Temperature and pH driven association in uranyl aqueous solutions
An association behavior of uranyl ions in aqueous solutions is explored. For this purpose a set of all-atom molecular dynamics simulations is performed. During the simulation, the fractions of uranyl ions involved in dimer and trimer formations were monitored. To accompany the fraction statistics on...
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Інститут фізики конденсованих систем НАН України
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irk-123456789-1203062017-06-12T03:04:53Z Temperature and pH driven association in uranyl aqueous solutions Druchok, M. Holovk, M. An association behavior of uranyl ions in aqueous solutions is explored. For this purpose a set of all-atom molecular dynamics simulations is performed. During the simulation, the fractions of uranyl ions involved in dimer and trimer formations were monitored. To accompany the fraction statistics one also collected distributions characterizing average times of the dimer and trimer associates. Two factors effecting the uranyl association were considered: temperature and pH. As one can expect, an increase of the temperature decreases an uranyl capability of forming the associates, thus lowering bound fractions/times and vice versa. The effect of pH was modeled by adding H⁺ or OH⁻ ions to a "neutral" solution. The addition of hydroxide ions OH⁻ favors the formation of the associates, thus increasing bound times and fractions. The extra H⁺ ions in a solution produce an opposite effect, thus lowering the uranyl association capability. We also made a structural analysis for all the observed associates to reveal the mutual orientation of the uranyl ions. Проведено дослiдження процесiв асоцiативностi у водних розчинах уранiлу. Для цього проведено низку моделювань методом молекулярної динамiки. Пiд час моделювання проведено монiторинг фракцiй iонiв уранiлу, що беруть участь у формуваннi димерiв та тримерiв. Також зiбрано розподiли iмовiрностi за часами життя уранiлових димерiв та тримерiв. Розглянуто вплив двох факторiв – температури та рiвня рН середовища – на здатнiсть iонiв уранiлу до формування асоцiатiв. Виявлено, що збiльшення температури знижує асоцiативнiсть, при цьому також скорочуються часи життя асоцiатiв i зменшуються фракцiї iонiв уранiлу у асоцiатах. Вплив рiвня рН середовища змодельовано додаванням iонiв H⁺ чи OH⁻ до “нейтрального” розчину. Виявлено, що наявнiсть у розчинi iонiв OH⁻ є сприятливим чинником для асоцiативностi, в той час як наявнiсть iонiв H⁺ призводить до протилежного ефекту. Для усiх розчинiв проведено конфiгурацiйний аналiз взаємної орiєнтацiї iонiв уранiлу, що перебувають у асоцiативному станi. 2012 Article Temperature and pH driven association in uranyl aqueous solutions / M. Druchok, M. Holovko // Condensed Matter Physics. — 2012. — Т. 15, № 4. — С. 43602:1-9. — Бібліогр.: 21 назв. — англ. PACS: 61.20.Ja, 61.20.Qg, 82.20.Wt, 82.30.Hk, 82.30.Nr DOI:10.5488/CMP.15.43602 arXiv:1212.6357 http://dspace.nbuv.gov.ua/handle/123456789/120306 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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An association behavior of uranyl ions in aqueous solutions is explored. For this purpose a set of all-atom molecular dynamics simulations is performed. During the simulation, the fractions of uranyl ions involved in dimer and trimer formations were monitored. To accompany the fraction statistics one also collected distributions characterizing average times of the dimer and trimer associates. Two factors effecting the uranyl association were considered: temperature and pH. As one can expect, an increase of the temperature decreases an uranyl capability of forming the associates, thus lowering bound fractions/times and vice versa. The effect of pH was modeled by adding H⁺ or OH⁻ ions to a "neutral" solution. The addition of hydroxide ions OH⁻ favors the formation of the associates, thus increasing bound times and fractions. The extra H⁺ ions in a solution produce an opposite effect, thus lowering the uranyl association capability. We also made a structural analysis for all the observed associates to reveal the mutual orientation of the uranyl ions. |
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Article |
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Druchok, M. Holovk, M. |
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Druchok, M. Holovk, M. Temperature and pH driven association in uranyl aqueous solutions Condensed Matter Physics |
| author_facet |
Druchok, M. Holovk, M. |
| author_sort |
Druchok, M. |
| title |
