Crystal structure of 20-methyl-nonatriacontane ((C19H 39)2CHCH3 and its compatibility with nonatriacontane (C39H80)

Article abstract of DOI:10.1021/jp035629d

We synthesized a branched n-alkane sample M39 with a carbon number of the main chain n = 39 with the methyl group at the middle of the chain, and studied its crystalline structure and also compatibility with the corresponding linear homologue of C39. Results of SEM, DSC, X-ray diffraction, IR absorption, and Raman scattering measurements on solution-grown crystallized sample (SG-M39) and bulk-crystallized sample (BK-M39) reveal along with computer simulation that a triclinic type of crystal belonging to the space group PI is predominantly realized in BK-M39, whereas an orthorhombic type of P2₁2 ₁2₁ is found to be coexistent for SG-M39 crystal. The phase diagram is constructed for the BK-M39/C39 binary system, which shows that the system does not form a solid solution and the respective molecules with a small amount of contaminant of another component develop their own crystal structures separately over the entire range of molar fraction of M39 from 0.05 to 0.95 studied.

Full text of DOI:10.1021/jp035629d

J. Phys. Chem. B 2004, 108, 5827-5835
5827
Crystal Structure of 20-Methyl-Nonatriacontane ((C19H39)2CHCH3) and Its Compatibility
with Nonatriacontane (C39H80)
†
Hiroko Yamamoto and Norio Nemoto*
Department of Molecular and Material Sciences, IGSES, Kyushu UniVersity,
Hakozaki, Fukuoka 812-8581, Japan
Kohji Tashiro‡
Department of Macromolecular Science, Graduate School of Science, Osaka UniVersity,
Toyonaka, Osaka 560-0043, Japan
ReceiVed: June 9, 2003; In Final Form: March 8, 2004
We synthesized a branched n-alkane sample M39 with a carbon number of the main chain n ) 39 with the
methyl group at the middle of the chain, and studied its crystalline structure and also compatibility with the
corresponding linear homologue of C39. Results of SEM, DSC, X-ray diffraction, IR absorption, and Raman
scattering measurements on solution-grown crystallized sample (SG-M39) and bulk-crystallized sample (BK-
M39) reveal along with computer simulation that a triclinic type of crystal belonging to the space group P1h
1 1 1
is predominantly realized in BK-M39, whereas an orthorhombic type of P2 2 2 is found to be coexistent for
SG-M39 crystal. The phase diagram is constructed for the BK-M39/C39 binary system, which shows that the
system does not form a solid solution and the respective molecules with a small amount of contaminant of
another component develop their own crystal structures separately over the entire range of molar fraction of
M39 from 0.05 to 0.95 studied.
Introduction
branch is likely to influence the crystallization kinetics as well
as the equilibrium crystalline structure.
Polyethylene (PE) is known to have various lengths of
branches depending on catalysts used for polymerization along
with different molecular weight distributions from the most
As the main chain carbon number n decreases, alkanes tend
to crystallize, taking the extended chain conformation as the
energetically most favored one. In this case, introduction of
small functional groups such as the carbonyl or hydroxyl group
at the middle of the main chain hardly disturbs the crystalline
structure of their linear homologues. In earlier studies, this was
probable one to a very broad one with a ratio of weight-average
to number-average molecular weight as large as 20.1-4 Branched
groups and chain ends are both considered as defects to hinder
formation of a chain-folded single crystal. Nevertheless, it has
been long believed that such short branches as the methyl or
the ethyl group are mainly incorporated into lamellae of PE
crystals.
Quite recently it became clear from a morphological study
on two branched alkanes of high purity, C96H193CHRC94H189
with R being methyl or butyl, that such short branches are
rejected to the lamellar surface and eventually affect molecular
9
-18
proved indeed to be the case.
The subcell structure was the
same for all samples examined, and their melting temperatures
became higher than those of the linear homologues because of
dipole-dipole interaction or hydrogen bonding, whereas the
solid-solid-phase transitions characteristic of pure linear alkanes
1
9-24
disappeared.
On the other hand, attachment of bulky side
groups such as hydrocarbons or aromatic compounds without
having any specific interaction to the center of the main chain
does not contribute to stabilization of the crystal structure but
greatly impedes crystallization kinetics with an increase in a
size of the branch. In an extreme case, the main chain might
fold so that the bulky side groups may be rejected on the
lamellar interface, or the sample cannot crystallize any more
but might become a glass-forming material. Thus it seems
meaningful to perform a systematic study on effects of a branch
size on solid structures of alkanes that contain branches with
various lengths in the center of an otherwise linear chain taking
the extended conformation in the crystalline state.
deposition on the crystal growth face so as to take a once-folded
chain conformation.5-7 The branching effect was further studied
on asymmetric methyl-branched alkane C191H383CH(CH3)-
C99H199 by the same group. They showed that the sample can
8
be crystallized in two different semicrystalline forms depending
on crystallization temperature, which on further cooling trans-
form to a double layer and a triple layer crystalline structure,
respectively. In both structures, the methyl branches are found
to be always located on the lamellar interface, and the whole
main chain took the extended conformation in the double layer
structure, whereas the longer part of the chain was once-folded
at the position of the methyl branch in the triple one. Thus
topological restriction brought by introduction of the methyl
In this paper, we report results on the methyl-branched alkane
M39 with n ) 39 obtained from SEM, DSC, X-ray diffraction,
Raman scattering, and IR absorption measurements as well as
computer simulation. We find that M39 crystallizes as an
extended chain with the methyl group being inside the lamellae
and the crystals obtained with the solution-grown and bulk-
*
To whom correspondence should be addressed. Tel: +8192-642-3821.
