The Hofmeister series
a review of
monatomic ions near the hydrophobic/water interface
Student: Ernst Rösler
Student ID: 0105813
Course: Literature Study
MSc Chemistry, track Theoretical and Computational Chemistry
University of Amsterdam, FNWI, HIMS
Supervisor: Prof. Dr. E.J. Meijer
July 26th 2013
Hofmeister series, anion, cation, hydrophobic surface, in silico, density profile, PMF, CPMD, MD
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Cover figure: N. Schwierz, D. Horinek, R. R. Netz, Langmuir 2010, 26, 7370–7379
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MSc Chemistry
Molecular Simulation
Literature Thesis
The Hofmeister series
a review of
monatomic ions near the hydrophobic/water interface
by
Ernst Rösler
July 2013
Supervisor:
Prof. Dr. E.J. Meijer
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4
Abstract
In 1888, Hofmeister discovered ion specific effects on precipitation of purified egg white. According
to the efficiency of the different ions, they can be ordered reproducibly. This is known as the
Hofmeister series. Nowadays, the mechanism of the Hofmeister series is still not clear. In silico
studies, mostly molecular dynamics (MD), provide new insight about the effects of ions on the
water/hydrophobic interfaces. The aim of this review is to compare the setup, the used methods and
the results of in silico studies on the effect of ions on water/hydrophobic interfaces. Studies show a
great variety of aqueous biphasic systems, investigated ions and the used computational methods.
The setup is crucial for a proper description of the system. The kind of surface and the ion
concentration affects the behaviour of the ions. Many characteristics can be explained by the first
solvation shell of the ion.
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Table of contents
1.
Introduction ..................................................................................................................................... 9
1.1. Hydrogen bonds ...................................................................................................................... 9
1.2. Water molecules close to the interface .................................................................................. 9
1.2.1. Gibbs’ dividing surface and interfacial width .............................................................. 10
1.3. Ions in water .......................................................................................................................... 10
1.3.1. The periodic table: Anions and cations ....................................................................... 10
1.3.2. Polyatomic ions............................................................................................................ 10
1.4. Charge density of ions ........................................................................................................... 10
1.4.1. Charge density of monatomic ions .............................................................................. 11
1.4.2. Polyatomic ions............................................................................................................ 11
1.5. Solvation shells ...................................................................................................................... 11
1.5.1. Solvation shells of monatomic ions ............................................................................. 11
1.5.2. Solvation shells of polyatomic ions.............................................................................. 12
1.5.3. Hydrogen bond strength ............................................................................................. 12
1.5.4. Residence time and diffusion ...................................................................................... 12
1.6. Ions near the interface .......................................................................................................... 12
1.6.1. Water/hydrophobic interfaces .................................................................................... 13
1.7. Hofmeister series .................................................................................................................. 13
1.8. Computational simulations ................................................................................................... 13
2.
Computational methods ................................................................................................................ 14
3.
Results and discussion ................................................................................................................... 15
3.1. Density profiles and potentials of mean force ...................................................................... 15
3.1.1. Anions .......................................................................................................................... 16
3.1.2. Cations ......................................................................................................................... 16
3.1.3. Same trend, different results....................................................................................... 16
3.2. Residence time and Diffusion coefficient.............................................................................. 17
3.2.1. Water near the interface ............................................................................................. 17
3.2.2. Water in first solvation shell ........................................................................................ 18
3.3. Interfacial tension and Interfacial widths.............................................................................. 18
3.4. Ion-water coordination and solvation shells of monatomic ions .......................................... 19
4.
Summary ........................................................................................................................................ 23
References ............................................................................................................................................. 25
Acknowledgments ................................................................................................................................. 27
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1. Introduction
Water is one of the most important molecules in life on earth; water is vital for all known forms of
life. Due to its importance in life and its special structure and reactivity, water is a solvent of
increasing interest in modern (bio)chemistry. The most important characteristics of water are the
unique solvating abilities and the formation of hydrogen bonds between water molecules and
between water and solvated molecules.
1.1.
