Effects of Functional Groups and Ionization on the Structure of

Aug 27, 2014 - *E-mail: [email protected]., *E-mail: [email protected]. ... We study both neutral and charged end groups for two different chain ...
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Effects of Functional Groups and Ionization on the Structure of Alkanethiol-Coated Gold Nanoparticles Dan S. Bolintineanu,* J. Matthew D. Lane, and Gary S. Grest* Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: We report classical atomistic molecular dynamics simulations of alkanethiol-coated gold nanoparticles solvated in water and decane, as well as at water/vapor interfaces. The structure of the coatings is analyzed as a function of various functional end groups, including amine and carboxyl groups in various ionization states. We study both neutral and charged end groups for two different chain lengths (9 and 17 carbons). For the charged end groups, we simulated both mono- and divalent counterions. For the longer alkanes, we find significant local bundling of chains on the nanoparticle surface, which results in highly asymmetric coatings. In general, the charged end groups attenuate this effect by enhancing the water solubility of the nanoparticles. On the basis of the coating structures and density profiles, we can qualitatively infer the overall solubility of the nanoparticles. This asymmetry in the alkanethiol coatings is likely to have a significant effect on aggregation behavior. Our simulations elucidate the mechanism by which modulating the end group charge state can be used to control coating structure and therefore nanoparticle solubility and aggregation behavior.

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INTRODUCTION There has been significant interest in nanoparticles due to their size-dependent properties, and the ability to tune optical,1−3 mechanical,3−5 and thermal properties6,7 of materials by the addition of nanoparticles. Aggregation is often a significant concern for bulk handling of nanoparticle-based materials. In many applications, coating nanoparticle surfaces with short polymer chains has proven to be effective in promoting dispersion. Conversely, self-assembly of nanoparticles into complex superstructures is often desirable, and can be controlled with similar strategies. Fundamental studies of nanoparticle surface coatings are therefore essential to understanding the solubility, dispersion, and self-assembly properties relevant to many nanoparticle-based technologies. Numerous combinations of nanoparticle core materials and coatings have been reported. The nanoparticle core is often composed of a relatively hard material, such as a metal, metal oxide, or solid polymer (e.g., gold, silver, copper, silica, various semiconductors and metal oxides, and cross-linked polymers). Coatings can consist of an additional hard material layer, as in applications where optoelectronic or magnetic properties of individual particles are of interest, or a soft material layer, typically consisting of a polymer, biologically functional material, or short organic molecule. In the present work, we focus exclusively on nanoparticles composed of a metallic gold core surrounded by a uniform coating of short alkanethiol molecules. The gold−alkanethiol system has been studied extensively, both in the context of self-assembled monolayers (SAMs) on flat substrates8−13 and in the context of alkanethiolstabilized nanocrystallite or nanoparticle systems.14−18 There© 2014 American Chemical Society

fore, alkanethiol-coated gold nanoparticles represent an excellent model system for the study of core/shell nanoparticles in general. Additionally, many technological applications have emerged for this system, including various optoelectronic applications,19 biochemical sensors,19−21 nanofiltration,22 medical imaging,23 and drug delivery.18,24 Alkanethiol-coated gold nanoparticles have been studied both experimentally and computationally. For a detailed review of experimental studies, including synthesis, characterization, and applications, the interested reader is referred to the review by Daniel and Astruc.25 While significant computational effort based on quantum/DFT calculations has focused on the adsorption of thiols on gold surfaces,26−28 we are not aware of work that has reached the length scales of an entire nanoparticle. Thus, we focus here exclusively on classical simulations. Some of the earliest molecular dynamics simulations of gold nanocrystallites were reported by Luedtke and Landman,17 who studied the self-assembly of alkanethiol molecules on gold nanoparticles. Subsequent simulation works have investigated various aspects of alkanethiol ligand selfassembly,29−32 coating stability,33 chain structure,30,31,34 dynamics,33,34 as well as interaction and assembly of multiple particles.29,35−37 Many of these studies employed coarsegrained or united atom models to represent the nanoparticle and implicit methods to represent the solvent. Atomistic simulations of passivated gold nanoparticles in supercritical Received: July 15, 2014 Revised: August 26, 2014 Published: August 27, 2014 11075

