THERMODYNAMIC CHARACTERISTICS OF THE SORPTION OF GLYCEROL ETHERS
1591
ple volume was 0.2
was 100–180 C.
µ
L. The thermostat temperature
RESULTS AND DISCUSSION
°
Experimental data on the retention indices and
their temperature dependences are listed in Table 1.
For all monoꢀ and diethers, changes in the retention
indices per 10 K are positive and increase as the
number of atoms in the substituent grows, but do
not exceed 1 i.u. For triethers, changes in the indiꢀ
ces as the temperature rises are negative and range
up to 2 i.u.
All measurements were performed in the isotherꢀ
mal mode, according to the procedures described in
[4, 5]. Samples of 1
ethers and 1 L of
methanol. Samples of 0.2
µ
L of the investigated mixtures of
ꢀalkanes were dissolved in 1 mL of
L were injected into the
µ
n
µ
evaporator using an autosampler.
Retention indices were calculated according to the
Kovats formula [6],
°
during
Hi
Our calculated values for
and Δsp
sorption (kJ/mol) are listed in Table 2, along with
coefficients in Eq. (3).
log(tx' ) − log(tz')
,
100 + 100z
(1)
Ix =
B
Consideration of the data shows that the changes
in the enthalpy of sorption depend linearly on the
number of carbon atoms in the alkyl substituent.
As the chain length grows by one СН2 group, the
enthalpy of sorption falls by 3.2 kJ/mol for monoetꢀ
hers (both with linear and isoalkyl substituents),
2.9 kJ/mol for diethers, and 2.5 kJ/mol for triꢀ
ethers. This indicates that even with a nonpolar colꢀ
umn, polar molecules exhibit weak interaction with
the SLP that declines in moving from glycerol
monoethers to diethers and to triethers. We specuꢀ
'
'
tz
'
where tx
of the ether and
+ 1, respectively.
Retention indices were determined from five to
,
,
and tz + 1 are the adjusted retention times
nꢀalkanes of carbon numbers and
z
z
seven measurements with confidence intervals of no
more than 1.0 index unit (i.u.).
Sorption equilibria in the gas phase–stationary liqꢀ
T
Vg
uid phase (SLP) system were characterized by
valꢀ
ues related to the constants of sorbate sorption from late that these interactions are due to hydroxyl
the gas phase by the SLP. The specific retention volꢀ groups. Note that for glycerol 2ꢀmonoꢀ and
T
1,2ꢀdiethers, the enthalpies of sorption are lower
ume of a substance,
formula [7]
(cm3/g), was calculated via the
Vg
than those for glycerol 1ꢀmonoꢀ and 1,3ꢀdiethers.
As the number of carbon atoms in the substituent
increases from 2 to 5, the difference increases from
0.4 to 1.3 kJ/mol, for monoethers, and decreases
from 1.3 to 0.5 kJ/mol for diethers. Calculations
(WinGamess [10], DFT, B3LYP/6ꢀ31G basis) show
that in the case of 2ꢀmonoethers, oxygen atoms in
the 1ꢀ and 3ꢀpositions have a higher negative charge
than oxygen atoms in the 2ꢀ and 3ꢀpositions in
1ꢀmonoethers, due to the displacement of electron
density. The difference increases as the chain length
of the substituent grows; this is the reason for the
lower enthalpy of sorption of glycerol 2ꢀmonoetꢀ
hers.
T
col 3( pi /pa)2 − 1
(tR − tM)F
VgT
=
pa,Ta
,
(2)
2( pi /pa)3 − 1
g
Ta
where tR is the retention time of the substance under
investigation, min; tM is the time for the emergence of
a nonsorbed substance from the column (dead time),
is the volumetric flow rate of the carrier gas
at pressure pa and temperature Тa, cm3/min;
mass of the SLP in the column, g; Тa is the temperaꢀ
ture of the carrier gas, 298.2 K; pi is the pressure of the
carrier gas at the column inlet, atm; and рa is the atmoꢀ
spheric pressure, atm.
min;
F
pa,Ta
g is the
With glycerol diethers, the difference between the
enthalpies of sorption for 1,2ꢀ and 1,3ꢀdiethers diminꢀ
ishes as the chain length of the substituent grows, due
to the screening of the remaining hydroxyl group.
Standard values for the changes in internal energy
°
(
) and enthalpy (Δsp , kJ/mol) during sorption
Hi
were determined via the relations [8, 9]
CONCLUSIONS
lnVgT = B −
,
(3)
(4)
A procedure for calculating enthalpies of vaporizaꢀ
tion from the temperature dependences of retention
times was described in [11]. In [12–14], it was shown
that retention indices can be used in calculating the
enthalpies of vaporization for compounds of the same
class no matter what their structure. We may conseꢀ
RT
,
where
is the change in internal energy during
sorption, kJ/mol;
(8.314 J/(mol K)).
R
is the universal gas constant quently assume that there is a relationship between
enthalpies of sorption and retention indices. To test
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 88
No. 9 2014
Zhabina
