end, where T means trans and G means gauche. The annealed specimen consists of almost all-trans molecule but partial molecules have the TGT conformation.

Full text of DOI:10.1016/j.saa.2006.05.025

Spectrochimica Acta Part A 66 (2007) 1048–1051
Infrared study on annealing effect on conformation of zinc stearate
Tsutomu Ishioka∗, Atsushi Kiritani, Takuya Kojima
Department of Chemistry, Faculty of Science, Toyama University, Gofuku, Toyama 930-8555, Japan
Received 29 March 2006; accepted 1 May 2006
Abstract
The molecular conformation and thermal transition behavior of two zinc stearate specimens, unannealed one and annealed one, were compared.
The unannealed specimen has one thermal transition at 134 ◦C. Annealing was made by increasing temperature to 150 ◦C and cooling to room
temperature slowly. This annealed specimen has an exothermic peak at 103 ◦C, and endothermic shoulders and a peak at 118, 124 and 131 ◦C,
respectively. The observed frequencies of all bands of the unannealed specimen at room temperature are assigned to the all-trans conformation.
We found new bands at 858, 823, 793, 766, 688, and 604 cm−1 for the annealed specimen. Based on the normal mode analyses, these bands are
assigned to the TGT conformation at the COO end, where T means trans and G means gauche. The annealed specimen consists of almost all-trans
molecule but partial molecules have the TGT conformation.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Zinc stearate; Infrared spectra; Normal mode analyses
1. Introduction
into a refrigerator one night. Precipitate was filtered and washed
by distilled water, ethanol, and acetone and recrystallized from
benzene. The precipitate was dried under vacuum at 60 ◦C over
night. Resultant material was confirmed by the infrared spec-
trum. Differential scanning calorimetry (DSC) measurements
were made by a Rigaku DSC 8230 with a heating rate 10 K/min.
Infrared spectra were measured with a JASCO IR-810 with KBr
disk method.
Zinc soaps are useful for industrial usage as lubrication pow-
der, but their molecular level structure and thermal transition
behavior have not been fully clarified yet. We have carried out
the study on the structure of the alkyl chains and the coordi-
nation structure of the COO groups around the zinc cation for
zinc stearate by the infrared and XAFS study. As a result we
found the conformation of the alkyl chain takes the all-trans,
and the coordination structure was bridging bidenteta type [1].
The thermal transition behavior of zinc soaps depends on the
chain length and thermal history [2]. In this study we investigate
anneling effect on the thermal structural transition behavior of
zinc stearate.
3. Results and discussion
Fig. 1 shows the DSC traces of two zinc stearate speci-
mens: trace (a) is for the unannealed specimen and trace (b)
for the annealed specimen. The annealed specimen was made
by increasing the temperature to 150 ◦C and then cooled to room
temperature slowly. The trace (a) has one phase transition peak at
134 ◦C. This is ascribed to the structural phase transition from
the all-trans conformation of the alkyl chain and the bridging
bidentate coordination structure of the COO groups at room
temperature to the liquid-like conformation of the alkyl chain
and slightly distorted bridging bidentate coordination structure
of the COO groups [1]. The trace (b) has an exothermic peak
at 103 ◦C, and endothermic shoulders and a peak at 118, 124
and 131 ◦C, respectively. The structural change in this tem-
perature range of these DSC peaks of anneadled specimen is
unknown.
2. Experimental
Zinc stearate was synthesized from stearic acid (ꢀ company,
99% purity) as follows. Potassium hydroxide aqueous solution
was added dropwise to stearic acid ethanol solution slowly and
stirred 2 h. Then, zinc sulphonate aqueous solution was added to
the above solution and stirred for 2 h. Resultant solution was put
∗
Corresponding author. Tel.: +81 76 445 6610; fax: +81 76 445 6549.
E-mail addresses: ishioka@sci.toyama-u.ac.jp, ishioka@sci.u-toyama.ac.jp
(T. Ishioka).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.05.025