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CO238,39 and in water40,41 have also been reported. Of particular interest here is the recent work of Yang and Weng,40 who simulated alkyl-passivated gold NPs with various neutral end groups (COOH, NH2, CH3, and OH), and of Heikkilä et al.,42 who simulated 2 nm gold NPs coated with alkyl chains terminated with COO− and NH3+ end groups. In the work of Yang and Weng, significant focus was placed on the orientation of water molecules near the alkyl−water interface; Heikkilä et al. also reported results on water orientation, and focused on a thorough comparison to continuum electrostatic theory. Additionally, Sandberg et al. studied the conformations of spherical polyelectrolyte brushes with ionized chains and explicit counterions in salt-free solutions.43 Lane and Grest carried out molecular dynamics simulations of alkanethiolcoated gold nanoparticles with CH3 and COOH end groups.16 Despite uniform and symmetric grafting densities, they observed spontaneous asymmetry of coating structures depending on alkane chain length and particle size.16 We use the term “asymmetry” here and in the remainder of the work to describe coatings that have significant azimuthal and/or polar structural variations, i.e., are not spherically symmetric. This has significant implications for solubility and self-assembly behavior. We extend the work of Lane and Grest to analyze the effects of charged ligand end groups on coating structures in similar nanoparticles. In particular, we study alkanethiol-coated gold nanoparticles where the CH3 and COOH end groups of the alkane chains are substituted with carboxylic acid and amine groups in various ionization states. Since these ionization states can be controlled in experiments by adjusting solution pH, understanding their effects on coating structures may provide a simple method of controlling nanoparticle coating structure and therefore aggregation behavior in solution. We have selected amine and carboxylic acid end groups in order to test the effects of both cationic and anionic groups that are readily ionized in an aqueous environment. Furthermore, these moieties are commonly synthesized, well-studied, and prevalent in many applications, including biological systems.18 We have carried out simulations of individual gold nanoparticles with diameters of 4 nm coated with alkanethiol molecules, both in bulk water and at water/vapor interfaces. The ligand molecules investigated were HS(CH2)9X and HS(CH2)17X, where X is one of the following end groups: CH3, COOH, COO−Na+, COO−Ca2+, NH2, and NH3+Cl−. Experimentally, upon physisorption, the alkanethiol molecule binds to the Au nanoparticle through the sulfur head atom and loses the mercaptan hydrogen atom, transforming itself into an alkanethiolate.44 In all cases, coatings were fully homogeneous with respect to ionization state and alkane chain length. In addition, the uncharged cases (CH3, COOH, and NH2) were simulated in bulk decane to investigate the effects of solvent interactions. We observe significant coating asymmetry, particularly for nanoparticles coated with S(CH2)17X ligands solvated in water. The asymmetry is attenuated by charged end groups (X = COO−Na+, COO−Ca2+, or NH3+Cl−), consistent with the propensity of charged S(CH2)9X particles to enter the water phase in water-vapor simulations. Decane-solvated particles also show some coating asymmetry, which we show to be largely due to crystallization of the ligands. The remainder of the paper is organized as follows: In the Methods section, we discuss construction of the simulation systems and the choice of simulation parameters. In the Results section, we present a detailed analysis of coating structures in

bulk solvents, at elevated temperatures and at water/vapor interfaces.

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METHODS

The nanoparticle starting states were constructed using a protocol similar to that described by Lane and Grest.16 The gold atoms in the nanoparticle core were not represented explicitly, since neither solvent nor alkane chains are expected to interact in a significant way with the gold core. Instead, thiol sulfur atoms were placed at the carbon positions of a C-240 fullerene structure scaled to a diameter of 4 nm. This yields a homogeneous surface coverage of approximately 4.8 chains/nm2 to match experimental estimates of 4.67 chains/nm2 for full coverage of gold single crystals,11 which is also in good agreement with other reported values.14,45 The sulfur atoms are constrained to move together as a rigid body throughout the simulation; although diffusion and rearrangement of adsorbed sulfur atoms along the gold surface does occur in real systems, excluding this effect in the present case of full coverage does not affect coating structure, since the grafting coverage is fully saturated and homogeneous in all cases. To prevent any artificial penetration of solvent or alkane chains past the sulfur atoms, a Lennard-Jones particle with σ on the order of the gold core diameter was placed at the particle center. Alkane chains were attached to the sulfur atoms and initially placed in an outward radial orientation in all-trans configurations. The end groups were either all ionized or all neutral, corresponding to different extremes in pH relative to the isoelectric points of the acidic and basic groups. Simulating partially ionized coatings (i.e., having some end groups charged and others neutral in the same nanoparticle) would require accounting for both dynamic ionization as well as ligand mobility along the surface; these complications are beyond the scope of the present work. In charged systems, sufficient counterions were inserted to neutralize the overall charge of the systems, with no additional ions present (i.e., zero salt concentration). Counterions were initially placed within 1.2 nm of the end group along the particle surface normal. The OPLS-AA classical atomistic force field46 was used for all simulations, with the TIP4P/2005 water model.47 All systems were allowed to equilibrate for 0.1 ns in implicit solvent (Langevin dynamics at 300 K) with a damping constant of γ = 10 ps−1 prior to being solvated. The dielectric constant for the implicit solvent simulations was set to 80 for the water systems and 2 for the decane systems. Once solvated in atomistic solvent (water or decane), the dielectric constant was set to unity. Cubic simulation boxes with decane and water solvent were prepared and allowed to equilibrate prior to the addition of the nanoparticles. Spherical cavities slightly larger than the nanoparticles were created by removing solvent molecules in those regions. Nanoparticles were placed in the resulting cavities, and any coordinated counterions were placed at the locations resulting from the initial implicit solvent equilibration. All bulk systems were initially equilibrated at a constant pressure of P = 100 atm to eliminate cavities at the solvent−ligand interface. The final simulation box size was approximately 16 nm in all cases, whereas the largest dimensions of the short- and long-chain-coated nanoparticles were 8 and 9 nm, respectively. Production runs were subsequently carried out at constant volume, while maintaining the temperature using a Langevin thermostat with a damping constant of γ = 1 ps−1. Most of the systems presented here were simulated at temperature T = 300 K, and in a few cases, we also studied T = 400 and 500 K. The attractive (r−6) term in the Lennard-Jones interaction as well as the electrostatic interactions were calculated using a particle−particle particle−mesh algorithm.48 As a result, the Lennard-Jones interaction becomes a fully long-range potential.49 Interactions closer than 1.2 nm are calculated in real space; those outside this range are calculated in reciprocal Fourier space with a precision of 10−4. The repulsive (r−12) Lennard-Jones interaction is truncated at 1.2 nm. A simulation time step of 1 fs was used in all cases, with SHAKE constraints50 for the bond lengths and angles in water molecules. Simulations were performed using the LAMMPS software package.51 11076

