Crystal structure and solid-solid phase transition of 1-dodecylamine hydrobromide

Article abstract of DOI:10.1524/zpch.2010.5546

1-dodecylamine hydrobromide was synthesized. The crystal structure of the compound was determined by X-ray crystallography. Low-temperature heat capacities of the title compound were measured by an adiabatic calorimeter over the temperature range from 78 to 390 K. Three solid-solid phase transitions were observed at the peak temperatures of (329.278±0.234), (337.805±0.326), and (347.371±0.154) K. The molar enthalpies and entropies of the three phase transitions of the substance were calculated based on the analysis of heat capacity curves. Experimental values of heat capacities for the title compound were fitted to two polynomial equations. Three solid-solid phase transitions and a melting process of 1-dodecylamine hydrobromide were verified by DSC technique. In addition, the reversibility and repeatability of the three phase transitions were discussed. by Oldenbourg Wissenschaftsverlag, Mu?nchen.

Full text of DOI:10.1524/zpch.2010.5546

Z. Phys. Chem. 224 (2010) 1371–1387 . DOI 10.1524.zpch.2010.5546
© by Oldenbourg Wissenschaftsverlag, München
Crystal Structure and Solid–Solid Phase
Transition of 1-Dodecylamine Hydrobromide
*
By Yupu Liu, Youying Di , Wenyan Dan and Donghua He
College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059,
Shandong Province, P.R. China
(Received March 15, 2010; accepted May 27, 2010)
1-Dodecylamine Hydrobromide . X-ray Crystallography .
Adiabatic Calorimetry . Low-temperature Heat Capacity .
Solid–solid Phase Transition
1-dodecylamine hydrobromide was synthesized. The crystal structure of the compound was
determined by X-ray crystallography. Low-temperature heat capacities of the title compound
were measured by an adiabatic calorimeter over the temperature range from 78 to 390 K.
Three solid–solid phase transitions were observed at the peak temperatures of (329.278 0.234),
(337.805 0.326), and (347.371 0.154) K. The molar enthalpies and entropies of the three phase
transitions of the substance were calculated based on the analysis of heat capacity curves.
Experimental values of heat capacities for the title compound were fitted to two polynomial
equations. Three solid–solid phase transitions and a melting process of 1-dodecylamine hydro-
bromide were verified by DSC technique. In addition, the reversibility and repeatability of the
three phase transitions were discussed.
1. Introduction
With the increasing seriousness of the energy crisis, the study about the phase
change materials (PCMs) [1,2] has become the focus in recent years. The solid–
solid phase change materials (SSPCMs) [3,4] have the advantages of high energy
density, no liquid or gas generated during the phase transitions, simple device,
small size, environmental friendliness, and flexible design over other kinds of
phase change materials, and they also are user-friendly and easy to handle.
Therefore, the investigation on the solid–solid phase change materials in all kinds
of energy storage materials are most frequently conducted owing to the wide
application foreground of these materials.
*
Corresponding author. E-mail: diyouying@126.com
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Y. Liu et al.
The bis(n-alkylammonium) halometallates (II) (abbreviated as C12M) [1,5]
are important organometallic compounds of the generalized formula
(C12H25NH3)2MXn (n = 4 or 6), where M is a divalent or trivalent metal ion
(M = Zn2+, Cu2+, Mn2+, Cd2+, Co2+, etc.) and X is a negative haloid ion. They
are characterized by high enthalpy change and excellent reversibility of a solid–
solid phase transition between two polymorphic forms in the temperature range
of 273–393 K. As the necessary intermediate or raw material, 1-dodecylamine
hydrobromide (C12H25NH3Br) plays an important role in the formation of C12M
in which the C12H25NH3+ cation bonds with the MXn2K anion through ionic
bonds.
In recent years, it is found that n-alkylammonium halide can exhibit the
phenomena of the distinct solid–solid phase transitions. A great attention has
been paid to the application of 1-dodecylamine hydrobromide and dodecyl am-
monium salts in many fields such as energy storage and textile industry. In
1953, Gordon et al. investigated the crystal structure and some properties of n-
dodecylammonium bromide by X-ray diffractometer, automatic variable-temper-
ature moving-film powder camera, and single-crystal techniques [6]. In 1974,
Lunden gave a three-dimensional single-crystal study of n-dodecylammonium
bromide [7]. However, some important thermodynamic properties of the title
compound have not been found in the literature, which restricts the progress of
relevant theoretical studies and application development of the compound. Heat
capacity is one of the most fundamental thermodynamic properties of materials
and necessary for many theoretical studies in physics, chemistry, and engineering
technology which are related to materials.
