Aggregation Kinetics and Colloidal Stability of Amphiphilic Janus

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Materials and Interfaces

Aggregation Kinetics and Colloidal Stability of Amphiphilic Janus Nanosheets in Aqueous Solution Taiheng Yin, Zihao Yang, Meiqin Lin, Juan Zhang, and Zhaoxia Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06413 • Publication Date (Web): 02 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Industrial & Engineering Chemistry Research

Aggregation Kinetics and Colloidal Stability of Amphiphilic Janus Nanosheets in Aqueous Solution Taiheng Yin,†,§ Zihao Yang,†,§ Meiqin Lin,† Juan Zhang,† and Zhaoxia Dong*,†,‡

† Unconventional

Petroleum Research Institute, China University of Petroleum (Beijing), Beijing,

102249, People’s Republic of China ‡

China University of Geosciences (Beijing), Beijing, 100083, People’s Republic of China

Abstract: Although amphiphilic Janus nanosheets have great potential applications in diverse fields to provide improved performance over conventional homogenous nanoparticles, its aggregation characteristics are still mostly uncovered. Anisotropic shape and chemistry should play a critical role in the interactions between two colloidal particles. Hence, the amphiphilic Janus nanosheets are expected to have unique aggregation behavior different from homogenous nanoparticles. In this paper, a representative silica-based amphiphilic Janus nanosheets was synthesized, and the influences of temperature, pH, ionic strength, ion valence on their aggregation and stability were probed by batch static multiple light scattering, zeta potential, and electrophoretic mobility measurements. Particularly, the interactions energy between two adjacent

amphiphilic

Janus

nanosheets

were

calculated

based

on

the

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation kinetics and colloidal stability of amphiphilic Janus nanosheets were systematically investigated for the first time. The

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

aggregation of amphiphilic Janus nanosheets depends on the ionic strength, pH and temperature of aqueous. Amphiphilic Janus nanosheets exhibited a stronger tendency to aggregate compared with hydrophilic Janus nanosheets, and the classical DLVO theory was verified to be inaccurate in predicting its aggregation, which was caused by the additional hydrophobic attraction. A reliable extended DLVO model containing hydrophobic interactions was developed to interpret the aggregation behaviors of amphiphilic Janus nanosheets.

Keywords: amphiphilic Janus nanosheets; aggregation kinetics; colloidal stability; static multiple light scattering; DLVO theory

1. Introduction Amphiphilic Janus nanosheets have two surfaces with distinctly different wettability, an ultra-high ratio of the lateral size to the thickness, and exhibited unique behaviors and properties at the immiscible fluid interfaces.1-4 With the recent development of synthesis techniques, the efficient large-scale preparation of amphiphilic Janus nanosheets has been realized.5,6 More importantly, current researches have shown that the amphiphilic Janus nanosheets can provide improved performance over conventional homogenous nanoparticles in diverse fields, including emulsification, enhanced oil recovery, and many others.7-9 When dispersed into a polar solvent, nanoparticles are likely to aggregate and may subsequently lose its stability.10,11 Numerous studies published in the literature have reported that the practical application effect of nanoparticles relies on their stability in aqueous.12-14 For instance, although the use of nanofluid in enhanced oil recovery has tremendous potential, the


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harsh environment (e.g., high temperature and high salinities) of reservoirs are expected to lead to nanoparticles aggregation and sedimentation, which would ultimately cause severe formation damage.15 As a novel and unique nanomaterial, the amphiphilic Janus nanosheets may have different aggregation behavior from conventional nanoparticles. Therefore, knowledge of the aggregation behaviors of amphiphilic Janus nanosheets in aqueous solution is critical for practical applications. Nevertheless, to date, the majority of studies still have focused on the synthesis methods of amphiphilic Janus nanosheets.16-18 The study on the aggregation behavior of amphiphilic Janus nanosheets is still in its infancy, and only limited amounts of studies of this topic have been conducted recently.19 At present, it is very urgent to clarify the aggregation behaviors of amphiphilic Janus nanosheets for further practical applications. The aggregation behaviors of surface chemically homogeneous nanomaterials, such as graphene oxide, bentonite and silica nanoparticles, have been extensively investigated in aqueous environment.20-23 Summing up these studies, we could find that the aggregation of nanoparticles was strongly dependent upon the solution environmental (e.g., temperature, pH, ionic strength and electrolyte ion valence) and the surface chemical characteristics (e.g., the type and content of surface