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Figure 1. Four nm nanoparticles coated with S−(CH2)9X and S−(CH2)17X in water at T = 300 K (water molecules not shown for clarity). Sulfur atoms are shown in yellow, carbon in cyan, oxygen in red, nitrogen in blue, hydrogen in white, and counterions for all charged cases in green. In order to investigate the effect of various end groups on solubility, we also carried out simulations of nanoparticles at water/vapor interfaces. The configurations of the nanoparticles and solvent at the end of the bulk water simulations were used as starting states. Water molecules were removed if their z-coordinates were between the nanoparticle center z-coordinate and the closest simulation box edge in the z-direction (i.e., water molecules located approximately in the top or bottom half of the simulation box). This procedure worked well for the uncharged systems which are not soluble and remain at the surface. For the charged systems which are absorbed into the water, the film was too thin, so we studied thicker systems by compressing the system in the xy plane. For the charged systems, the counterions in the newly created vapor phase were retained. They quickly condensed on the nanoparticle end groups or entered the aqueous phase. The ionization state of all the end groups was maintained even in the vapor in order to create the most soluble coating. For nanoparticles that are sufficiently soluble to enter the water phase, the fully ionized state is realistic (corresponding to low pH for carboxylates and high pH for amines). For nanoparticles that are not soluble and remain on the surface, this fully ionized state is not a physically likely configuration but is the configuration which most promotes nanoparticle miscibility. We can therefore safely conclude that the inability of the particles to enter the aqueous phase would persist even if partial ionization were fully modeled. In order to ensure that the nanoparticles were not trapped at the water/vapor surface in a metastable or local free energy minimum, we initially allow the nanoparticles to reach a stable configuration with respect to the water and then apply a harmonic constraint between the nanoparticle and the center of the water film to drive the nanoparticle into the water film. After equilibrating the system for a few nanoseconds, the constraint is then released, and the nanoparticle is allowed to reach a new stable state. This approach, while intrusive to the dynamics, assures that particles at the liquid surface are not simply trapped in a metastable state over the time scales of our simulations.

Figure 2. Four nm nanoparticles coated with S−(CH2)9X and S− (CH2)17X in decane (not shown for clarity) at T = 300 K. The color scheme is the same as that in Figure 1.

by considering hydrophobicity and geometric constraints: in combination with ligand−ligand and solvent−ligand interactions, the packing of the chains is dictated by the free volume available to each chain. This in turn is a function of the curvature (radius) of the particle core and the length of the chains. In the case of the S(CH2)9 chains, the free volume is sufficiently close to the volume of the chain, so that a symmetric, uniformly distributed arrangement that maintains hydrophobic regions of the chains in contact with one another is possible. For the longer S(CH2)17 chains, the free volume is much larger, and a uniform, symmetric arrangement would result in significantly more available volume for the coating ligands, which would expose the hydrophobic regions to water. As a result, the chains bundle so as to minimize the area exposed to water. Since the grafted thiol ends are constrained to the nanoparticle surface, the size of the bundles is limited, which gives rise to the localized, discrete bundles observed. A more detailed analysis of free volume and packing in similar systems was given by Lane and Grest.16 We also note recent experimental evidence suggestive of similar asymmetry.52 In order to quantify the extent of asymmetry of the coatings observed, we employ an analysis similar to that reported by Lane and Grest,16 with some minor modifications. We define asymmetry based on the same general measure, namely, the

