pH-Responsive Pickering Foams Generated by Surfactant-Free Soft

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pH-Responsive Pickering Foams Generated by Surfactant-Free Soft Hydrogel Particles Dalin Wu, Voichita Mihali, and Andrei Honciuc Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03342 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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pH-Responsive Pickering Foams Generated by Surfactant-Free Soft Hydrogel Particles

Dalin Wu†, Voichita Mihali† and Andrei Honciuc†* †Institute

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

*Corresponding

author:

[email protected],

tel.:+41589345283,

ICBC,

ZHAW,

Einsiedlerstrasse 31, 8820 Waedenswil, Switzerland

Abstract Pickering foams are foams stabilized by particles and are generally known to have a good stability. A special sub-class of particle stabilized foams is the stimuli-responsive Pickering foams that can be formed or de-constructed by applying an external stimuli or changing the environmental conditions; such intelligent particles could find use in many practical applications. Here, we synthesized the surfactant-free

biocompatible

poly[2(diethylamino)ethylmethacrylate]

(PDEAEMA)

hydrogel

particles (HGPs) by emulsion polymerization. The morphology, structure and surface charge of the HGPs were characterized by TEM, DLS and Zeta potential, respectively. We have observed that the pH values of the aqueous solution have great influence on the formation of the Pickering foams in presence of PDEAEMA HGPs. Namely, at pH values ≤ 4.0 no Pickering foams were produced, while at pH values > 4.0, stable Pickering foams were formed. Moreover, the height, size and bubble size distribution of Pickering foams are strongly influenced by the pH values of aqueous solution and PDEAEMA HGPs concentration. The formed Pickering foams in basic aqueous solution can all be conveniently de-constructed by changing pH values below 4.0. Interestingly, the dried lamellas of the Pickering foams were constituted by either monolayers or multi-layers of PDEAEMA HGPs as demonstrated by SEM.

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1. Introduction Foams are constituted by the gas bubbles that are dispersed in liquid or solid in presence of interfacially active molecules, such as surfactants, which can effectively decrease the interface tension (IFT) of the gas and liquid phase and stabilize the foams.1,2 Owing to their high surface/volume ratio, foams are being broadly used as intermediates and end products in decontamination,3 food manufacturing,4 electrocatalysis,5 cosmetic formulations,6 etc.7 Typically, the foams stabilized by interfacially active molecules have a short term stability, generally due to the drainage of water and fast adsorption/desorption kinetics and low adsorption/desorption energies of surfactants at air-H2O interface, which is comparable to the thermal energy of ~1 kT.1,8 However, in some cases, extending the stability of foams as well as its deconstruction is an important technological issue. For example, at the end of the decontamination, the foams should be destabilized in a controlled way, or on-demand, in order to make the decontamination waste easier to be dealt with.9 On the other hand, particles with suitable surface hydrophobicity can also stabilize foams that are referred to as Pickering foams,2,10,11 which exhibit a better stability as compared with those stabilized by surfactants.12–14 This is in part due to the irreversible adsorption of particles at the air-H2O interface. For example the desorption energy ΔG required to remove a particle from air-H2O interface is several orders of magnitude greater than the thermal energy,10,15 while, the surfactants molecules can absorb and desorb reversibly. The hydrophobicity of the surface of particles determines the wettability and contact angle of particles at air-H2O interface, as result, the stability and de-foaming of the formed Pickering foams can be adjusted by switching the surface hydrophobicity of the particles. Destabilization of Pickering foams can also be achieved by using stimuli-responsive (temperatureresponsive and magnetic-responsive) particles.15–23 For example, S. Lam prepared the Pickering foams stabilized by a mixture of oleic acid coated carbonyl iron particles (4.5-5.2 μm average diameter) and hypromellose phthalate (HP-55) particles, which can be destabilized by applying a magnetic field because the iron particles can be removed away from the air-H2O interface easily and ultimately can be quickly and easily recycled back.24 However, it can be debated whether their Pickering foams have broad application due to biological incompatibility of iron particles. Additionally, temperature was also utilized to fulfill a similar effect. Also, controlling pH value of aqueous solution is another important way to adjust the Pickering foams.17,25–27 Comparing with magnetic field and temperature, alternating the pH values of aqueous solution is much easier to accomplish, especially more effective when large amounts of Pickering foams are used on industrial platforms. The common strategy to fabricate the pH-responsive particles is by physical or chemical grafting of pH-responsive molecules or polymers on the surface of hard particles.16,17 For example, Q. Lin et al.25 modified the surface of SiO2 particles by using dodecyl dimethyl carboxyl betaine (C12B, C12H25-N+(CH3)2-CH2-COO-, M = 271 g/mol) through the ion-ion interaction. Their result demonstrated that the Pickering foams show pH-responsivity, namely, 2 ACS Paragon Plus Environment

