Molecular Dynamics Simulations of Metal Nanoparticles in Deep

Article Cite This: J. Phys. Chem. C 2018, 122, 18029−18039

pubs.acs.org/JPCC

Molecular Dynamics Simulations of Metal Nanoparticles in Deep Eutectic Solvents Mert Atilhan*,†,‡ and Santiago Aparicio*,§ †

Department of Chemical Engineering, Texas A&M University at Qatar, Doha, Qatar Gas and Fuels Research Center, Texas A&M University, College Station, Texas United States § Department of Chemistry, University of Burgos, 09001 Burgos, Spain ‡

J. Phys. Chem. C 2018.122:18029-18039. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/30/18. For personal use only.

S Supporting Information *

ABSTRACT: A molecular dynamics study on the solvation of metal nanoparticles in deep eutectic solvents is reported in this work. The solvation process was analyzed in terms of the type of metal, geometry of the nanoparticles and properties of the deep eutectic solvent. Simulations results in the microsecond range allowed to infer the properties of the solvation shells and the effects of the nanoparticles on the liquid structuring. The possible aggregation of metal nanoparticles in the studied solvents was analyzed and discussed in terms of the screening effect of the solvents and the efficient nanoparticle−solvent intermolecular forces. The reported results show deep eutectic solvents acting as metal nanoparticle stabilizers, thus providing a new platform for nanoparticles technologies.

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INTRODUCTION Metal nanoparticles (MNPs, i.e. particles with diameters lower than 100 nm) have led to a large collection of technological applications1,2 in diverse fields such as physics,3,4 chemistry,5,6 electronics,7,8 optical science,9−11 materials science,12−15 and biomedical applications.16,17 These applications rise from their outstanding physicochemical properties,18 which are largely correlated with MNPs size and shape19 leading to a way for tailoring MNPs properties via morphology control.20−23 The successful application of MNPs for catalytic purposes is wellknown,24,25 and it has led to great advances in petrochemicals manufacturing.26 Physical phenomena such as surface plasmon on MNPs have also great relevance,27,28 and they offer many opportunities for application in several technologies via observing nanometer scale alterations in thickness, density oscillations or molecular absorption on metal surfaces.29,30 For medical applications, the use of MNPs for cancer therapy,31 for diagnostic and imaging,32 as drug delivery agents,33 or as antifungal−antimicrobial agents34−36 have been widely studied. The development of new methods for MNPs synthesis and surface modification has allowed to produce MNPs with tailored sizes, shapes and composition,37−39 i.e., tuning the MNPs by their morphology.40,41 The preparation and properties of stable MNPs colloidal dispersions have been studied in different solvents,42−45 which is of great relevance in applications such as catalysis.46,47 Likewise, MNPs synthetic methods in solution are highly convenient and versatile,48−50 and thus, MNPs fabrication in polar and nonpolar solvents has been considered. The use of water as solvent for MNPs synthesis has been widely reported © 2018 American Chemical Society

because of environmental, economic and physicochemical properties,51,50 but other solvents such as supercritical fluids,52 carbonates,53 and most recently ionic liquids (ILs)50,52,54,55 have also been considered as new solvents for the same purpose. For the case of MNPs in ILs has attracted great attention because of the possibility of tailoring IL properties to fulfill requirements for MNPs synthesis and colloidal stabilization. It is well-known that MNPs induce large restructuring in surrounding solvents56 due to the solvent− MNP interface and the involved intermolecular forces. This phenomenon is also relevant especially for ILs due to the complex intermolecular forces.57,58 Because of such ionic character of the solvent it leads to a remarkable structuring at the interfaces. Despite the promising properties of ILs for developing MNPs, some concerns59,60 about their toxicity and biodegradability,61 economic viability,62 large scale application63,64 or unsuitable physicochemical properties65 have been reported, which have shifted, at least partially, the interest toward other related relevant solvent alternatives. Deep eutectic solvents (DESs)66 are a type of fluids, which are produced by a combination of a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), leading to a mixture with low melting temperature. DESs have been proposed as sustainable media for the development of nanoscale materials67 because of their to low toxicity,68 suitable biodegradability,69,70 and low production costs, Received: March 16, 2018 Revised: July 4, 2018 Published: July 6, 2018 18029