Hydrogen bonds
In liquid water, hydrogen bonds are continuously formed and broken. Each water molecule can form
up to four hydrogen bonds, see Figure 1.1. The oxygen atom of water can form two hydrogen bonds
with hydrogen atoms of other molecules and each hydrogen atom of water can form one hydrogen
bond with oxygen atoms of other molecules. The formation of a hydrogen bond between two water
molecules yields an energy of 2.6 ± 0.1 kcal/mol.[1] The formation of a hydrogen bond network in the
bulk of water is energetically favourable and affects the structure of bulk water. Due to the hydrogen
bonds, the bulk water is dominated by a tetrahedron structure and the bulk water is a fully isotropic
system.
Figure 1.1 The four possible hydrogen bonds of a water molecule.
1.2.
Water molecules close to the interface
The orientation of water molecules near the interface differs from the isotropic bulk phase. The bulk
phase of water has a homogeneous density, whereas close to the interface the density of water is
reduced. Due to the reduced number of water molecules close to the interface, water needs a special
arrangement to maximize the probability to form hydrogen bonds with the bulk. Whereas the
molecules in the bulk are in the isotropic orientation, the molecules close to the interface need to be
ordered. The orientation of the water molecules dependents on the surface charge and polarity, the
temperature, the pressure and the kind of solution.
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1.2.1.
Gibbs’ dividing surface and interfacial width
Depending on the kind of surface media, the density of the media is also reduced close to the
interface.[2] Like the water molecules, the orientation of surface molecules will differ from their
orientation in the bulk phase. The region where the densities change is called the interfacial width,
with in the middle the Gibbs’ dividing surface (GDS).
1.3.
Ions in water
If salts are solvated in water, they quickly separate into anions and cations. These ions disturb the
highly structured liquid: the natural hydrogen bond network is disrupted. Breaking hydrogen bonds is
energetically unfavourable, however, depending on the kind of ion, the loss of hydrogen bonds
between water molecules can be compensated. Ions can form hydrogen bonds with water and the
formation of solvation shells around ions can result in a negative total free energy of solvation.
1.3.1.
The periodic table: Anions and cations
The periodic table arranges all chemical elements on basis of their characteristics and gives insight
into the behaviour of ions. Ions are divided into two main groups: the negatively charged anions and
the positively charged cations. The anions are found in the upper right corner of the table, whereas
the rest of the table represent cations. Both, anions and cations, are monovalent or multivalent. The
alkali metals in the first group form ions with a 1+ charge, the alkaline earth metals of the second
group form 2+ ions. The halogens in group 17 form 1- ions, whereas the non-metal ions in group 16
and 15 have 2- and 3- charges, respectively.
The atomic radii of elements in a group increase from top to bottom. The same holds for the radii of
ions in the same group; from top to bottom the ionic radii increase. The ionic radius of a cation is
smaller compared to the atomic radius of the element, whereas the ionic radius of an anion is larger
compared to the atomic radius.
1.3.2.
Polyatomic ions
Like monatomic ions, polyatomic ions are divided into anions and cations, which both contain
monovalent and divalent ions. Polyatomic ions contain at least 2 atoms and have a less symmetric
geometry than monatomic ions. The arrangement of polyatomic ions on basis of their characteristics
however is not as straightforward as for the monatomic ions.
1.4.
Charge density of ions
The size and the charge of the ion determine a very important characteristic of the ion; the charge
density. Many properties of ions can be explained by the charge density. The charge density affects
the strength and the number of hydrogen bonds with water molecules, as well as the attraction
between counter ions.
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1.4.1.
Charge density of monatomic ions
Monatomic ions, both anion and cation, have a symmetric charge density, independent of the total
charge. The charge density depends on the radius and the total charge of the ion. Within groups of
the periodic table the charge density decreases from top to bottom because of the increase of ion
radius. For example: Na+ has a larger charge density than Cs+.
1.4.2.
Polyatomic ions
More complex is the correlation between the total charge and the geometry of polyatomic ions.[3]
Like monatomic ions, polyatomic ions can be anionic and cationic, which, in turn, can be mono- and
multivalent. The geometry of polyatomic ions consists of at least two atoms up to many atoms, like
OH- and
respectively. Due to the structure, polyatomic ions do not have a uniform charge
density.
