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J. Phys. Chem. C 2009, 113, 9230–9238
Influence of Residual Silanol Groups on Solvent and Ion Distribution at a Chemically Modified Silica Surface Sergey M. Melnikov,† Alexandra Ho¨ltzel,‡ Andreas Seidel-Morgenstern,†,§ and Ulrich Tallarek*,‡ Max-Planck-Institut fu¨r Dynamik komplexer technischer Systeme, Sandtorstrasse 1, 39106 Magdeburg, Germany, Department of Chemistry, Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany, and Institut fu¨r Verfahrenstechnik, Otto-Von-Guericke-UniVersita¨t Magdeburg, UniVersita¨tsplatz 2, 39106 Magdeburg, Germany ReceiVed: NoVember 8, 2008; ReVised Manuscript ReceiVed: April 7, 2009
We report on molecular dynamics simulations of solvent and ion distribution at a prototypic alkyl-modified silica surface under explicit consideration of residual silanol group activity. The model contains two β-cristobalite silica walls with dimethyloctylsilyl (C8) ligands as the main modification and trimethylsilyl groups for end-capping, grafted at surface densities of 2.95 µmol/m2 and 0.85 µmol/m2, respectively. Residual silanol groups are present at a surface density of 3.8 µmol/m2. The mobile phase consists of water/acetonitrile mixtures over the whole range of volumetric compositions. We have studied the two limiting cases of residual silanol group activity: (i) undissociated silanol groups, and (ii) dissociated silanol groups with sodium ions as counterions. Solvent and ion distribution in the system as well as the orientational arrangement of solvent molecules and the conformation of the bonded phase are presented for both cases of residual silanol activity. We analyze the influence of mobile phase composition on system conformation to illustrate that the presence of residual silanol groups in their ionized, sodium salt form induces larger changes than a variation of the water/acetonitrile mixture. 1. Introduction Reversed-phase liquid chromatography (RPLC) is extensively used today in various industrial and academic fields for the efficient analytical and preparative separation of a wide range of compounds.1,2 Retention of solute molecules (analytes) in RPLC occurs in an extremely thin interfacial region between the solid support structure of the stationary phase, most often composed of porous silica, and the bulk mobile phase, usually water/alcohol or water/acetonitrile mixtures. The chromatographic interphase encompasses the surface of the porous silica support, the alkyl chains of the bonded phase, and the solvent molecules that are present near the silica surface, intercalated between the bonded-phase chains, and adsorbed on the hydrocarbon layer of the bonded phase.3 Various experimental and theoretical approaches4-6 have established the environmental and compositional variables, such as temperature and mobile phase composition, and surface density and chain length of the alkyl ligands, respectively, that influence the morphology of the interphase, which is of primary importance in the chromatographic retention mechanism. Yet despite the attention the subject has received, the underlying retention mechanism in RPLC is not entirely understood. As the thickness of the interfacial layer is on the order of nanometers, an entire comprehension of the retention mechanism in RPLC requires a molecular-scale description of the processes occurring in this layer. The continuum theories fail to describe solvent and ion partitioning at the nanometer scale because of their inherent ignorance of the discontinuity of matter,7,8 whereas treating * Corresponding author. Phone: +49-(0)6421-28-25727; fax: +49(0)6421-28-22124; e-mail:
[email protected]. † Max-Planck-Institut Magdeburg. ‡ Philipps-Universita¨t Marburg. § Otto-von-Guericke-Universita¨t Magdeburg.
