Amphiphilic silica nanoparticles at the decane-water interface: insights from atomistic simulations

The properties of 3 nm-diameter silica nanoparticles with different surface chemistry were systematically investigated at the decane-water interface using molecular dynamics simulations. Our results show that the decane-water interfacial tension is not much influenced by the presence of the nanoparticles. The three-phase contact angle increases with nanoparticle surface hydrophobicity. Contact angles observed for the nanoparticles at 300 and at 350 K differ very little. The contact angle of the nanoparticle with randomly dispersed hydrophobic groups is smaller than that observed in Janus nanoparticles of equal overall surface chemistry composition. The energy necessary to desorb Janus nanoparticles from the interface is usually higher than that required to desorb the corresponding homogeneous nanoparticles. Desorption from the interface into the aqueous phase is preferred over that into the organic phase for all except one of the nanoparticles considered. Structural and dynamic properties including nanoparticle rotational relaxation, solvent density profiles, and solvent residence autocorrelation functions near the nanoparticles are also presented. The data are useful for designing Pickering emulsions.     

Influences of surfactant and nanoparticle assembly on effective interfacial tensions

We have studied assembly at air-water and liquid-liquid interfaces with an emphasis on systems containing both surfactants and nanoparticles. Anionic surfactants, sodium dodecyl sulfate (SDS) and non-ionic surfactants, Triton X-100 and tetraethylene glycol alkyl ethers (C(8)E(4), C(12)E(4) and C(14)E(4)), effectively decrease the surface tension of air-water interfaces. The inclusion of negatively charged hydrophilic silica nanoparticles (diameters of approximately 13 nm) increases the efficiency of the SDS molecules but does not alter the performance of the non-ionic surfactants. The former is likely due to the repulsive Coulomb interactions between the SDS molecules and nanoparticles which promote the surfactant adsorption at air-water interfaces. For systems involving trichloroethylene (TCE)-water interfaces, the SDS and Triton X-100 surfactants effectively decrease the interfacial tensions and the nanoparticle effects are similar compared to those involving air-water interfaces. Interestingly, the C(12)E(4) and C(14)E(4) molecules, with or without the presence of nanoparticles, fail to decrease the TCE-water interfacial tensions. Our molecular dynamics simulations have suggested that the tetraethylene glycol alkyl ether molecules tend to disperse in the TCE phase rather than adsorb at the TCE-water interfaces.     

Umbrella Sampling Simulations of Carbon Nanoparticles Crossing Immiscible Solvents

We use molecular dynamics to compute the free energy of carbon nanoparticles crossing a hydrophobic–hydrophilic interface. The simulations are performed on a biphasic system consisting of immiscible solvents (i.e., cyclohexane and water). We solvate a carbon nanoparticle into the cyclohexane layer and use a pull force to drive the nanoparticle into water, passing over the interface. Next, we accumulate a series of umbrella sampling simulations along the path of the nanoparticle and compute the solvation free energy with respect to the two solvents. We apply the method on three carbon nanoparticles (i.e., a carbon nanocone, a nanotube, and a graphene nanosheet). In addition, we record the water-accessible surface area of the nanoparticles during the umbrella simulations. Although we detect complete wetting of the external surface of the nanoparticles, the internal surface of the nanotube becomes partially wet, whereas that of the nanocone remains dry. This is due to the nanoconfinement of the particular nanoparticles, which shields the hydrophobic interactions encountered inside the pores. We show that cyclohexane molecules remain attached on the concave surface of the nanotube or the nanocone without being disturbed by the water molecules entering the cavity.     