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RESULTS Nanoparticles in Bulk Solvents. Figures 1 and 2 present a visual summary of the coating structures from simulations in bulk solvents at T = 300 K. Figure 1 shows results from the water-solvated systems after 4−8 ns of simulation time. The most significant qualitative difference in the coating structures arises from the differences in chain lengths. For the 9-carbon alkane chains, coating structures are relatively symmetric, with most chains aligned along the surface normal. In contrast, the 17-carbon alkane chains consistently form localized bundles that result in highly asymmetric coatings. This can be explained 11077

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Table 1. Measures of Coating Asymmetry for All Nanoparticles at T = 300 Ka Systems Solvated in Water

a

X=

CH3

NH2

S(CH2)9X S(CH2)17X

33.8% 63.5%

23.6% 55.8%

NH3+ Cl− 16.5% 46.0% Systems Solvated in Decane

COOH

COO− Na+

COO− Ca2+

32.1% 57.2%

17.8% 36.5%

21.1% 52.5%

X=

CH3

NH2

COOH

S(CH2)9X S(CH2)17X

28.4% 52.7%

25.0% 50.3%

26.5% 53.5%

Larger values correspond to higher asymmetry. See text for an explanation.

groups, which leads to localized but relatively stable clusters of alkane chains within the coating. An example of such a cluster is shown in Figure 3a. We note that, in order to capture this

standard deviation in the coating density distribution along angular coordinates relative to the particle. The implementation here differs slightly, in that we randomly select unit vectors with the origin at the particle center, and compute the total number of carbon atoms within a conical region centered about each unit vector. This can be implemented in a computationally expedient manner, where the dot product of the randomly generated unit vector and a vector connecting each atom to the particle center is compared to the cosine of the angle that defines the conical region. This angle, which must be the same for all systems tested, should be selected to be large enough so that most trials (i.e., randomly generated unit vectors) yield a conical region that contains a total number of carbon atoms equivalent to at least one ligand molecule but small relative to the angular extent of the coating. While the choice is still relatively arbitrary and affects the absolute values of asymmetry obtained, it does not generally change the relative rankings of the systems. Table 1 shows the standard deviations in the carbon density scaled by the mean value for all systems investigated. Larger values are indicative of higher asymmetry. The values shown correspond to an angle of 18° defining the conical region; we also tested an angle of 9°, and found the same relative ranking of asymmetry (data not shown). For all cases, data are averaged over several hundred configurations separated by 10 ps near the end of the simulations, with 1000 randomly selected unit vectors per time point. As expected, the measured asymmetry of the long-chain alkane coatings is significantly higher in all cases. Several other features of the coating structures are worth noting. First, the coating structures corresponding to charged end groups tend to be more symmetric, regardless of chain length. The effect is somewhat apparent in the visual representations in Figure 1, and is quantitatively confirmed in Table 1. This is not surprising, since charged end groups repel one another, which attenuates the tendency of the chains to form large bundles. However, since counterions readily condense at the interface and largely neutralize the charges, the effect is not strong enough to result in completely symmetric, uniform coatings. Furthermore, the presence of charged moieties (end groups and their associated counterions) also increases the overall hydrophilic character of the chains, which allows more water penetration, leading to higher solubilities and more uniform coatings. Following the same argument, one might expect Ca2+neutralized systems to be more asymmetric than Na+neutralized systems, due to the higher ionic strength of the solution near the interface. However, visual inspection of the coating structures in Figure 1 and computed asymmetries in Table 1 suggest the opposite. This is a result of the strong tendency of Ca2+ to be coordinated by multiple carboxylate

Figure 3. Examples of localized clustering of ligands caused by (a) hydrogen bonding in S(CH2)17COOH solvated in decane and (b) coordination to Ca2+ in S(CH2)17COO−Ca2+ solvated in water. Sulfur atoms are shown in yellow, oxygen in red, carbon in light blue, hydrogen in white, and calcium in green.

effect, explicit atomistic representations of the counterions and end groups are necessary. While the Ca2+ systems exhibit more localized bundling compared to the Na+ systems, they are still less asymmetric than the nonionized COOH systems. Therefore, although the divalent ion coordination does tend to increase the coating asymmetry, the effect is small compared to the increased chain dispersion caused by solvation of the ionized end groups. Comparing the structures of coatings between NH2 and COOH end groups in water, we note a slight tendency for higher asymmetry in COOH. The differences are small, and likely due to small variations in relative hydrophobicity and electrostatic character of these end groups. Although hydrogen bonding between end groups leading to localized clustering can potentially occur, the interaction is very weak in an aqueous environment, and rarely seen in visual inspection of the structures. Similarly, comparing monovalent NH3+Cl− and COO−Na+ end groups shows only slight differences in coating asymmetries. Finally, the CH3 terminated ligands give rise to the most asymmetric coatings for both the short- and longchain cases, as expected given the hydrophobic nature of this group. Overall, we can conclude that specific details of end group chemistry are secondary in their effects on coating structure as compared to chain length and overall electric charge of the end groups and their associated counterions. To investigate the effects of solvent interactions, we also simulated nanoparticles with neutral end groups solvated in decane. The resulting coating structures are depicted in Figure 2. Significant asymmetry in the coating structure is still 11078