Adiabatic calorimetry is the most reliable method to obtain heat capacities
and various thermodynamic data of substances. It is an important and effective
experimental method used for the study of the solid–solid phase transitions by
measuring low-temperature heat capacities and calculating relevant thermody-
namic data. Much work has been done to measure heat capacities with adiabatic
calorimetric instruments all over the world [8,9].
In the present work, the crystal structure of the title compound was deter-
mined by X-ray crystallography. Low-temperature heat capacities and thermody-
namic properties of the title compound were determined by a precision auto-
mated adiabatic calorimeter over the temperature range from 78 to 390 K. The
phase transition of the title compound was investigated by means of the analysis
of the heat capacity curve. Furthermore, three solid–solid phase transitions and
the melting process were further studied by DSC technique.
2. Experimental
2.1 Synthesis and characterization of 1-dodecylamine hydrobromide
(1-C12H25NH3Br)(s)
1-dodecylamine and hydrobromic acid (≥ 40 mass %) chosen as the reactants
and anhydrous ethyl alcohol (A. R.) used as the solvent were of analytical grade.
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Crystal Structure and Solid–Solid Phase Transition
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1-dodecylamine was dissolved into the right amount of the absolute alcohol in a
three-neck flask, and excess of hydrobromic acid was gradually added to the
above solution under sufficient stirring. The solution was boiled and refluxed for
2 h, whereafter the mixture was condensed by boiling off some of the liquid
until crystal membrane emerged on the surface of the solution. The final solution
was naturally cooled to room temperature, filtered, and colorless crystal was
obtained. The product was re-crystallized three times with anhydrous ethyl alco-
hol and colorless transparent crystal was gained. Finally, the sample was placed
in a vacuum desiccator to dry in vacuum at the temperature of 303 K for 6 h.
The final product was put into a weighing bottle and preserved in a desiccator.
The mass fraction purity of the compound was determined to be > 0.995 by
HPLC. The contents of the bromide in the compound were determined by chemi-
cal analysis. Elemental analysis (C, H, and N) was carried out on a Vario EL III
CHNOS instrument made in Germany. These results showed that the mass frac-
tion purity of the compound was > 0.995.
A single crystal suitable for X-ray crystallography was glued to a fine glass
fiber. All diffraction data for the compound were collected on a Bruker Smart-
1000 CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at T =
(298 2) K using the program SMART and processed by SAINT-plus [10]. Ab-
sorption corrections were applied by SADABS. The structure was solved by
direct methods and expanded using Fourier techniques with SHELXTL-97 pro-
gram [11], and refined with full-matrix least-squares technique on F2. All non-
hydrogen atoms were refined anisotropically. All H-atoms were located theoreti-
cally and refined. The structural plots were drawn using the SHELXTL program.
2.2 Adiabatic calorimetry
A precision automatic adiabatic calorimeter was used to measure heat capacities
of the compound over the temperature range 78 ≤ (T .K) ≤ 400. The calorimeter
was established in the Thermochemistry Laboratory in the College of Chemistry
and Chemical Engineering, Liaocheng University, China. The principle and
structure of the adiabatic calorimeter were described in detail elsewhere [12,13].
The calorimetric system was composed of a calorimetric cryostat, a data
collection system, an adiabatic controlling system and a high vacuum pump
system. The calorimetric cryostat mainly comprised a sample cell, a platinum
resistance thermometer, an electric heater, inner, middle and outer adiabatic
shields, three sets of six-junction chromel-copel thermocouple piles installed be-
tween the calorimetric cell and the inner shield, between the inner and middle
shields, and between the middle and outer shields, respectively. The miniature
platinum resistance thermometer (IPRT No.2, produced by the Shanghai Institute
of Industrial Automatic Meters, 16 mm in length, 1.6 mm in diameter and a
nominal resistance of 100 Ω) was applied to measure the temperature of the
sample. The thermometer was calibrated on the basis of ITS-90 by the Station
of Low-Temperature Metrology and Measurements, Academia Sinica.
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The data collection system consisted of a multi-channel data acquisition.
switch unit (Agilent, 34970A) for electric energy collection, a 7 1.2 digit nano-
volt.microohm Meter (Agilent, 34420A) for acquisition of the temperature of
the sample cell, and a P4 computer equipped with a matched module and inter-
face card GPIB (IEEE 488). The adiabatic controlling system consisted of a high
precision temperature controller (Model: 340 Lake Shore), inner, middle and
outer adiabatic shields, and three sets of six-junction chromel-copel thermocou-
ple piles. The high vacuum pump system consisted of a combined rotational
mechanical pump and oil diffusion pump (Edwards, Model NXK 333000).