Fax: +8192-642-3825. E-mail: nemo@mm.kyushu-u.ac.jp.
†
‡
E-mail: yamamoto@mm.kyushu-u.ac.jp.
E-mail: ktashiro@chem.sci.osaka-u.ac.jp.
1
0.1021/jp035629d CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/13/2004

5828 J. Phys. Chem. B, Vol. 108, No. 18, 2004
Yamamoto et al.
SCHEME 1
reacted in a nitrogen atmosphere at room temperature for 24 h.
Hydrolysis of the solution of (5) by dropwise addition of
aqueous solution saturated with ammonium chloride yielded the
2
7
product (6). After evaporation of solvent the product was
confirmed as a tertiary alcohol (6) from IR measurement and
gas chromatography. The tertiary alcohol (6) was reduced in a
water/ethyl alcohol mixture containing iodine and phosphorus
as reducing agents for 24 h at room temperature. In this reaction,
however, a double bond was occasionally introduced as a result
of a side reaction in the main or the side chain through an
intermediate stage of carbocation generation; thus the additional
hydrogenation reaction became necessary for conversion of the
double bond into the single one (reaction (7) f (8)) adding
hydrogen molecules in an autoclave. Products obtained were
finally purified with elution and recrystallization through a silica
column using hexane as solvent.
After solubility tests of pure C39 and M39 using several
organic solvents, we chose toluene and butanone for solution-
grown crystallization of C39 (SG-C39) and M39 (SG-M39)
respectively, because each solvent is not a bad solvent to C39
and M39 and crystallization took place at around room
temperature. It is to be noted that we could not obtain a
crystallized M39 sample from 3% toluene solution at a
temperature as low as 250 K. Bulk-crystallized C39 and M39
denoted BK-C39 and BK-M39, respectively, were prepared
with a slow cooling rate of 2.0 K/h from the melt at 120 °C.
Eight binary M39/C39 samples with a molar fraction fm in the
range 0.05-0.95 were prepared from respective homogeneous
solutions of C39 and M39 in hexane by evaporating the solvent
on a hot plate and then slowly cooling to ambient temperature.
crystallization methods both exhibit polymorphs taking two
different crystalline structures belonging to the space groups
P 1h and P212121. The difference between them is that an
orthorhombic type of crystal structure is mainly formed for the
solution-grown M39, whereas a triclinic type of crystal structure
is dominantly realized in the bulk-crystallized M39. The
crystalline structures determined in this work are in appearance
similar to the double layer structure of C191H383CH(CH3)C99H199
Homologous purities of C39 and M39 were determined by a
capillary gas chromatograph (GC-14A, Shimazu) as 99.9% and
99.6% respectively. The density was measured with a Gay-
8
proposed by Ungar et al. where all long alkane chains take
3
extended conformation, but are unique for packing of the central
methyl branch in cooperation with end methyl groups, which
gives rise to staggered alignment between the neighboring
extended chain with the relative translation of a half main chain
length. Compatibility between M39 and its linear homolouge
C39 is also examined from DSC measurements, which indicate
that the respective molecules, allowing a small amount of
contamination of another component, tend to develop their own
crystal structures separately over the entire range of molar
fraction of M39 from 0.05 to 0.95 studied.
Lussac pycnometer as 0.944 g cm for SG-C39 and 0.915 g
3
cm for SG-M39.
Methods. SEM was made for morphology observation of SG-
C39 and SG-M39 at room temperature with JSM-T100 (Ni-
hondenshi). DSC measurements were performed using a calo-
rimeter (DSC-8240B with a TAS-100 controller, Rigaku) for
SG- and BK-M39, SG-C39, and C39/M39 binary mixtures from
-
1
293 to 393 K. The standard heating rate of 1.0 K min was
employed under dry nitrogen atmosphere for a sample mass of
about 1.0 mg. The heating rate dependence was examined for
several samples of the M39/C39 system chosen randomly and
was found negligible. Temperature calibration was performed
using indium (In) and also shorter n-alkanes with known
equilibrium melting temperatures. The heat of transition or
fusion was calibrated using In and gallium.
Experimental Section
Materials. The long chain n-alkane with a branch at the
middle, 20-methyl-nonatriacontane ((C19H39)2CHCH3) denoted
M39, and the corresponding linear n-alkane C39 were used for
this investigation. The branched alkane was synthesized through
ketene dimerization and Grignard reactions using the corre-
sponding carbonic acid chlorides free from other homologues.