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apparent but is clearly of a different nature than in the watersolvated particles. The earlier explanation of asymmetry in terms of geometric constraints to minimizing alkane-solvent exposure no longer applies, so it is not surprising that different structures with asymmetries dictated by a different mechanism are observed in decane. The longer (CH2)17 alkane chains still appear bundled, but the bundles are smaller and distributed evenly along the particle surface. For COOH and NH2, this is likely a result of strong hydrogen bonding among end groups, which is much more prominent in a nonaqueous, low-dielectric environment. An example of hydrogen bonding is shown in Figure 3b. In the case of the shorter S(CH2)9 chains, geometric constraints lead to more symmetric coatings than S(CH2)17 systems, but some localized bundling is still observed, again as a result of hydrogen bonding. In both the short- and long-chain cases, there are no significant differences between the NH2 and COOH end groups. The CH3 groups show comparable asymmetries, but the localized bundling appears to be less pronounced in the S(CH2)17 case, which is consistent with the inability of the CH3 end group to form hydrogen bonds. Simulations at Elevated Temperatures. All systems discussed so far were simulated at 300 K, which is close to the melting point of the ligands (for reference, the melting point of octadecane is 303 K, and the melting point of octadecanoic acid is 343 K; however, the effects of confinement on a curved surface will certainly change the melting point of the ligands). As such, the bundled coating structures noted for all decanesolvated ligands may also be due to partial crystallization of the ligands. We have investigated this by carrying out simulations at T = 400 and 500 K for all long-chain ligands with neutral end groups, in both decane and water. The resulting coating structures are shown in Figure 4 in water and in Figure 5 in

Figure 5. Four nm nanoparticles coated with S−(CH2)17X ligands solvated in decane at elevated temperatures. The color scheme is the same as that in Figure 1.

Table 2. Measures of Coating Asymmetry for Nanoparticles Coated with S(CH2)17X as a Function of Temperaturea Systems Solvated in Water X= CH3 NH2 COOH

a

T = 300 K

T = 400 K

63.5% 45.4% 55.8% 43.6% 57.2% 47.0% Systems Solvated in Decane

T = 500 K 15.6% 16.8% 18.8%

X=

T = 300 K

T = 400 K

T = 500 K

CH3 NH2 COOH

52.7% 50.3% 53.5%

21.6% 23.3% 31.0%

17.0% 17.0% 17.9%

See Table 1 and related text for an explanation.

asymmetry measures in Table 2. In water, asymmetry is still apparent at 400 K, due to the hydrophobic ligands bundling together to minimize solvent interactions. At sufficiently high temperature (500 K), this effect is overcome, and the coating is symmetric. These symmetric coatings presumably also indicate the particles becoming water-soluble at the elevated temperatures. In the decane systems, the coating structures become almost fully symmetric at 400 K, and only minor differences are noted with a further increase in temperature to 500 K. Notably, these differences are more pronounced for COOH-terminated ligands (see Table 2), most likely due to the higher propensity for hydrogen bond formation among COOH groups. Furthermore, the rapid transition to symmetric coatings with elevated temperatures in decane suggests that the asymmetry noted in these systems at 300 K is due to partial crystallization of the ligands, rather than a result of ligand−solvent interactions. We also note that previous simulations by Peters et al.32 of amorphous silica particles coated with S−(CH2)17− CH3 ligands solvated in decane showed little to no asymmetry at temperatures of 300 and 400 K. However, the highest coverage in that study (3.0 chains/nm2) is much lower than that for the systems studied here, which explains in part why they did not note partial crystallization and asymmetry of ligands.

Figure 4. Four nm nanoparticles coated with S−(CH2)17X ligands solvated in water at elevated temperatures. The color scheme is the same as that in Figure 1.

decane, along with the previous structures at T = 300 K for ease of comparison. Similarly, in Table 2, we present asymmetry measures analogous to those in Table 1 as a function of temperature. As expected, elevated temperatures cause a decrease in coating asymmetry in all cases, as seen from the visual representations in Figures 4 and 5 as well as the coating 11079

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Figure 6. Radial density profiles for key atomic species for several systems at T = 300 K.

