Evaluation of the Effect of Citronellol Group on Functionalized Mesogenic Materials by Capillary GC

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DOI 10.1007/s10337-015-2938-3

ORIGINAL

Evaluation of the Effect of Citronellol Group on Functionalized Mesogenic Materials by Capillary GC

Ouassila Ferroukhi¹ · Azzedine Addoun¹ · Saliha Guermouche¹ · Jean Pierre Bayle² · Moulay Hassane Guermouche¹

Received: 2 March 2015 / Revised: 25 June 2015 / Accepted: 6 July 2015

Keywords Capillary gas chromatography · Terpenoid liquid crystal · Stationary phase · Shape selectivity

Introduction

Since the introduction of their use in gas chromatography (GC) [1], liquid-crystalline (mesogenic) materials have been widely studied and used in various research fields [2].

Their applications as stationary phases in chromatographic analyses (GC and high-performance liquid chromatography) have attracted much attention, and they have been used in the separation of toxic isomers of compounds in the environment [3–5].

The molecular structure of rod-shaped liquid crystals (LCs) usually consists of a rigid main core, two or more rings, terminal substituents, and, in some materials, a lateral group attached to the rigid structure [6]. However, other configurations can be obtained by addition of a linking group to increase the flexibility or of a metal to impart a desired property [7]. The LC state or mesophase is obtained by heating in a temperature range between the solid and liquid states. The mesogenic state starts from the melting point and ends at the clarification point, corresponding to the passage of molecules from the LC state to the isotropic liquid state. The selectivity of the mesogenic stationary phase is mainly determined by the molecular structure of the LC and the degree of ordering of its mesophase (nematic or smectic). These two important parameters influence specific intermolecular interactions between the LC and a chromatographed analyte, and this significantly affects the analytical behavior of an LC when it is used as a stationary phase [4]. Much research has therefore been performed on the influence of the linking group [8], the lateral group [9, 10], and the terminal substituents [11], and their Abstract In this paper, the effects of functionalization

with terpenes on two new liquid-crystalline stationary phases for gas chromatography (GC) are described. Cit- ronellol was used as the terminal group in the first material, and tetrahydrogeraniol was used with a second material. Inverse GC showed that the new materials have wide liquid-crystalline ranges (mesophases), 371–500 and 395–

501 K, respectively. Moreover, they show good thermal stability up to 523 K and good potential as stationary phases for capillary GC. To clarify the effects of the liquid crystal structures and functional groups on retention and separation, the chromatographic behaviors of the two stationary phases were compared by eluting alkylbenzenes, poly- cyclic aromatic hydrocarbons, aromatic compounds, and terpenoids. The selectivities for a wide range of analytes achieved using the citronellol column were significantly better than those obtained using the tetrahydrogeraniol column. The columns showed different retention behaviors and fine resolutions for some of the main constituents of essential oils. Introduction of the double bond of citronellol greatly improved the polarization interactions involved in the shape recognition of the liquid-crystalline state for isomers. The new citronellol liquid-crystalline stationary phase, therefore, has a high affinity for natural compounds.

* Ouassila Ferroukhi

[email protected]; [email protected]

1 Laboratoire de Chromatographie, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediene, B.P. 32, El-Alia, Bab-Ezzouar, 16111 Alger, Algeria

2 Laboratoire de Chimie Structurale Organique, Université Paris Sud, Bt. 410, 91405 Orsay, France

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relationships with specific properties such as the nematic range and its stability, thermal stability, selectivity, and polarity.

Capillary GC is an appropriate technique for the analysis of natural active ingredients, because of their high volatili- ties. However, these extracts are complex mixtures, and this makes their identification via GC challenging. The functionalization of a mesogenic compound by a natural product, namely a terpenoid, increases its selectivity for natural materials when it is used as a GC stationary phase.

In our study, two novels LCs with similar mesogenic cores and different terpene terminal groups (Fig. 1) were synthesized. Citronellol was used as the terminal group for the first LC, denoted by LCUT, and tetrahydrogeraniol was used as the terminal substituent for the second LC, denoted by LCST. To the best of our knowledge, there have been no previous reports of the use of terpenoid-type LCs as GC stationary phases. The aim of this study was to evaluate the effects of these terpene substituents on the thermal and

analytical properties of these two liquid-crystalline materials, and to observe the potential effects of the presence of double bonds in the terminal terpene group of LCUT and of its absence in the LCST column.