Details of synthesis are reported elsewhere.25 As is shown in
Scheme 1, carboxylic acid chloride (2) synthesized from
corresponding carboxylic acid (1) (SIGMA Co., purity; mini-
mum 99.0%) and thionyl chloride in argon atmosphere was
further reacted in ether solution with tertiary amine to obtain
alkyl ketene dimer (3). Symmetrical ketone (4) with a carbonyl
group at the middle of the main chain was easily obtained from
hydrolysis of (3). C39 was synthesized by reducing the
The X-ray diffraction measurements were performed for SG
and BK samples of C39 and M39 with a diffractometer using
Ni-filtered Cu KR radiation (Rigaku, Geigerflex 2027) at room
temperature. Structural changes in respective crystals with
increasing T were studied from room temperature to T just below
the melting temperature of SG-M39 at twelve or fifteen
temperatures. Raman spectra were obtained for SG-C39 and
SG-M39 samples with an NR-1800 Raman spectrometer (Japan
Spectroscopic Co). A beam line with 514.5 nm of wavelength
from an argon ion laser was used as an excitation light source.
IR spectra were obtained at room temperature using a FT-IR
spectrometer (FTS-6000, Bio-Rad) with 64 scans at a resolution
2
6
corresponding ketone (4) using the Wolff-Kishner method.
-1
Tertiary alcohol (6) was synthesized through the product (5)
of the meso compound from symmetrical ketone (4). Powders
of symmetrical ketone (4) were poured into 3 M tert-butyl ether
solution of a Grignard reagent (CH3-MgBr; Aldrich Co.) and
of 2 cm . To find the energetically most stable crystal structure,
a commercial software Polymorph Predictor, a module of
2
Cerius (version 4.0, Accelrys Inc.), was used. The unit cell
parameters of the crystal structural models obtained were slightly

(C19H39)2CHCH3 and Its Compatibility with C39H80
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5829
Figure 2. DSC curves of C39, SG-M39, and BK-M39. The weight of
-
1
the samples used is 1.0 mg, and the scanning rate, 1.0 K min
.
Figure 1. SEM photographs for (a) SG-C39 and (b) SG-M39.
modified so that they may give better reproduction of X-ray
diffraction data measured at room temperature keeping essential
features of the structure unchanged. These modified structures
were minimized again under the assumption of rigid body
molecules.
Figure 3. X-ray diffraction profiles of BK-M39, SG-M39, and C39
at room temperature.
Results and Discussion
Crystal Structure of M39. Direct ObserVation. Figure 1
shows SEM photographs of (a) SG-C39 and (b) SG-M39. C39
forms a typical lozenge shape of thin single crystal mat for
which the crystal grows along the (110) plane. In contrast, M39
takes a needlelike crystal different from C39, indicating that
the attachment of the methyl residue to the middle of the main
chain significantly affects a mechanism of crystal growth. The
difference shall be discussed later after the crystalline structure
of M39 is determined. Because the M39 crystal was obtained
as fine powders, it was not possible to use a four-circle X-ray
diffractometer to determine the crystal structure.
Thermal BehaViors. Figure 2 shows DSC curves of SG-C39,
SG-M39, and BK-M39. BK-C39 gave a DSC curve with almost
the same characteristic feature as SG-C39. The curve of C39
unambiguously exhibits the B and C solid-solid-phase transi-
tions at 333 and 341 K, respectively, and a sharp melting peak
at 353 K. On the other hand, melting peaks observed for SG-
and BK-M39 are quite broad as compared with the peak of C39,
and their melting temperatures Tm are 331.4 and 330.7 K,
respectively, being lower by more than 20 K in comparison
with Tm of C39. Lowering of Tm and broadening of the
endothermic peak are undoubtedly related so that M39 could
not form the single crystal but was forced to take a more or
less disordered crystal due to a difficulty of M39 molecules in
achieving close parallel chain packing. A small dip is observed
in the DSC curve near the bottom of the BK-M39 data. Repeated
measurements, however, showed that the position of the dip on
the temperature axis as well as its depth varied every run. At
present we have no explanation why this kind of distortion is
observed for the BK-M39 sample.
X-ray Diffraction Profiles. Figure 3 shows X-ray diffraction
profiles of SG-M39 and BK-M39 measured at room tempera-
ture, and also that of SG-C39 for comparison in a range of
diffraction angle 2θ from 3° to 40°. SG-C39 gives a sequence
of sharp diffraction peaks over a wide diffraction angle range
corresponding to (00l) reflections of the long spacing with even
number of l along the main chain c axis as well as two peaks
at 2θ ) 21.5° and 2θ ) 23.8° from the (110) and (200) planes
characteristic of linear alkanes with odd carbon number at room
2
2
temperature, i.e., of the A structure. On the other hand, only
a small number of broad diffraction peaks are observed in the

5830 J. Phys. Chem. B, Vol. 108, No. 18, 2004
Yamamoto et al.
