Effect of the concentration of organic modifier in an aqueous-ethanol mobile phase on the chromatographic retention and thermodynamic characteristics of the adsorption of enantiomers of α-phenylcarboxylic acids on silica gel with immobilized eremomycin antibiotic

Article abstract of DOI:10.1134/S0036024414100057

Regularities of the chromatographic retention and thermodynamics of the adsorption of enantiomers of α-phenylcarboxylic acids on a chiral stationary phase with immobilized macrocyclic antibiotic eremomycin under conditions of reversed-phase liquid chromatography with aqueous-ethanol mobile phases are studied. Relationships between the retention characteristics of the acids, the enantioselectivity of their separation, and the concentration of organic modifier in the mobile phase are found. It is shown that the sterical structure of substituents on the chiral atoms of the acids affect the mechanism of retention. The compensation effect in the studied systems is considered.

Full text of DOI:10.1134/S0036024414100057

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 10, pp. 1778–1784. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © A.S. Blinov, E.N. Reshetova, 2014, published in Zhurnal Fizicheskoi Khimii, 2014, Vol. 88, No. 10, pp. 1591–1597.
PHYSICAL CHEMISTRY OF SEPARATION PROCESSES.
CHROMATOGRAPHY
Effect of the Concentration of Organic Modifier
in an Aqueous–Ethanol Mobile Phase on the Chromatographic
Retention and Thermodynamic Characteristics of the Adsorption
of Enantiomers of ꢀPhenylcarboxylic Acids on Silica Gel
α
with Immobilized Eremomycin Antibiotic
A. S. Blinov and E. N. Reshetova
Institute of Technical Chemistry, Russian Academy of Sciences, Ural Branch, Perm, 614990 Russia
eꢀmail: lenire@yandex.ru
Received November 8, 2013
Abstract—Regularities of the chromatographic retention and thermodynamics of the adsorption of enantioꢀ
mers of ꢀphenylcarboxylic acids on a chiral stationary phase with immobilized macrocyclic antibiotic ereꢀ
α
momycin under conditions of reversedꢀphase liquid chromatography with aqueous–ethanol mobile phases
are studied. Relationships between the retention characteristics of the acids, the enantioselectivity of their
separation, and the concentration of organic modifier in the mobile phase are found. It is shown that the sterꢀ
ical structure of substituents on the chiral atoms of the acids affect the mechanism of retention. The compenꢀ
sation effect in the studied systems is considered.
Keywords: chiral chromatography, enantiomers,
DOI: 10.1134/S0036024414100057
αꢀphenylcarboxylic acids, eremomycin.
INTRODUCTION
tography (HPLC). In [16], an attempt was made to
separate enantiomers of derivatives of mandelic acid
With the development of experimental and clinical
pharmacology, data have been accumulated on the difꢀ
ferent roles of the Rꢀ and Sꢀenantiomers of many raceꢀ
mic compounds used in practice with regard to the
occurrence of positive and negative effects. For examꢀ
ple, the antiꢀinflammatory activity of derivatives of
2ꢀarylpropionic acid (profens) is associated with the
by reversedꢀphase HPLC using
αꢀ
and ꢀcyclodexꢀ
β
trins as the chiral selectors. High selectivity was
achieved only in the separation of chlorineꢀsubstituted
derivatives of mandelic acid in eluent with added ꢀ
β
cyclodextrins at pH 2.1. The authors of [17] succeeded
in successfully separating enantiomers of mandelic
acid on CSP containing molecularly imprinted polyꢀ
mers. Considerable disadvantages of the above separaꢀ
tion conditions were the high values of the retention of
S
ꢀconfiguration of chiral centers, and
in some cases have side effects [1, 2]. (
R
ꢀenantiomers
R)ꢀmandelic
acid is the main intermediate compound for preparing
semiꢀsynthetic penicillins and cephalosporins; it is
used as a chiral synthon for the synthesis of antitumor
and antiobesity medications, along with other pharꢀ
maceutical substances [3–8]. Enantiomers of manꢀ
delic acid are used for the separation of racemic alcoꢀ
hols and amines [9–12]. Enantiomerically pure manꢀ
delic acid was prepared via diastereomeric
crystallization in [13], asymmetric synthesis in [3, 14],
and kinetic and enzymatic separation in [3, 14, 15].
However, such methods either fail to provide high proꢀ
ductivity and the required degree of purity for the
enantiomeric product, or are inefficient due to the
need to use of expensive metals in asymmetric syntheꢀ
sis [3].
enantiomers (the time required for Rꢀmandelic acid
elution from a chromatographic column was 35 min).
