The structure and thermal behaviour of some long chain cerium(III) carboxylates

Article abstract of DOI:10.1039/a800326b

The even chain length cerium(III) carboxylates from the octanoate to the octadecanoate have been synthesised by metathesis. Thermogravimetry shows the presence of coordinated water for the short chain homologues, whereas the longer chain ones only contain adsorbed water. X-Ray diffraction and IR spectral measurements show that the solid phase has a lamellar, bilayer structure with planes of the cerium(III) ions coordinated to the carboxylate groups. The phase behaviour of the carboxylates has been studied by DSC and polarized-light microscopy. One or more mesophases are observed over the temperature range 70-120°C and melting occurs between 130 and 150°C. The textures observed on the polarizing microscope clearly show the anisotropic nature of the mesophases. Although the overall enthalpy and entropy of melting of these compounds increase with increasing chain length, the values are considerably lower than expected for complete fusion of the alkyl chains. Competition between melting of the chains and changes in the metal-carboxylate coordination region is the major factor responsible for the differences observed in the phase behaviour between the short and long chain derivatives.

Full text of DOI:10.1039/a800326b

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The structure and thermal behaviour of some long chain cerium(III)
carboxylates
Eduardo F. Marques, Hugh D. Burrows*¤ and Maria da Grac
Ó a Miguel
Departamento de Qu•mica, Universidade de Coimbra, 3049 Coimbra, Portugal

The even chain length cerium(III) carboxylates from the octanoate to the octadecanoate have been synthesised by metathesis.
Thermogravimetry shows the presence of coordinated water for the short chain homologues, whereas the longer chain ones only
contain adsorbed water. X-Ray di†raction and IR spectral measurements show that the solid phase has a lamellar, bilayer
structure with planes of the cerium(III) ions coordinated to the carboxylate groups. The phase behaviour of the carboxylates has
been studied by DSC and polarized-light microscopy. One or more mesophases are observed over the temperature range 70È
120 ¡C and melting occurs between 130 and 150 ¡C. The textures observed on the polarizing microscope clearly show the aniso-
tropic nature of the mesophases. Although the overall enthalpy and entropy of melting of these compounds increase with
increasing chain length, the values are considerably lower than expected for complete fusion of the alkyl chains. Competition
between melting of the chains and changes in the metalÈcarboxylate coordination region is the major factor responsible for the
di†erences observed in the phase behaviour between the short and long chain derivatives.
The amphiphilic character of the long chain carboxylates of
polyvalent metal ions is valuable for their incorporation into a
variety of organic matrices. These systems Ðnd extensive
applications as catalysts, dryers, polymer stabilisers, etc.1,2
Many of these applications are related to the thermal proper-
ties of these compounds.3 Because of their catalytic activity,
spectroscopic and magnetic properties, it is anticipated that
the long chain carboxylates of trivalent lanthanide ions will be
of considerable interest.
However, surprisingly little has been reported on these
compounds. R. C. Mehrotra and co-workers have reported
the synthesis of the laurates, palmitates and stearates of LnIII,
CeIII, PrIII and NdIII.4,5 Thermal decomposition, and some
magnetic measurements have been discussed.4h7 Electronic
spectral measurements have been described for neodymium
carboxylates,8 and lanthanum(III) has been used as a releasing
agent in atomic absorption spectroscopy.9 K. N. Mehrotra et
al. have reported results on some trivalent lanthanide(III) car-
boxylates, with particular interest in their solution proper-
ties.10h14 Further, it has recently been reported that long
chain cerium(III) carboxylates can be used to make photosensi-
tive polymer Ðlms.15 However, in spite of the value of these
results, there is a clear need for further systematic study in this
area. We have prepared the even chain cerium(III) carbox-
ylates, from the octanoate to the octadecanoate, and the odd
chain length heptadecanoate. Structural data and thermal
properties of these systems are reported.
tris(carboxylate); (iii) washing the precipitate with triply dis-
tilled water and ethanol and drying in a vacuum desiccator for
several days. The synthesised soaps are insoluble in water and
it has been reported that they can be recrystallised from
benzene.4,6 However, after testing this and other organic sol-
vents, it was found that they could only readily be solubilised
in hot n-hexanol. The soaps were recrystallised at least twice
in this solvent; a brownish hexanolic solution forms for tem-
peratures ranging from 60 ¡C (CeC ) to 80 ¡C (CeC ), from
which crystals precipitate after several hours, or days for the
shorter chain soaps. Melting temperatures as obtained by
heated stage optical microscopy are: octanoate, 153 ¡C; deca-
noate, 143 ¡C; dodecanoate, 146 ¡C; tetradecanoate, 130 ¡C;
hexadecanoate, 130 ¡C; heptadecanoate, 128 ¡C; octadeca-
noate, 132 ¡C.
8
18
Elemental analysis of hydrogen, carbon and cerium of the
synthesised soaps are presented in Table 1. C and H analysis
was done by Butterworth Laboratories Ltd (UK). Cerium was
determined by thermogravimetric analysis in an oven at a
temperature of 800 ¡C. These soaps are stable until ca.