Figure 4. Changes in X-ray diffraction profiles induced by heat treatment. (a) SG-M39: (1) 298.3 K, (2) 313.1 K, (3) 327.6 K, and then cooled
to (4) 297.6K. (b) BK-M39: (1) 294.6 K, (2) 312.1 K, (3) 327.4 K, and then cooled to (4) 294.6K.
low diffraction angle region for the M39 samples, and corre-
sponding intensities are very low, especially for BK-M39. This
is not an unexpected result for a powder-like crystal, whereas
broadness of the peaks might be due to superposition of two or
even three peaks. Even though the presence of these low angle
peaks must be taken into account in determining their crystalline
structures, the X-ray diffraction measurement after all does not
permit accurate determination of the long spacing L, and it
remains uncertain whether the main chain of M39 is fully
extended or once folded due to presence of the bulky methyl
group.
SG-M39 gives three strong peaks at around 2θ ) 19.3°, 21.1°,
and 23.5° with the largest intensity at 2θ ) 21.1°, of which the
peak at 2θ ) 19.3° is a new one with the spacing d ) 0.458
nm not present for the orthorhombic type of the subcell structure
as C39 and the peak at 2θ ) 23.5° appears to have a shoulder
at 2θ ) 23.0°. On the other hand, the sample gives two weak
peaks at 2θ ) 35.9° and 39.6° close to the positions where peaks
from (020) and (310) reflections are observed for C39. These
results lead to supposition that the SG-M39 crystal may be
polymorphous. BK-M39 also gives three strong peaks at around
the patterns in different manners. For SG-M39, two peaks at
2θ ) 19.3° and 21.1° remain essentially at the same positions
with a rise in T, whereas the shoulder observed for the broad
peak at 2θ ) 23.5° of the virgin sample (1) at T ) 298.3 K
tends to be split as an independent peak with increasing
temperature and this new peak at 2θ ) 23.2° is irreversibly
maintained after the sample is cooled to T ) 297.6 K in profile
4. Furthermore, comparison profiles 4 and 1 indicates that
intensities at 2θ ) 21.1° and 23.5° measured with peak height
noticeably increased with this heat treatment. Thus it seems
plausible that the broad endothermic peak of SG-M39 shown
in Figure 2 is related to formation of a rather distorted crystal
lattice with the solution-grown method or disordering of
molecular alignment brought by competitive formation of two
different crystalline structures, or to the both.
In the X-ray diffraction patterns of BK-M39 shown in Figure
4b, on the other hand, three characteristic peaks simply shift to
the lower θ with raising temperature probably due to thermal
expansion and, after annealing, they return to almost the same
angles with the peak shapes unchanged. There appears to be
no change in the crystal structure with increasing temperature
before the melting point is reached, even though a dip was
observed in the DSC curve in Figure 2.
2
1
θ ) 19.3°, 21.1°, and 23.1° with the largest intensity at 2θ )
9.3°, but no peaks at higher 2θ. Three main peaks imply that
the unit cell of BK-M39 is presumably a triclinic or a monoclinic
form, but we surely need to collect more information using other
experimental techniques to determine a type of the crystal lattice
and unit cell parameters, and also to examine a possibility if
BK-M39 might be polymorphous as SG-M39.
Because not only X-ray diffraction patterns but also DSC
peaks are broad for M39 samples, changes in diffraction profiles
with raising T were studied at twelve temperatures from room
temperature to T just below respective melting points and also
at room temperature after the samples were cooled. Parts a and
b of Figure 4 show X-ray diffraction patterns at four selected
temperatures for SG-M39 and BK-M39, respectively. As is clear
from the figure, thermal treatments appear to have influenced
Raman Scattering. The small-angle X-ray diffraction mea-
surement on M39 samples did not provide information sufficient
for determination of the long spacing L. Alternatively, confor-
mation of the main chain or the chain length in the crystalline
state can be unambiguously determined from Raman scattering
measurement using vibration modes of the longitudinal acoustic
mode (LAM), if the chain is depicted as a continuous elastic
rod. The jth LAM frequency of the vibration, νj, is given
1
/2
ν ) (j/2cL)(E /F)
(1)
j
c
where c is the speed of light in the medium, Ec is Young’s
modulus of the chain in the solid state, F is the density, and j

(C19H39)2CHCH3 and Its Compatibility with C39H80
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5831
Figure 5. Raman spectra from longitudinal acoustic modes (LAM)
for SG-C39 and SG-M39.
Figure 6. IR spectra for SG-C39, SG-M39, and BK-M39 in the CH
rocking mode region at 303.1K.
2
is the vibration order (only odd numbers are Raman active).
Figure 5 shows results of Raman scattering measurements on
SG-M39 and SG-C39 at room temperature. Three peaks are
whereas there occurs no splitting for the former. We performed
IR absorption measurements on powder samples of SG-C39,
SG-M39, and BK-M39, which were carefully sandwiched
between two KBr plates to minimize possible disturbances or
alteration of crystalline structures of the fragile samples by
applied pressure. It is to be noted that completely different IR
spectra were indeed obtained for M39 samples prepared with
such a conventional method that the samples are mixed well
with KBr powder and then pressed to make the disk.