atomic interactions explicitly, molecular dynamics (MD) simulations can be utilized for that purpose. Owing to the amazing computational possibilities of modern supercomputers and their relative accessibility as well as numerous available software packages, MD simulations have become a widely used method for studying spatial and dynamic aspects in molecular ensemble evolution.9 Over the last two decades, several groups have focused on the development of adequate molecular models of the retention mechanism in RPLC.10-26 The influence of temperature,10,15,17 alkyl chain length and surface density of the ligands,11,17 and mobile phase composition12-14,19,20 on the conformation and mobility of the bonded phase have been investigated, and free energy profiles for the transfer of simple solute molecules from the mobile to the stationary phase were presented.12,13,19,20 Siepmann and co-workers have provided an excellent summary of previous simulations of RPLC systems20 and revealed a novel, molecular-level insight into the differences in the retention of polar and nonpolar solutes.22 Progress has also been made with molecular-level simulations of stationary phases functionalized with polar end groups,24 with polar-embedded stationary phases,25,26 and with shape-selective stationary phases.16,17,21 In RPLC the dissociation state of residual silanol groups and the type of counterions present in the system is controlled by the composition and pH of the mobile phase. Experimentally, the silanol activity of stationary phases, i.e., the accessibility, type, and dissociation state of the residual silanol groups on the silica surface as well as the degree of end-capping, and how it affects analyte retention is an intensely studied subject.27-30 Several types of interactions are possible between the residual silanol groups and analytes: hydrogen bonding, dipole interaction, and ion exchange. The latter is the predominant mode of interaction with basic compounds, where most of the undesirable
10.1021/jp8098544 CCC: $40.75 2009 American Chemical Society Published on Web 05/01/2009
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Figure 1. Snapshot of the final configuration of the studied RPLC system for the ionic case at a mobile phase composition of 40/60 (v/v) water/ acetonitrile, depicted as a ball-and-stick model alongside the corresponding number density profiles of the solvent molecules (oxygen atoms of water, red line; nitrogen atoms of acetonitrile, blue line) and sodium ions (gray-green line). Atoms and ions in the snapshot are color-coded as follows: oxygen, red; hydrogen of water and silanol, white; carbon and hydrocarbon groups, cyan; silicon, yellow; nitrogen, blue; sodium ions, gray-green. The simulation box is displayed in the y-z plane. The dashed line at the level of the bottom surface silicon atoms indicates z ) 0.
influence of the silanols becomes apparent: They act as cationexchange groups of intermediate strength, i.e., their activity is a function of the mobile phase pH. At around pH 3, most silanols are protonated and therefore do not undergo the ion-exchange interaction with positively charged analytes. As the pH increases, more and more silanols become negatively charged and interact with the analytes. This results in an increase in retention and in peak tailing.1 The role of the silica surface as an integral part of the chromatographic interphase has only rarely been considered in previous simulations of RPLC systems. Sophisticated RPLC models using explicit silica surfaces and solvent molecules have shown how the presence of residual silanol groups on the surface affects the solvent distribution in the interfacial layer.18,20,22-26 In particular, Zhang et al.20 analyzed in detail the structure of the hydrogen-bonding network between water and methanol molecules of the mobile phase with residual silanol groups at the silica surface. So far, charged surfaces and ions have not been included in molecular models of RPLC systems, but Qiao31 examined the distribution of ions and water molecules in his study of electroosmotic flow past a polymer-coated surface. The simulated system included amphiphilic (CH2)17COOH chains grafted onto a solid wall containing additional surface charges, aqueous KCl electrolyte, and an external electrical field. In this contribution we investigate how the dissociation of residual silanol groups affects the structure of the chromatographic interphase. We study a prototype of a reversed-phase alkyl-modified silica surface. Dimethyloctylsilyl (C8) groups as the main modification and trimethylsilyl groups for end-capping were grafted on an explicit silica