ISSN 0036�0244, Russian Journal of Physical Chemistry A, 2012, Vol. 86, No. 8, pp. 1308–1317. © Pleiades Publishing, Ltd., 2012.Original Russian Text © L.A. Onuchak, T.S. Burmatnova, E.A. Spiryaeva, Yu.G. Kuraeva, Z.P. Belousova, 2012, published in Zhurnal Fizicheskoi Khimii, 2012, Vol. 86, No. 8,pp. 1424–1434.

1308

INTRODUCTION

The results from experimental investigations of thethermodynamic characteristics of non�mesogen–chiral liquid crystal (LC) systems were presented in [1,2]. It was demonstrated that their helically twistedstructure and special interactions play an importantrole in the mechanism of recognizing optical isomersof butanediol�2,3 under gas–liquid chromatographyconditions. This work is a continuation of investiga�tions in this field and is devoted to an experimentalstudy of the thermodynamic characteristics of sorp�tion from the gaseous phase of non�mesogen organiccompounds of different classes by a binary sorbentbased on achiral nematic LC and a chiral macrocyclicadditive inducing the formation of a helically twistedcholesteric mesophase (the chiral nematic phase N*).

EXPEREMENTAL

To prepare binary sorbent, we used nematic LC, 4�methoxy�4'�ethoxyazoxybenzene (MEAB),

CH3O N N

O

OC2H5

(tCN = 91°C, tNI = 150°C, Mr = 272)

and heptakis(2,3,6�tri�O�acetyl)�β�cyclodextrin(Acetyl�β�CD),

synthesized from the initial β�CD hydrate in a one�stage reaction through acetylation with acetic anhy�dride.

O

ORRO

OR

O

O

OR

ROOR

O

OOR

OR

OR

O

O

OROR

OR

OO

OR

OR

ROO

OOR

ORRO

O

OOR

RO

RO

O

(tm = 175–178°С, Mr = 2017)

R = –СОСН3

PHYSICAL CHEMISTRY OF SEPARATION PROCESSES:CHROMATOGRAPHY

Sorption and Selective Properties of Binary Liquid�Crystalline Sorbent Based on 4�Methoxy�4'�ethoxyazoxybenzene

and a Cetylated β�CyclodextrinL. A. Onuchak, T. S. Burmatnova, E. A. Spiryaeva, Yu. G. Kuraeva, and Z. P. Belousova

Samara State University, Samara, 443011 Russiae�mail: onuchak@ssu.samara.ru

Received August 4, 2011

Abstract—The mesomorphous, sorption, and selective properties of a binary sorbent based on achiral liquidcrystal 4�methoxy�4'�ethoxyazoxybenzene and macrocyclic heptakis(2,3,6�tri�O�acetyl)�β�cyclodextrin(10.25 wt %) are investigated. Macrocyclic additive was found to induce the formation of a helically twistednematic phase (N*). The standard and excess thermodynamic functions of sorption by binary sorbent fromgaseous phase are determined for 29 organic compounds (n�alkanes, cycloalkanes, arenes, monoatomic alco�hols, heterocycles, optical isomers of camphene, limonene, and butanediol�2,3). The investigated sorbent isfound to have high structural selectivity over the wide temperature range of the N* mesophase (αp/m = 1.11–1.06, 95–120°C) and moderately expressed enantioselectivity within the narrow region of transition from thesolid crystalline state to the N* mesophase (α+/– = 1.024–1.088, 91–100°C).

Keywords: Gas–liquid chromatography, liquid crystals, modified β�cyclodextrines, structural selectivity,enantioselectivity.

DOI: 10.1134/S0036024412080080

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

SORPTION AND SELECTIVE PROPERTIES 1309

Phase transfer temperatures for the binary MEAB–Acetyl�β�CD sorbent were determined by visual poly�thermal analysis, while the heats of phase transfer weremeasured by differential scanning calorimetry on aDSC Q20 apparatus (TA Instruments) and the types offormed mesophases were identified by polythermalpolarization microscopy using a MIN�8 polarizationmicroscope.

Our gas chromatography (GC) experiment wasperformed on a Tsvet�500 chromatograph equippedwith a flame�ionization detector, using a steel column(1.2 m × 2 mm) packed with MEAB–Acetyl�β�CDsorbent (89.75 : 10.25 wt %) on a Chromaton NAWsolid carrier (impregnation was 11.1%). Nitrogen wasused as the carrier gas. The volume velocity at the col�umn outlet was no more than 15 mL/min. Our exper�iments were carried out in the isothermal mode over atemperature range of 85 to 170°С, including the crys�tal (С), chiral nematic (N*) and isotropic (I) phases ofthe sorbent. Small samples of sorbates (0.01–0.03 μL)taken from the equilibrium vapor phase were intro�duced. The GC experiment for a column with neatMEAB was performed under analogous conditions.

We used 29 organic compounds of different classesas sorbates. Their physicochemical properties, i.e.,polarizability (α), dipole moment of molecules (μ),boiling point (Тb) [3], as well as A, B, C, and D coeffi�cients [4] for calculating the saturated vapor pressurefor each sorbate at the temperature of study are listedin Table 1. We could find no literature data for calcu�lating saturated vapor pressure in the case of 1�methyl�4�isopropenyl�cyclohexene�1 (limonene).