-
1
clearly observed for SG-C39 at 62.6, 176, and 283 cm ,
corresponding to the LAM modes with j ) 1, 3, and 5,
respectively. Stroble and Eckel pointed out, from Raman
scattering experiments on linear alkanes CnH2n+2 with 33 e n
e 94, that weak interlamellar forces result in an upward shift
2
8
of νj given by eq 1, and proposed an empirical equation
νj ) 2236j/(n - 1.6) + 2.2/j
(2)
Figure 6 gives IR spectra of three samples in a relatively
-
1
narrow wavenumber region of 700-750 cm at 303.1 K. The
measurements were also made at elevated temperatures below
respective Tm’s, and essentially the same shape of the spectrum
was obtained for each sample. The well split IR bands are seen
in C39 as expected, but not for the two M39 samples. The
spectrum of SG-M39 is broad, but a small peak is found at 730
Equation 2 gives ν1 ) 62.0 cm-1 for n ) 39, which is in
excellent agreement with the experimental value of ν1 ) 62.6
-1
-1
-1
cm , and also ν2 ) 180 cm and ν3 ) 299 cm , being slightly
larger than the values measured. Three peaks are also observed
-
1
for SG-M39 at 62.3, 176, and 285 cm , which are in excellent
agreement with corresponding νj values of C39. Because C39
molecules are known to take the extended chain conformation,
forming the orthorhombic type of crystal, we conclude that M39
molecules are not folded but extended so as to take the planar
zigzag form in the solid state. In addition, close agreement of
νj values between C39 and M39 indicates that intermolecular
interactions responsible for symmetrical longitudinal backbone
vibration modes are almost the same for the two samples, even
though the methyl group attached to the center of the main chain
of M39 must be located inside the lamellae and is expected to
perturb the chain packing to some extent.
-
1
cm corresponding to the vibration mode parallel to the a axis
intrinsic for the orthorhombic type of subcell structure and the
-
1
peak at 720 cm is broader compared with the corresponding
one of SG-C39. Similar trends were also observed for the CH2
-1
deformation mode at 1465 cm . These IR results are in accord
with X-ray diffraction profiles of this sample, indicating
polymorphs composed of the orthorhombic and the triclinic or
monoclinic subcells for SG-M39. The IR spectra of BK-M39
look broader toward the low wavenumber direction, which may
be adressed to (1) more distorted chain packing in the local
subcell size when M39 is bulk crystallized and (2) superpositions
of the normal orthorhombic doublet and some additional band-
(s) from another phase, or to the both. Issue 1 may be supported
by that the molar enthalpy of fusion, ∆Hm, of BK-M39 is
IR Absorption. Results of X-ray diffraction measurement on
SG-M39 have suggested that the structure of the subcell is likely
to consist of two different types; one is a triclinic or a monoclinic
type supposed to be identical to the BK-M39 case and the other
an orthorhombic type. For the latter, the IR absorption band at
-
1
-1
lowered to 114 kJ mol from the value of ∆Hm ) 120 kJ mol
of SG-M39 and both are much smaller than ∆Hm ) 146 kJ
mol of SG-C39. Issue 2 comes from the observation of the
shoulder in the spectra at around 730 cm . We performed IR
-
1
-1
7
20 cm corresponding to the CH2 rocking mode is known to
be split into two components due to intermolecular interaction,
-
1

5832 J. Phys. Chem. B, Vol. 108, No. 18, 2004
Yamamoto et al.
measurement on the another sample BK-M39 obtained by
melting of BK-M39 at T ) 333.5 K slightly above Tm of BK-
M39 and subsequently slow cooling to 303.1 K, and the result
is shown in the same figure. Obviously, the heat treatment
-
1
improved local chain packing so that the peak at 720 cm
increased not only in intensity but also became sharper and
furthermore the shoulder shifted to the lower wavenumber side
with a decrease in intensity. Combining the two IR results
mentioned above, we should consider that BK-M39 predomi-
nantly takes a triclinic or a monoclinic type of the crystalline
structure with rather distorted local chain packing and may
contain a small fraction of the orthorhombic type of the crystal
as in SG-M39. However, we did not attempt to estimate the
fraction, because of a drastic change in the spectra by heat
treatment, lacking of a clear theoretical guide for decomposition,
and poor resolution of 2 cm-1 of the instrument used.
Computer Simulation. X-ray diffraction, Raman scattering,
and IR data described in previous sections strongly suggested
that SG-M39 and probably BK-M39 are present in polymorphic
forms, one being the orthorhombic one and another a triclinic
or a monoclinic one, whereas the main chain takes the planar
zigzag form. Owing to a small number of diffraction peaks,
another technique such as computer simulation may be helpful
for determining how extended main chains are aligned and how
the central methyl groups are packed in cooperation with end
methyl groups in respective unit cells. We performed computer
simulation to find the energetically most stable crystalline
structure choosing five space groups of P21, P212121, P21/c,
C2/c, and P 1h as potential candidates for the M39 crystal. X-ray
diffraction profiles were calculated from lattice parameters
predicted for each space group and compared with those
experimental curves shown in Figure 3. Two space groups of
P212121 and P 1h were found to reproduce experimental results
fairly well, whereas others belonging to the monoclinic group
failed to predict one or two main peaks at positions of 2θ
observed.
