Self-Diffusion of Sodium, Chloride and Iodide Ions
in Acetonitrile-Water Mixtures
E. H aw licka
Institute of Applied Radiation Chemistry, Technical University, Lodz, Poland
Z. Naturforsch. 42a, 1014-1016 (1987); received June 22, 1987
The self-diffusion coefficient of sodium, chloride and iodide ions in acetonitrile-water mixtures at 25.0
±
0.05 °C has been measured in dependence on the salt m olarity in the range
1.0-10-4 1.0-10 m ol/dm 3. The ionic self-diffusion coefficients in infinitely diluted solutions have been
computed. The influence of the solvent composition on the solvation of the ions is discussed. Preferential
solvation of the ions by acetonitrile above 15m ol% of acetonitrile has been found. An effect o f the
sodium ions on the form ation of acetonitrile globules is postulated.
The relation between the structure o f a solvent
and the solvation o f solutes is a fundam ental q ues­
tion in solu tion chem istry. A acetonitrile-w ater
m ixtures as solvents seem to be suitable system s to
study that problem because they exhibit tw o struc­
tural discontinuities. From the viscosity and
density investigations M oreau and Douheret [1, 2]
have postulated the existence o f three fundam ental
structural regions: U p to 15m ol% o f acetonitrile
the water structure remains unchanged and the
acetonitrile m olecules occupy cavities in that struc­
ture. A b o v e that concentration the water structure
is disrupted and a segregation o f water and
acetonitrile m olecules occurs, the m ixture exhibit­
ing phase segregation at a critical com p osition o f
about 38 m ol% C H 3CN [3] and with an upper
critical tem perature o f about 272 K [ 3 - 5 ] . In the
acetonitrile rich solution (above 75 mol^o C H 3 C N )
the water m olecules accom m odate in the structure
o f pure acetonitrile. Studies o f the proton chem ical
shift [6 ] and self-d iffu sion [7, 8 ] have confirm ed
those structural regions.
The aim o f the present work was to determ ine
the self-d iffu sion coefficients o f sodium , chlorine
and iodine ions in acetonitrile-w ater mixture and
to study the ion-solvent interactions.
distilled over P 2 O 5 , and double distilled, degassed
water were used to prepare the solu tion s. 2 2 N a, 3 6 C1
and 125I were used as radioactive tracers. The
radioactivities o f the sam ples were measured in
toluene-ethan ol so lu tio n o f 2,5-d iph en yloxazole
(P P O ) with a liquid scintillation counter.
For the self-d iffu sio n m easurem ents the openend capillary m ethod was applied. The details o f
the experim ental procedure have been described in
[9].
R esults
The self-d iffu sio n experim ents were carried out
at 2 5 .0 ± 0 .0 1 °C . T he m easurem ents have covered
the w hole com p o sitio n range o f the solvent. The
m olarity o f the salts w as varied between 1 . 0 • 1 0
and 1.0 • 1 0 ' 2. A ll D -values have been calculated
from 9 independent experim ents. The statistical er­
ror o f the D -value is less than 1 %.
A s it was derived previously [10], the depen­
dence o f the ionic self-d iffu sion coefficient on the
salt m olarity m can be expressed in form:
(1)
Experim ental
N aC l (suprapur, Merck), N al (suprapur,
M erck), acetonitrile (spectroscopy grade, M erck),
Reprint requests to Dr. E. Hawlicka, Institute of Applied Radia­
tion Chemistry, Technical University, Wroblewskiego 15,
P-93-590 Lodz, Poland.
0932-0784 / 87 / 0900-1014 $ 01.30/0. -
w here T, £ 0 and D°° denote the tem perature, the
static dielectric constant o f the solvent and the
ionic self-d iffu sio n coefficient in the infinitely
diluted solu tion (lim iting self-d iffu sion co e ffi­
cient), respectively, and B , = D ° ° / D f , where D°°
and D f relate to the ion under study and the cou n ­
terion, respectively.
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1015
E. Hawlicka • Self-D iffusion o f Sodium , Chlorine and Iodine Ions
Table 1. The limiting self-diffusion coefficients o f sodium ,
D Ncf
chloride and iodide ions in acetonitrile-water mixtures. XAN is om2s-i
the mole fraction of acetonitrile in the solvent. The static dielec­
tric constants of the solvent were taken from [11].
-^AN
^N a+ ' 105
c m 's - 1
Nal
NaCl
0.000
0.103
0.147
0.256
0.408
0.508
0.579
0.733
0.805
0.912
1.000
1.333
1.18
1.15
1.09
1.10
1.26
1.40
1.67
1.79
1.91
2.05
• 105
cm2 s " 1
1.333
1.18
1.16
1.09
1.10
1.25
1.40
1.67
1.80
1.92
2.06
2.060
1.60
1.54
1.58
1.71
1.77
1.89
2.15
2.30
2.48
2.70
D ” • 105
2 -t
cm s
2.090
1.65
1.60
1.64
1.78
1.86
1.96
2.19
2.35
2.53
2.72
Fig. 1. The dependence of the limiting self-diffusion coefficients D Na + (a), DC\- (b) and D \- (c) on A"an at 2 5 °C.
T o calculate D°° and D°° from (1) a com puter
program was written to m inim ize sim ultaneously
the tw o mean square deviations
<7/ = l / l l Di (cal) - D , (exp)]'
(2 )
for / = cation and / = anion.