surface at densities of 2.95 µmol/m2 and 0.85 µmol/m2, respectively, leaving residual vicinal silanol groups on the surface at a density of 3.8 µmol/m2. The surface densities of alkyl ligands, end-capping, and residual silanol groups represent typical values of real-life end-capped C8-bonded phases.1 Mimicking typical experimental conditions, the simulations were performed over a broad range of water/ acetonitrile mobile-phase compositions. Two extreme cases for the residual silanol group activity are considered: In the first case, all surface silanol groups are undissociated, resulting in an uncharged but polar surface; in the second case, all residual silanol groups are deprotonated so that the negatively charged
oxygen atoms on the surface produce a charge density of -0.36 C/m2. To maintain electroneutrality, the system is supplemented with sodium ions equaling the number of residual silanol groups on both surfaces. In the following, we will refer to the two cases of residual silanol activity as the neutral and the ionic case. The first case considers only undissociated silanol groups and the second case only dissociated silanol groups with sodium ions as counterions. This situation resembles cation-exchange chromatography where the negatively charged surface groups are employed in their sodium ion form for the separations.28 We consider the presented results as a first step in our strategy to configure a realistic model of an RPLC system, focusing here on the question of how the dissociation state of residual silanol groups on an alkyl-modified and end-capped silica surface influences the solvent and ion distribution in the chromatographic interphase. 2. Investigated System and Computational Methods The studied RPLC model consists of a mobile phase of water and/or acetonitrile confined between two parallel silica walls with attached bonded phase and residual silanol groups on the surface (Figure 1). Each silica wall is a two-layered slab of β-cristobalite with its (111) surface exposed toward the mobile phase. The (111) surface of β-cristobalite provides dangling silanol groups at a surface density of 7.6 µmol/m2 and has been shown to be a good structural model for the chromatographic silica that is used as stationary support in RPLC columns.32 Water/acetonitrile mobile phases were simulated for six volumetric compositions (v/v): 100/0, 80/20, 60/40, 40/60, 20/80, and 0/100. The size of the simulation box was Lx ) 6.16 nm, Ly ) 7 nm, and Lz ) 9.14 nm. The dimensions were chosen to satisfy the conditions that (i) the simulation box is long enough to contain an area which is free of surface influence, and (ii) that lateral box dimensions are large enough to accommodate the cavities in the bonded phase which naturally occur as a result of the grafting process. Polymer chains (78 C8 ligands and 20 trimethylsilyl groups on each wall) were covalently attached to random positions at the surface avoiding steric overlap, resulting in 98 residual silanol groups per wall. The minimum distance between C8 ligands was 0.5 nm, and the average distance was
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about 0.55 nm. Before placing molecules of the mobile phase into the simulation box, the bonded phase was allowed to relax in vacuum from an initially upright all-trans conformation to the equilibrated conformation.11,18 Solvent molecules were then placed in the simulation box in random positions uniformly over the whole accessible volume between the two walls. A penalty potential was used for the initial placement of water molecules to ensure that both types of solvent molecules, water and acetonitrile, had the same minimum distance to the silica surface (ca. 0.6 nm) at the start of the simulation. For the ionic case, 196 sodium ions were added to the system prior to the solvent molecules and placed at regular intervals in the bulk region of the simulation box. The MD study of this system was performed for a canonical NVT-ensemble using the molecular dynamics package Gromacs 3.3.1.33 Periodic boundary conditions were used in x and y directions. Energy minimization was carried out before the simulation start to remove bad contacts between atoms originating from initial model building. Initial velocities were randomly assigned according to a Maxwell-Boltzmann distribution at a temperature of T ) 300 K. The system was maintained at 300 K by means of a Berendsen thermostat with a coupling constant of 0.1 ps.34 Because of the (quasi) twodimensional geometry of the examined system long-range Coulombic interactions were calculated by the PME algorithm with a pseudo-2D summation.35 This algorithm