Dodecanol Pickering Emulsions

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Meso-Microscale Study of Glycerol/Dodecanol Pickering Emulsions Stabilized by Polystyrene-Grafted Silica Nanoparticles for Interfacial Catalysis Guolin Zhao, Bing Hong, Bo Bao, Shuangliang Zhao, and Marc Pera-Titus J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01876 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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The Journal of Physical Chemistry

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Meso-Microscale Study of Glycerol/Dodecanol Pickering Emulsions Stabilized by Polystyrene-Grafted Silica Nanoparticles for Interfacial Catalysis Guolin Zhao,1,2 Bing Hong,2 Bo Bao,1 Shuangliang Zhao,1* and Marc Pera-Titus,2*

1 State

Key Laboratory of Chemical Engineering and School of Chemical Engineering, East China

University of Science and Technology, 200237 Shanghai, China 2

Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464 CNRS-Solvay, 201108

Shanghai, China AUTHOR INFORMATION

Corresponding Author * [email protected] (SZ), [email protected] (MPT)

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ABSTRACT: In this study, we report a combined meso-microscale computational study of Pickering emulsions stabilized by sulfonated polystyrene-grafted silica nanoparticles for the glycerol/dodecanol system using Dissipative Particle Dynamics. Different G/D emulsification regimes could be ascertained as a function of the length of polystyrene brushes, as well as the surface density, sulfonation degree and distribution of sulfonic acid groups in the brushes, matching experimental results. Local nanomixing effects between glycerol and dodecanol could be clearly identified in the vicinity of the catalytic sulfonic acid groups, unravelling potential reactivity zones for etherification.


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Introduction In the past decades, the field of Pickering or particle-stabilized emulsions has become a transversal topic for a broad variety of applications in the oil, pharmaceutical, cosmetic and food industries.1,2 Unlike surfactants, the use of colloidal nanoparticles (NPs) as emulsifiers affords easy separation and reuse, as well as a lower energy footprint.3 Despite these general benefits, it was not until the earlier studies of Resasco4 and Qiu5 in 2010 that Pickering emulsions were devised as a potential platform for catalytic reactions between immiscible reagents at the liquidliquid (L/L) interface. In this concept, Pickering emulsions can be used to generate large interfacial contact areas and short diffusion paths, enhancing mass/heat transfer between the liquid phases.6 The self-assembly of colloidal NPs at the L/L interface depends primarily on hydrophiliclipophilic properties of the NPs, as well as on their size and shape.7,8 Amphiphilic NPs can be easily prepared by grafting alkyl chains or polymer brushes (e.g., polystyrene, methacrylate) on silica.9-14 By adjusting the length, composition and surface density of the alkyl chains or brushes, it is possible to tune the adsorption and packing properties of the NPs at the L/L interface. By incorporating catalytic centers (e.g., acid sites, metal NPs) during or after the synthesis, alkyl- or polymer-grafted NPs can be used as a platform for engineering amphiphilic catalysts for conducting reactions at the L/L interface.15-19 In earlier studies, sulfonated polystyrene (PS)-grafted silica NPs revealed as active catalysts for conducting the etherification reaction between glycerol and dodecanol, targeting the synthesis of (poly)glyceryl dodecanoate as biobased surfactant.20 Interestingly, the PS chain length and sulfonation degree conditioned to an important extent the emulsification properties of the NPs. Specifically, silicas with long PS brushes (MW 4900-6800 g/mol, ca. 40-50 styrene units) at an




optimal sulfonation degree of 28% exhibited the highest catalytic activity due to the generation of very high interfacial areas ascribed to the formation of dodecanol/glycerol/dodecanol double Pickering emulsions. Opposing this behavior, silicas with short PS brushes (MW 2100-2700 g/mol, 10-20 styrene units) only exhibited partial stability for a sulfonation degree about 50%, favoring the genesis of G/D emulsions. Using Dissipative Particle Dynamics (DPD), here we present a combined meso- and microscale computational study of Pickering emulsions stabilized by sulfonated PS-grafted silica NPs for the glycerol/dodecanol system. DPD is a reliable method for predicting the self-assembly of NPs at the L/L interface and the stability of Pickering emulsions.21-26 DPD has already been applied with success to rationalize the glycerol/dodecanol emulsification properties of alkylgrafted silicas bearing propylsulfonic acid groups, as well as local distribution of glycerol and dodecanol in the vicinity of the acid groups.27 In this study, we investigated first the effect of the PS chain length, surface density of PS brushes and distribution of sulfonic acid groups in the brushes on the glycerol/dodecanol emulsification pattern, and compared the simulation results with earlier experiments. Subsequently, we explored the local distribution of glycerol and dodecanol, unraveling an enhanced glycerol/dodecanol nanomixing near the sulfonic acid groups favoring the contact between both reagents for etherification.