chemical

groups)

of

themselves.20-23

particles

Moreover,

the

classical

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was usually used to interpret the colloidal properties of charged particles in polar solvents.24-26 This theory considers the interactions between two colloidal particles are only related to the attractive van der Waals (vdW) and repulsive electrostatic (EL) interactions.27 However, for colloids with strong hydrophilic or hydrophobic surfaces, the additional non-DLVO interactions usually play a critical role in the 3 ACS Paragon Plus Environment


colloidal stability.28-32 In comparison to those chemically homogeneous nanoparticles, amphiphilic Janus nanosheets have a highly anisotropic shape as well as chemistry, which will lead to their aggregation behaviors in aqueous solutions more complicated. In a word, this is a great challenge to elucidate the aggregation behaviors of amphiphilic Janus nanosheets based on the experimental measurements and traditional colloidal theory. The overall objective of this paper is to enhance our understanding of the aggregation behaviors of amphiphilic Janus nanosheets in aqueous solution. A representative silica-based amphiphilic Janus nanosheets was synthesized and used as examples during this investigation. Batch static multiple light scattering measurement experiments were carried out to quantify the aggregation and stability of amphiphilic Janus nanosheets in aqueous solutions under a wide range of water environments (including the pH, temperature and ionic strength and electrolyte ion valence). The aggregation kinetics of amphiphilic Janus nanosheets at different electrolyte solutions were directly compared with hydrophilic Janus nanosheets for the first time. The aggregation behaviors of Janus nanosheets in electrolyte solutions were predicted by classical DLVO theory. In addition, a reasonable theoretical model containing hydrophobic effect was verified to interpret the aggregation behaviors of amphiphilic Janus nanosheets further.

2. Experimental section 2.1. Preparation of Janus nanosheets The synthesis of silica-based Janus nanosheets was introduced in the previous work,8,33 as illustrated in Scheme 1. Briefly, a silica Janus shell coated calcium carbonate (CaCO3@SiO2) 4 ACS Paragon Plus Environment

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was first synthesized by the self-assembled and sol-gel of amphiphilic silane (AS) on the surface of calcium carbonate. Then, the silica-based Janus nanosheets (SJN) were obtained by acid etching the CaCO3@SiO2. Further, the surface of the CaCO3@SiO2 was modified with octadecyltrichlorosilane (OTS) to introduce the hydrophobic groups, which was then etched to obtain the amphiphilic Janus nanosheets (CSAJN). The Janus nanosheets powders were dispersed in deionized water and sonicated to form a stock solution for further studies. The source and purity of all chemicals used were listed in the Supporting Information.

Scheme 1. Schematic of the synthesis of Janus nanosheets. 2.2. Characterization of Janus nanosheets suspensions Atomic force microscopy (AFM, Agilent 5500) analysis showed that SJN and CSAJN are flake-like and have a similar thickness of approximately 2.5 and 2.6 nm (Figure S1), respectively. The hydrodynamic diameter distribution of SJN and CSAJN in deionized water were measured using the Malvern Zetasizer Nano ZS, and the results showed that the average hydrodynamic diameters are about 616.9 and 674.1 nm, respectively (Figure S2). Automatic refractometer (J457, Rudolph Research Analytical) measurements showed the refractive indices of CSAJN and SJN are about 1.48 and 1.51, respectively. The zeta (ζ) potential and electrophoretic mobility (EPM) of Janus nanosheets suspensions were also measured by Malvern Zetasizer Nano ZS 5 ACS Paragon Plus Environment


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under different experimental conditions, three measurements were conducted for each of at least triplicate samples, and the final test results were expressed as mean and standard deviation values.