was applied in combination with introducing empty compartments along the z axis above and beneath the system. For nonbonded interaction potentials a cutoff radius of 1 nm was used. Water was modeled by using the so-called SPC/E model.36 Sodium ions were treated as point-charged Lennard-Jones particles. Acetonitrile was considered as a three-interaction-site linear rigid molecule.37 The united atom approach was used for the methyl and methylene groups of the alkyl chains and end-capping groups, but residual silanol groups were treated by modeling each atom explicitly. Force field parameters for all bonded and nonbonded interactions were taken from the Gromacs database (Gromacs Forcefield),33 with the exception of acetonitrile for which parameters were taken from Gee and van Gunsteren.37 Parameters for a mixed pair of atoms were calculated using the conventional combination rule. The equations of molecular motion were integrated with a time step of 2 fs. After starting from a random conformation, the system was simulated for some time to reach equilibrium, and then data collection followed. In our study, the system was at equilibrium after approximately 300 ps in the neutral case but required about 1.5 ns in the ionic case. The total simulation trajectory was 5 ns in each case (periods of up to 50 ns have been investigated, without revealing further changes). Calculations took about 29 h for a mobile phase of 100% water (32879 atoms in the system) and almost 17 h for a mobile phase of 100% acetonitrile (19721 atoms in the system) using 16 processors of a HP Superdome supercomputer. 3. Results and Discussion A simulation snapshot of the final configuration of the modeled RPLC system (y-z plane) for the ionic case of residual silanol activity is shown in Figure 1 at a mobile phase composition of 40/60 (v/v) water/acetonitrile. The number density profiles of the solvent molecules (oxygen atoms of water, nitrogen atoms of acetonitrile) and sodium ions in the system are shown alongside the molecular model. Because of the intrinsic symmetry of the model, we will describe only the bottom half. Between the stationary support and the bulk mobile phase, three layers can be distinguished in the system: (i)
Melnikov et al.
Figure 2. Number density profiles of solvent molecules and ions in the simulated RPLC system. The left panel (A) shows the neutral case, the right panel (B) the ionic case. For reference, the number density profiles of the bonded phase (yellow) are also shown for the neat solvent cases. Atoms and ions are color-coded as follows: oxygen atoms of water, blue; nitrogen atoms of acetonitrile, green; carbon and silicon atoms of the bonded phase, yellow; sodium ions, red. The water (acetonitrile) fraction in the mobile phase decreases (increases) from top to bottom.
the silica surface containing the siloxane bonds that tether the alkyl chains of the bonded phase to the stationary support, as well as the end-capped and the residual silanol groups (nearsurface region, z < 0.5 nm); (ii) the layer containing the major part of the bonded-phase alkyl chains (core bonded-phase region, 0.5 nm < z < 1 nm); (iii) the interface region (1 nm < z < 1.5 nm) where the distal ends of the hydrophobic alkyl chains meet the bulk mobile phase. Figure 1 already depicts the key characteristics of the interphase structure in the ionic case. Sodium ions and water molecules have high peak densities at and near the silica surface, and water is present alongside acetonitrile in the core bonded-phase region. Acetonitrile has its maximum density in the interface region, layering above the distal ends of the hydrophobic alkyl chains. In the following, we will investigate and analyze the features of the modeled RPLC system for both cases of residual silanol activity in terms of solvent and ion distribution, the orientational arrangement of the solvent molecules, and the conformation of the bonded phase. Additionally, the preferential solvation of sodium ions in mixed mobile phases will be discussed. 3.1. Solvent and Ion Distribution. Solvent and ion distribution in the system are discussed for both examined cases of residual silanol activity over the whole range of mobile phase compositions by the number density profiles presented in Figure 2. Profiles were calculated using a bin width of 0.033 nm and averaged over a time span of 3-5 ns. Because of the intrinsic symmetry of the model, only one-half of the calculated profiles is shown in Figure 2. For reference, the number density profiles of the bonded phase (yellow) are included for the neat solvent cases. The two large peaks in the bonded-phase profiles at z ) 0.28 nm and z ) 0.44 nm represent the end-capping groups, the dimethyl-substituted silicon anchors, and the C1 methylene groups of the C8 chains. C2 and C3 methylene groups cor-