Simulation Details Models and interaction parameters The DPD simulation details can be found in the SI. The system under study consisted primarily of glycerol (G), dodecanol (D), a PS-grafted silica NP with variable chain length, sulfonation degree and distribution of sulfonic acid groups in the PS brushes, and in some cases a reaction product (G-D) (Figure 1). The interaction parameters used for conservative forces are listed in Table 1. G 4 Environment ACS Paragon Plus

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was modeled as a single bead, while D was modeled by the assembly of four beads, i.e. three propane beads (Pe) and one propanol bead (Pol). G-D was modeled by the combination of five beads, i.e. three Pe beads, one Pol bead and one G bead. The beads within the same molecule were connected by a harmonic spring with constant ks = 4 (DPD unit) and equilibrium distance r0 = 0.7rc.26 The bead volume was set to 120 Å3 with a G/D molar ratio of 4:1 matching previous emulsification studies.20 The density of the G and D phases was set to 1.26 and 0.83 g/cm3, respectively.28 The silica NPs were represented as a planar substrate based on an assembly of closely packed silica beads (S) grafted with PS brushes constituted by a variable number of styrene beads (N=530), sulfonic acid groups (SH), which were modeled as a single bead, and silanol groups (SiOH), also represented by a single bead. The sulfonation degree of the PS brushes was defined as the percentage of styrene beads sulfonated by assuming one SH group per sulfonated styrene bead. The PS brushes were anchored randomly with one end on the surface (i.e. monopodal grafting). Unless otherwise stated, the nominal surface density of PS brushes was 1.0 groups/nm2, whereas the surface density of SiOH groups was 2.8 groups/nm2. These densities were chosen to match the experimental surface density of SiOH groups in the parent Aerosil®200 silica (3.8 groups/nm2), which was earlier used for the synthesis of the NPs.20 Computational details The DPD simulation was performed using the Materials Studio 6.1 version of the Mesocite module.29 The simulation box was set to 10 x 20 x 25 rc3 (Lx x Ly x Lz), where Li refers to the length of the simulation box along the i-direction. Orthorhombic boxes were used and periodic boundary conditions were applied in the X and Y directions. The Z-direction was perpendicular to the G/D interface, which was considered as planar, and the Y-direction was perpendicular to the



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NP surface and parallel to the interface. The bead number density of the system was set to 3, so the entire simulation box contained about 15,000 beads. The constants for the dissipative and random forces were set to  = 4.5 and  = 3 to keep the temperature constant (kBT=1). The simulation runs were established at 200,000 steps with a time step of 0.05. The entire simulation process was divided into two steps: (1) the first 100,000 steps were used for system equilibrium, while (2) the last 100,000 steps for data analysis.

Calculation of interfacial and surface tensions The G/D interfacial tension, GD, and the solid-G and solid-D surface tensions (i.e. SG and SD) were calculated with and without the product (G-D) through the Irving-Kirkwood (IK) equation by integrating the stress difference30-32 𝛾𝑆𝐺: = 𝛾𝑆𝐷: = γ𝐺𝐷: =

∫[

]

1 𝑃𝑧𝑧 ― (𝑃𝑥𝑥 + 𝑃𝑦𝑦) 𝑑𝑧 2

(1)

where Pzz represents the normal pressure tension along the direction perpendicular to the interface/surface, and Pxx and Pyy represent the tangential pressure tensions (Pxx=Pyy). The Pzz and Pxx pressures can be computed along the direction perpendicular to the interface/surface using the expressions 1 𝜙 (𝑟 ) ∑ [ 𝑟 |𝑧 |Θ[

1 𝑃𝑧𝑧(z) = 𝜌(z)𝑘𝐵𝑇 ― 2𝐴

1 𝑃𝑥𝑥(z) = 𝜌(z)𝑘𝐵𝑇 ― 4𝐴

𝑧2𝑖𝑗

𝑖10), matching earlier experimental observations.20 We further computed |(γSG-γSD)C| and Ep,dim as a function of the sulfonation degree for a curved NP surface assuming an upper distribution of sulfonic acid groups, a surface density of PS brushes of 1.0 groups/nm2, and a chain length of N=10 (Figure 3A1,A2) and N=30 (Figure 3B1,B2). On the guidance of previous experimental studies, the NP radius was set at 100 nm.18,20 Overall, the line tension broadens the emulsion stability region, affording stabilization at lower sulfonation degrees, namely 40% and 20% for N=10 and N=30, respectively, for GD = 1000 pN. In particular, for N=30, Ep,dim approaches 1 at 60% sulfonation degree, suggesting a contact angle of ca. 90o. This result suggests a phase inversion leading potentially to D/G/D emulsions as reported in a previous study, even if the inversion occurred at a lower sulfonation degree (28%).20 In a next step, we investigated the effect of the surface density of PS brushes (range 0.25-1.0 groups/nm2) on the G/D emulsification properties. To this aim, we computed |(γSG-γSD)C| and Ep,dim 9 Environment ACS Paragon Plus