DOI: 10.1021/acs.jpcc.8b02582 J. Phys. Chem. C 2018, 122, 18029−18039

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The Journal of Physical Chemistry C

icosahedral nanoparticles with 6 (Au6, diameter = 28 Å) and 8 (Au8, diameter = 44 Å) shells were also built. Gold spherical nanoparticles with 14 Å diameter (Au_sph) were also built. The number of atoms for each studied MNP are reported in Table S1 (Supporting Information). Systems and conditions for simulating DES + MNPs were built as defined in Table S2 (Supporting Information) with the objectives of analyze the effects of HBD type, metal type, and MNPs size and shape effect. Initial simulation boxes with the characteristics reported in Table S2 (Supporting Information) were built with the Packmol software.89 In the case of systems containing a single MNP, boxes were built with the corresponding MNP placed in the center of a cubic simulation box and surrounded by the corresponding molecules forming the DES. For systems containing five MNPs (for the study of MNP aggregation), these nanoparticles were initially distributed in the cubic simulation boxes as reported in Figure S1 and surrounded by the corresponding DES molecules. MD simulations were carried in two steps for all the systems reported in Table S2 (Supporting Information): (i) equilibration runs using 20 ns long simulations in the NVT ensemble with fixed MNPs (for equilibrating the surrounding DES solvent) at 300 K, and (ii) production runs using 1 μs long simulations in the NPT ensemble at 300 K and 1 bar. Temperature was controlled with a Langevin type thermostat,90 with 0.1 ps−1 damping constant, and pressure with a Berendsen barostat,91 with 4 ps relaxation time. Nonbonded cross interaction terms were calculated using Lorentz− Berthelot mixing rule. The particle mesh Ewald method was used for handling electrostatic interactions.92 All of the studied systems were treated by considering periodic boundary conditions.

which lead to agreement on DESs as suitable platforms for developing MNPs within a sustainable chemistry framework.71−73 Nevertheless, studies on MNPs at DESs are in their infancy and the mechanisms of MNPs growth, solvation and stabilization in DESs remain largely unexplored. Lee74 recently reviewed the use of DESs for the synthesis of MNPs showing great potential for developing MNPs applications. Moreover, Wang et al. 75 studied the electrochemical preparation of copper MNPs in choline chloride−urea DES and showed how DESs can successfully replace traditional methods for preparing copper nanoparticles. Several studies have also reported successful preparation of different types of metallic nanoparticles76−78 and, thus, confirming the suitability of these solvents for preparing MNPs. The large number of possible HBA:HBD combinations leading to DESs is advantageous on one side since it allows to tailor DESs properties for the selected process, but on the other side it requires a deep knowledge of the properties of these solvents as a function of the involved HBA:HBD and their interaction with MNPs. Therefore, the analysis of solvation and stability of MNPs in selected solvents require a know-how on the intermolecular forces at microscopic level and structuring at particle−solvent (DESs) interfaces. Theoretical studies using molecular dynamics simulation (MD) have proven to be useful for characterizing MNPs in different solutions,79−81 but MD studies for DESs-MNPs are absent in the literature. MD studies on DESs-MNPs would allow to obtain molecular level information on the relationships between the involved HBA:HBD and its ability to solvate and interact with MNPs, thus contributing to the development of structure−property relationships for screening purposes of DESs regarding MNPs. Therefore, we report herein a MD study on the solvation and aggregation of gold, silver, palladium, platinum and nickel nanoparticles in choline chloride (ChCl) based DES with urea (URE), ethylene glycol (EG), glycerol (GLY), malonic acid (MAL), levulinic acid (LEV), and phenyl acetic acid (PAA) as HBDs, which were selected to consider different molecular functionalities to analyze their effect on the interaction with the selected MNPs. ChCl is selected as HBA since it is a nontoxic and biodegradable compound and it can be obtained at very low costs.82,83 MNPs size and shape effects on the mechanism of solvation were also studied. The objective of the study is to analyze for the first time the mechanisms of MNPs solvation and aggregation in DESs as a function of the type of metal, size, and shape of the nanoparticles, and involved HBDs.