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Pickering foams were stable at pH values ≤ 3.4 but unstable at pH values ≥ 10.0 and could be cycled between stable and unstable for many cycles by alternating the pH values of the aqueous solution, however, the presence of free small interfacially active molecules at air-H2O interface can influence the accuracy of the results and conclusions. For example, if small interfacially active molecules and particles are used these can be present in a different concentration ratio in the foam than in bulk, resulting in a complex and difficult to control system. In order to eliminate the interference from the interfacially active molecules, S. Fujii et al. grafted covalently pH-responsive polymers poly[2-(dimethylamino)ethylmethacrylate] (PDMAEMA)22 and poly[2(diethylamino)ethylmethacrylate] (PDEAEMA)26 on the surface of polystyrene (PS) microsized particles through ATRP. They found that the Pickering foams’ stability and formation was greatly influenced by the pH values of the aqueous solution. Under acidic condition, the PDMAEMA and PDEAEMA were protonated and the surface of PS was too hydrophilic to stabilize the Pickering foams. But under basic condition, the PDMAEMA and PDEAEMA were deprotonated and the surface of PS could be wetted by both water and air, which favored the formation of Pickering foams with long term stability. Additionally, the heights of the Pickering foam and shell structure of foams in their experiments were also influenced by pH values. Obviously, chemical modification of the surface of particles increases the workload invested in the preparation of pH-responsive particles. Here, we are reporting one type of pH-responsive Pickering foams generated by pH-responsive PDEAEMA hydrogel particles (HGPs), which were synthesized by surfactant-free emulsion polymerization. PDEAEMA with pKa value of 7.0-7.3,28 is a useful pH-responsive polymer and is being used broadly in pharmaceutical applications due to its very good biocompatibility.29 The synthesized PDEAEMA HGPs have the following notable features: i) the surface of HGPs is covered only by positive charges without any interfacially active molecules and stabilizers; ii) the diameter and softness of HGPs can be controlled by pH values;30 iii) the surface polarity HGPs can be effectively switched by pH values of aqueous solution between the hydrophobic and hydrophilic states. We found that the pH values of the aqueous solution plays a critical role on the formation of Pickering foams. When the pH values are lower than 5.0 (2.0 - 4.0), no Pickering foams were observed, while stable Pickering foams can be produced at the pH values of solution are above 5.0 (5.0 - 10.0). In this condition, the formed Pickering foams have kinetic stability for over one week at room temperature. Moreover, the size of Pickering foams and distribution are greatly influenced by pH values of the solution and PDEAEMA HGPs concentration. At last, the de-foaming process can be achieved by adjusting pH values of the aqueous solution.


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2. Experimental Section PDEAEMA HGPs synthesis: the list of chemicals and their acronyms is given in the SI. 500 mg (vinylbenzyl)trimethylammonium

chloride

(VBTMAC),

100

mg

2,

2′-Azobis(2-

methylpropionamidine) dihydrochloride (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 ethylene glycol dimethacrylate (EGDMA) were added. Then the solution was de-oxygenated by bubbling Ar gas 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 HGPs with cross-linking degree 1% were purified by washing with ethanol four times and UPW four times.30 Characterization Instruments and Methods: The morphology of synthesized HGPs at pH 6.3 was characterized with the transmission electron microscopy (TEM). A 5 μL amount of HGPs 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 structure was acquired on a Philips CM100 TEM at an acceleration voltage of 80 kV. The Feret average diameter of the HGPs was determined using the ImageJ software. The hydrodynamic diameter of PDEAEMA HGPs in concentration 0.5 mg/mL at pH values 6.3 (without adjustment) was also 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 Zeta potential values of PDEAEMA HGPs in concentration 0.5 mg/mL at different pH values (3.0 – 10.0) were measured Zeta Sizer Nano ZS device at 25 °C. The pKa value of the PDEAEMA HGPs was determined experimentally to be between 6.8-8.2 by titration and change in zeta potential with pH, Figure S6. The titration experiments were carried on PDEAEMA HGPs solution (46.5 mg/mL) with a 10-1 M HCl solution. The Pickering foams were prepared by using the following two methods. 1. Using high speed homogenizer (IKA, T25 Digital). The 9 mL PDEAEMA HGPs aqueous solution at a specific HGPs concentration (10 mg/mL - 100 mg/mL) and at different pH values (2.0 - 10.0) were homogenized at 20 x 103 rpm/min for 2 minutes at room temperature (r.t.). After that, the formed Pickering foams were carefully handled for the next characterization stage. 2. Bubbling the argon through the PDEAEMA HGPs aqueous solution. The sketch of the setup is presented in supporting information (SI) (Figure S2). The 2 mL PDEAEMA HGPs (10 mg/mL) at pH 3.0 and 9.0 were first carefully added into a glass tube with a ceramic frit at a lower side. Then the argon gas with stable flow rate of 4 ± 0.8 mL/s (2 Ar bubble/second) was pumped from the bottom side of the tube, passing to the frit to generate small gas bubble into the solution containing PDEAEMA HGPs and finally lead to Pickering foam formation. The Ar-gas was bubbled for 10 seconds after which the gas flow was immediately stopped, glass tubes 4 ACS Paragon Plus Environment