300 ¡C,4 after which they slowly decompose to give cerium(IV)
oxide, CeO , as the Ðnal product.6 After 1È2 h in an oven at
2
800 ¡C, full decomposition occurs and the yellowish oxide
residue is weighed. Since the elemental analysis was not con-
clusive on whether the soaps were anhydrous or hydrate
(bearing either adsorbed or structural water), thermogravi-
metry (TG) on a microbalance was carried out, for the tem-
perature range of 50È200 ¡C using a Polymer Laboratories PL
STA 1500 TGA apparatus. For the X-ray powder di†raction,
a Philips di†ractometer was used, equipped with a PW-1730
generator and PW-1840 goniometer, using monochromatic
Cu-Ka (j \ 1.54 Ó) radiation. Estimated errors in are nor-
mally ^0.02 Ó, but are slightly higher in the small angle
region. IR spectra of the crystalline samples as KBr pellets
were obtained at room temperature using a PU9800 FTIR
Philips Analytical spectrometer over the wavenumber range
3200È400 cm~1, using 16È32 scans, with a resolution of 4
cm~1. UVÈVIS absorption spectra of thin solid Ðlms (O0.5
cm) were measured on a quartz slide with a Shimadzu
UV-2100 spectrometer. Luminescence excitation and emission
spectra for CeC and CeC were recorded on a Spex Flu-
Experimental
Even chain length carboxylates of cerium(III) (octanoate to
octadecanoate), and the odd chain length heptadecanoate,
were synthesised. These compounds have the general structure
CeIII[CH (CH ) CO ] (n [ 2 \ 6, 8, 10, 12, 14, 15, 16) and
3
2 n~2
2 3
are designated CeC , where n is the total number of carbon
n
atoms of the chain. These soaps were prepared by metathesis
in alcoholic solution,4,6 involving: (i) neutralisation of the acid
with KOH in boiling ethanol; (ii) slow addition of an aqueous
solution of cerium(III) nitrate and precipitation of the cerium
1
0
18
orolog 111 spectroÑuorimeter at room temperature and 77 K
¤
E-Mail: burrows=cygnus.ci.uc.pt
(with samples in a quartz Dewar Ñask containing liquid N ),
2
J. Chem. Soc., Faraday T rans., 1998, 94(12), 1729È1736
1729

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Table 1 Results (%) for the elemental analysis of cerium(III) carboxylates (n is the number of C atoms per chain)
Ce
H
C
H O
2
calc.
anhydr.
calc.a
monohydr.
calc.
anhydr.
calc.
monohydr.
calc.
anhydr.
calc.
monohydr.
calc.
monohydr.
n
found
found
found
foundb
8
7.69
8.66
9.26
7.96
8.79
9.42
8.06
8.85
9.46
49.06
53.81
57.59
60.81
62.16
62.65
64.65
50.60
55.10
58.59
61.35
63.61
64.59
65.47
49.04
53.63
57.19
60.04
62.37
63.38
64.31
25.45
21.29
19.57
17.56
16.05
15.60
14.49
24.59
21.43
18.98
17.04
15.46
14.77
14.15
23.84
20.85
18.53
16.68
15.16
14.50
13.89
3.03
2.80
2.00
0.60
0.55
È
3.07
2.68
2.38
2.14
1.95
È
1
1
1
1
1
1
0
2
4
6
7
8
9.71
9.93
9.96
10.72
10.71
10.32
10.34
10.52
10.68
10.36
10.53
10.69
0.58
1.79
a Value for monohydrate soap, Ce(OOCR) É H O. b Weight % loss for the range 50È150 ¡C, as obtained by TG.
3
2
with 1.25 mm slits for both monochromators. For CeC1
, the
X-ray di†raction. X-Ray di†ractograms of the solid soaps
show clear peaks for the low angle region, up to Ðfth order
reÑection, with the corresponding d Bragg spacing values in
the ratio 1 : 1/2 : 1/3 . . . 1/n. A typical di†ractogram for
cerium(III) octanoate is shown in Fig. 2. The d values corre-
spond to the successive (00l) reÑections through planes of
atoms of a lamellar bilayer structure, whose separations are
proportional to twice the length of the carboxylate molecule.
The alternative monolayer molecular organisation, which has
been suggested for some long chain lead(II) carboxylates,16 is
ruled out. However, these peaks are rather weak and broad, in
comparison to the sharp and intense peaks expected for car-
boxylates of heavy metals bearing a well deÐned crystalline
structure.17 This e†ect could be due to the presence of struc-
tural or adsorbed water as an “impurityÏ, which causes defects
on the structure of the crystalline lattice, to a high X-ray
absorption by the CeIII ion, or, even, to a combination of both
e†ects. The wide angle region, corresponding to reÑections
through planes separated by short distances parallel to the
molecular long axis, shows very weak and rather ill deÐned
peaks. In a crystalline lamellar bilayer structure the two alkyl
chains are totally stretched in an all-trans conformation and
orthogonal to the metal ion basal plane. For the cerium soaps,
8
spectra of a hexanolic solution (90 ¡C) was also recorded, with
the sample cell inserted in an aluminium block, heated by a
pre-adjustable temperature set.
The thermotropic behaviour of the cerium soaps was
studied by di†erential scanning calorimetry (DSC) and
polarized-light microscopy. For DSC studies a DSC-7 Perkin
Elmer calorimeter was used, with sensitivity 0.001 ¡C and
0.001 lW. Finely divided powder samples were sealed in per-
forated 25 ll Al pans and run at heating/cooling rates of
2
.5 ¡C min~1. Thermograms were recorded in N atmosphere,
2
using empty aluminium pans as reference. For calibration of
temperature and enthalpy an In standard was used. Polarizing
microscopy studies used a Nikon OpthiphotÈPol polarized
light microscope. Samples between cover slips were put on top
of an aluminium heating block, with a PT 100 3F probe,
coupled to a temperature control unit DSTR-R2-P12 (0.5 ¡C)
and observed under polarized light on heating over the tem-
perature range 25È200 ¡C. Temperature calibration used a
benzoic acid standard (mp 122È123 ¡C).
Results and Discussion
the predicted lamellar spacings d are:
max
d
\ 2d
] 2(n [ 1)d
sin 55¡ ] 2d
] 2r
3
`
max
ChH
ChC
ChO
Ce
Characterisation of the solid phase
where n is the total number of carbon atoms in the chain.