The sample cell used for the heat capacity measurement was made of 0.3 mm
thick gold-plated copper, 20 mm in length, and 20 mm in diameter with an inner
volume of about 6 cm3. Three sheaths were fixed at the bottom of the cell for
inserting the platinum resistance thermometer and thermocouples. Electric heat-
ing wire (insulated Karma wire of 0.12 mm in diameter, R = 120 Ω) used as the
heater of the cell was bifilarly wound on the outer surface of the cell. The heater
was covered with about 50 μm thick aluminum foil to decrease the heat leakage
by radiation and the temperature nonuniformity on the surface of the cell. The
heater and aluminum foil were insulated and fixed by W30–11 varnish. A small
amount of thermally conductive silicone sealant (Type HT916, produced by
Shanghai Huitian New Chemical Material Company, Limited Shanghai, China)
was used to seal the lid to the main body of the cell. On the lid, there was a
section of copper capillary used for pumping out the air in the cell and introduc-
ing the helium gas to promote thermal equilibrium. The capillary was pinched
off and the resultant fracture was soldered to ensure the sealing of the cell.
To verify the accuracy of the calorimeter, heat capacities of synthetic sap-
phire (α-Al2O3, Standard Reference Material 720) were measured over the tem-
perature range 78 ≤ (T .K) ≤ 400. The sample mass used was 1.7143 g, which
was equivalent to 0.0168 mol based on its molar mass, M(Al2O3) =
101.9613 g · molK1. In order to compare the values with those recommended by
NIST [14], the molar heat capacities of α-Al2O3 in the temperature range of 80
to 400 K were calculated with an interval of 5 K using the non-linear inserting
method based on the measured molar heat capacity data. Temperature variability
of balance period is an important indicator used to weigh the stability and relia-
bility of the adiabatic system in the experimental process. The variability of the
temperature of all the measuring points in the equilibrium period was within
K0.30 mK · minK1 to +0.30 mK · minK1 in the temperature range of 78 to 400 K.
Most of the deviations of our values from the NIST recommended values were
within 0.30 %, which proved that the performance of the newly constructed
calorimetric apparatus had been improved compared with previous calorimeters.
Heat capacity measurements were continuously and automatically carried out
by means of the standard method of intermittently heating the sample and alter-
nately measuring the temperature. The heating rate and temperature increments
were generally controlled at 0.1 K · minK1 to 0.4 K · minK1 and 1 K to 3 K. The
heating duration was 10 min, and the temperature drift rates of the sample cell
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Crystal Structure and Solid–Solid Phase Transition
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measured in an equilibrium period were always kept within 10K3 K · minK1 to
10K4 K · minK1 during the acquisition of all heat-capacity data. The data of heat
capacities and corresponding equilibrium temperature had been corrected for heat
exchange of the sample cell with its surroundings [13]. The sample mass used
for calorimetric measurements was 1.62363 g, which was equivalent to
0.006098 mol in terms of its molar mass, M = 266.26 g · molK1
.
2.3 Differential scanning calorimetry (DSC)
DSC analysis was carried out in a Perkin-Elmer diamond DSC made by Perkin-
Elmer Corporation, USA. The phase transition temperature and the latent heat
were calibrated using high-pure indium (purity: 99.999 %) as a standard. The
heating rate was 5 K · minK1. The material was protected by high purity nitrogen
with a discharge rate of 40 cm3 · minK1. Because the measurement on energy
storage characteristics of the 1-dodecylamine hydrobromide was carried out at
the same condition, the results of the temperatures and the enthalpies of the
phase transitions were reliable.
3. Results and Discussion
3.1 Description of crystal structure
The molecular structure of 1-dodecylamine hydrobromide (1-C12H25NH3Br) is
plotted in Fig. 1. The crystal parameters, data collection, and refinement results
for the compound are listed in Table 1. The crystal system of the compound is
monoclinic, space group is P21.m, unit cell dimensions are a = 6.0450(7) Å, b =
6.9681(8) Å, c = 17.850(2) Å; α = 90°, ꢀ = 91.8920(10)°, γ = 90°, and Z = 2.
The calculated density of the compound is 1.177 Mg · mK3 and the volume of
3
formula unit is 751.47(15) Å . Non-hydrogen atomic coordinates ( 104) and
!
equivalent isotropic displacement parameters (Å2 103) are shown in Table 2.
!
The geometries of the hydrogen bonding are listed in Table 3. Each BrK ion
forms three N-H…Br hydrogen bonds with three neighboring 1-dodecylammo-
nium cations with N…Br distances of: 3.284 Å, 3.499 Å and 3.499 Å, as seen in
Table 3.