1.5.
Solvation shells
Water molecules form solvation shells around the ions by hydrogen bonds with solvated ions. The
kind of solvation shells depends on the charge density and the geometry of the ion; ions can be
monatomic or polyatomic. The charge of the ion determines the orientation of the water molecules,
whereas the geometry of the ion determines the structure of the solvation shell.
1.5.1.
Solvation shells of monatomic ions
Due to its structure, monatomic ions can form symmetric solvation shells, see Figure 1.2. Near a
cation, see Figure 1.2 a), the oxygen of the water molecules is closest to the cation, which forms a
less favourable hydrogen covered shell around the cation. Whereas in the first solvation shell of an
anion, the water molecules are orientated with one oxygen-hydrogen bond toward the anion, see
Figure 1.2 b). This is comparable with the natural solvent character and makes anions more
favourable than cations. The number of coordinating water molecules and the geometry of the
coordination sphere depends on the ion.
Figure 1.2 Solvent orientation around a) cation and b) anion
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1.5.2.
Solvation shells of polyatomic ions
The solvation shells of polyatomic ions are more complex, due to the less symmetric geometry of the
ion, as for example in the solvation shell of hydroxide; the hydrogen forms a weak hydrogen bond
with one solvent molecule, whereas the oxygen of the hydroxide forms strong hydrogen bonds with
roughly four neighbouring waters.
1.5.3.
Hydrogen bond strength
The size of the ion determines the disruption of the hydrogen bond network of the bulk and the
delocalisation of the charge. The larger the ion, the larger the disturbance of the local water network
and the larger the delocalisation of the charge. Strong hydrogen bonds between water and ions are
caused by a high charge density on the ion, whereas a lower charge density on the ion results in
weaker hydrogen bonds with water molecules. On the other hand, a larger solvation shell can form
more hydrogen bonds with solvent molecules compared to a smaller solvation shell. The geometry
and size of the solvation shell and the sign of the charge of the specific ion determine the net effect.
1.5.4.
Residence time and diffusion
Another correlated effect of the first solvation shell and the kind of ions is their mobility in the bulk.
The strength of the hydrogen bond between the water molecules and the ion determines the
residence time and the diffusion of the water molecules in the first solvation shell of the ion. The
stronger the hydrogen bond between the water molecule and the ion, the longer the water molecule
will stay in the solvation shell of the ion. A low degree of water interchange implies a higher
residence time. A high residence time results in a low diffusion of the ion and therefore the ion will
stay in the bulk. Whereas weaker hydrogen bonds between the ion and the waters in the first
hydration shell causes a high interchange and a low residence time of water molecules in the first
shell, which results in a high diffusion of the ion. Ions with weak hydrogen bonds can diffuse from the
bulk to the surface.
1.6.
Ions near the interface
Ions not only change the hydrogen bond network and the orientation of water molecules in the bulk,
they also disturb the arrangement of water molecules close to the interface. Due to the reduced
density of water molecules near the interface, the presence of ions close to the interface achieves a
reordering of the water molecules. The number of water molecules available for the solvation shell
of the ion near the interface is reduced compared with the bulk water. This affects the ion
distribution near the surface, depending on the strength of the solvation shell. The kind of hydration
shell has immediate consequences for the surface tension and the interfacial width; ions can increase
or decrease the surface tension and the interfacial width.
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1.6.1.
Water/hydrophobic interfaces
Water/hydrophobic interfaces are important in a wide range of fields as protein folding, ion
transport, drug delivery and industry. Hydrophobic surfaces have only very little or no tendency to
adsorb water and have no active groups to form hydrogen bonds with water. There are many
different kinds of hydrophobic surfaces, which differ in phase and molecular structure. A well known
hydrophobic liquid is CCl4, which was historically used in proton NMR. Other hydrophobic liquids are
heptane and decane. Long alkane chains like icosane can form solid hydrophobic membranes. On the
other hand, proteins contain hydrophobic and hydrophilic patches on their surface.
