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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Contrasting Mechanisms of Spontaneous Adsorption at Liquid-Liquid Interfaces of Nanoparticles “Constituted of” and “Grafted with” pH-Responsive Polymers Dalin Wu, and Andrei Honciuc Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00877 • Publication Date (Web): 05 May 2018 Downloaded from http://pubs.acs.org on May 6, 2018

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Contrasting Mechanisms of Spontaneous Adsorption at Liquid-Liquid Interfaces of Nanoparticles “Constituted of” and “Grafted with” pH-Responsive Polymers

Dalin Wu† and Andrei Honciuc†* †

Institute of Chemistry and Biotechnology, Zurich University of Applied Sciences, Einsiedlerstrasse 31, 8820 Waedenswil, Switzerland

*Corresponding author: [email protected], tel.:+41589345283, ICBT/ZHAW, Einsiedlerstrasse 31, 8820 Waedenswil, Switzerland

Abstract Elucidating the mechanisms responsible for spontaneous adsorption of nanoparticles (NPs) at interfaces is important for their application as emulsifiers, bubble stabilizers or foaming agents. In order to investigate the key factors that control spontaneous adsorption of NPs at Liquid-Liquid interfaces, we synthesized seven different types of NPs from pH-responsive polymers poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) and poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) via surfactant-free emulsion polymerization or via “grafting from” polystyrene (PS) NPs. The dynamic interfacial tension (IFT) measurements at toluenewater (Tol-H2O) interface reveal that when PDEAEMA and PDMAEMA are grafted from the surface of PS NPs, the solubility of the grafted pH-responsive polymers in toluene is the key factor determining the NPs’ interfacial adsorption. In acidic condition (pH < 6.0), PDEAEMA and PDMAEMA are protonated, show no solubility in toluene and as a result the grafted NPs do not adsorb at Tol-H2O interface. Oppositely, in basic condition (pH > 7.0), PDMAEMA dissolves in toluene and therefore the PDMAEMA grafted NPs can adsorb at Tol-H2O interface. Interestingly, when NPs are constituted of PDEAEMA, they can adsorb spontaneously at Tol-H2O interface in acidic condition (pH < 6.0), but not in basic conditions (pH > 7.0). In this case, the key factor determining the NPs’ spontaneous adsorption at Tol-H2O interface is the softness degree of NPs rather than the solubility of PDEAEMA in toluene. Further, we found that the adsorption of NPs constituted of PDEAEMA (pH 2.0 - 6.0) and PDMAEMA grafted PS NPs at (pH 7.0 - 10.0) at Tol-H2O interface is a combination of diffusion controlled and energy barrier controlled. The opposite trends observed for the interfacial attachment ∆E and activation energies for the “constituted of” and “grafted from” NPs with pH suggest an opposite mechanisms of adsorption at Tol-H2O interface. Finally, the synthesized NPs prove to be effective emulsifiers, where the phase of the Pickering emulsions can be changed dynamically by pH adjustment.

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1. Introduction Understanding the mechanisms of adsorption of NPs at interfaces and the governing particle-interface interactions that eventually lead to their interfacial adsorption are of significant fundamental interest and also of great practical importance across many industries. Similar to surfactants and amphiphilic macromolecules, particles with appropriate surface properties, such as solvent wettability, can also adsorb spontaneously at Liquid-Liquid (L-L) interfaces, thus decreasing the interfacial tension or in the presence of convection currents with the formation of Pickering emulsions.1–6 Currently, there is an increased interest in designing particles that exhibit spontaneous, meaning both thermodynamic and kinetically favored, interfacial adsorption at L-L interface from bulk without any energy input, from a sonotrode or high speed homogenizer.7–11 The groups of Richtering & To Nagi have reported that the poly(N-isopropylacrylamide) (PNIPAM) microgel particles can spontaneously adsorb at heptane-water interface9,12 decreasing the IFT by 33 mN/m from initially 43 mN/m.13 It was shown that the spontaneous adsorption of PNIPAM microgel at heptane-water interface with measurable decrease in the IFT values was essentially caused by the degree of softness and deformation ability of the PNIPAM microgel particles in a temperature interval from 13 to 44 ºC.14,15 The softer PNIPAM microgel caused a stronger reduction of the IFT and this explanation was backed by the theoretical calculations and simulation by others.16,17 On the other hand particles which were subsequently surface grafted with various polymers brushes, surfactants or amphiphilic block copolymers can also adsorb spontaneously at L-L interface with decreasing the IFT.18,19 For example, Kan Du et al.20 reported that the (1-mercaptoundec-11-yl)tetra-(ethylene glycol) (TEG) grafted gold NPs can adsorb spontaneously at 2,2,3,3,4,4,5,5-octafluoropentyl acrylate-water interface with decreasing the IFT value from initially 25 mN/m to 12 mN/m. Similarly, Yi Zhang et al.8 found that the methylsilyl capped SiO2 NPs can spontaneously adsorb at the air-water interface with decreasing the IFT value from 72 mN/m to 60 mN/m in 1800 s, while unfunctionalized SiO2 did not adsorb at the air-water interface. While both soft and grafted particles appear to spontaneously adsorb at L-L interfaces, the underlying mechanisms of interfacial adsorption and decreasing the IFT may be different. Here we attempt a comparative study with respect to the effectiveness of these two classes of NPs to decrease the IFT of the Tol-H2O interface. To the best of our knowledge such comparative studies have not been yet performed using soft and grafted NPs created from the same or very similar polymers in one report. For this study, we prepared two types of NPs. The first type, has a bulk composition based on poly(2(diethylamino)ethyl methacrylate) (PDEAEMA), which will be referred to hereinafter as PDEAEMA constituted NPs. PDEAEMA is a popular pH-responsive polymer with pKa value 7.0-7.3,21 which is often used in drug-delivery research.22 The PDEAEMA NPs change from hard to soft gradually with pH value changing from 10.0 to 2.0 due to the protonation in aqueous solution. In addition, through controlling the cross-linking degree (1% - 3%) and co-polymerizing strategy with a pH-inert and hydrophobic monomer, butyl methacrylate, the softness of PDEAEMA NPs could be effectively controlled. 2 ACS Paragon Plus Environment

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The second type of NPs were made by grafting solid polystyrene (PS) NPs with the poly(2dimethylamino)ethyl methacrylate) (PDMAEMA), which will be referred hereinafter as PDEAEMA grafted PS NPs. The PDMAEMA is a polymer with pKa value 7.4-7.8,23 but it is a water soluble polymer in both its protonated and de-protonated states (at low temperatures). Its hydrophilicity can further increase by protonation in acidic conditions (pH < 6.0). The two types of NPs, exhibit a contrasting behavior with respect to their L-L interfacial adsorption function of pH. The first type, PDEAEMA constituted NPs can decrease the IFT of Tol-H2O only in acidic condition (pH < 6.0). The second type, the PDMAEMA grafted PS NPs behaved oppositely, namely, the IFT decreased only in basic condition (pH > 7.0) but not in acidic condition (pH < 6.0). Moreover, the decreasing in the IFT value was more dramatic upon the adsorption of the latter type of NPs, ~ 30 mN/m at pH 10.0, than in the case of the former ones ~ 15 mN/m in pH 2.0. Both PDEAEMA constituted NPs and PDMAEMA grafted PS NPs were excellent emulsifiers for toluene and water. Even more, the emulsion phase could be dynamically changed by changing the pH, to either Tol/H2O or H2O/Tol phases. This property could be a significant advantage in encapsulations and triggered release applications.24,25 The results obtained from the dynamic IFT measurements and Pickering emulsion phase correlate well with particle surface properties. Another important advantages of Pickering emulsions, compared to surfactant stabilized emulsions, is their high stability due to the strong attachment of particles at the L-L interface.26 For example, the adsorption free energy of a spherical solid NPs of 10 nm diameter at the hydrocarbon-water interface (IFT = 50 mN/m) having a contact angle of 90° degree is 3.89 x 103 kT, which is much larger than that of the corresponding molecular surfactants, only slightly larger than the thermal energy ~1 kT at 293 K.27 In the presence of convection currents due to mechanical agitation in emulsification experiments, the NPs acquire high kinetic energy, increased bulk diffusivities, thus easily overcome the interfacial adsorption energy barriers adsorbing irreversibly at the interface. In other words, in such case, the existing particle-interface interactions play a less significant role in the interfacial adsorption of particles than the magnitude of the external energy input. The types of Pickering emulsion (w/o and o/w) are mainly determined by the surface wetting-ability of particles at interface by the organic solvent and water.28 Generally speaking, particles with hydrophilic surface, for example, un-modified SiO2, having better wetting-ability in water than in organic solvent, can generate o/w Pickering emulsion.29 While, particles with hydrophobic surface, for example, PS particles, having better surface wetting-ability in organic solvent than in H2O, which results in w/o Pickering emulsion.30 However, if the surface of particles is too hydrophilic or too hydrophobic, the activation energy for their L-L interfacial attachment may be too high to overcome by mechanical stirring, thus failing to emulsify and produce Pickering emulsions. NPs in this manuscript, due to their ability to adsorb at the L-L interfaces they proved to be efficient emulsifiers that can be used to create pH-responsive Pickering emulsions from toluene and water.