The specific retention volume at the tempera�ture of our experiment (column) was the main experi�mentally obtainable characteristic of sorbate retentionin the column with LC sorbent. Dimensionless con�stants of sorption ( = 1 atm, =

) [6], activity coefficients (a symmetricreference system) characterizing deviation fromRaoult’s law when small samples of sorbate are dis�solved in a stationary liquid phase, and the standardand excess thermodynamic characteristics of sorptionalong with factors of isomer separation,

(1)

were calculated from the value of .

RESULTS AND DISCUSSION

Our choice of nematic MEAB as the main compo�nent of the sorbent was governed by its high structuralselectivity in the mesophase range, distinguished by ahigh degree of separation of isomeric para� and meta�xylenes ( = 1.11–1.13, depending on the natureof the solid carrier [7, 8]). Macrocyclic Acetyl�β�CD

TgV

st / HP K stP HK

/,, 0( )

i Li i L xP x→ i

∞

γ

(1)1/2

(2)

TgTg

V

Vα =

TgV

α p m

is a product of complete acetylation of 21 hydroxylgroups in the initial β�CD composed of seven residuesof D�(+)�glucopyranose, connected to each other byα�1,4�bonds.

It was found that when chiral macrocyclic Acetyl�β�CD was introduced into achiral nematic MEAB inthe amount of 10.25 wt % (1.52 mol %), a helicallytwisted structure, i.e., induced chiral nematicmesophase N* was formed (Fig. 1).

Two phase transitions were found for the MEAB–Acetyl�β�CD system by DSC, the enthalpies andentropies of which are presented in Table 2 for com�parison with the analogous data for MEAB.

The enthalpy of melting for the MEAB–Acetyl�β�CD system is higher by 2.57 kJ/mol than that ofMEAB, indicating the enhancement of intermolecu�lar attraction in a solid crystalline phase containing1.52 mol % of the macrocyclic additive.

Helically twisted mixed mesophase N* of theMEAB–Acetyl�β�CD sorbent has rather high thermalstability ( = 91 ↔ 142°С), and transition entropy

= 2.0 kJ/(mol K) is comparable to the analo�gous value for MEAB. This indirectly indicates highorientational ordering of the induced helically twistedmesophase N*.

The temperature dependences of specific retention

volumes for para� and meta�isomers of xyleneobtained on columns with the MEAB–Acetyl�β�CDand MEAB sorbents are presented in Fig. 2. The val�

ues of increase dramatically near the meltingpoint due to transitioning from an adsorp�tion/absorption mechanism of sorption to a distribu�tive one, and they reach a maximum at temperatureof 98°С. The sorption of xylenes by the binary sor�bent is appreciably lower than that of the initial

*NTΔ

*N I SΔ

TgV

TgV

Fig. 1. Fingerprint texture of N* mesophase for the binaryMEAB–Acetyl�β�CD sorbent (heating cycle, crossedpolaroids, 200 power magnification).

1310

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

ONUCHAK et al.

