Ultrasonic Velocity and Isentropic Compressibility Studies of Monoalkylammonium Salts in Binary Mixtures of Acetonitrile and N,N-Dimethylacetamide at Variable Temperature and Atmospheric Pressure

Article abstract of DOI:10.1007/s10953-020-00991-3

Ultrasonic velocities and densities of alkyl substituted ammonium perchlorates-RNH₃ClO₄, where R is methyl (Me), ethyl (Et)_,n-propyl (Pr)_,n-butyl (Bu)_,n-hexyl (Hx) and n-octyl (Oc), have been measured in the concentration range 0.03–0.28?mol·kg^?1 in binary mixtures of acetonitrile (AN) and N,N-dimethylacetamide (DMA) containing 0, 20, 40, 60, 80 and 100?mol% DMA at temperatures 298–328?K in 10?K intervals, using an Anton Paar density and sound velocity meter (DSA 5000?M). The experimental data of ultrasonic velocities and densities have been used for calculating isentropic compressibilities (K_S) and apparent molal isentropic compressibilities (K_S,?). Limiting apparent molal isentropic compressibilities (KS,?o) for all the electrolytes have been evaluated from K_S,? against m^1/2 plots and split into contributions of individual ion (KS,?,±o). The variation of KS,?,±o values with solvent composition shows preferential solvation of alkylammonium (RNH3+) ions by DMA in the AN rich region and by AN in the DMA rich region. The extent of solvation of the alkylammonium ions, at all studied temperatures, was found to increase with decreasing alkyl chain length; that is OcNH3+ < HxNH3+ < BuNH3+ < PrNH3+ < EtNH3+ < MeNH3+. KS,?,±o values for all RNH3+ ions increase with increasing temperature, indicating a decrease in the solvation of the ions with increasing temperature.

Full text of DOI:10.1007/s10953-020-00991-3

Journal of Solution Chemistry
https://doi.org/10.1007/s10953-020-00991-3
Ultrasonic Velocity and Isentropic Compressibility
Studies of Monoalkylammonium Salts in Binary Mixtures
of Acetonitrile and N,N‑Dimethylacetamide at Variable
Temperature and Atmospheric Pressure
Vivek Pathania1 · Shrutila Sharma1 · Shashi Kiran Vermani2 · B. K. Vermani1 ·
Navya Grover1
Received: 26 July 2019 / Accepted: 3 March 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Ultrasonic velocities and densities of alkyl substituted ammonium perchlorates-RNH3ClO4,
where R is methyl (Me), ethyl (Et), n-propyl (Pr), n-butyl (Bu), n-hexyl (Hx) and n-octyl
(Oc), have been measured in the concentration range 0.03–0.28 mol·kg−1 in binary mix-
tures of acetonitrile (AN) and N,N-dimethylacetamide (DMA) containing 0, 20, 40, 60,
80 and 100 mol% DMA at temperatures 298–328 K in 10 K intervals, using an Anton
Paar density and sound velocity meter (DSA 5000 M). The experimental data of ultrasonic
velocities and densities have been used for calculating isentropic compressibilities (KS)
and apparent molal isentropic compressibilities (KS,ꢀ). Limiting apparent molal isentropic
compressibilities (Ko ) for all the electrolytes have been evaluated from KS,ꢀ against m1/2
S,ꢀ
plots and split into contributions of individual ion (Ko ). The variation of Ko
values
S,ꢀ,
S,ꢀ,
with solvent composition shows preferential solvation of alkylammonium (RNH+3 ) ions by
DMA in the AN rich region and by AN in the DMA rich region. The extent of solvation of
the alkylammonium ions, at all studied temperatures, was found to increase with decreas-
ing alkyl chain length; that is OcNH+3 < HxNH3+<BuNH3+ < PrNH+3 <EtNH3+<MeNH+3 .
Ko values for all RNH+3 ions increase with increasing temperature, indicating a decrease
S,ꢀ,
in the solvation of the ions with increasing temperature.