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were carefully covered by rubber stoppers and the foams immediately photographed to analyze their stability. Surface tension measurements were carried out via pendat-drop and equilibrium drop-shape analysis method. The experiments were done using a DataPhysics OCA 15Pro 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 PDEAEMA HGPs aqueous solution with concentrations 10 mg/mL in different pH values. The dosing rates for generating the droplet at the apex of the tip were kept high, in order to achieve good reproducibility and eliminate any lag time during the droplet expansion. If the expansion is slow then the beginning values will be different from the surface tension 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 ~ 72 mN/m the surface tension of pure air-water interface. The real |𝐸𝑟𝑒| (dilatational elasticity) and imaginary |𝐸𝑖𝑚| (dilatational viscosity) components of the complex dilatational viscoelastic modulus were measured by the oscillating pendant droplet method with the DataPhysics OCA 15Pro instrument. The measurement was done on a colloid solution containing PDEAMA HGPs (10 mg/mL). The surface tension of the pendent drop volume of ~4-6 mL was measured at a frequency of 1.025 Hz and ~0.5 mm longitudinal amplitude. The features of Pickering foams were characterized by using an inverted fluorescence microscope (IX51, Olympus, Tokyo, Japan) equipped with a 100 W mercury discharge burner (lamp: USH-103OL, Ushio, Tokyo Japan), a 460 nm - 495 nm excitation filter and a CPLN-PH 10X microscope objective lens. Before taking images of Pickering foams, three drops of fluorescent dye ATTO 488 containing water solution (2 mM) was first added dropwise on surface of glass slide, and then the Pickering foams were gently deposited with a spatula on the microscope slide. After that, the microscope was quickly focused and measured the morphology of Pickering foams. The morphologies of dried lamella of Pickering forms in different pH values were characterized with the scanning electron microscope (SEM) (FEI Quanta FEG 250), operating at 5 – 30 kV accelerating voltage in the secondary electron (SE) mode under low vacuum (0.40 mbar). Before the measurement, the samples were sputtered with Au in a sputter-coater (Q15OR-S Sputter Coater, Quorum, 20 mA for 15 sec), under Ar atmosphere (sputter vacuum: 5  10–2 mbar).