As shown in Fig. 2, hydrogen bonds of N(1)-H(1A)…Br(1), N(1)-H(1B)…
Br(1) and N(1)-H(1C)…Br(1) together with the van der Waals forces exist in the
crystal to stabilize the structure. All hydrogen atoms are placed in geometrically
idealized positions and constrained to ride on their parent atom (N). Chemical
bonds N(1)-H(1A), N(1)-H(1B), and N(1)-H(1C) have almost same bond length,
0.8900 Å.
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Y. Liu et al.
Fig. 1. Molecular structure of the title compound 1-dodecylamine hydrobromide.
Fig. 2. Packing diagram of the compound 1-dodecylamine hydrobromide.
In comparison with the literature [6,7], it can be seen that the crystal data and
the structure of 1-dodecylamine hydrobromide determined in this paper basically
agrees with that in references.
3.2 Heat capacities of the sample
All experimental results for the first experiment are listed in Table 4. The 59
experimental points in the temperature region from 78 to 300 K before the first
phase transition, and 17 experimental points between 357 and 390 K after the
third phase transition are fitted by means of least squares method, respectively,
and two polynomial equations of the experimental molar heat capacities (Cp,m
.
J · KK1 · molK1) vs. the reduced temperature (X), X = f(T), have been obtained:
(1) Before the first phase transition in the temperature range from 78 to
300 K,
–1
–1
2
3
4
!
!
C
p,m.(J K mol ) = 267.242+114.143X+16.625X +6.283X –0.877X (1)
where X is the reduced temperature, X = (T.K K 189).111, in which 189 is
half of the upper limit (300 K) plus the lower limit (78 K) in the temperature
region of 78–300 K, and 111 is half of the upper limit (300 K) minus the lower
limit (78 K) in the same region. The reduced temperature (X) obtained by the
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Crystal Structure and Solid–Solid Phase Transition
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Table 1. Crystal data and structure refinement for 1-C12H25NH3Br(s).
Empirical formula
Formula weight
Temperature
C12H28BrN
266.26
298(2) K
Wavelength
0.71073 Å
Crystal system, space group
Monoclinic, P2(1).m
a = 6.0450(7) Å α = 90°
b = 6.9681(8) Å ꢀ = 91.8920(10)°
c = 17.850(2) Å γ = 90°
751.47(15) Å 3
Unit cell dimensions
Volume
Z, Calculated density
Absorption coefficient
F(000)
2, 1.177 Mg.m3
2.708 mmK1
284
Crystal size
0.30! 0.28! 0.27 mm
2.28 to 25.01 °
Theta range for data collection
Limiting indices
K6 ≤ h ≤ 7, K8 ≤ k ≤ 5, K21 ≤ l ≤ 21
3903 . 1429 [R(int) = 0.0895]
99.4 %
Reflections collected . unique
Completeness to theta = 25.01
Absorption correction
Max. and min. transmission
Refinement method
Semi-empirical from equivalents
0.5284 and 0.4972
Full-matrix least-squares on F2
1429 . 0 . 94
Data . restraints . parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
1.012
R1 = 0.1076, wR2 = 0.2733
R1 = 0.1249, wR2 = 0.2839
1.892 and K1.198 e · Å K3
Largest diff. peak and hole
method is placed between +1 and K1, and deviations of the smoothed heat
capacities from the experimental values would become smaller and smaller with
the increase of the power of the fitted polynomial equation according to the
statistical principle. The correlation coefficient for the fitting R2 = 0.99995.
(2) After the third phase transition in the temperature range from 357 to
390 K,
–1
–1
2
!
!
C
p,m.(J K mol ) = 495.955+2.121X+0.22169X
+0.09734X3–0.23587X4
(2)
where X = (T.K K 373.5).16.5, in which 373.5 is half of the upper limit (390 K)
plus the lower limit (357 K) in the temperature region of 357–390 K, and 16.5
is half of the upper limit (390 K) minus the lower limit (357 K) in the same
region. The correlation coefficient for the fitting R2 = 0.99971.
Three series of the experiments in the region of three phase transition of the
title compound are carried out so that the reversibility and repeatability of the
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Y. Liu et al.
Table 2. Non-hydrogen atomic coordinates (! 104) and equivalent isotropic displacement pa-
rameters (Å2 ! 103) for 1-C12H25NH3Br(s).
Atom
N(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
Br(1)
x
y
z
U(eq)
67(4)
71(5)
98(7)
73(5)
96(7)
78(5)
79(10)
74(5)
78(10)
77(5)
78(11)
83(6)
105(9)
60(1)
7650(20)
8050(30)
6220(40)
6430(30)
4620(30)
4650(30)
2890(40)
2990(30)
1150(40)
1210(30)
K600(30)
K620(30)
K2460(30)
2566(2)
2500
5448(7)
2500
6265(9)
2500
6720(11)
7544(9)
2500
2500
8017(11)
8818(10)
9307(12)
10113(10)
10577(12)
11390(10)
11875(12)
12665(10)
13150(12)
4718(1)
2500
1890(40)
2500
1880(40)
2500
3070(40)
2500
2500
2500
Table 3. Geometry of the hydrogen bond (distance and angle).