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TABLE 1: Solvent Structure in the Near-Surface Region for the Neutral Casea H2O, %
0D0A
1D0A
2D0A
0D1A
1D1A
2D1A
0D2A
1D2A
2D2A
100 80 60 40 20
-
0.02 0.04 -
0.01 -
0.12 0.04 0.05 0.17 0.19
0.75 0.71 0.80 0.62 0.65
0.04 0.12 0.06 0.12 0.09
0.03 0.02 0.04 0.06 0.05
0.03 0.06 0.05 0.02 0.02
0.01 0.01 -
a The relationship between near-surface water molecules (z < 0.35 nm) and residual surface silanol groups was analyzed according to geometric hydrogen-bonding criteria.38
respond to the two peaks discernible at z ) 0.54 nm and z ) 0.67 nm, respectively. Because of their less restricted movement, hydrocarbon moieties from C4 onward compose a smooth curve devoid of sharp peaks at z > 0.7 nm. 3.1.1. Near-Surface Region (z < 0.5 nm). The near-surface region in the neutral case (Figure 2A) contains peak densities of solvent molecules at all mobile phase compositions. The total solvent number density in this surface layer is nearly constant for all mobile phases (slight fluctuations are statistical). In the absence of water, vacant positions at the silica surface are taken up by acetonitrile molecules. The near-surface solvent peaks are followed by a solvent depletion region of ca. 0.1 nm width around z ) 0.36 nm (neat water) or z ) 0.38 nm (acetonitrilecontaining mobile phases), where the methyl groups from the end-capping and the silicon anchors crowd the available space. Water molecules (z ) 0.22 nm) come closer to the surface than acetonitrile molecules (z ) 0.28 nm), reflecting the stronger electrostatic attraction of residual silanol groups for water molecules as well as the relative bulkiness of the acetonitrile methyl group. According to geometric hydrogen-bonding criteria,38 all surface water molecules are hydrogen-bonded to residual silanol groups, predominantly (71% averaged over all water-containing mobile phase compositions) in a one-donor-oneacceptor (1D1A) configuration (Table 1). Water molecules that reach the silica surface during a simulation remain in a fixed position, while acetonitrile molecules can exchange their positions near the surface with other acetonitrile molecules in close proximity after several hundred picoseconds. A significant fraction of residual silanol groups is isolated, i.e., not engaged in hydrogen-bonding with surface water nor in the vicinity of surface acetonitrile molecules, because of the steric hindrance imposed by the alkyl chains of the bonded phase. In acetonitrile-containing mobile phases, the fraction of solvent-inaccessible surface silanol groups is larger (∼43%) than in neat water (∼21%), reflecting the relative bulkiness of acetonitrile. The presence of solvent-inaccessible silanol groups and their increase at increasing organic solvent content has been observed before by Zhang et al.20 for a C18modified silica surface and a water/methanol mobile phase. In the ionic case (Figure 2B), the near-surface region is strongly populated with sodium ions (large peak at z ) 0.15 nm trailing to a shoulder at z ) 0.36 nm) and water molecules in two high-density layers (peaks at z ) 0.22 nm and z ) 0.4 nm). Most sodium ions reach the charged surface within 1-1.5 ns after the simulation start. A small fraction (around 5%) remains in the bulk region during the whole simulation period, uniformly distributed over the whole bulk space. Two layers of sodium ions can be distinguished in the near-surface region. The majority of sodium ions is located on the same plane as the surface charges (z ) 0.15 nm). Sodium ions of this layer undergo ionic bonding with the charged oxygen atoms of two adjacent silanol groups and are hydrated by one to two molecules of water from the bulk side (water peak at z ) 0.22 nm). A minor fraction of sodium ions is distributed within the
Figure 3. Enlarged part of Figure 1 showing an instant conformation of the near-surface region for the ionic case at a mobile phase composition of 40/60 (v/v) water/acetonitrile. Silicon and oxygen atoms of the β-cristobalite support and the hydrocarbon moieties of the bonded phase are displayed as yellow, red, and cyan sticks, respectively. Oxygen and hydrogen atoms of water are shown as red and white balls, and the negatively charged oxygen atoms of the ionic silanol groups as transparent red balls. Sodium ions of the first type (contact-adsorbed on the surface) are depicted as blue balls, and sodium ions of the second type (with complete hydration shell) as gray-green balls.