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2. Experimental Section Materials: Styrene (Sty) (˃ 99%), divinylbenzene (DVB) (80%), 2-(dimethylamino)ethyl methacrylate (DMAEMA) (> 98%), 2-(diethylamino)ethyl methacrylate (DEAEMA) (> 99%), 2, 2 ′ -Azobis(2methylpropionamidine) dihydrochloride (V-50) (˃ 97%), 2,2′-Azobis(2-methylpropionitrile) (AIBN) (> 98%), (vinylbenzyl)trimethylammonium chloride (VBTMAC) (˃ 99%), 4-vinylbenzyl chloride (4-VBC) (> 90%), butyl methacrylate (BMA) (> 99%), copper(I) bromide (99.999%), copper(II) bromide (> 99%), ethanol (EtOH) (˃ 99%), tetrahydrofuran (THF) (> 99.9%), toluene (Tol) (˃ 99%), ethylene glycol dimethacrylate (EGDMA) (> 98%), ammonium hydroxide solution (NH4OH) (28%), N,N,N´,N´´,N´´pentamethyldiethylenetriamine (PMDETA) (> 99%), and aluminum oxide (Al2O3 basis) (≥ 98%) were purchased from Sigma-Aldrich. Sty, DMAEMA, DEAEMA, 4-VBC, BMA and DVB were passed through Al2O3 to remove the stabilizer before usage. AIBN was re-crystalized from ethyl ether twice before usage. Other reagents were used as received. Ultrapure water (UPW) (conductivity at 298 K: 0,055 µS/cm) was obtained from an Arium 611 VF water purification system (Startorius Stedim Biotech, Aubagne, France). Instruments and Characterization Methods: The morphologies of synthesized NPs were characterized with the SEM (FEI Quanta FEG 250), operating at 5 – 30 kV accelerating voltage in the secondary electron (SE) mode under high vacuum (3 × 10–6 – 1.8 × 10–5 mbar). Before the measurement, the NPs were sputtered with Au in a sputter-coater (Q15OR-S Sputter Coater, Quorum, 20 mA for 30 sec), under Ar atmosphere (sputter vacuum: 5 × 10–2 mbar). The FTIR analyses of synthesized NPs were performed with a Perkin Elmer spectrometer (Spectrum 1000). For each sample 1000 spectra between 400 cm-1 to 4000 cm-1 were acquired and averaged. The hydrodynamic diameter of NPs in concentration 0.5 mg/mL at different pH values (2 - 10) were measured by dynamic light scattering (DLS) (Malvern Instruments, Worcestershire, UK), equipped with a 4 mW He-Ne laser at a wavelength of 633 nm and a set scattering angle of 173° at 25 °C. The IFT measurements were carried out via pendat-drop and equilibrium drop-shape analysis method. The experiments were done using a DataPhysics OCA 15Pro contact angle goniometer, equipped with an automatic dosing system. The images of the drop shape were captured in real time and then digitally analyzed using the edge-detecting DataPhysics SCA 22 software module by fitting the contour of the droplet to the Young-Laplace equation. For the measurement process, we prepared a NPs stock aqueous solution with concentrations: 10 mg/mL and 20 mg/mL in different pH values. The liquid droplets of the PDEAEMA constituted NPs aqueous solution were generated at the apex of a straight dosing needle within toluene phase contained in a 20 cm cubic quartz cuvette and corresponding dosing volume 35 µL and dosing rate 35 µL/s, and for the PDMAEMA grafting NPs, the dosing volume is 15 µL and dosing rate 35 µL/s. The dosing rates for generating the droplet at the apex of the tip were kept high, in order to achieve good reproducibility and


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eliminate any lag time during the droplet expansion. If the expansion is slow then the beginning values will be different from the IFT of the pristine interface, the same is valid if there are surface active molecular impurities that adsorb significantly faster, on the order of milliseconds, thus pushing the starting value already significantly lower than for example ~ 35 mN/m the IFT for pure Tol-H2O interface. The penetration measurement for the PDEAEMA measured directly on dried PDEAEMA bulk polymer samples, which have the same cross-linking degree (1%) with PDEAEMA-1 NPs. The procedure of synthesizing this bulk material was described in SI. The PDEAEMA bulk polymer sample was cut in small cubes (1 x 1 x 1 cm). The PDEAEMA cubes were then added into a 500 mL beaker containing water and ensuring that the pH values remained constant for 48 hours. The penetration rate was measured with a computer controlled DCAT 21 Tensiometer (DataPhysics Instruments GmbH) equipped with a solid brass penetration cone. The tensiometer measures the compensated weight (the resistance of the material to penetration) with time due to lowering of the brass cone (25 g) at a constant speed into the soft material. During the measurement, the following important parameters were used: motor speed 0.05 mm/s, surface detection threshold 0.05 mg, diving speed of the penetration cone, 0.05 mm/s and maximum immersion depth of 1 mm. Transmission electron microscopy (TEM): A 5 µL amount of NPs solution (0.1 mg/mL) was absorbed on 400 mesh copper grids. The grids were further stained with 2% uranyl acetate and the negatively stained image of nanostructures were aquired on a Philips CM100 TEM at an acceleration voltage of 80 kV. Synthesis of PDEAEMA constituted NPs with different cross-linking degree: 500 mg VBTMAC, 100 mg V-50 were first added into 100 mL ultrapure water (UPW) in a 250 mL round-bottom flask. Subsequently, 10 mL DEAEMA and EGDMA (100 µL or 300 µL) were added. Then the solution was de-oxygenated by bubbling argon for 20 minutes. The reaction was further initiated by heating to 60 °C under Ar. The reaction was carried out for 24 hours under stirring at 700 rpm. Finally, the PDEAEMA constituted NPs with cross-linking degree 1% (PDEAEMA -1) and 3% (PDEAEMA -2) were purified by washing with EtOH four times and UPW three times continually before IFT measurements. Synthesis of PDEAEMA constituted NPs with 200 nm diameter and 1% cross-linking degree: 1000 mg VBTMAC, 100 mg V-50 were first added into 100 mL ultrapure water (UPW) in a 250 mL round-bottom flask. Subsequently, 10 mL DEAEMA and 100 µL EGDMA were added. Then the solution was deoxygenated by bubbling Ar for 20 minutes. The reaction was further initiated by heating to 60 °C under Ar. The reaction was carried out for 24 hours under stirring at 700 rpm. Finally, the PDEAEMA constituted NPs with cross-linking degree 1% (PDEAEMA -3) were purified by washing with EtOH four times and UPW three times before IFT measurements. Synthesis of PDEAEMA-co-PBMA copolymerized NPs: 500 mg VBTMAC, 100 mg V-50 were first added into 100 mL UPW in a 250 mL round-bottom flask. Subsequently, 5 mL DEAEMA, 5 mL BMA and 100 µL 5 ACS Paragon Plus Environment