2.3. Multiple light scattering measurements of nanosheets suspensions The Turbiscan Lab (Formulaction Company, France) based on the principle of static multiple light scattering was employed to study the aggregation kinetics and colloidal stability of nanosheets suspensions. The Turbiscan is equipped with a pulsed near-infrared light source (λ = 880 nm) and synchronous detectors which determine the intensity of transmission light. Since the transmission is directly related to the photon mean free path (λ*) which is the average distance between scatterers, the mean hydrodynamic diameter (D) of particles in media can be calculated from transmission intensities by the following equations: –

𝑇 r = T 0e λ* =

2ri λ*

2D 3ϕQs

(1) (2)

where Tr is the transmission intensities, T0 is the transmission intensity of continuous phase, λ* is the transport mean free path of photon in the suspension, ri is the radius of the sample bottle, ϕ is the volume fraction of particles, and Qs is the optical parameters given by the Mie theory.34,35 The colloidal stability of nanosheets suspensions was quantified by the Turbiscan stability index (TSI), which can be calculated by


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TSI =

∑

∑l|scani(l) – scani - 1(l)| H

i

(3)

where scan i(l) is the transmission intensity of i-th scan at the scanning probe height of l, and H is the total height of the sample. The larger TSI value indicates less stability of the suspension. In the current study, approximately 20 mL of nanosheets suspension with a concentration of 50 mg/L was used for each multiple light scattering experiment. The nanosheets concentration of 50 mg/L was chosen based on our previous application studies,8 and this concentration could provide a strong light scattering signal. The hydrodynamic diameter evolution and colloidal stability of the nanosheets were monitored over time at different aqueous solutions environmental, including different pH, temperature, salt types, and ionic strength.

2.4. Mathematical models The total interaction between two colloid particles is described as a balance of van der Waals (vdW) interactions and electrostatic double layer (EL) interactions according to the classical DLVO theory as follows:27 WDLVO = WvdW + WEL

(4)

The amphiphilic Janus nanosheets have two sides with different wettability, so there may be three different encounters modes between adjacent nanosheets, i.e., hydrophilic surface (HS) and lipophilic surface (LS), LS and LS, and HS and HS, for each of which the constituent DLVO interactions may be different. But there is no well-accepted theory to directly calculate the DLVO forces between adjacent anisotropic plates. In general, the surfaces of the anisotropic plates are 7 ACS Paragon Plus Environment


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assumed to be homogeneous for the calculation of DLVO forces.19 However, the non-DLVO force of hydrophobic forces may have a significant effect on the coagulation and dispersion of particles with hydrophobic surfaces.36 For amphiphilic Janus nanosheets, the extended DLVO (eDLVO) theory containing hydrophobic interactions (WS) should be investigated and the formulation as shown: WeDLVO = WvdW + WEL + WS

(5)

The vdW interactions between two nanosheets can be approximated by the potential law for two flat plates as follows:37

WvdW = –

[

A 1 1 2 + – 2 2 12π d (d + 2t) (d + t)2

]

(6)

where A is the Hamaker constant, t is the thicknesses of nanosheets, and d is the separation distance. The Hamaker constant between two identical nanosheets 1 interacting across a medium 3 can be calculated based on Lifshitz theory, as expressed in eq 7.27

A ≈

3hνe

(n12 – n32)2

16 2(n12 + n32)3/2

(7)

where h is the Planck’s constant (6.626 × 10-34 J·s ), n1 and n3 is the refractive index of nanosheets and water (1.33) in visible regime, respectively. νe denotes the main absorption frequency of nanosheets in the UV regime, νe can be calculated by νe = νb 3/(n12 + 2),27 where νb is the absorption frequency of a Bohr atom (3.3 × 1015 s-1). For two parallel plate-like nanosheets, the electrostatic double layer interactions can be 8 ACS Paragon Plus Environment

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approximated by the Poisson-Boltzmann equation as follows:38

WEL =

2σ2 exp(–κd) εε0κ

(8)

where σ is the surface charge density, which can be estimated by27

σ =

2εε0κkT e

eψ0

( )

sinh

kT

(9)

where ε denotes the relative permittivity of water (ε = 87.73 × 10-0.002(T-273.15)),39 ε0 denotes the permittivity of vacuum (8.854 × 10-12 F/m), k is the Boltzmann constant (1.381 × 10-23 J/K), T is the absolute temperature, ψ0 denotes the surface potential of nanosheets, which is often approximated to ζ potential,40 κ is the Debye-Hückel parameter and is defined by38 κ-1 = (ε0εkT/2NAIe2)1/2