The separation of enantiomers of mandelic acid and
its derivatives was achieved on an anion exchange CSP
obtained via the immobilization of quinine tertꢀbutyl
carbamate and quinidine tertꢀbutyl carbamate on surꢀ
faces of silica gel [18]. The authors of [18] studied the
effect selector structure on enantioselectivity, and its
relationship to the thermodynamic characteristics of
adsorption was demonstrated.
High values of selectivity and the resolution factor
were obtained only on CSP based on quinine tertꢀbutyl
carbamate [18]. Enantiomers of mandelic acid and its
derivatives were separated for the first time with high
resolution factors on a new CSP with immobilized celꢀ
lulose; however, high selectivity and low retention facꢀ
Another method of preparing enantiomers is the
separation of racemic mixtures on chiral stationary
phases (CSP) via highꢀperformance liquid chromaꢀ tors were not attained [19].
1778

EFFECT OF THE CONCENTRATION OF ORGANIC MODIFIER
1779
Some of the most promising chiral selectors used in acid from SigmaꢀAldrich were used in our experiꢀ
liquid chromatography and capillary electrophoresis ments. The purity of the 2ꢀphenylbutanoic, ꢀhydroxyꢀ
(CE) for separation of the enantiomers of bioactive phenylacetic, and ꢀmethoxyphenylacetic acids was
α
α
compounds are macrocyclic glycopeptide antibiotics. 99%, and the purity of the 2ꢀphenylpropionic acid
Their molecules contain multiple functional groups was 97%.
and chiral centers that ensure successful separation of
the enantiomers of compounds of different classes,
particularly those containing carboxyl groups [20].
Mixtures obtained by diluting ammonium acetate
and acetic acid in an aqueous–ethanol solution with
different ethanol : water ratios (30 : 70, 40 : 60, 50 : 50.
The use of one such antibiotic (eremomycin) as a 60 : 40, and 70 : 30) were used as our mobile phases.
chiral selector was proposed by Russian scientists in The concentration of ammonium acetate for all eluꢀ
[21]. The effectiveness of using eremomycin was ents was 0.1 M, and the concentration of acetic acid
proved in the separation of enantiomers of profens was 1 vol %. Components of chemically pure grade,
using CE, but higher values of selectivity were bidistilled water, and rectified ethanol (96% high
observed during enantioseparation by HPLC [22]. A purity grade) were used in preparing the mobile
CSP of silica gel with immobilized eremomycin moleꢀ phases.
cules was successfully used in chromatographic sepaꢀ
ration of the enantiomers of amino acids [23] and proꢀ
fens [24, 25]. The authors [24, 25] studied the patterns
of chromatographic retention and the thermodynamꢀ
ics of adsorption of some derivatives of 2ꢀarylproꢀ
pionic acid. No information on separation of the
enantiomers of mandelic acid and its analogues on
CPS with immobilized antibiotic eremomycin was
found in the available literature.
The few works dealing with the enantioselective
adsorption of chiral compounds on the above adsorꢀ
bent indicates that it remains poorly understood, and
that further study of its properties and potentials is
needed.
Retention was assessed on the basis of the magniꢀ
tude of retention factor . Dead time was determined
using sodium nitrite. Thermodynamic characteristics
of adsorption (standard enthalpy , and enthropy
) were calculated using the linear relationship
between the magnitude of retention factor and
reciprocal temperature 1/
The of geometry of molecules was calculated using
the HyperChem 6.03 software package (Hypercube,
Inc.) via semiꢀempirical AM1 method.
k
'
ΔН°
ΔS°
k'
T
.
RESULTS AND DISCUSSION
General Patterns of Retention and Selectivity
In this work, we studied the effect of experimental
conditions on the chromatographic behavior and the
The successful enantioseparation of carboxylic
acids using eremomycin as an adsorbent is due to the
presence of carboxyl groups in their molecules, and
enantiorecognition increases if the separated comꢀ
pound contains carbonyl groups or aromatic ring in
thermodynamics of adsorption of
acids on CSP with immobilized antibiotic eremoꢀ
mycin. ꢀHydroxyphenylacetic (mandelic) acid (I),
ꢀmethoxyphenylacetic acid (II), 2ꢀphenylpropionic
αꢀphenylcarboxylic
α
α
close proximity to the steric center (i.e., in the
positions) [26]. An additional contribution to the
enantioselectivity of the separation of aromatic acids
comes from interactions with the aromatic rings
α,
β, or
acid (III), and 2ꢀphenylbutanoic acid (IV) were used
as the test adsorbates.
