INTRODUCTION:
The discovery of liquid crystals dates back to 1888, when botanist Friedrich Rheinitzer observed a double fusion for cholesterol benzoate. At the first fusion, he observed the formation of a viscous liquid and opaque against the second the formation of a liquid fluid2,3.
It is the physicist Otto Lehmann who is the first to talk about liquid crystal after observing this compound using a polarized light microscope. By observing the viscous liquid, he found that he had both the birefringence of solids and the fluidity of liquids4,5.
Mesogens are composed of two parts, one tending to organize through intermolecular interactions and the other tends to move because of its flexibility. The response of the system to this stress will be to generate a liquid crystalline phase for which we will have both a crystal type order in some directions of the space resulting from intermolecular interactions, and a liquid type disorder in the other directions generated by the flexible parts6.
For almost a century, researchers worked with liquid crystals without finding any application. It was not until 1968 that Heilmeier, a researcher at RCA (Radio Corporation of America), developed the first liquid crystal display device, and another five years for the development of a stable technology, using cyanobiphenyl as a mesomorphic derivative, allowing the sale of the first consumer device a Seiko watch7.
The development of liquid crystal science technology has led to the preparation and study of many mesogens in particular, thermotropic liquid crystals8-10.
GC gas chromatography is one of the most common methods of analysis, which has been successful with the development of capillary columns and the variety of stationary phases. Kelker11 was the first who used the liquid crystal as stationary phase in gas chromatography for separating the hydrocarbons isomers in 1963. While many studies confirmed that nematic phases of Liquid crystals have higher efficiency in the separation of isomers12-19.
The synthesis of these new stationary phases is followed by an application in chromatography to better correlate the domain of existence of the mesophases studied to characterize them20.
In this study, we propose to synthesize a new liquid crystalline phase and study their performance for the separation of some aliphatic alkanes by GC gas chromatography, as well as the thermal and thermodynamic study where we used chromatography in phase CPGI inverse gas to determine the solid-nematic-liquid phase change and to study the influence of their terminal groups on the evolution of thermodynamic parameters.
MATERIALS AND METHODS:
Chemicals:
The starting materials (reagents and solvents) have a high analytical purity. They are used as delivered by Fluka (Germany), Aldrich (Germany), and BHD (UK) including Volatile aromatics delivered by Meyreau-Beiveau (France).
The intermediate products (I), (II), (III) and (IV) as well as the final product (V) are characterized by the analytical techniques listed below:
1. Thin-layer chromatography
2. Melting point (type Buchi B-545).
3. The FT-IR spectra (Perkin Elmer FT-IR 1720x, Japan).
4. NMR spectra (Bruker Avance DPX, USA).
5. Differential scanning calorimetry DSC (NETZSCH DSC 204F1, Switzerland).
6. Gas chromatography GC (Auto System XL Perkin Elmer, USA).
Synthesis of the liquid crystal:
A liquid crystal is synthesized according to a reaction mechanism that comprises five essential steps represented in Figure 1.
Figure 1. Synthesis steps of the liquid crystal.
Name and structure of the liquid crystal:
The liquid crystal synthetized is the final compound of this synthesis. Figure 2 represents the structure of the liquid crystal. The name of the liquid crystal is: (5-(4-ethoxyphenyl)-azophenyl)-2-butylthio-1,3,4-oxadiazole).
Figure 2. Structure of the liquid crystal.
Statistical analysis:
All measurements were performed using Origin PRO 8.
RESULTS AND DISCUSSION:
Characterization of compounds:
The structure of the liquid crystal synthesized (Fig. 2) was confirmed by IR spectroscopy (Fig. 3), 1H-NMR analysis (Fig. 5) and 13C-NMR analysis (Fig. 7).
IR Spectroscopy:
IR Spectrum of the liquid crystal (Figure 3), IR (KBr); ʋ (cm-1): 2940-3000 (C-H aliphatic), 3000-3100 (C-H aromatic), 1450-1600 (C=C aromatic), 800-850 (substitution para aromatic), 1000-1250 (aro-O-R(aryl ethers)) 1185 (C-O oxadiazole); 2780 (C=N oxadiazole); 600-700 (C-S); 1250 (N=N).
Figure 3. IR Spectrum of the liquid crystal.
1H--NMR spectroscopy
Figure 4. Structure of the liquid crystal to the 1H spectroscopy.
1H NMR Spectrum of the liquid crystal (Figures 4 and 5), RMN-1H (CDCl3,δ ppm):0.86 (t, 3H; J = 7.8 Hz; H-14), 1.32 (t, 2H; J = 7.6 Hz; H-1), 1.61 (m, 2H; J = 7.2 Hz; H-13), 1.84 (m, 2H; J = 7.2 Hz; H-12), 3.09 (t, 2H; J = 7.8 Hz; H-11), 4.19 (q, 2H; J = 7.4 Hz; H-2),7.18 (d, 2H; J = 7.6 Hz; H-3, H-5), 7.45 (d, 2H; J = 7.6 Hz; H-4, H-6), 8.04 (d, 2H; J = 7.6 Hz; H-7, H-9) 8.12 (d, 2H; J = 7.6 Hz; H-8, H-10).