Ions, which are ubiquitous in living organisms, affect the structural and dynamical properties of
aqueous interfaces. Low concentrations of about 0.1 to 0.2 M of monovalent ions, such as Na+, K+
and Cl-, are present inside and outside living cells. But even in the absence of salt ions, water is able
to autoionize; bulk water contains very low concentrations of hydronium and hydroxide, 10 -7 M for
pH neutral.[4]
1.7.
Hofmeister series
In 1888, Hofmeister discovered ion specific effects on precipitation of purified egg white. According
to the efficiency of the different ions, they can be ordered reproducibly and are known as the
Hofmeister series. As Hofmeister discovered, anions and cations behave different in solution close to
a surface of egg white. Anions have a larger effect than cations, and therefore the Hofmeister series
are divided into anionic and cationic series. Both series contains monovalent and divalent ions and
monatomic and polyatomic ions. The ion adsorption at or repulsion from the surface is determined
by the characteristics of the surface and the ion. The behaviour of ions near the air-water interface
and uncharged hydrophobic solid surfaces has been the subject of many experimental studies and
simulations of ions at the air-water interface gave crucial insight into the order of the Hofmeister
series. Due to the specific effects the ions are ordered in what is known as the direct order of the
Hofmeister series.
1.8.
Computational simulations
The Hofmeister ordering of soft inorganic anions and cations near hydrophobic aqueous interfaces
has been subject of many studies. Surface selective spectroscopic techniques and extended
molecular simulations have made it possible to study the phenomena occurring at
water/hydrophobic interfaces. In silico studies, mostly Molecular Dynamics (MD), provide new insight
about the effects of ions on the water/hydrophobic interfaces. Studies show a great variety of
aqueous biphasic systems, investigated ions and the used computational method. The used force
fields are crucial for a proper description of the system. The behaviour of cationic or anionic series
are simulated in the presence of a counter ion and are compared with experimental data to get a
detailed understanding of the molecular structure of aqueous interfaces. The sodium cation and the
chloride anion are commonly used counter ions. However, nowadays, the mechanism of the
Hofmeister series is still not clear. The aim of this review is to compare the setup, the used methods
and the results of in silico studies on the effect of ions on water/hydrophobic interfaces.
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2. Computational methods
In order to study the behaviour of ions in water near the interface between water and a hydrophobic
media, the systems are simulated by Molecular Dynamics (MD) simulations using the AMBER or
GROMACS software packages. The systems are formed by rectangular unit cells, which contain a
water slab between two surface layers, i.e. each system contains two interfaces. The size of the used
unit cell ranges from 4.1 x 101 nm3 to 5.8 x 102 nm3 and the thickness of the water slab varies
between 18 Å and 100 Å. The water/alkane interface is used as a model to simulate the water/lipid
membrane system. In the studies under comparison, different chain lengths are used. For short chain
lengths the orientation is isotropic, whereas the surface of long alkane chains consists of terminally
fixed, unconstrained self-assembled monolayers (SAMs) in a gold(111) lattice spacing with a tilted
angle of 30°, which is close to experimental values.[5–7] The water molecules are simulated explicit
and are modelled by different models like POL3, Dang-Chang[8] and SPC/E. In the reviewed studies
the anions F-, Cl-, Br-, I- and the cations Na+, Li+, K+, Cs+, Zn2+, Mg2+ were examined close to a
hydrophobic alkane surface.[2,3,5–7,9–11] The used concentrations range from 0.015 M to 1.4 M. The
force fields used to describe the alkanes, the waters and the ions differ in the reviewed studies. All
simulations were performed at a constant temperature close to T = 300K and a constant pressure of
1 atm, using thermostats and barostats. The periodic boundary conditions were applied to all
systems. See Table 2.1 in the Supporting Information for all system details.
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3. Results and discussion
125 years after Hofmeister discovered that ions affect the precipitating of gen-egg white proteins,
the mechanism of the Hofmeister series is still not clear. Surface selective spectroscopic techniques
and molecular simulations provide new insight into the sequence of the Hofmeister series. Like the
Hofmeister series, the simulations distinguish between anions and cations. The focus of this review is
on the behaviour and effect of ions near the interface between water and several hydrophobic media
and only monatomic ions are involved in this review. The results of different studies will be
compared and discussed based on the most important characteristics:




3.1.
