effect of charge distribution on the electrostatic adsorption of janus nanoparticles onto charged surface

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[.]

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,1Q Q Cao,2, aand C C Zuo1,2, a

1College of Mechanical Science and Engineering, Jilin University, Changchun 130022,

People’s Republic of China

2College of Mechanical and Electrical Engineering, Jiaxing University, Jiaxing 314001,

People’s Republic of China

(Received 15 December 2016; accepted 22 February 2017; published online 3 March 2017)

We carried out coarse-grained molecular dynamics simulations to study the electro-static adsorption of Janus nanoparticles which consist of oppositely charged hemi-spheres 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 investi-gated in detail When the surface is highly charged, Janus nanoparticles tend to form single particles or small clusters In these cases, the surface charge distribu-tion plays an important role in regulating the process of electrostatic adsorpdistribu-tion 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 nanoparti-cles 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

oth-erwise 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.1,2For 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,4pressure-controlled particle assembly,5pickering emulsions,6and 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,9catalyst manufacture,10interfacial stabilization,11 optical probe,12chemical and biological sensing,13etc

Adsorption of JNPs can be applied in polymer blend compatibilization,14electrocatalytic activ-ity,15interfacial compatibilization,16drug delivery,17and 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

investigation of Janus particles.8The interfacial activity of amphiphilic Janus gold nanoparticles was experimentally investigated.18Moreover, 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 clus-ters.20 Besides, MC simulations of the adsorption between polyelectrolyte and JNPs were also reported.21Recently, dissipative particle dynamics (DPD)22–24and 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.28,29The adsorp-tion, orientaadsorp-tion, 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.34In this work, we use coarse-grained molecular dynamics (CGMD) simulations to explore the electrostatic adsorption of charged JNPs onto solid surfaces Distinguished from Langevin molecular dynamics which use stochas-tic 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 experimen-tal 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 SectionIII Finally, we summarize our conclusions in SectionIV

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.35Five kinds of nanoparticles are selected which

is distinguished by the fraction ρ+of positive charges on the JNP surface, are chosen as shown in Figure1 The charged surface is modeled through two layers of uniformly distributed beads with different charge distributions which are characterized by ρw (see Figure1) 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

r i(t) is

m i d

2

dt2ri= −∂r∂

i

where m i denotes the mass of particle i The mass of all coarse-grained beads with radius of 1.0σ is set to m 0 U totalis the interaction potential between coarse-grained particles which includes

Lennard-Jones (LJ) potential U LJ and Coulombic potential U coul,

035006-3 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017)

FIG 1 Schematic diagram of JNPs and charged surfaces which are characterized by ρ + and ρ w , respectively Color code: negatively (red) and positively charged beads (blue) on the JNPs, neutral (grey) and charged (yellow) surface beads.

U LJis used to describe the short-range pair interactions between any two particles,

U LJr ij =









4εij







σij

r ij

!12

− σij

r ij

!6



 , r ij < r c

(3)

where εij is the depth of potential well and is the distance at which the LJ potential is zero Here, ε ij

and σijbetween any two particles are set to 1.0ε and 1.0σ, respectively, where ε is reduced energy unit.35r 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 r c= 2.5σ The electrostatic interactions between charged particles are molded via Colombic potential

U coulr ij  = k B TZ i Z jλB

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 λB = e2/(4πε0εr k BT ) is the distance at which the electrostatic energy

between two elementary charges is comparable in magnitude to the thermal energy k BT 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 3.

In the initial configuration, 120 JNPs are added in a cubic box of length L= 50σ In our

sim-ulations, 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.35A time step of 0.0005τ was

chosen, where τ= (mσ2/ε)1/2is the reduced time unit.35Nose-Hoover thermostat was used to main-tain 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 Figure2 For the sake of clarity, only JNPs and the surface are displayed The aggre-gation 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 ρwand ρ+ In the following sections, the electrostatic adsorption of charged JNPs on the surface is discussed in detail

A Adsorption structure and density distribution

Figures3– show the typical density distribution of positively ( ρnp+(z)) and negatively ( ρ np− (z))

charged beads in JNPs, as well as positive ( ρc+(z)) and negative ( ρ c− (z)) counterions TableIIgives 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 3 for the cases of ρw= 0.25σ−2, the first peaks at different ρ+ appear

near z= 2.5σ ρnp+(z) and ρ np− (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+<0.5, the magnitude

FIG 2 Snapshots of adsorption conformations of JNPs for different ρ and ρ +

035006-5 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017)

FIG 3 Density distribution of positively (ρnp+(z)) and negatively (ρ np− (z)) charged beads in JNPs, positive (ρ c+(z)) and

negative (ρc− (z)) counterions for (a) ρ+ = 0.1, (b) 0.2, (c) 0.3, (d) 0.4, and (e) 0.5 The surface charge density is set to

