Article Cite This: J. Phys. Chem. C 2017, 121, 22926-22938 pubs.acs.org/JPCC Solvation of Carbon Nanoparticles in Water/Alcohol Mixtures: Using Molecular Simulation To Probe Energetics, Structure, and Dynamics Kevin R. Hinkle and Frederick R. Phelan, Jr.* Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States *S Supporting Information ABSTRACT: Molecular dynamics simulations were used to examine the solvation behavior of buckminsterfullerene and single-walled carbon nanotubes (SWCNT) in a range of water/alcohol solvent compositions at 1 atm and 300 K. Results indicate that the alcohols assume the role of pseudosurfactants by shielding the nanotube from the more unfavorable interactions with polar water molecules. This is ΔΔ evident in both the free energies of transfer ( Gwater→xOH = − − 68.1 kJ/mol and 86.5 kJ/mol for C60 in methanol and ΔΔ − − ethanol; Gwater→xOH = 345.6 kJ/mol and 421.2 kJ/mol for the (6,5)-SWCNT in methanol and ethanol) and the composition of the solvation shell at intermediate alcohol concentrations. Additionally, we have observed the retardation of both the translational and rotational dynamics of molecules near the nanoparticle surface through use of time correlation functions. A 3-fold increase in the residence times of the alcohol molecules within the solvation shells at low concentrations further reveals their surfactant-like behavior. Such interactions are important when considering the complex molecular environment present in many schemes used for nanoparticle purification techniques. ■ INTRODUCTION within populations of the same chirality.16 Other techniques 1 involve first dispersing the SWCNTs in one surfactant and then Since their discovery in 1991, single-walled carbon nanotubes 17 (SWCNTs) have been extensively studied due to their using alcohol to replace with another as certain alcohols have demonstrated a stabilizing effect on the surfactant-dispersed interesting mechanical and electrical properties. They have 18 been used in many applications including molecular,2 electro- systems. The solution environment around these nano- chemical,3 and optical sensors,4 nanocomposite materials,5 drug particles is complex, as multicomponent solvents are often used delivery agents,6 and DNA sequencing.7,8 The main barrier to tune the colloidal solvation and improve the separation. It is inhibiting the widespread use of this material is the with this motivation that we examine the interaction between nonuniformity of the nanotube fabrication process that aqueous solutions of methanol/ethanol and bare carbon produces polydisperse mixtures of different size (diameter nanoparticles. and length), thickness (single- and multiwalled), chirality, and The energy of solvation of carbon nanoparticles is an handedness. This wide range of species means that significant extremely important quantity because many of their applica- fi tions are as sensors in aqueous environments. Much work has postsynthesis puri cation steps must be performed before 19−24 specific topologies can be isolated for metrology or industrial been done to examine fullerene particles such as C60 in water, but relatively few studies have examined SWCNTs in the use. Various separation methodologies address this problem 25 through the use of surfactants to disperse the tubes in aqueous same detail. Similarly, the behavior of the solvent surrounding − 9 the nanoparticle has been extensively examined for water C60 media before using techniques such as ultracentrifugation, ion- 23,26,27 28,29 exchange chromatography (IEX),10 and aqueous two-phase systems and occasionally for other solvents, but to extraction (ATPE)11 to separate the SWCNTs based on their our knowledge, work with nanotubes is nonexistent as is that physiochemical properties. Many surfactants have shown with mixed solvents. The purpose of this study is to understand promise in these separation protocols, including anionic the energetics, structure, and dynamics of the solvation of bare surfactants such as sodium dodecyl sulfate (SDS),12 sodium carbon nanoparticles in multicomponent water/alcohol sys- deoxycholate (DOC) among other bile salts,9,11,12 and perhaps tems. It is a precursor to an ongoing larger study examining the most interestingly, single-stranded DNA (ssDNA).10,13 This sequence-specific ssDNA-based approach has not only shown Received: August 4, 2017 success in separating particular SWCNT chiralities14,15 but also Revised: September 15, 2017 has demonstrated the ability to separate individual enantiomers Published: September 20, 2017 This article not subject to U.S. Copyright. Published 2017 by the American Chemical 22926 DOI: 10.1021/acs.jpcc.7b07769 Society J. Phys. Chem. C 2017, 121, 22926−22938 The Journal of Physical Chemistry C Article effects of such multicomponent solvent systems on surfactant calculated. The simulated free energy can then be obtained adsorption of ssDNA and the effect on the dispersion and via thermodynamic integration as in eq 1. separation characteristics of SWCNTs. 