纳米SiO2与阳离子表面活性剂的相互作用及其诱导的正辛烷-水乳状液的双重相转变

研究了3种不同结构的水溶性阳离子表面活性剂对纳米二氧化硅颗粒的原位表面活性化作用, 它们分别是单头单尾的十六烷基三甲基溴化铵(CTAB)、单头双尾的双十二烷基二甲基溴化铵(di-C12DMAB)和双头双尾的Gemini型阳离子三亚甲基-二(十四酰氧乙基溴化铵)(II-14-3), 并通过测定Zeta电位、吸附等温线及接触角等参数对相关机理进行了阐述. 结果表明, 阳离子表面活性剂吸附到颗粒/水界面形成以疏水基朝向水的单分子层, 从而增强了颗粒表面的疏水性是原位表面活性化的基础. 通过吸附CTAB和II-14-3, 颗粒的疏水性适当增强, 能吸附到正辛烷/水界面稳定O/W(1)型乳状液; 而通过吸附di-C12DMAB所形成的单分子层更加致密, 颗粒的疏水性进一步增强, 进而使乳状液从O/W(1)型转变为W/O型; 当表面活性剂浓度较高时, 由于链-链相互作用, 表面活性剂分子将在颗粒/水界面形成双层吸附, 使颗粒表面变得亲水而失去活性, 但此时体系中游离表面活性剂的浓度已增加到足以单独稳定O/W(2)型乳状液的程度. 因此当采用纳米二氧化硅和di-C12DMAB的混合物作乳化剂时, 通过增加di-C12DMAB的浓度即可诱导乳状液发生O/W(1)→W/O→O/W(2)双重相转变.     

Molecular dynamics simulations of charged nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces

Nanoparticle self-assembly at liquid-liquid interfaces can be significantly affected by the individual nanoparticle charges. This is particularly true at ionic liquid (IL) based interfaces, where Coulombic forces play a major role. Employing 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) as a model IL, we have studied the self-assembly of hydrophobic nanoparticles with different surface charges at the IL/water and IL/oil (hexane) interfaces using molecular dynamics simulations. In the IL/water system, the nanoparticles were initially dispersed in the water phase but quickly equilibrated at the interface, somewhat in favor of the IL phase. This preference was lessened with increased nanoparticle charge. In the IL/hexane system, all charged nanoparticles interacted with the IL to some extent, whereas the uncharged nanoparticles remained primarily in the hexane phase. Potential of mean force calculations supported the observations from the equilibrium studies and provided new insights into the interactions of the nanoparticles and ionic liquid based interfaces.     

Hierarchical positioning of gold nanoparticles into periodic arrays using block copolymer nanoring templates

We report a simple and versatile self-assembly method for controlling the placement of functional gold nanoparticles on silicon substrates using micellar templates. The hierarchical positioning of gold nanoparticles is achieved in one-step during the spontaneous phase inversion of spherical poly(styrene)-block-poly(2-vinylpyridine) copolymer micelles into nanoring structures. The placement is mainly driven by the establishment of electrostatic interactions between the nanoparticle ligands and the pyridine groups exposed at the interface. In particular, we show the formation of ordered arrangements of single gold nanoparticles or nanoparticle clusters and demonstrate that their morphologies, densities and periodicities can be tuned by simply varying the initial block copolymer molecular weight or the deposition conditions. Besides gold nanoparticles, the method can be used for controlling the assembly of a large variety of nanoscale building blocks, thus opening an attractive pathway for generating functional hybrid surfaces with periodic nanopatterns.     

Wetting properties of passivated metal nanocrystals at liquid-vapor interfaces: a computer simulation study

Molecular dynamics simulations have been employed to determine the contact angles of alkylthiol passivated gold nanocrystals adsorbed at the air-water interface. Simulations were performed using butane-, dodecane-, and octadecanethiol passivated nanoparticles. We demonstrate how the length of the surfactant chain can profoundly influence the wetting behavior of these nanoparticles. All particles were found to be stable at the air-water interface, possessing large, well-defined contact angles. We find that the shape of the dodecane- and octadecanethiol particles is strongly perturbed by the interface. We also present an analysis of the orientational ordering of water molecules at the dodecane-water interface and around butane- and dodecanethiol passivated nanoparticles. The orientational ordering translates into an electrostatic field around the nanoparticles, the magnitude of which corresponds with that of the water liquid-vapor interface.     

Molecular dynamics simulations of nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces

We have studied the self-assembly of hydrophobic nanoparticles at ionic liquid (IL)-water and IL-oil (hexane) interfaces using molecular dynamics (MD) simulations. For the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)])/water system, the nanoparticles rapidly approached the IL-water interface and equilibrated more into the IL phase although they were initially in the water phase. In contrast, when the nanoparticles were dispersed in the hexane phase, they slowly approached the IL-hexane interface but remained primarily in the hexane phase. Consequently, the IL-hexane interface was rather undisturbed by the nanoparticles whereas the IL-water interface changed significantly in width and morphology to accommodate the presence of the nanoparticles. The equilibrium positions of the nanoparticles were also supported and explained by potential of mean force (PMF) calculations. Interesting ordering and charge distributions were observed at the IL-liquid interfaces. At the IL-hexane interface, the [BMIM] cations preferentially oriented themselves so that they were immersed more in the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributed to a heightened IL density and a slightly positive charge at the IL-hexane interface. In contrast, the cations at the IL-water interface were oriented isotropically unless in the presence of nanoparticles, where the cations aligned across the nanoparticle surfaces.     