Taking d
\ 1.09 Ó, d \ 1.54 Ó, d
ChC ChO
values obtained experimen-
\ 1.36 Ó and
ChH
Elemental analysis. Results for C, H and Ce elemental
analysis of the synthesised carboxylates are shown in Table 1,
together with the calculated values for the two most probable
forms, the anhydrous and the monohydrate soaps. From the
comparison between experimental and calculated values the
structure cannot unambiguously be assigned. Therefore, TG
measurements were carried out to determine the water
content in terms of any weight loss over the range 50È150 ¡C.
In the thermograms, all samples show loss of weight over this
region, but there are marked di†erences between the soaps
r
\ 1.18 Ó, the average d
3`
Ce
max
tally by X-ray di†raction, corresponding to the distance
between two consecutive CeIII basal planes, are in good agree-
ment with the calculated values for a lamellar bilayer arrange-
ment (Fig. 3). A further feature of the di†ractograms is the
occurrence of a broad, intense peak at 4.1 Ó, for CeC14 and
(
Table 1). CeC8
and CeC show a sharp weight loss in the
10
region 80È110 ¡C, with the total loss equivalent to that
expected for the monohydrate state. CeC shows a contin-
1
2
uous weight loss over the full temperature range, in reason-
able agreement with the presence of one water molecule. The
longer chain soaps, however, show a gradual loss for 60È
100 ¡C, much smaller than expected for the monohydrate
state. Representative thermograms are shown in Fig. 1 for the
short chain decanoate and for the long chain octadecanoate.
It is clear that CeC and CeC are monohydrates (one coor-
8
10
dinated water), while the remaining soaps do not have coordi-
nated water, but have adsorbed water, varying between 1
molecule for CeC1
and much less for CeC
. Some di†er-
2
14h18
ences are anticipated in the structure and thermal behaviour
of the short and long chain length soaps, with the dodeca-
noate being an intermediate case.
Fig. 1 Typical TG curves obtained for cerium decanoate (CeC
0)
1
and octadecanoate (CeC
8)
1
1730
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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absence for CeC and CeC might indicate that the coordi-
8
10
nated water molecule in these cases packs well into the crys-
talline lattice.
IR spectroscopy. Vibrational spectroscopy of long chain car-
boxylates provides information on metal ion-carboxylate
coordination, chain conformation and chain packing (see ref-
erences in ref. 3). The IR spectra of all the cerium(III) soaps
were recorded. Absorption due to water OH stretching modes
occurs for the range 3500È3300 cm~1 and was observed for
the octanoate and decanoate as a broad band of medium
intensity centred at ca. 3450 cm~1. This supports a hydrate
structure for these soaps. For the longer chain soaps the rela-
tive intensity of this band decreases signiÐcantly (becoming
vestigial for the octadecanoate) in agreement with the pres-
ence of a small amount of adsorbed water.
In Fig. 4(a) spectra for the region 2000È400 cm~1 are com-
pared and in Table 2 the most intense vibrational bands and
their assignment are listed. For detailed discussion, we divide
spectra into four regions: 3000È1600, 1600È1350, 1350È1110
and 1110È400 cm~1.
Fig. 2 Typical X-ray di†ractogram observed for cerium(III) octa-
noate
the higher chain length compounds. For CeC and CeC it is
3000È1600 cm~1 region. The symmetric and antisymmetric
stretching modes for the methyl and methylene groups occur
in this region as strong bands.19,20 The absence of a strong
band at ca. 1700 cm~1, assigned to the CxO stretching vibra-
tion of carboxylic acids, conÐrms the equivalence of the two
CO bonds and absence of free acid in the synthesised soaps. A
weak band is observed at 1650 cm~1 for the octanoate and
decanoate, and only a very weak trace is seen with the dode-
canoate. Weak absorptions in the 1750È1600 cm~1 range are
also seen for the longer chain soaps. The band at 1650 cm~1
has been assigned to the bending (angular deformation) mode
of adsorbed water (1600 cm~1 for free water) and suggested to
be indicative of the hydrate state of soaps.21,22 This provides
8
10
not possible to detect this peak and for CeC it is rather
1
2
weak and broad. The wide angle region (d \ 5 Ó) is associated
with the di†raction of X-rays by planes parallel to the molecu-
lar long axis (lateral planes). An intense and di†use band at
4.5 Ó in heavy metal carboxylates is typical of disordered
alkyl chains.17,18 The occurrence for the longer chain cerium
soaps again suggests the possibility of a perturbation of the
crystalline order by the presence of adsorbed water. Its
further support for a monohydrate form for CeC and CeC
For the remaining soaps, the vestigial IR absorption is attrib-
uted to the small amount of adsorbed water.
.
8
10
1
600È1350 cm~1 region. The absorption bands for this
range come from two sources: the carboxylate (COO~) group
and the methylene (CH ) groups. The antisymmetric and sym-
metric stretching modes for the carboxylate vibration,
2
l (COO) and l (COO), show as very strong bands in the
as
s
ranges 1530È1545 cm~1 and 1406È1414 cm~1, respectively,
for the di†erent soaps. According to several authors, the fre-
quencies and relative position of these two bands, *l \ l
as
[
l are of particular relevance for the elucidation of the type
s
of carboxylateÈmetal ion coordination.23h25 The frequency
shift of these bands relative to those of free carboxylate ion,
Fig. 3 Comparison between the experimental X-ray di†raction
l
(taken as those of Na` or K` soaps), strongly depends on
the coordination. The three main types of carboxylateÈmetal
(
circles) and calculated (triangles) values for the lamellar spacing d
ion
max
of the solid phase of cerium(III) carboxylates as a function of chain
length (number of C atoms per chain)
ion coordination are illustrated in Fig. 5.