(10)

where NA is the Avogadro’s number (6.02 × 1023), I is the ionic strength of the solution, and e is the elementary electric charge (1.602 × 10-19 C). One can estimate the hydrophobic interaction by an empirically derived model as follows:41 WS = –2γfe -d/D0

(11)

where γ is the interfacial tension between the solvent and the hydrocarbon on the lipophilic surface of nanosheets (i.e., octadecane-water, ~50 mN/m), D0 is the decay length for the hydrophobic interaction, which is usually assumed to be 1 nm based on many previous studies,19,27,41 f is the ratio of hydrophobic area to total surface area of nanosheets. When f = 1, the nanosheets is fully hydrophobic, and when f = 0, there is no effect contribution from the 9 ACS Paragon Plus Environment


hydrophobic interaction. The f of amphiphilic Janus nanosheets is estimated to be 0.5 due to it has two completely different wettability sides.

3. Results and discussion 3.1. The aggregation and migration characteristics of CSAJN In order to observe the whole process of aggregation and migration of CSAJN, and considering that water usually contains different kinds of cations, the static multiple light scattering experimental measurements were conducted in a high salt solution (683.8 mM NaCl + 90.1 mM CaCl2). The transmitted light profile of CSAJN in high salt aqueous solution is shown in Figure 1a, where the abscissa is the sample height; the ordinate (T) is the difference of transmitted light intensity relative to the initial value and the curves colors represent different testing time. According to the equations (1) and (2), the transmitted light intensity increased with the increase of particle size and the decrease of particle volume concentration. As shown in Figure 1a, the transmitted light intensity was distributed evenly and increased with time in the areas of sample height above 2.5 mm, indicating the particle size grows gradually with the continues aggregation of nanosheets. The transmitted light intensity decreased, and the peak width increased with time when the height below 2.5 mm, which indicates that the volume fraction of nanosheets increases owing to the sedimentation of nanosheets in the bottom of the suspension. The sedimentation thickness (i.e., the height of the sediment) of the sample was monitored and analyzed by the Turbiscan Lab software. Figure 1b shows the stratified thickness kinetic 10 ACS Paragon Plus Environment

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curve of the nanosheets in high salt aqueous solution. It can be seen that the sedimentation of nanosheets occurred after about 2.7 hours, and then the thickness of the sediment increased gradually and finally approached an equilibrium value of ~0.21 mm. The migration rate of the nanosheets could be calculated by the slope of the linear variation in the stratified thickness kinetic curve.42 The Janus nanosheets exhibited a migration rate of 6.138 × 10-2 mm/hr in high salt aqueous solution. (a)

(b)

(c)

Figure 1. (a) Transmitted light profile, (b) stratified thickness kinetic curve and (c) schematic diagram of aggregation and migration process of CSAJN in high salt aqueous solution. 11 ACS Paragon Plus Environment


Based on the above results of the transmitted light profile and stratified thickness kinetic curve, the aggregation and migration characteristics of CSAJN in aqueous solution could be described as Figure 1c.

3.2. pH effect Figure 2a presents the TSI variation with time of CSAJN suspensions at different pH values levels. The TSI values of the CSAJN suspensions increased with time and tended to a lower constant value when the pH was higher than 5. But when the pH of CSAJN suspensions was less than 5 (e.g., 1.5 and 3), the TSI values kept increasing with time and the platform area disappeared. Moreover, The TSI values of the CSAJN suspensions decreased with the increase of pH and the increasing of pH had almost no effect on the TSI when the pH was over 5. Figure S3 shows the visual image of CSAJN suspensions at different pH levels were after 24 hours. At low pH values levels (e.g., 1 and 3), the CSAJN suspensions were unstable with large visible aggregates. At medium and high pH values levels, the suspensions were homogeneous, indicating excellent stability of the CSAJN particles. This result was consistent with the multiple light scattering experiments. To further analyze the effect of pH on the CSAJN stability, the Zeta potentials of the CSAJN suspensions at different pH values were measured (Figure 2b). As the pH values decreased from 10 to 6, the Zeta potential of the CSAJN suspensions was approximate -36 mV. As the pH values decreased from 6 to 1, the Zeta potential of the CSAJN suspensions increased dramatically with 12 ACS Paragon Plus Environment