3. Results and Discussion 3.1. PDEAEMA HGPs Characterization 5 ACS Paragon Plus Environment

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The procedure for synthesizing PDEAEMA HGPs was previously reported by our group.30 The use of the PDEAEMA polymer, with pKa value of 7.0 - 7.3,28 is currently widespread in the research of drug delivery. PDEAEMA HGPs surface becomes very hydrophilic due to protonation of more than half of the tertiary amine groups on PDEAEMA at pH values lower than its pKa 6.8-8.2, and becomes relatively hydrophobic at higher pH values. The TEM image (Figure 1A) shows the synthesized PDEAEMA HGPs with diameter 1890 ± 390 nm (measured and statistically counted from TEM image with n = 35) and with very soft hydrogel structure at pH 6.3 (HGPs aqueous solution without adjusting pH value). The PDEAEMA HGPs are as stated gel particles that are relatively hydrophilic compared to polystyrene particles, and are soft as demonstrated by the penetration measurements performed in the previous work.30 In solution they adopt a dynamic shape presumably close to perfectly spherical but which can be presumably modulated by shear currents. Upon drying the capillary forces further deform these HGPs and their form slightly deviates from perfectly spherical and appear to adopt a kinetically stable shape. We hypothesize that the reason for the preservation of this stable kinetic shape is due to their softness and interaction with the substrate that prevents them from returning to a perfectly spherical shape. The diameters of the PDEAEMA HGPs were determined from TEM images (Figure 1A). At the pH 6.3, the hydrodynamic diameter of the PDEAEMA HGPs measured by DLS is 920 ± 190 nm (Figure 1A, embedded), which is much smaller than the one determined from TEM image. This difference in size of the HGPs determined by TEM and DLS reveals indeed that the HGPs suffer deformation on a hard substrate during drying, in this case the TEM grid. The hydrodynamic diameter of the PDEAEMA HGPs increases from 640 ± 190 nm to 1530 ± 340 nm with decreasing pH values from 11.0 to 2.0 (Figure 1B). In addition, the zeta potential values (Figure 1B) of PDEAEMA HGPs at pH values from 3.0 to 10.0 remain above +15 mV demonstrating that the particle surface is positively charged. With decreasing the pH values from 10.0 to 6.3, the absolute zeta potential values increase gradually from +16 mV to + 40 mV, which is clearly the effect of the protonation of tertiary amine of PDEAEMA. With the increase in the surface charge of the particles, their mobility also increases with the decrease in pH of the solution, Figure S8. Further, the zeta potential values did not experience a significant change (from + 40 mV to 42 mV) in the pH range between 6.3 and 3.0. It should be noted that even though the PDEAEMA HGPs have zeta potential values below +30 mV at pH values above 8.0, because the PDEAEMA HGPs are relatively hydrophilic comparing with very strong hydrophobic polystyrene nanoparticles, the PDEAEMA HGPs have long flocculation time and even then they can easily be redispersed by gentle agitation. From Figure 1B it is clear that the PDEAEMA HGPs remain monodisperse and do not flocculate in the pH range 3.0-10.0.


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Figure 1. (A) TEM image of the PDEAEMA HGPs at pH 6.3 with the curve depicting the hydrodynamic diameter measured by DLS (embedded). (B) The hydrodynamic diameter (pH 2-11) and zeta potential values (pH: 3-10) of PDEAEMA HGPs. Scale bar is 2000 nm.

3.2. The Influence of pH Values on the Pickering Foams PDEAEMA HGPs are pH-responsive; when the pH values of aqueous solution drop below the experimentally determined value of the pKa 6.8-8.2, more than half of tertiary amines on PDEAEMA will be protonated gradually and their hydrophilicity and surface polarity increases. As a result, the surface hydrophobicity of PDEAEMA HGPs and wettability by water will undergo dramatic change when the pH of the solution changes from basic to acidic. The digital images in Figure 2 depict freshly prepared Pickering foams generated by PDEAEMA HGPs (0 day) and clearly show that Pickering foams form only at pH values higher than pH 7.0. In contrast, at pH values lower or equal to 4.0, no stable Pickering foams form. It could be debated that the PDEAEMA HGPs have a too small contact angle θ (to water) at the air-H2O interface to adsorb at the interface, which is indeed the case here. But according to our interfacial measurements previously reported30 the PDEAEMA HGPs do adsorb spontaneously at Toluene-H2O interfaces at pH < 7.0. From this we tempted to think that the Pickering foam formation/stability may not only be related to the interfacial activity of the particles but also with the stability of the particles in the foam lamella, due to strong electrostatic repulsion at low pHs. Surprisingly, the Pickering foams can form at the pH value of 5.0 and 6.3 which are also lower than the pKa value of PDEAEMA. The possible explanation is that at these pH values, PDEAEMA units are not totally protonated, meanwhile, PDEAEMA also retain a certain amphiphilicity and due to the softness of the particle the polymer chain can re-arrange at the interface exposing the hydrophilic units to water and the hydrophobic ones to air.31 S. Nakayama et al.22 reported that the PDEAEMA grafted PS particles with repeating unit of DEAEMA from 30 to 90 cannot emulsify Pickering foams at pH 5.0. The different behaviors of stabilizing Pickering foams between our PDEAEMA HGPs and PDEAEMA grafted PS particles synthesized by S. Nakayama et al. at pH 5.0 probably were caused by 7 ACS Paragon Plus Environment

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the difference of PDEAEMA chains structure and polymer density on the surface of particles, as we have already discussed in a previous publication.30