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as a function of the sulfonation degree for an upper distribution of sulfonic acid groups and N=30 by assuming both a planar (Figure 4) and a curved NP surface (Figure 5). In both cases, poor emulsion stability is observed for a surface density of PS brushes in the range 0.25-0.75 groups/nm2, except for higher sulfonation degrees (>40%), favoring D/G emulsions. The line tension (curved surfaces) broadens the emulsion stability region to sulfonation degrees lower than 40% and increases Ep,dim in the range 40-80%. Opposing this behavior, at the highest surface density (1.0 groups/nm2), the Ep,dim curve exhibits a maximum at 60% sulfonation degree, which becomes broader with the line tension, promoting in turn emulsion inversion from D/G to G/D (Figure 2B2, Figure 3B2). Emulsification behavior in the presence of G-D We further computed |(γSG-γSD)C| and Ep,dim as a function of the sulfonation degree for a planar NP surface at variable G-D volume fractions (Figure 6). The presence of G-D on the NP surface only affected slightly the emulsion stability region for NPs with shorter PS brushes (N=10) and an upper distribution of sulfonic acid groups. For such NPs, Ep,dim only shifts to lower sulfonation degrees at higher G-D volume fractions (i.e. 5% and 10%). In contrast, for NPs with longer PS brushes (N=30), G-D expands the emulsion stability region to lower and higher sulfonation degrees, even at low G-D volume fractions (1%). We also computed |(γSG-γSD)C| and Ep,dim as a function of the sulfonation degree for a curved NP surface (Figure 7) at a constant G-D volume fraction of 5%. The line tension expands the emulsion stability region to lower sulfonation degrees for both N=10 and N=30, especially at GD = 1000 pN. Moreover, G-D reinforces the intrinsic capacity of NPs with N=30 to generate D/G/D emulsions, as Ep,dim approaches 1 at 60% sulfonation degree. This observation matches

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qualitatively the emulsion inversion observed during the biphasic etherification reaction of G and D over PS-grafted silica NPs with N=45 and 28% sulfonation degree.20 Microscale analysis: Local distribution of G and D G/D nanomixing effects in the absence of G-D In a second step, we computed the density profiles of PS brushes, sulfonic acid groups, G and D along the distance (Y) perpendicular to the NP surface as a function of the PS chain length, surface density of sulfonated PS brushes, sulfonation degree and distribution of sulfonic acid groups (Figure 8, Figure 9, S2-S8). At 1.0 groups/nm2, for N=10, the brushes exhibit a narrow distribution in the range Y = 0.4-8.0rc (Figure 8B, S2A, S3A). The sulfonic acid groups appear distributed in the same Y range, but with two main peaks centered at Y = 2rc and Y = 4rc. Opposing these trends, NPs with longer PS brushes (N=30) exhibit broader distributions of PS brushes and sulfonic acid groups at a sulfonation degree higher than 40% (Figure 9B, S4A, S5A). This behavior is more pronounced at higher surface density of PS brushes (i.e. 0.75 and 1.0 groups/nm2) (Figure S5A-S8A). All NPs favor D adsorption in the glycerol phase, but with a different pattern depending on the PS chain length. On the one hand, for longer brushes (N=30), the D concentration exhibits a Gaussian distribution regardless of the sulfonation degree and distribution of sulfonic acid groups (Figure 9D1, S4C1, S5C1). The peak values shift to higher Y positions at higher sulfonation degrees and surface densities of PS brushes (>0.75 groups/nm2) (Figure 9D1, S6C1-S8C1). Partial D enrichment is observed near the acid centers, especially at lower sulfonation degrees (80%) for upper and bottom distributions of sulfonic acid groups (Figure 8C1, S2B1, S3B1). PS brushes favor D and G adsorption in the glycerol and dodecanol phases, respectively, whereas sulfonic acid groups show weak repulsion. At zero sulfonation degree, the microscopic interactions between the PS brushes and G and D dominate, and in this circumstance, a neat G/D interface appears near the NP surface. By increasing the sulfonation degree, the G/D interface becomes gradually indistinct, and both G pockets in the dodecanol phase (Figure 8C2, Figure 9C2, S2B2-S8B2), and D pockets in the glycerol phase are generated (Figure 8D1, Figure 9D1, S2C1S8C1). As a result, the density zone of D in the glycerol phase and the density zone of G in the dodecanol phase becomes enlarged. However, a further increase of the sulfonation degree overbalances the interactions from the PS brushes, and the interactions with sulfonic acid groups are expected to dominate, reducing the size of G and D pockets. Consequently, the density zone of D and G in the glycerol and dodecanol phases, respectively, shrinks. D adsorption in the dodecanol phase is favored near the acid groups, especially at a sulfonation degree higher than 60% for N=10 (upper distribution) and higher than 80% for N=30 (all distributions). From the G and D profiles, we computed the (D/G)surf and (G/D)surf molar ratios in the vicinity of the sulfonic acid centers in the glycerol and dodecanol phases, respectively (1.0 groups/ 12Environment ACS Paragon Plus