interval from 0.3-0.45 nm (shoulder at z ) 0.36 nm). These sodium ions are surrounded by a complete hydration shell (water peak at z ) 0.4 nm). Figure 3 shows a snapshot of sodium ions and water molecules near the charged surface. Contact-adsorbed sodium ions and their accompanying water molecules remain in fixed positions during a simulation run. Sodium ions of the second layer and their hydration shells show some degree of translational motion but retain a fixed distance to the charged silanol groups. The high density of the negative surface charges in combination with steric hindrance from the bonded-phase chains firmly prevents ions and their water shells from leaving the surface region. Contact adsorption of sodium ions to a negatively charged surface and immobility of the adsorbed ions was also reported by Qiao and Aluru in their MD simulations of electroosmotic flow in carbon nanotubes39 and silicon nanochannels.40 They observed two sodium ion density peaks close to the surface, one of them approximately four times larger than the other.40 Ions of the larger peak were closer to the surface than water molecules and were surrounded by water molecules only from the bulk side, while ions of the second peak had a complete hydration shell. Immobility of the adsorbed sodium ions and
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Melnikov et al.
Figure 4. Snapshots of the simulated RPLC system showing a top view onto the hydrocarbon layer and the silica surface (solvent molecules and ions are not shown) for the neutral case at a 100/0 (v/v) water/acetonitrile mobile phase (A) and at a 60/40 (v/v) water/acetonitrile mobile phase (B), and for the ionic case at a 100/0 (v/v) water/acetonitrile mobile phase (C). Atoms are color-coded as follows: oxygen, red; hydrogen (of silanol), white; hydrocarbon group, cyan; silicon, yellow.
their surrounding water molecules persisted even at applied external electrical field strengths of 0.55 V/nm (at a surface charge density of -0.285 C/m2). 3.1.2. Core Bonded-Phase Region (0.5 nm < z < 1 nm). For all acetonitrile-containing mobile phases, the core bonded-phase region in the neutral case (Figure 2A) is populated nearly exclusively by acetonitrile molecules (shoulder at z ) 0.8 nm). Even a small fraction of acetonitrile in the mobile phase drives water molecules from this region, because acetonitrile molecules interact favorably with the hydrophobic alkyl chains, while acetonitrile-water interactions that could pull water molecules into this region are weak.13 The solvent density profile for the neat water case is very similar to that of neat acetonitrile. In the core bonded-phase region, solvent molecules reside in cavities in the hydrocarbon layer that result from clustering of the bonded-phase alkyl chains.16,17,21,30 In the absence of organic solvent, cavities in the hydrocarbon layer are filled with hydrogen-bonded clusters of water molecules. Because of repulsion from the hydrophobic chains, water maintains a distance of ca. 0.35 nm to the cavity walls. Figure 4 shows a top view onto the hydrocarbon layer for neat water and for a mixed mobile phase (Figures 4A and 4B, respectively). In neat water there are fewer but larger cavities in the hydrocarbon layer compared to acetonitrile-containing mobile phases. Because of the large dimension of the cavities compared to the small size of water molecules, water in these cavities behaves like in bulk solution, maintaining a tetrahedral coordination. During a simulation, water molecules from the bulk can reach the silica surface only through cavities. In acetonitrile-containing mobile phases, the cavities are filled with acetonitrile molecules; therefore, only those water molecules reach the surface that were placed initially in its vicinity. Previous simulations of the more hydrophobic C18 chains in neat water found the core bonded-phase region to be solvent-depleted,14,15,18 probably because the highly