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EGDMA were added. Then the solution was de-oxygenated by bubbling argon for 20 minutes. The reaction was further initiated by heating to 60 °C under Ar. The reaction was carried out for 24 hours under stirring at 700 rpm. Finally, the PDEAEMA-co-PBMA NPs were purified by washing with EtOH four times and UPW three times continually before IFT measurements. Synthesis of ATRP initiator PS-Cl NPs: 50 mg VBTMAC, 100 mg V-50 were first added into 100 mL UPW in a 250 mL round-bottom flask. Subsequently, 9 mL styrene, 1 mL 4-VBC and 100 µL DVB were added. Then the solution was de-oxygenated by bubbling argon for 20 minutes. The reaction was further initiated by heating to 60 °C under Ar. The reaction was carried out for 24 hours under stirring at 700 rpm. Finally, the synthesized NPs were purified by washing with THF four times before ATRP of DEAEMA and DMAEMA. Synthesis of PDEAEMA grafted PS NPs: 672 mg PS-Cl NPs were dispersed in the mixture of THF (10 mL) and EtOH (10 mL) first. Then 3.4 mg CuBr2, 66 µL PMDETA and 43 mg CuBr were added into the above solution. After the mixture was bubbled with argon for 20 minutes under ice cooling bath, DEAEMA (1 mL, 2 mL or 3 mL) was added into the solution. The final mixture was bubbled with argon for another 15 minutes under ice cooling bath before initiating the polymerization at 60 ºC under Ar for 17 hours. Synthesis of PDMAEMA grafted PS NPs: The procedure and amount of chemicals used in the synthesis were the same as in the case of PDEAEMA grafted PS NPs except that DMAEMA (3 mL) was used instead of DEAEMA.

Scheme 1. The synthesis routes of different nanoparticles used in the current work. 6 ACS Paragon Plus Environment

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Pickering emulsion preparation. A stock solution containing 4 mg/mL NPs in UPW was first prepared. The pH value of the stock solution was adjusted by HCl and NH4OH. Next, 5 mL of the stock solution containing NPs was mixed with 4 mL Tol containing 0.01 wt. % hydrophobic Hostasol Yellow 3G dye. Finally, the mixture was homogenized using a Branson 450D sonifier equipped with a ½ inch horn for 30 seconds at room temperature or 70 °C. 3. Results and Discussion 3.1. Synthesis and Characterization of NPs The aim of this work is to investigate the main factors influencing the adsorption of NPs at L-L interface. Scheme 1 shows the synthesis procedures of the seven types of NPs used in this work. The tertiary amine groups of PDEAEMA (pKa = 7.0 - 7.3) is protonated when the pH value is below its pKa, as a result, the hydrophilicity of PDEAEMA, the swelling degree and the softness of the PDEAEMA constituted NPs also increase. Four PDEAEMA constituted NPs were synthesized with 1% (PDEAEMA-1, PDEAEMA-3) and 3% (PDEAEMA-2) crosslinking degree or co-polymerized with the hydrophobic monomer BMA (PDEAEMAco-PBMA). Four grafted PS NPs were made from starting PS-Cl NPs via ATRP grafting of PDEAEMA (PDEAEMA-g-PS-1, PDEAEMA-g-PS-2 and PDEAEMA-g-PS-3, differing in the grafted polymer lengths) and PDMAEMA (PDMAEMA-g-PS). The PS-Cl NPs had a 1% cross linking degree to ensure that they do not dissolve in Tol during the IFT measurement and emulsification experiments.25,30 PDMAEMA-g-PS was synthesized because PDMAEMA is hydrophilic below pKa 7.4 - 7.8 and remains hydrophilic (water soluble) above its pKa, in contrast PDEAEMA becomes hydrophobic (water insoluble) above its pKa (7.0 - 7.3), which causes the PDEAEMA grafted PS NPs to strongly aggregate. This made the IFT vs. time measurement of PDMAEMA grafted PS NPs possible in both acidic and basic condition. The composition of synthesized NPs is given in Table S1.

3.2. Characterization and pH-responsiveness of PDEAEMA constituted NPs SEM images of PDEAEMA constituted NPs with typical soft nanogel morphologies are presented in Figure 1. Figure 2A and Table S2 show the evolution of the hydrodynamic diameters of the PDEAEMA constituted NPs with pH; the diameters increase from 500 – 800 nm to 1100 – 1350 nm with decreasing the pH values from 10.0 to 2.0. Correspondingly, the swelling degree expressed as the change in the change in the fractional the diameter of the NPs at different pH with respect to the diameters at pH 10, calculated with equation . .

× 100% (HD: hydrodynamic diameter). The swelling degree increases with decreasing pH

values from 10.0 to 2.0 (Figure 2B and Table S3). Comparatively, the swelling degree of PDEAEMA-1 increases from 0 to 162 % with decreasing pH from 10.0 to 2.0, but the swelling degree of PDEAEMA-2 and PDEAEMA-co-PBMA increases only with ~ 42% in the same pH interval. Evidently, the cross-linking degree


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and co-polymerization of a hydrophobic monomer proved very effective in controlling the swelling behavior and softness of the PDEAEMA constituted NPs.

Figure 1. SEM images of PDEAEMA constituted NPs at pH 7. (A) PDEAEMA-1, (B) PDEAEMA-2 and (C) PDEAEMA-co-PBMA copolymerized NPs. Scale bar is 5 µm.

Additionally, in the 10.0 to 2.0 pH range, the DLS results in SI (Figure S1 A, B, C and Table S2) show that PDEAEMA constituted NPs are well-dispersible in the water and reasonably monodisperse, which allowed us to perform the dynamic IFT measurements at Tol-H2O interface. Interestingly, the zeta potential values of PDEAEMA-1 NPs remains positive and unchanged, well above +30 mV from pH 2.0 to 10.0 (Figure S2A).

Figure 2. pH-responsive behavior of PDEAEMA constituted NPs. (A) The influence of pH values in solution (2.0 – 10.0) on hydrodynamic diameters of PDEAEMA constituted NPs; (B) The relationship of swelling degree of PDEAEMA constituted NPs with pH values in solution (2.0 – 10.0).

3.3. Characterization of PDEAEMA and PDMAEMA grafted PS NPs The synthesis of the PDEAEMA and PDMAEMA grafted PS NPs was performed via ATRP. The -Cl initiator was immobilized on the surface of the starting PS NPs. The SEM images of the obtained PS-Cl, PDEAEMA-


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g-PS-1, PDEAEMA-g-PS-2, PDEAEMA-g-PS-3 and PDMAEMA-g-PS are presented in Figure 3 A-E, which show round and well-defined spherical particles. The diameters of the PDEAEMA grafted NPs can be controlled by the grafted polymer length, increasing with 10 to 30 nm (Figure 3F) compared with the original PS-Cl NPs (diameter ~ 150 nm). The hydrodynamic diameter of the PS-Cl and grafted NPs were also measured in native solution by DLS at pH = 2, when presumably the polymer chains in the brush are fully stretched; the data is shown in Table S4. The thickness of the brush shell in the grafted NPs increases significantly with ~200 to ~300 nm as compared with the starting PS-Cl NPs. The zeta potential of the

PDMAEMA-g-PS NPs decreases from +64 to +8 mV with increasing pH from 2.0 to 10.0 (Figure S2 B) demonstrating the protonation of PDMAEMA at pH < 7.0.

Figure 3. SEM images of PDEAEMA and PDMAEMA grafted PS nanoparticles. (A) PS-Cl ARTP initiator NPs, (B) PDEAEMA-g-PS-1, (C) PDEAEMA-g-PS-2, (D) PDEAEMA-g-PS-3, (E) PDMAEMA-g-PS and (F) Summary of evolution of the grafted NPs’ diameters. Scale bar is 1 µm.