Density Profiles and the Ligand−Solvent Interface. In this section, we provide a more detailed, quantitative description of overall coating structures, with a particular emphasis on the ligand−solvent interface. Figure 6 shows density profiles of various atomic species measured radially from the particle center for T = 300 K. We limit the discussion to several representative cases that exhibit interesting features; the remaining data are shown in the Supporting Information. Unless noted otherwise, all density profile data have been averaged over the entire NVT portion of the simulations, sampled every 10 ps. There are several features common to all density profiles. The sulfur profile (black line) is a single sharp peak that marks the particle core boundary. The fixed position is due to the fact that sulfur atoms move as a rigid body throughout the simulation. The alkane carbon profiles (red line) show several well-defined peaks near the core surface, which are smoothed

out at larger r. This is indicative of a high degree of order near the gold core, with the alkane chains exhibiting characteristics of the solid state near the thiol attachment point and liquid-like characteristics near the solvent interface. For comparison, the experimentally measured melting points of nonanoic acid (CH3(CH2)7COOH), octadecanoic acid (CH3(CH2)16COOH), and decane are 305, 343, and 243 K, respectively, while the results in Figure 6 are all for T = 300 K. Considering the effects of confinement and solvation, it is therefore not surprising that we observe both liquid-like and solid-like behavior for the alkane chains in these systems. Additionally, the density profiles are averaged over angular directions, which effectively broadens the density profile features, particularly for systems with asymmetric coating structures (compare parts b and c of Figure 6). In the case of the decane simulations (e.g., Figure 6e), ordering in the carbon atoms persists at large r, which is consistent with the well11080

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Figure 7. Representative distributions of water orientation cos(θ) as a function of distance r from the particle center. All r values are in nm.

Figure 7 shows distributions of the cosine of the angle θ, which describes the relative orientation of water molecules with respect to the particle surface normal. We define θ as the angle between the bisector of the H−O−H angle in a water molecule and a vector connecting the oxygen atom to the center of the nanoparticle (this is equivalent to the angle α defined by Yang and Weng40 and by Heikkilä et al.42). For each case, distributions are shown for several values of r, the distance outward from the particle center (compare these values to Figure 6 to identify the location of the interface). For netural end groups such as COOH in Figure 7a, there is a preferred orientation at small r values where cos(θ) ∼ −0.25, or θ ∼ 105°, which shifts toward θ ∼ 90° before a uniform distribution is achieved at the alkane−water interface (r ∼ 3.5). The distributions at low r values correspond to preferred orientations of water molecules in the hydrophobic alkane region. This is in good agreement with the results of Yang and Weng for neutralized end groups.40 In the case of cationic end groups such as NH3+Cl− in Figure 7b, we note the same trends at the lowest r values (in the alkane ligand region) but a strong preference for cos(θ) ∼ 1 near the interface, indicative of small values of θ in this region. This corresponds to orientations where the bisector of the water H−O−H angle is aligned with the surface normal, with the oxygen atom closest to the nanoparticle. This is consistent with the expected orientation of the water dipole moment near the positively charged water− ligand interface in the case of NH3+Cl−. The opposite trend is observed for anionic end groups such as COO−Ca2+ in Figure 7c, where there is a preference for cos(θ) ∼ −1 near the interface. This indicates the same preferred alignment of the H−O−H bisector as for the cationic interface but in the

ordered, well-solvated structures observed in these systems (see Figure 1). The density profiles are also instructive with regard to features of the ligand−solvent interface. The extent of solvent penetration into the alkane region is apparent in all cases, and can be quantified by the overlap between the solvent (green) and alkane (red) curves. Clearly, all water-solvated systems exhibit much lower solvent penetration as compared to decane (compare parts a−d to part e of Figure 6). In the case of S(CH2)17−COO−Na+ (Figure 6c), which has a highly asymmetric coating structure (see Figure 1), the water penetration is somewhat misleading, as it averages regions of low and high alkane density in the asymmetric coating. The general location of the ligand−solvent interface can also be ascertained from the density profiles of the end group atoms, in these cases both carboxyl oxygens (blue line). For the charged end groups, the density profiles of the counterions (orange lines) closely mirror those of the end group atoms (blue lines), indicating strong ionic association. As expected, this is more pronounced in the case of the divalent Ca2+ cation (compare the orange and blue lines in Figure 6b and d). We also note a small bump in the water density profile near the interface for the two short-chain anionic end groups. This is likely a result of strong association between cationic Na+ and Ca2+ counterions and the negatively charged oxygen atom in water. The effect is stronger in the divalent case (compare the green lines in Figure 6b and d), and is largely obscured by angular averaging of the asymmetric coating structure in the case of the longer ligands (Figure 6c). Water structural features near the ligand−solvent interface and counterion binding are discussed in greater detail below. 11081