Density profiles and potentials of mean force
Residence time and diffusion coefficient
Interfacial tension and interfacial widths
Ion-water coordination and solvation shells of monatomic ions
Density profiles and potentials of mean force
Density profiles and potentials of mean force (PMF) profiles contain a lot of information about the
effect and behaviour of the ions near the hydrophobic surfaces. Density profiles show the
concentrations of the system’s components perpendicular to the surface, see Figure 3.1 a). The
concentrations are represented in either density or scaled to be 1 for their bulk density. The ion
densities were multiplied for better visibility of low concentrated ions. A maximum in the density
profile shows the most likely position of the ion close to the surface. In contrast to the density
profiles, the PMF profiles only contains information about the adsorption strength of the ions as
function of the surface separation, see Figure 3.1 b). A negative PMF energy indicates an attraction,
whereas a positive energy means repulsion from the surface. Local minima show the most likely
locations of the ions. In the bulk, the system becomes isotropic and the average PMF energy goes to
zero.
a)
b)
[9]
[7]
Figure 3.1: a) Density profile and b) Potential of mean force profile
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3.1.1.
Anions
Studies of inorganic anions used different selections of halides, see Table 3.1.[5,7,9,10] Despite the
variety of selected halides, all the results show the same trend and reproduce the direct Hofmeister
series; F- < Cl- < Br- < I-. The large iodide ion is attracted to the surface and has the largest
concentration peak, whereas the smaller halide anions become more repelled from the hydrophobic
interface with a decreasing concentration peak. These findings are comparable to the air/water
interface simulations, although the interfacial concentrations are somewhat lower at the
hydrophobic interface.[12,13]
Table 3.1: Inorganic anions for each study
Vazdar et al[9]
F-, Cl-, Br-, I[7]
Schwierz et al
F-, Cl-, IWick et al[10]
Cl-, Br-, I[5]
Horinek et al
F-, Cl-, Br-, I3.1.2.
Cations
The behaviour of cation adsorption at the hydrophobic surface is comparable to the anion behaviour
at the water/air and the water/alkane interfaces and is correlated to the ion size; Cs+ > Li+ > K+ >
Na+.[6,11,12] The largest cation, Cs+, adsorbs strongest, whereas the smaller K+ and Na+ are increasingly
repelled from the surface. The small Li+ ion is irregular and a closer look to the solvation shells will
explain this, this will be explained in paragraph 3.4.
The divalent cations Mg2+ and Zn2+ are more repelled from the surface than Na+, due to their
solvation shells, see paragraph 3.4.[2] Both divalent cations form ionic layers with the chloride
counter ion.
3.1.3.
Same trend, different results
Although the anionic and cationic studies show the same trend in ion adsorption, the results differ
for both the density and the PMF profiles. The positions of the minimum and maximum in the
concentration and force differ between the compared studies. These differences are most notable
for chloride.
The cause of these differences can be found in the setup of the simulation. In addition to the
different force fields and molecular models, the ion concentrations used in the simulations are very
different, ranging from 0.015 M to 1.4 M. In contrast, the ionic concentrations inside and outside
living cells vary between 0.1 to 0.2 M.
Solvated ions can form ionic interactions with their counter ions, however simulations with very low
ion concentration only insert a single ion into the water layer. Due to the lack of counter ions, it is not
possible to form ionic double layers.
The position of the peaks in the density profile and the PMF profile depend on the kind of ion, the
presence and kind of the counter ion, the surface and the concentration.
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3.2.
Residence time and Diffusion coefficient
The water molecules are not fixed to the surface or the solvation shells of ions and will exchange
with bulk water molecules. The average time water molecules stay close to their initial position is
called the residence time τres. The diffusion coefficient D and τres are correlated by
~ D. An
increase in τres causes a decrease in D. The diffusion coefficient is given by equation (1):
(1)
where
is the mean square displacement of the centre of mass at time t.
3.2.1.