Temperature and pH driven association in uranyl aqueous solutions |
| title_short |
Temperature and pH driven association in uranyl aqueous solutions |
| title_full |
Temperature and pH driven association in uranyl aqueous solutions |
| title_fullStr |
Temperature and pH driven association in uranyl aqueous solutions |
| title_full_unstemmed |
Temperature and pH driven association in uranyl aqueous solutions |
| title_sort |
temperature and ph driven association in uranyl aqueous solutions |
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Інститут фізики конденсованих систем НАН України |
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2012 |
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http://dspace.nbuv.gov.ua/handle/123456789/120306 |
| citation_txt |
Temperature and pH driven association in uranyl aqueous solutions / M. Druchok, M. Holovko // Condensed Matter Physics. — 2012. — Т. 15, № 4. — С. 43602:1-9. — Бібліогр.: 21 назв. — англ. |
| series |
Condensed Matter Physics |
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AT druchokm temperatureandphdrivenassociationinuranylaqueoussolutions AT holovkm temperatureandphdrivenassociationinuranylaqueoussolutions |
| first_indexed |
2025-07-08T17:37:51Z |
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2025-07-08T17:37:51Z |
| _version_ |
1837101242777600000 |
| fulltext |
Condensed Matter Physics, 2012, Vol. 15, No 4, 43602: 1–9
DOI: 10.5488/CMP.15.43602
http://www.icmp.lviv.ua/journal
Temperature and pH driven association in uranyl
aqueous solutions
M. Druchok, M. Holovko
Institute for Condensed Matter Physics, NAS of Ukraine, 1 Svientsitskii St., 79011 Lviv, Ukraine
Received July 3, 2012, in final form September 19, 2012
An association behavior of uranyl ions in aqueous solutions is explored. For this purpose a set of all-atom molec-
ular dynamics simulations is performed. During the simulation, the fractions of uranyl ions involved in dimer
and trimer formations were monitored. To accompany the fraction statistics one also collected distributions
characterizing average times of the dimer and trimer associates. Two factors effecting the uranyl association
were considered: temperature and pH. As one can expect, an increase of the temperature decreases an uranyl
capability of forming the associates, thus lowering bound fractions/times and vice versa. The effect of pH was
modeled by adding H+ or OH− ions to a “neutral” solution. The addition of hydroxide ions OH− favors the for-
mation of the associates, thus increasing bound times and fractions. The extra H+ ions in a solution produce
an opposite effect, thus lowering the uranyl association capability. We also made a structural analysis for all the
observed associates to reveal the mutual orientation of the uranyl ions.
Key words: molecular dynamics, uranyl aqueous solution, association, pH, temperature
PACS: 61.20.Ja, 61.20.Qg, 82.20.Wt, 82.30.Hk, 82.30.Nr
1. Introduction
For the last decades, the unresolved problem of nuclear fuel wastes containing actinides and other ra-
dionuclides contacting with water urged intensive theoretical and experimental studies. Comprehension
of the association processes in such systems can provide onewith important data for chemical technology,
medicine, environmental ecology [1]. The actinides An in water can easily form dioxides called actynils
(AnO2)
Z+. Both An=O bond lengths are usually 1.7–1.8 Å and O=An=O angle is close to 180◦. Actynil
is usually hydrated by five water molecules, the so-called ligands, located in an equatorial plane (nor-
mal to O=An=O axis) at the distances of 2.5–2.6 Å. Such a finding was confirmed by quantum-chemical
investigations. In particular, in [2, 3] it is shown that the number of ligands of uranyl, neptunyl and plu-
tonyl is equal to five. It was also found that a hydrolysis reaction with one proton loss from one of five
uranyl ligands is energetically favorable. However, the quantum-chemical calculations cannot provide
a scrupulous understanding of the role of the surroundings beyond the hydrated-hydrolyzed complex,
which is essential for a correct interpretation of many structural, dynamic, and thermodynamic proper-
ties of these complexes [4]. In [5–7], the molecular dynamics simulations were carried out for aqueous
solutions of actinides using rigid TIP3P or SPC water models. Neither the deformation of water molecules
nor the hydrolysis effects in a hydration shell were possible due to constrained rigid models of water
molecules. No hydrolysis evidence was also found in a recent investigation of uranyl hydration by the ab
initiomolecular dynamics simulation [8].