Table 1. Physicochemical properties of sorbates

No. Sorbate α, Å3 μ, D Tb, °C –А –B С D × 106

1 n�Hexane 11.78 0.0 68.9 13.9993 7284.572 105.9605 14.1032

2 n�Heptane 13.62 0.0 98.4 14.12488 8030.07 108.1461 12.0485

3 n�Octane 15.45 0.0 125.8 7.37874 6981.936 65.77825 3.38092

4 n�Nonane 17.29 0.0 150.9 8.327399 7739.415 72.54661 3.89483

5 n�Decane 19.12 0.0 174.0 7.768817 8163.335 69.76469 2.62033

6 Methanol 3.25 1.7 64.7 9.372816 7029.198 79.09415 7.99273

7 Ethanol 5.08 1.7 78.5 5.089412 6606.453 53.17030 0.59540

8 Propanol�1 6.92 1.7 97.2 7.702226 8002.693 71.71697 0.39504

9 Butanol�1 8.75 1.7 117.9 9.882614 9127.496 86.72214 1.42848

10 Pentanol�1 10.59 1.7 138.0 19.38194 12196.40 149.3463 9.39571

11 Propanol�2 6.92 1.7 82.3 7.694051 7690.896 71.34113 0.76564

12 2�Methyl�propanol�1 8.75 1.7 108.4 29.61931 14106.20 214.5658 21.26

13 Tetrahydrofuran 7.98 1.9 66.0 9.608804 6339.983 78.35769 8.18347

14 Cyclohexane 11.01 0.3 81.0 9.200978 6354.898 75.65086 7.37481

15 1,4�Dioxane 8.61 0.0 101.4 9.151525 7075.938 76.91763 5.90158

16 Benzene 10.43 0.0 80.1 8.433613 6281.040 71.10718 6.19841

17 Toluene 12.27 0.4 110.8 8.799548 6918.798 74.1358 5.75491

18 Ethylbenzene 14.10 0.4 136.3 9.553938 7638.082 79.79371 5.65318

19 p�Xylene 14.10 0.1 138.4 9.527348 7637.951 79.55720 5.74897

20 m�Xylene 14.10 0.3 139.1 9.106679 7556.611 76.86698 5.40363

21 o�Xylene 14.10 0.5 144.6 10.06590 7946.229 83.32184 5.93974

22 cis�Decalin 17.58 0.01 194.6 –4.00047 –1594.687 –69.7310 –

23 trans�Decalin 17.58 0 185.5 –3.99304 –1572.899 –65.9470 –

24 (2S,3S)�(+)�butanediol�2,39.39 2.13 179.7 17.82816 12286.40 137.8910 13.8579

25 (2R,3R)�(–)�butanediol�2,3

26 (+)�Camphene17.38 0.26 160 –4.7386 –1893.48 –33.855 –

27 (–)�Camphene

28 (+)�Limonene17.97 0.19 178 – – – –

29 (–)�Limonene

Note: Values of α and μ were calculated using the HyperChem program with semiempirical AM1; saturated vapor pressures were calculatedby the equation log [kPa] = A lnT – B/T + C + DT2 with constants A, B, C, and D [4]; in the case of camphenes (2,2�dimethyl�3�

methylene�bicyclo[2.2.1]heptane) and decalines, we used equation log = A – B/(T + C) with constants A, B, and C [5].

Pi°

Pi°

Table 2. Temperatures, enthalpies and entropies of phase transitions determined by DSC method

Sorbate Mr

Melting point Clearing point

tCN*, °С ΔCN*H,kJ/mol

ΔCN*S,J/(mol K) tN*I, °С ΔN*IH,

kJ/molΔN*IS,

J/(mol K)

MEAB 272 91 19.73 54 150 0.85 2.0

MEAB–Acetyl�β�CD 298 91 22.30 61 142 0.85 2.0

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

SORPTION AND SELECTIVE PROPERTIES 1311

MEAB, with the exception of a narrow temperatureregion near the melting point.

Analogous regularities were also found for other

investigated arenes, 1,4�dioxane, tetrahydrofuran,

and methanol, while the dependencies (Т) for non�aromatic hydrocarbons with linear (С6–С10) andcyclic structure (cyclohexane, decalines, camphenes,limonenes) are located lower over the investigatedtemperature range when binary sorbent is used, ascompared to the column with MEAB, as we can see inFig. 3 in the instance of n�decane.

The values of for the investigated С1–С5�alco�hols in pretransition regions C ↔ N* and N* ↔ I areconsiderably higher on the column with the binarysorbent than on the one with MEAB (in Fig. 3, thesorbate is propanol�2). However, the retention ofmesophase by the binary sorbent is comparable to oreven slightly higher than the retention on the columnwith the MEAB in the temperature region of 98–130°С only for the С2–С3 alcohols.

Organic compounds capable of inductive, dipole–dipole, and specific interactions with components ofthe binary sorbent thus proved to be the ones most sen�sitive to the Acetyl�β�CD additive, with the increasedretention for such sorbates manifesting itself maxi�mally in the transition temperature region C↔ N*,where the adsorption/absorption mechanism of reten�tion is realized. The presence in LC sorbate of a mac�rocyclic additive containing 21 electron releasingacetyl groups outside the cavities facilitates the

TgV

TgV

155

90 110 130 150 170t, °C

115

75

35

VgT, cm3/g

1

2

3

4

N

IC

N*

Fig. 2. Dependences of on temperature for xylene iso�mers obtained on a column with neat MEAB (1, 2) andbinary MEAB–Acetyl�β�CD sorbent (3, 4): 1 and 3 corre�spond to para�xylene, 2 and 4 correspond to meta�xylene.

TgV

190 110 130 150 170

T, °C

5

9

~~25

49

73

97

121

VgT, cm3/g

1

2

3

4

N

I

C

N*

N

N*

IC

Fig. 3. Dependences of on temperature for n�decane (1, 2) and propanol�2 (3, 4): 1 and 4 correspond to neat MEAB; 2 and3 correspond to MEAB–Acetyl�β�CD.

TgV

1312

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

ONUCHAK et al.

increased sorption of proton�donating sorbates, par�ticularly alcohols with chain lengths of С2–С3, overthe temperature region of distribution chromatogra�phy (t ≥ 98°С). Sorption contributions δ (%) along

with activity coefficients of sorbates for differenttemperatures of mesophase N* (95, 110, and 140°С)and of isotropic phase I (160°С) were calculated toevaluate the influence of the Acetyl�β�CD additive onthe retention of sorbates (see Table 3).

i∞

γ

Sorption contributions δ due to incorporatingmacrocyclic Acetyl�β�CD into MEAB were calcu�lated using the equation

, (2)

where and are the specific retention volumeson columns with the binary sorbent and neat MEAB,respectively.

,2 ,1

,1

100T Tg g

Tg

V V

V

−δ =

,2TgV ,1

TgV

Table 3. Specific retention volumes ( ), contributions (δ, %) to retention and activity coefficients ( ) of sorbates ob�tained using the MEAB–Acetyl�β�CD sorbent

Sorbates

95°C (N*) 110°C (N*) 140°C (N*) 160°C (I)