D-H…A d(D-H). Å d(H…A) . Å <DHA.o d(D…A). Å Symmetry code
N(1)-H(1A)…Br(1) 0.890
N(1)-H(1B)…Br(1) 0.890
N(1)-H(1C)…Br(1) 0.890
2.396
2.788
2.769
174.92
137.86
140.14
3.284
3.499
3.499
[ x+1, y, z ]
[ Kx+1, Ky+1, Kz+1]
[ Kx+1, Ky, Kz+1]
phase transitions of the sample are verified. The results of the three series of
heat capacity measurements are plotted in Fig. 3. The results in Fig. 3 are ob-
tained from the repeated measurements of the same sample undismantled from
the adiabatic system.
Fig. 3 shows that the structure of the compound is stable over the temperature
range between 78 and 300 K. However, there are three endothermic peaks ap-
pearing in the curve of experimental molar heat capacities when the sample is
heated for the first time, but only one obvious peak appears when the sample is
heated in the second and third repeated experiments over the temperature range
from 300 to 357 K. Two small peaks before the obvious peak can not almost be
observed, the enthalpy changes of the two phase transitions can be estimated to
be much less than 0.1 kJ · molK1 according to the method of diagrammatic area
integration, and may be omitted in comparison with the big peak.
The melting point of the title compound is measured to be about 462.0–
463.5 K according to the results obtained from the microscopic melting point
device, therefore, the three peaks appearing in the curve of experimental molar
heat capacities are ascribed to the solid–solid phase transitions. A reasonable
explanation may be that [15] the first and second phase transitions (Ttrs,1, Ttrs,2
)
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Crystal Structure and Solid–Solid Phase Transition
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Table 4. Experimental molar heat capacities of 1-C12H25NH3Br (s) (M = 266.26 g · molK1).
T (K)
Cp, m
.
T (K)
Cp, m
.
T (K)
Cp, m
.
T (K)
Cp, m.
(J · KK1 · molK1
162.34
164.71
168.12
170.75
174.51
177.66
180.81
183.70
187.73
192.28
195.43
198.58
201.30
204.54
)
(J · KK1 ·molK1
255.22
259.51
263.80
268.00
272.21
276.23
279.73
284.90
289.71
293.65
298.03
302.84
307.22
311.86
317.03
321.58
326.83
331.12
336.64
341.71
346.35
351.17
355.28
359.90
364.66
368.73
375.27
)
(J · KK1 ·molK1
381.03
386.41
391.41
397.53
403.97
411.19
418.94
426.25
433.21
441.65
456.08
473.34
491.68
509.83
532.83
556.46
585.06
618.62
686.24
800.26
1129.19
1423.65
1734.86
1004.53
1099.9
1632.7
2506.6
)
(J · KK1 · molK1
547.32
518.12
629.67
1476.7
3327.4
1226.8
549.33
501.39
493.86
494.18
494.51
494.71
494.98
495.27
495.47
495.78
495.98
496.27
496.49
496.81
497.04
497.37
497.61
497.88
498.11
)
78.229
80.215
83.904
87.380
90.856
94.260
97.666
100.86
105.18
109.72
113.13
116.68
120.36
123.91
177.68
181.44
185.34
189.18
193.22
197.05
200.88
205.56
210.31
214.14
218.19
221.95
225.85
229.75
233.72
237.70
241.81
246.28
250.82
254.65
258.55
262.38
264.94
268.60
272.43
276.27
280.27
284.10
288.11
291.94
296.01
299.84
303.17
305.16
306.65
307.99
309.91
311.85
313.69
315.38
317.12
318.94
320.83
322.75
324.64
326.51
327.79
328.78
329.54
330.16
332.68
334.81
336.87
338.33
340.767
342.825
344.771
346.370
347.425
350.982
353.820
355.806
357.792
359.850
361.836
363.822
365.808
367.794
369.852
371.838
373.824
375.810
377.797
379.783
381.769
383.755
385.671
387.657
389.643
127.601 207.86
130.94
134.62
138.31
142.00
145.76
149.45
154.13
158.81
162.64
166.33
170.16
173.85
210.58
213.55
217.32
221.52
225.72
228.79
233.25
237.89
241.04
244.02
247.08
250.58
from the low temperature phase (P21.m) to the intermediate temperature phase
is possibly due to the breaking of hydrogen bonds and leads to a dynamic order–
disorder transition of rigid alkyl chains, whereas the third phase transition (Ttrs,3
)
from the intermediate to the high temperature phase is related to cooperative
conformational changes of the alkyl chains and the complete disorder of the
conformation is assumed to be similar to a “molten” state. In the intermediate
phase the rigid alkyl chains are flipping around their long axes between two
equivalent orientations separated by 90°. The two transitions are somewhat anal-
ogous to the ones found in the lipid bilayer membranes and can be described by
order parameters used for smectic liquid crystals. In the second and third experi-
ments, the first two peaks almost disappear. The breaking of hydrogen bonds
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Y. Liu et al.
