Russian Journal of General Chemistry, Vol. 73, No. 3, 2003, pp. 327 330. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 3, 2003,
pp. 353 356.
Original Russian Text Copyright
2003 by Aladko, Dyadin, Mikina.
Clathrate Hydrates
in the System Tetraisopentylammonium Iodide Water
L. S. Aladko, Yu. A. Dyadin, and T. V. Mikina
Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk, Russia
Received March 11, 2001
Abstract Two clathrate hydrates i-Pent4NI 36H2O and i-Pent4NI 32H2O were revealed in the system
(i-Pent)4NI H2O. The hydrates melt incongruently at 14.2 and 14.8 C, respectively. Along with the poly-
hydrates, tetraisopentylammonium dihydrate was found.
All earlier attempts [1, 2] to obtain clathrate
hydrates of ammonium iodides were unsuccessful.
Later it was shown that certain iodides, such as tetra-
isopentylammonium iodide (i-Pent4NI) [3, 4] and
tributylisopentylammonium iodide (i-Bu3PentNI) [3]
form by one polyhydrate at atmospheric pressure. The
hydrate of i-Bu3PentNI melts at 7.1 C and comprises
25 to 35 water molecules per one salt molecule.
According to [4], the hydrate in the i-Pent4NI H2O
system melts at 13.3 C, and the hydrate number, by
very rough estimates, is 0.18 to 75.6. However, ac-
cording to [3], one hydrate, i-Pent4NI (36 3)H2O is
formed at atmospheric pressure, and it melts incon-
gruently (with decomposition into salt and water) at
14.7 C. The density and unit cell parameters were
determined. At 26 MPa, one more hydrate was found,
with a lower hydrate number. The composition of the
hydrate was established preparatively, and the large
error in its determination was associated with the fact
that crystalline tetraisopentylammonium iodide
hydrate, unlike hydrates of other quaternary am-
monium salts, quite readily lose water. Extensive
studies on quaternary ammonium base water systems
showed that these systems, as a rule, contain several
compounds with very close compositions but different
structures (clathrates with the hydrate numbers 39, 38,
36, 35, 34, 32, etc. are known). Thus, the error limits
in above estimates for the i-Pent4NI H2O system
include several hydrate numbers, and to find out
whether this indeed relates to the determination error
or we deal here with a mixture of different hydrates,
the composition of the hydrate should be detemined
as exactly as possible. To this end, in the present work
we made use of the Schreinemakers method [5] which
allows accurate and reliable detemination of the com-
position and number of compounds formed in a
system.
system at 0 C (Fig. 1, see table). As a result, two
polyhydrates, with 1:36 and 1:32 salt:water ratios,
were revealed. The compositions were calculated by
the program [6] that takes account of the intersects of
the Schreinemakers rays with the axis of the
i-Pent4NI H2O binary system and equals 1:(35.94
0.14) (number of Schreinemakers rays n = 15) and
1:(32.02 0.17, n = 9). The solubility of both the
hydrates in water is rather low (0.07 1.5 wt%);
moreover, both the compounds, rather than forming
two separate fields, are present throughout the entire
crystallization range of clathrate hydrates, and this
makes their stability (metastability) ranges difficult to
separate. Most likely, the 1:36 hydrate is stable until
some MeCO2H concentration and then it crystallizes
as a metastable phase, whereas the 1:32 hydrate gets
more stable. We performed a special experiment,
where the initially formed 1:32 hydrate was stirred in
the range where its metastable existence was proposed
(the solubility with respect to MeCO2H was 8.8 wt%)
for 5 days (shorter time was insufficient), after
which it passed into the 1:36 hydrate. This fact
implies that it is the latter hydrate which is stable in
the indicated concentration range. The same pattern
was observed at a 36% concentration of MeCO2H.
But in the latter case we first observed crystallization
of the 1:36 hydrate which then, after 5 days, too,
passed into the 1:32 hydrate. On addition of a seed
crystal of the corresponding hydrate, stable equilib-
rium is much faster attained (1 2 days). Then we
studied several mixtures with compositions inter-
mediate between 1:32 and 1:36 with gradually
approaching stability regions, and, finally, the eutonic
point between the 1:36 and 1:32 hydrates was ob-
tained by mixing roughly equal quantities of these
hydrates in a solution with a roughly specified
MeCO2H concentration. The eutonic point corres-
ponds to 0.30 wt% of i-Pent4NI and 23.9 wt% of
We studied the i-Pent4NI MeCO2H H2O ternary
1070-3632/03/7303-0327$25.00 2003 MAIK Nauka/Interperiodica