The FTIR analysis of the grafted NPs there show additional vibration bands appearing at 1728 cm-1 and 1146 cm-1 originating from the vibration of –C=O and –N-C-, as compared with the spectrum of the PS-Cl NPs, see Figure S3, demonstrate the successful grafting of PDEAEMA and PDMAEMA on the surface of the PS NPs via ATRP.

3.4. Adsorption of NPs at the Tol-H2O interface The dynamic IFT measurements for slow adsorbing particles are typically done via pendant drop tensiometry and drop-shape analysis. Here we emphasize that in such measurements two important points should be carefully considered. The first one is the good dispersibility of particles in one of the phases; if the particles 9 ACS Paragon Plus Environment

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are not well dispersed in the bulk solution, with the formation of aggregates, the IFT vs. time results are difficult to interpret. The second one is the need to carefully eliminate the interfacially active impurities from the solution, especially when using polymers, surfactants and amphiphilic block copolymers as capping agents or stabilizers that are only adsorbed on the particle surface through week interaction, such as double-layer interaction, hydrogen bonding or hydrophobic interaction. Presence of such impurities will greatly interfere with the IFT measurements and very often invalidate the results. Therefore, we took great care to ensure that no impurities were present. Figure 4 shows the evolution of the IFT with time as a consequence of PDEAEMA-1 NPs adsorption from the aqueous phase at the Tol-H2O interface, measured via pendant-drop method. In basic condition (curve a, pH 10.0, 10 mg/mL), the IFT value remains constant at ~34 mN/m without significant decrease with time, while the IFT decreases dramatically in the acidic condition (curves b, c, d, e & f at pH ≤ 6.5, 10 mg/mL). Furthermore, the decreasing of IFT values of PDEAEMA-1 NPs in acidic conditions strongly depends on the pH value of the aqueous solution. Lower pH values result in lower final IFT values (b, c, d, e and f), or larger ∆IFT values. The ∆IFT is the difference between the initial value of the IFT at t = 0 s and the plateau IFT value at t = 180 minutes. In the last section, we discussed that a gradual increase in the swelling degree implies that PDEAEMA-1 NPs (Figure 2), become softer at lower pHs, here understood as compressible and deformable. Because all the other parameters in the system remain constant, except pH, it is likely that the increasingly larger ∆IFT values observed at lower pHs are a consequence of an enhanced interfacial activity of PDEAEMA-1 NPs due an increased degree of swelling or softness, in agreement with the observations of Richtering et al.15 The lowest IFT value (21.1 mN/m) was obtained at pH 2.0 (curve f, 10 mg/mL), the dynamic IFT of of PDEAEMA-1 NPs were also measured at pH 2.0 at higher concentration (curve g, 20 mg/mL). A further increase in the concentration of the PDEAEMA-1 NPs to 30 mg/mL resulted in a gel (SI, Figure S4), which may be due to the fact that all the free water has been absorbed by the swelling PDEAEMA-1 NPs. The highest PDEAEMA-1 NPs concentration for which we could perform the dynamic IFT measurement was 20 mg/mL with the final IFT ~19.2 mN/m, Figure 4 (curve g). From Figure 4, it is obvious that both lower pH values and higher PDEAEMA-1 NPs concentrations lead to larger ∆IFT values. In order to demonstrate that the PDEAEMA-1 NPs become softer with decreasing the pH value of the solution, we ran penetration experiments on the PDEAEMA bulk materials with the same chemical composition and cross-linking degree as the PDEAEMA-1 NPs at different pH values. The preparation method and the experimental setup are given in the SI. The result in Figure S5 shows that PDEAEMA bulk material at pH 2.0 is soft, showing the fastest penetration rate (g/s). The hardness of the PDEAEMA material increases progressively with increasing the pH values of the aqueous solution.


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Figure 4. The dynamic IFT results of PDEAEMA-1 at Tol-H2O interface in different pH values. (a) pH = 10 (10 mg/mL), (b) pH = 6.5 (10 mg/mL), (c) pH = 5.0 (10 mg/mL), (d) pH = 4.0 (10 mg/mL), (e) pH = 3.0 (10 mg/mL), (f) pH = 2.0 (10 mg/mL) and (g) pH = 2.0 (20 mg/mL). The error bars represent the standard deviation of three measurements. By carefully analyzing the IFT vs. time curves in Figure 4 (b, c, d, e, f and g) it becomes evident that there are two stages describing the PDEAEMA-1 NPs adsorption at Tol-H2O interface. The first stage happens at the very beginning, short time 0-1 min. At short time the PDEAEMA-1 NPs exhibit very fast adsorption kinetic with rapid reduction in the IFT. At this stage of the adsorption process, short time 0-1 min, the incoming PDEAEMA-1 NPs diffusing from the bulk solution meet a pristine Tol-H2O interface and will easily adsorb causing the IFT to rapidly decrease. During the second stage, at later time > 1 min, the IFT decreases slowly and reaches the final plateau value in very long time. This can be explained by the increased occupancy of the interface with the NPs, which impose an additional adsorption barrier due to the increasing electrostatic repulsion between PDEAEMA-1 NPs already present at the interface and those in the bulk, prohibiting the fast adsorption of PDEAEMA-1 NPs from the H2O bulk solution. As already mentioned Richtering et al.15 demonstrated that soft particles adsorb at L-L interface easier than solid particles. Similar results were obtained here by using pH-responsive NPs. The PDEAEMA-1 NPs are softer in acidic than in basic solution, due to the protonation of PDEAEMA. Additionally, the fact that ∆IFT value is strongly pH-dependent also demonstrates that the softness of PDEAEMA-1 NPs does play a key role 11 ACS Paragon Plus Environment

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in NPs adsorption at the Tol-H2O interface. In order to further prove this hypothesis, we synthesized two other types of PDEAEMA constituted NPs: PDEAEMA-2 with 3% crosslinking degree and PDEAEMA-co-PBMA NPs with 1% crosslinking degree, but in which PDEAEMA was copolymerized with a pH inert and hydrophobic monomer. From Figure 2 and Table S3, we see that the swelling degrees of PDEAEMA-2 NPs and PDEAEMA-co-PBMA NPs at each pH values are lower than that of PDEAEMA-1 NPs, meaning that the strategy in restricting swelling degree was successful. Figure 5 shows that in basic conditions (pH 10.0), both PDEAEMA-2 and PDEAEMA-co-PBMA NPs are not capable of lowering the IFT values (Figure 5A-a and Figure 5B-a), but in acidic condition (pH < 5.0), can decrease the IFT of the Tol-H2O interface significantly. The lowest plateau IFT value 22.7 mN/m (Figure 5A-d) and 29.0 mN/m (Figure 5B-d) were attained by PDEAEMA-2 and PDEAEMA-co-PBMA NPs at pH 2.0 with concentration 20 mg/mL respectively, which are higher than the final IFT value of PDEAEMA-1 NPs (19.2 mN/m) in the same pH and concentration condition (Figure 4, curve g). The above results demonstrate that: (1) increasing the cross-linking degree and co-polymerized inert hydrophobic monomer reduces the swelling degree of NPs and the softness of the NPs and (2) the softer the PDEAEMA-1 NPs (Figure S5), the lower the final IFT values of Tol-H2O interface observed upon NPs adsorption.

Figure 5. The dynamic IFT results of PDEAEMA-2 NPs (A) and PDEAEMA-co-PBMA NPs (B) at Tol-H2O interface in different pH values: (a) pH = 10 (10 mg/mL), (b) pH = 4.0 (10 mg/mL), (c) pH = 2.0 (10 mg/mL) and (d) pH = 2.0 (20 mg/mL). The error bars represent the standard deviation of three measurements.