ρ w = 0.25σ −2

of ρnp+(z) is smaller than that of ρ np− (z) With the increase of ρ+, ρnp+(z) gradually increases and

approaches ρnp− (z), which is also found in other cases Because the adsorptive surfaces are positively

charged, a thin layer of negative counterions appears near the surface In addition, along with the thin layer of negative counterions a thin layer of positive counterions is also found near the surface, which is more apparent when ρ+ is large ( ρ+<0.5) Except for the thin layer of counterions, the variation of ρc+(z) and ρ c− (z) is consistent with ρ np− (z) and ρ np+(z), respectively Moreover, when

ρ+<0.4, with the increase of ρ+ the first peaks of ρnp− (z) and ρ np+(z) decrease because the first

FIG 4 Density distribution of positively (ρnp+(z)) and negatively (ρ np− (z)) charged beads in JNPs, positive (ρ c+(z)) and

negative (ρc− (z)) counterions for (a) ρ+ = 0.1, (b) 0.2, (c) 0.3, (d) 0.4, and (e) 0.5 The surface charge density is set to

ρ w = 0.06σ −2

concentrated area of JNPs becomes thinner and the amount of JNPs near the surface decreases (see Figure 2) The second concentrated area of JNPs firstly moves to the adsorptive surface and then moves away from the adsorptive surface when ρ+increases from 0.1 to 0.3 As ρ+continues to increase, there is no obvious adsorption layer near the surface Moreover, clusters of JNPs move away from the surface as ρ+increases from 0.4 to 0.5 Combining with Figure2, we found that clusters formed by JNPs become larger when ρ+ is increased We presume that if the amount of positive and negative charges in JNPs is extremely uneven ( ρ+= 0.1), repulsive electrostatic interactions

035006-7 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017)

FIG 5 Density distribution of positively (ρnp+(z)) and negatively (ρ np− (z)) charged beads in JNPs, positive (ρ c+(z)) and

negative (ρc− (z)) counterions for (a) ρ+ = 0.1, (b) 0.2, (c) 0.3, (d) 0.4, and (e) 0.5 The surface charge density is set to

ρ w = 0.01σ −2

between negative charges in JNPs suppress the aggregation of JNPs With the increase of ρ+, the suppression effect becomes weak For the cases of ρw= 0.1σ−2, the change of adsorption structure with the change of ρ+is similar to that of ρw= 0.25σ−2 Comparing to the cases of ρw= 0.25σ−2,

at ρ+= 0.1, the first peak of ρc+(z) is larger than that of ρ c− (z) At ρ+= 0.2, the first peak of ρc+(z) is

comparable to that of ρc− (z) When ρ+continues to increase, the first peak of ρc+(z) becomes smaller

than the peak of ρc− (z) Moreover, at large ρ+, the concentrated area of negative charges becomes obvious

For the cases of ρw= 0.06σ−2 as shown in Figure4, when ρ+increases from 0.1 to 0.4, the adsorption layer of JNPs near the surface becomes thinner, and the amount of JNPs belonging to the thin adsorption layer decreases (see TableII) The first peak of ρc+(z) is larger than that of ρ c− (z) at

ρ+= 0.1 and 0.2 What is different is that when ρ+increases from 0.1 to 0.4, the main concentrated area of JNPs continues to move toward the surface However, when ρ+increases from 0.4 to 0.5, Janus clusters move away from the adsorptive surface Since for neutrally charged JNPs, the electrostatic repulsions between positively charged beads of JNPs and the surface are approximately balanced by the attractions between negatively charged beads in JNPs and the surface leading to that neutrally charged JNPs are less possible to be adsorbed on the adsorptive surface For the cases of ρw= 0.04σ−2, the change of density distribution is similar to the cases of ρw= 0.06σ−2 The first peak of ρc+(z)

is larger than that of ρc− (z) at ρ+= 0.1, 0.2 and 0.3 In contrast to the results discussed above, the main distribution area of JNPs changes little as ρ+is increased For the cases of ρw= 0.02σ−2, the change in the locations of the main concentrated area of JNPs is similar to the cases of ρw= 0.06σ−2

at ρ+= 0.1 ∼ 0.3 However, when ρ+is larger ( ρ+= 0.4 and 0.5), large clusters are formed and the amount of JNPs in the thin adsorption layer decreases especially at ρ+= 0.4 Moreover, the first peak of ρc+(z) is larger than that of ρ c− (z) at ρ+= 0.1, 0.2 and 0.3 which is similar to the cases of