In computer simulations, in order to explicitly treat the cation hydrolysis effect, water should be con-
sidered in the framework of a non-constrained flexible model. In our previous studies [9–13], water was
treated within a slightly modified version of central force model CF1 [14, 15]. It was found that with an
increase of the cation-water interaction, some water molecules in the cation hydration shell lost some
protons. This effect was treated as the hydrolysis of water caused by a high valency of the cation. In
[13, 16], the CF1 model for water was also used for the investigation of the hydration structure of the
© M. Druchok, M. Holovko, 2012 43602-1
http://dx.doi.org/10.5488/CMP.15.43602
http://www.icmp.lviv.ua/journal
M. Druchok, M. Holovko
uranyl ion UO2+
2
. It is found that the uranyl hydration shell has bipyramydal pentacoordinated structure
with five ligands. It includes four water molecules and one hydroxide OH− ion. Usually the cation hy-
drolysis process does not stop at the creation of the hydrated-hydrolyzed complexes and proceeds to a
condensation reaction creating polynuclear ions [17].
Apparently, the tendency for the cation hydrolysis depends on the pH of an aqueous solution. One can
expect a hydrolysis effect in an alkaline solution stronger than in an acidic one. Some previous results for
the computer modeling of the effect of pH on the cation hydrolysis of an uranyl ion UO2+
2
were presented
in [13]. In this paperwe continue the investigation of the formation of the ionic associates between uranyl
ions. In particular, we study the effect of the temperature and pH of an aqueous solution on the structure
of the uranyl dimers and trimers.
2. Model and method
Three different solutions are considered. The first one is a solution with 1600 water molecules and
16 uranyl ions. To mimic an acidic or alkaline conditions, we add 100 H+ or 100 OH− ions to the initial
“neutral” solution, respectively. For simplicity we will further refer to these solutions as alkaline, neutral,
and acidic.
The model is similar to the one used in our study of the uranyl hydration [16]. Central force model
CF1 was engaged in water description. For the uranyl-water interaction we took the potentials from [7].
These potentials are of the “1–12–6” type:
Ei j =
Zi Z j
4πǫ0r
+
Ai A j
r 12
−
Bi B j
r 6
. (2.1)
The corresponding potential parameters are listed in table 1.
Table 1. Potential parameters for uranyl-water interaction.
Z A (kcal Å12/mol)1/2 B (kcal Å6/mol)1/2
O in H2O –0.65966 793.322 25.010
H 0.32983 0.1 0.0
U 2.50 629.730 27.741
O in UO2 –0.25 793.322 25.010
In order to preserve the uranyl intramolecular geometry, additional constraints are used for the U=O
bond length in the form Ei j ∼ (r − 1.75)2 and for the O=U=O angle in the form Ei j ∼ (θ− 180)2 . The
distances are measured in Å, the angles – in degrees, the energies – in kcal/mol.
All the species were allowed to move freely across the MD cell. The cut-off radius for the short-range
interactions is chosen to be 15 Å. The long-range Coulomb interactions were taken into account by the
Ewald summation technique. One has to note that all the systems considered are physically non-neutral
but still can be effectively treated because the charges are implicitly compensated by a neutralizing back-
ground in the Ewald formulation. The pressure (1 bar) and the temperature (278 K or 318 K) were con-
trolled by means of a Nose-Hoover barostat and thermostat in an isotropic N PT ensemble [18, 19]. The
particles were placed into a cubic box (Lx = Ly = Lz ≈ 37 Å) with periodic boundary conditions. The
length of the production runs ranged from 7 to 10 ns. As before [10–13], we used the velocity Verlet algo-
rithm with a time step 0.2 fs to integrate the classical equations of motion.