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the decreases of pH values. It is known that the CSAJN had a hydrophilic side covered by carboxyl groups. We previous studies showed that the degree of protonation of carboxyl groups gradually increased with the decrease of pH values when the pH was below 6,8 which led to the loss of negative charge on the CSAJN surface. Meanwhile, the electrostatic repulsion between adjacent CSAJNs decreases due to the charge screening effect of hydrogen ions (Figure S4). These results suggested that the aggregation of amphiphilic Janus nanosheets was closely relevant to the aqueous pH. (b)

(a)

Figure 2. (a) The TSI-time curves of the CSAJN suspensions at different pH; (b) Zeta potential dependence on the pH of the CSAJN suspensions (0.01%). All experiments were conducted at 25 °C.

3.3. Temperature effect The TSI-time cures of CSAJN suspensions at different temperature are presented in Figure 3a. TSI-time curves of CSAJN suspensions had a similar profile at different temperature, and the TSI values increased slightly with the increase of temperature, indicating the CSAJN gradually lost stability in aqueous solution with the increase of temperature. The stability of CSAJN 13 ACS Paragon Plus Environment


suspensions was also determined by visual observation experiments (Figure S5a). There was no observable aggregation in the suspensions after 24 hours, indicating the CSAJN have high stability at mild temperature levels. (a)

(b)

Figure 3. (a) TSI-time curves of the CSAJN suspensions at different temperature; (b) Classical DLVO interaction energy between two CSAJNs at different temperature. All experiments were conducted at pH 7. The classical DLVO interaction energies between two adjacent CSAJNs at different temperature are displayed in Figure 3b. A significant energy barrier against aggregation was observed and the energy barrier decreased with increasing temperature. Furthermore, for hydrophobic particles, hydrophobic interactions derived from hydrophobic groups on the surface of the particle may cause aggregation and loss of stability in aqueous solution.36 We added the hydrophobic force to the calculations of interaction energy and the results are shown in Figure S5b. The strong hydrophobic attraction led to the increases in the minimal requirement for stable intersheet distances and the decreases of energy barrier against aggregation. Interestingly, the curves of intersheet interaction energies versus distance were more similar than those without considering the hydrophobic force. The theory analysis results were consistent with the multiple 14 ACS Paragon Plus Environment

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light scattering and visual observation experiments, and the effect of temperature on the aggregation and stability of CSAJN suspensions was further certified.

3.4. Effect of cation types and concentrations Electrolytes are ubiquitous in nature water environmental, and the interactions between nanoparticles and these electrolytes may trigger uncertain aggregation effects.43 Therefore, it was crucial to investigate the aggregation kinetics and stability of the amphiphilic Janus nanosheets in the presence of electrolytes. In this section, to our knowledge, the aggregation behaviors of amphiphilic Janus nanosheets were investigated by direct comparison with the non-amphiphilic Janus nanosheets for the first time. Two representative cations (Na+ and Ca2+) were selected to prepare the nanosheets suspensions with different ionic strength at an unadjusted pH (approximate 6.5). The test temperature was set to 40°C to avoid the effect of room temperature changes on the measurement results. The TSI-time curves of SJN at NaCl solution are shown in Figure 4a. When the electrolyte concentration at a relatively low concentration regime (i.e., below 30 mM), the stability of nanosheets suspension decreased with the increase of electrolyte concentration. However, when the electrolyte concentration at higher concentration regime (i.e., above 40 mM), the stability did not change with the electrolyte concentration. Comparatively, a similar law was also observed during the study of the stability of SJN and CSAJN at different electrolytes solutions (Figure 4b and S6).