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nm2, Table 2). In these calculations, we integrated the Z-averaged G and D concentrations in the Y range displaying the distribution of sulfonic acid groups. Regardless of the sulfonation degree and distribution of sulfonic acid groups in the PS brushes, the average (D/G)surf molar ratio in the glycerol phase for NPs with N=10 exhibits much higher values (range 0.84-2.59) than the bulk ratio, i.e. (D/G)bulk = 0.002 (Table 2, Figure S9). This result points out an enhanced ‘nanomixing’ between G and D in the glycerol phase. Nonetheless, for N=30, (D/G)surf ratios exhibit divergent values as a function of the sulfonation degree. As a matter of fact, the (D/G)surf ratios show higher values (0.40-2.86) than the bulk ratio (up to 0.12) at lower and higher sulfonic acid ratios, i.e. 020% and 80-100%, respectively. In contrast, the (D/G)surf ratio approaches the bulk value at intermediate sulfonation degrees (range 0.05-0.22), for which the NPs display a maximum value for Ep,dim (Figure 2A2, 3A2). With regards to the dodecanol phase, for all distributions of sulfonic acid groups, the average (G/D)surf molar ratios approach the bulk ratio for 1.0 groups/nm2 and N=10 at low sulfonation degrees (0-20%), i.e. (G/D)bulk = 0.13-0.68 (Table 2, Figure S9). At higher degrees (40-100%), the average (G/D)surf ratio is higher than the bulk ratio for an upper distribution of sulfonic acid groups (range 2.29-8.31). In the case of uniform and bottom distributions, an enhanced (G/D)surf ratio is also observed at higher sulfonation degrees (range 4.19-7.61 and 5.05-5.52, respectively), except for 80%, approaching the bulk ratio. Similar conclusions can be drawn for longer PS brushes (N=30), even if the (G/D)surf ratio exhibits much higher values at low and high sulfonation degrees, especially for an upper distribution of sulfonic acid groups (range 0.47-5.71 at 20-40% and 13.5-15.2 at 80-100%). In contrast, irrespective of the type of sulfonic acid groups, the (G/D)surf ratio approaches the bulk value at 60% sulfonation degree (range 0.47-1.02), corresponding to the maximum value for Ep,dim (Figure 2B2, 3B2), favoring emulsion inversion.



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We further investigated the effect of the surface density of PS brushes in the range 0.25-1.0 groups/nm2 on the local distribution of G and D on the NP surface for N=30 as a function of the sulfonation degree and distribution of sulfonic acid groups (Table S1). The (D/G)surf molar ratio in the glycerol phase falls into the range 0.08-1.03, 0.17-2.88, 1.13-2.77 and 0.06-2.90 for 0.25, 0.50, 0.75 and 1.0 groups/nm2, respectively, whereas the corresponding (G/D)surf molar ratio in the dodecanol phase falls into the range 0.46-4.67, 0.27-10.2, 4.04-12.6 and 0.54-22.6. Overall, this body of results points out an enhanced nanomixing effect between G and D on the NP surface at a higher surface density of PS brushes, this effect being more pronounced in the dodecanol phase. G/D nanomixing effects in the presence of G-D We further computed the density profiles of G, D and G-D along the distance from the NP surface as a function of the G-D volume fraction (range 0-10%) for N=10 and N=30. (Figure 10, Figure 11, S10-S13). In these calculations, the surface density of PS brushes was kept at 1.0 groups/nm2 with an upper distribution of sulfonic acid groups. The presence of G-D on the NP surface affects to an important extent the density profiles and local distribution of G and D near the sulfonic acid groups. Starting our analysis on the glycerol phase, for N=10, the D distribution becomes narrower at 1% G-D and broader at 5% and 10% G-D for 40% sulfonation degree, shifting to higher Y positions (Figure 10C1). In contrast, at 20% and 60% sulfonation degree, a broad band appears at 1% G-D, which becomes narrower with a peak approaching the sulfonic acid groups at 5% and 10% G-D (Figure S10C1, S11B1). In parallel, for all sulfonation degrees, the G distribution shifts to higher Y positions, especially at higher G-D volume fractions (Figure 10B1, S10B1, S11A1). On the other hand, for N=30, the G and D density profiles in the glycerol phase become only slightly modified by G-D at 40-60% sulfonation degree (Figure 11C1, S12C1, S13B1). However,