Water-in-trichloroethylene emulsions stabilized by uniform carbon microspheres

Uniform hard carbon spheres (HCS), synthesized by the hydrothermal decomposition of sucrose followed by pyrolysis, are effective at stabilizing water-in-trichloroethylene (TCE) emulsions. The irreversible adsorption of carbon particles at the TCE-water interface resulting in the formation of a monolayer around the water droplet in the emulsion phase is identified as the key reason for emulsion stability. Cryogenic scanning electron microscopy was used to image the assembly of carbon particles clearly at the TCE-water interface and the formation of bilayers in regions of droplet-droplet contact. The results of this study have potential implications to the subsurface injection of carbon submicrometer particles containing zero-valent iron nanoparticles to treat pools of chlorinated hydrocarbons that are sequestered in fractured bedrock.     

Organic mediator-induced structural transformation in superlattices of monolayer-protected gold nanoparticles

N-acetylglutathione (NAG)-protected gold nanoparticles self-assemble into three-dimensional (3D) face-centered cubic (fcc)-type superlattices at an air/water interface under highly acidic conditions. To prepare the well-defined superlattices, 1month's incubation is at least necessary since the size growth of the as-prepared nanoparticles is essential. Addition of 4-pyridinecarboxyic acid (PyC), a bifunctional hydrogen-bonding mediator, promotes the formation of the superlattices, which are created for about 2weeks' storage. Interestingly, PyC-induced nanoparticle superlattices are in a body-centered tetragonal (bct) structure. The fcc-to-bct phase transformation would be due to stronger interaction between NAG and PyC than that between NAG molecules on the gold nanoparticle surfaces.     

Structure and dynamics of halogenoethanol-water mixtures studied by large-angle X-ray scattering, small-angle neutron scattering, and NMR relaxation

To clarify the structure of solvent clusters formed in halogenoethanol-water mixtures at the molecular level, large-angle X-ray scattering (LAXS) measurements have been made at 298 K on 2,2,2-trifluoroethanol (TFE), 2,2,2-trichloroethanol (TCE), and their aqueous mixtures in the TFE and TCE mole fraction ranges of 0.002 < or = x(TFE) < or = 0.9 and 0.5 < or = x(TCE) < or = 0.9, respectively. The radial distribution functions (RDFs) for TFE-water mixtures have shown that the structural transition from inherent TFE structure to the tetrahedral-like structure of water takes place at x(TFE) approximately 0.2. In the TCE-water mixtures inherent TCE structure remains in the range of 0.5 < or = x(TCE) < or = 1. Small-angle neutron scattering (SANS) experiments have been performed on CF(3)CH(2)OD- (TFE-d(1)-) D(2)O and CF(3)CD(2)OH- (TFE-d(2)-) H(2)O mixtures in the TFE mole fraction range of 0.05 < or = x(TFE) < or = 0.8. The SANS results in terms of the Ornstein-Zernike correlation length have revealed that TFE and water molecules are most heterogeneously mixed with each other in the TFE-water mixture at x(TFE) approximately 0.15, i.e., both TFE clusters and water clusters are most enhanced in the mixture. To evaluate the dynamics of TFE and ethanol (EtOH) molecules in TFE-water and ethanol-water mixtures, respectively, (1)H NMR relaxation rates for the methylene group within alcohol molecules have been measured by using an inversion-recovery method. The alcohol concentration dependence of the relaxation rates for the TFE-water and ethanol-water mixtures has shown a break point at x(TFE) approximately 0.15 and x(EtOH) approximately 0.2, respectively, where the structural transition from alcohol clusters to the tetrahedral-like structure of water takes place. On the basis of the present results, the most likely structure models of solvent clusters predominantly formed in TFE-water and TCE-water mixtures are proposed. In addition, effects of halogenation of the hydrophobic groups on clustering of alcohol molecules are discussed from the present results, together with the previous ones for ethanol-water and 1,1,1,3,3,3-hexafluoro-2-propanol- (HFIP-) water mixtures.     