,

cm3/gδ, % ,

cm3/gδ, % ,

cm3/gδ, %

n�Hexane 1.8 –71.4 25.7 (8.1) 3.0 –43.4 11.0 (7.1) 3.5 –13.5 5.4 (5.1) 5.0 (3.4)

n�Heptane 5.3 –62.4 21.4 (8.6) 7.0 –37.5 10.9 (7.5) 7.1 –15.2 5.5 (5.1) 4.7 (3.5)

n�Octane 11.9 –51.4 21.8 (11.3) 13.0 –44.2 12.8 (7.9) 12.6 –16.6 6.1 (5.6) 5.0 (3.6)

n�Nonane 26.9 –46.1 22.1 (12.6) 25.3 –44.6 14.3 (8.7) 21.6 –21.0 7.1 (6.2) 5.4 (3.9)

n�Decane 58.0 –45.7 22.9 (13.1) 47.4 –46.6 16.2 (9.5) 39.2 –19.5 7.6 (6.7) 5.8 (4.3)

Methanol 6.8 183.3 5.0 (15.4) 5.7 –30.5 4.4 (3.0) 5.2 10.9 2.1 (2.5) 2.2 (1.9)

Ethanol 7.1 184.0 7.7 (23.9) 8.4 1.2 4.7 (4.5) 6.5 34.5 2.4 (3.5) 2.3 (2.6)

Propanol�1 13.1 151.9 8.5 (23.7) 16.4 0.6 4.4 (4.5) 11.8 11.7 2.4 (2.9) 1.9 (2.0)

Butanol�1 25.0 80.0 9.8 (19.4) 32.3 –6.9 4.4 (4.5) 23.3 8.2 2.4 (2.8) 1.9 (1.9)

Pentanol�1 52.7 58.3 10.0 (17.3) 62.5 –11.7 6.4 (4.5) 43.9 6.9 2.4 (2.9) 2.0 (1.9)

Propanol�2 5.8 107.1 10.8 (24.6) 8.8 3.5 1.9 (5.0) 7.4 34.1 2.3 (3.4) 2.1 (2.2)

2�Methyl�propanol�1 16.4 100.0 10.1 (22.2) 22.4 –1.3 1.8 (4.8) 16.7 12.0 2.5 (3.0) 2.0 (2.1)

Tetrahydrofuran 8.5 41.7 5.1 (7.9) 17.5 –15.9 4.1 (1.6) 14.6 –2.6 1.2 (1.2) 1.1 (0.9)

Cyclohexane 2.6 –50.9 26.2 (14.0) 7.4 –36.2 3.9 (4.5) 7.2 –23.3 3.6 (3.0) 3.1 (2.0)

1,4�Dioxane 23.3 89.4 5.3 (11.0) 42.0 –11.6 4.1 (1.9) 32.0 0.0 1.2 (1.4) 1.2 (1.0)

Benzene 8.6 24.6 7.6 (10.4) 20.7 –24.7 2.2 (1.8) 18.6 –5.8 1.3 (1.4) 1.3 (1.0)

Toluene 16.5 12.2 10.0 (12.0) 41.7 –22.8 2.6 (2.2) 35.0 –5.4 1.5 (1.6) 1.4 (1.1)

Ethylbenzene 27.2 26.5 13.2 (18.1) 68.0 –24.9 3.3 (2.7) 56.1 –6.6 1.8 (1.9) 1.7 (1.3)

o�Xylene 38.2 13.0 12.5 (15.5) 96.3 –25.3 3.0 (2.5) 76.6 –5.4 1.7 (1.8) 1.5 (1.2)

m�Xylene 30.3 31.7 12.9 (18.7) 77.0 –26.3 3.2 (2.6) 63.5 –4.8 1.7 (1.8) 1.6 (1.3)

p�Xylene 31.8 27.2 12.5 (16.7) 84.2 –26.0 3.1 (2.3) 62.8 –8.9 1.8 (1.7) 1.6 (1.2)

(2S,3S)�(+)�Butanediol�2,3 244.3 – 14.6 196.0 – 8.9 107.5 – 4.7 3.5

(2R,3R)�(–)�Butanediol�2,3 224.5 – 15.9 194.7 – 9.0 108.2 – 4.7 3.5

(+)�Camphene 23.0 –50.9 36.7 (19.7) 55.1 –35.9 9.2 (6.4) 54.1 –5.7 3.7 (3.8) 2.9 (2.2)

(–)�Camphene 22.9 –51.1 36.7 (19.7) 54.4 –36.7 9.1 (6.4) 55.2 –3.8 3.6 (3.8) 2.9 (2.2)

Note: Values for neat MEAB are presented in parentheses.

TgV ∞

γ i

TgV

∞

γ i

TgV

∞

γ i

TgV

∞

γ i∞

γ i

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

SORPTION AND SELECTIVE PROPERTIES 1313

The values of δ at a temperature of 95°С are posi�tive for all the polar sorbates with linear and cyclicstructure, as they are for arenes. There are goodgrounds for believing that interactions LC···LC at lowmesophase temperatures, resulting in the formation ofa highly oriented structure, impede the formation ofthe LC···macrocycle bonds, whereupon the interac�tions of polar sorbates and arenes with macrocyclemolecules, particularly in a gas–sorbent surface layer,make a substantial contribution to sorption. Suchinteraction can occur, among other things, accordingto a guest–host mechanism. The solvation of macro�cyclic Acetyl�β�CD in polar solvent MEAB increaseswith a rise in temperature and a drop in the LC orderparameter, which is favored by the presence of21 acetyl groups at the entrance of a macrocycle cavity.