For structural analysis, the radial distribution functions (RDFs) are collected. The angular distribu-
tions of uranyl-uranyl mutual orientation are also accumulated during the simulations. To accompany
structural details, we have also collected distributions of lifetimes of the uranyl dimers and trimers.
43602-2
Uranyl assocation: pH and temperature influence
3. Numerical results
In this section we present the results of the simulations. It includes the RDFs describing the uranyl-
solution and uranyl-uranyl correlation. As it is mentioned above, we also collected angular distributions.
Further we discuss distributions of lifetimes of the uranyl dimers and trimers.
3.1. Radial distribution functions
In figure 1 we present the RDFs for the uranium-oxygen correlation for T = 278 K (left hand panel)
and 318 K (right hand panel). The red solid lines denote the results for the alkaline, the green dashed lines
Figure 1. (Color online) The uranium-oxygen (of water and hydroxide ions) RDFs for T = 278 K (left) and
318 K (right). Results for the alkaline solution are shown by solid red lines, neutral – by dashed green
lines, and acidic – by dotted blue lines.
– for the neutral, and the blue dotted lines – for the acidic solutions. If one compares the RDFs between
the two temperatures, it is clear that T = 278 K case is characterized by the higher first peaks, indicating
more stable formations under a lower temperature. One can also make another apparent observation: for
both considered temperatures, extra OH− ions in the solution favor the uranium-oxygen attraction while
the neutral and acidic conditions demonstrate roughly the same weaker correlations. The roots of such
a specific behavior in the alkaline case originate from a competition between OH− ions and the water
molecules to be bound to the uranyl ions. Since the hydroxide ions possess a negative charge, they are
capable of pushing out the water molecules from the uranyl shell, occupying the vacancies and becoming
preferential neighbors of the uranyls. To illustrate this fact, we show the uranium-hydrogen RDFs in
figure 2. The distribution belonging to the alkaline case demonstrates the lower first peak reflecting a lack
Figure 2. (Color online) The uranium-hydrogen (of water, OH− and H+ ions) RDFs for T = 278 K (left) and
318 K (right). The color scheme is the same as in figure 1.
43602-3
M. Druchok, M. Holovko
of hydrogens in the uranyl hydration shell in comparison with the neutral and acidic solutions. Another
consequence is the U–H peak shift toward larger distances since the O-H axis in the OH− ions tends to be
oriented directly from the uranium in contrast to water molecules. An acidic case yields the U–H peaks
higher than the neutral peak due to extra H+ ions in the bulk, preventing the uranyl ligands from being
hydrolyzed. A similar tendency to replace water molecules by OH− ions and to keep the number of the
ligands unchanged was previously found in [13].
Next we analyze the uranyl-uranyl coordination in terms of the uranium-uranium and the uranium-
oxygen (of uranyl) RDFs. In figure 3 the U–U distributions for T = 278 K and 318 K are presented. The
Figure 3. (Color online) The uranium-uranium RDFs for T = 278 K (left) and 318 K (right). The color
scheme is the same as in figure 1.
temperature effect follows the same pattern as the one observed for the uranyl-water correlation. Also,
the neutral and the acidic cases behave similarly, while the alkaline one appears to differ from these two.
Contrary to the neutral and the acidic solutions, one can see a prominent tendency for the uranyl-uranyl
association in an alkaline solution. Such an intense uranyl correlation (attraction) in the alkaline solu-
tions is a consequence of an increased screening of the uranyls by OH− ions and the attraction between
the uranyl and shells (negatively charged by the hydroxide ions) of the other uranyls. That is why the al-
kaline solution demonstrates the highest association ability in spite of a weaker association in the neutral
and the acidic solutions.
Next we explore the correlation between the uranium and the oxygens of the uranyls. In figure 4 the
corresponding RDFs for T = 278 K (left hand panel) and 318 K (right hand panel) are collected. As one can
expect, the RDFs for the alkaline solution indicate a stronger correlation than the RDFs for the neutral
and acidic cases. Nevertheless, all these RDFs show the peaks near 4.5–5.5 Å and 7–8 Å. Temperature
produces a very similar effect by decreasing the peak heights.