Table 2 Frequency (in cm~1) of the main absorption bands observed for the IR spectra of cerium(III) soaps and their vibrational assignment
na
vibrational
8
10
12
14
16
17
18
assignmentb
2
2
2
2
1
1
1
1
1
1
957
925
877
855
532
464
451
406
382
106
2957
2925
2877
2853
1530
1464
1458
1410
1382
1110
722
2957
2921
2875
2851
1537
1468
1447
1412
1380
1110
722
2957
2921
2874
2851
1545
1468
1453
1412
1385
1112
721
2957
2917
2876
2851
1541
1468
1458
1412
1380
1110
721
2955
2919
2877
2851
1541
1468
1458
1420
1384
1111
723
2957
2919
2877
2849
1543
1468
1456
1414
1384
1106
721
l (CH )
as
2
l (CH )
s
3
l (CH )
s
3
l (CH )
s
2
l (COO)
as
d(CH )
2
d (CH )
as
3
l (COO)
s
d (CH )
s
3
l(CwC)
7
21
78
o(CH )
2
6
678
678
682
680
684
682
d(COO)
a Number of C atoms per carboxylic chain. b l, stretching; d, bending; o, rocking; as, antisymmetric; s, symmetric.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
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Fig. 4 IR spectra of cerium(III) carboxylates: (a) comparative view for all compounds in the region 2000È400 cm~1; (b) expanded view of the
region 1350È1000 cm~1, where the relation between the number of bands of the wagging progrssion (dots) and chain length (number of C atoms
per chain) is illustrated, for the even chain decanoate (10), tetradecanoate (14) and octadecanoate (18) and the odd chain heptadecanoate (17); (c)
expanded view of the region 800È600 cm~1 for the octanoate (8), decanoate (10) and dodecanoate (12)
Monodentate coordination removes the equivalence of the
is known to be of the bridging type;27 (iii) for CeIII l and *l
s
CO bonds, and results in an increase in l (COO) and a
values are comparable to those for ZnII and PbII soaps, which
as
decrease in l (COO), such that *l
is much greater than
have a chelating (asymmetric) coordination.26 Since there is
no vibrational evidence for the non-equivalence of the CO
bonds in the carboxylate, we propose symmetrical chelating
coordination for the cerium carboxylates.
s
mon
*
l
(ca. 138 cm~1). For the two symmetrical bidentate struc-
tures, Fig. 5(b) and (c), the frequencies shift in the same direc-
tion, since both CO bond orders change by the same amount.
ion
From studies with metal haloacetates, an increase of l (COO)
In addition to the COO stretching modes, CH and
as
2
CH bending modes are also encountered in this region.27
3
in relation to the values for the ionic soaps is indicative of a
bridging coordination, whereas a decrease correlates with a
The absorption peak for 1464È1468 cm~1 is assigned to the
chelating coordination.25 Further, *l
(100 cm~1) @
CH deformation, as supported by the increase in relative
chel
(150È170 cm~1). Table 3 presents a summary of the *l
2
*
l
intensity of this peak with an increase in the number of
br
values for the carboxylates of CeIII (this work), Na`, ZnII and
PbII,26 and CuII.27 From a comparison of the frequency
values, several conclusions can be drawn. Clearly, a mono-
dentate structure for the cerium soaps is ruled out, since both
vibrational modes are shifted to lower frequencies than in the
ionic soaps. Further, (i) l (Ce) is lower than l (Na); (ii) l (Ce)
methylene groups in the chain. The band immediately follow-
ing at 1447È1458 cm~1 is assigned to the antisymmetric
bending of CH while the symmetric bending mode occurs as
3
a weak band at 1380È1385 cm~1.
1350È1100 cm~1 region. For all the CeIII soaps, a progres-
sion of regularly spaced bands is found in the range 1350È
as
as
as
is much lower than l (Cu), in which the CuwO coordination
1150 cm~1 corresponding to the wagging modes of CH
as
2
groups, typical for long aliphatic chains.21,26h30 Previous
studies with carboxylic acids and metal soaps gave an empiri-
cal relation between the number of wagging bands and the
alkyl chain length.21,28 As with sodium, silver and barium
soaps,28 for the even chain length Ce soaps the following
relationship is observed: number of bands \ number of C
atoms/2. This is illustrated in Fig. 4(b) for the decanoate,
tetradecanoate and octadecanoate. For the octanoate and
decanoate, in addition to the wagging progression, additional
Fig. 5 Schematic representation of the main types of metalÈ
carboxylate coordination: (a) monodentate; (b) chelating bidentate
(
symmetric) and (c) bridging bidentate
Fig. 6 Typical DSC curves for cerium(III) carboxylates, where heating (h), cooling (c) and reheating (r) runs are shown: (a) cerium octanoate; (b)
cerium decanoate and (c) cerium octadecanoate
1732
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Table 3 Frequency (in cm~1) of the COO~ symmetric and antisymmetric stretching modes and their separation for the carboxylates of di†erent
metals
na
metal
8
10
12
14
16
18
Nab
l (COO)
1565
1427
138
1532
1406
126
1587
1416
171
1541
1400
141
È
È
È
1563
1427
136
1537
1412
125
1587
1417
170
1543
1401
143
È
È
È
1545
1412
133
È
1563
1425
138
1541
1412
129
1585
1416
169
1541
1400
141
1563
1425
138
1543
1414
129
È
È
È
1544
1400
144
1531
1421
110
as
l (COO)
s
*
l
CeIII
l (COO)
1530
1410
120
1586
1416
170
1543
1400
142
1532
1416
116
as
l (COO)
s
*
l
CuII c
ZnII b
PbII b
l (COO)
as
l (COO)
È
È
s
*
l
l (COO)
1542
1400
141
1528
1417
111
as
l (COO)
s
*
l
l (COO)
1532
1415
117
1532
1418
114
1532
1419
113
as
l (COO)
s
*
l
a Number of C atoms per carboxylic chain. b Values from ref. 22; for Na, values are for n \ 7, 11, 15,17, respectively. c Values from ref. 23.
and weaker regularly spaced bands are observed which corre-
Luminescence study. The cerium(III) ion shows efficient lumi-
nescence at room temperature in variety of solid
spond to coupled twistingÈrocking modes of CH groups
a
2
(
superimposed on the wagging modes for higher chain
compounds33 and in solution,34h36 due to totally allowed
5dÈ4f electronic transitions within the ligand Ðeld. This is
highly sensitive to the coordination environment.37 Since the
possibility of using luminescent amphiphilic molecules as
structural probes in lipid membranes and organic matrices is
of fundamental interest, it was natural for us to study the
luminescent properties of the synthesised cerium soaps.