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RESULTS AND DISCUSSION The addition of MNPs to the studied DESs should lead to changes in the liquid structure because of changes in intermolecular forces to produce efficient solvation around MNPs, and thus, changes in the intermolecular forces between the molecules forming the DESs should be expected with largest changes in solvation shells around the MNPs. The properties of DESs stand on the hydrogen bonding between the salt (ChCl in this work) and the considered HBDs. Therefore, for the first DESs studied in this work, ChCl:URE, the Ch to Cl and Cl to URE hydrogen bonding93,94 should experience changes upon adding MNPs to the fluid. Radial pair distribution functions, RDFs, for relevant molecular pairs of ChCl:URE DES are reported in the presence and absence of one MNP (Au4, Ag4, Pd4, Pt4, and Ni4), Figure 1. The characteristic narrow RDF peaks, showing hydrogen bonding is maintained upon addition of the MNPs, although minor shifting and certain rearrangement of structuring at distances larger than 7 Å are inferred. Therefore, the ChCl:URE DES is able to solvate efficiently the MNPs maintaining most of the features of their liquid structuring. Likewise, there are no differences in the type of considered MNPs, with all of them leading to the same minor changes in the DES. Nevertheless, slightly visible minor changes as reported in Figure 1 are indicative of molecular rearrangements in the solvation shells around the MNPs. The structuring of the molecules around the MNPs is analyzed from the RDFs between the centers-ofmass of the MNPs and those for cation, anion and URE (for ChCl:URE DES), Figure 2. The results reported in Figures 2a

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METHODS MD simulations were carried out using the ACEMD84 program running on GPUs. Force field parametrizations for the considered DESs were previously reported.85 Metal atoms (Au, Ag, Pd, Pt and Ni) were described as noncharged 12−6 Lennard-Jones sites according to the parametrization reported by Heinz et al.86 Icosahedral and spherical MNPs were built using the nanoparticle builders available in openMD software.87 Icosahedral MNPs with four shells (Au4, Ag4, Pd4, Pt4, and Ni4) were built with the corresponding experimental lattice constants (Au = 4.062 Å, Ag = 4.062 Å, Pd = 3.876 Å, Pt = 3.913 Å, and Ni = 3.508 Å),88 and thus leading to nanoparticles with diameters Au4 = 22.0 Å, Ag4 = 18.8 Å, Pd4 = 17.8 Å, Pt4 = 17.9 Å and Ni4 = 16.1 Å. Gold icosahedral nanoparticles with larger sizes were also built to analyze the MNPs size effect on properties, and thus Au 18030


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to 2c show the formation of a first solvation layer around the MNPs with additional disrupting features produced by the presence of the MNPs on DES liquid structure beyond this first layer. The analysis of the properties of this first solvation layer shows that they are largely dependent on the type of metal forming the MNP and they are richer in URE (HBD) than in salt ions, Figures 2d to 2f. Two regions may be defined around the MNPs, the first one with r < 12 Å (where r stands for the distance to the MNPs center-of-mass), corresponding to the limit of the first solvation layer, and the second one with r in the 12 Å to 15 Å range, which can be assigned roughly to the second solvation shell. The quantification of the molecules in these solvation shells is reported in Figure 3. Results in Figure 3a shows a large excess of URE molecules in the first solvation shell, especially for Au MNPs, which almost vanishes in the second solvation shell. This excess of URE molecules is almost negligible for Ni MNPs. Likewise, the first solvation shell is characterized by a deficit of Ch cations in comparison with Cl anions, especially for Au and Ag MNPs, which almost vanishes on going to the second solvation spheres. Therefore, the solvation of the studied MNPs is characterized by a first strong and well-defined solvation sphere, rich in URE molecules, followed by a second weak solvation shell with properties close to the bulk liquid DES, and with very minor perturbations of the liquid structure beyond these two solvation spheres, Figure S2 (Supporting Information). Regarding the orientation of molecules around the MNPs, using vectors as defined in Figures 4 and S3 (Supporting Information). Results in Figure 4a for URE molecules show that in the first layer they tend to be parallel to the surface, whereas URE molecules are skewed in the second solvation shell. In the case of cholinium cations, Figure 4b, ions are