Figure 4. (Color online) The uranium-oxygen (of uranyl) RDFs for T = 278 K (left) and 318 K (right). The
color scheme is the same as in figure 1. Note that the intramolecular U=O bonds do not contribute to the
RDFs shown.
43602-4
Uranyl assocation: pH and temperature influence
One has to note that the RDFs in figures 3 and 4 are properly normalized and converge to unity at
large distances but a shorter distance range is chosen in the plots to show the structural details.
3.2. Uranyl-uranyl mutual orientation
We made the angular analysis in order to clarify how the uranyls are mutually oriented being in-
volved in the associates. During the simulation, the coordinates of the uranyls were stored and then
processed. A straight and simple criterion to test whether the uranyls are forming an associate is the dis-
tance between uraniums: the location of the first minima of the U–U RDFs (figure 3) can be used for this
purpose. Since the minima locations are smeared for different temperatures, a single distance threshold
r = 7.5 Å was utilized. The angular analysis protocol is as follows: at every simulation step, one can as-
sign a vector along the uranyl axis O=U=O. Next we consider a vector between two uranium ions when
the distance between them is less than 7.5 Å. A description of the mutual uranyl orientations can be
treated in terms of two angles α, β between each of O=U=O axis and the U–U vector (see figure 5). The
corresponding symmetrized probability distributions are shown in figure 6.
Figure 5. The uranyl-uranyl mutual orientation and characteristic angles.
As one can see from figure 6, the angular distributions show a pronounced dependence on pH of
the solution, while the temperature effect is less apparent. We start with the acidic solutions: a bright
spot in the symmetric range 30–65◦ indicates a configuration when both uranyls are tilted to the vector
connecting the uraniums. A wide range of available angles speaks in favor of a “breathing” behavior of
such associates. For a neutral solution at T = 278 K, one can discriminate two probability spots at α= 25–
45◦, β= 55–75◦ and vice versa. At 318 K, the distribution demonstrates the similar shape though slightly
smeared. The alkaline distribution for 278 K shows the symmetric spot at α = β = 45–60◦ with a tiny
possibility for configurations with both angles equal to 90◦ (parallel orientation of uranyls). Similarly to
the neutral case, the temperature makes a smearing effect when going to 318 K. One can see that the
distributions for the alkaline solutions are characterized by more strict and localized spots. This is due
to the hydroxide ions in the uranyl hydration shells when more electrostatic players take part in the
stabilization of the polynuclear associate.
All the distributions indicate no associates with the arrangement of uranyls on a straight line (both
angles are 0◦), and no associates with the perpendicular orientation (one angle is 0◦ and the other – 90◦).
3.3. Lifetimes and associate fractions
Apart from the structural information on the ordering of the uranyls in an associate, it is also inte-
resting to measure the average time spent by them in the associated state. To obtain an information on
the lifetimes of the uranyl associates, we monitor their trajectories during the simulation. Our strategy
is: for any observed dimer (the distance between two uraniums is less than 7.5 Å) we start its individual
“stopwatch” and let the stopwatch go during the timewhen the distance criterion remains satisfied.When
an uranyl leaves an associate, the stopwatch is stopped. The actual procedure involves several steps:
first (i), we determine the atom distributions and identify the uranyls involved in the associates (t = 0),
next (ii) we repeat this procedure (checking the configuration) every 5 elementary steps, that is on 1.0
fs interval. (iii) When the associate is broken (t = τ), we update the lifetime distribution f (τ). If another
uranyl attaches to a dimer (the distance to any of dimer uraniums is less than 7.5), this associate becomes
a trimer and a corresponding “stopwatch” is ascribed.