Figure 2. The digital images of formed Pickering foams at different pH values with 10 mg/mL PDEAEMA HGPs aqueous solution (up) (HGPs volume: 9 mL) and the fluorescence microscopy images of the corresponding Pickering foams (down). Scale bar is 50 µm. Except for surface hydrophobicity, the types of charge on the surface of particles may also play a critical role on their adsorption ability at the air-H2O interface and thus their capability to generate Pickering foams.32 For example, S. L. Kettlewell et al.32 reported that the positive charged PS particles and poly(ethylene glycol)-grafted PS particles can generate Pickering foams with gratifying stability, but the negatively charged PS particles with similar diameter cannot accomplish the same. The reason why the types of charge on the particles surface have an influence on the adsorption of particles at air-H2O interface and stability of Pickering foams is the presence of the so called images charge effects at the air-H2O interface (repulsion force)33 and the broadly accepted phenomena of negatively charged air-H2O interface (attraction force).34,35 Obviously, an increase in the images charge effect (repulsion force with the interface) caused by decreasing pH values may be one of the reason explaining why the PDEAEMA HGPs cannot produce stable Pickering foams with long term kinetic stability at pH values lower than 4.0. We are also clearly showing that the PDEAEMA HGPs do indeed spontaneously adsorb at the air-H2O interface in basic pH but not in acidic pH by measuring the dynamic surface tension of a pendant drop, Figure 3A. The initial value of the air-H2O surface tension ~72.5 mN/m shows that the interface is initially pristine; no molecular impurities are present in the system, which have adsorbed much faster than the particles, on the order of milliseconds. We have previously reported that the PDEAEMA HGPs are able to spontaneously adsorb at the Toluene-H2O interface without external energy input at pH values lower than its pKa (6.8-8.2), opposite behavior than that at the air-H2O interfaces; the corresponding final plateau IFT values are presented in Figure S1.


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We have also prepared Pickering foams by bubbling Ar in the PDEAEMA HGPs aqueous solution at pH 3.0 and pH 9.0. The difference between bubbling gas and using a homogenizer to create foams is that in the former case less mechanical energy input is applied to the sample thus less kinetic energy is transferred to the particles. In other words the former situation is much closer to the conditions of foam creation due to spontaneous adsorption of particles rather than forced adsorption in the latter case, as discussed in our previous reports on Pickering emulsions.13,30 Therefore, when Ar gas is bubbled, the Pickering foams should indeed also form if the PDEAEMA HGPs have interfacial activity, i.e. are capable of spontaneous adsorption at the air-H2O interface. The digital images of the formed Pickering foams by this method are presented in Figure S2. It is clear that at the very beginning (Figure S2A, 0 second after stopping the Ar flow), the PDEAEMA HGPS at pH 3.0 are also successful in generating Pickering foams; likewise at pH 9.0. But the height (~ 2.3 cm) and the survival time (~ 10 s) of Pickering foams at pH 3.0 are significantly shorter at pH 3 than at pH 9.0 (Figure S2). In other words, the Pickering foams formed at pH 3.0 are much more fragile than at pH 9.0. It is obvious that at pH lower than or equal to 4.0, the fast disappearance of formed Pickering foams (Figure 2 and Figure S2) indicate that the interfacial activity of the nanoparticles is the key parameter responsible for the process of the Pickering foams formation and its kinetic stability. We also hypothesize that the stability of the Pickering foam is controlled, among others, also by the nature of interaction between PDEAEMA HGPs constituting the foam lamella, see Scheme S1. The interaction between two PDEAEMA HGPs originates from the electric “double-layer” interaction (repulsion force) and the van der Waals force (attraction force), depicted in Scheme S1. In order to determine the overall interaction energy between two PDEAEMA HGPs at different pH values we have used equations S1 and S2 and calculated the attractive van der Waals interaction energy (negative) and the electric double-layer repulsive interaction (positive by convention), see Figure 3B. The resultant of the both interaction energies represented by the green dashed curve in Figure 3B demonstrate that the interaction between two PDEAEMA HGPs is repulsive below pH 7.0 and becomes attractive above pH 7.0 with the overall interaction energy increasing from -2.82 x 102 kT to 1.18 x 103 kT with decreasing the pH values from 10 to 3. Especially, when the pH value is below 5, the repulsion energy between PDEAEMA HGPs steeply increases above ~350 kT, which may be sufficiently large repulsion to cause destabilization of the Pickering foams. The stability of the foams is attributed to a variety of factors among which, a low air-H2O interfacial tension and strong repulsive DLVO or non-DLVO (steric) type of interactions that exist between the two planes of a foam lamella (Scheme S1).36 Generally, it is thought that strong electrostatic repulsion or steric stabilization between lamella planes prevent a quick drainage and foam destruction, whereas the good stability of the foam is correlated with a thick single foam film.37 However, Wierenga & Gruppen38 pointed out that such statements are based on correlations and may not be a true causal relationship between such factors and foam stability. 9 ACS Paragon Plus Environment