disordered and partially backfolded C18 chains form a hydrocarbon layer at the interface that is largely impenetrable to water. In a previous simulation of a C8-bonded phase, penetration of water molecules up to 0.6 nm into the bonded-phase in neat water and water/acetonitrile 70/30 (v/v) was observed, but this model did not include an explicit silica surface.13 In the ionic case (Figure 2B) a significant amount of water is present in the core bonded-phase region at all mobile phase compositions, decreasing the amount of acetonitrile in this region compared to the neutral case. The top views shown in Figure 4
reveal that cavity formation in the ionic case (Figure 4C) results in a larger accessible surface area compared to the neutral case. The presence of water molecules in the hydrophobic cavities is supported by a hydrogen-bonded network of transient water clusters, connecting water molecules in the cavities with surface and bulk water. Acetonitrile molecules in this region are accumulated in one big slab which is penetrated by the temporal water aggregates (fingers and bridges). 3.1.3. Interface Region (1 nm < z < 1.5 nm). In the interface region, both systems show a similar behavior (Figure 2). Acetonitrile is preferentially enriched here, forming a layer of several angstroms width above the hydrocarbon bonded phase.13,24 In the neutral case, acetonitrile (maximum peak density at z ) 1.12-1.25 nm) is present from a volume fraction of 40% onward with almost the bulk density of the neat solvent, relegating the majority of water molecules to the bulk. With increasing acetonitrile content in the mobile phase, water is driven further outward to the bulk region, extending the interface width. In previous simulations of RPLC systems partitioning of the aqueous/organic mobile phase at the hydrophobic interface was observed for a C18-bonded phase in water/methanol,12,20,23-26 and for a C8-bonded phase in water/methanol and in water/ acetonitrile.13,23 Solvent partitioning at the hydrophobic interface and the retreat of water toward the bulk region at increasing acetonitrile content in the mobile phase also persist in the ionic case (Figure 2B). The acetonitrile density profile is shifted by 0.15 nm toward the bulk region (maximum peak density at z ) 1.3-1.4 nm), because the high density of sodium ions and water molecules at and near the charged surface drives acetonitrile molecules toward the bulk. Solvent partitioning is also less pronounced than in the neutral case because of the higher water and consequently lower acetonitrile density in the core and interface regions. Thus, while the silanol group activity generally determines the solvent distribution at the silica surface, the presence of charged silanol groups changes not only the solvent distribution at the silica surface drastically but also influences solvent distribution and partitioning in the core bonded-phase and the interface region. 3.2. Solvent Orientation. The orientational arrangement of the solvent molecules in the modeled RPLC system was probed by the cosine of the dipole-wall angle β, defined as the angle between the dipole vector of the solvent molecules and the surface normal pointing toward the bulk.13 Distributions of cos(β) for both solvents along the z axis are depicted in Figure
Solvent and Ion Distribution at a Silica Surface
Figure 5. Orientation of solvent molecules in the simulated RPLC system for the neutral (A and B) and the ionic case (C and D) at a mobile phase composition of 60/40 (v/v) water/acetonitrile. Distributions of cos(β), the cosine of the angle between the surface normal and the solvent dipole vector, are shown for water (blue bars, A and C) and acetonitrile (red bars, B and D) molecules. The corresponding number density profiles (atoms/nm3) for the oxygen atoms of water (blue lines) and the nitrogen atoms (green lines) and methyl groups (violet lines) of acetonitrile are also given for reference.