43602-5
M. Druchok, M. Holovko
α
β
0 10 20 30 40 50 60 70 80 90
0
10
20
30
40
50
60
70
80
90
α
β
0 10 20 30 40 50 60 70 80 90
0
10
20
30
40
50
60
70
80
90
α
β
0 10 20 30 40 50 60 70 80 90
0
10
20
30
40
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80
90
α
β
0 10 20 30 40 50 60 70 80 90
0
10
20
30
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60
70
80
90
α
β
0 10 20 30 40 50 60 70 80 90
0
10
20
30
40
50
60
70
80
90
α
β
0 10 20 30 40 50 60 70 80 90
0
10
20
30
40
50
60
70
80
90
Figure 6. (Color online) The uranyl-uranyl angular distributions for the acidic (top), neutral (center) and
alkaline (bottom row) solutions at T = 278 K (left) and 318 K (right hand column). The colors correspond
to the probability from low (dark) to high (bright) to find a corresponding configuration. Color scheme is
scaled in such a way that the maximum values of all the distributions are of the same intensity.
43602-6
Uranyl assocation: pH and temperature influence
Next, based on the lifetime distribution f (τ), we can estimate the average lifetimes as:
〈τ〉 =
∫τmax
0
τ f (τ)dτ
∫τmax
0
f (τ)dτ
, (3.1)
where τmax = 200 ps is taken as the integration limit (∞) since the distributions f (τ) have completely
decayed at these times.
In table 2, we present the average lifetimes for the uranyl associates (dimers and trimers) at T = 278
and 318 K. One can see that a decrease of the temperature extends the average lifetimes 〈τ〉. Another
factor favoring a formation of the associates is the presence of OH− ions. That is why the strongest associ-
ation uranyl capability is found for the alkaline solution at 278 K. As it was shown above, the association
capability in the alkaline case is sufficiently increased due to the uranyl screening by the hydroxide ions.
In the absence of OH− ions, the acidic and neutral solutions behave in a similar way demonstrating a
relatively close results within the uncertainties. Due to this, the lifetime for the acidic case at 278 K is
longer than the one for the neutral case at 278 K. The average trimer lifetimes for the acidic case at 278
and 318 K do not fit the overall temperature tendency but the differences originate from the relatively
short association events and do not change the main trends.
Table 2. The average lifetimes of the uranyl associates. Time units are – ps.
dimers trimers
278 K 318 K 278 K 318 K
acidic 14.18 5.71 1.84 2.55
neutral 13.79 6.21 3.15 2.65
alkaline 20.98 10.57 10.14 5.00
Sometimes [20, 21] in the studies of the association lifetimes, besides the distance criterion, the au-
thors introduce an additional tolerance time. During this time, an associate is allowed to be broken and
to be united back so that the lifetime “stopwatch” keeps running. The purpose of such a deployment is
to avoid an overestimation of the shorter lifetimes due to sequent broken/united events. In our study we
did not utilize this formalism because it is not well defined and does not affect the general conclusions.
Other quantities to consider are the fractions of the uranyls taking part in the formation of dimers
and trimers. These fractions are defined as the number of the uranyls in the corresponding type of as-
sociates divided by the total number of the uranyls in the solution. The criteria for dimer and trimer
creation events are the same as before. In table 3 the fractions are collected. As one can expect, the same
tendencies can be found here as above for the average lifetimes: the low temperature and the presence
of OH− ions are the factors increasing the capability of the uranyls to associate.
Table 3. The fractions of the uranyls involved in the associates.
dimers trimers
278 K 318 K 278 K 318 K
acidic 0.132 0.077 0.008 0.006
neutral 0.154 0.133 0.019 0.023
alkaline 0.411 0.350 0.218 0.145
Despite the similar conclusions drawn from the lifetimes and the fractions, one has to note that the
average lifetime does not contain the same information as the fraction of the associated uranyls: the same
fraction of the associated uranyls can be obtained as a result of many short or, alternatively, fewer but
longer (in time) formations.