Fig. 3. Plot of experimental molar heat capacities (Cp, m) against the absolute temperature (T.
K) of the compound C12H25NH3Br (s), in which “o”, “Δ”, and “*” represent the first, the
second, and the third series of heat capacity measurements for the same sample, respectively.
and an irreversible dynamic order–disorder transition of rigid alkyl chains take
place when the sample is heated in the first experiment. In crystal structure of
the compound, the formed hydrogen bonds are the type of N-H…Br, which has
a weak strength and low bond energy different from hydrogen bond of the type
N-H…Cl with a strong bond energy in the compound C12H25NH3Cl(s) [16]. After
the first heat capacity measurements are fulfilled and the temperature is reduced
to < 300 K by cooling the adiabatic system using natural temperature falling, the
hydrogen bonds in the sample are not completely formed, the dynamic order–
disorder transition of rigid alkyl chains do not almost take place during the
second and third repeated measurements. The third peak appears in all three
experiments, and only some slight differences in heights and widths, which ex-
hibit good reversibility and repeatability. This also verifies that the third phase
transition (Ttrs,3) is related to cooperative conformational changes of the alkyl
chains and the complete disorder of the conformation similar to a “molten” state.
3.3 Molar enthalpies and entropies of phase transitions
Fig. 4 shows the plots of experimental molar heat capacities (Cp, m) against the
absolute temperature (T.K) of the compound C12H25NH3Br(s) for three experi-
ments, and new sample is used for each heat capacity measurements. It can be
seen from Fig. 4 that all three endothermic peaks appearing in the first experi-
ment of Fig. 3 appear if each series of heat capacity measurements are carried
out by using new sample. Although there are some slight differences in heights
and widths of peaks corresponding to each series of heat-capacity measurements
during the phase changes, the three phase transitions basically exhibit good re-
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Crystal Structure and Solid–Solid Phase Transition
1381
Fig. 4. Plot of experimental molar heat capacities (Cp, m) against the absolute temperature (T)
of the compound C12H25NH3Br(s), in which “o”, “Δ”, and “*” represent the first, the second,
and the third series of heat capacity measurements for the different samples in the same batch,
respectively.
versibility and repeatability. A nearly agreement in Cp, m values of each series of
repeated experiment is obtained.
The molar enthalpy changes of the first and second phase transitions in Cp, m
~ T curve are calculated with the method of diagrammatic area integration [17]
because the two peaks are partly folded together. The molar enthalpy of the third
phase transition ΔtrsHm (kJ · molK1) is evaluated in terms of the equation (3)
[17], and molar entropies ΔtrsSm(J · KK1 · molK1) of the three phase transitions
are calculated with the following thermodynamic Equation (4):
Ttrs
Tf
Tf
i
–1
!
ΔtrsHm = [Q–n$ET Cp,m(i)d T-n$ET Cp,m(f)dT–ET H0dT].n (kJ mol )
(3)
(4)
i
trs
–1
–1
!
!
ΔtrsSm = ΔtrsHm.Ttrs (J K mol )
where Ti in Equation (3) is a temperature slightly lower than the starting phase
transition temperature, Tf is a temperature slightly higher than the finishing tran-
sition temperature, Cp, m (i) is the heat capacity at the temperature Ti, Cp, m (f) is
the heat capacity at the temperature Tf, Q is the total heat quantity introduced to
the calorimeter from temperature Ti to Tf, Ttrs is the transition temperature of the
sample measured, n is the mole number of the sample, and Ho is the heat capacity
of the empty sample cell. Values of Q and Ho are calculated with the program
stored in the computer linked with the adiabatic calorimetric system, and printed
along with experimental results of heat capacities. The results of phase transition
temperatures, phase transition enthalpies, and phase transition entropies of three
series of repeated measurements are presented Table 5.
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1382
Y. Liu et al.
Table 5. Thermodynamic properties of three phase transitions obtained from three series of
repeated heat capacity measurements of 1-dodecylamine hydrobromidea.