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at 20% sulfonation degree, G pockets appear on the NP surface far from the PS brushes at 1% GD, decreasing the D concentration in the range Y =10-20rc (Figure 11B1, S12B1, S13A1). Turning our attention into the dodecanol phase, G pockets are still generated far from the PS brushes for both short (N=10) and long (N=30) PS brushes and all sulfonation degrees (Figure 10B2, 11B2, S10B2, S11A2, S12B2, S13A2). The G concentration in the pockets approaches the G density at higher G-D volume fractions (5-10%). In contrast, for both N=10 and N=30, the D distribution decreases drastically with the G-D volume fraction (Figure 10C2, 11C2, S10C2, S11B2, S12C2, S13B2). However, partial D enrichment is observed in the vicinity of the sulfonic acid groups. We further computed the (D/G)surf and (G/D)surf molar ratios in the glycerol and dodecanol phases, respectively (upper distribution of sulfonic acid groups) (Table S2). Overall, the presence of G-D enhances G/D nanomixing in the glycerol phase for NPs with longer PS brushes (N=30) at intermediate sulfonation degrees (40-60%). However, the effect is detrimental at lower sulfonation degrees (20%). In the case of NPs with shorter PS brushes (N=10), no remarkable nanomixing effects are observed near the sulfonic acid groups at 40-60% sulfonation degree, except at 1% G-D for 20% sulfonation degree. With regards to the dodecanol phase, an enhanced G/D nanomixing is also observed for NPs with N=30 for a sulfonation degree in the range 20-60%, whereas the effect is more moderate for NPs with N=10.

Summary and Conclusions Along this study we investigated by dissipative particle dynamics the effect of sulfonated polystyrene brushes grafted on silica nanoparticles on the emulsification properties and local nanomixing effects near the sulfonic acid groups for the glycerol/dodecanol system. The emulsion stability and type depend strongly on the length of polystyrene brushes, surface density of brushes,



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sulfonation degree and distribution of sulfonic acid groups. In particular, for nanoparticles with longer brushes (N=30) and 1.0 groups/nm2, the contact angle approaches 90o, suggesting a possible phase inversion leading potentially to D/G/D emulsions, which is in line qualitatively with the experimental observations. The G-D product further promotes this phenomenon, providing a solid explanation on the emulsion inversion observed experimentally in the biphasic etherification reaction of glycerol with dodecanol. Local nanomixing effects between glycerol and dodecanol near the sulfonic acid groups are especially prominent in the dodecanol phase. The (D/G)surf and (G/D)surf molar ratios in the glycerol and dodecanol phases, respectively, approach the bulk values for nanoparticles with longer polystyrene brushes (N=30) at a sulfonation degree about 60% where emulsion inversion occurs. For such nanoparticles, the high reactivity observed experimentally is governed by a large interfacial surface area rather than to nanomixing effects. The presence of the reaction product (glyceryl dodecanoate) enhances the (D/G)surf and (G/D)surf molar ratios in the glycerol and dodecanol phases, respectively, boosting the contact between both reagents and favoring accordingly their reactivity.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: Simulation details for DPD; evolution of |(γSGγSD)C| and Ep,dim as a function of the sulfonation degree and distribution of sulfonic acid groups for a planar surface for N=5 and N=20; additional local concentration profiles (Z-averaged) of polystyrene (PS) and SO3H (SH) moieties, as well as G, D and G-D along the Y-direction from the grafted SiNPs surface for the glycerol and dodecanol phases; and local concentration profiles of G


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and D in the bulk phase along the Z-direction; (D/G)surf and (G/D)surf molar ratios in the glycerol and dodecanol phases on the NP surface both in the absence of presence of G-D.

Notes The authors declare no competing financial interests.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21808056, U1707602, and 21878078), and the 111 Project of China (No.B08021). SZ acknowledges the support of Fok Ying Tong Education Foundation (151069). CNRS and Solvay are also acknowledged for funding.

