5
78
PORTNOVA et al.
The resulting compounds were purified via frac- All values obtained using Eqs. (2) and (3) correspond
tional distillation under vacuum. The reaction mass to the average experimental temperature for each com-
and obtained hydroxycarboxylic acid esters were ana- pound. They were reduced to the standard tempera-
lyzed via GLC using the Khromatek-Analytic software ture (298.2 K) using the equation
and a hardware complex based on a Kristall-2000M
T
gas chromatograph equipped with a flame ionization
n
ꢀ
ΔsorbH(T ) = ΔsorbH(T ) + (−Δ C )dT
detector and a 100 m × 0.2 mm × 0.5 μm capillary col-
umn with the grafted liquid stationary phase (LSP)
dimethylpolysiloxane or DB-1. The temperature of
the injector was 250°C, the temperature of the detec-
tor was 280°C, the carrier gas was helium, the split
ratio was 1/80, and the volume of each sample was
av
liq
p
Tav
(5)
n
liq
ꢀ
=
ΔsorbH(T ) + (−Δ C )Δ(T −T ),
av
p
av
n
liq
ꢀ
where Δ C is the change in the heat capacity of the
p
liquid‒gas transition. The correctness of using Eq. (5)
was substantiated in [8]. Because there are no experi-
mental values for the change in the heat capacities of
0
.2 μL. The following column temperature conditions
were used for our analysis: the initial temperature was
maintained for 20 min (100 or 150°C for the analysis of
n
ꢀ
lactic acid esters of C ‒C alcohols), and the thermo- the liquid‒gas transition, values Δ C were calcu-
3
5
liq
p
stat was then heated to 260°C at a rate of 5 K/min. The lated using the procedure proposed in [12].
purity of the obtained samples was 98‒99 wt %.
The sorption characteristics were determined on
RESULTS AND DISCUSSION
the same instrument in the isothermal mode, using
the procedures described in [7, 8]. The temperature of
the injector was 250°C, the temperature of the detec-
tor was 280°C, the carrier gas was helium, the split
ratio was 1/80, the sample volume was 0.2 μL, and the
temperature of the thermostat was 90‒260°C. When
preparing a sample in 1 mL of methanol, 1 μL of the
test compound, and 1 μL of n-alkanes with numbers
of carbon atoms z and z + 1 were dissolved. They were
selected such that the retention time of the test com-
pound was between those of the alkanes.
Retention Indices
Table 1 shows the experimental values of the reten-
tion indices and the coefficients of their temperature
dependences for all of the investigated compounds.
The error in measuring I was less than 1.2 units.
Unfortunately, we cannot compare the obtained val-
ues to the ones in the literature, since the available data
[
13, 14] were determined for the nonpolar phase
during temperature programming.
It should be noted that for lactic acid esters, the
increment of indices upon a 10°C increase in tempera-
ture did not exceed 1 unit. For malic acid, it was
The retention indices were calculated using the
Kovats formula [9]:
2
‒3 units; for tartaric acid, it was 3‒4 units. Each sub-
ln(t' ) − ln(t')
x
z
sequent hydroxyl or ester group thus gave an incre-
ment of ~1 u.m. for every 10°C. Our results agree with
the data on the change in the retention index values for
esters of dicarboxylic acids and mono- and dialkyl
Ix =
100 +100z,
(1)
ln(t' ) − ln(t')
z+1
z
where t' , t', and t' are the retention times of the test esters of glycerol studied in [7, 8].
x
z
z+1
compound and n-alkanes with the number of carbon
The temperature dependences of the retention
indices for all of the investigated compounds were lin-
ear, indicating there was no interaction between the
molecules of the substance in the sorbent [15]. The
coefficients of our equations are given in Table 1. They
allow us to calculate indices and identify the esters
considered in this work at other temperatures near the
investigated interval.
atoms z and z + 1, respectively.
The values of the change in internal energy Δsorb
U
(
kJ/mol) and enthalpy of sorption Δ H (kJ/mol) at
sorb
the average temperature of an experiment were deter-
mined using the dependences [8, 10, 11]
k
= C − Δsorb
U
We obtained the dependences of the retention indi-
ln
,
(2)
(3)
(
)
ces on the length of the linear alkyl substituent (n ) at
C
temperatures of 120°C for glycolic and lactic acids and
ΔsorbH = ΔsorbU − RT,
2
30°C for malic and tartaric acids (Fig. 1). For glycolic
acid esters,
where R is the universal gas constant (8.314 J/(mol K))
and k is the retention factor, calculated using the for-
mula
2
I120 = 100.3n + 556.1, R = 0.999,
(6)
(7)
C
for lactic acid esters,
I120 = 98.75n + 595.5, R = 0.999,
t − t
k = R
M
,
(4)
2
tM
C
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 93
No. 3
2019