Experimental

Materials and Equipment

Chloroform, dichloromethane, methanol, p-toluenesul- fonic acid, p-toluenesulfonyl chloride, pyridine, N,N′- dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine, citronellol, KOH, poly(ethylene glycol) (PEG 200), dioxane, and KHCO₃ (all analytical grade, purity 98 % or better) were obtained from Sigma-Aldrich (Steinheim, Germany). Aromatic hydrocarbon derivatives, essential oil constituents (EOCs), n-alkanes, ketones, phenol derivatives, and polyaromatic hydrocarbons (all analytical grade,

HO

O OH

HO

O OMe

OH OTs

p-TsOH MeOH

TsCl Pyridine/

CHCl3

O

O OMe

O

O 2 OH

OH + HO

KHCO₃ PEG200/diox

1) KOH/MeOH + H₂O 2) H₃O⁺Cl^-

O O O O

O

O DCCDMAP CHCl³

(1)

(2)

(3)

(4)

(5) (a) (b)

(a) (b) (b)

(b) (a)

(a)

(c) (d) (e)

(c) (d)

(f) (g) (h)

(i) (j) (k)

(l) (m)

(c)

(c) (d)

(e) (d) (f) (g)

(h) (i) (k) (j) (m)

(l)

Fig. 1 Synthetic route to terpenoid liquid crystal LCUT and LCST.¹H NMR (δ, ppm): a 7.88, b 6.86, c 8.13, d 6.97, e 3.85, f 1.67, g 1.65, h 0.92, i 1.35, j 1.95, k 5.05, l 1.69, m 1.71

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purity 99 % or better) were provided by Fluka Chemika (Buchs, Switzerland). All analytical solutions were pre- pared in dichloromethane and kept in amber glass vials at 277 K. Treated fused-silica capillary columns of medium polarity (30 m, 0.32 mm id) were obtained from Supelco (Bellefonte, PA, USA). Thermogravimetric analysis (TGA) was performed using a TGA 4000 analyzer (PerkinElmer, Waltham, MA, USA) equipped with a compact ceramic furnace. The heating runs were performed at a heating rate of 283 K min⁻¹ from 303 to 823 K under a nitrogen gas flow. The transition temperatures of the mesogenic materials were determined using differential scanning calorim- etry (DSC; PerkinElmer Technology, Waltham, MA, USA), at a heating rate of 283 K min⁻¹ from 313 to 473 K. The different phase states of LCUT and LCST were identi- fied using a polarizing microscope (Olympus), fitted with a heated turntable, at a heating rate of 283 K min⁻¹. The GC system (Clarus^®500, PerkinElmer, Boston, MA, USA) had automated pressure control and was equipped with a split/splitless injector and a flame ionization detector (FID).

High-purity nitrogen (99.995 %, Linde Gas (Algiers, Alge- ria) was used as the carrier gas. The raw data were acquired using Total Chrome software. The conditions used for all analyses were as follows: split ratio mode 60:1, injector chamber temperature 523 K, FID temperature 553 K, and carrier gas flow rate 1 mL min⁻¹. Manual injections of diluted solutions (0.2 μL) were performed using a 5 μL Hamilton syringe, (Sigma-Aldrich, Steinheim, Germany).

The gas hold-up time (2.9 min at 373 K) was obtained by injection of methane gas.

Methods

Synthesis of Mesogenic Materials

The synthetic routes to LCUT and LCST are shown in Fig. 1. Methyl-4-hydroxybenzoate (1) was obtained by esterification of p-hydroxybenzoic acid with methanol in the presence of p-toluene sulfonic acid. Treatment of the citronellol alcohol group with p-toluenesulfonyl chloride in the presence of a mixture of pyridine/chloroform yielded the sulfonate ester 2. The reaction between 1 and 2 under weak basic conditions gave methyl 4-(3,7-dimethyloct- 6-enyloxy)benzoate (3). Saponification of 3 with KOH in water/methanol (1:3 v/v), followed by acidification with HCl, gave 4-(3,7-dimethyloct-6-enyloxy)benzoic acid (4).

Compound 4 was reacted with 4,4′-bisphenol in the pres- ence of DCC and 4-dimethylaminopyridine in chloroform for 24 h at ambient temperature to give the final product.

After removal by filtration of the hydrated DCC and solvent evaporation, the crude product was purified chromato- graphically using dichloromethane as the eluent. The final

pure product, 4,4′-bis[4-(3,7-dimethyloct-6-enyloxy)benzoate]biphenyl (5), was recrystallized several times from an ethanol/chloroform (95/5,v/v) mixture until constant transition points were obtained. The second terpenoid LC was synthesized similarly, except for an additional step involv- ing citronellol hydrogenation in the presence of palladium on activated carbon after 4,4′-bis[4-(3,7-dimethyloctyloxy]

benzoate)biphenyl production. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopies were used to determine the purities and properties of the obtained compounds.

