Effect of charge distribution on the electrostatic adsorption of Janus nanoparticles onto charged surface D M Hu, Q Q Cao, and C C Zuo Citation AIP Advances 7, 035006 (2017); doi 10 1063/1 4978220 Vie[.]
Effect of charge distribution on the electrostatic adsorption of Janus nanoparticles onto charged surface D M Hu, Q Q Cao, and C C Zuo Citation: AIP Advances 7, 035006 (2017); doi: 10.1063/1.4978220 View online: http://dx.doi.org/10.1063/1.4978220 View Table of Contents: http://aip.scitation.org/toc/adv/7/3 Published by the American Institute of Physics AIP ADVANCES 7, 035006 (2017) Effect of charge distribution on the electrostatic adsorption of Janus nanoparticles onto charged surface D M Hu,1 Q Q Cao,2,a and C C Zuo1,2,a College of Mechanical Science and Engineering, Jilin University, Changchun 130022, People’s Republic of China College of Mechanical and Electrical Engineering, Jiaxing University, Jiaxing 314001, People’s Republic of China (Received 15 December 2016; accepted 22 February 2017; published online March 2017) We carried out coarse-grained molecular dynamics simulations to study the electrostatic adsorption of Janus nanoparticles which consist of oppositely charged hemispheres onto charged surfaces Films with different conformations were formed by Janus nanoparticles The effects of charge distributions of Janus nanoparticles and the surface on the film structures and dynamic adsorption behavior were investigated in detail When the surface is highly charged, Janus nanoparticles tend to form single particles or small clusters In these cases, the surface charge distribution plays an important role in regulating the process of electrostatic adsorption When the amount of surface charges is reduced, the effect of charge distribution of Janus nanoparticles becomes significant The repulsive interactions between Janus nanoparticles determine the aggregation behavior of Janus nanoparticles as well as the shape of adsorption structures, which tends to separate Janus nanoparticles and results in a thin adsorption layer and small clusters When the number of positive charges on the surface of Janus nanoparticle approaches that of negative charges, Janus nanoparticles aggregate into large clusters close to charged surface The charge distribution of Janus nanoparticles becomes pronounced in the process of electrostatic adsorption © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4978220] I INTRODUCTION The electrostatic adsorption of nanoparticles onto charged surfaces is an effective and promising approach to modify specific surface with various kinds of materials.1,2 For instance, it has been used to prepare Janus magnetic nanoparticles coated with a bicompartmental polymer brushes.3 Janus particles can be synthesized by grafting polymers,4 pressure-controlled particle assembly,5 pickering emulsions,6 and coating metallic particles,7 and so on In recent decades, much attention has been paid to the aggregation morphology and interaction mechanism of Janus nanoparticles (JNPs) Janus particles were first introduced as Janus grains and Janus beads by Casagrande et al.8 JNPs are the nanosized particles which consist of two or more asymmetric physically and/or chemically distinct surfaces Due to functional diversities, various applications of Janus particles have been studied in the past decades, such as in the fields of nanomedicine,9 catalyst manufacture,10 interfacial stabilization,11 optical probe,12 chemical and biological sensing,13 etc Adsorption of JNPs can be applied in polymer blend compatibilization,14 electrocatalytic activ15 ity, interfacial compatibilization,16 drug delivery,17 and so on In the past decades, the adsorption behavior of Janus particles has been experimentally and theoretically investigated Casagrande et al first reported that spherical Janus glass particles with one hydrophilic hemisphere and the other hydrophobic hemisphere are prepared, which opens up the way to the investigation of the a Cao Q Q and Zuo C C are corresponding authors Electronic mail: qqcao@mail.zjxu.edu.cn (QC); zuocc@jlu.edu.cn (CZ) 2158-3226/2017/7(3)/035006/13 7, 035006-1 © Author(s) 2017 035006-2 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017) investigation of Janus particles.8 The interfacial activity of amphiphilic Janus gold nanoparticles was experimentally investigated.18 Moreover, bio-inspired Janus gold nanoclusters consist of lipid and amino acid functional capping ligands were synthesized.19 It is verified that the electron transfer through Janus gold clusters at alveolar interfaces is enhanced due to double functional feature of JNPs In the field of electrochemistry, AgAu bimetallic JNPs have been prepared to improve the adsorption of oxygen in the fabrication of electrodes.15 Theoretically, Monte Carlo (MC) simulations of the assembly of Janus spherical particles with opposite charges on each hemisphere have been performed to study the shape of assembled clusters.20 Besides, MC simulations of the adsorption between polyelectrolyte and JNPs were also reported.21 Recently, dissipative particle dynamics (DPD)22–24 and molecular dynamics (MD)25–27 simulations have been used to investigate the properties of JNPs The adsorption behavior of JNPs with different shape, surface chemistry, and charge density have been investigated.28,29 The adsorption, orientation, and self-assembly behavior of JNPs on the interface between two immiscible liquids have also been reported using MD simulations.30–32 Our group has studied electrostatic adsorption between polyelectrolytes and spherical polyelectrolyte brushes,33 as well as between polyelectrolytes and charged nanoparticles through MD simulations.34 In this work, we use coarsegrained molecular dynamics (CGMD) simulations to explore the electrostatic adsorption of charged JNPs onto solid surfaces Distinguished from Langevin molecular dynamics which use stochastic differential equations to simplify system models, C GMD method describes a system by a reduced number of degrees of freedom