Figure 7 illustrates the predicted molecular model belonging
to the P212121 space group whose unit cell dimensions are a )
1
.515 nm, b ) 0.505 nm, c ) 5.358 nm, and R ) â ) γ ) 90°,
-3
and the density is 0.919 g cm , being close to the experimental
-3
value of 0.915 g cm obtained for SG-M39. The model predicts
that the unit cell contains four M39 molecules as found for C39
taking the double layer structure, but the lattice parameters a,
b, and c are different from a ) 0.749 nm, b ) 0.496 nm, and
c ) 5.197 nm of C39. The closest packing of end methyl groups
at the interface in the C39 crystal gives rise to the relative
translation of linear alkane molecules along the a axis in the
upper and the lower layers; thus c becomes a double of the
main chain length. In the M39 crystal, adjacent four extended
chains surrounding one M39 molecule relatively translate by a
half of the main chain length along the c axis to accommodate
the central methyl group under cooperation of end methyl
groups, which affect directions of chain planes of the next
nearest neighbor chains, resulting in the unit cell with the large
a, as illustrated in the projection on the a-b plane. According
to this model, nevertheless, introduction of the central methyl
group very slightly loosens the local chain packing to a similar
extent along all three axial directions. The model illustrated in
Figure 8 belongs to the P 1h space group. The lattice parameters
estimated for this triclinic type of crystal are a ) 1.050 nm, b
Figure 7. Molecular model of M39 simulated for the space group
P2
1 1 1
2 2 . a ) 1.515 nm, b ) 0.505 nm, c ) 5.358 nm, R ) â ) γ )
9
0.0°.
length and chain planes of all molecules are oriented to the same
direction. The triclinic structure proposed is different from those
2
9-34
in even alkanes from C H to C H .
1
0
22
26 54
The number of
molecules in the unit cell is 1 up to C22 and becomes 2 for
C24 and C26. Typical values of lattice parameters below C22
are, for example, a ) 0.483 nm, b ) 0.429 nm, c ) 2.482 nm,
R ) 67.8°, â ) 85.1°, and γ ) 72.5° for C18 with the density
-
3
of 0.929 g cm , and for C26, a ) 0.767 nm, b ) 0.526 nm, c
) 3.528 nm, R ) 68.4°, â ) 83.6°, and γ ) 83.0° with the
-
3
density of 0.926 g cm . X-ray diffraction patterns shown in
Figure 3 were indexed with the two molecular models and
results are listed in Table 1. It is to be noted that, if unit cell
structures given by the two models are realized in M39 crystals,
intensities from (00l) reflections should be considerably lowered
in comparison with those of C39. The orthorhombic model
successfully predicts four peaks of SG-M39 at 2θ ) 21.1°,
23.5°, 35.9°, and 39.6°, and also two broad peaks at low angles
as reflections from (002) and (004) planes but no peak to (003)
)
0.470 nm, c ) 5.176 nm, R ) 72.6°, â ) 68.0°, and γ )
-
3
6
5.0°, and the density is 0.88 g cm . The unit cell contains
two M39 molecules due to staggered alignment between
adjacent chains with the relative translation of a half main chain

(C19H39)2CHCH3 and Its Compatibility with C39H80
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5833
Figure 9. DSC curves of bulk-crystallized M39/C39 compared with
those of C39, SG-M39, and BK-M39. The weight of the samples used
-
1
is 1.0 mg, and the scanning rate, 1.0 K min
.
to conclude that SG-M39 is present in polymorphic forms, one
being the orthorhombic one and another the triclinic one with
approximate lattice parameters obtained from the simulation.
Three strong peaks of BK-M39 at 2θ ) 19.3°, 21.1°, and 23.1°
appear to be represented by the triclinic model. The model gives
(
002) and (003) reflections at 2θ ) 3.73° and 5.60°, but no
peak is seen at those angles. Instead, very weak and broad peaks
are seen at 2θ ) 3.2° and 6.5°, which are close to (002) and
(004) reflections of the orthorhombic model whose main peak
is located at 2θ ) 21.2°. Thus it seems legitimate to consider
that the BK-M39 crystal is also present in polymorphic forms,
taking the triclinic form predominantly and the orthorhombic
one as a minor component.
Figure 8. Molecular model of M39 simulated for the space group P 1h .
a ) 1.050 nm, b ) 0.470 nm, c ) 5.176 nm, R ) 72.6°, â ) 68.0°,
γ ) 65.0°.