43602-7
M. Druchok, M. Holovko
4. Conclusions
The purpose of this study is to clarify in what way the temperature and pH effect the formation of the
polynuclear associates in the uranyl aqueous solutions. For this purpose, we performed a series of MD
simulations of the uranyl aqueous solutions at temperatures 278 and 318 K. To model different pH’s the
neutral system (containing waters and uranyls only) was mixedwith the additional H+ or OH− ionsmimi-
cking an acidic, or alkaline solutions. During the simulations we collected the necessary statistics on the
radial distributions functions describing the uranyl hydration shells and the uranyl-uranyl correlation.
Comparing the RDFs at the temperatures 278 and 318 K one can conclude that the increase of temperature
leads to a decrease of the correlation (the peaks of the RDFs become lower). It is also found that in the
acidic and the neutral solutions, the uranyl hydration shells stay unchanged, while in the alkaline case,
the uranyls are preferably hydrated by the hydroxide ions. The modified hydration in the alkaline case
results in the increased uranyl attraction contrary to a weak uranyl correlation in the neutral and the
acidic solutions. The negatively charged OH− ions screen the UO2+
2
ions allowing them to form more
stable associates. In addition to the RDFs, we introduced the angular distributions reflecting the mutual
orientation of the uranyls involved in the polynuclear associates. A temperature effect on the angular
distributions is less apparent than the effect made by pH: indeed one can see that the distributions change
the shape from smeared to a strict one when going from the acidic to the neutral and then to the alkaline
case. This is due to the presence of the hydroxide ions in the uranyl hydration shells, that stabilize the
uranyl-uranyl associates. For every solution we calculated the fraction of the uranyls taking part in the
associates. Besides the fractions we also monitored the lifetimes of the uranyl dimers and trimers, built
corresponding lifetime distributions, and extracted the average lifetimes for each case. The conclusions
drawn agree with the ones from the RDFs: the low temperature and the alkaline environment are the
factors favoring the long-lasting uranyl associates.
Acknowledgements
The MD calculations were performed on the clusters of Ukrainian Academic Grid.
43602-8
Uranyl assocation: pH and temperature influence
References
1. Ahearne J.F., Phys. Today, 1997, 50, 27; doi:10.1063/1.881763.
2. Spencer S., Gagliardi L., Handy N.C., Ioannou A.G., Skylaris C.-K., Willetts A., Simper A.M., J. Phys. Chem. A, 1999,
103, 1831; doi:10.1021/jp983543s.
3. Tsushima S., Suzuki A., J. Mol. Struct. THEOCHEM, 2000, 529, 21; doi:10.1016/S0166-1280(00)00526-1.
4. Liu C.X., Zachara J.M., Qafoku O., McKinley J.P., Headd S.M., Wang Z.W., Geochim. Cosmochim. Acta, 2004, 68,
4519; doi:10.1016/j.gca.2004.04.017.
5. Guilbaud P., Wipff G., J. Phys. Chem., 1993, 97, 5685; doi:10.1021/j100123a037.
6. Guilbaud P., Wipff G., J. Mol. Struct. THEOCHEM, 1996, 366, 55; doi:10.1016/0166-1280(96)04496-X.
7. Greathouse J.A., O’Brien R.J., Bemis G., Pabalan R.T., J. Phys. Chem. B, 2002, 106, 1646; doi:10.1021/jp013250q.
8. Nichols P., Bylaska E.J., Schenter G.K., de Jong W., J. Chem. Phys., 2008, 128, 124507; doi:10.1063/1.2884861.
9. Holovko M.F., Kalyuzhnyi Yu.V., Druchok M.Yu., J. Phys. Stud., 2000, 4, 100.
10. Druchok M.Yu., Bryk T.M., Holovko M.F., J. Phys. Stud., 2003, 7, 402.
11. Holovko M., Druchok M., Bryk T., J. Chem. Phys., 2005, 123, 154505; doi:10.1063/1.2064582.
12. Holovko M., Druchok M., Bryk T., J. Mol. Liq., 2007, 131–132, 65; doi:10.1016/j.molliq.2006.08.029.
13. Holovko M., Druchok M., Bryk T., In: NATO Science for Peace and Security Series – A: Chemistry and Biology.
Self-Organization of Molecular Systems, ed. by Russo N., Antonchenko V., Kryachko E., Springer, 2009, 221–253.