and elimination of fine interaction details, which reduces the computational resources and simulation time comparing to the all-atom description Moreover, CGMD extends the simulated time and length scale of classic molecular dynamics (all-atom MD) which may be a possible method to bridge the gap between molecular modeling and experimental techniques In our simulations, JNPs were molded as monodispersed hard spheres as reported in the work of Hong et al.20 The effect of charge distribution of JNPs and the surface on the electrostatic adsorption processes is analyzed and discussed in detail This work shed light on the understanding of electrostatic adsorption and aggregation states of JNPs near the surface on molecular level The remainder of this paper is organized as follows The simulation method is described in the next section Simulation results are illustrated and discussed in Section III Finally, we summarize our conclusions in Section IV II METHOD We use CGMD simulations to study the electrostatic adsorption behavior of JNPs on a charged surface Single JNP consists of 50 coarse-grained beads distributing on the surface of a sphere with a diameter of 5σ, where σ is reduced distance units.35 Five kinds of nanoparticles are selected which is distinguished by the fraction ρ+ of positive charges on the JNP surface, are chosen as shown in Figure The charged surface is modeled through two layers of uniformly distributed beads with different charge distributions which are characterized by ρw (see Figure 1) Here, ρw denotes the number density of charges on the surface Note that only some top layer beads are positively charged All surface beads were fixed at their initial position Additionally, positively and negatively charged counterions are added to neutralize the system The equation of motion for particle i at position ri (t) is mi ∂ d2 ri = − Utotal ({ri }) ∂ri dt (1) where mi denotes the mass of particle i The mass of all coarse-grained beads with radius of 1.0σ is set to m0 U total is the interaction potential between coarse-grained particles which includes LennardJones (LJ) potential U LJ and Coulombic potential U coul , Utotal = ULJ + Ucoul (2) 035006-3 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017) FIG Schematic diagram of JNPs and charged surfaces which are characterized by ρ+ and ρw , respectively Color code: negatively (red) and positively charged beads (blue) on the JNPs, neutral (grey) and charged (yellow) surface beads U LJ is used to describe the short-range pair interactions between any two particles, ! σ ! 12 σij ij 4ε ij − , rij < rc ULJ rij = rij rij rij ≥ rc (3) where ε ij is the depth of potential well and is the distance at which the LJ potential is zero Here, ε ij and σij between any two particles are set to 1.0ε and 1.0σ, respectively, where ε is reduced energy unit.35 r ij is the distance between the ith and jth particle r c is the cutoff radius of LJ potential, and U LJ is shifted at rc = 2.5σ The electrostatic interactions between charged particles are molded via Colombic potential λB (4) Ucoul rij = kB TZi Zj rij where k B is the Boltzmann constant Z i is the charge valence of ith particles Here, all charged beads are monovalent The Bjerrum length λ B = e2 /(4πε ε r kB T ) is the distance at which the electrostatic energy between two elementary charges is comparable in magnitude to the thermal energy k B T Electrostatic interactions are calculated using the particle-particle particle-smesh (PPPM) algorithm.36 To treat electrostatic interaction of the systems with non-periodic direction, an empty volume with the height of nLz is inserted along the z axis For all simulations, we set n to In the initial configuration, 120 JNPs are added in a cubic box of length L = 50σ In our simulations, periodic boundary conditions were applied along the x- and y-directions, and the top wall along the z-direction was simulated by a virtual wall with reflect boundary condition The virtual wall reflects particles when they attempt to move through it Reflection means that if a particle moves outside the wall on a time step by a distance delta then it is put back inside the face by the same delta, and the sign of the corresponding component of its velocity is flipped.35 A time step of 0.0005τ was 035006-4 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017) chosen, where τ = (mσ /ε)1/2 is the reduced time unit.35 Nose-Hoover thermostat was used to maintain the system temperature to 1.0 with a damp parameter of 0.05τ The whole simulation process runs for 5,000,000 time steps The simulation data after 4,500,000 time steps were used to analyze the equilibrium properties of the system All simulations were conducted by LAMMPS packages.35 III RESULTS AND DISCUSSION The simulation snapshots of JNPs for various ρw and ρ+ from the equilibrium trajectory are summarized in Figure For the sake of clarity, only JNPs and the surface are displayed The aggregation state of JNPs at ρ+ = 0.5 is comparable to the results in Ref 20 Thin adsorption layer and “mushroom” adsorption structures were found in different cases The aggregation degree of JNPs varies with ρw and ρ+ In the following sections, the electrostatic adsorption of charged JNPs on the surface is discussed in detail A Adsorption structure and density distribution Figures –5 show the typical density distribution of positively (ρnp+ (z)) and negatively (ρnp− (z)) charged beads in JNPs, as well as positive (ρc+ (z)) and negative (ρc− (z)) counterions Table II gives the equilibrium amount of JNPs in the thin adsorption layer near the surface the thickness of which is less than 5σ In this section, if the distance between the center of a JNP and the surface is less than 6σ, the JNP is considered to belong to the adsorption layer As shown in Figure for the cases of ρw = 0.25σ −2 , the first peaks at different ρ+ appear near z = 2.5σ ρnp+ (z) and ρnp− (z) show a similar peak location and range Here, we count the peaks of all analyzed parameters starting from the low z to high z When ρ+