The PDEAEMA-1, PDEAEMA-2 and PDEAEMA-PBMA NPs were all capable of spontaneous interfacial adsorption at Tol-H2O after the protonation of PDEAEMA polymers in acidic condition (pH < 6.0). Next, we investigate if the other type of particles, namely the PDEAEMA grafted PS NPs can also adsorb spontaneously at the Tol-H2O interface in the same pH conditions. We have synthesized three PDEAEMA-g-PS NPs with different PDEAEMA polymer chain lengths on the surface (10 nm, 20 nm and 30 nm, Figure 3F-b, c, d). The dynamic IFT measurement of Tol-H2O in acidic condition (pH 2.0) with the NPs concentration 20 mg/mL are represented in Figure 6A (a, b and c). The IFT vs. time remained unchanged for five hours, which means that PDEAEMA-g-PS NPs did not adsorb at TolH2O interface regardless of the PDEAEMA polymer chain length grafted on the surface of PS NPs. We also 12 ACS Paragon Plus Environment

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attempted to measure the Tol-H2O IFT value of PDEAEMA-g-PS NPs in basic condition (pH = 10.0), but the PDEAEMA-g-PS NPs did not disperse well in water at this pH value. In order to be able to measure the graftet NPs in basic conditions, we grafted instead PDMAEMA on the surface of PS NPs (PDMAEMA-g-PS). The length of PDMAEMA is around 20 nm (Figure 3F-e). PDMAEMA has a pKa 7.4-7.8 and a similar chemical structure with PDEAEMA but is more hydrophilic.31 Its water solubility in acidic aqueous solution (pH < 6.0) is higher than in basic aqueous solution (pH 10.0), because of the tertiary amine protonation. Figure S1-D shows that PDMAEMA-g-PS NPs are well-dispersible in both acidic (pH 2.0) and basic conditions (pH 10.0), which allowed the measurement the IFT of PDMAEMA-g-PS at Tol-H2O interface in the pH 2.0 to 10.0 interval. The dynamic IFT measurement results obtained are presented in Figure 6B-a and these show that PDMAEMA-g-PS NPs with concentration 10 mg/mL behave similarly with PDEAEMA-g-PS NPs at pH 2.0 without a noticeable decrease in the IFT value of Tol-H2O with time. On the other hand, the IFT value decreases greatly in the presence of PDMAEMA-g-PS (at 10 mg/mL), from 34 mN/m to 17.2 mN/m, 15.0 mN/m and 5.5 mN/m respectively at pH 7.2, pH 8.0 and pH 10.0 in only 60 minutes (Figure 6B-b, c and d). Moreover, the final IFT value was 4.1 mN/m (Figure 6B-e) when PDMAEMA-g-PS concentration was 20 mg/mL at pH 10.0. Because of the rapid lowering of the IFT value at pH 10.0 when the water phase contains NPs in higher concentration, ~20 mg/mL, the pendent drop of PDMAEMA-g-PS NPs solution quickly elongates and detaches from the tip of needle (Figure S6). Additionally, the PS-Cl NPs do not adsorb at the Tol-H2O interface at pH 10.0 (at 10 mg/mL), the IFT values remained constant at ~34 mN/m (Figure 6B-h), demonstrating that the decreasing of the IFT for PDMAEMA-g-PS NPs is caused by the grafted PDMAEMA brush. In addition, the IFT vs. time values of homopolymer PDEAEMA and PDMAEMA at Tol-H2O interface at different pH values were also investigated. Figure 6A-d and Figure 6B-f demonstrate that at pH 2.0, homopolymer PDEAEMA and PDMAEMA in their polyelectrolyte form cannot adsorb at Tol-H2O interface and thus decrease the corresponding IFT, behaving similarly to the PDEAEMA and PDMAEMA grafted PS NPs. However, PDMAEMA at pH 10.0 with concentration 2 mg/mL can decrease the IFT of Tol-H2O to lower than 1 mN/m just in 30 seconds (Figure 6B-g). After 30 second, the polymer solution pendant drop detaches from the tip of the needle (Figure S7), which indicates that the non-protonated homopolymer PDMAEMA with some solubility in Tol has a relatively high interfacial activity.


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Figure 6. The dynamic Tol-H2O IFT results of PDEAEMA-g-PS NPs, PDMAEMA-g-PS NPs, PDEAEMA and PDMAEMA homopolymer. (A) Dynamic Tol-H2O IFT with 20 mg/mL NPs in the water phase at pH 2.0: (a) PDEAEMA-g-PS-1, (b) PDEAEMA-g-PS-2, (c) PDEAEMA-g-PS-3 and (d) dynamic IFT of PDEAEMA homopolymer with concentration 20 mg/mL at pH 2.0. (B) Dynamic Tol-H2O IFT of PDMAEMA-g-PS NPs with 10 mg/mL at: (a) pH = 2.0, (b) pH = 7.2, (c) pH = 8.0, (d) pH = 10.0, (e) PDMAEMA-g-PS NPs with concentration 20 mg/mL at pH 10.0, (f) PDMAEMA homopolymer with concentration 20 mg/mL at pH 2.0 , (g) PDMAEMA homopolymer with concentration 2 mg/mL at pH 10.0 and (h) PS-Cl NPs with concentration 10 mg/mL at pH 10.0. The error bars represent the standard deviation of three measurements.

Comparing the IFT results from Figure 4f and Figure 6B-d, it is clear that PDMAEMA-g-PS NPs at pH 10.0 (10 mg/mL) have stronger ability to decrease the IFT of Tol-H2O than PDEAEMA-1 (10 mg/mL) at pH 2.0. However, PDEAEMA-1 NPs and PDMAEMA-g-PS NPs also have very different diameters. In order to test whether the diameter of NPs has an influence on the decreasing the IFT of Tol-H2O, we have synthesized the smaller PDEAEMA constituted NPs, namely PDEAEMA-3, which have the same cross-linking degree (1%) with PDEAEMA-1 NPs. The PDEAEMA-3 NPs are smaller, around 200 nm as determined from the TEM image (Figure S8), than the PDEAEMA-1 NPs, similar with the diameter of PDMAEMA-g-PS NPs (Figure 3E). Because the smaller PDEAEMA-3 NPs are very difficult to purify, due to their small sizes, we were only able to measure the dynamic IFT of Tol-H2O interface at 1 mg/mL NPs concentration in the aqueous solution. The dynamic IFT result is shown in Figure S9. It is clear that PDEAEMA-3 NPs can decrease the IFT at pH 2.0 with ∆IFT 2.5 mN/m after three hours (Figure S9-b) and no interfacial activity at pH 10.0 (Figure S9-a). Therefore, compared to the performance PDEAEMA-1 NPs ∆IFT = 4 mN/m after three hours for (Figure S9b), we conclude that the size of the particles in this case had little influence on the ability of the particle to lower the IFT. On the other hand, the PDMAEMA-g-PS NPs with concentration 1 mg/mL at pH 10.0 can decrease IFT from initially 34 mN/m to 9 mN/m in 50 minutes (Figure S9-d). Therefore, we can conclude that the stronger interfacial activity of PDMAEMA-g-PS NPs at pH 10.0 compared with the PDEAEMA-1 NPs at pH 2.0 is not caused by the difference in diameter. It is therefore reasonable to assume that because the PDEAEMA and PDMAEMA are hydrophilic at pH 2.0 and in their protonated form cannot be solubilized or wetted by toluene, the PDEAEMA grafted PS NPs and PDMAEMA grafted PS NPs cannot spontaneously adsorb at Tol-H2O interface. However, PDEAEMA constituted NPs can spontaneously adsorb at interface in its protonated form (pH < 6.0), meaning that 14 ACS Paragon Plus Environment