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To summarize the discussion, in the current system, in acidic conditions at low pH the PDEAEMA HGPs are positively charged, therefore a high electrostatic particle-particle exist, they do not adsorb at the air-water interface as demonstrated by surface tension measurements, Figure 3A&B, yet they are able to generate Pickering foam, but with poor kinetic stability (Figure S2). In contrast, in basic conditions at high pH the PDEAEMA HGPs have a low zeta potential, can spontaneously adsorb at the air-H2O interface as demonstrated by surface tension measurements, have low electrostatic particle-particle repulsion, Figure 3A&B and they can generate stable Pickering foams. We therefore, conclude that the interfacial activity and surface charge density of the particles may be the key parameters for the foam formation and stability. The real |𝐸𝑟𝑒| (dilatational elasticity) and imaginary |𝐸𝑖𝑚| (dilatational viscosity) components of the complex dilatational viscoelastic modulus39 were measured at the interface of the pendant water droplet containing PDEAEMA HGPs (10 mg/mL). The values of |𝐸𝑟𝑒| and |𝐸𝑖𝑚| steeply increase around pH 6 due to adsorption of the HGPs at the interface. The interface remains however elastic

|𝐸𝑟𝑒| >> |𝐸𝑖𝑚| with the dilatational viscosity slightly decreasing with the pH from 6.0-10.0, Figure S7. In addition, the foam structure can also be discussed as a function of the particle dispersibility. By taking a zeta potential value of +30 mV at pH 7 as threshold above which the HGPs are in a dispersed state and below which the HGPs are in their flocculation range of the zeta potential one can observe that population of small air bubbles increases significantly above pH 7, Figure 4. The foam bubble size analysis, Figure 4, implies that HGPs in their flocculation range of the zeta potential form thicker foam walls and produce foams with better stability. The survival time (~ 6 h) of Pickering foams prepared by bubbling Ar-gas is much shorter (Figure S2) than the one prepared by homogenizer (~ 1 week) (Figure 2) in the same conditions. The PDEAEMA HGPs adsorption at interfaces is an energy activated process; external mechanical energy input (such as homogenizer or ultrasonication) promotes the interfacial adsorption of PDEAEMA HGPs at the airwater interface.30 In contrast, bubbling Ar-gas provides a lower energy input, and this could explain why Pickering foams prepared by homogenizer have longer stability. Therefore, only the PDEAEMA HGPs stabilized Pickering foams prepared with the homogenizer were used for further investigations.


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Figure 3. (A) Dynamic surface tension measurements of a water pendant drop containing PDEAEMA HGPs 10 mg/mL at (a) pH 2 and (b) pH 9. (B) The calculated interaction energy of two PDEAEMA HGPs located at each air-H2O interfaces of the lamella, Scheme S1, at different pH values (3.0 – 10.0): (a) repulsion energy due to electrostatic double-layer interactions; (b) van der Waals attraction energy and (c) the resultant energy between the two types of interactions. The model and equations used for calculations are given in SI. From Figure 2, Figure 4 and Figure S3, it is worth noting that the bubble diameters (30 µm-7000 µm), the diameter distributions and the heights of the Pickering foams (1.7 cm-3.2 cm) vary at different pH values (PDEAEMA HGPs concentration 10 mg/mL). At pH 5.0, the height of the Pickering foam is around 3.2 cm which is higher than the foam at all other pH values (6.3–10.0) (Figure S3). This is probably caused by the presence of large populations of big Pickering foam air bubbles (Figure 2, pH 5.0). We also statistically compared the Pickering foams by counting the foam bubbles and measuring their diameter and diameter distribution at different pH values. The result in Figure 4 shows that the diameter of Pickering foams range from ~30 µm to ~7000 µm with a broad and discrete size distribution. Interestingly, the fraction of small size Pickering foam bubbles (size < 100 µm) increases with increasing the pH values of aqueous solution from 5.0 to 10.0 and the fraction of bigger size foam bubbles (size > 1000 µm) decrease simultaneously.