However, it was observed that both the decanoate and octade-
canoate only show very weak emission when excited at 265
nm, at room temperature, 77 K or in hexanol solution (at
100 ¡C for the octadecanoate). Thus, the carboxylate group in
the soaps appears to act as a strong quencher of cerium(III)
luminescence. This is not entirely unexpected, and quenching
of cerium(III) luminescence by the carboxylate group has pre-
viously been reported for a variety of compounds.33,35 This
suppression has been ascribed to an electron transfer process,
in which the excited 5d electron decays to a closely lying n*
orbital of the ligand.33 The implication of this quenching is
that the ceriumÈcarboxylate coordination in the present com-
pounds is not totally ionic but must have some degree of
covalence. These results are consistent with a bidentate chelat-
ing coordination of cerium(III) carboxylate as proposed from
the infrared spectral data.
lengths). For the odd chain length heptadecanoate [also
shown in Fig. 4(b)] the corresponding relationship is:28
number of bands \ (number of C atoms ] 1)/2. Finally, the
relatively strong band in the range 1106È1112 cm~1 is assign-
ed to a CwC stretching mode.30,31
1
000È400 cm~1 region. A band progression in the region
1
050È700 cm~1 is assigned to CH coupled twistingÈrocking
2
modes.21,26h30 This, however, consists of weaker and less
regularly spaced bands than the wagging progression. A weak
band at 940 cm~1 for all carboxylates is assigned to the CwC
stretching mode of the bond adjacent to the carboxylic ter-
minal of the chain, l(CC)COO.31 The band of medium inten-
sity at 721È723 cm~1, shown in Fig. 4(c) for the short chain
soaps, is assigned to the in-phase rocking mode of CH
groups. This band appears for crystalline compounds with
2
more than four consecutive CH groups and its shape is
2
highly dependent on the alkyl chain packing geometry for ali-
phatic compounds.29,32 For triclinic or hexagonal chain
packing a single band is observed, whereas the presence of a
splitting for this band is indicative of orthorhombic or mono-
clinic packing.29,32 For all the CeIII carboxylates studied a
singlet is observed, suggesting either a triclinic or a hexagonal
unit subcell.
The vibrational band at 678È684 cm~1 is assigned to the
Di†erential scanning calorimetry and thermal behaviour. It is
well known that the fusion of long chain metallic carbox-
ylates, such as those of divalent transition metals and lantha-
nides, is frequently a stepwise process in which one or more
intermediate liquid crystalline phases form upon heating and
before transition to the isotropic melt.3,38 Several lanthanide
ion soaps have been shown to possess thermotropic
mesomorphism3 and cerium(III) soaps are expected to follow
this trend. Observations with polarizing microscopy clearly
conÐrm the appearance of anisotropic textures for all cerium
soaps before complete fusion and well below the onset of
thermal decomposition (250È300 ¡C). To gain further under-
standing of their thermotropic behavior, the soaps were inves-
tigated by DSC over the range 25È200 ¡C, at a heating rate of
2.5 ¡C min~1. Cycles of heatingÈcoolingÈreheating were run
for all compounds. The thermodynamic parameters calculated
for endothermic processes taking place during the heating run
(associated in principle with phase transitions) are listed in
Table 4. For CeC , CeC and CeC the values for the
carboxylate group bending mode, d(COO).27 Its intensity rela-
tive to the neighbouring band o(CH ) decreases markedly
2
with the chain length, as can also be seen in Fig. 4(c). Further,
it appears as a doublet for CeC
, but it shows as a weak,
8h12
broad singlet from CeC onwards. These di†erences are
14
almost certainly due to the presence of structural or adsorbed
water for the shorter chain soaps, which a†ects the
carboxylateÈmetal interaction. Further support for this inter-
pretation comes from the fact that l (COO), and l (COO) to
as
s
a lesser extent, which are highly sensitive to di†erences in the
coordination region, are signiÐcantly lower for the shorter
chain soaps CeC8h12 than for the longer homologues, espe-
cially compared with its invariance with chain length for other
metallic soaps (Table 3). However, a di†erence in the type of
metalÈcarboxylate coordination or even, though less likely, in
the packing of the alkyl chains, cannot be excluded for the
two groups of soaps. The remaining bands occurring until 400
cm~1 may be due to stretching modes of CewO bond,
although only isotopic substitution studies will allow a deÐni-
tive assignment.27
8
10
12
reheating run are also listed. The temperatures of the Ðnal
transition are, in all cases, in good agreement with the
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1733

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Table 4 DSC transition temperatures, enthalpies and entropies for
cerium(II) carboxylates (n is the number of C atoms per chain)
pic peak for the octanoate is that at 377.7 K; the Ðrst tran-
sition at 367.0 K has a very low enthalpy change, and may
either correspond to a solidÈsolid type of transition or (since it
persisted upon reheating) to some other kind of pre-melting
transition. In contrast, for the decanoate the main enthalpic
peak is the Ðrst one at 350.7 K. This implies a direct transition
to a mesophase without pre-melting phenomena. Further, the
fusion is more gradual since a transition to a second meso-
phase occurs before the isotropic phase.