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hydrophilicity is not the only parameter that controls the adsorption process of particles at L-L interface. One possible explanation why PDEAEMA constituted NPs can spontaneously adsorb at Tol-H2O interface and decrease IFT at pH < 6.0 is that PDEAEMA chains in the NPs with higher swelling degree and softness have enough flexibility such that the chains can re-arrange themselves to make their hydrophobic sides face the toluene and hydrophilic side face water. In this way, the surface of PDEAEMA constituted NPs could be better wetted by toluene. Another explanation may be that the soft particles have a more efficient way of expelling the water layers separating the particle surface and Tol-H2O interface, for example water can freely circulate through the particle. In contrast for the hard NPs the surface must be de-wetted/dehydrated before resolvated by toluene. When PDEAEMA constituted NPs are at pH = 10.0, the PDEAEMA polymers in the NPs with relative hard structure may lose their flexibility and ability to re-arrange the polymer chains, which results no adsorption at Tol-H2O interface at all, even though PDEAEMA polymer has some solubility in toluene. Although plausible, this remains however a conjecture at this point. However, the ability of decreasing the IFT of Tol-H2O for the PDEAEMA and PDMAEMA grafted PS could be attributed to the better solubility/wettability of grafted polymers in toluene. In order to further test if the solubility of PDMAEMA on the surface of PDMAEMA-g-PS NPs in organic solvent is the key factor controlling the interfacial adsorption of these NPs, we have performed dynamic IFT measurements at heptane-H2O and ethyl acetate-H2O interfaces. The corresponding results are given in Figure S10 and show that at heptane-H2O and ethyl acetate-H2O interfaces, PDMAEMA-g-PS NPs can decrease the IFT at pH 10.0 (Figure S10A-c and S10B-c) and loses the ability adsorb at pH 2.0 (Figure 10A-b and S10B-b). The solubility of the grafted PDMAEMA polymer chains (brush) in the organic phase may control both the adsorption ability of the PDMAEMA-g-PS NPs at the interfaces and the plateau IFT values that can be reached at longer times, > 2 minutes. For example, compare the progressively lower IFTs obtained at longer times for the PDMAEMA-g-PS NPs at heptane-H2O (~18 mN/m, Figure S10A), Tol-H2O (~ 8 mN/m, Figure 6B) and ethyl acetate-H2O interfaces (~2 mN/m, Figure S10B), with the progressively larger solubility of the PDMAEMA polymer in heptane (4.9 mg/mL), toluene (198 mg/mL) and ethyl acetate (243 mg/mL). The solubility of the PDMAEMA in the organic solvent was determined according to the procedures described in the SI.

3.5. Activation and Adsorption Energies of PDEAEMA constituted NPs and PDMAEMA-g-PS NPs The interfacial attachment energy (∆E) can be calculated if the contact angle of the three-phase line is known, which is however difficult to determine in situ requiring specialized equipment to perform water freezing and vitrification.32 Instead, the following equation was applied by Dinsmore et al20 to calculate the interfacial attachment energy (∆E) of particles at the interface from the IFT vs. time measurements: ΔE = −(γ − γ() )πR /η (1) 15 ACS Paragon Plus Environment

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where γ is the IFT value of clean L-L interface, γ() is the plateau IFT value at the maximum

concentration of particles above which γ does not decrease anymore, η is the packing parameter (0.91) and

R is the radius of the particles. In our calculation we take η = 0.91 at the highest concentration of NPs at which no further decrease in γ with increase in NPs concentration is observed.20 We have stated in the last section that the PDEAEMA-1 NPs water solution with concentrations above 20 mg/mL in pH 2.0 produce a gel (Figure S4) and therefore the dynamic IFT could not be measured above 20 mg/mL. As a result, we consider that the IFT plateau values (γ ), meaning interface saturation, are attained at this concentration. Table S5 in SI show the ∆E values of the synthesized NPs, which were calculated according to the ∆IFT measured from Figures 4, 5 & 6. The calculated values for the ∆E show a clear trend (Table S5). Among the PDEAEMA constituted NPs, the PDEAEMA-1 NPs at pH = 2.0 exhibited the strongest interfacial activity as judged from their largest absolute value of ∆E (6.07 x 106 kT) at Tol-H2O interface, followed by PDEAEMA-2 NPs (3.3 x 106 kT) and PDEAEMA-PBMA NPs (1.6 x 106 kT). Among the PDMAEMA grafted PS NPs, the PDMAEMA-g-PS NPs at pH 10.0 have the largest absolute value ∆E (1.8 x 105 kT). Many researchers use the magnitude of ∆E alone to assess the interfacial activity of particles. However, because of their different diameters of PDEAEMA constituted NPs and PDMAEMA-g-PS NPs, it is not reasonable to compare only the ∆Es for two reasons: (1) because ∆E scales with R and predictabily the largest particle, to the extent of macroscopic object, may appear as the most interfacially active and (2) an energy barrier to interfacial adsorption may present. It has been reported that diffusion controlled, energy barrier controlled, or a combination of two are the three main types of adsorption kinetics for particles adsorption at interfaces.33 Pendant drop dynamic IFT measurements and the Ward & Tordai theory34 are normally used to model the adsorption kinetics of NPs in the absence of an adsorption barrier. The so called Ward & Tordai theory considers that adsorption is controlled by the particle’s concentration and bulk diffusivity followed by immediate adsorption of any particles colliding with the interface. The adsorption of NPs at interface is much slower in presence of an energy barrier than that described by the pure diffusive models, because it is reasonable to assume that not every particle colliding with the interface also adsorbs. Liggieri et al.35 and Ravera et al.36 proposed an effective diffusion model to account for the activation energy barrier in interfacial adsorption. Therefore, an observed effective diffusion coefficient ( ), which is different from the Stokes’ bulk diffusivity ( ), can be calculated from the dynamic IFT curves with the following equation:33. =

− 2"# $ % &

'()) * +

(2)

where $ is the concentration of NPs in bulk, is the measured IFT a time t,

is IFT value of the clean Tol-

H2O interface, % the attachment energy (here calculated values are in Table 1) and is the effective 16 ACS Paragon Plus Environment

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Langmuir

diffusion coefficient. If only a smaller number of all the particles arriving at the interface do also adsorb, the effective diffusion coefficient should always be smaller than the Stokes-Einstein bulk diffusivity, signaling that indeed an energy barrier at the interface might be present. We also emphasize that if the effective diffusivity determined from the IFT vs. time curves with the Equation 2 is larger than the StokesEinstein diffusivity ( ) this represents an un-physical situation37 and experimental conditions must be rechecked, especially with respect to the presence of smaller and fast adsorbing interfacially active impurities, or particles with broad distributions. Correspondingly, the activation energy of attachment ( ) can be calculated according to the equation (3), as the logarithm of the ratio between the observed bulk-to-surface NPs diffusivity and bulk NPs Stokes-Einstein diffusivity . By fitting the earlier portion of the IFT vs. √- curves, t ≤ 200 s, at all interfaces with NPs concentration 10

mg/mL, see SI (Figure S11-S14), we were able to extract the values of the corresponding to all NPs. Fitting the earlier portion of the curves is justified by the fact that the incoming NPs meet a pristine interface, at a later time the electrostatic repulsion between the adsorbed and incoming NPs dominates and thus imposes

a additional energy barrier.33 Here we want to learn about the NPs-pristine Tol-H2O interactions. The obtained results for the PDEAEMA-1 NPs and PDMAEMA-g-PS NPs are given in Table 1, and for PDEAEMA2 and PDEAEMA-PBMA NPs are given in SI, Table S6. From this, it can be observed that changes

significantly for different NPs at the same Tol-H2O interface. The obtained effective diffusion coefficient can be compared with calculated from the Stokes-Einstein equation (4). 1

= exp (− 3 25) 4

(3)

where is the activation energy of attachment at the interface. 3 5

4 = 6+78

(4)

where μ is the viscosity of water and : is the radius of the NPs. The values obtained from Equation (4) for each NPs are given in Table 1 (PDEAEMA-1 NPs and PDEMAEMA-g-PS NPs) and SI Table S6 (PDEAEMA-2 NPs and PDEAEMA-PBMA NPs). Basavaraj et al.33 obtained differences between vs. as large as three orders of magnitude for 10 nm silica particles

at dodecane-water interface. They claimed that the bigger difference between and is in fact caused by

the presence of a large activation energy barrier at the L-L interface. Wu & Honciuc25 obtained up to three orders of magnitude differences between vs. for 200 nm Janus nanoparticles. In the current situation,

for PDEAEMA-1 NPs at pH from 2.0 to 6.5, the value ranges from 1.49 x 10-16 m2s-1 to 1.24 x 10-18 m2s-1, which is three to five times orders of magnitude larger than their corresponding (3.64 x 10-13 m2s-1 – 4.43 x 10-13 m2s-1). Therefore, it is clear that the adsorption kinetics of PDEAEMA-1 NPs at pH values from 2.0 to