14. Nyberg A., Haymet A.D.J., In: Structure and Reactivity in Aqueous Solutions, ed. by Trular D., Kramer C., Ameri-
can Chem. Soc., New York, 1994.
15. Duh D.M., Perera D.N., Haymet A.D.J., J. Chem. Phys., 1995, 102, 3736; doi:10.1063/1.468556.
16. Druchok M., Bryk T., Holovko M., J. Mol. Liq., 2005, 120, 11; doi:10.1016/j.molliq.2004.07.071.
17. Ramsay J.D.F., In: Water and Aqueous Solutions, ed. by Neilson G.N., Enderby J.E., Adam Hilger, Bristol, 1986,
p. 207.
18. Hayle J.M., Molecular Dynamics Simulations: Elementary Methods. Wiley, New York, 1992.
19. Melchionna S., Ciccotti G., Holian B.L., Mol. Phys., 1993, 78, 533; doi:10.1080/00268979300100371.
20. Spohr E., In: Solid-Liquid Electrochemical Interfaces – ACS Symposium Series, ed. by Jerkiewicz G., Soriaga M.P.,
Uosaki K., Wieckowski A., American Chem. Soc., Washington DC, 1997, 31–44; doi:10.1021/bk-1997-0656.ch003.
21. Laage D., Hynes J.T., J. Phys. Chem. B, 2008, 112, 7697; doi:10.1021/jp802033r.
Вплив температури та рiвня рН на асоцiативнiсть у водних
розчинах уранiлу
М. Дручок, М. Головко
Iнститут фiзики конденсованих систем НАН України, вул. Свєнцiцького 1, Львiв 79011, Україна
Проведено дослiдження процесiв асоцiативностi у водних розчинах уранiлу. Для цього проведено низ-
ку моделювань методом молекулярної динамiки. Пiд час моделювання проведено монiторинг фракцiй
iонiв уранiлу, що беруть участь у формуваннi димерiв та тримерiв. Також зiбрано розподiли iмовiрно-
стi за часами життя уранiлових димерiв та тримерiв. Розглянуто вплив двох факторiв – температури та
рiвня рН середовища – на здатнiсть iонiв уранiлу до формування асоцiатiв. Виявлено, що збiльшення
температури знижує асоцiативнiсть, при цьому також скорочуються часи життя асоцiатiв i зменшуються
фракцiї iонiв уранiлу у асоцiатах. Вплив рiвня рН середовища змодельовано додаванням iонiв H+ чи OH−
до “нейтрального” розчину. Виявлено, що наявнiсть у розчинi iонiв OH− є сприятливим чинником для
асоцiативностi, в той час як наявнiсть iонiв H+ призводить до протилежного ефекту. Для усiх розчинiв
проведено конфiгурацiйний аналiз взаємної орiєнтацiї iонiв уранiлу, що перебувають у асоцiативному
станi.
Ключовi слова: молекулярна динамiка, водний розчин уранiлу, асоцiативнiсть, pH, температура
43602-9
http://dx.doi.org/10.1063/1.881763
http://dx.doi.org/10.1021/jp983543s
http://dx.doi.org/10.1016/S0166-1280(00)00526-1
http://dx.doi.org/10.1016/j.gca.2004.04.017
http://dx.doi.org/10.1021/j100123a037
http://dx.doi.org/10.1016/0166-1280(96)04496-X
http://dx.doi.org/10.1021/jp013250q
http://dx.doi.org/10.1063/1.2884861
http://dx.doi.org/10.1063/1.2064582
http://dx.doi.org/10.1016/j.molliq.2006.08.029
http://dx.doi.org/10.1063/1.468556
http://dx.doi.org/10.1016/j.molliq.2004.07.071
http://dx.doi.org/10.1080/00268979300100371
http://dx.doi.org/10.1021/bk-1997-0656.ch003
http://dx.doi.org/10.1021/jp802033r
Introduction
Model and method
Numerical results
Radial distribution functions
Uranyl-uranyl mutual orientation
Lifetimes and associate fractions
Conclusions
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