Keywords Apparent molal volume · Isentropic compressibility · Preferential solvation ·
Acetonitrile · N,N-dimethylacetamide · Alkylammonium ion
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1095
3-020-00991-3) contains supplementary material, which is available to authorized users.
*
Shrutila Sharma
shrutilasharma@gmail.com
1
2
Department of Chemistry, D.A.V. College, Chandigarh 160011, India
Higher Education, Panchkula, Haryana 134109, India
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Journal of Solution Chemistry
1 Introduction
Compressibility measurements are usually used for predicting the pressure depend-
ence of properties of electrolyte solutions. Acoustical parameters have been found to
be a most powerful tool for understanding solvent structure, ion–ion association and
ion–solvent interactions in pure and binary mixtures of solvents. Data of physico-
chemical properties of various electrolytic solutions in binary mixtures of solvents are
necessary in many chemical processes, industrial applications and theoretical research
[1–7]. Such data have been found to be completely lacking for electrolyte solutions of
monoalkylammonium perchlorates (RNH3ClO4) in acetonitrile and N,N-dimethylacet-
amide solvent mixtures. A literature survey indicates that no work has been done on
these electrolyte solutions from the point of view of density and ultrasonic velocity
measurements. Alkylammonium perchlorates have been recently used as electrolytes
in developing electrical double layer capacitors [8]. Also, alkyl substituted ammonium
perchlorates are used as one of the main ingredients in rocket propellants and mis-
siles [9]. For the removal of waste explosives containing alkyl substituted ammonium
perchlorate from munitions, non-aqueous solvents or binary mixtures of solvents can
be used as solvating medium to recover the components (RNH3ClO4) from waste com-
posite propellants. Keeping this in view we have undertaken this research in which
compressibility studies of alkyl substituted ammonium perchlorates were carried out
in binary mixtures of acetonitrile and dimethylacetamide. These studies can be helpful
to enhance the extraction efciency of alkylammonium perchlorates from propellants
and explosives. The compressibility studies of electrolytes in acetonitrile and dimethy-
lacetamide, in general, is of interest because both solvents have high dipole moments,
high dielectric constants (µAN = 3.37 D, µDMA = 3.72 D, DAN = 36.0 and DDMA = 37.8).
Also, the Gutmann donor number of dimethylacetamide difers signifcantly from that
of acetonitrile (14.1 for AN and 27.8 for DMA). The large diference in the values
of Gutmann donor number of acetonitrile and dimethylacetamide plays an important
role in solvation of ions. These solvents are extensively used as extracting solvents,
because of their excellent solvating ability, which is why in relation of our work they
can also be utilized for extracting alkylammonium perchlorates from propellants and
explosives [10]; therefore it would be of great interest to extend electrolytic investi-
gation in their binary mixtures. In order to investigate the compressibility behavior
of monoalkylammonium perchlorates in binary mixtures of acetonitrile and dimeth-
ylacetamide, attempts have been made to measure ultrasonic velocities and densities
of monoalkylammonium perchlorates (CH3NH3ClO4, C2H5NH3ClO4, C3H7NH3ClO4,
C4H9NH3ClO4, C6H13NH3ClO4 and C8H17NH3ClO4) over the concentration range
0.03–0.28 mol·kg−1 at four diferent temperatures (298, 308, 318 and 328 K). Results
are interpreted in terms of limiting ionic apparent molal isentropic compressibility,
Ko . The sign and magnitude of Ko
will help us to understand the nature of vari-
S,ꢀ,
S,ꢀ,
ous types of molecular interactions prevailing in these solutions. To obtain individual
S,ꢀ,
ion contribution to solvation behavior Ko , ultrasonic velocities and densities of two
more salts, tetrabutylammonium perchlorate (C16H36NClO4) and tetrabutylammonium
tetraphenylborate (C40H56BNa) in binary mixtures of acetonitrile and dimethylaceta-
mide have also been measured. Results show how the alkyl chain length, temperature
and solvent composition afect the solvation behavior of these systems.