The molecular models illustrated in Figures 7 and 8 have a
characteristic feature that there exists no smooth flat interface
where methyl groups are disclosed. Instead, all methyl groups
are located inside the crystal, which gives rise to staggered
alignment between neighboring extended chains. Thus it does
not seem unreasonable to suppose that chain stacking along the
main chain axis is quite difficult. It is also to be noticed that, in
Figure 8, crystal growth along the b axis does not produce extra
surface, whereas more extra surface energy is needed along the
a axis. Anisotropy of rates of crystal growth along three
directions may explain why a needlelike crystal has been
obtained for M39 samples.
Compatibility of M39 with C39. Figure 9 shows eight DSC
curves of the bulk-crystallized M39/C39 binary system (BK-
M39/C39) with those of pure BK-M39 and C39. In the range
of the molar fraction fm of M39 from 0.05 to 0.80 studied, we
observe the solid-solid-phase transitions characteristic of pure
C39 followed by melting, and also another melting peak at a
lower T close to the melting temperature of pure M39. It is
ambiguous if the solid-solid-phase transition persists above fm
) 0.80, because of lowering of the melting temperature of the
C39-rich phase. The phase diagram was constructed for BK-
M39/C39 using peak temperatures on respective DSC curves
TABLE 1: Comparison of Diffraction Angles 2θ Measured
with Those Calculated from Computer Simulation
crystalline lattice simulated
experimental
2θ/deg
P1h
2θ/deg
P2
2/deg
1 1 1
2 2
sample
BK
d/nm
(hkl)
(hkl)
3.2 (vw)
2.79
1.36
3.3
6.6
(002)
(004)
6
.2 (vw)
1
2
2
9.3 (s)
0.459
0.421
0.385
2.79
19.5
21.6
23.2
(200)
(010)
(210)
1.1 (m)
3.1 (vs)
3.2 (w)
21.1
23.5
3.3
(210)
(400)
(002)
(004)
SG
6
.5 (vw)
1.35
6.6
1
2
2
2
3
3
9.3 (s)
1.1 (vs)
3.0 (s)
3.5 (s)
5.9 (m)
9.6 (vw)
0.458
0.420
0.386
0.380
0.250
0.228
19.5
21.2
23.2
(200)
(010)
(210)
21.1
(210)
23.5
35.6
39.9
(400)
(020)
(610)
reflection consistently. The strong peak at 2θ ) 19.3° and the
shoulder at 2θ ) 23.0° may be complemented by coexistence
of the triclinic form approximately. Thus, we may be allowed

5834 J. Phys. Chem. B, Vol. 108, No. 18, 2004
Yamamoto et al.
Figure 10. Phase diagram for bulk-crystallized M39/C39 system: (b)
solid-solid-solid transitions and melting of the C39-rich phase; (O)
melting of the M39-rich phase. The liquidus line is calculated from eq
3
, and horizontal and dotted lines are empirically drawn.
Figure 11. Heat of transition ∆H for the M39/C39 system as a function
of f : (b) ∆HC39 of the C39-rich phase; (O) ∆HM39 of the M39-rich
m
excluding the endothermic curve at the higher melting temper-
ature, to which the final melting temperature was used and is
shown in Figure 10. In the figure, filled circles correspond to
the transition and the melting points of the C39-rich phase and
unfilled circles correspond to that of the M39-rich phase. The
two solid-solid transition temperatures look to decrease very
slightly with increasing fm following empirically drawn dotted
lines. Effects of a small amount of impurity on the DSC
transition behaviors were studied using a homologous series of
phase.
TABLE 2: Melting Temperatures and Heats of Transition
for the M39/C39 System
H/kJ mol-
1
∆
m
T /K
M39 molar
fraction
at
m,M39
at
Tm,C39 A′ f B B f C total
M39 C39
T
0
0.05
0
0
0.40
0.60
0
0
0
1
1
354.4
331.5 354.1
328.0 353.7
331.6 353.0
331.7 351.1
331.8 348.2
331.8 343.7
136
2
2
2
4
3
3
2
8
7
6
7
6
5
146
139
136
138
129
130
120
114
116
114
120
3
5
7
12
24
49
76
96
124
117
103
72
47
22
4
n-alkanes. Mixing of 0.5 mol % C26 into C27 did not affect
the latter melting temperature as well as the transition temper-
ature to the C structure, but lowered the transition temperature
to the B structure by 1.7 K. Mixing of 0.8 mol % of C25 into
C26 gave another independent solid-solid transition peak.
Therefore it seems plausible that the amount of M39 contami-
nated in the C39-rich phase may be less than 1 mol %. On the
other hand, the melting temperature Tm,M39 of the M39-rich
phase remains constant independent of fm excluding the data
point given by the white circle at fm ) 0.10 and the endothermic
peaks are sharper than pure M39. These strongly suggest that
Tm,M39 should be regarded as the eutectic temperature. The triple
point may be located as the point where the two solid lines in
the figure tend to intersect. Because the melting point of pure
BK-M39 sample is slightly but definitely lower than the eutectic
temperature, this conclusion is ambiguous a little bit yet.