Water near the interface
The orientation of interfacial water molecules differ from the orientation of bulk water molecules.[13]
Water molecules of the bulk are isotropic orientated, whereas water molecules of the first and
second water layer of the interfacial region are orientated specifically in order to reduce the surface
area. The intermolecular forces determine the surface tension ɣ. The dipole moment of the water
molecules of the first layer is most probably parallel to the surface, an angle of θ ~ 90° between the
dipole moment of the water molecule and the vector normal to the interface, see A in Figure 3.2. The
water molecules of the second layer prefer an angle of θ ~ 45° or θ ~ 135°. Due to the preferred
orientation of the first two water layers in the interfacial region the exchange of interfacial water
molecules is much lower than in the bulk phase; the residence time τres of interfacial water
molecules is eight time longer than that of bulk water molecules.[13]
The addition of salts like KCl, NaCl, MgCl2 and ZnCl2 affects the orientation of the first two water
layers in the interfacial region.[2,11] The ions at the second layer modifies the water molecule ordering
and shifts the angle θ to negative values of cosθ, which results in an increase of surface forces and a
decrease in exchange of interface water molecules. The salt ions increase the τres and therefore
decrease D of interfacial water molecules.
Figure 3.2. Orientation distributions of water
molecules for pure water (A), KCl (B) and NaCl (C). Z is
the distance from the interface along the z axis.
Positive values refer to the water phase, negative
values refer to the hydrophobic surface. Inset: the
definition of angle θ.
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3.2.2.
Water in first solvation shell
The τres of water molecules in the first solvation shell of ions is proportional to the strength of the
hydrogen bond.[2] A strong hydrogen bond yields a high τres and a low exchange of water molecules
from the solvation shell. The diffusion coefficient
is correlated to the τres and the preferences to
the interface. The weak solvation shell of Na+ results in a high
compared to the diffusion
coefficients of Zn2+ and Mg2+;
~ 60
and ~ 18
.[2]
The diffusion coefficient of the Cl- depends on the counter ion. For the NaCl system, the
are comparable, in contrast to the diffusion coefficients of the MgCl2 and ZnCl2 systems;
and ~ 3
8
and
~
.[2] The chloride remains in the second solvation shell and forms partial ion
pairs with Na+ and Mg2+, in contrast to Zn2+, which has the strongest solvation shell and does not
form an ionic couple with Cl-. This explains the difference in diffusion coefficients.
3.3.
Interfacial tension and Interfacial widths
The addition of inorganic salts to water affects the surface tension ɣ and the interfacial widths σ,
which are related to the ion concentration. The interfacial tension ɣ is given by equation (2):
(2)
where px, py and pz are the diagonal components Pxx, Pyy and Pzz of the pressure tensor and Lz is the
length of the box in the z direction.
Adding ions to the solvent increases the interfacial tension and Δɣ positive for all ions. The Δɣ is most
remarkable for divalent ions. The order of the interface tension is inversely connected to the trend in
ion adsorption. The change in interfacial tension decreases by increasing interfacial concentration; Δɣ
F- > Cl- > Br- > I- and Zn2+ > Mg2+ > Na+.[2,7,9,10] The increase in surface tension is in the direct Hofmeister
series. An increase in salt concentrations raises the surface tension and results in a salting out of egg
white.[7] These results are also quite similar to the results of air/water interface simulations and
water/n-heptane measurements.[14–17] The same can be seen from PMFs; the less repulsive, the
smaller the slope of the interfacial tension. Due to the dependence on concentration, the studies
show very different results.
The interfacial widths of the hydrophobic/water system depend on the surface and the ion. The
divalent cations Mg2+ and Zn2+ decrease the interfacial widths more than the monovalent Na+.[2] The
σ is also dependent on the interfacial surface area and increasing alkane chain length decreases the
interfacial widths.
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3.4.
Ion-water coordination and solvation shells of monatomic ions
Solvated ions are surrounded by solvation shells of water molecules and a closer look to the first
solvation shells helps to explain the behaviour of ions in the bulk and near a surface. The number of
coordinating water molecules and the geometry of the coordination sphere in the first solvation shell
depend on the ion and change if the ions approach the hydrophobic surface.