Because the orientation of primary and secondaryacetyl groups is determined by the chiral structure ofthe D�(+)�glucopyranose residues, the formation ofdirected LC···macrocycle bonds in a near solvationenvironment induces a helically twisted chiral nematicstructure, due to the cooperative interaction of LCmolecules in the near solvation region with LC mole�cules of the remote environment as a result of theirmesogenity. The formation of such an induced highlyordered structure (N*) hinders the interaction of thesolute (sorbate) with sorbent components; the valuesof δ for most of the investigated substances (non�mesogens) are therefore negative or slightly positive(ethanol, propanol�1, propanol�2) at a temperature of110°С. Solvation effects weaken with a furtherincrease in temperature, and a drop of the orderparameter in the N* phase ensures the interaction ofsorbates with macrocyclic Acetyl�β�CD, which is whythe values of δ for most of the polar sorbates againbecome positive at a temperature of 140°С. Analysis

and comparison of the values of activity coefficients for non�mesogen sorbates in both the mixedmesophase N* and the initial nematic N supports theforegoing.

The interaction of sorbate molecules, includingthose optically active, with the macrocyclic Acetyl�β�CD is therefore more likely to occur over the low tem�perature region of mesophase N* (near the meltingpoint of the sorbent) and in transition range N* ↔ I.It is in these temperature regions that we might expectthe sorbent to reveal enantioselective properties.Indeed, the maximum values of enantioselectivity forall the three pairs of optical isomers (±limonene,±camphene, R/S�butanediol�2,3) are observed over anarrow temperature range of 91–100°С, with (+) iso�mers being sorbed more strongly than (–) isomers (seeFig. 4). This is probably due to the macrocycle formedby seven D�(+)�glucopyranose residues.

The maximum value of enantioselectivity for thebutanediol�2,3 optical isomers, αS/R, was 1.088(95°С), while the following results were obtained forcamphene and limonene: α+/– = 1.024 (98°С) and

i∞

γ

α+/– = 1.057 (91°C), respectively. Note that a changein the order of appearance of optical isomers isobserved for camphene at 120°C (Fig. 5), the value ofα–/+ being equal to 1.021 at a temperature of 140°C.

The investigated binary MEAB–Acetyl�β�CD sor�bent therefore possesses moderately expressed enanti�oselectivity toward optical isomers of both polar andlow polar compounds. The enantioselectivity isrevealed within the narrow pretransition temperatureregion (91–100°C) in which the adsorption/absorp�tion mechanism of sorption is realized, with the inter�action of proton�donating compounds with macrocy�clic Acetyl�β�CD most likely taking place accompa�nied by the formation of hydrogen bonds (alcohols,diols) with acetyl groups, while low polar compounds(arenes, camphenes, limonenes) interact with Acetyl�β�CD according to the guest–host mechanism. Theinvestigated binary sorbent possesses high para� andmeta�selectivity over a wide temperature range of themesophase (see Fig. 2); this is associated with the highorientational ordering of the mixed N* mesophase [9].The maximum value of selectivity to xylene isomers,αp/m, was 1.11 (98°C), comparable to the selectivity of

90 110 130 150 170t, °C

20

60

65

115

165

215

265

VgT, cm3/g

1

2

3

4

IC N*

40

~~

Fig. 4. Dependences of for optical isomers on a columnwith the binary MEAB–Acetyl�β�CD stationary phase:1 represents (2S,3S)�(+)�butanediol�2,3; 2 represents(2R,3R)�(–)�butanediol�2,3; 3 represents (–)�camphene;4 represents (+)�camphene.

TgV

1314

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

ONUCHAK et al.

MEAB deposited on an analogous solid carrier (αp/m =1.12, 98°C). The separation factor for cis� and trans�isomers of decalin, αc/t, changes slightly within thetemperature range of 95–170°C, being equal to 1.30.

Along with isomer selectivity, separation selectivityfor compounds of different classes possessing closeboiling points is of great importance for practicalchromatography. Fig. 5 illustrates relationship

between ln and boiling points of sorbates (Тb),

which were obtained using values of at 100°С (N*mesophase).

Analysis of the data presented in Fig. 5 shows thatwithin the N* mesophase range, the investigatedbinary sorbent possesses good separation selectivity forcompounds of different classes with a wide range ofboiling points. Sorbent affinity for the classes of ana�lyzed compounds in the helically twisted mesophase(N*) increases in the order n�alkanes → cycloalkanes →alkanols → arenes → heterocycles. The location ofpoints for the optically active compounds (24–29)corresponds on the whole to the observed regularities.

Standard thermodynamic functions (enthalpy

and entropy ) of sorption of organic com�pounds under conditions of a distributive mechanismof sorption are characterized by a change in thesefunctions upon transition of 1 mol of gaseous sorbateunder ideal gas conditions at Pst = 1 atm and columntemperature to an infinitely dilute solution in the sta�tionary liquid phase [6]. The calculated values of

and for mesophase N* and isotropicphase I of the binary sorbent are presented in Table 4.The values of the standard enthalpies and entropies of

TgV

TgV

sp0iHΔ sp

0iSΔ

sp0iHΔ sp

0iSΔ

sorption by neat MEAB are presented (in parentheses)in this table as well.