Capillary Column Preparation

Prior to use, the treated fused-silica capillary columns were rinsed consecutively with dichloromethane, methanol, and dichloromethane (~10 mL each), dried with a nitrogen flow for about 2 h, and conditioned under a nitrogen gas flow at 523 K for about 6 h. The coating solution contained 10 % of the LC in dichloromethane (3 mL). A dynamic coating process was used, in which a plug was pushed through the column by a nitrogen flow with a linear velocity of 1–2 cm s⁻¹. After exit of the coating solution, the column was dried, under a stream of nitrogen (40 psi or 2.75 10⁵ Pa) for 8 h to evaporate the remaining solvent in the column. The column was then placed in a GC oven under a nitrogen gas flow; the oven temperature was set from 333 K to the second transition temperature, related to the nematic–isotropic transition for each liquid-crystalline stationary phase (LCSP), and was kept constant at this final temperature for about 10 h.

Results and Discussion

Thermal Properties of LCUT and LCST

The two LCs have similar main core structures, and differ only in their terminal groups. In LCUT, the citronellol group (terminal group) is unsaturated, whereas that in LCST is saturated. This difference is expected to result in different thermal properties. TGA (Fig. 2) showed weight losses beginning at about 525 and 528 K for LCUT and LCST, respectively. As expected, the LCs had similar decomposition temperatures, because their molecular structures are similar. DSC analysis of LCUT (Fig. 3) showed two transition temperatures (heating run); the first point corresponds to the solid–nematic transition (371.5 K), which is the more energetic transition, and a weak transition appeared at about 500 K, which is related to the nematic–

isotropic liquid transition. Similar changes were observed for LCST, but the melting point was higher, 395 K. The clearing temperature was parallel to the LCUT clearing transition at 501 K. These results show that the citronellol

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terminal group caused a shift of the melting transition by about 23 K, from 401.5 K (LCUT) to 379 K (LCST), indi- cating a significant decrease in the range of the nematic- phase stability. For LCUT, the dispersion forces within the unsaturated terminal chain of the LC structure are weak, whereas for LCST, the electron cloud of the terminal group is important and enhances the dispersion forces responsible for this melting point shift.

Inverse Gas Chromatographic Study van’t Hoff Plot Study

Inverse GC was performed to clarify the responses of the mesogenic stationary phases to some specific compounds.

The van’t Hoff relationship (1), which is usually used to obtain thermodynamic data [12], was used to obtain infor- mation on the interactions of the columns with certain analyte probes. Various analytes of different shapes and polar- izabilities were analyzed by GC at different isothermal temperatures, which involved mesomorphic transitions.

The van’t Hoff equation is

where ΔH, ΔS, R, T, k, and ϕ represent the enthalpy of transfer, entropy of transfer, ideal gas constant, absolute temperature, retention factor, and column phase ratio, respectively. The plots of ln k vs 1/T for the LC columns are shown in Fig. 4a and b. Clear breaks can be seen at around 368 and 388 K in the curves 9 for LCUT and LCST, respectively. These discontinuities can be explained by the ability of the LCSP to interact with analytes via different types of interaction. Below the melting point, adsorption is the main phenomenon in the separation process. Dur- ing the melting step (i.e., the first transition), the solute probes dissolve into the bulk material (partition phenomenon), and this is the principal cause of the increases in the retention times of the chromatographed analytes. These different retention mechanisms explain the discontinuities observed above this transition (solid–nematic), where greater amounts of analytes are retained in the liquid stationary phase than on the solid column [13]. The highest point of the discontinuity (Fig. 4) is attributed to the melting temperature of the related terpenoid LCSP. The transitions obtained based on inverse GC are in good agreement with those obtained by DSC analysis; this can be explained by the selection of suitable solute probes with great affinity for the LC columns [14]. A comparison of aliphatic and aromatic probes shows that the selectivities were greater for aromatic analytes, because of the differences among the thermodynamic interactions with the probes. Previ- ous studies [13, 15] showed that aromatic probes are more compatible with LC structures than aliphatic probes are, possibly because the phenyl moiety of the core structure is mainly responsible for solute probe–LCSP interactions.