The radial distribution function (RDF) gIW(r) for the ion-oxygen distance shows the maxima and
minima of coordination water molecules around the ions. Each maximum represents a solvation shell
and the radius dependent coordination number nc(r) for each solvation shell is given by equation (3):
(3)
The average number of water molecules in the first hydration shell around an ion is denoted as n1 =
nc(r1), where r1 is the position of the first minimum of the bulk RDF. The RDF gIW(r) orders the ions in
size from small to large; F- < Cl- < I- and Li+ < Na+ < K+ < Cs+, see Figure 3.3.[6] By an increasing ion size
the peak in the RDF decreases and broadens, indicating a widening of the solvation shell. All cations
have smaller RDF than Cl-. None of the cations show a partially strip off of their first solvation shell at
the minimum of the potential mean force of Cs+, which is 7,5 Å away from the surface, see lower half
of Figure 3.6, or at the point of maximal surface attraction, in contrast to the large I- anion.[6,7]
−
−
−
+
+
+
+
Figure 3.3: Radial distribution function gIW(r) of the anions F , Cl , and I (A) and the cations Li , Na , K , and Cs (B) in bulk.
−
[6]
The dashed vertical line in (B) denotes the first maximum in the radial distribution function of Cl for comparison.
Figure 3.4 Cation coordination
number in the first solvation shell
1
n as a function of the distance z
1
from the CH3 surface. n
+
+
+
decreases for Cs , K , and Na as
the
ions
approach
the
hydrophobic surface indicating
reduced ion hydration. In contrast,
+
the hydration shell of Li remains
intact. The open symbols denote
the position of the minimum in
[6]
the ion−surface PMF.
19
+
Figure 3.5: Snapshots of the hydration shell structure of the Na (GROMOS
+
2+
2+
force field) (a), Na (KB force field) (b), Mg (c), Zn (d), and Cl (e) ions in
[2]
the bulk water phase (left) and in the interface (right).
20
The number of water molecules in the first solvation shell of the cations as function of the ionsurface separation helps to understand the unexpected behaviour of the Li+ cation, see Figure 3.4
and upper half of Figure 3.6.[6] The average number of water molecules in the first solvation shell of
cations is constant in the bulk phase. Due to the reduced number of available water molecules near
the interface compared with the bulk water, the coordination number is expected to be lower near
the surface. Between the PMF minimum and the surface, the three largest cations partially strip off
their solvation shell, whereas the first solvation shell of Li+ remains intact. This explains the
unexpected behaviour of the Li+ cation compared to the larger cations in water near a hydrophobic
surface and is identical to the air-water interface.
Figure 3.6 Upper half: Simulation snapshots of Li+ at different surface separations z = 0.3 nm, z = 0.525 nm, z = 0.7 nm, and
z = 0.875 (from left to right). For small separations, only the first hydration shell containing four water molecules is shown.
For larger separations, water molecules within 5.5 or 6.5 Å of the ion are shown. Lower half: Simulation snapshots of the
cations at the hydrophobic SAM are taken at the position of the minimum in the PMF of Cs+ at z = 0.75 nm. Snapshots of
[6]
Li+, Na+, and K+ at the hydrophilic SAM.
The change in the number of coordinated water molecules in the first solvation shell also affects the
structure of the first solvation shell, see Figure 3.5.[2] The Na+ cation is surrounded by five water
molecules in a square based pyramidal geometry in the bulk. Close to the interface the Na + cation
solvation shell contains four surrounding water molecules in a trigonal pyramid. The first solvation
shell of the two divalent cations Mg2+ and Zn2+ are formed by six water molecules in the bulk and the
water molecules are arranged in an octahedral geometry. The number of coordinated water
molecules for both divalent cations close to the surface is reduced to five and those are coordinated
in a square pyramidal geometry. In the bulk, the solvation shell of the chloride anion contains six
water molecules, which forms an octahedron coordination shell. When the chloride approaches the
interface, the shell is formed by four water molecules.
21
22
4. Summary
Water/hydrophobic interfaces are important in many biological and environmental processes and in
silico studies provide new insight in the effects of ions near the water/hydrophobic interface.
