The Hofmeister series

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 1

Cover figure: N. Schwierz, D. Horinek, R. R. Netz, Langmuir 2010, 26, 7370 7379 2

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 3

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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. 5

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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 7

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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. 9

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. 10

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 11

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. 12

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 CCl 4, 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. 13

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 10 1 nm 3 to 5.8 x 10 2 nm 3 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 +, Zn 2+, Mg 2+ 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. 14

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: 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 3.1. 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) Figure 3.1: a) Density profile [9] and b) Potential of mean force profile [7] 15

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 - Schwierz et al [7] F -, Cl -, I - Wick et al [10] Cl -, Br -, I - Horinek et al [5] F -, Cl -, Br -, I - 3.1.2. 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 Mg 2+ and Zn 2+ 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. 16

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 τ res. The diffusion coefficient D and τ res are correlated by increase in τ res causes a decrease in D. The diffusion coefficient is given by equation (1): ~ D. An (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 τ res of interfacial water molecules is eight time longer than that of bulk water molecules. [13] The addition of salts like KCl, NaCl, MgCl 2 and ZnCl 2 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 τ res 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. 17

3.2.2. Water in first solvation shell The τ res 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 τ res and a low exchange of water molecules from the solvation shell. The diffusion coefficient is correlated to the τ res and the preferences to the interface. The weak solvation shell of Na + results in a high compared to the diffusion coefficients of Zn 2+ and Mg 2+ ; ~ 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 MgCl 2 and ZnCl 2 systems; ~ 8 and ~ 3. [2] The chloride remains in the second solvation shell and forms partial ion pairs with Na + and Mg 2+, in contrast to Zn 2+, which has the strongest solvation shell and does not form an ionic couple with Cl -. This explains the difference in diffusion coefficients. and 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): where p x, p y and p z are the diagonal components P xx, P yy and P zz of the pressure tensor and L z 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 Zn 2+ > Mg 2+ > 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 Mg 2+ and Zn 2+ 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. (2) 18

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) g IW (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 n c (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 n 1 = n c (r 1 ), where r 1 is the position of the first minimum of the bulk RDF. The RDF g IW (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 g IW (r) of the anions F, Cl, and I (A) and the cations Li +, Na +, K +, and Cs + (B) in bulk. The dashed vertical line in (B) denotes the first maximum in the radial distribution function of Cl for comparison. [6] Figure 3.4 Cation coordination number in the first solvation shell n 1 as a function of the distance z from the CH3 surface. n 1 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 the ion surface PMF. [6] 19

Figure 3.5: Snapshots of the hydration shell structure of the Na + (GROMOS force field) (a), Na + (KB force field) (b), Mg 2+ (c), Zn 2+ (d), and Cl - (e) ions in the bulk water phase (left) and in the interface (right). [2] 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 Li+, Na+, and K+ at the hydrophilic SAM. [6] 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 Mg 2+ and Zn 2+ 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

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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 Mg 2+ and Zn 2+ 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 τ res is eight time longer than in the bulk phase. [11] The diffusion coefficient D and τ res are correlated by τ ~ D. Salt ions increase the τ res and therefore decreases D of interfacial water molecules. [11] The τ res 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 Zn 2+ > Mg 2+ > 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. 23

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References [1] Walrafen, G.E., Fischer, M.R., Hokmabidi, M.S., Yang, W.H., J Chem Phys 1986, 85, 6970. [2] F. Rodríguez-Ropero, M. Fioroni, J. Comput. Chem. 2011, 32, 1876 1886. [3] E. S. Shamay, G. L. Richmond, J. Phys. Chem. C 2010, 114, 12590 12597. [4] R. Vácha, D. Horinek, M. L. Berkowitz, P. Jungwirth, Phys. Chem. Chem. Phys. 2008, 10, 4975 4980. [5] D. Horinek, R. R. Netz, Phys. Rev. Lett. 2007, 99, 226104. [6] N. Schwierz, D. Horinek, R. R. Netz, Langmuir 2013, 29, 2602 2614. [7] N. Schwierz, D. Horinek, R. R. Netz, Langmuir 2010, 26, 7370 7379. [8] Chang, T.M., Dang, L.X., Chem. Rev. 2006, 106, 1305 1322. [9] M. Vazdar, E. Pluhařová, P. E. Mason, R. Vácha, P. Jungwirth, J. Phys. Chem. Lett. 2012, 3, 2087 2091. [10] C. D. Wick, T.-M. Chang, J. A. Slocum, O. T. Cummings, J. Phys. Chem. C 2012, 116, 783 790. [11] C. Zhang, P. Carloni, J. Phys. Condens. Matter 2012, 24, 124109. [12] Jungwirth, P., Tobias, D.J., Chem. Rev. 2006, 106, 1259 1281. [13] Petersen, P.B., Saykally, R.J., Annu. Rev. Phys. Chem 2006, 333 364. [14] Horinek, D., Herz, A., Vrbka, L., Sedlmeier, F., Mamatkulov, S.I., Netz, R.R., Chem. Phys. Lett. 2009, 479, 173 183. [15] Rivera J.L., McCabe, C., Cummings, P.T., Phys. Rev. E 2003, 011603. [16] Mitrinovic, D.M., Tiknonov, A.M., Li, M., Huang, Z., Schlossman, M.L., Phys. Rev. Lett. 2000, 85, 582. [17] Goebel, A., Lunkenheimer, K, Langmuir 1997, 13, 369. 25

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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! 27

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