There is a minimal decrease in overall enthalpy for the
reheating process for the dodecanoate (ca. 5%). As with the
decanoate, two main phase transitions are observed before
melting to the isotropic liquid. However, there are similarities
to the octanoate, in that the Ðrst main transition is preceded
by a weak enthalpic peak at 348 K. Since this latter peak is
absent upon reheating it may correspond to a solidÈsolid type
of transition. The longer chain length derivatives only display
two endothermic peaks upon heating, the Ðrst strongly endo-
thermic and the second of very low enthalpy. Only one meso-
phase is thus present for these chains. An enthalpy decrease in
the range of 10È15% is observed upon reheating (values not
listed in Table 4), owing either to the loss of the small amount
of adsorbed water or to the formation of a metastable state of
the solid after the heatingÈcooling run. Supercooling is
observed for all cerium soaps, and is greater for the octanoate
and decanoate (ca. 10 ¡C) than for the longer soaps (ca. 5 ¡C).
This observation indicates that the mesophases are not truly
liquid crystalline but still possess a considerable degree of
crystalline order.
In Fig. 7 the values for the overall enthalpy and entropy of
transition from solid to liquid for the di†erent soaps are
plotted vs. chain length. The removal of the coordinated water
is seen to reduce the values for the octanoate and decanoate
and to give roughly linear dependences of *H and *S on
chain length. Although strictly speaking, one should have
plotted the reheating values for the longer chain derivatives,
this makes no signiÐcant di†erence. These results imply that
melting of the paraffin chains plays a strong role in the fusion
of these soaps.
A way of interpreting and rationalising phase transitions in
long alkyl chain compounds consists of decomposing the
enthalpy and entropy changes into their main contributions
and then comparing theoretical estimates with the experimen-
tal results. The enthalpy change associated with the fusion
process in metallic soaps can be decomposed as follows:41
T
trans/K
*
H/kJ mol~1
*
S/J K~1 mol~1
trans
trans
n
heating reheating heating reheating heating reheating
8
344.2
È
53.9
2.1
6.1
1.3
105.4
È
È
0.8
8.8
1.0
18.2
1.5
1.2
È
156.6
5.8
16.2
3.1
300.6
È
2.4
È
2.2
23.5
2.3
51.9
3.9
3.0
3
3
4
67.0
77.1
26.3
367.0
377.7
428.2
350.7
375.5
415.0
È
1
0
2
350.7
368)
(
4
14.8
1.0
3.0
1
348.3
8.6
È
3
4
4
86.6
04.4
18.6
388.0
405.1
421.0
15.9
3.7
2.0
19.0
3.3
1.6
41.1
9.1
4.8
49.0
8.1
3.8
1
1
1
1
4
6
7
8
379.2
05.5
385.7
05.2
388.6
01.2
391.1
06.2
51.8
2.1
62.5
1.1
66.6
1.0
79.5
1.1
136.6
5.2
162.1
2.8
171.4
2.5
203.2
2.6
4
4
4
4
observed melting points.
Marked di†erences are clearly apparent in thermal behav-
iour between the shorter chain (CeC ) and the longer chain
8
h12
(
CeC
) compounds. Firstly, the short chain length deriv-
14h18
atives have higher melting temperatures than the longer ones,
with an apparent decrease of the melting point with chain
length. Since such a trend is typical of alkali metal soaps,39,40
this clearly suggests that the cerium carboxylate has consider-
able ionic character. Secondly, the presence of structural water
has an expected dramatic inÑuence on the thermal behaviour
of the octanoate and decanoate when a heatingÈcoolingÈ
reheating cycle is run. The initial heating removes the water
molecule from the soap and as a consequence a drastic
decrease in the overall enthalpy of the fusion process (ca. 85%)
is observed upon reheating. However, di†erent behaviour is
found for these two compounds. For the octanoate, the endo-
thermic peak at 344.2 K totally vanishes on reheating, as can
be seen in Fig. 6(a), and only the other two phase transitions
remain before complete fusion. For the decanoate [Fig. 6(b)],
the phase transitions previously superimposed on the dehy-
dration process are resolved on reheating into two clearly
spaced peaks, albeit of much lower intensity. The main enthal-
*
H(solid ] liquid) \ *U ] *U
] *U ] P(*V )
conf
vdW
o
where *U
and *U
are the energy variations associated
conf
vdW
with intramolecular conformational disorder (introduction of
gauche conformations in the chains) and intermolecular van
der Waals interactions, respectively, and *U incorporates all
o
other energetic terms, such as electrostatic interactions. The
P(*V ) term is small compared with the others and can be
neglected. Using similar values for appropriate parameters to
those in our study of melting of long chain lead(II) carbox-
ylates,19 we calculate the contribution of conformational dis-
ordering and decreased van der Waals interactions to the
melting of cerium(III) decanoate to be
*U
] *U
\ 90.3 kJ mol~1
conf
vdW
This is considerably greater than the overall enthalpy change
solid ] liquid observed on reheating this compound (20.9 kJ
mol~1, Table 4), suggesting either that there is incomplete
fusion of the chains on melting, or that this is counterbalanced
by other energy terms, such as contributions from changes in
electrostatic interactions. Further information on this comes
from the observed linear increase of the overall enthalpy of
fusion with chain length. This is associated with both the
decrease in van der Waals interactions and introduction of
some degree of conformational disorder, and good quantitat-
Fig. 7 Overall enthalpy (* H) and entropy (* S) change for the
tot
tot
solidÈliquid transition vs. chain length of cerium(III) carboxylates;
*
H/*S heat. and *H/*S reh. are the values for the heating and
reheating run (shown only for the octanoate, decanoate and dodeca-
noate in this case), respectively
1734
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
tion of the electrostatic interactions which appears to occur
with the fusion is probably due to a decrease in the repulsive
interactions between the cerium trivalent ions. Lateral relax-
ation of the lattice may be accompanied by a longitudinal
change at the level of the headgroups, i.e., there may be some
fusion of the polar region leading to a perturbation of the
metalÈcarboxylate interaction, in a process parallel to that
which has been proposed for the melting of carboxylic acid
chains.45
In addition to electrostatic e†ects, incomplete fusion of the
hydrocarbon chains may contribute to the smaller slope of the
enthalpyÈchain length plot of the soaps, i.e., some degree of
conformational or lateral order may be left in the chains in
the molten state, in contrast with the usual melt of paraffins.