Figure 1. Center-of-mass radial distribution functions, g(r), for the reported molecular pairs for pure ChCl:URE and for the ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar.

Figure 2. (a−c) Center-of-mass radial distribution functions, g(r), and (d−f) the corresponding solvation numbers, N, for the reported pairs for the systems of ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar. In panels a−c curves are shifted to improve visibility. r stands for the distance to the center-of-mass of the MNPs. 18031


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Figure 5. (a) Distribution of molecules and (b) snapshot of URE molecules around a central Au4MNP, for molecules inside MNP solvation sphere with r = 12 Å. The systems are formed by ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar.

liquid phase. These effects are quantified by the self-diffusion coefficients reported in Figure 6 (obtained from the mean

Figure 3. Relevant ratios of number of molecules, Ni, in solvation spheres around central MNPs as a function of the radius of the sphere, r, defined from the center-of-mass of MNPs. Dashed lines show values corresponding to stoichiometric ratios in bulk DES. The systems are formed by ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar.

Figure 6. Ratio of self-diffusion coefficients, D, for molecules in the first solvation sphere around MNPs (r < 12 Å) and for those in bulk liquid phase. The systems are formed by ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar.

square displacements and Einstein’s equation) and compared with the values in the bulk fluid (well-beyond the solvation spheres). These results show an increase in molecular mobility upon adsorption on MNPs for all the considered metals, which can be justified considering that the first solvation shell is largely composed of URE molecules (Figure 3a and 5a). The strength of MNP−DES interactions is quantified in Figure 7, following the ordering URE > Ch > Cl, in agreement with the composition of the first solvation shell around MNP, therefore the stabilization of MNPs in DES is produced mainly Figure 4. Orientation angles, φ and δ, as a function of the distance, r, between MNPs and (a) URE and (b) Ch, for vectors defined in the figure. The systems are formed by ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar.

skewed in the first shell, whereas except for Ni MNPs they lay parallel to the MNPs in the second solvation shell. The spatial structure in the first solvation shell around Au MNP is reported in Figure 5a (similar arrangements are inferred for the remaining MNPs) showing how most of the MNP is covered by URE molecules with isolated spots of chlorine and cholinium ions. A visual picture of the parallel arrangement of URE molecules on top of Au MNP is reported in Figure 5b. The adsorption of DES molecules, especially of URE, should lead to changes in the molecular mobility for those molecules in the first solvation shell in comparison with those in the bulk

Figure 7. Intermolecular interaction energies, Einter, for MNPmolecule pairs. The systems are formed by ChCl:URE containing one icosahedral MNP. Values at 300 K and 1 bar. 18032


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The Journal of Physical Chemistry C by the interaction with the hydrogen bond donor (e.g., URE). Nevertheless, the rearrangement of DES molecules leads to local ordering around the MNPs by the preferential solvation by URE molecules but the disruptive effect of the MNPs is limited to the liquid region close to the MNPs (Figures 1 and S2, Supporting Information), with minor changes in the structuring of the bulk liquid phase as showed by the extension of hydrogen bonding reported in Figure 8.