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Journal of Solution Chemistry
2 Experimental
2.1 Chemicals Used
Acetonitrile (99.8%, E.Merck), N,N-dimethylacetamide (99.8%, E.Merck) and acetone
(99.8%, E.Merck) were purifed further as reported earlier [11–14]. The chemicals,
silver perchlorate monohydrate (AgClO4·H2O, Alfa Aesar), sodium tetraphenylborate
(NaBPh4, 99.5%, CDH Chemicals) and tetrabutylammonium bromide (Bu4NBr, 99.5%,
TCI Chemicals) were used without any further purifcation.
2.1.1 Analysis of Water Content in Chemicals
The source, purity and water content of solvents used are described briefy in Table 1.
The density (ρ) and ultrasonic velocity (u) values used for analysis of the present
data for pure AN, pure DMA and their binary mixtures at temperatures 298 K, 308 K,
318 K and 328 K are reported in Table 2. AN and DMA were dried over 4 Å and 3 Å
molecular sieves for 72 h before use.
2.2 Preparation of Compounds
Bu4NClO4 and Bu4NBPh4 were prepared by methods reported earlier [20, 21]. The
purity of these compounds was confrmed by determining their melting points,
(Bu4NClO4, 211 °C and Bu4NBPh4, 225 °C) which agree well with the literature values
[22]. Alkylammonium perchlorates (RNH3ClO4) were prepared by reacting silver per-
chlorate monohydrate (AgClO4·H2O) with monoalkylammonium chloride (n-alkylamine
hydrochloride) (RNH3Cl where R = methyl, ethyl, propyl, butyl, hexyl and octyl) (TCI
Chemicals) dissolved in aqueous acetone mixtures in 1:1 molar ratio. Silver chloride
(AgCl) precipitated out, was fltered and the fltrate was concentrated by evaporation,
again fltered and heated untill the salt separated out. The salts so obtained was col-
lected, recrystallised twice from anhydrous acetone and dried at 60 °C under vacuum for
3 days. Characterization of the monoalkylammonium perchlorates was done by elemen-
tal analysis (C, H, N and O) using a Thermo Finnigan Flash EA 1112 Series CHNS–O
Analyzer. The perchlorate content in these compounds was calculated by gravimetric
analysis which relies on the insolubility of potassium perchlorate in absolute ethanol
[23] (Table 3).
Table 1 Provenance and mass
Chemical name
Source
Mass frac-
tion purity
Water content
fraction purity of solvents studied
Acetonitrile
N,N-Dimethylacetamide
Acetone
E.Merck
E.Merck
E.Merck
0.999a
0.999a
0.999a
0.00030b
0.00009b
0.00021b
aGas chromatography analysis
bKarl fscher titration method
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Table 3 Elemental analysis data of alkylammonium perchlorates (RNH3ClO4) and their perchlorate content
Salts
MeNH3ClO4 EtNH3ClO4 PrNH3ClO4 BuNH3ClO4 HxNH3ClO4 OcNH3ClO4
C
Found (%)
Theoretical (%)
9.372%
9.125%
16.022%
16.495%
21.347%
22.570%
26.477%
27.665%
35.101%
35.732%
42.006%
41.830%
H
Found (%)
Theoretical (%)
4.841%
4.599%
5.032%
5.542%
6.610%
6.319%
7.133%
6.971%
7.912%
8.003%
8.421%
8.784%
N
Found (%)
Theoretical (%)
11.111%
10.646%
9.412%
9.622%
8.449%
8.777%
7.816%
8.069%
6.116%
6.948%
6.691%
6.100%
O
Found (%)
47.606%
48.666%
42.101%
43.984%
39.608%
40.123%
37.608%
36.885%
31.009%
31.759%
28.002%
27.885%
Theoretical (%)
ClO−4
Found (%)
Theoretical (%)
75.53%
75.66%
68.19%
68.38%
62.09%
62.38%
56.94%
57.34%
50.47%
49.37%
43.01%
43.35%
2.3 Measurements
A vibrating U-tube Anton Paar digital Density and Sound velocity meter (Model DSA
5000M) with inbuilt Peltier–type thermostating unit for maintaining constant temperature
was used for measuring densities and ultrasonic velocities of the pure solvents and electro-
lyte solutions. The frequency at which ultrasonic velocities were measured was 3 MHz. The
uncertainty of the density and ultrasonic velocity was 5×10−3 kg·m−3 and 0.5 m·s−1,
respectively. The experiments were carried out at 298, 308, 318 and 328 K with an accu-
racy of 0.01 K. For the calibration of the instrument, triply distilled water and dry air at
atmospheric pressure (0.1 MPa) were used (as given in the apparatus manual). The reli-
abilities of density and sound velocity measurements were ascertained by comparing the
experimental data of pure solvents, AN and DMA with literature values (Table 2). Working
solutions were prepared by diluting stock solutions of appropriate concentrations. Stock
solutions were prepared by mass, weighed using an ACZET-602CY balance with pre-
cision of 0.1 mg. To avoid absorption of atmospheric moisture, the solutions were kept
in air tight stoppered glass bottles. The probable error in molality was found to be less
than 1×10−4 mol·kg−1.