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opposite direction, i.e., with the oxygen atom toward the aqueous phase. Once again, this is consistent with the orientation of the water dipole near an anionic interface. For both cationic and anionic end groups, the distributions of cos(θ) gradually decay to uniform distributions at larger values of r, as bulk water behavior is recovered. Similar trends are observed for the remaining end groups and chain lengths with regard to the effects of end group charge (see Figures S5 and S6 in the Supporting Information). This is in excellent qualitative agreement with the results of Heikkilä et al.42 for charged end groups (direct quantitative comparisons cannot be easily made, since their results are based on particle sizes of 2 nm). The orientational order parameter Sθ = ⟨(3 cos2(θ) − 1)/2⟩ is plotted in Figure 8 as a function of r for several cases. This

Table 3. Counterion Association Strength for All Charged Systems

S(CH2)9NH3+Cl− S(CH2)17NH3+Cl− S(CH2)9COO−Na+ S(CH2)17COO−Na+ S(CH2)9COO−Ca2+ S(CH2)17COO−Ca2+

mean assocation lifetime (ps)

fraction of simulation time association is active

59.1 65.2 64.2 63.4 1980 1810

0.073 0.098 0.051 0.060 0.63 0.61

monovalent ions, by more than an order of magnitude. This is consistent with the localized bundling of counterioncoordinated ligands noted earlier only in the case of Ca2+ (see Figure 3 and related discussion). Furthermore, we find no significant differences in the association lifetimes between the two monovalent ions, once again suggesting that overall charge rather than chemistry largely controls the interactions between end groups and counterions. Water/Vapor Interface. In order to investigate the overall water solubility characteristics of the nanoparticles studied here, we performed additional simulations of nanoparticles at water/ vapor interfaces. We investigate qualitative features of the coating structures at interfaces, similar to the analysis carried out by Lane and Grest.16 In addition to inferences about solubility, the behavior of nanoparticles at liquid/vapor interfaces is of general interest for applications that involve self-assembly at interfaces.53 In the case of the S(CH2)17 ligands, the strong coating asymmetry observed in all cases at 300 K clearly indicates that these particles are not soluble in water, regardless of the ligand end group. As such, we do not report simulation results at water/vapor interfaces for these systems. Figure 9 shows the coating structure and location relative to the water interface for several representative nanoparticles coated with S(CH2)9 ligands with charged and neutral end groups at T = 300 K. As discussed in the Methods section, we initially allow the particles to equilibrate near the water/vapor interface from a half-submerged state (Figure 9a and d). After several nanoseconds, we force the particles into the film using a harmonic constraint with a force constant of 100.0 kcal mol−1 Å−2 between the center of the particles and the center of the water film. The constraint for any given position is maintained for some time in order to allow the coating structures to equilibrate. Particles are then forced farther down into the film, so that the lower edge of the coating approaches the bottom water/vapor interface. Finally, all constraints are released, and particles are allowed to reach their equilibrium position. All uncharged end groups (COOH, NH2, CH3) showed similar behavior, where particles did not spontaneously insert into the water film (Figure 9b), and returned to a similar conformation after being forced into the film and subsequently released (Figure 9c). Although there are differences in the degree of insertion of the neutral end group particles, they all show the same qualitative behavior, with a large portion of the nanoparticle remaining outside of the water film. We therefore only show the COOH end group in Figure 9 as a representative case of the neutral ligands. In contrast, all nanoparticles coated with short, charged ligands (COO−Na+, COO−Ca2+, and NH3+Cl−) spontaneously enter the water film and become almost fully submerged within several nanoseconds (Figure 9d). In all three cases, a small portion of the particle remains

Figure 8. Water orientation order parameter Sθ as a function of distance r from the center of the nanoparticle.

definition of the order parameter is typically used to characterize order in liquid crystals. Generally, nonzero values of Sθ indicate orientational ordering. At large values of r, Sθ goes to zero, as expected for bulk solvent behavior, where no orientational order exists, and uniform distributions of cos(θ) are recovered (see Figure 7). At low r values in the region occupied by alkane chains, we see a sharp drop in Sθ for all systems, consistent with a preference for cos(θ) ∼ 0. In charged systems, there is a pronounced peak in the orientational ordering of water near the interface, where Sθ > 0. This indicates a preferred orientation of water along the surface normal (i.e., values of θ near zero or 180°) but does not distinguish between the two opposing directions corresponding to the anionic and cationic end groups, as noted in Figure 7. The orientational order parameter confirms the fact that charged systems induce order in the structure of water molcules near the interface. Additional plots of orientation order parameters are included in the Supporting Information (Figure S7). We have also quantified the association between charged end groups and counterions. Table 3 summarizes the strength of end group−counterion association for various cases. An association is defined on the basis of a simple cutoff distance, determined so as to fully include the first peak in the radial distribution function for a particular pairing (not shown). Throughout the simulation, any associated pairs of counterions and end groups are monitored, and the total duration of each association is logged. The mean association lifetime in Table 3 is a good indicator of the strength of association, and the fact that this is typically shorter than the time scale of the simulations indicates adequate sampling. The divalent Ca2+− COO− association is much stronger than any of the 11082