Controlled Self-Assembly of Metallacycle-Bridged Gold Nanoparticles for Surface-Enhanced Raman Scattering

In this work, well-defined two-dimensional metallacycles have been successfully employed for the well-controlled self-assembly of gold nanoparticles (AuNPs) into discrete clusters such as dimers, trimers, tetramers, pentamers and even hexamers at the water–oil interface for the first time. Furthermore, the modular construction of metallacycle molecules allows precise control of spacing between the gold nanoparticles. Interestingly, it was found that interparticle spacing below 5 nm created by molecular metallacycles in the resultant discrete gold nanoparticle clusters led to a strong plasmon coupling, thus inducing great field enhancement inside the gap between the NPs. More importantly, different discrete clusters with precise interparticle spacing provide a well-defined system for studying the hot-spot phenomenon in surface-enhanced Raman scattering (SERS); this revealed that the SERS effects were closely related to the interparticle spacing.     

A molecular dynamics simulation of the adsorption of water molecules surrounding an Au nanoparticle

This study uses molecular dynamics simulations performed in a parallel computing environment to investigate the adsorption of water molecules surrounding Au nanoparticles of various sizes. An observation of the oxygen and hydrogen atom distributions reveals that the adsorption of the water molecules creates two shell-like formations of water in close vicinity to the Au nanoparticle surface. These shell-like formations are found to be more pronounced around smaller Au nanoparticles. The rearrangement of water molecules in this region reduces the local hydrogen bond strength to below that which is observed in the bulk region. Finally, the simulation results indicate that the absolute value of the interaction energy between the water molecules and the Au nanoparticle is reduced when the water molecules surround a nanoparticle of larger diameter. This observation implies that a stronger adsorption effect exists between smaller Au nanoparticles and water molecules. Hence, the value of the adsorption constant increases for smaller Au nanoparticles.     

Quantifying the Electrocatalytic Turnover of Vitamin B12‐Mediated Dehalogenation on Single Soft Nanoparticles

We report the electrocatalytic dehalogenation of trichloroethylene (TCE) by single soft nanoparticles in the form of Vitamin B₁₂‐containing droplets. We quantify the turnover number of the catalytic reaction at the single soft nanoparticle level. The kinetic data shows that the binding of TCE with the electro‐reduced vitamin in the Co^I oxidation state is chemically reversible.     

Water-induced interactions between carbon nanoparticles

Molecular dynamics simulations were carried out in order to study the hydration of C60 fullerenes, carbon nanotubes, and graphene sheets in aqueous solution and the nature of water-induced interactions between these carbon nanoparticles. The hydration of these nonpolar carbon nanoparticles does not exhibit classical hydrophobic character due to the high density of surface atoms (carbon) resulting in strong water-surface dispersion interactions. Water was found to wet the nanoparticle surfaces independent of nanoparticle surface curvature, with the decrease in the extent of water-water hydrogen bonding with decreasing surface curvature being offset by stronger water-surface interactions. While all carbon nanoparticles investigated are anticipated to aggregate in water due to strong direct nanoparticle-nanoparticle interactions, the water-induced interactions between nanoparticles were found to be repulsive and, in contrast to the wetting behavior, were observed to exhibit strong dependence on surface curvature. The strength of the water-induced interaction between carbon nanoparticles was found to correlate well with the number of hydration water molecules displaced upon particle aggregation, which, relative to the amount of direct nanoparticle-nanoparticle contact engendered upon aggregation, decreases with decreasing surface curvature.     

Structure and phase transformations of DPPC lipid bilayers in the presence of nanoparticles: insights from coarse-grained molecular dynamics simulations

In this article, we investigate fluid-gel transformations of a DPPC lipid bilayer in the presence of nanoparticles, using coarse-grained molecular dynamics. Two types of nanoparticles are considered, specifically a 3 nm hydrophobic nanoparticle located in the core of the bilayer and a 6 nm charged nanoparticle located at the interface between the bilayer and water phase. Both negatively and positively charged nanoparticles at the bilayer interface are investigated. We demonstrate that the presence of all types of nanoparticles induces disorder effects in the structure of the lipid bilayer. These effects are characterized using computer visualization of the gel phase in the presence of nanoparticles, radial distribution functions, and order parameters. The 3 nm hydrophobic nanoparticle immersed in the bilayer core and the positively charged nanoparticle at the bilayer surface have no effect on the temperature of the fluid-gel transformation, compared to the bulk case. Interestingly, a negatively charged hydrophobic nanoparticle located at the surface of the bilayer causes slight shift of the fluid-gel transformation to a lower temperature, compared to the bulk bilayer case.     