3 Results and Discussion
3.1 Compressibility Studies
Ultrasonic velocities (u) and densities (ρ) of MeNH3ClO4, EtNH3ClO4, PrNH3ClO4,
BuNH3ClO4, HxNH3ClO4, OcNH3ClO4, Bu4NClO4 and Bu4NBPh4 were measured in the
concentration range 0.03–0.28 mol·kg−1 in AN + DMA solvent mixtures containing 0,
20, 40, 60, 80 and 100 mol% of DMA at 298, 308, 318 and 328 K. From plots of ultra-
sonic velocity (u) versus molal concentration (m) for MeNH3ClO4 at diferent temperatures
1 3

Journal of Solution Chemistry
(Fig. 1), it is observed that sound velocity increases linearly with increasing concentration
of electrolyte solution. This may be attributed to the greater association of the molecules
due to electrostatic bonding between solute and solvent molecules or to the presence of
efective solute–solvent interactions. Also, the sound velocity was found to decrease with
increasing temperature (Fig. 1). A perusal of Fig. 2 shows that, at constant temperature, the
density values increase with increasing concentration of salt in each of the binary systems,
at the experimental temperatures. As expected, the density values decrease with increas-
ing temperature (typical plots of variation of density with concentration are shown in
Fig. 2). Possible explanation for increase in the values of density with increase in concen-
tration would be the efect of replacing molecules of solvent containing atoms that are on
(b)1340
(a)
1300
1280
1260
1240
1220
1200
1180
1160
1140
AN:DMA
80:20
1320
1300
1280
1260
1240
1220
1200
Pure AN
0
0.05
0.1
m/mol kg-1
0.15
0.2
0.25
0
0.05
0.1
0.15
0.2
0.25
m/mol kg-1
(c) 1400
(d)1440
1380
1360
1340
1320
1300
1280
1260
1240
AN:DMA
60:40
1420
1400
1380
1360
1340
1320
1300
1280
AN:DMA
40:60
0
0.1
0.2
0.3
0
0.1 0.2
m/mol kg-1
0.3
m/mol kg-1
(e)
(f)
1500
1500
1480
1460
1440
1420
1400
1380
1360
1340
1320
AN:DMA
20:80
1480
1460
1440
1420
1400
1380
1360
1340
Pure
DMA
0
0.1
0.2
0.3
0
0.1
0.2
m/mol kg-1
0.3
0.4
m/mol kg-1
Fig. 1 Plots of ultrasonic velocity (u) against molal concentration (m) of MeNH3ClO4 in binary mixtures of
AN + DMA containing 0, 20, 40, 60, 80 and 100 mol% of DMA at 298, 308, 318 and 328 K. Filled rhom-
bus, 298 K; flled square, 308 K; flled triangle, 318 K;cross, 328 K
1 3

Journal of Solution Chemistry
(b) 0.845
0.84
(a)
0.8
0.795
0.79
0.785
0.78
0.775
0.77
0.765
0.76
0.835
0.83
0.825
0.82
0.815
0.81
0.805
0.8
0.755
0.75
0.745
0.795
0
0.05
m/mol kg-1
0.1
0.15
0.2
0.25
0
0.05
0.1
0.15
0.2
0.25
m/mol kg-1
(c)
0.885
0.88
0.875
0.87
(d)
0.91
0.905
0.9
0.895
0.89
0.865
0.86
0.855
0.85
0.845
0.84
0.885
0.88
0.875
0.87
0.835
0
0.05
0.1
0.15
0.2
0.25
0
0.1
0.2
0.3
m/mol kg-1
m/mol kg-1
(e) 0.95
(f) 0.955
0.95
0.94
0.93
0.92
0.91
0.9
0.945
0.94
0.935
0.93
0.925
0.92
0.915
0.91
0.89
0
0.1
0.2
0.3
0.4
0
0.05
0.1
m/mol kg-1
0.15
0.2
0.25
m/mol kg-1