The terminal alkyl group is suggested to have less effect on the column selectivity. A citronellol (bp 499 K) probe, acetophenone (bp 475 K), and propiophenone (bp 491 K) were injected into both columns. For the LCUT column, the probes were less resolved in the solid state than in the (1) lnk= −�H

RT +�S R +lnϕ,

0 200 400 600 800

0 20 40 60 80 100

255 ° C

Weight (%)

T (°C)

LCUT LCST 252 ° C

Fig. 2 TGA thermograms of LCUT and LCST

50 100 150 200 250

0 2 4

50 100 150 200 250

0 2 4

LCUT

HeatFlow/Endo up LCST

T (°C) Fig. 3 DSC thermograms of LCUT and LCST

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nematic state. Above the melting point, the resolutions of these analytes increased. For the LCST column, the sepa- rations between these probes were similar for the solid and nematic phases. It is suggested that the absence of a terminal double bond decreases the terminal group effect in the solid phase. In summary, the terpenoid LC contains four phenyl rings, which give strong π–π interactions with aromatic probes. This can be used to predict the selectivities for aromatic and polyaromatic analytes. When the oven temperature was increased, the retention times decreased until the second transition point (clearing temperature), at which a weak perturbation was noticed for both the LCUT and LCST columns. The passage through the isotropic liquid state results in slight molecular order destruction (loos- ening of the partial parallel arrangement), which weakly affects the solute probe–stationary phase interactions. Only the first transition was therefore clearly detected by inverse GC; the second transition was insignificant, because of the curve stability around this point (nematic–isotropic liquid).

Selectivity Study

The retention factors k of the LCUT and LCST stationary phases were also studied; the tetrahydrogeraniol LC had higher k values. The difference between the ϕ values (phase ratios) of the columns may explain this difference. The selectivity factors were plotted against the absolute temperature (α vs1/T) to remove the retention time–phase ratio

Fig. 4 van’t Hoff plots (lnk vs 1/T) for LCUT (a) and LCST (b)

0.3 0.6 0.9 1.2 1.5

0.00224 0.00240 0.00256 0.00272 0.00288

0.0023 0.0024 0.0025 0.0026 0.0027 0.0028

1.5 1.8

n-undecane citronellol acetophenone propiophenone butyrophenone

N S

ln k

(a)

N S

1/T (K^-1)

(b)

1,04 1,08 1,12

0,0023 0,0024 0,0025 0,0026 0,0027

1,08 1,12 1,16 1,20

1

Selectivity

2

LCUT LCUT

1/T (K^-1) α

α

Fig. 5 Selectivity plots (α vs 1/T) for α₁ = ^kacetophenone kn−undecane and α₂ =^kacetophenone

kcitronellol ; LCUT and LCST are represented by (black filled

square) and (red filled circle), respectively

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dependence, as recommended by Reid et al. [16]. n-Unde- cane, citronellol, and acetophenone probes were used to investigate this at 373, 393, 413, and 433 K. Figure 5 shows that the ratio of the selectivity factors of acetophenone and n-undecane (α₁) was higher with the LCUT column. Simi- larly, LCUT was more selective for acetophenone than for citronellol (α₂). These results show that the LCUT stationary phase gives high selectivities. Previous studies [17, 18]

showed that the greater the nematic range, the better the selectivity of the corresponding LCSP. Our results show that the selectivity of the LCUT column was better than that of the LCST column; the widths of the nematic ranges are 379 and 401 K, respectively. A second parameter that may affect the selectivity of the LC is the unsaturated terminal part; this enhanced the thermodynamic interactions responsible for the selectivity of the mesogenic stationary phase. The weak dipole of the double bond in the terminal group enhanced the intermolecular interactions with different chromatographed probes.

Analytical Study of Terpenoid Columns Efficiency and Durability of LCSPs

The efficiencies of the separation columns at 393 K were evaluated by injecting naphthalene as the analyte. The efficiencies were 2930 (k = 0.41) and 2472 (k = 0.54) plates/m for LCUT and LCST, respectively. This temperature was used to compare separation on the nematic phase, because the parallel arrangement of this rod-shaped mesophase enables the optimum properties of the LC column to be achieved. The durabilities of the separation columns were evaluated based on hundreds of injections [19]; the same test analyte (naphthalene) was used. The obtained efficiencies varied slightly with respect to those in the first test; the relative standard deviations (RSDs) for LCUT and LCST were 0.23 and 0.38 %, respectively. The retention factors also changed slightly; the corresponding RSDs were 3.9 and 2.7 %. These results indicate that the coated films on LCUT and LCST are stable, which enables their use for further applications, with good reproducibility.

Resolution of Isomeric Compounds

Table 1 lists the separation data for various positional isomers on the LC columns; the corresponding boiling points and structures are also given. The isomer pair 1,3/1,4-diethylbenzene was weakly resolved using LCUT (α = 1.24), and unresolved on the second column (α = 1.04). Their similar structures and solid-state stationary phases were unfavorable for separation. Estragol and anethol were well separated with LCUT (α = 1.69) and weakly resolved with LCST. Similar results were obtained for the isomer

pair thymol and carvacrol; the LCUT column showed better separation ability (α = 1.16) toward these compounds.