Thermodynamic
properties
Series 1
xi
Series 2
xi
Series 3
xi
Mean value
–
(x σa*)
Ttrs,1 (K)
Ttrs,2 (K)
Ttrs,3 (K)
ΔtrsHm,1
329.686
338.330
347.425
8.770
329.271
337.877
347.081
8.448
328.876
337.208
347.608
8.527
(329.278 0.234)
(337.805 0.326)
(347.371 0.154)
(8.582 0.097)
(kJ · molK1
)
)
)
ΔtrsHm,2
7.585
9.438
7.421
9.477
7.798
10.042
25.928
23.125
28.889
(7.601 0.109)
(9.652 0.195)
(26.062 0.281)
(22.503 0.338)
(27.787 0.553)
(kJ · molK1
ΔtrsHm,3
(kJ · molK1
ΔtrsSm,1
26.601
22.419
27.166
25.657
21.964
27.305
(J · KK1 · molK1
)
)
)
ΔtrsSm,2
(J · KK1 · molK1
ΔtrsSm,3
(J · KK1 · molK1
a
Ttrs,1 (K), Ttrs,2 (K), Ttrs,3 (K), ΔtrsHm,1 (kJ · molK1), ΔtrsHm,2 (kJ · molK1), ΔtrsHm,3
(kJ · molK1), ΔtrsSm,1 (J · KK1 · molK1), ΔtrsSm,2 (J · KK1 · molK1), and ΔtrsSm,3 (J · KK1 · molK1
represent the peak temperatures, molar enthalpies and molar entropies of the first, the sec-
ond, and the third phase transitions, respectively.
)
3.4 Smoothed heat capacities and thermodynamic functions of the
compound
The smoothed molar heat capacities and thermodynamic functions are calculated
based on the fitted polynomial equation of the heat capacities as a function of the
reduced temperature (X) according to the following thermodynamic Equations,
(HT – H298.15) = ET298.15
(ST – S298.15) = E2T98.15 p,m$T–1dT
(GT – G298.15) = ET298.15 p, m$dT–TET298.15 p,m$T–1dT
C
C
p, m$dT
(5)
(6)
(7)
C
C
The polynomial fitted values of the molar heat capacities and fundamental
thermodynamic functions of the sample relative to the standard reference temper-
ature 298.15 K are tabulated in Table 6 at intervals of 5 K.
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Crystal Structure and Solid–Solid Phase Transition
1383
3.5 Results of DSC analysis of the compound
The DSC results of 1-dodecylamine hydrobromide in the cycle of the first heat-
ing and succedent cooling are given in Fig. 5. It can be seen from Fig. 5 that
there are four endothermic processes in the temperature range from 300 to 470 K
when the sample is heated. Because the first and second peaks appearing in the
DSC curve are joined together, their thermodynamic properties may be calcu-
lated by coming down to one big peak. The peak temperatures and molar enthal-
pies of the endothermic peaks are determined to be 338.76 K, 16.025 kJ · molK1
for the first and second peaks, 345.85 K, 9.157 kJ · molK1 for the third peak, and
465.45 K, 8.121 kJ · molK1 for the fourth peak, respectively, as shown in Fig. 5.
The first, second, and third endothermic peaks in the DSC curve are in accord-
ance with the three phase transition peaks appearing in Cp, m ~ T curve. The
melting point of the compound is measured to be about 462.0–463.5 K using the
microscopic melting point device, so the fourth endothermic peak should be the
melting process of the substance.
It can be seen from the DSC curve in Fig. 5 that there are two exothermic
peaks appearing in the temperature range from 470 to 300 K when the sample
is cooled down, and the number of the peaks are not the same as that of endother-
mic peaks when the sample is heated. The peak temperatures and molar enthal-
pies of the exothermic peaks are determined to be 463.35 K, K7.792 kJ · molK1
for the first exothermic peak, and 342.70 K, K8.580 kJ · molK1 for the second
exothermic peak, respectively, as shown in Fig. 5. So, it can be found that the
first exothermic peak corresponds to the fourth endothermic peak during the
heating, and the second exothermic peak corresponds to the third endothermic
peak during the heating, while no more exothermic peak appears and corresponds
to the first and second endothermic peaks in the DSC curve in Fig. 5. This also
verifies the irreversibility of the first and second peaks appearing in the DSC
curve. In comparison with the Cp,m ~ T curve in Fig. 3 and Fig. 4, it can be seen
that the thermodynamic properties of 1-dodecylamine hydrobromide determined
by DSC technique almost agree with those from adiabatic calorimetric method.
4. Conclusions
1-dodecylamine hydrobromide was synthesized. The crystal structure of the title
compound was determined by X-ray crystallography.