Referring to the data presented in Table 4, we can see

that values for all the investigated sorbates in the

case of a mixed sorbent in the N* phase are lower by3⎯7 kJ/mol than for the N phase of MEAB, due obvi�ously to the solvation effects of MEAB···Acetyl�β�CDand the formation of the induced N* mesophase. Wethus observe reduced sorption for most of the sorbates(excluding the С2–С3 alcohols) over a temperaturerange of 98–130°С, relative to MEAB (Figs. 2, 3). Theabsence of orientational ordering in the isotropicregion raises the possibility of intermolecular interac�tions between components of the mixed sorbent andsorbate molecules, resulting in an increase in the heatof sorption. However, the heat of sorption of the mixedsorbent for most sorbates does prove to be lower thanfor neat MEAB, indicating the existence of solvationeffects in the isotropic phase of the MEAB–Acetyl�β�CD system and confining the interactions of sorbatemolecules with components of the binary sorbent.

Sorption enthalpy of (2S,3S)�(+)�butanediol�

2,3 in both phases of the sorbent (N* and I) is about 0.5kJ/mol higher than the sorption enthalpy of (2R,3R)�(–)�butanediol�2,3, which is insufficient for theappearance of noticeable enantioselectivity at temper�atures of >98°С. Increased sorption of the (+)�iso�mer, relative to the (–)�isomer for the optical isomersof camphene and limonene in the N* phase, is due to

the elevated (by about 1 kJ/mol) values of for

these isomers, relative to laevorotatory isomers.

sp0iHΔ

sp0iHΔ

sp0iHΔ

1.4

1701401108050 Tb, °C0

2.8

4.2

5.6

lnVgT

19

20

21

10

16

15

14

13

179

812

117

2

1

3

4

5

23

18

24, 25

28, 29

26, 27

6

I

IIIII

IVV

22

Fig. 5. Dependences of ln for sorbates on their boiling points Tb obtained on a column with binary MEAB–Acetyl�β�CD sor�bent (100°С, N*): I represents n�alkanes, II represents cycloalkanes, III represents alkanols, IV represents arenes, V representsheterocycles (sorbate numbering corresponds to Table 1).

TgV

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

SORPTION AND SELECTIVE PROPERTIES 1315

The reason for the inversion of the order of emis�sion of camphene isomers at a temperature of >120°С(Fig. 4) is probably that the sorption enthalpy of(⎯)�camphene in the I phase of the sorbent beinghigher (in absolute magnitude) than the sorptionenthalpy of (+)�camphene. It was shown in [1] that no

enantioselectivity toward isomers of slightly polarcamphene and limonene was observed when one com�ponent of chiral nematic N* was used as a sorbent. Thelow enantioselectivity toward optical isomers of cam�phene and limonene in the case of binary MEAB–Acetyl�β�CD sorbent is thus provided by guest–host

Table 4. Standard thermodynamic functions of the sorption of organic compounds by chiral nematic (N*) and isotropic (I)phases of the MEAB–Acetyl�β�CD system, along with the differences (ΔaddH) of standard enthalpies of sorption for theinvestigated compounds and n�alkanes whose molecules have the same values of polarizability

No. Sorbate

, kJ/mol , J/(mol K) –ΔaddH, kJ/mol

N*(98–130°C)

I(145–170°C)

N*(98–130°C)

I(145–170°C)

N*(98–130°C)

I(145–170°C)