ρw= 0.04σ−2

For the cases of ρw= 0.01σ−2, ρnp− (z), ρ np+(z), ρ c+(z), and ρ c− (z) are displayed in Figure5 The main concentrated area of JNPs moves toward the surface when ρ+is increased The duration

of the main concentrated area also becomes larger Combining with Figure 2, we found that the adsorption layer of small clusters of JNPs is only found in the cases of ρ+= 0.1 and 0.2 However,

in the cases of other ρw, small clusters of JNPs are adsorbed uniformly at ρ+= 0.3, which denotes that the adsorption ability of surface is weak at ρw= 0.01σ−2 As shown in Figure5, the first peak

of ρc+(z) is larger than that of ρ c− (z) at ρ+<0.5, which is also found in Figures3and4 Moreover, clusters formed by JNPs at ρw= 0.01σ−2are not as large as other cases at the same ρ+

Comparing the results of different surface charge distribution in Figures2– , we found that when

ρ+ is small ( ρ+= 0.1 and 0.2), a thin layer of JNPs appears near the surface In combination with TableIwe found that with the increase of ρ+the amount of JNPs in the adsorption layer decreases and then increases However, with the increase of ρwthe amount of JNPs in the thin adsorption layer changes little at ρ+= 0.1 and 0.2 Moreover, the peak range of ρnp− (z) and ρ np+(z) also increases

and then decreases with the increase of ρw for all ρ+ Note that at ρ+<0.4, with the increase of ρ+ the largest cluster tends to appear in the case of smaller ρw Moreover, when ρ+is increased from 0.4 to 0.5, the largest cluster also appears at larger ρw (0.02σ−2) When the net charge of JNPs

is large ( ρ+= 0.1), repulsive interactions between adjacent JNPs prevent the aggregation of JNPs, which reduces the size of JNP clusters If the net charge of JNPs is reduced, the repulsive interactions between adjacent JNPs are also weakened leading to larger JNP clusters Note that the decrease of

ρwalso affects the aggregation of JNPs JNPs tend to form into small clusters at large ρw, while form into large clusters at small ρw The decrease of ρw reduces the JNP amount in the thin adsorption layer near the surface Obviously, a very small ρw (0.01σ−2) may weaken the adsorptive ability of the surface

B Dynamic amount of adsorption

Figure 6shows the dynamic amount of adsorption D a(t) which denotes the number of JNPs

adsorbed on the surface In this section, the definition for the amount of adsorption is different from TABLE I Equilibrium amount of JNPs in adsorption layer.

ρ+ ρ w = 0.25σ −2 ρ w = 0.1σ −2 ρ w = 0.06σ −2 ρ w = 0.04σ −2 ρ w = 0.02σ −2 ρ w = 0.01σ −2

035006-9 Hu, Cao, and Zuo AIP Advances 7, 035006 (2017)

FIG 6 Dynamic amount of adsorption D a (t), for the cases of (a) ρw= 0.25σ −2 , (b) 0.1σ−2, (c) 0.06σ−2, (d) 0.04σ−2, (e) 0.02σ−2, (f) 0.01σ−2 Note that, the horizontal axis is expressed in 10 logarithm.

that in sectionIII A During the adsorption process, JNPs may aggregate into clusters If the distance between the center of mass of one JNP and the surface is less than 6σ, the nanoparticles are considered

to be adsorbed on the surface Here, we define that if a cluster contacts with the surface, the whole cluster is considered to be adsorbed on the surface

For the cases of ρw= 0.25σ−2 as shown in Figure6a, we found that during the initial stage of

adsorption process (t<20τ), D a (t) gradually increases over time at ρ+= 0.1, 0.2, and 0.3 At ρ+= 0.4

and 0.5, D a(t) changes little when t<20τ This indicates that when ρ+is small ( ρ+<0.4), JNPs touch the surface and then undergo conformational changes However, when ρ+is large ( ρ+= 0.4 and 0.5), JNPs aggregate into clusters and then adsorb on the surface in the form of clusters Obviously, the final amount of adsorption is related to the charge property of JNPs When ρ+is smaller than 0.4, only a part of JNPs is adsorbed on the surface At ρ+= 0.4 and 0.5, almost all JNPs are adsorbed, which is also observed in Figure2

For the cases of ρw= 0.1σ−2(see Figure6b), when ρ+is small ( ρ+= 0.1 and 0.2), D a(t) gradually

increases However, at larger ρ+, D a(t) begins to increase when t>10τ Again, this can be explained

by the formation of JNP clusters during the initial stage of adsorption D a(t) continues to increase

until the adsorption amount reaches a steady state Moreover, the steady adsorption amount for the cases of ρ+= 0.1 and 0.5 is small, while in other cases, all JNPs are attached to the surface Note that

for the cases in which all JNPs are adsorbed on the surface, D a(t) rapidly increases, which is due to

the self-assembly of JNPs