This elution order is similar to that obtained with PEG [20].This suggests that the separation columns had mod- erate polarities. For the ortho, para, and meta isomers of cresol, the difficult separation of m/p-cresol isomers was only achieved with the LCUT column; co-elution was observed with the LCST column (Fig. 6a). o-Cresol was first eluted, and the more elongated isomer (p-cresol) was retained on the nematic phase. These results are in agreement with previous studies in which increased solubil- ity of p-cresol was observed. This is typical of nematic phases [21, 22]. Dimethylphenol derivatives were better eluted with the LCUT column, in the order 2,6-, 2,4-, 2,3-, 3,5-, and 3,4-dimethylphenol. Only one co-elution was observed, for the pair 2,3- and 3,5 dimethylphenol. This elution order can be explained by the steric hindrance of the adjacent methyl group on the OH group, which pre- vents the formation of hydrogen bonds. The least retained isomer has a strong steric effect (2,6-dimethylphenol) in the vicinity of the OH group, and this effect decreases in 2,4- and 2,3-dimethylphenol. The most retained compound (3,4-dimethylphenol) has the lowest steric effect, because it has the freedom to establish the hydrogen bond responsible for its considerable retention. Poor separation factors were obtained with the LCST column; the α value of each dimethylphenol analyte (Table 1) was significantly higher with the LCUT column. Furthermore, the polar character of the stationary phase was confirmed by the separation of the phenol derivatives, for which the elution order is in accordance with those obtained using ordinary polar columns [23]. Baseline separation was obtained with 1- and 2-methylnaphthalene positional isomers on the LCUT column (α = 1.07), whereas they appeared as one peak on the LCST column. The 1,5- and 2,6-dimethylnaphthalene isomers were well resolved on the LCUT column (α = 2.03), and partially separated with the LCST column (α = 1.10); the 1-positional isomer (1,5-dimethylnaphthalene) was better retained. Although the 1-positional isomers are more volatile (Table 1) than the ortho positional isomers (2-methylnaphthalene and 2,6-dimethylnaphthalene), the stationary phases were more selective toward the 1-positional isomers. Similar results have previously been reported [24]; the explanation is that on a conventional stationary phase, the elution order of naphthalene homologs is mainly related to their boiling points, whereas with nematic phases, the shape has the main effect on the solute–solvent interactions responsible for the separation of isomers.

Geometric isomers were also investigated (Table 2);

cis- and trans-decalin were injected into both separation columns in the solid state. They were only well separated on the LCUT column, because their elutions corresponded

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Table 1 Parameters for resolution of positional isomers

on LCUT and LCST columns Structural isomers bp^a

(K) Structure LCUT LCST

T^b

(K) k α T

(K) k α

1,3- Diethylbenzene 455

353S^c

0.29 - 353S

0.22 -

1,4- Diethylbenzene 457 0.36 1.24 0.23 1.04

Estragol 488

403N^d

0.36 - 403N

0.42 -

Anethol 508 0.61 1.69 0.48 1.14

Thymol 505

OH 413 N

0.55 - 413N

0.44 -

Carvacrol 509 ^OH 0.64 1.16 0.49 1.11

o-Cresol 464 ^OH

393N

0.14 - 393N

0.25 - m-Cresol 475

OH

0.23 1.64 0.39 1.56

p-Cresol 475 ^OH 0.27 1.92 0.39 1.56

2,6-Dimethylphenol 476 ^OH

393N

0.20 -

393N

0.45 - 2,4-Dimethylphenol 484

OH

0.63 3.15 0.72 1.60 2,3-Dimethylphenol 490

OH

0.69 3.45 0.75 1.66

3,5-Dimethylphenol 495 ^OH 0.69 3.45 0.75 1.66

3,4-Dimethylphenol 499

OH

0.83 4.15 0.85 1.88

aboiling point

bcolumn temperature

csolid state of the LCSP

dnematic state of the LCSP 2-Methylnaphthalene 514.1

373N

0.96 - 393N

0.60 -

1-Methylnaphthalene 517.7 1.03 1.07 0.60 1.00

Dimethylnaphthalene2,6- 535

393N

1.45 - 403N

1.48 -

Dimethylnaphtalene1,5- 538 2.95 2.03 1.63 1.10

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1 2 3 4 5 2 4 6 8 10 12

0 2 4 6

1 2 3 4

1 2 LCUT 3

(a)

min min

min 1

2

(b)

min

2 1 2+3

LCST 1

Fig. 6 Chromatographic separation of isomers on LCUT and LCST stationary phases: a 1 o-cresol, 2 m-cresol, and 3 p-cresol; and b 1 phenanthrene and 2 anthracene