This has been previously suggested for the isotropic phases of
some divalent metal soaps.3,42 Evaluation of entropic changes
can be elucidating on this. The best Ðt to the *S plot
(
r \ 0.996) gives a slope of 2.4 J K~1 (mol CH )~1, consider-
2
ably lower than that expected for complete fusion of an alkyl
chain [10 J K~1 (mol CH )~1]. The isotropic liquid phase
2
may then be some kind of “organised solutionÏ, probably con-
taining small molecular aggregates. The fact that both the
enthalpy and entropy values for the transition to this phase
are very low indicates that these aggregates, if existing, may be
closely related in structure to the immediately preceding
mesophase. A third plausible explanation for the particularly
low melting enthalpy and entropy observed for CeC and
8
CeC on reheating, can be found in the possibility that, upon
1
0
water removal and cooling, these compounds have re-
crystallised into a metastable phase, where the chain arrange-
ment is not necessarily all-trans lamellar type or has a
di†erent chain packing. A combination of all these e†ects may
explain why, for example, the melting enthalpy of cerium
decanoate, 21 kJ mol~1, is considerably smaller than that for
n-octanosane (C
H
), 77 kJ mol~1, or for three moles of
2
8 58
decanoic acid, 3 ] 24 kJ mol~1.
The value of polarized-light microscopy in the assignment
of mesomorphic phases has long been recognised.46,47 From
our microscopy observations on the synthesised cerium(III)
carboxylates [Fig. 8(a)È(c)], it was not possible to unam-
biguously assign phase structures for the di†erent textures
that were observed. For the tetradecanoate and higher chain
soaps, an anisotropic texture with a grainy appearance and
some ray-like domains is clearly visible, in a range of 10È15 ¡C
before the melting transition [Fig. 8(c)]. For the dodecanoate,
the observations were similar and it was not possible to detect
two di†erent textures, as would be expected from the DSC
data. However, this was possible for the decanoate. A Ðrst
phase texture, somewhat reminiscent of focal conic and
homeotropic textures of a typical smectic A phase, occurred
for the range 363È378 K, thus corresponding to the Ðrst DSC
mesophase [Fig. 8(a)]. A second texture, less well deÐned and
with a higher density of isotropic domains, was visible for the
range 385È415 K, corresponding to the second DSC meso-
phase [Fig. 8(b)]. Finally for the octanoate, an incipient aniso-
tropic texture shows for the range 363È383 K, corresponding
to the Ðrst mesophase detected by DSC; for temperatures
higher than 393 K a viscous and pseudoÈisotropic texture
(showing a very weak birefringence between cross polars) is
apparent until complete melting at 423È428 K.
Although a deÐnitive assignment of the mesophases is not
possible at present, on the basis of the combined DSC and
polarizing microscopy data we may tentatively assign the
mesophase detected for the longer chain cerium soaps to a
disordered smectic phase. This phase consists of a lamellar
arrangement of the chains bearing some conformational dis-
order and larger lateral distances than in the crystalline state,
i.e., higher area per carboxylic headgroup. For the octanoate,
this mesophase is preceded by one of considerable solid-like
character, given the low enthalpy change involved for the
Fig. 8 Typical textures observed for cerium(III) soaps between
crossed polars on the hot stage of a polarizing microscope: (a)
cerium(III) decanoate at 100 ¡C, corresponding to mesophase I, similar
to typical focal conic and homeotropic texture of smectic A phase; (b)
cerium(III) decanoate at 125 ¡C corresponding to mesophase II with a
less deÐned texture and a higher density of isotropic domains; (c)
cerium(III) tetradecanoate at 120 ¡C with the texture having a granular
appearance and some ray-like domains typical of all the longer chain
length cerium(III) soaps 10È15 ¡C below the melting point
ive correlations with experimental data have been observed,
for example, with lead(II) soaps,19,42 where the metalÈ
carboxylate bond is relatively covalent. However, for the
cerium(III) soaps the best Ðt for the *H plot (r \ 0.981) gives a
slope of 2.5 kJ (mol CH )~1, a value somewhat lower than
2
that 3.8 kJ (mol CH )~1 associated with complete fusion of
2
aliphatic chains from their fully crystalline state.43 Further-
more, extrapolation to zero methylene groups gives an inter-
cept of *H \ [49 kJ mol~1 which implies that electrostatic
interactions give a strong negative contribution to melting.
JustiÐcation for the key role of electrostatics comes from the
facts that, Ðrstly, the ion is trivalent and, secondly, the
ceriumÈcarboxylate bond has an appreciable ionic character,
since the 4f valence orbitals of lanthanide ions have a limited
radial extension and therefore do not signiÐcantly overlap the
ligand orbitals to yield a covalent bond.44 The net stabilisa-
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1735

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8
9
R. C. Vickery, J. Mol. Spectrosc., 1958, 2, 308.
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chains in a state of higher rotational disorder.48 For the deca-
noate, the putative disordered smectic phase is followed by a
second mesophase, which is closely related in structure to the
molten phase. The dodecanoate seems to present the richest
thermal behaviour: the smectic mesophase is preceded by an
irreversible low enthalpy transition (probably of solidÈsolid
nature) and followed by a transition to another mesophase
closely related to the molten state in structure.