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Figure 9. Four nm nanoparticles with S(CH2)9X ligands at a water/vapor interface at T = 300 K. The S(CH2)9COOH nanoparticle is shown as a representative case of the neutral end groups (a) at the start of the water/vapor simulation, (b) after 6 ns of simulation, and (c) after being pushed into the water film, released, and allowed to equilibrate. Panel d shows the S(CH2)9COO−Na+ nanoparticle after freely diffusing and becoming mostly submerged in the original, thinner water film. In the thicker film, the S(CH2)9COO−Na+ nanoparticle becomes fully submerged (e), whereas the S(CH2)9COO−Ca2+ nanoparticle remains near the surface (f). Note that, in panels e and f, the image of the water film has been cropped at the bottom.

outside of the water film. In order to test whether this is a metastable state, we followed the same procedure outlined above with regard to forcing the nanoparticles into the water and releasing them. Similar conformations were recovered, with all charged nanoparticles mostly submerged but very close to the water/vapor interface. To ensure that these results were not affected by water thickness, we compressed the water film in the xy plane and repeated the procedure (see the Methods section). The COO−Na+ nanoparticle became fully submerged (Figure 9e), but both the COO−Ca2+ and NH3+Cl− nanoparticles remained near the surface with a small portion exposed to the vapor phase (COO−Ca2+ shown in Figure 9f). We cannot infer any significant differences in solubility behavior among charged end groups on the basis of these small differences in behavior; however, the stark differences between charged and neutral end groups suggest that the former are far more soluble, consistent with the expectation based on overall charge as well as the coating asymmetry differences noted in bulk solvent.

group. However, the divalent Ca2+ counterion leads to stronger association of multiple COO− groups to the same counterion, which leads to localized bundling of ligands. Simulations in bulk decane show more symmetric structures, with some localized bundling of ligands due to a combination of hydrogen bonding among end groups and crystallization of the ligands. This is consistent with the suppression of the asymmetry in higher temperature simulations (T = 400 and 500 K). We have also provided a quantitative description of the nanoparticle coating structures, including measures of asymmetry, radial density profiles of various atomic species, and radial profiles of water orientation near the ligand−water interface. The alkane segments of the ligands exhibit significant solid-like packing near the thiol tethering point, with more liquid-like ordering near the water interface. Charged end groups result in increased ordering of water in this region, and cationic counterions (COO−Na+ and COO−Ca2+) result in increased water density near the interface. We have also reported simulations of nanoparticles at a water/vapor interface. In the case of short-chain ligands, particles with charged ligand end groups remain submerged in water, whereas those with neutral end groups do not readily enter the water film. This suggests that the charged end groups in this case likely result in water-soluble nanoparticles. For long-chain ligands, all particles appear to be relatively insoluble. These results are fully consistent with the coating asymmetries observed in bulk water simulations, wherein a high degree of asymmetry correlates with poor solubility. Overall, this work suggests the possibility of controlling nanoparticle solubility and aggregation behavior by changing the ionization state of ligand end groups, which can be readily achieved by modulating the solution pH. The effects of chain length and nanoparticle size clearly play a significant role in the resulting coating structures, and can drastically alter the solubility properties of the particles. For the systems tested

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CONCLUSIONS We have performed classical atomistic molecular dynamics simulations of 4 nm gold nanoparticles coated with alkanethiol molecules, where the ligand end group and chain length were varied. Our simulations in bulk water show spontaneous asymmetry of the nanoparticle coating structure, similar to what was observed by Lane and Grest.16 This asymmetry is always more significant in the case of longer chains (S(CH2)17 vs S(CH2)9), as expected from simple geometric packing arguments. Charged end groups (NH3+Cl−, COO−Na+, and COO−Ca2+) attenuate some of this coating asymmetry as compared to neutral end groups (NH2, COOH, and CH3). This is consistent with the overall hydrophilic character of these groups. With regard to overall coating structure, the charge of the end group has a much stronger effect than the specific end 11083

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here, the particular chemistry of the end groups appears to have a negligible effect as compared to their total charge.

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ASSOCIATED CONTENT

* Supporting Information S

Additional density profile plots and water orientation distributions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research used resources obtained through the Advanced Scientific Computing Research (ASCR) Leadership Computing Challenge (ALCC) at the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the United States Department of Energy under Contract No. DE-AC02-05CH11231. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

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