Figure 5. 1H NMR Spectrum of the liquid crystal.
13C-NMR spectroscopy
Figure 6. Structure of the liquid crystal to the 13C-NMR spectroscopy.
13C NMR Spectrum of the liquid crystal (Figures 6 and 7), RMN-13C (CDCl3,δ ppm): 13.17 (1C; C-20), 14.44 (1C; C-1), 21.78 (1C; C-19), 33.38 (1C; C-18), 36.91 (1C; C-17), 64.61 (1C; C-2),114.76 (2C; C-4, C-7),123.78(2C; C-5, C-8),124.29(2C; C-10, C13),128.73(1C; C-12),130.84 (2C; C-11, C-14), 140.012 (1C; C-16),145.27 (1C; C-6), 153.17 (1C; C-9), 158.56 (1C; C-3) 163.35 (1C;C-15).
Figure 7. 13C NMR Spectrum of the liquid crystal.
The analysis of the NMR spectra showed:
- Carbon resonance at 114.76 ppm and 123.78, which corresponds to the carbon of the first aromatic ring correlates with a proton at 7.18 and 7.45 ppm.
- Carbon resonance at 130.84 ppm and 124.29, which corresponds to the carbon of the second aromatic ring correlates with a proton at 8.04 and 8.12 ppm.
The results confirmed that the desired liquid crystal has been successfully obtained.
Differential scanning calorimetry (DSC)
The results obtained by measuring the DSC (Fig. 8) are as follows: transition temperature solid phase to nematic mesophase at 112 °C, and the nematic mesophase to the isotropic phase at 151 °C.
Figure 8. DSC thermograms of the liquid crystal.
Thermal study by IGC
In the thermal study, we evaluated the evolution of the capacity factor (TR’) and the specific retention volume as a function of the inverse of the temperature (Fig. 9) as well as the calculation of the slope "b" eqn. (1).
Figure 9: Variation of the ‘b’ in function of inverse absolute temperature (1/T).
The evolution curve of parameter "b" (Fig. 9), show a clear discontinuity at the mesophase transition temperatures, in other words, there are sharp breaks at 112 °C and 151 °C, with a better transition for these injected probes.
Thermodynamic Study:
For the thermodynamic study, we calculated the thermodynamic quantities with infinite dilution (Lnγ∞, ΔH∞, ΔS∞ and ΔG∞) as a function of the inverse of the temperature as well as the solution quantities (ΔHsol, ΔSsol and ΔGsol) according to the inverse of the temperature.
From the infinite dilution activity coefficients at different temperatures, enthalpies and entropies and free energies of interaction 3 are determined by the following equations eqn. (2) and eqn. (3):
(2)
… (3)
For an increase in temperature, there is an increase in the coefficient of infinite dilution activity that is to say that the activity coefficient in the isotropic phase is greater than that in the nematic phase and that the latter is greater to that in the solid phase.
At a fixed temperature, the infinite dilution coefficient of activity increases with the growth of the carbon number of the homologous series (Fig. 10).
Figure 10. Evolution of Ln γ∞ alkanes according to 1000 / T (K).
In our study, there is a sharp break at 112 °C corresponding to the transition temperature of the nematic phase to the isotropic phase, is another less clear break at 151 °C corresponding to the transition temperature of the solid phase to the nematic phase.
Study according to ΔH∞ and ΔS∞
In the nematic phase, the ΔH∞ values for an individual probe were more positive and higher compared to the isotropic phase and the solid phase indicating that the interactions were lower. This can be a result of the nematic order limiting the conformation of the probe and preventing the adoption of its optimal interaction. However, the relatively weak interactions due to the ΔH∞ values impose less restriction of movement of the probe so that its translational entropy will be lower; thus ΔS∞ is negative in the nematic phase.
The results of excess molar partial enthalpies and the entropy of the solutions are listed in the Table 1.
Table 1. The partial molar enthalpies (kJ.mol-1) and the entropies (J.mol-1.K-1) with infinite dilution in LC.
|
Alkan |
Solid |
Nematic |
Isotropic |
|||
|
∆S∞ |
∆H∞ |
∆S∞ |
∆H∞ |
∆S∞ |
||
|
C13 |
3.01 |
-16.52 |
7.42 |
-46.6 |
6.08 |
-53.26 |
|
C14 |
3.34 |
-16.28 |
7.91 |
-57.42 |
5.26 |
-36.21 |
|
C15 |
7.38 |
-21.75 |
12.34 |
-54.42 |
7.27 |
-27.76 |
|
C16 |
6.22 |
-20.5 |
10.86 |
-51.19 |
4.79 |
-20.2 |
|
C17 |
5.45 |
-18.82 |
9.9 |
-62.67 |
3.22 |
-21.31 |
|
C18 |
5.36 |
-18.68 |
6.24 |
-35.81 |
10.58 |
-23.52 |
|
C19 |
6.06 |
-18.86 |
8.59 |
-63.52 |
2.97 |
-19.1 |
Values of ΔH∞ were more positive, and are very high in the nematic phase, indicating that strong probe-CL interactions occur in this mesomorphic phase. On the contrary, the ΔS∞ values were negative, indicating that the probe undergoes a restricted conformation movement.