R. A. Mostyn, B. T. N. Newland and W. E. Hearn, Anal. Chim.
Acta, 1970, 51, 520.
1
0
K. N. Mehrotra, A. S. Gahlaut and M. Sharma, J. Colloid Inter-
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chem., 1988, 4, 513.
2
3
4
1
1
1
K. N. Mehrotra, M. Chauhan and R. K. Shukla, T enside, Sur-
factants, Deterg., 1992, 29, 432.
K. N. Mehrotra, R. K. Shukla and M. Chauhan, Bull. Chem. Soc.
Jpn., 1995, 68, 1825.
K. N. Mehrotra, M. Chauhan and R. K. Shukla, T enside, Sur-
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Concluding remarks
1
1
1
1
5
6
7
8
Y. Lin, J. Rare Earths, 1996, 14, 98.
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A combination of thermogravimetric, spectroscopic and calo-
rimetric data clearly shows that among the synthesised tri-
valent cerium carboxylates, the octanoate and decanoate are
in the monohydrate form while the remaining longer chain
ones are anhydrous compounds but contain a small amount
of adsorbed water. The solid phase of these metallic soaps
consists of a lamellar bilayer arrangement of the carboxylates
having all-trans chains, packed either in a triclinic or hexago-
nal lattice. A bidentate chelating coordination is proposed for
the metal ionÈcarboxylate pair and it has been found that the
ligand acts as a luminescence quencher for the trivalent
cerium, suggesting some degree of covalence of the metal
oxygen bonds. Some observations from the X-ray and IR data
suggest structural di†erences in pair coordination and/or
chain packing between the short and the long chain soaps,
certainly reÑecting the presence of the coordinated water mol-
ecule in the former. Consistent with this fact, the thermal
behaviour of the soaps is found to depend on chain length.
For the tetradecanoate and higher chain soaps only one meso-
phase occurs, suggested to be of a disordered smectic type.
The shorter chain soaps have a more gradual melting pattern,
involving two or more intermediate phases and higher melting
temperatures. Transition enthalpies show that electrostatic
interactions in the polar region (owing to the essentially ionic
character of the metalÈcarboxylate bond) play an important
role in the melting process, while the corresponding entropies
indicate an incomplete fusion of the chains and are thus con-
sistent with the presence of molecular aggregates in the melt.
Complementary crystallographic and spectroscopic studies
are in progress to further characterise the structural and
thermal properties of cerium(III) carboxylates.
19 C. G. Bazuin, D. Guillon, A. Skoulios, A. M. A. Costa, H. D.
Burrows, C. F. G. C. Geraldes, J. J. C. Teixeira-Dias, E. Black-
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0
N. B. Colthup, L. H. Daly and S. E. Wiberly, Introduction to
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975, p. 223.
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1
E. M. Kirby, M. J. Evans-Vader and M. A. Brown, J. Am. Oil
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22 D. D. Saperstein, J. Phys. Chem., 1987, 91, 2922.
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2
2
3
4
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K. Nakamoto, Infrared Spectra of Inorganic and Coordination
Compounds, Wiley, New York, 1963, p. 198.
F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry,
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G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33, 227.
26 M. A. Mesubi, J. Mol. Struct., 1982, 81, 61.
27 D. P. Strommen, A-M. Giroud-Godquin, P. Maldivi, J-C.
Marchon and B. Marchon, L iq. Cryst., 1987, 2, 689.
2
8
R. A. Meiklejohn, R. J. Meyer, S. M. Aronovic, H. A. Schuette
and V. W. Meloch, Anal. Chem., 1957, 29, 329.
R. G. Snyder, J. Mol. Spectrosc., 1961, 7, 117.
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3
9
0
31 M. F. R. Moita, M. L. T. S. Duarte and R. Fausto, Spectrosc.
L ett., 1994, 27, 1421.
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3
D. Chapman, in Colloquium Spectroscopcum Internacionale V I
(
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3
34 G. Blasse, G. J. Dirksen, N. Sabbatini and S. Perathoner, Inorg.
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S. T. Frey and W. DeW. Horrocks, Jr, Inorg. Chem., 1991, 30,
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073.
Financial support from PRAXIS XXI (project 2/2.1/QUI/411/
3
6
Y. Kaizu, K. Miyakawa, K. Okada, H. Kobayashi, M. Sumitani
94) is gratefully acknowledged. We are also grateful to Profs.
and K. Yoshihara, J. Am. Chem. Soc., 1985, 107, 2622.
J. J. Cruz-Pinto and M. H. Gil for the DSC facilities, Pro-
fessor M. T. Vieira for the TG experiments and Prof. M. L. P.
37 P. N. Hazin, C. Lakshminarayan, L. S. Brinen, J. L. Knee, J. W.
Bruno, W. E. Streib and K. Folting, Inorg. Chem., 1988, 27, 1393.
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8
H. D. Burrows, in T he Structure, Dynamics and Equilibrium
Properties of Colloidal Systems, ed. D. M. Bloor and E. Wyn-
Jones, Kluwer, Dordrecht, 1990, p. 415.
Leita
8 o for the FTIR spectral study. One of the authors
(
E.F.M.) also thanks JNICT and PRAXIS XXI for Ðnancial
support (grant BD/9295/96).
3
9
M. J. Vold, M. Macomber and R. D. Vold, J. Am. Chem. Soc.,
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940, 63, 168.
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Paper 8/00326B; Received 12th January, 1998
1736
J. Chem. Soc., Faraday T rans., 1998, V ol. 94