γ
π–π
of chiral selector [23]. All of the studied compounds
were of similar structure. The chiral carbon atom of
each molecule is bonded to a hydrogen atom, phenyl
and carboxyl groups, and a substituent (A) that is difꢀ
ferent for each compound: A = OH (1), OCH3 (II),
CH3 (III), and C2H5 (IV).
EXPERIMENTAL
Chromatographic analysis was performed via
reverseꢀphase HPLC on an Agilent 1100 Series liquid
chromatograph (Agilent Technologies, United States)
equipped with a precision pump, a diode array detecꢀ
tor, an autosampler, and a column thermostat ensuring
A
a stable temperature in the range of 0.2
ChemMack S&T chromatographic column (250
mm) was filled with silica gel that had eremomycin
molecules covalently bonded to its surface. The diamꢀ
eter of sorbent particles was 5 m. Our study was perꢀ
formed in the temperature range of 22–50
°
C. A Bioꢀ
×
COOH
4
H
μ
We may assume that retention and the selectivity of
. The eluꢀ optical isomer separation is determined by the nature
°С
ent flow rate was 0.8 mL/min. The concentration of of substituent in the side chain of the
the studied enantiomers was 0.5 mg/mL. The injecꢀ xylic acids.
αꢀphenylcarboꢀ
tion volume was 2 L. Detection was performed at a
μ
Comparison of chromatographic characteristics in
a series of related compounds (Fig. 1, Table 1) allowed
us to find certain patterns of retention. The elution
wavelength of 254 nm. The ChemStation Rev.
A.08.03Rus software package was used in the registraꢀ
tion of chromatrograms and processing of the results.
times of the
S
ꢀenantiomers of
α
ꢀphenylcarboxylic
Enantiomers of
α
ꢀphenylcarboxylic acids from acids that emerge first from the column were: III
≈
Alfa Aesar and ꢀenantiomer of 2ꢀphenylbutanoic IV < II < I. The stronger retention of I and II can be
R
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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1780
BLINOV, RESHETOVA
explained by the presence of polar hydroxy and methꢀ
oxy groups in their structure that are able to interact
with the active centers of chiral selectors.
k
3
'
I
II
III
IV
Analysis of the obtained data (Fig. 1) showed that
Rꢀenantiomers were in all cases retained more strongly
and their elution time grew with an increase in the
chain length (by CH2 group) of the substituents at the
chiral centers of homologous analytes: III < IV and I <
2
1
0
II.
(R)ꢀ ꢀmethoxyphenylacetic acid had the highest
α
'
'
retention factor. Parameter (
) reflecting the
kR − kS
difference in the retention of enantiomers due to
'
selective binding was 58% of the
value and
kR
depended only slightly on the conditions of each
experiment. It was apparently associated with addiꢀ
tional interactions between the
(R
)ꢀ ꢀmethoxypheꢀ
α
nylacetic acid and different polar groups of the chiral
selector, forming more stable adsorption complexes
30
40
50
60
70
[
Ethanol], vol %
with
The highest values of selectivity (Table 1) for 2ꢀ
phenylbutanoic and ꢀmethoxyphenylacetic acids
were apparently associated with features of their steriꢀ
cal structure. Calculations of the molecules’ geometry
in fact showed that these acids have branched strucꢀ
Rꢀenantiomers.
Fig. 1. Dependence of
'
retention factors (black symbols)
kS
α
and
'
(white symbols) of optical isomers of
α
ꢀphenylcarꢀ
boxylic acids on the concentration of ethanol in the mobile
phase at 22
kR
°
C.
tures in which each bulky substituent (C6H5–
,
COOH–, and A–) on a chiral carbon atom is located
in its own plane. We may assume that such configuraꢀ
tions favor stereoselective interactions between moleꢀ
cules of 2ꢀphenylbutanoic and ꢀmethoxyphenylaceꢀ
tic acids and the eremomycin molecules’ centers of
chiral recognition.
the carboxyl group is always located in planes different
from the aromatic rings. Such configurations are
apparently less favorable for stereoselective interacꢀ
tions. The authors of [27] explained the higher values
α
of selectivity for
to ꢀhydroxyphenylacetic acid by the hydrogen atom
α
ꢀmethoxyphenylacetic acid relative
α
Low values of selectivity are in turn characteristic
in hydroxyl group being replaced with bulkier methyl
radical. This could affect the hydrogen bonds between
the adsorbate and CSP, thereby determining chiral
recognition.
of 2ꢀphenylpropionic and
αꢀhydroxyphenylacetic
acids. In 2ꢀphenylpropionic acid, the angle of dihedral
between carboxylic group, chiral carbon atoms, and
methyl group is thus 176.8°, indicating that the above
substituents are located in virtually one plane with
chiral atoms.