T. Ishioka et al. / Spectrochimica Acta Part A 66 (2007) 1048–1051
1049
Fig. 1. DSC traces for (a) unannealed and (b) annealed specimens. Trace (a) has one endothermic peak at 134 ◦C. Trace (b) has one exothermic peak at 103 ◦C, and
endothermic shoulders at 118, 124 ◦C and a peak at 131 ◦C.
Fig. 2 shows infrared spectra of (a) the unannealed specimen
and (b) the annealed specimen in the 400–1500 cm−1 region at
room temperature. This infrared region is sensitive to the confor-
mational change of the alkyl chain and the COO group. All bands
that appeared in Fig. 2(a) are ascribed to the all-trans conforma-
tion of the alkyl chain, as confirmed by normal mode analysis
[1]. Almost all bands that appear in Fig. 2(b) are also ascribed
to the all-trans conformation, but several new bands appear at
858, 823, 793, 766, 688, and 604 cm−1. Fig. 2(b) indicates that
in the annealed specimen, almost all molecules take the all-trans
conformation but partial molecules have some conformational
defects.
We want to clarify what kind of conformational disorder
appeared in the annealed specimen. Maroncelli et al. have inves-
tigated thermal structural transitions of a series of odd n-alkanes
whose carbon numbers from 17 to 29 by the CH2 progres-
sive region [3]. The structural transition temperatures depend
on the carbon number. As a result, they found that in the low
temperature phase I, n-alkanes take all-trans form, but in the
high temperature phase II, n-alkanes have disorder at the mainly
methyl end such as end-gauche, kink, and double gauche confor-
mations. This case of n-alkanes, conformation disordered bands
appeared in the wide frequency region from 700 to 1400−1, But
in the case of zinc stearate, conformational disordered bands
appear mainly in the methylene rocking progressive region from
858 to 604 cm−1. Some normal fatty acids have a polymorphism
having the gauche conformation at C␣–C␤ position where ␣ and
␤ means O2C–C␣–C␤ [4]. We postulate that the conformational
disorder in the annealed specimen of zinc stearate also occurs
at the COO end. In order to confirm this idea, we carried out
normal mode analyses of the molecules having conformational
disorder around the COO group as follows.
We carried out normal mode analyses for an all-trans
molecule and three molecules having conformational defects at
COO end where O2C–C␣–C␤–C␥. The detail of the calculation
has been reported [1]. We used two sets of force constants for
the alkyl chain: one is for all-trans [5] and the other is for disor-
deredmolecule[6]. Theall-transmoleculeisrepresentedasTTT.
This means O2C–C␣, C␣–C␤, and C␤–C␥ are all-trans confor-
mation. The second molecule is indicated as CTT. It means the
OOC group is rotated by 90◦ around O2C–C␣ bond, and C␣–C␤
and C␤–C␥ are trans. The third model TGT means C␣–C␤ tak-
ing gauche conformation, and OOC–C␣, C␤–C␥ are trans. The
forth model is CGT. This means the COO group is rotated by
90o, C␣–C␤ bond takes gauche, and C␤–C␥ is trans. We will
discuss these four models, TTT, CTT, TGT, and CGT as follows.
All models are shown in Fig. 3.
Table 1 lists the observed frequencies (cm−1) and the calcu-
lated frequencies for the TTT, CTT, TGT, and CGT models in
Fig. 2. IR spectrum of zinc stearate of (a) unannealed and (b) annealed specimen
in the 400–1500 cm−1 region. In the annealed spectrum, defect bands appear at
858, 823, 793, 766, 688, and 604 cm−1 as shown by arrows.
the CH2 wagging progressive region from 1180 to 1370 cm−1
.
All calculated frequencies for the four models agree with the