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6.5 at Tol-H2O interfaces is combination of diffusion controlled and energy barrier controlled. Comparing the and results in Table S6, the adsorption kinetics of PDEAEMA-2 and PEDAEMA-PBMA are same with PDEAEMA-1 at pH values from 2.0-6.5. From Table S5 in SI, it should be noted that PDMAEMA-g-PS NPs at pH 10.0 with concentration 10 mg/mL have the lowest final IFT value (5.5 mN/m) from all the grafted PS NPs. The calculated (1.67 x 10-12 m2s-1) for the PDMAEMA-g-PS is quite similar with the (2.94 x 10-12 m2s-1), which means that there is almost no energy barrier to their adsorption at the interface of Tol-H2O at pH 10.0. However, at pH 8.0 and 7.2, the values are 5.60 x 10-15 m2s-1 and 2.92 x 10-15 m2s-1 are three

orders of magnitude smaller than their corresponding (2.94 x 10-12 m2s-1), signaling the presence of a large energy barrier experienced by the PDMAEMA-g-PS NPs while diffusing from water bulk to the Tol-H2O interface at pH 8.0 and 7.2, which vanishes at pH 10.0, in which case the adsorption is only diffusion

controlled. It can be imagined that the adsorption of NPs at the Tol-water interface, from the water phase, requires displacement of water molecules from the NPs’ surface by toluene. Therefore, we hypothesize that the magnitude of the energy barrier is the energy cost of NPs’ surface dehydration and solvation. Table 1. Activation and adsorption energies of the PDEAEMA-1 constituted NPs and PDMAEMA-g-PS NPs at Tol-H2O interface and their diffusivity, effective vs. actual with concentration 10 mg/mL at different pH values. NPs

PDEAEMA-1

PDMAEMA-g-PS

pH Radius (nm) D0 (m2s-1) Deff (m2s-1) Ea (kBT) 2

678.5

3

632

4

629.5

5

557.5

6.5

394.5

10

259

2

84

7.2

84

8

84

10

84

3.6×10-13 1.5×10-16

3.9×10-13 8.6×10-17 3.9×10-13 3.1×10-17

4.4×10-13 3.3×10-17 4.4×10-13 1.2×10-18 9.5×10-13 2.9×10-12

Slope

∆E (kBT)

7.8

-1.79

-5.3×106

8.5

-1.05

9.5 9.6 13.2

~0

∞

~0

∞

2.9×10-12 2.9×10-15

2.9×10-12 5.6×10-15 2.9×10-12 1.7×10-12

6.9 6.3 0.57

-4.1×106

-5.63×10-1 -3.7×106 -3.44×10-1 -2.2×106 -2.44×10-2 -8.0×105 -1.78×10-2 -6×10-2

0 0

-1.6×10-1 -1.0×105

-2.4×10-1 -1.2×105 -6.2

-1.7×105

Here we also note that the most interfacially active NPs are clearly the PDMAEMA-g-PS and the PDEAEMA-1, for which the interfacial attachment energy is the highest, the activation energy barrier the lowest and the ∆IFT the largest. From the IFT data in Figure 4, 5 & 6 and Ea data in Table 1 it is apparent that energy barriers between 1 – 10 kT can be overcome by the NPs at room temperature. In only one instance, for PDEAEMA-1 at pH = 6.5 the Ea is larger than 10 kT, Table 1. We also emphasize that any of the parameters above taken alone cannot define entirely the NPs interfacial activity. It has been proposed that the interfacial activity is the magnitude of NPs’ attachment energy at the interface1 but here we believe that also the ability to 18 ACS Paragon Plus Environment

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lower the interfacial tension, ∆IFT is equally important to consider in making such statement on a particle’s interfacial activity. As already state ∆E scales with the size of the NPs and becomes significant for microparticles and for macroscopic objects and it may not be reflecting a true difference in the interfacial activity of objects that have different sizes, therefore the activation energy of attachment and ∆IFT that does not scale with size should also be considered. The most interesting and surprising result can be observed when the adsorption parameters summarized in the Table 1 are compared for the PDEAEMA-1 NPs and PDMAEMA-g-PS NPs. Notice that with increasing the pH value from 2.0 to 10.0 at NPs concentration 10 mg/mL, the absolute interfacial attachment energy ∆E of PDEAEMA-1 NPs decreases from 5.3 x 106 kT to 0 kT and the corresponding activation energy of interfacial attachment increases from 7.86 kT to very large values. On the other hand, the absolute interfacial attachment energy ∆E of PDMAEMA-g-PS NPs increases from 0 kT to 1.7 x 105 kT and the corresponding activation energy of attachment decrease from very large to 0.57 kT with increasing the pH value from 2.0 to 10.0 for the same NPs concentration. Therefore, there are totally opposite decreasing and increasing trends of ∆E and for PDEAEMA-1 NPs and PDMAEMA-g-PS NPs with increasing pH values (Table 1 and Figure S15), which demonstrate the contrasting interfacial adsorption mechanisms at the Tol-H2O interface. As commented earlier, the softness of the PDEAEMA-1 NPs on one hand appears to be crucial for the particles’ spontaneous interfacial adsorption, while the solubility of the grafted polymer on the surface of the NPs in both phases, i.e. toluene and water, is responsible for PDMAEMA-g-PS spontaneous interfacial adsorption. While both materials have similar chemical nature it is apparent that the conformational freedom of the pH responsive polymer chains in the constituted of NPs vs. surface grafted NPs, may be responsible for the markedly different interfacial activities observed in this work.

3.6. Pickering emulsions stabilized by pH–responsive nanoparticles Controlling the pH of the solution is an efficient way to adjust the surface wetting property of pH-responsive particles and the phase of stabilized Pickering emulsion.24,25,38–41 For example, Hengqun Yang et al. 40 reported that

after

surface

modification

of

SiO2

NPs

with

pH-responsive

molecules

((MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2), the surface hydrophilicity of NPs can be tuned by changing the pH values and the Tol-H2O Pickering emulsion phase could be controlled. In heterogeneous catalysis phase inversion of Pickering emulsion can be key to recycling of catalysts.40 Tu & Lee24 also observed pH switching behavior in Pickering emulsions stabilized by micron sized particles and underlined their potential use in encapsulation and triggered release applications. Previously, we have reported phase inversion in Pickering emulsion stabilized by pH-responsive PS-PDIPAEMA Janus nanoparticles and thoroughly described the mechanisms involved.25 The main factor leading to the emulsion phase inversion is the change in polarity of the particles and consequently their affinity change with respect to the organic vs. aqueous phase, a