Effect of the Concentration of Organic Modifier
A similar structure is found in
αꢀhydroxyphenylꢀ
In reverseꢀphase HPLC, the eluotropic strength of
acetic acid, where the hydroxyl and carboxyl groups a solvent increases with a decrease in the polarity of
are nearly coplanar with chiral atoms (the angle of mobile phase; this leads to faster desorption of comꢀ
dihedral between them is 156.3°). However, the pheꢀ pounds [28, 29]. The experimental data show that an
nyl group, chiral carbon atom, and carbon atom of increase in the content of organic modifier (ethanol)
carboxylic group are also coplanar, with an angle of in the mobile phase lowers the retention and resoluꢀ
dihedral between them of 176.9°. This distinguishes tion factors of all enantiomers of the studied comꢀ
αꢀhydroxyphenylacetic acid from other acids in which pounds (Fig. 1, Table 1). At higher concentrations of
Table 1. Coefficients of selectivity of separation
tions of ethanol in the mobile phase (30–70 vol %);
α
and resolution
C
R
S of
α
ꢀphenylcarboxylic acids at different concentraꢀ
60 vol % 70 vol %
T
= 22
°
30 vol %
40 vol %
50 vol %
RS
Acid
α
RS
α
RS
α
α
RS
α
RS
I
1.24
2.38
1.09
1.33
2.09
9.03
0.69
2.49
1.26
2.41
1.09
1.38
2.15
8.76
0.45
1.81
1.26
2.38
1.10
1.40
1.91
7.84
0.33
1.20
1.27
2.32
1.10
1.45
1.74
6.73
0.23
0.79
1.28
2.18
1.09
1.41
1.71
5.91
0.18
0.64
II
III
IV
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EFFECT OF THE CONCENTRATION OF ORGANIC MODIFIER
1781
selectivity with an increase in the ethanol concentraꢀ
tion in the mobile phase is observed, allowing us to
accelerate enantioseparation and reduce consumption
of the eluent without reducing the separation of peaks
or lowering of the selectivity. Similar patterns were
observed by the authors of [30] for several derivatives
of triptophanes on CSP with teicoplanin under condiꢀ
tions of reverseꢀphase HPLC.
−Δ
H°, kJ/mol
18
(a)
16
14
12
10
8
Effect of Ethanol Concentration on the Thermodynamic
Characteristics of Adsorption
The temperature dependences of retention factors
k
'
for both enantiomers are linear (correlation coeffiꢀ
cients, r2
0.967) in the coordinates of Van’t–Hoff
equation, which for is written as
6
≥
k'
30
, kJ/mol
40
50
60
70
ln(k' ) = – /(RT) +
/ϕ
ΔH°
ΔS°
/R,
(1)
[
Ethanol], vol %
−
T
ΔS°
16
14
12
10
8
where
Δ
H
°
and are the standard enthalpy and
Δ °
S
(b)
entropy of adsorption, is the universal gas constant,
R
T
is the mean harmonic temperature (here,
T
=
310 K), and
phases in the column.
ϕ
is the ratio of stationary and mobile
It can be seen from the dependences presented in
Fig. 2a that a reduction in the effect of heat upon an
increase in the concentration of ethanol in the mobile
phase is observed for the Sꢀ and
R ꢀisomers of all the
investigated compounds, indicating a weakening of
the bonding between the adsorbates and chiral selecꢀ
tor. In mobile phases with high concentrations of ethꢀ
anol, the sorption and desorption of analytes occur
rapidly and the structural changes in the complex
enantiomer–chiral selector are negligible. This is conꢀ
firmed by the low values of adsorption entropy and
agrees with the magnitudes of the retention factor
(Fig. 1). When eluents with low quantities of organic
modifier are used, the stability of the adsorption comꢀ
plex adsorbate–adsorbent is in turn higher, and it
becomes more difficult to brake the bond between its
components, leading to an increase in entropy
(Fig. 2b).