Table 2 Parameters for resolution of geometric isomers on LCUT and LCST columns

Geometric isomers bp

(K) Structure LCUT LCST

T k α T k α

trans-Decalin 460

H

H 353

S

0.26 1.00 353S

0.13 1.00

cis-Decalin 469

H

H 0.32 1.23 0.15 1.15

Nerol 499

OH

393N

0.37 1.00 393N

0.26 1.00 Geraniol 503

OH

0.62 1.67 0.26 1.00

cis-Myrtanol 506

CH₂OH 393 N

0.23 1.00 393S

0.41 1.00

trans-Myrtanol 508 _CH

2OH 0.54 2.34 0.42 1.02

Phenanthrene 613

453N

2.21 1.00 473N

0.70 1.00

Anthracene 613 2.46 1.11 0.80 1.14

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to their boiling points. The isomer pair nerol (cis) and geraniol (trans), which have similar boiling points, were clearly resolved on the LCUT column (α = 1.67) within the nematic range, whereas they were co-eluted with the LCST column. Similarly, cis- and trans-myrtanol were only successfully separated with the LCUT column (α = 2.34). Again, the nematic stationary phase interacted more with the trans isomers (trans-myrtanol and trans- geraniol), because the trans shape is more elongated, which favors analyte–stationary phase interactions. The isomer pair phenanthrene and anthracene was chromatographed within the nematic range on both columns; resolution was achieved with both LC columns, and the separation factor was 1.11 and 1.14 with LCUT and LCST, respectively (Fig. 6b). For mesogenic stationary phases, retention of the isomer with the higher length-to-breadth ratio (L/B) is higher in the nematic mesophase, regardless of specific interactions. Anthracene (L/B = 1.56) retention was higher than that of phenanthrene (L/B = 1.46). Although they have similar boiling points (613 K), their different shapes enable their separation on both columns. In summary, the nematic state was one of the main factors that improved the separation ability of the mesogenic stationary phases; however, the natures of the terminal groups of the liquid-crystalline materials were also important, because they contributed to the differences between the separation abilities of the columns. The unsaturated terminal group considerably enhanced the efficiency of the LCUT column, whereas its absence weakened the separating properties of the LCST column.

Separation of Mixtures of n‑Alkanes and Natural Components

The separation abilities of the two terpenoid LC columns were evaluated using mixtures of alkanes and EOCs. Fig- ure 7a and b shows the chromatograms for each group of compounds under different chromatographic conditions.

For each complex mixture, good resolution was obtained with the LCUT and LCST columns; however, increased efficiency was achieved with the LCUT column, because the nematic state was more involved. The non-polar compound n-nonane was distinguished on the LCUT station- ary phase, but was not retained on the LCST column. Good separation was also achieved for the more polar EOCs on both columns. For the LCUT column, most of the terpenes (Fig. 7b) were separated, except the co-eluted pair linalool and borneol; weak resolution of geranyl acetate and eugenol compounds was observed. With the LCST column, the efficiency decreased significantly, because two co-elutions occurred, i.e., for borneol/terpineol and citral/carvone.

Also, the analysis time was shorter with the LCUT column. Betts et al. [25] studied the polarities of certain GC stationary phases and proposed the use of volatile oils as solute probes, because the separation ability is considerably affected by the polarity. In our study, the behaviors of both stationary phases toward EOCs showed pronounced polarity, which was already observed for the LCUT column. As expected, the presence of the double bond increased the polarizability of the stationary phase, which improved the separation ability of the corresponding LC column.

Fig. 7 Chromatographic separation of n-alkanes (a) and EOCs (b) on LCUT and LCST columns. (a) 1 n-nonane, 2 n-decane, 3 n-undecane, 4 n-dodecane, 5 n-tridecane, 6 n-tetradecane, 7 n-pentadecane, and 8 n-hexadecane. GC conditions for LCUT and LCST, respectively:343 K at 277 K min⁻¹ and 363 K at 279 K min⁻¹. (b)1 eucalyptol, 2 linalool, 3 borneol, 4 terpineol, 5 citronellol, 6 citral, 7 carvone, 8 geranyl acetate, 9 eugenol, and 10 cis-jasmone.