1 n�Hexane 13.1 (16.1) 16.8 (20.1) 63.0 (67.3) 69.9 (74.8) – –

2 n�Heptane 14.4 (20.8) 19.4 (18.3) 60.3 (73.8) 69.8 (67.2) – –

3 n�Octane 18.0 (23.2) 21.7 (22.4) 64.0 (73.9) 70.6 (69.9) – –

4 n�Nonane 21.5 (27.2) 22.6 (25.9) 67.6 (78.7) 68.0 (73.4) – –

5 n�Decane 25.1 (32.0) 28.2 (28.5) 71.7 (85.7) 76.4 (75.1) – –

6 Methanol 13.4 (25.7) 31.0 (34.2) 59.1 (89.1) 101.6 (107.8) 11.1 26.6

7 Ethanol 20.0 (25.8) 27.7 (34.5) 73.1 (89.3) 91.7 (108.3) 19.2 20.7

8 Propanol�1 22.3 (25.6) 20.7 (31.7) 73.8 (83.1) 69.2 (95.3) 18.3 11.1

9 Butanol�1 23.1 (26.4) 27.3 (33.2) 70.2 (78.8) 79.1 (92.8) 16.0 15.1

10 Pentanol�1 24.6 (28.9) 27.8 (34.6) 68.5 (79.5) 75.5 (90.9) 14.4 13.0

11 Propanol�2 19.2 (24.1) 22.6 (34.6) 70.7 (84.4) 78.2 (106.8) 15.2 13.0

12 2�Methyl�propanol�1 21.2 (23.2) 23.3 (32.2) 68.4 (74.0) 72.4 (93.3) 14.1 11.1

13 Tetrahydrofuran 17.6 (21.3) 22.2 (22.3) 60.9 (70.0) 71.4 (69.7) 11.8 11.1

14 Cyclohexane 12.0 (17.4) 17.9 (15.9) 53.2 (64.5) 66.2 (58.2) 1.1 2.5

15 1,4�Dioxane 21.8 (24.4) 25.2 (27.1) 64.5 (70.9) 71.7 (75.1) 15.0 13.2

16 Benzene 14.1 (20.6) 24.2 (19.0) 50.1 (65.8) 73.6 (60.0) 4.2 9.6

17 Toluene 16.3 (23.0) 25.2 (24.0) 50.3 (66.2) 70.9 (66.6) 3.3 8.0

18 Ethylbenzene 19.0 (25.6) 27.0 (25.8) 53.2 (68.7) 71.4 (66.6) 2.9 7.2

19 p�Xylene 21.8 (28.0) 28.5 (28.5) 58.8 (73.1) 74.0 (72.2) 5.7 8.7

20 m�Xylene 19.0 (26.5) 27.6 (27.8) 52.1 (70.0) 71.8 (70.5) 2.9 7.8

21 o�Xylene 19.6 (27.1) 28.8 (27.6) 51.8 (69.7) 73.0 (68.6) 3.5 9.0

22 cis�Decalin 15.2 30.3 35.5 71.2 –6.8 5.5

23 trans�Decalin 15.8 29.5 39.0 71.3 –6.2 4.7

24 (2S,3S)�(+)�Butanediol�2,3 35.7 32.8 87.8 80.3 27.6 19.7

25 (2R,3R)�(–)�Butanediol�2,3 35.2 32.4 86.8 79.4 27.1 19.3

26 (+)�Camphene 14.9 (22.9) 22.6 (27.4) 44.2 (62.2) 60.9 (69.9) –7.4 –1.9

27 (–)�Camphene 13.6 (22.9) 23.4 (27.4) 40.8 (62.2) 62.7 (69.9) –8.4 –1.1

28 (+)�Limonene 14.9 14.5 – – –7.8 –10.8

29 (–)�Limonene 13.8 13.4 – – –8.9 –11.9

Note: Analogous values for a column with neat MEAB are presented in parentheses. Standard enthalpies of limonene sorption were calcu�

lated using the formula = – RT at temperatures of 110°C (for N*) and 160°C (for I); the value of was found

from the slope of the linear dependence between ln and reciprocal temperature [6].

ΔspHi°– ΔspSi°–

ΔspHi° ΔspUi° ΔspUi°

TgV

1316

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

ONUCHAK et al.

type complexing under the conditions of competingwith molecules of the main component of the sorbent(MEAB).

To evaluate additional interactions (i.e., additionalto dispersion) ( ) between sorbate molecules andcomponents of the mixed sorbent, experimental datawere plotted in the sorption enthalpy–polarizabilitycoordinates of a sorbate molecule. A linear depen�

dency of sorption enthalpy on the polarizability(α) of molecules was found for n�alkanes. The valuesof were calculated as the differences between

addHΔ

sp0iHΔ

addHΔ

sorption enthalpies for compounds not belonging tothe class of n�alkanes, and for a hypothetical n�alkanehaving the same value of polarizability (see Table 4).The values of for monoatomic alcohols anddiols, dioxane and tetrahydrofuran are high in both theN* and I phases of the sorbent and are 11–27 kJ/mol,testifying to the notable contribution from specific andorientational sorbate–sorbent interactions to theirretention. However, the interaction of molecules ofthese polar sorbates with acetyl groups of macrocyclein the case of distribution gas�liquid chromatography

addHΔ

Table 5. Excess partial molar enthalpies and entropies of sorbates in an infinitely dilute solution of the station�ary phase (calculated without regard to virial coefficients)

No. Sorbate, kJ/mol , J/(mol K)

N* (98–130°C) I (145–170°C) N* (98–130°C) I (145–170°C)

1 n�Hexane 16.0 (13.1) 12.4 (9.2) 22.3 (17.7) 15.5 (11.1)

2 n�Heptane 18.4 (12.1) 12.1 (9.8) 28.1 (14.8) 15.3 (12.3)

3 n�Octane 25.6 (14.3) 14.4 (11.2) 45.6 (20.2) 20.1 (15.1)

4 n�Nonane 26.9 (14.1) 17.3 (13.7) 48.1 (18.8) 26.1 (20.2)

5 n�Decane 27.8 (13.4) 15.3 (14.9) 49.4 (16.2) 20.9 (22.0)

6 Methanol 22.5 (10.3) 4.4 (1.2) 47.6 (17.7) 4.0 (–2.2)

7 Ethanol 19.1 (13.3) 9.8 (3.9) 38.3 (22.1) 15.6 (1.3)

8 Propanol�1 19.8 (16.5) 17.7 (7.6) 39.85 (30.6) 35.7 (11.6)

9 Butanol�1 22.3 (19.1) 15.7 (9.1) 45.8 (37.3) 29.6 (15.4)

10 Pentanol�1 23.5 (19.1) 16.7 (9.7) 48.4 (37.4) 32.7 (16.8)

11 Propanol�2 20.7 (15.8) 14.8 (9.7) 41.7 (27.9) 28.1 (15.6)

12 2�Methyl�propanol�1 21.2 (19.2) 16.2 (7.1) 42.9 (37.3) 31.6 (10.5)

13 Tetrahydrofuran 12.0 (8.4) 6.9 (6.9) 26.9 (18.2) 15.1 (16.8)

14 Cyclohexane 21.0 (12.9) 11.7 (13.7) 39.4 (21.3) 18.0 (26.0)

15 1,4�Dioxane 13.2 (10.7) 8.7 (7.0) 29.1 (22.7) 19.1 (16.0)