Molecular dynamics studies of the kinetics of phase changes in clusters III: structures, properties, and crystal nucleation of iron nanoparticle Fe331

Molecular dynamics (MD) computer simulations have been carried out to study the structures, properties, and crystal nucleation of iron nanoparticles with 331 Fe atoms or with diameter around 2 nm. Structure information for the nanoparticles was analyzed from the MD simulations. Three crystalline phases and one amorphous phase were obtained by cooling the nanoparticles from their molten droplets at different cooling rates or with different lengths of cooling time periods. Molten droplets froze into three different solid phases and a solid-solid transition from a disordered body-centered cubic (BCC) phase to an ordered BCC phase were observed during the slow cooling and the quenching processes. Properties of nanoparticle Fe₃₃₁, such as melting point, freezing temperature, heat capacity, heat of fusion, heat of crystallization, molar volume, thermal expansion coefficient, and diffusion coefficient, have been estimated. Nucleation rates of crystallization to two solid phases for Fe₃₃₁ at temperatures of 750, 800, and 850 K are presented. Both classical nucleation theory and diffuse interface theory are used to interpret our observed nucleation results. The interfacial free energy and the diffuse interface thickness between the liquid phase and two different solid phases are estimated from these nucleation theories.     

PBD/PDMS共混物分散相的聚并捕获行为

借助显微-剪切装置在线研究了低速剪切场下SiO2纳米粒子含量、分散相聚丁二烯(PBD)浓度和剪切速率对PBD/聚二甲基硅氧烷(PDMS)不相容体系中聚并捕获行为的影响.结果表明,聚并捕获所形成的液滴尺寸与形状规整度由粒子含量、分散相浓度和剪切速率等因素共同决定.在较低的SiO2纳米粒子含量或较高的分散相浓度下,PBD液滴在低剪切场下发生聚并捕获,形成尺寸较大、形状不规则的液滴.增加SiO2纳米粒子含量或减小分散相浓度,能够减小分散相的尺寸并提高分散相的规整度.增加剪切速率能有效地减小分散相的尺寸并提高分散相的规整度.     

Forced desorption of nanoparticles from an oil-water interface

While nanoparticle adsorption to fluid interfaces has been studied from a fundamental standpoint and exploited in application, the reverse process, that is, desorption and disassembly, remains relatively unexplored. Here we demonstrate the forced desorption of gold nanoparticles capped with amphiphilic ligands from an oil-water interface. A monolayer of nanoparticles is allowed to spontaneously form by adsorption from an aqueous suspension onto a drop of oil and is subsequently compressed by decreasing the drop volume. The surface pressure is monitored by pendant drop tensiometry throughout the process. Upon compression, the nanoparticles are mechanically forced out of the interface into the aqueous phase. An optical method is developed to measure the nanoparticle area density in situ. We show that desorption occurs at a coverage that corresponds to close packing of the ligand-capped particles, suggesting that ligand-induced repulsion plays a crucial role in this process.     

纳米SiO2/十二烷基氨基丙酸钠协同稳定的pH响应性Pickering乳状液

用纳米SiO₂颗粒与微量氨基酸型两性表面活性剂十二烷基氨基丙酸钠作复合乳化剂, 以正癸烷为油相, 制备了pH响应性O/W型Pickering乳状液. 室温下该乳状液在pH≤4.0 时稳定, 在pH≥6.0时不稳定, 因此, 可以通过改变水相的pH值使乳状液在稳定和破乳之间多次循环. 在酸性水介质中, 氨基酸型两性表面活性剂分子呈阳离子状态, 可通过静电作用吸附到带负电荷的SiO₂颗粒表面, 产生原位疏水化作用, 使其转变为表面活性颗粒; 而在中性和碱性水介质中, 氨基酸型两性表面活性剂呈两性或阴离子状态, 不能产生原位疏水化作用, 因而导致乳状液破乳. 相关作用机理通过吸附量、 Zeta电位及接触角等实验数据得以论证. 该刺激-响应性Pickering乳状液在乳液聚合、 油品输送以及燃料生产等领域具有重要的应用价值.