6
4
30
40
50
60
70
[
Ethanol], vol %
Fig. 2. Dependence of the change in the (a) enthalpy and
(b) entropy of adsorption of enantiomers of ꢀphenylcarꢀ
boxylic acids on the concentration of ethanol in the mobile
phase. For denotations, see Fig. 1.
α
ethanol in an eluent, the molecules likely to be
retained on the stationary phase apparently displace
molecules of adsorbates from the active centers of the
adsorbent more quickly. The character of relationships
is similar for homologue compounds: for Sꢀ and
The changes in thermodynamic characteristics
coincide in a number of related compounds. For
α
ꢀhydroxyphenylacetic and
αꢀmethoxyphenylacetic
acids, the dependences of enthalpy on the content of
R
ꢀenantiomers of 2ꢀphenylbutanoic and 2ꢀphenylꢀ ethanol reach their maxima at 40% ethanol in the
propionic acids, a sharper reduction in the retention
factor is observed with an increase of the ethanol
concentration than for
α
mobile phase, while a monotonous decline in enthalpy
upon an increase in the ethanol concentration in the
eluent is observed for 2ꢀphenylbutanoic and 2ꢀphenylꢀ
propionic acids (Fig. 2a). The dependences of the
entropy of adsorption reach their maxima at 50% ethꢀ
anol in the mobile phase for the enantiomers of 2ꢀpheꢀ
nylpropionic and 2ꢀphenylbutanoic acids and at 40%
α
ꢀhydroxyphenylacetic and
ꢀmethoxyphenylacetic acids. Based on the obtained
relationships, we may assume that the mechanism of
adsorption of homologous compounds is similar.
Our data also indicate that the concentration of
organic modifier in the mobile phase affects the selecꢀ
tivity of separation only slightly (Table 1). For 2ꢀpheꢀ
for the enantiomers of
α
ꢀhydroxyphenylacetic and
α
ꢀ
methoxyphenylacetic acids, indicating the formation
nylbutanoic acid, a tendency toward an increase in of a strongly bound adsorbate–adsorbent complex.
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BLINOV, RESHETOVA
Table 2. Thermodynamic characteristics of the adsorption of
ganic modifier in the mobile phase
α
ꢀphenylcarboxylic acids at different concentrations of orꢀ
Δ
G°, kJ/mol
[Ethanol],
vol %
ΔΔ
H
°,
TΔΔ
kJ/mol
S
°,
–
ΔΔ
G°,
kJ/mol
kJ/mol
S
R
I
30
40
50
60
70
–4.13
–2.98
–2.28
–1.78
–1.57
–4.62
–3.51
–2.80
–2.32
–2.12
–0.78
–1.16
–1.07
–1.50
–1.62
–0.29
–0.64
–0.55
–0.96
–1.07
0.50
0.52
0.52
0.54
0.55
II
30
40
50
60
70
–3.23
–2.09
–1.37
–0.94
–0.83
–5.14
–3.99
–3.24
–2.72
–2.47
–5.98
–6.82
–6.90
–7.26
–6.87
–4.07
–4.91
–5.03
–5.48
–5.22
1.91
1.90
1.86
1.78
1.65
III
IV
30
40
50
60
70
–2.91
–1.17
0.28
–3.15
–1.41
0.02
0.28
0.16
0.52
0.40
0.91
0.23
0.15
0.24
0.24
0.26
0.25
0.22
0.65
1.15
0.90
–0.02
–0.07
1.49
1.27
30
40
50
60
70
–3.23
–1.17
0.64
–3.92
–1.90
–0.16
0.94
–1.05
–1.70
–1.64
–2.56
–2.45
–0.36
–0.96
–0.84
–1.75
–1.71
0.68
0.73
0.80
0.81
0.74
1.75
2.20
1.46
Effect of the Thermodynamics of Adsorption
on Retention and Selectivity
The selectivity of separation depends on the differꢀ
ence between the Gibbs energies of enantiomers,
The retention of adsorbates is associated with the
change in the standard Gibbs energy of adsorption,
G :
ΔΔ = –RTln ΔΔ ΔΔ °.
G
°
α
=
H
°
–
T
S
(3)
The ΔΔH°
and ΔΔ parameters diminish with a
T
S
°
Δ
°
rise in the content of organic modifier in the mobile
phase. The enthalpic term is in most cases negative
ΔG°
=
ΔH°
–
TΔS°.
(2)
The changes in the standard enthalpy and entropy
and exceeds the entropic contribution for
phenylacetic and ꢀmethoxyphenylacetic acids.