5. We observed for both examined cases of residual silanol activity that the mobile phase composition influences only the solvent density at a particular location but not the solvent orientation per se. Therefore, results are presented at one selected mobile phase composition chosen as 60/40 (v/v) water/acetonitrile. At far distance from the interfacial region (z > 2.5 nm) all orientational arrangements disappear, as there is no preferred molecular orientation in the bulk. In the neutral case (Figures 5A and 5B), the most noticeable feature is demonstrated by the acetonitrile molecules (Figure 5B). A sharp rise in the distribution curve toward positive values of cos(β) at z < 0.5 nm indicates that the majority of acetonitrile molecules approaches the polar surface silanol groups with their nitrogen end. Separate number density profiles for the nitrogen atom (green, peak at z ) 0.28 nm) and the methyl group of acetonitrile (violet, peak at z ) 0.5 nm) also given in Figure 5B reveal a minor fraction (ca. 10%) of surface acetonitrile molecules with the opposite orientation. Water molecules of the near-surface layer around z ) 0.22 nm (Figure 5A) are inclined to layer with their molecular planes over the surface, sampling dipole-wall angles between approximately 70° and 120°. Solvent molecules in the interface and toward the bulk region show only a slight tendency for orientation: acetonitrile molecules (z > 1 nm) for pointing their hydrophobic methyl groups toward the distal ends of the alkyl chains; the few water molecules present in the interface region for pointing with their dipole moment toward the surface. In the ionic case (Figures 5C and 5D), the presence of negative charges and sodium ions at the surface evidently imposes far-reaching restrictions on the arrangement of the solvent molecules, particularly on the water dipoles (Figure 5C). Water molecules of the near-surface region (at z ) 0.22 nm and z ) 0.4 nm) are oriented with their hydrogen atoms toward the surface and this arrangement only gradually decreases to isotropic distribution deep in the bulk region. Acetonitrile (Figure 5D) displays an antiparallel arrangement: molecules closest to the near-surface region (0.4 < z < 0.7 nm) are oriented with their nitrogen ends toward the surface as in the neutral case, whereas in the intermediate layer (0.8 < z < 1.25 nm) the majority of acetonitrile molecules exhibits opposite orientation. Acetonitrile molecules at and beyond the interface region (z >
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Figure 6. Conformation of the bonded phase in the simulated RPLC system. Number density profiles (atoms/nm3) of the bonded phase in the neutral (A) and the ionic case (B). The color-code refers to the mobile phase composition of 100/0 (gray), 80/20 (blue), 60/40 (red), 40/60 (green), 20/80 (violet), and 0/100 (black) water/ acetonitrile (v/v).
1.25 nm) are again, if to a lesser degree, preferentially oriented with their nitrogen ends toward the surface. The presence of water in the core bonded-phase and interface regions also enforces the orientation of acetonitrile molecules in the ionic case, while in the neutral case the polar silanol groups can only influence solvent dipoles near the surface. 3.3. Conformation of the Bonded Phase. The conformation of the bonded-phase chains was assessed in terms of the number density profiles, the end-to-end distance Rend between the silica surface (z ) 0) and the terminal methyl groups C8, the chainwall normal angle χ (defined as the angle between the vector pointing out from a coordinate of the first methylene group C1 toward a coordinate of the terminal methyl group C8 and the surface normal pointing toward the bulk),13 and the diffusional mobility of the terminal C8 groups. The number density profiles of the bonded phase are generally very similar for both cases of residual silanol activity (Figure 6). In the ionic case (Figure 6B) the bonded-phase profiles are more structured in the distal region (three shoulders are discernible at z > 0.7 nm) and extended by ca. 0.15 nm compared to the neutral case (Figure 6A). In both cases of residual silanol activity, the number density profiles in neat water differ from those of acetonitrile-containing phases indicating a slight shrinking of the alkyl chains in neat water. The effect of a purely aqueous mobile phase on the bondedphase conformation is also reflected in the probability distributions of cos(χ) and Rend shown in Figure 7. The curves for neat water (gray lines) are shifted from the curves of the acetonitrilecontaining mobile phases, featuring a larger chain-wall normal angle and shorter end-to-end distances. In the neutral case (Figures 7A and 7B), the cos(χ) probability distributions (Figure 7A) reflect a bonded-phase conformation with largely nonuniform tilt angle, in which the alkyl chains are mostly bent toward the surface with a chain-wall angle