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(15) Yang, H.; Zhou T.; Zhang, W. A Strategy for Separating and Recycling Solid Catalysts Based on the pH-Triggered Pickering Emulsion Inversion. Angew. Chem. Int. Ed. 2013, 52, 7455-7459. (16) Zhang, W.; Fu L.; Yang, H. Mixcrometer-Scale Mixing with Pickering Emulsions: Biphasic Reactions without Stirring. ChemSusChem. 2014, 7, 391-396. (17) Zhou, W-J.; Fang, L.; Fan, Z. Y.; Albela, B.; Bonneviot, L.; De Campo, F.; Pera-Titus, M.; Clacens, J-M. Tunable Catalysts for Solvent-Free Biphasic Systems: Pickering Interfacial Catalysts over Amphiphilic Silica Nanoparticles. J. Am. Chem. Soc. 2014, 136, 4869-4872. (18) Fan, Z. Y.; Tay, A.; Pera-Titus, M.; Zhou, W.; Benhabbari, S.; Feng, X.; Malcouronne, G.; Bonneviot, L.; De Campo, F.; Wang, L.; Clacens, J-M. Pickering Interfacial Catalysis for Solvent-Free Biomass Transformation: Physicochemical Behavior of Non-Aqueous Emulsions. J. Colloid Interface Sci. 2014, 427, 80-90. (19) Kirillova, A.; Schliebe, C.; Stoychev, G.; Jakob, A.; Lang H.; Synytska, A. Hybrid Hairy Janus Particles Decorated with Metallic Nanoparticles for Catalytic Applications. ACS Appl. Mater. Interfaces 2015, 7, 21218-21225. (20) Shi, H.; Fan, Z. Y.; Ponsinet, V.; Sellier, R.; Liu, H. L.; Pera-Titus, M.; Clacens, J-M. Glycerol/Dodecanol Double Pickering Emulsions Stabilized by Polystyrene-Grafted Silica Nanoparticles for Interfacial Catalysis. ChemCatChem. 2015, 7, 3229-3233. (21) Lin, Y-L.; Chiou, C-S.; Kumar, S. K.; Lin, J-J.; Sheng, Y-J.; Tsao, H-K. Self-Assembled Superstructures of Polymer-Grafted Nanoparticles: Effects of Particle Shape and Matrix Polymer. J. Phys. Chem. C 2011, 115, 5566-5571. (22) Fan, H.; Striolo, A. Mechanistic Study of Droplets Coalescence in Pickering Emulsions. Soft Matter 2012, 8, 9533-9538. (23) Luu, X-C.; Yu, J.; Striolo, A. Ellipsoidal Janus Nanoparticles Adsorbed at the Water-Oil Interface: Some Evidence of Emergent Behavior. J. Phys. Chem. B 2013, 117, 13922-13929.



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(24) Luu, X-C.; Yu, J.; Striolo, A. Nanoparticles Adsorbed at the Water/Oil Interface: Coverage and Composition Effects on Structure and Diffusion. Langmuir 2013, 29, 7221-7228. (25) Zhao, S.; Zhan, B.; Hu, Y-F.; Fan, Z. Y.; Pera-Titus, M.; Liu, H. L. Dynamics of Pickering Emulsions in the Presence of an Interfacial Reaction: a Simulation Study. Langmuir 2016, 32, 12975-12985. (26) Xiang, W.; Zhao, S.; Song, X.; Fang, S.; Wang, F.; Zhong, C.; Luo, Z. Amphiphilic Nanosheet Self-Assembly at the Water/Oil Interface: Computer Simulations. Phys. Chem. Chem. Phys. 2017, 19, 7576-7586. (27) Zhao, G.; Li, Y.; Hong, B.; Zhao, S.; Pera-Titus M.; Liu, H. L. Nanomixing Effects in Glycerol/Dodecanol Pickering Emulsions for Interfacial Catalysis. Langmuir 2018, 34, 15587-15592. (28) Chemical Rubber Company, Lide, D. R. ed, CRC Handbook of Chemistry and Physics, 85th ed, CRC Press, Boca Raton, 2004. (29) Accelrys Software Inc., Materials Studio Release Notes, Release 6.1, San Diego: Accelrys Software Inc., 2012. (30) Billy, D. T.; Denis, J. E.; Peter, J. D. Pressure Tensor for Inhomogeneous Fluids. Phys. Rev. E, 1995, 52, 1627-1638. (31) Varnik, F.; Baschnagel, J.; Binder, K. Molecular Dynamics Results on the Pressure Tensor of Polymer Films. J. Chem. Phys., 2000, 113, 4444-4453. (32) Aziz, G.; and Patrice, M. Calculation of the Surface Tension from Multibody Dissipative Particle Dynamics and Monte Carlo Methods. Phys. Rev. E, 2010, 82, 016706. (33) Bresme, F.; Quirke, N. Computer Simulation Study of the Wetting Behavior and Line Tensions of Nanometer Size Particulates at a Liquid-Vapor Interface. Phys. Rev. Lett. 1998, 80, 3791-3794. (34) Aveyard, R.; Clint, J. Liquid Droplets and Solid Particles at Surfactant Solution Interfaces. J. Chem. Soc., Faraday Trans. 1995, 91, 2681-2697. 20Environment ACS Paragon Plus