The com puted Z)-values are sum m arized in
Table 1. A s it was expected, the sam e lim iting self­
d iffu sion coefficients o f sod ium ions resulted from
the experim ents with N aC l and N a l. The influence
o f the solvent com p osition on the lim iting self­
d iffu sion coefficients o f the ions is presented in
Figure 1. As it can be seen, all fun ction s are sim ilar
and pass through a m inim um . The sm allest values
o f D q \ - and D
are observed at x AU = 0 .1 5 , and
that o f £>Na+ at x AN = 0.36.
A s com pared with G ill’s original equation, in
the first term o f (3), resulting from the viscous fric­
tion forces, perfect slipping is assum ed for the
solvated ion-solvent boundary con d itions [10, 13].
Based on the data given in Table 1 the ionic radii
have been calculated from (3). The results are
presented in Figure 2. For all studied ions the de­
pendence o f the r,-values on x AN is non-linear and
exhibits a positive deviation from linearity, which
m ight indicate a preferential solvation o f ions by
acetonitrile m olecules. This con clusion disagrees
with a weak preferential hydration o f halide ions
postulated from nmr studies [14] and with a prefer­
ential hydration o f sodium ions concluded from
upper critical tem perature changes [3], free energy
o f transfer between water and w ater-acetonitrile
D iscussion
In order to investigate an influence o f the
solvent com p osition on ion ic solvation one can
com pute ionic radii. A s it has been show n previ­
ously [10], G ill’s m odification [12] o f S to k es’ law
seem s to be m ore accurate then the sim ple Einstein-Stokes approach to calculate ion ic radii
because both viscous and dielectric friction forces
are taken into account. G ill’s equation, applied to
self-d iffu sion coefficients instead o f the ionic con ­
ductance, can be rewritten as
r, =
kT
■+ 0.0103 £q ,
4nnoD ?
where r]0 denotes the viscosity o f the solvent.
(3)
icrO-O—oJo-o-<
QO
0.5 x
AN
1.0
Fig. 2. Ionic radii calculated from (3). (The viscosity of the
acetonitrile-water mixture was taken from [1]). The vertical
lines mark the structural regions: I: water like structure, II:
microphase coexistence, III: acetonitrile like structure.
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1016
E. Hawlicka • Self-D iffusion o f Sodium , Chlorine and Iodine Ions
m ixture [15] and nmr studies [16]. Experim ental Bviscosity coefficien ts o f alkali metal halides in acetonitrile-w ater solu tions [17] suggest, however,
that preferential solvation by acetonitrile occurs.
A s it can be seen from Fig. 2, in the regions I and
III, i.e. below 1 5m ol% C H 3C N and above
75 m ol% C H 3 C N , the radii o f the ions are the
sam e as in the pure solvents. One can suppose that
the presence o f the cosolven t, acetonitrile and
water, respectively, does not disturb the form ation
o f the sam e solvation shell as in the pure solvent.
This confirm s also the previous conclusion [6 , 7]
that acetonitrile, w hile m ethanol [ 1 8 - 2 0 ] does,
does not change the water structure.
In the region o f m icrophase coexistence
(0.15 < x AN < 0.75) the interactions between the
solvent m olecules and sodium ions differ from
those with the anions. In this range a linear in­
crease o f the radii o f both anions is observed,
whereas the radius o f the sodium ions reaches its
upper values, the sam e as in pure acetonitrile,
already at x AN = 0.38. The difference can be ex­
plained as follow s: O ne can suppose that the
critical com p osition (38 m ol% C H 3 CN) divides the
m icroheterogenous region into two sections. Below
that point the structure is form ed by water globules
and acetonitrile m olecules a n d /o r globules are dis­
tributed between them , whereas above that point
the acetonitrile globules form the structure. A lso in
[6 , 7] it was postulated that the structure o f the
water glob ules is m ore ordered than that o f pure
water. If the disagreem ent betw een the spherical
structure o f the hydrated cation and the structure
o f the water globules is great, then sodium ions
cannot from hydration shells and are rem oved
from the w ater globules. The bare sodium ions
cannot exist and are “ fo rced ” to create a solvation
shell con taining acetonitrile m olecules. A t the
critical co m p o sitio n the concentration o f aceto­
nitrile is great enough to form the sam e solvation
shell as in pure acetonitrile. In such a case on e can
su ppose that the presence o f the sodium ions in­
duces the form ation o f the acetonitrile globules,
because the cation-w ater interactions are weaker
then w ater-w ater ones.
T his “ arranging e ffe c t” o f sodium ions might
explain the increase o f the upper critical tem pera­
ture observed in the presence o f sodium nitrate [5].
T he interactions o f the an ions with the solvent,
both w ater and acetonitrile, are weaker them those
o f sod ium ions. T herefore the anions do not
induce the form ation o f acetonitrile glob ules, as
can be seen from the linear dependence o f the re­
values on jcan in the m icroheterogenous region.
It w as foun d previously [21] that, if a preferen­
tial so lvation occurs, the self-d iffu sio n o f the com ­
p onent form ing the solvation shell is strongly a f­
fected by the presence o f electrolyte. In order to
con firm the concluded preferential solvation o f
ions by acetonitrile, studies o f the self-d iffu sion o f
both co m p on en ts o f the solvent in the presence o f
electrolytes are under way.
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