Fig. 2 Plots of density (ρ) against molal concentration (m) of MeNH3ClO4 in binary mixtures of
AN + DMA containing 0, 20, 40, 60, 80 and 100 mol% of DMA at 298, 308, 318 and 328 K. Filled rhom-
bus, 298 K; flled square, 308 K; flled triangle, 318 K; cross, 328 K
average lighter than the atoms in the solutes by the solute molecules. Exactly same trends
of ultrasonic velocity (u) against molality (m) and density (ρ) against molality (m) plots are
obtained for EtNH3ClO4, PrNH3ClO4, BuNH3ClO4, HxNH3ClO4 and OcNH3ClO4.
The isentropic compressibility (KS) of electrolytes in pure AN, pure DMA and their
binary mixtures were calculated from the experimental values of ultrasonic velocities and
densities (ρ) using Eq. 1
1 3

Journal of Solution Chemistry
1
u2ꢀ
KS =
(1)
The KS values of pure AN, pure DMA and their binary mixtures at 298, 308, 318
and 328 K are reported in Table 1. The plots of KS against molal concentration (m) for
MeNH3ClO4 in pure solvents and in all binary mixtures of solvents at diferent tempera-
tures are presented in Fig. 3. From these plots, it can be seen that the KS values decrease
linearly with increasing salt concentration at all studied temperatures, indicating increasing
structural efects in the solutions due to the infuence of the solute on surrounding solvent
(b)
(a) 120
90
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
0
0.05
0.1
m/mol kg-1
0.15
0.2
0.25
0
0.1
m/mol kg-1
0.2
0.3
(c)
(d)80
80
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
0
0.1
0.2
0.3
0
0.1
0.2
m/mol kg-1
0.3
m/mol kg-1
70
(e)
(f)
70
60
50
40
30
20
10
0
60
50
40
30
20
10
0
0
0.1
0.2
0.3
0.4
0
0.05
0.1
0.15
0.2
0.25
m/mol kg-1
m/mol kg-1
Fig. 3 Plots of isentropic compressibility (KS) against molal concentration (m) of MeNH3ClO4 in binary
mixtures of AN + DMA containing 0, 20, 40, 60, 80 and 100 mol% of DMA at 298, 308, 318 and 328 K.
Filled rhombus, 298 K; flled square, 308 K; flled triangle, 318 K; cross, 328 K
1 3

Journal of Solution Chemistry
molecules or, perhaps, due to the greater intrinsic KS values of the solute compared to the
solvent [24]. Also, it is observed (Fig. 3) that KS increases with increasing temperature.
Exactly the same behavior of variation of KS with salt concentration was observed for the
other alkylammonium perchlorates (EtNH3ClO4, PrNH3ClO4, BuNH3ClO4, HxNH3ClO4
and OcNH3ClO4) at all studied temperatures.
Apparent molal volumes (Vɸ) and apparent molal isentropic compressibilities (KS,ꢀ) for
all the electrolytes have been calculated using Eqs. 2 and 3
(
)
103 ꢁ − ꢁo
M
(2)
Vꢀ =
−
ꢁ
mꢁꢁo
(
)
103 KS − KS,o
KS,ꢀ = VꢀKS +
(3)
mꢁo
where m is molality of the solution, M is the molecular weight of solute, KS, KS,o are
isentropic compressibilities of solution and that of pure solvent, ρ and ρo are the densities
of solution and pure solvent, respectively.