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(35) Drehlich, J. The Significance and Magnitude of the Line Tension in Three-Phase (SolidLiquid-Fluid) Systems. Colloids Surf. A 1996, 16, 43-54. (36) Bresme, F.; Oettel, M. Nanoparticles at Fluid Interfaces. J. Phys.: Condens. Matter. 2007, 19, 413101. (37) Law, B. M.; McBride, S. P.; Wang, J. Y.; Wi, H. S.; Paneru, G.; Betelu, S.; Ushijima, B.; Takata, Y.; Flanders, B.; Bresme, F.; et al. M. Line Tension and its Influence on Droplets and Particles at Surfaces. Progr. Surf. Sci. 2017, 92, 1-39.

FIGURE CAPTIONS Figure 1. Coarse-grained models for (A) dodecanol, (B) glycerol, (C) G-D, (D) sulfonated PSgrafted silica NPs, and (E-G) distribution of sulfonic acid groups in PS brushes (uniform, upper, bottom). Figure 2. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and distribution of sulfonic acid groups for a planar surface. Chain length: N=10 (A1, A2) and N=30 (B1, B2). The red-solid, green-dashed and blue-dotted curves refer to upper, uniform and bottom distribution of sulfonic acid groups, respectively. The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively. Figure 3. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree for a curved surface with GD = 10-1000 pN. Chain length: N=10 (A1, A2) and N=30 (B1, B2). Results based on upper distribution of sulfonic acid groups. The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively. Figure 4. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and surface density of PS brushes for a planar surface with an upper distribution of sulfonic acid groups and N=30. Figure 5. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and surface density of PS brushes for a curved surface, an upper distribution of sulfonic acid groups and N=30 with GD = 10-1000 pN. Surface density of PS brushes: 0.25 groups/nm2 (A1, A2), 0.50 groups/nm2 (B1, B2) and 0.75 groups/nm2 (C1, C2). Figure 6. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and G-D volume fraction for a planar surface and an upper distribution of sulfonic acid groups. Chain length: N=10 (A1, A2) and N=30 (B1, B2). The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively. 21Environment ACS Paragon Plus


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Figure 7. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree for a curved surface and an upper distribution of sulfonic acid groups with GD = 10-1000 pN. Chain length: N=10 (A1, A2) and N=30 (B1, B2). The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively, while the G-D volume fraction was set at 5%. Figure 8. (A) Snapshot of the G (red) and D (blue-pink) equilibrium distribution at the surface of PS-grafted NPs (PS, green; SO3H, yellow) with a sulfonation degree of (A1) 0%, (A2) 40%, (A3) 80% and (A4) 100%; local concentration profiles (Z-averaged) of (B) polystyrene (PS) and SO3H (SH) moieties (in the figure insert), (C) G and (D) D along the Y-direction for the glycerol (B1, C1, D1) and dodecanol (B2, C2, D2) phases. NPs: N=10, upper distribution of sulfonic acid groups, variable sulfonation degree at 0% (black-solid), 40% (red-dashed), 80% (blue-short dashed) and 100% (green-dotted). The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively. Figure 9. (A) Snapshot of the G (red) and D (blue-pink) equilibrium distribution at the surface of PS-grafted NPs (PS, green; SO3H, yellow) with 40% sulfonation degree; local concentration profiles (Z-averaged) of (B) polystyrene (PS) and SO3H (SH) moieties (in the figure insert), (C) G and (D) D along the Y-direction for the glycerol (B1, C1, D1) and dodecanol (B2, C2, D2) phases. NPs: N=30, upper distribution of sulfonic acid groups, variable sulfonation degree at 0% (blacksolid), 40% (red-dashed), 80% (blue-short dashed) and 100% (green-dotted). The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively. Figure 10. (A) Snapshot of G (red), D (blue-pink) and G-D (bright green-bright blue) equilibrium distribution at the surface of PS-grafted NP (PS, green; SO3H, yellow) and 10% G-D concentration. Local concentration profiles (Z-averaged) of (B) G, (C) D and (D) G-D along the Y-direction for the glycerol phase (B1, C1, D1) and dodecanol phase (B2, C2, D2) at a G-D volume fraction of 0% (black-solid), 1% (red-dashed), 5% (blue-short dashed) and 10% (greendotted). NPs: N=10, upper distribution of sulfonic acid groups, 40% sulfonation degree. The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively. Figure 11. (A) Snapshot of the G (red), D (blue-pink) and G-D (bright green-bright blue) equilibrium distribution at the surface of PS-grafted NP (PS, green; SO3H, yellow) and 10% G-D concentration. Local concentration profiles (Z-averaged) of (B) G, (C) D and (D) G-D along the Y-direction for the glycerol phase (B1, C1, D1) and dodecanol phase (B2, C2, D2) at a G-D volume fraction of 0% (black-solid), 1% (red-dashed), 5% (blue-short dashed) and 10% (greendotted). NPs: N=30, upper distribution of sulfonic acid groups, 40% sulfonation degree. The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively.