GC conditions for LCUT and LCST, respectively: 353 K at 277 K min⁻¹ and 363 K at 278 K min⁻¹

0 2 4 6 8 10 12 14 0 2 4 6 8

0 2 4 6 8 10 0 2 4 6 8

(b) _N

N S N

S

min min 8

7

6 5

4 3 2 1

N

LCUT

(a)

min min

1 2+3

4 56 7 8 9

10 S

LCST 2

3

4 5

6 7 8

s + 1

1 2

3+4 5

6+78 9

10

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Thermal Stabilities of LCSP columns

The two LCSPs were conditioned for at least 3 h at 463, 483, and 503 K. A homologous series was injected after each conditioning run. This method is adequate for evaluat- ing the thermal stabilities of the studied materials, because it permits determination of their sustainability and efficiency after being subjected to high oven temperature runs [26]. Table 3 shows the retention times for a homologous series of n-alkanes on LCUT and LCST after each conditioning step. The data show that the obtained retention parameters were almost the same for the LCUT and LCST separation columns. A weak baseline drift was noticed dur- ing the analysis run after each conditioning temperature.

The RSD (%) for three retention times was calculated, and the results showed that both columns had good stability in high-temperature runs.

Conclusion

The separation ability and mesophase stability of an LCSP greatly depend on the structure of the terminal substituent, which affects the intermolecular interactions of the LC, and the LC molecule–analyte interactions. Based on the shape selectivity of the mesogenic materials, the terpenoid LC with an unsaturated terminal group (LCUT) showed better efficiency, as a result of enhancement of specific interactions and the mesophase stability, compared with the LCST column. The durabilities and thermal stabilities of both columns had interesting features, which enabled analysis of complex mixtures with high boiling points in elevated temperature runs. Functionalization of the mesogenic materials enhanced the selectivity for EOCs, especially in the case of the citronellol LC. Other LCs with natural compounds with more polar terminal groups incorporated as terminal and lateral groups will be investigated in future studies.

References

1. Kelker H (1963) Kristallin-flüssige Schmelzen als stationäre Phasen in der Gas-Flüssigkeits-Verteilungschromatographie. Z Anal Chem 198(3):254–266. doi:10.1007/BF00486441

2. Emad Ghanem SA-H (2014) Separation of isomers on nematic liquid crystal stationary phases in gas chromatography: a review. Chromatographia 77(9–10):653–662. doi:10.1007/

s10337-014-2675-z

3. Ferroukhi O, Atik N, Guermouche S, Guermouche MH, Berda- gué P, Judenstein P, Bayle JP (2000) High performance liquid chromatography of aromatic and polyaromatic hydrocarbons on a new chemically bonded liquid crystal phase. Chromatographia 52(9–10):564–568. doi:10.1007/BF02789751

4. Witkiewicz Z (1989) Application of liquid crystals in chromatography. J Chromatogr A 466:37–87. doi:10.1016/

S0021-9673(01)84616-1

5. Witkiewicz Z, Oszczudłowski J, Repelewicz M (2005) Liquid- crystalline stationary phases for gas chromatography. J Chroma- togr A 1062(2):155–174. doi:10.1016/j.chroma.2004.11.042 6. Demus D, Goodby JW, Gray GW, Spiess HW, Vill V (1998)

Handbook of liquid crystals fundamentals. Wiley, VCH Verlag GmbH, USA, pp 133–187

7. Berdagué P, Perez F, Courtieu J, Bayle JP, Abdelhadi O, Guer- mouche S, Guermouche MH (1995) Gas chromatographic study of the thermal and analytical properties of a nematic liquid crystal and its cupric complex. J High Resolut Chromatogr 18(5):304–308. doi:10.1002/jhrc.1240180508

8. Witkiewicz Z, Wacławczyk A (1979) Some properties of high- temperature liquid crystalline stationary phases. J Chromatogr A 173(1):43–52. doi:10.1016/S0021-9673(01)80444-1

9. Dahmane M, Athman F, Sebih S, Guermouche M-H, Bayle J-P, Boudah S (2010) Effect of the chain length on the thermal and analytical properties of laterally biforked nematogens. J Chroma- togr A 1217(42):6562–6568. doi:10.1016/j.chroma.2010.08.036 10. Addoun A, Ferroukhi O, Dahmane M, Guermouche S, Bayle

JP, Guermouche M (2014) Three nematogen azobenzene-based stationary phases for capillary GC: synthesis and comparative study. Chromatographia 77(19–20):1367–1377. doi:10.1007/

s10337-014-2743-4

11. Ammar-Khodja F, Guermouche S, Guermouche MH, Berdagué P, Bayle JP (1999) Gas chromatographic properties of some liquid crystals containing dioxyethylene ether terminal chains.