around MNPs is always characterized by the presence of an excess of HBD molecules. Nevertheless, the type of HBD leads to subtle differences for the properties of the solvation shells, especially for the first solvation shell because of the size and shape of HBD molecules. The first peak in RDFs corresponding to MNP−HBD pairs, Figure 9c, provided for all the studied HBDs and the structure of the peak changes depending on the considered HBD. Likewise, the characteristics of the distribution of Ch and Cl ions around the MNPs is also dependent on the type of HBD. Nevertheless, first solvation shells are always rich in HBD for all the considered DES (Figure 9d−f), but these heterogeneities vanish beyond the second solvation shells. These heterogeneities are quantified by the molecular ratios reported in Figure 10 with a large excess of HBDs in the first solvation shells but almost vanishing on going to the second shell and, thus, confirming that the disruptive effect of MNPs is very local and the bulk liquid structure in DES is mostly maintained for the studied MNPs concentrations. The HBD rich solvation shells around Au MNPs are reported in Figure 11 showing that PAA leads to a sphere almost covered by the HBD and MAL leads to a lower presence of the HBD in the solvation shell. Nevertheless, strong MNP−HBD interactions are reported for all the considered HBDs, Figure 12, with the weakest interactions obtained for MAL in agreement with results in Figures 10a and 11c. Regarding the molecular orientation of HBDs in the first solvation shell around Au MNP, results in Figure 13 show parallel arrangement of HBDs on top of MNP for HBDs in the first solvation shell (with the exception of slight skewing for MAL and EG), whereas molecules are skewed in the second shell. Therefore, HBDs lay parallel to MNPs surface for all the

Figure 8. Average number of hydrogen bonds per molecule between the reported molecular pairs in systems containing ChCl:URE and one icosahedral MNP. The criterion for defining hydrogen bond was 3.5 Å and 60° for donor−acceptor separation and angle. Values are calculated per Cl molecule (URE-Cl pairs) or per Ch molecule, for the remaining interactions. Values at 300 K and 1 bar.

MNPs solvation and stabilization is characterized by the MNP−HBD interaction for all the studied MNPs, the effect of the type of involved HBD was also analyzed in this work. RDFs reported in Figure 9a−c show that the first solvation shell

Figure 9. (a−c) Center-of-mass radial distribution functions, g(r), and (d−f) the corresponding solvation numbers, N, for the reported pairs for the systems of ChCl:HBD (where HBD stands for URE, EG, GLY, MAL, LEV or PAA) containing one icosahedral Au4MNP. Values at 300 K and 1 bar. In panels a−c, curves are shifted to improve visibility. r stands for the distance to the center-of-mass of the Au4. 18033


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Figure 12. Intermolecular interaction energies, Einter, for Au4− molecule pairs. The systems are formed by ChCl:HBD (where HBD stands for URE, EG, GLY, MAL, LEV or PAA) containing one icosahedral Au4MNP. Values at 300 K and 1 bar.

Figure 10. Relevant ratios of number of molecules, Ni, in solvation spheres around central Au4 as a function of the radius of the sphere, r, defined from the center-of-mass of Au4. Dashed lines show values corresponding to stoichiometric ratios in bulk DES (NHBD/NCH = 2 for all the DES except when HBD = MAL). The systems are formed by ChCl: HBD (where HBD stands for URE, EG, GLY, MAL, LEV, or PAA) containing one icosahedral Au4MNP. Values at 300 K and 1 bar.

Figure 13. Orientation angle, φ, as a function of the distance, r, between Au4 center-of-mass and HBDs. The systems are formed by ChCl:HBD containing one icosahedral Au4MNP. Values at 300 K and 1 bar. Vectors are defined in Figure S3 (Supporting Information).

studied DES, increasing Lennard-Jones interactions and thus leading to the large interaction energies reported in Figure 12. Nevertheless, the MNPs solvation is almost limited to the first sphere, and the hydrogen bonding structuring of DES is maintained for all the considered HBDs, Figure 14. The effect of the MNP size is analyzed and provided in Figure 15. The prevailing presence of URE (HBD) in the first solvation shell is maintained for the studied MNP sizes. Nevertheless, RDFs peaks corresponding both to Ch and Cl ions in the first solvation shells around the MNPs increase with increasing MNP size, thus decreasing the heterogeneities in the first solvation shells. Therefore, the increase in the size of MNPs allows the presence of a large number of URE molecules in the first solvation shell, but on the other side a remarkable presence of ions is also possible, which was not present in small Au8MNPs. Nevertheless, the disrupting effect of the MNPs is almost limited to the DES region in close contact with the MNP, even for the large Au8MNPs.