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Figure 4. Air bubble size distribution in Pickering foams at different pH values (5.0 - 10.0). The statistics was constructed from the digital images and fluorescence microscopy images of Pickering foams produced by PDEAEMA HGPs at a concentration 10 mg/mL. (Y axis is the number of PDEAEMA HGPs with certain diameter divided by the total number of calculated PDEAEMA HGPs). (A) pH 5.0, (B) pH 6.3, (C) pH 8.0, (D) 9.0 and (E) pH 10.0. Noteworthy is that not only the foaming ability, but also de-foaming process and ability to deconstruct the Pickering foam in a controlled way is also needed for evaluating the particle’s applicability in various processes as a solid-state foaming/de-foaming agent.9,17 The Pickering foams generated by 12 ACS Paragon Plus Environment

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PDEAEMA HGPs at pH 6.3 can be de-constructed by adjusting pH values to 2.0 easily (Figure 5A). Obviously, de-foaming of Pickering foams is achieved by simply adjusting pH has much more practical significance in scaling-up processes in plants as compared with the de-foaming by increasing temperature for thermo-responsive particles or applying magnetic field for magnetic particles. The addition of electrolyte to the aqueous solution containing particle can increase the ionic strength, thus screening the particles’ surface charges and reduce the electrostatic repulsive forces. It has been reported that the presence of electrolytes in the aqueous solution not only influences the foaming ability of particles15 but also the stability of the Pickering foams.40 In the current case, HCl electrolyte is added into the aqueous solution to adjust the pH values. To test the influence of electrolytes on the emulsifying capability of PDEAEMA HGPs on Pickering foams we prepared the PDEAEMA HGPs aqueous solution (HGPs concentration: 10 mg/mL) with 0.01 M NaCl at pH 6.3. In this condition, the concentration of NaCl in aqueous solution is same with the concentration of HCl at pH 2.0. The image in Figure 5A shows that the presence of 0.01 M NaCl has a minor influence the formation of Pickering foams; for example the height of the Pickering foams (Figure 5B) in the presence of NaCl (Figure 5A, pH 6.3) (1.5 cm) is only somewhat smaller than in the absence of electrolyte at the same pH (1.7 cm). In addition, it is clear that the foam bubble size distribution in Pickering foams with addition of 0.01 M NaCl is still broad (Figure 5B) (60 µm-3000 µm). However, this result confirms that at pH 2.0, the absence of Pickering foam formation is a result of increasing the density of positive charges of PDEAEMA HGPs surfaces (increasing the repulsion force between PDEAEMA HGPs and the images charge effect) rather than the presence of HCl as the electrolyte.

Figure 5. (A) The digital image of Pickering foams emulsified by PDEAEMA HGPs before and after adding HCl to adjust pH value from 6.3 to 2.0. (B) Digital image and fluorescence microscopy image of Pickering foams emulsified by PDEAEMA HGPs (10 mg/mL) at pH 6.3 with concentration of NaCl 0.01M. Scale bar is 20 µm. 3.3. The Influence of HGPs Concentration on the Pickering Foams


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The influence of concentration of PDEAEMA HGPs on the Pickering foams was also investigated at pH 6.3. The concentration of PDEAEMA HGPs was varied from 10 mg/mL to 100 mg/mL. The digital images and corresponding fluorescence microscopy images (Figure 6) show that the Pickering foams form in the tested PDEAEMA HGPs concentration range. However, the concentration has a non-negligible influence on the diameter and composition of Pickering foams. The populations of big Pickering foam bubbles with diameter above 1000 µm decreased from 24% to 2% and the populations of small Pickering foam bubbles with diameter smaller than 1000 µm increased from 76% to 98% with increasing the PDEAEMA HGPs concentration according to the statistical analysis (Figure S5). Moreover, the heights of Pickering foams do not show regular changing trend with increasing the concentration of PDEAEMA HGPs (Figure S4). The increasing the population of small Pickering foams (from 76% to 98%) with enhancing the concentration of PDEAEMA HGPs (10 mg/mL to 100 mg/mL) means the interface energy of the whole system and total interface area between H2O and air increasing, which is favored by the increasing the concentration of PDEAEMA HGPs.