1 ∂H Δ=Gsim ∫ dλ ■ METHODS 0 ∂λ (1) System Description and Simulation Details. This study Shirts et al.38 have developed a relationship between the examines a variety of carbon nanoparticles (see Table 1)in Δ simulated solvation free energy, Gsim, and the actual solvation Δ free energy, Gsolv, that corrects for the change in the system Table 1. Properties of Fullerene Nanoparticles Studied volume upon the insertion/deletion of the solute (eq 2). length (Å) no. of ⎛ ⎞ V* abbreviated diameter (one unit carbon Δ=Δ−GGkTln⎜ ⎟ nanoparticle name (Å) cell) atoms solv sim ⎝ ⎠ V1 (2) buckminsterfullerene C60 7.1 60 (6,5)-SWCNT T65 7.47 40.64 364 Here, V* denotes the system volume at with full solute/solvent (8,3)-SWCNT T83 7.71 41.96 388 interaction and V1 is the volume of a box of pure solvent with (7,6)-SWCNT T76 8.82 48.01 508 the same number of molecules. In our simulations, the largest (8,6)-SWCNT T86 9.53 25.91 296 volume change observed was roughly 3%, which results in a (9,7)-SWCNT T97 10.88 59.18 772 correction factor on the order of ∼80 J/mol (see Table S2 for details). This correction value is much lower than the statistical uncertainty of our simulations and can therefore be neglected in all cases. water/alcohol mixtures. The SWCNTs were treated as Another method to calculate free energy differences is the − infinitely long tubes that cross the periodic boundaries of the Bennet acceptance ratio (BAR),39 42 which is expressed in eq 3 simulation box. Bucky ball systems were built containing 2134 for the free energy difference between two adjacent λ-states, n total solvent molecules while SWCNT systems contained 3857 and n+1 (see Bennet,39 Pohorille et al.,40 or Kim and Allen42 total solvent molecules. The composition of these solvent for detailed equation development). molecules was varied in increments of 10 mol % in order to examine the effects of water/alcohol mixtures (see Table S1 for ⟨−+Δfu()() Gnn→+11 ⟩ n + =⟨ fuG −Δ nn →+ 1 ⟩ n (3) details of individual simulations). − ff Here, u = Un+1 Un is the energy di erence between the two Simulations were performed using the open source software −1 30−32 states, and f(x) = (1 + exp(x/kT)) is the Fermi function. package GROMACS (ver. 5.1.2) applying the SPC/E ff 33 fi 34 Recursively solving eq 2 yields the free energy di erence. This model for water and the CHARMM36 force eld for technique is attractive as it allows for better estimation of the treatment of the alcohols. All carbon atoms in the nanoparticles fi statistical error (eq 4) by comparing the histograms of the were treated identically and given the force eld parameters of energy difference between the two adjacent λ states: aromatic sp2 carbon and carried no partial charge. The LINCS 35 constraint algorithm was used to maintain the correct bond 21kT22⎛ ⎞ ⟨Δδ2()G ⟩=⎜⎟ −1 lengths of all hydrogen-containing bonds, thereby allowing a nn→+1 NS⎝ 2 ⎠ time step of 2 fs. Equilibration was carried out first for 200 ps in s (4) 36 ⟨δ2 Δ ⟩ the NVT ensemble using a velocity rescaling thermostat to Here, ( Gn→n+1) is the mean square error in the free maintain the temperature at 300 K, followed by 200 ps in the energy estimation, Ns is the number of times each energy state NPTensemblemakinguseoftheParrinello−Rahman is sampled, and S is a measure of the overlap between the two 37 barostat to maintain a pressure of 1 atm. In the case of the densities of u and equals zero for no overlap and 0.5 for two bucky ball system, this barostat was applied in an isotropic identical distributions (eq 5). manner, allowing all box dimensions to vary; however, in the systems containing SWCNTs it was applied in a semi-isotropic ρρ()uu+ () S = nn1 du fashion in order to maintain the correct box size in the axial ∫ ρρ()uu+ + () (5) dimension corresponding to the length of the periodic SWCNT nn1 unit cell. Following these equilibration steps, data was collected This acceptance ratio technique was used in the current work Δ over production runs of 1 ns. via the g_bar module within GROMACS. All Gsolv values Free Energy Calculation Details. The change in Gibbs were found using 50 evenly spaced λ states and were performed Δ free energies of solvation, Gsolv, of the carbon nanoparticles in a decoupling manner to avoid particle overlap and to prevent was calculated using free energy perturbation (FEP) by the encapsulation of solvent molecules within the nanoparticle. applying a coupling parameter, λ, to insert/remove the solute Entropic and Enthalpic Contribution to Solvation. from the surrounding solution.38 By gradually changing this Given the definition of Gibbs free energy, it is possible to parameter between 0 and 1, the nanoparticle can be “grown calculate the change in entropy for the solvation process by into” or “faded out of” the solution.