αꢀhydroxyꢀ
have the same sign, so the enthalpic term is compenꢀ
sated for by the entropic term. The contribution from
entropy to the Gibbs energy is substantial at more than
α
The adsorption of their enantiomers is thus an
enthalpyꢀcontrolled process. Lowering the temperature
improves enthalpyꢀcontrolled separation, while the
opposite is true for entropyꢀcontrolled processes [31].
half the
Δ
H value at the mean experimental temperaꢀ
°
ture. This influence is maximal and crucial for 2ꢀpheꢀ
nylpropionic and 2ꢀphenylbutanoic acids; it is miniꢀ
Our experimental data showed that parameter
from 2.41 to 1.89 for ꢀmethoxyphenylacetic acid
when the temperature was raised from 22 to 50
α
fell
mal for
αꢀmethoxyphenylacetic acid. The Gibbs
α
energy (Table 2) needed for the molecules of the invesꢀ
tigated adsorbates to transition from the mobile phase
on the surface of the adsorbent declines in the order
°С,
while the selectivity of separation of 2ꢀphenylpropiꢀ
onic acid remained unchanged within the experimenꢀ
tal limits of error. The entropy term governed the
enantioseparation of 2ꢀphenylpropionic acid, since
I > II > IV > III for the
ferent order is observed for
S
ꢀenantiomers of acids. A difꢀ
Rꢀenantiomers that is in
complete agreement with the order of elution of optiꢀ
cal isomers from chromatographic column (Fig. 1).
ΔΔH
° >
0 for this acid.
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EFFECT OF THE CONCENTRATION OF ORGANIC MODIFIER
1783
The graph of dependence
Δ
H
°
–
Δ
S
°
(Fig. 3) shows
−Δ
H°
, kJ/mol
that at different contents of organic modifier in the
mobile phase, there is a linear correspondence
between the thermodynamic characteristics of the
20
investigated compounds’
Sꢀenantiomers (Table 3),
except for mandelic acid, for which the character of
the given dependence differs considerably from that of
the other studied acids. When the concentration of
15
10
0
I
I
ethanol in the eluent was raised, the point of the Sꢀ
I
enantiomer of mandelic acid shifted away from linear
dependences. This allowed us to conclude that the
mechanism of adsorption of mandelic acid differs
I
1
2
3
4
5
I
from that of other
αꢀphenylcarboxylic acids, which
agrees with our assumptions made on the basis of anaꢀ
lyzing the geometry of molecules. In addition, a
reduction in the compensation dependences’ correlaꢀ
tion coefficients was observed upon an increase in the
ethanol content (Table 3). No compensation effect
15
20
25
30
35
40
−Δ
45
, J/(mol K)
50
S
°
was observed for the Rꢀ ꢀphenylcarboxylic acids, due
α
Fig. 3. Compensation dependencies for
ꢀphenylcarboxylic acids at ethanol content of ethanol in
mobile phase: , 30, , 40, , 50, , 60, and , 70%. Points
ꢀhydroxꢀ
Sꢀenantiomers of
to the special mechanism of the adsorption of
tiomers.
Rꢀenanꢀ
α
1
2
3
4
5
marked by symbol I belong to Sꢀenantiomers of
yphenylacetic acid.
α
CONCLUSIONS
The reality of the compensation effect for the
ꢀenantiomers of ꢀphenylcarboxylic acids confirms
the great difference between compensation temperaꢀ
ture (Table 3) and the mean harmonic temperature
Comparing the calculated values of ΔΔ
of ꢀphenylcarboxylic acids, it is clear that the selecꢀ
tivity of separation declines in the order II > IV > I >
III, as is confirmed by the experimental data (Table 1).
G (Table 2)
°
S
α
α
β
of the investigated temperature interval (310 K) [32].
ACKNOWLEDGMENTS
Enthalpy–Entropy Compensation
This work was supported by the Russian Foundaꢀ
tion for Basic Research, project nos. 13ꢀ03ꢀ00464ꢀA
and MKꢀ675.2013.3.
Enthalpy–entropy compensation is usually used in
chromatography to study mechanisms of the retention
of adsorbates from the point of view of thermodyꢀ
namic characteristics. The linear relationship (comꢀ
pensation effect) between the enthalpy and entropy
characteristics in a group of relative compounds indiꢀ
cates that their adsorption mechanisms are similar
[32–34]. The equation for the compensation effect
has the form
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