(b)

(a)

Figure 4. TSI-time curves of (a) SJN and (b) CSAJN at NaCl solution. The changes of mean particles size in the middle area of the samples were used to investigate the aggregation kinetics of amphiphilic Janus nanosheets. The aggregation profile of Janus nanosheets in electrolyte solutions is shown in Figure S7. Notably, the aggregation rates of Janus nanosheets increases with the increase of electrolyte until a critical concentration of electrolyte was reached, which indicates that electrostatic force is essential for maintaining the colloidal stability of Janus nanosheets. Above this critical concentration, the aggregation rate almost did not change with the electrolyte concentration. Particle attachment efficiency (α) was employed to quantify the aggregation kinetics of Janus nanosheets in aqueous solutions, which was defined as the ratio of the initial aggregation rate at a specific electrolyte concentration to the initial aggregation rate at diffusion-limited (fast) aggregation conditions,44,45 and the calculation details were included in Supporting Information. Representative attachment efficiencies of SJN and CSAJN at different electrolyte solutions are presented in Figure 5a. As shown in Figure 5a, the variation of attachment efficiency with electrolyte concentration had two distinct regimes, namely reaction-limited (α < 1) and 16 ACS Paragon Plus Environment

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diffusion-limited (α = 1) regimes, respectively.46 At the reaction-limited regime, the increase of electrolyte would enhance the charge shielding effect and weakens the electrostatic interactions between two nanosheets, resulting in an increase of attachment efficiency. At the diffusion-limited regime, the charges of Janus nanosheets were completely screened, and the attachment efficiency would no longer change. Critical coagulation concentration (CCC) is defined as the electrolyte concentration of the transition from slow (reaction-limited regime) to fast (diffusion-limited regime) aggregation occurs.43-46 The experimental CCC of the Janus nanosheets were determined from the intersection of the extrapolated lines through the diffusion- and reaction-limited regimes. The experimental CCC values of SJN suspensions were about 41.6 mM and 8.9 mM for NaCl and CaCl2, respectively. The CSAJN had CCC values about 18.6 mM NaCl or 3.8 mM CaCl2. The CCC of CSAJN was lower than SJN, indicating the amphiphilic Janus nanosheets are more likely to aggregate than hydrophilic Janus nanosheets in the presence of the electrolyte. EPM was defined as the ratio of the drift velocity of a dispersed particle to the applied electric field strength. The EPM of SJN and CSAJN suspensions were measured at different NaCl and CaCl2 concentrations (Figure 5b). When the electrolyte concentrations were low, the charges of CSAJN were less negative than SJN, which could be a consequence of the CSAJN was synthesized by the single-surface alkyl functionalization of SJN. Also, it was clear that the divalent ions (Ca2+) were more efficient than monovalent ions (Na+) to neutralize the negative surface charges of Janus nanosheets.



(b)

(a)

Figure 5. (a) Attachment efficiencies and (b) EPM of Janus nanosheets as a function of NaCl and CaCl2 concentration.

3.5. DLVO theory prediction for aggregation The classical DLVO theory was widely used to predict the aggregation behaviors of nanoparticles in aqueous solution.24-26 Figure S8 shows that DLVO interactions energy between two nanosheets at different electrolyte concentrations. In the case of increasing of the ionic strength, the electrical double layer was condensed by the counterions, and the corresponding energy barrier against aggregation also decreases. When the ionic strength was above CCC, the van der Waals interaction energy would exceed the electrostatic interaction energy, the energy barrier disappears, and fast aggregation appears. The aggregation behaviors of SJN in the NaCl and CaCl2 solutions predicted by classical DLVO theory were consistent with the experimental results (Figure S8a and S8b). Interestingly, when the ionic strength reached 20 mM NaCl or 5 mM CaCl2, the existence of large energy barriers against aggregation between two CSAJNs was apparently inconsistent with the experimental results (Figure S8c and S8d). When the ionic strength reached the CCC, the DLVO interactions between two nanosheets 18 ACS Paragon Plus Environment

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satisfied the following conditions:38 WDLVO = 0 ;

∂ WDLVO = 0 ∂d

(12)

Accordingly, we could obtain the CCC by applying these conditions to the equation of classical DLVO interaction energy. The calculated CCC values of SJN were 37.7 mM Na+ or 9.2 mM Ca2+, which is agree with the experimental CCC, i.e., 41.6 mM Na+ or 8.9 mM Ca2+ (Figure 6a). However, the calculated CCC values of CSAJN (34.9 mM Na+ or 9.7 mM Ca2+) deviated largely from the experimental value. The classical DLVO theory could accurately predict the aggregation behaviors of hydrophilic Janus nanosheets even quantitatively. On the contrary, it was failed to predict the aggregation behaviors of amphiphilic Janus nanosheets through the classical DLVO theory. (a)