Figure 6. Digital images and fluorescence microscopy images of the Pickering foams emulsified by PDEAEMA HGPs at pH 6.3 with different HGPs concentrations (10 mg/mL - 100 mg/mL). Scale bar is 50 µm. 3.4. Kinetic Stability of Pickering Foams One of the advantages of Pickering foams is their higher stability caused by the irreversible adsorption of particles at air-H2O interface as compared with the usual foams stabilized by small molecules surfactants. For example, the foams emulsified by sodium dodecyl sulfate (SDS) only have a lifetime of around 24 hours.26 However, Pickering foam destabilization takes place through the same type of mechanisms, such as Ostwald ripening and water drainage. Therefore, we have also investigated the kinetic stability of Pickering foams generated by PDEAEMA HGPs. The results presented in Figure 7 show that at r.t. the height of Pickering foam slowly decreases over the course of 8 days unrelated with pH values (5.0-10.0) of solution and concentration of HGPs (10 mg/mL-100 mg/mL), for example, the 14 ACS Paragon Plus Environment

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height of the Pickering foam decreases from 7.5 cm (0 day) to 1 cm (8 days) with HGPs concentration 10 mg/mL. Moreover, the corresponding digital images show that over time the finer foams consisting of smaller bubbles are slowly converted into coarser foams consisting of larger bubble. This indicates that the coalescence of foam bubbles leads to the slow disappearance of the Pickering foams. In addition, a clear influence of the pH value and concentration of PDEAEMA HGPs on the kinetic stability of Pickering foams could not be observed.

Figure 7. The kinetic stability of Pickering foams generated by PDEAEMA HGPs at r.t. (A) Digital images of Pickering foams generated by PDEAEMA HGPs at a concentration of 10 mg/mL at different pH values (5.0-10.0). (B) Digital images of Pickering foams generated by HGPs at different concentrations (10 mg/mL - 100 mg/mL) and at pH 6.3. 3.5. Lamella Structure of the Pickering Foams The morphologies of the dried lamella of Pickering foams generated by PDEAEMA HGPs were investigated by SEM. For this analysis Pickering foams were generated with PDEAEMA HGPs at 10 mg/mL and the corresponding pH values of aqueous solution between 5.0 and 10.0. Because the PDEAEMA HGPs have a soft hydrogel structure (Figure 8) individual particles are less clearly resolved as compared to the reported images of foam lamella formed by solid hard particles.22,26,32,41,42 However, representative SEM images (Figure 8) clearly show that Pickering foams lamella are constituted by a single layer of PDEAEMA HGPs (Figure 8A, B and C) and the junction of two different Pickering foams after drying are constituted by a bilayer and multilayer of PDEAEMA HGPs (Figure 8D, E and F). Interestingly, the dried lamella of the Pickering foams generated by PDEAEMA 15 ACS Paragon Plus Environment

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HGPs appear to withstand well mechanical stress; SEM images in Figure 8 A-C show monolayers of PDEAEMA HGPs forming low and high curvature structures, demonstrating a good mechanical strength and flexibility of these foam lamella. From Figure 1B it is clear that the PDEAMA HGPs remain monodisperse and do not flocculate in the pH range 3.0-10.0 and from the SEM image in Figure 8 it is clear that only single PDEAMA HGPs and not aggregates adsorb in a monolayer at airwater interface.

Figure 8. The SEM images of the Pickering foam lamella structures formed by PDEAEMA HGPs at a concentration of 10 mg/mL at different pH values. (A) pH 6.3, (B) pH 8.0, (C) pH 9.0, (D) pH 5.0, (E) pH 5.0 and (F) 10.0. 4. Conclusion The synthesized PDEAEMA HGPs can be applied to the production of pH-responsive Pickering foams. The emulsifying ability is strongly related with the pH values of the aqueous solution. At pH values lower or equal to 4.0, PDEAEMA HGPs are not able to generate Pickering foams due to their high density of surface positive charges caused by protonation of tertiary amine of PDEAEMA. However, PDEAEMA HGPs are able to generate Pickering foams, with a good kinetic stability for 7 days, at pH values above 4.0. The pH value appears to influence not only the foaming ability of PDEAEMA HGPs, but also the properties of Pickering foams, such as height and bubble size distribution. The important outcome is that the obtained Pickering foams are pH responsive, can be easily de-foamed by adjusting pH values of solution to be acidic (pH < 4.0) and re-formed in basic 16 ACS Paragon Plus Environment

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conditions, at pH > 4.0. These make the PDEAEMA HGPs very interesting for their significant potential in practical applications as solid emulsifier and foaming agent. In addition, due to the PDEAEMA’s biocompatibility, the PDEAEMA HGPs could also potentially be used in food manufacturing, pharmaceutical formulations as well as personal care products.

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). References (1)

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