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phenomenon that we have thoroughly investigated and reported in several occasions.3,25,30 In this report, we have already seen that the surface polarity of the PDEAEMA-1 and PDMAEMA-g-PS NPs, can be effectively influenced by the pH value. Low pH values (pH < pKa) lead to protonation of polymer (NPs becoming more hydrophilic) and consequently deprotonation at higher pH values (pH > pKa) (NPs becoming more hydrophobic). In Figure 7A, the creaming up of Pickering emulsions and the formation of droplets with very strong fluorescence demonstrates the formation of Tol/H2O (i.e. o/w) Pickering emulsion at pH values below 7.0. Interestingly, at pH 8.0, a mixture of Tol/H2O and H2O/Tol Pickering emulsions can be observed in the fluorescence images, signaling that the surface of PDEAEMA-1 NPs slowly changed from hydrophilic to hydrophobic when the pH increased from 7.0 to 8.0. At pH 10.0, the Pickering emulsion settle down and only H2O/Tol type Pickering emulsions were formed as seen from the fluorescence microscopy images. For the PDMAEMA-g-PS NPs, the Tol-H2O phase separated very fast at pH 2.0 (Figure 7B), no stable Pickering emulsion could be obtained, which demonstrates that PDMAEMA-g-PS NPs have little or no emulsification ability. This result is likely caused by the very hydrophilic nature of the NPs’ surface; after protonation of tertiary amine groups the surface of PDMAEMA-g-PS NPs becomes very polar, the energy cost for dewetting the surface and re-solvating it by Tol is probably very large, therefore no interfacial adsorption was observed even with external mechanical energy input. These results are in agreement with the large Ea predicted for these PDMAEMA-g-PS NPs at pH 2.0 in Table 1. For pH 4.0 and 6.0 Tol/H2O type emulsion were obtained, while H2O/Tol type emulsions were obtained for pH values above 7.0 (Figure 7B). Pickering emulsions stabilized by other NPs synthesized in this work, at pH 2.0 and pH 10.0 are presented in SI, Figure S16. Interestingly, the PDMAEMA-g-PS NPs at pH 2.0 are the only ones that have shown a very large Ea , Table 1, and were not able to emulsify Tol and water even under strong shearing by ultrasonication, Figure 7B. In contrast, PDEAEMA-1, PDEAEMA-2 and PDEAEMA-PBMA that have all exhibited large Ea (Table 1 & Table S6) at pH 10 can still emulsify Tol and water under ultrasonication at room temperature, Figure 7A & S16, but no emulsion could be obtained upon gentle shaking. Upon increasing the temperature to ~70 °C the PDEAEMA-1 NPs are able to emulsify Tol and water at pH 10 without any ultrasonication, just gentle shaking, see Figure S17, signaling that an energy barrier to adsorption at Tol/H2O is indeed present. Note that the PDEAEMA-1 NPs did not show any interfacial adsorption at room temperature in the absence of convection. From the fluorescence microscopy images we observe that the diameter of H2O/Tol Pickering emulsion droplets stabilized by PDMAEMA-g-PS are around 200 µm at pH 10.0, which are much bigger than others grafted NPs, e.g. PDMAEMA-g-PS-1 (Figure S16). We hypothesize that this observed effect could be due to the more hydrophilic nature of PDMAEMA than that of PDEAEMA at same pH value, which affects the immersion depth NP’s into the Tol phase resulting in a smaller curvature of the H2O/Tol droplets.27,42 The emulsification experiments in Figure 7 demonstrate that regardless of the NP-type, both “constituted of” and “grafted from” pH responsive polymer NPs are hydrophilic at pH = 2 (higher affinity to water) and more hydrophobic at pH = 10 (higher affinity to toluene). 20 ACS Paragon Plus Environment

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Figure 7. The digital and fluorescence images of Pickering emulsion emulsified by synthesized NPs at pH values from 2.0 to 10.0. (A) PDEAEMA-1 NPs and (B) PDMAEMA-g-PS NPs. Scale bar in the fluorescence microscope images is 200 µm.

In addition to the static phase inversion observed, meaning that the emulsions were prepared at the corresponding pH, the phase of Pickering emulsion can also be changed dynamically by adding acid or base. For example, the Tol/H2O Pickering emulsion stabilized by PDEAEMA-1 NPs at pH 2.0 can be changed to H2O/Tol by adding NH4OH and oppositely by adding HCl , see Figure 8. Here we note that after adding HCl or NH4OH, gentle shaking by hand of the emulsion is necessary to induce the phase inversion in the Pickering emulsion. Clearly, the pH induced phase inversions of Pickering emulsions is a consequence of NPs’ surface polarity change due to protonation-deprotonation of PDEAEMA and PDMAEMA polymers, in agreement with the previous reports.25

Figure 8. pH responsive Pickering emulsion stabilized by PDEAEMA-1 NPs and dynamic phase inversion by addition of an acid or a base. (A) Tol/H2O (pH 2.0) changes to H2O/Tol (pH 10.0) by adding NH4OH and (B) H2O/Tol (pH 10.0) changes to Tol/H2O (pH 2.0) by adding HCl. Scale bar in the fluorescence microscope images is 100 µm.


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4. Conclusion We have systematically investigated the spontaneous adsorption behavior of the PDEAEMA and PDMAEMA grafted PS NPs and PDEAEMA constituted NPs at Tol-H2O interface. Although the “constituted of” and “grafted from” have the same chemical properties we concluded that these exhibit a contrasting mechanism of spontaneous interfacial adsorption at L-L interfaces. Dynamic IFT results from the pendant drop seem to indicate that the solubility of PDEAEMA and PDMAEMA in toluene is likely the key parameter controlling the spontaneous adsorption of PDEAEMA and PDMAEMA grafted PS NPs at Tol-H2O interface. Namely, when PDEAEMA and PDMAEMA are in their protonated form at pH < 6.0, PDEAEMA, PDMAEMA and their grafted PS NPs remain in bulk water without adsorbing at Tol-H2O interface, though they can adsorb spontaneously at Tol-H2O interface in their de-protonated form at pH > 7.0, because of increased solubility of PDEAEMA and PDMAEMA in toluene. However, PDEAEMA constituted NPs behave oppositely. When PDEAEMA is in its protonated form at pH < 6.0, they can adsorb spontaneously at Tol-H2O interface and in its deprotonated form at pH > 7.0, interfacial adsorption is not observed. One possible reason for the observed behavior is that at pH < 6.0, the PDEAEMA constituted NPs become soft due to water intake and significant swelling. We cannot explain at the moment how softness of the NP can be mechanistically responsible for the adsorption of particles at interfaces than for the solid particles. However, one could hypothesize that the lower of interfacial adsorption observed as the NPs become softer at lower pH have a more efficient way of expelling the water layers separating the particle surface and toluene phase as the particle approaches the interface; for example water can freely circulate through the softer swollen particle. In contrast for the hard NPs the surface must be de-wetted/dehydrated before re-solvated by toluene. Another possibility is that polymer chains in soft particles acquire higher flexibility to re-arrange and presumably expose the hydrophobic domains preferentially toward toluene. If the magnitude of IFT drop can be used as parameter to gauge interfacial activity, comparing the final IFT values achieved by each type of NPs leads to the conclusion that the method of grafting polymers, which have some solubility in organic solvents, is the most effective method to endow the NPs with L-L interfacial activity. In addition, adsorption kinetics for PDEAEMA constituted NPs at Tol-H2O interfaces in the pH range of 1.0 to 6.0 is combination of diffusion controlled and energy barrier controlled. For PDMAEMA grafted PS NPs, in the pH from 7.0 to 8.0, the adsorption of PDEAEMA constituted NPs at Tol-H2O interfaces is combination of diffusion controlled and energy barrier controlled, while at pH 10.0 it appears to be only diffusion controlled. The most important finding is that by comparing the trends of ∆E and with pH of PDEAEMA constituted NPs with PDMAEMA grafted PS NPs, their adsorption behavior at Tol-H2O interface appears to follow a contrasting mechanism of interfacial attachment. Finally, the PDEAEMA constituted NPs and PDEAEMA and PDMAEMA grafted PS NPs can effectively emulsify Tol-H2O with the formation of Pickering emulsions. The Pickering emulsion phase can be controlled by the pH values of the solution. In acidic solution (pH = 2.0), the Tol/H2O Pickering emulsion are formed, while in basic conditions (pH = 10.0), only H2O/Tol Pickering emulsions were observed. The main factor 22 ACS Paragon Plus Environment

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leading to the emulsion phase inversion is the change in polarity of the particles and their affinity with respect to the organic and aqueous phase in agreement with the previously reported data.3,25,30

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications Website.

AUTHOR INFORMATION Corresponding Author: *[email protected], Tel.: +41(0)589345283 ORCID Andrei Honciuc: 0000-0003-2160-2484;

ACKNOWLEDGEMENTS We are especially grateful for the financial support of Metrohm Foundation (Herisau, Switzerland).

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