16 Benzene 17.0 (10.4) 6.1 (11.2) 37.9 (22.2) 12.4 (26.0)

17 Toluene 18.4 (11.8) 8.5 (9.7) 40.0 (24.3) 16.7 (21.5)

18 Ethylbenzene 19.2 (12.6) 9.8 (11.0) 40.1 (24.6) 18.6 (23.2)

19 p�Xylene 16.5 (10.4) 8.0 (9.6) 33.9 (20.2) 14.4 (20.1)

20 m�Xylene 19.7 (12.2) 9.8 (8.4) 41.9 (24.0) 18.6 (17.7)

21 o�Xylene 19.8 (12.3) 9.1 (10.2) 42.3 (24.5) 17.5 (21.8)

22 cis�Decalin 27.2 12.3 55.7 20.7

23 trans�Decalin 26.1 12.5 51.9 20.1

24 (2S,3S)�(+)�Butanediol�2,3 22.5 23.9 40.6 44.6

25 (2R,3R)�(–)�Butanediol�2,3 22.9 23.3 41.7 43.1

26 (+)�Camphene 31.7 (20.6) 20.1 (15.3) 64.5 (38.5) 37.7 (28.6)

27 (–)�Camphene 33.0 (20.6) 19.3 (15.3) 67.8 (38.5) 35.9 (28.6)

HiE ∞,

SiE ∞,

HiE ∞,

SiE ∞,

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 86 No. 8 2012

SORPTION AND SELECTIVE PROPERTIES 1317

is hindered considerably, due to the pronouncedMEAB···Acetyl�β�CD solvation effects. The interac�tion between molecules of the polar sorbates and mol�ecules of the main sorbent component (MEAB) there�fore make the main contribution to value bothunder conditions of a helically organized structure ofthe mixed N* mesophase and in the isotropic I phase.Hydrocarbons with cyclic and bicyclic rigid molecularstructures interact with the binary sorbent morestrongly than the corresponding conformationallyflexible n�alkanes, as evidenced by the positive valuesof (see Table 4).

Our investigations of excess thermodynamic func�tions included determining the Raoult activity coeffi�

cients of sorbates (Table 3) and their excess (partial)

enthalpy and entropy in an infinitely dilutethree�component solution with N*�type ordering andunder isotropic conditions (Table 5). Positive devia�

tions from ideality, i.e., ( ) falling when moving fromN* to the I phase, were found for all of the sorbates.Comparison of enthalpic and entropic contributions

to the ( value showed that positive deviations fromideality were determined by the enthalpy factor, i.e.,

by positive partial enthalpies ( > 0) of sorbates inthe mixed MEAB–Acetyl�β�CD (see Table 5). Intro�ducing Acetyl�β�CD in the amount of 1.52 mol % into

MEAB raises the positive values of and ofsorbates, along with their Raoult activity coefficient

. This appears to be related to the interactionbetween sorbate molecules and components of themixed sorbent in the N* phase being hindered, due tothe LC ··· macrocycle solvation effects and the forma�tion of an ordered helically twisted structure.

CONCLUSIONS

Introducing the chiral macrocyclic Acetyl�β�CDadditive (10.25 wt %) into a MEAB achiral nematicliquid crystal thus induces the formation of a highly

ordered, helically twisted N* mesophase. The guest–host complexing of sorbate molecules is hampered dueto LC ··· macrocycle solvation effects, and the enanti�oselectivity of the investigated binary MEAB–Acetyl�β�CD sorbent therefore manifests itself only over thenarrow temperature range of 91–100°С, where theadsorption/absorption mechanism of sorption is real�ized. At the same time, high structural selectivity ofthe sorbent is observed over the wide temperaturerange of the N* mesophase. Based on our results, wemay assume that the LC ··· LC intermolecular associ�ation characteristic of supramolecular liquid crystalscan impart more pronounced enantioselectivity over awide temperature range to a mixed mesophase con�taining the macrocyclic additive.

ACKNOWLEDGMENTS

This work was supported by federal target programScientists and Science Teachers of an Innovative Rus�sia for 2009–2013, project no. 02.740.11.0650.

REFERENCES

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3. R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, TheProperties of Gases and Liquids (McGraw�Hill, NewYork, 1977).

4. Korea Thermophysical Prperties Data Bank,http://www.cheric.org/kdb

5. NIST Chemistry WebBook, http://webbook.nist./gov/6. L. A. Onuchak, S. Yu. Kudryashov, and E. A. Davankov,

Russ. J. Phys. Chem. A 77, 1508 (2003).7. M. S. Vigdergauz, R. V. Vigalok, and G. V. Dmitrieva,

Russ. Chem. Rev. 50, 498 (1981).8. L. A. Onuchak, G. V. Surzhikova, and N. E. Maslova,

Zh. Fiz. Khim. 68, 127 (1994).9. E. P. Sokolova, A. Yu. Vlasov, and P. P. Kozak, Russ.

Chem. Bull. 45, 521 (1996).

addHΔ

addHΔ

i∞

γ

,EiH ∞ ,E

iS ∞

i∞

γ

i∞

γ

,EiH ∞

,EiH ∞ ,E

iS ∞

i∞

γ