The solid, nematic and isotropic phases present trends identical to those observed in the stationary phase (A). Once again, enthalpy contributions are paramount in determining behavior.
Study according to ΔG∞
The results of infinite dilution free energy variations (ΔG∞) are shown in Table 2.
Table 2: The free energy at infinite dilution of alkanes probes in LC.
|
Alkane |
Temperature (°C) |
|||||||||
|
100 |
110 |
112 |
120 |
130 |
140 |
150 |
153 |
160 |
165 |
|
|
ΔG∞ (kJ.mol-1) |
||||||||||
|
C13 |
11,75 |
10,5 |
9,9 |
9,9 |
9,6 |
9,8 |
9,3 |
9,2 |
9,8 |
10,4 |
|
C14 |
25,9 |
18,6 |
15,8 |
12,1 |
11,3 |
9,6 |
8,86 |
9,02 |
9,2 |
9,5 |
|
C15 |
13,6 |
9,9 |
9,1 |
9,3 |
9,4 |
9,8 |
9,3 |
9,5 |
9,4 |
9,5 |
|
C16 |
16,4 |
12,5 |
11,6 |
9,6 |
9,7 |
10,2 |
9,4 |
9,1 |
9,4 |
9,6 |
|
C17 |
15,65 |
14,9 |
12,7 |
9,3 |
10,01 |
10,1 |
9,3 |
9,3 |
9,5 |
9,6 |
|
C18 |
14,2 |
12,2 |
12,1 |
9,8 |
9,9 |
9,3 |
9,1 |
9,2 |
9,4 |
9,4 |
|
C19 |
13,3 |
12,5 |
11,5 |
10,3 |
10,3 |
9,9 |
9,2 |
9,3 |
9,6 |
9,7 |
Results of Tables 1 and 2 showed that the values of ΔH∞, ΔG∞ are positive over the temperature studied for liquid crystal. These values confirmed that the reaction process was endothermic.
Study according to ΔHsol and ΔSsol
Indeed, Chow and Martire [21] demonstrated that in families of probes such as the homologous series of n-alkanes, there is a linear relationship between ΔHsol and ΔSsol. Thus, all the values of the thermodynamic parameters are a complex function of these factors, the values also contain a contribution of the LC - LC interactions, but they will be everywhere the same.
The values of the molar partial enthalpies and entropies of the liquid crystal solution are listed in Table 3.
Table 3. The partial molar enthalpies (kJ.mol-1) and the entropies (J.mol-1.K-1) with solution dilution in LC.
|
Alkane |
Solid |
Nematic |
Isotropic |
|||
|
∆Hsol |
∆Ssol |
∆Hsol |
∆Ssol |
∆Hsol |
∆Ssol |
|
|
C13 |
-4.78 |
-24.38 |
-2.34 |
-24.44 |
-0.32 |
-24.33 |
|
C14 |
-5.06 |
-24.32 |
-2.38 |
-24.25 |
-0.3 |
-24.4 |
|
C15 |
-5.43 |
-24.23 |
-2.54 |
-24.31 |
-0.35 |
-24.15 |
|
C16 |
-5.39 |
-25.25 |
-2.46 |
-25.53 |
-0.51 |
-24.98 |
|
C17 |
-4.83 |
-25.3 |
-2.25 |
-25.39 |
-0.33 |
-25.22 |
|
C18 |
-4.29 |
-24.14 |
-2.05 |
-23.76 |
-0.19 |
-24.52 |
|
C19 |
-4.57 |
-25.24 |
-2.17 |
-24.99 |
-0.23 |
-25.49 |
The results showed that the values of partial enthalpy and entropy solution molars are all negative.
The n-alkane probes, their ability to undergo dispersion interactions increases with the length of the chain so that ΔHsol tends to increase (ie become more negative) along the series. However, a longer elongated molecule suffers from a greater restriction in terms of translation and conformation and thus ΔSsol would also be expected to be more negative.
Consideration of ΔHsol values, largely reflecting the differences in potential energy, would be to predict that the values for a given probe would pass from the isotropic phase to nematic in the solid state to supercooled mesophase whereas if the restriction on the movement of the probe was the dominant factor, the opposite trend would be predicted. The observed values indicate that the term energy is the dominant contribution.
Study according to ΔGsol
The results of solution dilution free energy variations (ΔGsol) are shown in Table 4.