1050
T. Ishioka et al. / Spectrochimica Acta Part A 66 (2007) 1048–1051
Table 2
Onserved νobs and calculated frequencies (cm−1) in the C–C stretching region
νobs
TTT
CTT
TGT
CGT
1116
1127
1097
1127
1096
1126
1091
1126
1092
1074
1063
1073
1071
1069
1066
1071
1070
1068
1066
1084
1071
1070
1067
1077
1071
1070
1067
1063
1062
1061
1064
1061
1056
1065
1062
1057
1065
1061
1056
1052
1032
1048
1038
1019
1010
1049
1032
1016
1011
1045
1035
1016
1005
1045
1032
1014
1005
Fig. 3. TTT, CTT, TGT, and CGT molecular models where open circles are
ascribed carbon or oxygen, and filled circles are hydrogen atom. They have
conformational disorder at the COO end.
993
972
992
975
989
976
989
973
983
973
observed frequencies within 10 cm−1. This means the all mod-
els explain the observed wagging frequencies.
969
968
963
960
For the TTT, CTT, TGT, and CGT, see the text.
Table 3
Table 2 shows the observed frequencies (cm−1) and calcu-
lated frequencies for the four models in the C–C stretching
region from 960 to 1130 cm−1. In this region, the observed
frequencies also agree with the calculated frequencies for all
models within 12 cm−1. This result indicates that we cannot
identify which model corresponds to the observed disordered
conformation bands for the annealed specimen. So, we carry
out a comparison of the frequencies in the CH2 rocking region
from 700 to 950 cm−1, where the new bands appeared at 858,
823, 793, and 766 cm−1 of the annealed zinc stearate specimen.
Table 3 shows the result of the observed frequencies in the
CH2 rocking region from 700 to 920 cm−1, where νall corre-
sponds to all-trans conformation, νannealed represents the defect
bands. The calculated frequencies (cm−1) are also shown as
TTT, CTT, TGT, and CGT. The observed new frequencies
(cm−1) that appeared of the annealed specimen at 858, 823,
793, and 766 cm−1. These frequencies of new bands correspond
fairly well to those of the CGT and TGT models. Based on the
results described above, partial disordered state expected from
the observed infrared spectrum of the annealed specimen may
be assigned to the TGT or CGT conformation at the COO end.
In order to distinguish between the TGT and CGT models, we
Observed and calculated frequencies (cm−1) in the CH2 rocking region
νall
νannealed
TTT
CTT
TGT
CGT
913
878
844
810
778
750
724
703
910
875
840
807
775
746
722
701
919
898
861
825
790
758
730
707
900
885
861
824
790
758
730
707
875
840
808
780
758
744
858
823
793
766
For the TTT, CTT, TGT, and CGT, see the text.
use the observed new band at 688 and 604 cm−1 of the annealed
specimen.
Table 4 shows the results of the observed frequencies of the
COO group vibrations. That is, the COO bending, the COO
out-of-plane and the COO rocking. Observed bands ascribed to
all-trans conformation are νall, and defect band is designated as
νannealed as in the case of Table 3. νall corresponds well to TTT
conformation. The νannealed at 688cm−1 can be ascribed to both
TGT or CGT models, but 604 cm−1 band should be ascribed
only to TGT.
Table 1
Observed νobs and calculated frequencies (cm−1) in the CH2 wagging region
where TTT, CTT, TGT, and CGT, see the text
In conclusion, zinc stearate have the all-trans conformation at
room temperature, but its annealed specimen has almost all-trans
νobs
TTT
CTT
TGT
CGT
1360
1340
1326
1306
1287
1266
1247
1228
1208
1188
1364
1348
1331
1314
1282
1266
1248
1227
1203
1178
1364
1349
1331
1315
1284
1268
1250
1229
1205
1180
1361
1344
1325
1308
1290
1273
1255
1231
1206
1180
1361
1344
1325
1308
1291
1262
1252
1231
1207
1180
Table 4
Observed νall and νannealed (cm−1), and calculated frequency region in the COO
bending, the COO out-of-plane, and the COO rocking region
Description
νall
νannealed
TTT
CTT
TGT
CGT
COO bending
COO out-of-plane
COO rocking
722
580
550
688
604
712
579
548
717
652
526
695
584
561
688
652
572
For the TTT, CTT, TGT, and CGT, see the text.

T. Ishioka et al. / Spectrochimica Acta Part A 66 (2007) 1048–1051
1051
conformation but partially disordered conformation of TGT at
the COO end.
[2] I. Konkoly-Thega, I. Ruff, S.O. Adeosum, S.J. Sime, Thermochim. Acta 24
(1978) 89.
[3] M. Maroncelli, S.P. Qi, H.L. Strauss, R.G. Snyder, J. Am. Chem. Soc. 104
(1982) 6237.
[4] M. Goto, E. Asada, Bull. Chem. Soc. Jpn. 51 (1978) 2456.
[5] J.H. Schachtschneider, R.G. Snyder, Spectrochim. Acta A 19 (1963)
117.
References
[1] T. Ishioka, K. Maeda, I. Watanabe, S. Kawauchi, M. Harada, Spectrochim.
Acta A 56 (2000) 1731.
[6] R.G. Snyder, J. Chem. Phys. 47 (1967) 1316.