To evaluate the limiting apparent molal isentropic compressibilities (Ko ) of various
S,ꢀ
electrolytes, extrapolation of linear plots of KS,ꢀ against m1/2 was made and Ko values
S,ꢀ
were obtained by the least squares treatment using equation
√
KS,ꢀ = Ko + AS,ꢀ
m
(4)
S,ꢀ
The Ko values for all the salts studied at diferent temperatures are reported in Table 4.
S,ꢀ
These values for MeNH3ClO4, EtNH3ClO4, PrNH3ClO4, BuNH3ClO4, HxNH3ClO4,
OcNH3ClO4 are not available in AN + DMA solvent mixtures; therefore, a comparison
S,ꢀ,
of these values could not be made. There are two factors which contribute to Ko : (a)
the positive efect of larger solute molecules, which can have some intrinsic compressibil-
ity due to which intermolecular free space is created making the solution more compress-
ible and (b) the negative efect from the insertion of solvent molecules into the intra-solute
free space, which results in the interactions of solute molecules with neighboring solvent
molecules [24]. Lower Ko values in the AN rich region of AN + DMA binary mixtures
(Table 4) specially in theS,cꢀase of MeNH3ClO4, EtNH3ClO4, PrNH3ClO4 and BuNH3ClO4
may be attributed to the predominance of the negative efect over the positive efect due
to which solvent surrounding solute ofers greater resistance to compression than the bulk
solvent.
3.2 Evaluation of Limiting Ionic Apparent Molal Isentropic Compressibilities (Kso,ɸ
)
To gain better insight of the tendency of each ion to produce solvation or structural efects,
S,ꢀ,
the Ko values of various salts, including the reference electrolytes Bu4NClO4 and
S,ꢀ,
Bu4NBPh4, have been split into individual ion contribution, Ko . As reported earlier [22]
S,ꢀ
Ko values are additive, so they can be directly split into the contribution of individual ions
to obtain structural efects. Milero had used Bu4AsBPh4 as the reference electrolyte [25]
for splitting the partial molar volume of electrolytes into individual ion contribution. A
similar model in which the use of Bu4NBPh4 as reference electrolytes to evaluate limiting
ionic apparent molal isentropic compressibility was suggested by Gill and coworkers [26].
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Journal of Solution Chemistry
This model has been used in the present work, which uses following equations to split Ko
S,ꢀ
values into its ionic contributions.
(
)
(
)
Ko Bu4N+
rc3 Bu4N+
5.003
5.353
S,ꢀ
=
=
(
)
(
)
(5)
(6)
Ko Ph4B−
rc3 Ph4B−
S,ꢀ
(
)
(
)
(
)
Ko Bu4NBPh4 = Ko Bu4N+ + Ko Ph4B−
S,ꢀ
S,ꢀ
S,ꢀ
where rc represents the crystallographic radius of the ion. Using Eqs. 4 and 5, frstly,
Ko values of Bu4NBPh4 were split into the ionic contribution for Bu4N+ and Ph4B− ions
S,ꢀ
and then Ko values of Bu4N+ and Ph4B− were further used to calculate Ko values for
S,ꢀ,
RNH+3 ionsS(,ꢀT,able 5) on the basis of the additive principle. A perusal of Table 5 shows that
Bu4N+ and Ph4B− have positive values of Ko throughout the whole composition range.
This may be attributed to some special type So,ꢀf,ion–solvent interactions, which may be due
to solvophobic, dispersive or solvent–solvent interactions. Also, these special interactions
of Bu4N+ and Ph4B− with AN and DMA were reported by Gill and Rana [18] and for
Ph4B− with AN were reported by Bose and Kundu from conductance measurements [27].