The melting point Tm,C39 of the C39-rich phase is appreciably
depressed with increasing fm. It is likely that the depression is
caused by melting of the M39 component at elevated temper-
atures.36 Because M39 and C39 are considered to mix athermally
in the liquid state to a very good approximation and the C39-
rich phase is exclusively composed of pure C39, eq 3 was
applied to examine the above presumption.
.10
.20
.80
.90
.95
331.9 337.9 111
331.8 333.3 114
2
.00(BK) 330.7
.00(SG) 331.4
114
120
into eq 3 gives the solid curve in Figure 10, which successfully
reproduces the fm dependence of Tm,C39.
We calculated enthalpy ∆H associated with melting or the
solid-solid-phase transitions from area of respective endother-
mic peaks on DSC curves for this system and results are given
in Table 2 with values of two melting temperatures. Because
∆
H of the A f A′ transition was only observed for pure C39
and was very small, it has not been listed. Total enthalpies ∆H
of the binary system listed in the last column of Table 2 indicate
that they tend to decrease almost linearly with an increase in fm
-1
from ∆Hm,C39* ) 146 kJ mol of pure C39 to ∆Hm,M39* )
-1
1
20 kJ mol of pure M39; thus ∆H may be given by
∆H ) ∆HM39 + ∆HC39
ln f ) ∆Hm,C39*/R(1/Tm,C39* - 1/Tm,C39
)
(3)
) f ∆H * + (1 - f )∆Hm,C39*
(4)
c
m
m,M39
m
Here fc ) 1 - fm is the molar fraction of C39 in the mixture
prepared, Tm,C39* and ∆Hm,C39* are the melting point and the
molar enthalpy of fusion of pure BK-C39, and R is the gas
constant. Substitution of measured Tm,C39* and ∆Hm,C39* values
Here ∆HM39 is the enthalpy corresponding to the peak at Tm,M39
and ∆HC39 a sum of enthalpies due to melting and solid-solid
transitions of the C39 component. Figure 11 shows dependences
of ∆HM39 and ∆HC39 on fm as unfilled and filled circles,

(C19H39)2CHCH3 and Its Compatibility with C39H80
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5835
respectively. The both quantities linearly vary with fm over the
entire range of 0.05 e fm e 0.95, which confirms the second
equality in eq 4. The results establish that a great majority of
C39 and M39 molecules are present separately in this binary
system, taking crystal structures different from each other.
Two crystalline structures proposed for M39 appear consider-
ably different. A closer look at Figures 7 and 8, however,
suggests that molecules are aligned as a pair of extended chains
at their closest distance with the methyl groups being arranged
toward the same direction for the P 1h group and along the
opposite direction for the P212121. This may partially explain
why the two crystalline structures appear to have almost the
same melting temperature as seen on the DSC curves.
(2) Boor, T. Ziegler-Natta Catalysts and Polymerization; Academic
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(
A final remark is concerned with an interrelationship between
the branching effect on the crystal structure and the main chain
length of alkane molecules. Ungar and Zeng have pointed out
3
(
8
in their review that, because of the surface overcrowding effect,
(
lamellar crystals of very long alkane chains cannot grow laterally
without chain folding at the conventional experimental condition
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center of the main chain facilitates chain folding at the branching
point for sufficiently long alkanes and results in rejection of
the branch to the lamellar surface. This does not seem to apply
for symmetric branched alkanes with a carbon number of the
main chain as low as 39, whose linear homologue exclusively
takes a planar zigzag conformation in the crystalline state due
to a heavy restriction that extra energy is needed for chain
folding. As demonstrated in this work for the first time, main
chains pack taking the extended conformation, whereas all
central methyl branches are accommodated inside lamellae in
cooperation with end methyl groups, which gives rise to
staggered alignment between adjacent extended chains. We shall
report that a similar crystalline structure is realized for symmetric
branched alkanes with various long branches in a forthcoming
paper. However, chain folding may take place for a highly
symmetrical branched alkane such as a three- or four-arm star
as the energetically most favorable state, even if the carbon
number of the main chain is low, and this possibility should be
definitely examined.
2
(
(
(18) Kimoto, H.; Yamamoto, H.; Urabe, Y.; Shiokawa, K.; Nemoto, N.
Manuscript in preparation.
(
(
(
(
19) Muller, A. Proc. R. Soc. A 1932, 138, 514.
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(
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25) Yamamoto, H.; Nemoto, N. Eng. Sci. Rep. Kyushu U. 2000, 22,
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1
5
(
(
(
9
(
(
(
(
29) Muller, A.; Lonsdale, K. Acta Crystallogr. 1948, 1, 129.
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Acknowledgment. We thank Accelrys Inc. for kindly
allowing us to use a software Polymorph Predictor. They also
thank Mr. Mitsuo Ohama of Graduate School of Science, Osaka
University, for measurements of Raman spectra.
1240.
(33) Gerson, A. R.; Roberts, K. J.; Sherwood, J. N. Acta Crystallogr.
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Press: London, 1998, Chapter 7.
1
(
(
References and Notes
(
1) Keii, T. Kinetics of Ziegler-Natta Catalysts; Kodansha: Tokyo,
1
972.