Chromatographia 50(5–6):338–345. doi:10.1007/BF02490839 12. Chester TL, Coym JW (2003) Effect of phase ratio on van’t

Hoff analysis in reversed-phase liquid chromatography, and Table 3 Retention parameters

for n-alkane mixtures after each conditioning step

Analytes 463 K 483 K 503 K RSD %

LCUT LCST LCUT LCST LCUT LCST LCUT LCST

t_r (min)

n-Nonane 3.15 – 3.19 – 3.17 – 1.25 –

n-Decane 3.36 3.63 3.39 3.62 3.37 3.62 0.61 0.27

n-Undecane 3.92 4.15 3.90 4.15 3.93 4.17 0.39 0.48

n-Dodecane 4.70 4.74 4.69 4.77 4.71 4.75 0.24 0.62

n-Tridecane 6.22 5.43 6.22 5.41 6.23 5.43 0.18 0.28

n-Tetradecane 8.16 6.33 8.15 6.31 8.17 6.32 0.12 0.32

n-Pentadecane 10.29 7.53 10.31 7.54 10.31 7.55 0.19 0.47

n-Hexadecane 14.65 9.52 14.62 9.54 14.63 9.51 0.17 0.58

(11)

phase-ratio-independent estimation of transfer enthalpy. J Chro- matogr A 1003:101–111. doi:10.1016/S0021-9673(03)00846-X 13. Price GJ, Hickling SJ, Shillcock IM (2002) Applications of

inverse gas chromatography in the study of liquid crystalline stationary phases. J Chromatogr A 969(1–2):193–205. doi:10.1016/

S0021-9673(02)00889-0

14. Llorente MA, Menduiña C, Horta A (1979) Inverse gas chromatography of poly(cyclohexyl methacrylate). J Polym Sci 17(2):189–197. doi:10.1002/pol.1979.180170201

15. Gritti F, Félix G, Achard MF, Hardouin F (2000) Investigation of the nematic—isotropic transition of a liquid crystalline polymer and determination of molecular diffusion coefficients using gas chromatography. J Chromatogr A 893(2):359–366. doi:10.1016/

S0021-9673(00)00772-X

16. Reid VR, Crank JA, Armstrong DW, Synovec RE (2008) Charac- terization and utilization of a novel triflate ionic liquid stationary phase for use in comprehensive two-dimensional gas chromatography. J Sep Sci 31(19):3429–3436. doi:10.1002/jssc.200800251 17. Kelker H, VS E (1968) Advances in chromatography, vol 6. Mar-

cel Dekker, New York

18. Schroeder J (1974) Scientific and technological application of liquid crystals. In: G.W. G, P.A. W (eds) Liquid crystals and plastic crystals: preparation, constitution and applications, vol 1.

E. Horwood, New York

19. Sun X, Zhu Y, Wang P, Li J, Wu C, Xing J (2011) High temperature and highly selective stationary phases of ionic liquid bonded polysiloxanes for gas chromatography. J Chromatogr A 1218(6):833–841. doi:10.1016/j.chroma.2010.12.036

20. Breckler PN, Betts TJ (1970) Relative retention time changes with temperature for the gas chromatographic identification of volatile oil components. J Chromatogr A 53(2):163–170.

doi:10.1016/S0021-9673(01)98456-0

21. Boudah S, Sebih S, Guermouche MH, Rogalski M, Bayle JP (2003) Thermal properties and chromatographic behavior of a mixture of two liquid crystals. Chromatographia 57(1):S307–

S311. doi:10.1007/BF02492121

22. Athman F, Dahmane M, Boudah S, Guermouche M-H, Bayle J, Sebih S (2009) Evaluation of thermal and analytical properties of two liquid crystals in capillary GC. Chromatographia 70(3–

4):503–510. doi:10.1365/s10337-009-1212-y

23. Belaïdi D, Sebih S, Guermouche MH, Bayle JP, Boudah S (2003) Comparison of two liquid crystals as stationary phases in capillary gas chromatography. Chromatographia 57(3–4):207–

212. doi:10.1007/BF02491718

24. Ghatge BB, Bhalerao NV (1991) Application of substituted liquid crystals as stationary phases in gas-liquid chromatography for the separation of mono-and dimethyl naphthalenes. J Chro- matogr A 549:423–428. doi:10.1016/S0021-9673(00)91454-7 25. Betts TJ, Finucane GJ, Tweedie HA (1981) Practical sys-

tem for polarity rating of packed gas-liquid chromatography columns. J Chromatogr A 213(2):317–322. doi:10.1016/

S0021-9673(00)81914-7

26. Qiao L, Lu K, Qi M, Fu R (2013) Separation performance of guanidinium-based ionic liquids as stationary phases for gas chromatography. J Chromatogr A 1276:112–119. doi:10.1016/j.

chroma.2012.12.039