Low-temperature heat capacities of the title compound were measured by the
adiabatic calorimeter over the temperature range from 78 to 390 K. Three solid–
solid phase transitions were found in the Cp,m ~ T curve, and the possible mecha-
nism of the phase transition was discussed. Three solid–solid phase transitions
were observed at the peak temperatures of (329.278 0.234), (337.805 0.326),
and (347.371 0.154) K. The molar enthalpies of the three phase transitions of
the substance were determined to be (8.582 0.097), (7.601 0.109), and
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1384
Y. Liu et al.
Table 6. Smoothed heat capacities and thermodynamic functions of 1-C12H25NH3Br(s).
T (K)
Cp, m
HT−H298.15
ST−S298.15
GT−G298.15
(J · K−1 · mol−1
)
(kJ · mol−1
K59.448
K58.614
K57.758
K56.878
K55.975
K55.050
K54.016
K53.131
K52.137
K51.120
K50.080
K49.017
K47.931
K46.822
K45.689
K44.533
K43.353
K42.148
K40.920
K39.667
K38.389
K37.086
K35.757
K34.403
K33.022
K31.615
K30.182
K28.721
K27.232
K25.715
K24.170
K22.595
K20.991
K19.357
K17.693
K15.998
K14.271
K12.512
)
(J · K−1 · mol−1
)
(kJ · mol−1
K33.310
K31.700
K30.142
K28.633
K27.172
K25.756
K24.386
K23.058
K21.773
K20.529
K19.325
K18.162
K17.037
K15.951
K14.904
K13.894
K12.923
K11.988
K11.091
K10.231
K9.4069
K8.6196
K7.8684
K7.4530
K6.4733
K5.8289
K5.2196
K4.6450
K4.1049
K3.5989
K3.1268
K2.6882
K2.2830
K1.9108
K1.5716
K1.2650
)
80
164.42
169.05
173.65
178.24
182.81
187.37
191.94
196.50
201.07
205.66
210.26
214.88
219.52
224.20
228.90
233.65
238.44
243.27
248.16
253.10
258.09
263.15
268.27
273.46
278.72
284.06
289.47
294.97
300.55
306.21
311.97
317.82
323.77
329.81
335.95
342.20
348.55
355.01
K326.72
K316.64
K306.84
K297.32
K288.04
K278.99
K270.14
K261.50
K253.03
K244.73
K236.57
K228.56
K220.67
K212.90
K205.24
K197.67
K190.19
K182.79
K175.46
K168.21
K161.01
K153.87
K146.78
K139.74
K132.75
K125.79
K118.87
K111.98
K105.12
K98.293
K91.490
K84.710
K77.951
K71.210
K64.485
K57.775
K51.075
K44.384
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
165
170
175
180
185
190
195
200
205
210
215
220
225
230
235
240
245
250
255
260
265
K0.99120
K0.75006
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Crystal Structure and Solid–Solid Phase Transition
Table 6a. (continued)
1385
T (K)
Cp, m
HT−H298.15
ST−S298.15
GT−G298.15
(J · K−1 · mol−1
)
(kJ · mol−1
K10.720
K8.8959
K7.0376
K5.1451
K3.2177
K1.2549
0
)
(J · K−1 · mol−1
)
(kJ · mol−1
)
270
275
280
285
290
295
298.15
300
305
310
315
320
325
330
335
340
345
350
355
360
365
367
370
375
380
385
390
361.58
368.26
375.06
381.97
389.00
396.15
400.71
403.42
K37.699
K31.016
K24.332
K17.644
K10.947
K4.2363
0
K0.54172
K0.36641
K0.22450
K0.11650
K0.043092
K0.005176
0
0.74396
2.4927
K0.0038590
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
Phase transition
494.21
27.605
30.078
31.068
32.554
35.033
37.516
40.002
42.491
79.113
79.092
79.084
79.071
79.051
79.031
79.012
78.993
27.526
29.999
30.989
32.475
34.954
37.437
39.923
42.412
494.89
495.14
495.51
496.15
496.82
497.52
498.16
(9.652 0.195) kJ · molK1, and their corresponding molar entropies were calcu-
lated as (26.062 0.281), (22.503 0.338), and (27.787 0.553) J · molK1 · KK1, re-
spectively, based on the analysis of heat capacity curves. The relevant thermody-
namic functions were determined based on the fitted polynomial equations.
Further researches of the thermochemical properties for this compound were
carried out by means of DSC technique and the reversibility and repeatability of
the three phase transitions were discussed.
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1386
Y. Liu et al.
Fig. 5. DSC curve in the heating and cooling of 1-dodecylamine hydrobromide with a scanning
rate of 5 K.min.
Acknowledgement
This work was financially supported by the National Natural Science Founda-
tions of China under the contract NSFC No. 20673050 and 20973089.
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