Figure 14. Average number of hydrogen bonds per molecule between the reported molecular pairs in systems containing ChCl:HBD and one icosahedral Au4MNP. The criterion for defining hydrogen bond was 3.5 Å and 60° for donor−acceptor separation and angle. Values are calculated per Cl molecule (URE-Cl pairs) or per Ch molecule, for the remaining interactions. Values at 300 K and 1 bar.

Figure 11. Distribution of molecules around a central icosahedral Au4MNP, for molecules inside Au4 solvation sphere with r = 12 Å. The systems are formed by ChCl:HBD (where HBD stands for URE, EG, GLY, MAL, LEV or PAA) containing one icosahedral Au4MNP. Values at 300 K and 1 bar. 18034


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Figure 15. (a−c) Center-of-mass radial distribution functions, g(r), and (d−f) the corresponding solvation numbers, N, for the reported pairs for the systems of ChCl:URE containing one icosahedral Au MNP with different sizes (Au4, Au6, and Au8). Values at 300 K and 1 bar. r stands for the distance to the center-of-mass of the Au MNPs.

Figure 16. (a−c) Center-of-mass radial distribution functions, g(r), and (d−f) the corresponding solvation numbers, N, for the reported pairs for the systems of ChCl:URE containing one icosahedral Au6 or Au MNP with spherical shape, Au_sph. Values at 300 K and 1 bar. r stands for the distance to the center-of-mass of the Au MNPs.

The effect of the MNP shape is analyzed in Figure 16 by comparison of the solvation of Au MNP with icosahedral and

spherical geometries, both with similar size. RDFs results in parts a−c of Figure 16 show that for both geometries similar 18035


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Figure 17. Time evolution of the reported distances between the centers-of-mass of icosahedral Au4MNPs in ChCl:URE. Values at 300 K and 1 bar.

Figure 18. Time evolution of the minimum distance between the centers-of-mass of two MNPs (the remaining MNP−MNP distances are larger than those reported in each case). (a) Systems with ChCl:URE and different icosahedral MNPs and (b) systems containing icosahedral Au4MNPs and different ChCl:HBD. Values at 300 K and 1 bar.

Likewise, the authors showed the templating effect of DES for solvating MNPs. The behavior of a group of MNPs, initially arranged in a tetrahedral formation (reported in Figure S1 of Supporting Information) was studied in the considered model DES. The initial distance between the centers-of-mass of nanoparticles was roughly 34 Å, which leads to 12 Å surface-tosurface distance, which is almost the double to that experimentally reported by Raghuwanshi et al.78 for the aggregates obtained experimentally (5 Å). The evolution of the interparticle distances was studied for 1 μs, showing a slight decrease, mainly in the initial stages of the simulation, the nanoparticles stay at distances larger than those reported by Raghuwanshi et al.;78 i.e., no aggregation is inferred by MD within the studied simulation time frame, Figure 17. The formation of stable solvation shells around MNPs hinders the MNP aggregation, maintaining MNPs beyond the aggregation limit; the large MNP−DES interaction energies reported in Figure 7 stabilizes the MNP upon solution in DES, and leads to a screening effect delaying particle aggregation. Nevertheless, this effect seems to be mainly kinetic because experimental results (Raghuwanshi et al.78) showed MNP aggregation after time which is beyond the limits of atomistic MD simulations. MD results confirm that efficient MNP solvation is produced even for highly concentrated MNP solutions in DES as studied in this work. The efficient MNPs