ClO−4 has negative Ko
values throughout the whole composition range indicating that
S,ꢀ,
the ClO−4 ion shows better solvation in AN + DMA mixtures. It can be seen from Table 5
S,ꢀ,
that, there is an increase in Ko
values for RNH+3 ions with increase in temperature and
S,ꢀ,
with increases in the alkyl chain length. Negative or lower Ko values may be attributed
to the smaller size of the ion or due to the predominance of the penetration of solvent mol-
ecules into the intra ionic free space of the solute, over the efect of their solvent intrinsic
compressibility or due to electrostriction as a result of which, the solvent molecules are
directed by the electrostatic feld of the solute molecules in the primary solvation shell and
solvent molecules become more compressed leading to decrease in Ko
values. Also, at
S,ꢀ,
temperatures 308, 318 and 328 K, the Ko
values for all the RNH+3 ions are large and
S,ꢀ,
positive throughout the whole composition range except at 40 mol% DMA in AN + DMA
binary mixtures, showing weak solvation of these ions at higher temperatures (308, 318
and 328 K).
The variation of Ko
for RNH+3 ions with change in solvent composition shows
S,ꢀ,
+
interesting behavior. A perusal of Table 5 shows that for RNH3 ions the Ko
val-
S,ꢀ,
ues decrease with increasing DMA composition up to 40 mol% DMA and beyond this
solvent composition, Ko
values start increasing again, passing through a minimum
S,ꢀ,
(Fig. 4) showing preferential solvation of RNH+3 ions by DMA in the AN rich region
and by AN in the DMA rich region of AN + DMA solvent mixtures. The non-linear
behavior of Ko
with solvent composition has been attributed to the existence of sol-
S,ꢀ,
ute–solvent and solvent–solvent interactions, which are maximized at about 40 mol%
of DMA in AN + DMA mixtures. This can be hypothesized to be due to the fact
that for 20 mol% of DMA in AN + DMA binary mixtures, the frst solvation shell of
RNH+ ions, which is formed when AN molecules in pure AN are partially replaced
by la3rger DMA molecules with higher donicities (defnite solvent coordination occurs
with solvents having large donicities); with increasing mol% composition of DMA the
frst solvation shell is signifcantly enriched with DMA and up to 40 mol% DMA, the
frst solvation shell is formed exclusively by DMA molecules and at this composition
maximum interactions between AN and DMA solvent molecules take place. Above
this composition the solvation shell further incorporates AN molecules leading to a
decrease in ion–solvent and solvent–solvent interactions and increasing the values of
1 3

Journal of Solution Chemistry
Table 5 Limiting ionic apparent molal isentropic compressibility 104 ×Ko /cm3·mol−1·bar−1 of some ions
S,ꢀ,
in AN + DMA solvent mixtures at diferent temperatures
mol % DMA Ph4B− Bu4N+ ClO4−
MeNH+3 EtNH3+ PrNH3+ BuNH+3 HxNH+3 OcNH3+
298 K
0
58.6
47.8
−25.4
39.8
32
52.4
43
72.4
38.6
−1.6
81.4
42.6
3.4
126.2
50.6
35.7
129.8
55.8
39
59.1a
48.3a
−25.5a
20
40
77.6
63.4
−27
63.7a
88.7
72.3
−26.4
−29.1
−18.1
73.4a
60
80
100
101.3
122.2
135
82.7
99.8
110.3
−25.1
−24.1
−20.1
31.7
35.1
40.1
62.7
71.1
80.3
63.2
78.1
85
85.3
90.1
99.7
89.1
95.1
129.1
98.6
113.6
132.3
135.1a 110.2a
308 K
0
20
40
60
70.3
100
111.9
124
144.9
163.7
57.3
81.6
91.4
101.3
118.3
133.6
−24.2
−26.4
−25.6
−24
−22.5
−19.7
41.1
34.4
−6.4
52.5
70.5
76.7
4.7
47.4
5.1
63.2
81.5
82.7