(b)

Figure 6. Comparison between the experimental CCC values and the predicted CCC values by (a) classical DLVO and (b) eDLVO theory. This inconsistency of aggregation behaviors of amphiphilic Janus nanosheets between experimental observations and classical DLVO predictions in the current study suggested that 19 ACS Paragon Plus Environment


there are non-DLVO interactions among amphiphilic Janus nanosheets. There have been numerous reports about the hydrophobic interactions between nanoparticles containing hydrophobic surfaces.36,41,47 Usually, the hydrophobic surfaces strongly attract each other to eject water into the bulk to minimize the free energy of the system.48 Thus, the existence of hydrophobic forces may be critical for solving the difficulty in predicting the stability of amphiphilic Janus nanosheets using the DLVO theory. With this in mind, we attempted to predict the aggregation behaviors of amphiphilic Janus nanosheets by using the extend DLVO theory model, which considers the effect of hydrophobic forces. The predicted CCC values of CSAJN in aqueous solutions by eDLVO theory model were 21.4 mM Na+ or 4.7 mM Ca2+, which is closes to the experimental CCC. In other words, the eDLVO theory model successfully predicted the aggregation behaviors of amphiphilic Janus nanosheets in aqueous solutions. The hydrophobic forces should be considered when analyzing the aggregation behaviors of amphiphilic Janus nanosheets.

4. Conclusion Understanding the aggregation characteristics of amphiphilic Janus nanosheets in aqueous solution is the prerequisite for further practical applications,8,9,33 but little has been known about it at present.19 The aggregation kinetics and colloidal stability of amphiphilic Janus nanosheets have been systematically investigated in this paper for the first time through the combinations of experiments and theoretical analysis. Amphiphilic Janus nanosheets gradually aggregated in aqueous, and then slowly migrated to the bottom of the suspensions to formed precipitation. The 20 ACS Paragon Plus Environment

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aggregation of amphiphilic Janus nanosheets was closely relevant to the aqueous pH due to the ionization of hydrophilic groups on the surface of particles and was more pronounced as the temperature increases. The aggregation characteristics of amphiphilic Janus nanosheets described above were similar to the conventional nanoparticles.20-23 However, the direct comparisons of aggregation kinetics of amphiphilic Janus nanosheets with hydrophilic Janus nanosheets at different electrolyte solutions revealed that the aggregation tendency of amphiphilic Janus nanosheets was stronger than that of non-amphiphilic Janus nanosheets, which was inconsistent with the predictions of the classical DLVO theory. An extended DLVO theory model containing hydrophobic interactions was developed, which could accurately predict the aggregation behaviors of amphiphilic Janus nanosheets in aqueous solutions. This study will enhance the understanding of the colloidal behavior of amphiphilic Janus nanosheets and facilitate their applications in unfriendly environments. In the future, the microscopic morphology of the aggregation will be observed through the scanning electron microscope to determine the aggregate process of amphiphilic Janus nanosheets.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions §T.Y.

and Z.Y. contributed equally to this work.

Notes 21 ACS Paragon Plus Environment


The authors declare no competing financial interest.

Supporting Information The chemicals; calculation of particle attachment efficiency; AFM images and hydrodynamic diameter distribution of nanosheets; stability testing and electrostatic interaction energy curve of CSAJN at different pH; stability testing and eDLVO interaction energy curve of CSAJN at different temperature; TSI-time curves, aggregation profiles and DLVO interaction energy curves of nanosheets in various electrolyte solutions.

Acknowledgements This work was supported by the National Key Technologies R&D Program of China (2017ZX05009-004).

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For Table of Contents (TOC) Graphic Only

Interaction Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

van der Waals Electrostatic Hydrophobic DLVO eDLVO

Intersheet Distance (nm)


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Aggregation Kinetics and Colloidal Stability of Amphiphilic Janus

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