The order in ion adsorption at the hydrophobic surface is as predicted by the Hofmeister series:
I- > Br- > Cl- > F- and Cs+ > Li+ > K+ > Na+.[5–7,9,10] The large ions are attracted to the surface and have the
largest concentration peaks, whereas the smaller ions are more repelled from the hydrophobic
interface with a decreasing concentration peak. The small Li+ ion is irregular, due to the solvation
shell properties.[6] The divalent cations Mg2+ and Zn2+ are more repelled from the surface than Na+
and both divalent cations form ionic layers with the counter ion.[2] The water molecules of the first
two water layers of the interfacial region are orientate specifically and determine the surface tension
ɣ. The exchange of interfacial water molecules is much lower and the residence time τres is eight time
longer than in the bulk phase.[11] The diffusion coefficient D and τres are correlated by
τ
~ D. Salt
[11]
ions increase the τres and therefore decreases D of interfacial water molecules. The τres of water
molecules in solvation shells is proportional to the strength of the hydrogen bonds. The diffusion
coefficient of the Cl- depends on depends on the counter ion.[2] The change in interfacial tension
decreases by increasing interfacial concentration; Δɣ F- > Cl- > Br- > I- and Zn2+ > Mg2+ > Na+.[2,7,9,10] The
interfacial widths of the hydrophobic/water system depends on the surface and the ion. Solvation
shells, which can change in coordination and structure between bulk and near the surface, are
important in the behaviour and probabilities of the ions.
Although studies show the same trend and reproduce the direct Hofmeister series, the simulated
systems use different setups, leading to different results. The results depend on the model, the force
fields and the used ion concentration and the use of counter ions; ion concentrations are crucial for
reliable results. For a proper description of the system, the force fields and ion concentration are
crucial.
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References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Walrafen, G.E., Fischer, M.R., Hokmabidi, M.S., Yang, W.H., J Chem Phys 1986, 85, 6970.
F. Rodríguez-Ropero, M. Fioroni, J. Comput. Chem. 2011, 32, 1876–1886.
E. S. Shamay, G. L. Richmond, J. Phys. Chem. C 2010, 114, 12590–12597.
R. Vácha, D. Horinek, M. L. Berkowitz, P. Jungwirth, Phys. Chem. Chem. Phys. 2008, 10, 4975–
4980.
D. Horinek, R. R. Netz, Phys. Rev. Lett. 2007, 99, 226104.
N. Schwierz, D. Horinek, R. R. Netz, Langmuir 2013, 29, 2602–2614.
N. Schwierz, D. Horinek, R. R. Netz, Langmuir 2010, 26, 7370–7379.
Chang, T.M., Dang, L.X., Chem. Rev. 2006, 106, 1305–1322.
M. Vazdar, E. Pluhařová, P. E. Mason, R. Vácha, P. Jungwirth, J. Phys. Chem. Lett. 2012, 3,
2087–2091.
C. D. Wick, T.-M. Chang, J. A. Slocum, O. T. Cummings, J. Phys. Chem. C 2012, 116, 783–790.
C. Zhang, P. Carloni, J. Phys. Condens. Matter 2012, 24, 124109.
Jungwirth, P., Tobias, D.J., Chem. Rev. 2006, 106, 1259–1281.
Petersen, P.B., Saykally, R.J., Annu. Rev. Phys. Chem 2006, 333–364.
Horinek, D., Herz, A., Vrbka, L., Sedlmeier, F., Mamatkulov, S.I., Netz, R.R., Chem. Phys. Lett.
2009, 479, 173–183.
Rivera J.L., McCabe, C., Cummings, P.T., Phys. Rev. E 2003, 011603.
Mitrinovic, D.M., Tiknonov, A.M., Li, M., Huang, Z., Schlossman, M.L., Phys. Rev. Lett. 2000,
85, 582.
Goebel, A., Lunkenheimer, K, Langmuir 1997, 13, 369.
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Acknowledgments
On this final page of my literature review, I would like to thank everyone who was involved in my
literature study. I will not mention everyone, but some people I want to thank especially. First of all I
would like to thank Prof. Dr. E.J. Meijer for guiding my literature study and for all discussions about
the model and results. Secondly, I would like to thank Dr. D. Dubbeldam as second reviewer.
Finally I would like to thank my friends and family for being there during the good and bad times of
my literature study; Luctor et Emergo!
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