mechanisms of solvation are inferred, with preferential presence of URE molecules in the first solvation shell. The main difference between the solvation of spherical and icosahedral MNPs stands on a minor decrease of the position of the maxima at which RDFs corresponding to the first solvation shell appears (∼1 Å shift), showing that spherical MNPs allows a closer approach of URE molecules (and of ions) to the surface of the particle leading to a larger number of solvating molecules, especially for URE, close to the surface, Figure 16d−f. Therefore, this seems to be a purely geometrical factor controlling also the geometry, and thus, availability of space of the first solvation shells around MNPs. In the last stage of this work, the solvation of a larger number of MNPs (i.e., larger MNP concentration) and possible MNP aggregation was analyzed using ChCl:URE as model DES. Raghuwanshi et al.78 studied the formation and self-assembly of Au MNP in ChCl:URE using an experimental approach showing that the formation of Au MNP with a regular size of 5 nm on average can be obtained in this DES. Likewise, these authors showed that MNP tend to selfaggregate leading to short-range ordering with MNP to MNP separation as close as 0.5 Å. Nevertheless, these experimental results showed that self-aggregation is largely dependent on MNP concentration, i.e., occurring for concentrated solutions, and requires several hours to form the MNP aggregates. 18036



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solvation is inferred for all the studied metals, although some metals such as Pd leads to closer MNP interdistances in the studied simulation time, Figure 18a. Likewise, the analysis of HBD effect on these highly concentrated MNP solutions shows that some HBDs such as MAL lead to lower interparticle distances, in agreement with the weaker MNP− DES intermolecular interactions reported in Figure 12, but for all the systems interparticle distances are larger than those experimentally obtained for aggregates. Therefore, MD results confirm that solutions with large MNP concentrations can be efficiently solvated by DES molecules, delaying MNP aggregation with DES screening MNP−MNP intermolecular forces.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.A.). *E-mail: [email protected] (S.A.). ORCID

Santiago Aparicio: 0000-0001-9996-2426 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge SCAYLE (Supercomputación Castilla y León, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

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CONCLUSIONS The solvation of metal nanoparticles in deep eutectic solvents was studied using molecular dynamics simulations. The effects of type of metal, type of deep eutectic solvent, particle size and shape were analyzed. Efficient solvation was inferred for all the considered cases, with a solvation structure characterized by a first solvation shell very rich in molecules of the corresponding hydrogen bond donor, followed by a second shell with properties close to the bulk liquid phases. The disruptive effect of the nanoparticles is almost limited to these two solvation layers, with the remaining fluid maintaining its hydrogenbonding network. The formation of this strong first solvation layer stands on the large nanoparticle−hydrogen bond donor Lennard-Jones interactions, and in minor extensions intermolecular forces with available anions and cations. The effective solvation of the nanoparticles is maintained for all the studied metals (Au, Ag, Pd, Pt and Ni) and for the considered hydrogen bond donors, although in some cases such as Ni nanoparticles or malonic acid the solvation is less effective. The increase of nanoparticles size decreases the heterogeneity in the composition of the first solvation shell, allowing both the preferential presence of hydrogen bond donor molecules but also of hydrogen bond acceptors. Likewise, the analysis of shape effect of nanoparticles on solvation shows that spherical nanoparticles leads a slightly more efficient solvation in comparison with icosahedral ones. Finally, the molecular dynamics study of the solvation of concentrated nanoparticles solution in deep eutectic solvents showed that even for these concentrated solutions, deep eutectic solvents can solvate the metal nanoparticles screening the interaction between nanoparticles and hindering nanoparticle aggregation in the studied microsecond simulation range. Therefore, the molecules of deep eutectic solvents can efficiently solvate different types of metal nanoparticles, acting as nanoparticle stabilizers, and delaying nanoparticles aggregation, and thus, they can be considered as suitable matrix for developing stable solution of metal nanoparticles with controlled particle size.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02582. Table S1 (MNPs used in this work), Table S2 (systems used for MD simulations), Figure S1 (initial arrangement of MNPs), Figure S2 (MNP−DES radial distribution functions), and Figure S3 (definition of orientation angles) (PDF) 18037


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Molecular Dynamics Simulations of Metal Nanoparticles in Deep

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