76.2
51.4
6.6
72.5
91
96.2
53.4
14.6
97.7
98.5
101.7
137.2
59.4
55.7
119.1
131.4
149.6
169.1
65
59.6
139.4
146.5
171.6
80
100
318 K
0
20
40
60
80
100
328 K
0
20
40
60
80
92.3
81.4
66.4
99.1
110
118.8
136.3
151.9
−22.4
−25.8
−25.1
−23.2
−22.1
−19.1
43.7
36.8
7.1
60.8
79.1
83.1
56.4
50.8
14.6
69
86.1
90.1
79.4
55.8
17.1
81.2
93.7
94.5
108.4
57.8
22.1
101.2
107.1
109.1
148.4
61.8
57.1
122.2
137.6
152.4
199.4
73.8
64.1
143.8
169.4
204.1
121.5
134.9
145.5
167.1
186.1
93.2
76
−21.7
−24.7
−24.3
−23
−21.6
−18.7
45.5
40.7
16.3
69.2
86.6
91.7
57.7
51.7
20.6
73.8
90.6
93.7
94.7
59.7
23.3
86
101.1
103.2
113.1
62.7
28.3
112.7
114.6
116.3
173.7
65.7
61.3
133.9
156.8
179.7
233.7
78.2
65.3
154.4
198.9
236.7
144.4
157.1
166.9
190.7
210.9
117.9
128.2
136.3
155.6
172.1
100
Maximum uncertainity in these values is 2.7×10−4 cm3·mol−1·bar−1
aRef. [27]
Ko . These results are similar to those found by Gill and coworkers in AN + DMA
S,ꢀ,
mixtures using conductance measurements [28] and by Gill and Cheema in AN + DMF
mixtures using conductance and viscosity measurements [29]. So in accordance with
our Ko
data collected for RNH+3 ions, notable changes in RNH3+ solvation have been
produSc,ꢀe,d at 40 mol% DMA in AN and DMA solvent mixtures.
1 3

Journal of Solution Chemistry
(b)100
(a) 100
80
60
40
20
0
80
60
40
20
0
+
EtNH3
+
MeNH3
-20
-40
-20
-40
mol% DMA
0
20
40
mol% DMA
60
80
100
0
20
40
60
80
100
(c)
(d) 140
120
100
80
120
100
80
60
40
20
0
60
+
PrNH3
40
+
BuNH3
20
0
-20
0
50
mol% DMA
100
0
50
mol% DMA
100
(f)
(e) 200
250
180
160
140
120
100
80
200
150
100
50
+
60
OcNH3
+
HxNH3
40
20
0
0
0
20
40
60
80
100
0
20
40
mol% DMA
60
80
100
mol% DMA
Fig. 4 Limiting ionic apparent molal isentropic compressibility Ko
versus mol% of DMA for alkylam-
S,ꢀ,
monium ions (RNH+) in AN + DMA mixtures at 298, 308, 318 and 328 K. Filled rhombus, 298 K; flled
square, 308 K; flled3triangle, 318 K; cross, 328 K
4 Conclusion
+
Monoalkylammonium ions (RNH3 ) do not appear to interact strongly with pure DMA
and AN. Negative Ko
values at 298 K for MeNH+3 , EtNH3+, PrNH+3 ions and at 308 K
for the MeNH+3 ion Sa,ꢀt, 40 mol% DMA in AN + DMA binary mixtures indicate struc-
tural efects arising due to solute–solvent and solvent–solvent interactions. The extent of
ion–solvent interactions for RNH+3 ions at all studied temperatures decreases in the order;
1 3

Journal of Solution Chemistry
MeNH3+>EtNH+3 >PrNH3+>BuNH+3 >HxNH3+>OcNH+3 . The non-linear variation of Ko
S,ꢀ,
values of RNH+3 ions with solvent composition shows preferential solvation of these ions
by DMA in the AN rich region and by AN in the DMA rich region. Positive Ko
val-
S,ꢀ,
ues of Bu4N+ and Ph4B− show some special type of solvophobic interactions in DMA and
AN + DMA binary mixtures. Large positive values of HxNH+3 and OcNH+ throughout the
whole composition range except at 40 mol% DMA shows very weak solvat3ion of these ions
in AN + DMA mixtures, which may be due to their large size and small charge density.
The ClO−4 ion, however, shows better solvation throughout the whole composition range
and the extent of solvation is greater in the AN rich region of the AN + DMA mixtures.
Also, the increase in Ko values of RNH3+ ions with increasing temperature indicates that
S,ꢀ,
the extent of solvation of these ions decreases with increasing temperature.
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