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TABLE CAPTIONS Table 1. Interaction parameters aij in kBT/rc units. The symbols G, Pe, Pol, SH, PS and S represent glycerol, propane, propanol, sulfonic acid groups, styrene and the silica surface, respectively. Table 2. D/G and G/D molar ratios on the NP surface as a function of the PS chain length, sulfonation degree and distribution of sulfonic acid groups in the PS brushes. The surface density of PS brushes and SiOH groups was 1.0 groups/nm2 and 2.8 groups/nm2, respectively.



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FIGURES

Figure 1. Coarse-grained models for (A) dodecanol, (B) glycerol, (C) G-D, (D) sulfonated PSgrafted silica NPs, and (E-G) distribution of sulfonic acid groups in PS brushes (uniform, upper, bottom).


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Figure 2. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and distribution of sulfonic acid groups for a planar surface. Chain length: N=10 (A1, A2) and N=30 (B1, B2). The red-solid, green-dashed and blue-dotted curves refer to upper, uniform and bottom distribution of sulfonic acid groups, respectively. The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively.



Page 26 of 37


Figure 3. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree for a curved surface with GD = 10-1000 pN. Chain length: N=10 (A1, A2) and N=30 (B1, B2). Results based on upper distribution of sulfonic acid groups. The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively.


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(A)

(B)

Figure 4. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and surface density of PS brushes for a planar surface with an upper distribution of sulfonic acid groups and N=30.



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Figure 5. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and surface density of PS brushes for a curved surface, an upper distribution of sulfonic acid groups and N=30 with GD = 10-1000 pN. Surface density of PS brushes: 0.25 groups/nm2 (A1, A2), 0.50 groups/nm2 (B1, B2) and 0.75 groups/nm2 (C1, C2).


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Figure 6. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree and G-D volume fraction for a planar surface and an upper distribution of sulfonic acid groups. Chain length: N=10 (A1, A2) and N=30 (B1, B2). The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively.



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Figure 7. Evolution of |(γSG-γSD)C| (A) and Ep,dim (B) as a function of the sulfonation degree for a curved surface and an upper distribution of sulfonic acid groups with GD = 10-1000 pN. Chain length: N=10 (A1, A2) and N=30 (B1, B2). The surface density of PS brushes and SiOH groups were kept at 1.0 and 2.8 groups/nm2, respectively, while the G-D volume fraction was set at 5%.


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Figure 8. (A) Snapshot of the G (red) and D (blue-pink) equilibrium distribution at the surface of PS-grafted NPs (PS, green; SO3H, yellow) with a sulfonation degree of (A1) 0%, (A2) 40%, (A3) 80% and (A4) 100%; local concentration profiles (Z-averaged) of (B) polystyrene (PS) and SO3H (SH) moieties (in the figure insert), (C) G and (D) D along the Y-direction for the glycerol (B1, C1, D1) and dodecanol (B2, C2, D2) phases. NPs: N=10, upper distribution of sulfonic acid groups, variable sulfonation degree at 0% (black-solid), 40% (red-dashed), 80% (blue-short dashed) and 100% (green-dotted). The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively.



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(A)

Figure 9. (A) Snapshot of the G (red) and D (blue-pink) equilibrium distribution at the surface of PS-grafted NPs (PS, green; SO3H, yellow) with 40% sulfonation degree; local concentration profiles (Z-averaged) of (B) polystyrene (PS) and SO3H (SH) moieties (in the figure insert), (C) G and (D) D along the Y-direction for the glycerol (B1, C1, D1) and dodecanol (B2, C2, D2) phases. NPs: N=30, upper distribution of sulfonic acid groups, variable sulfonation degree at 0% (blacksolid), 40% (red-dashed), 80% (blue-short dashed) and 100% (green-dotted). The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively. 32Environment ACS Paragon Plus

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(A)

Figure 10. (A) Snapshot of G (red), D (blue-pink) and G-D (bright green-bright blue) equilibrium distribution at the surface of PS-grafted NP (PS, green; SO3H, yellow) and 10% G-D concentration. Local concentration profiles (Z-averaged) of (B) G, (C) D and (D) G-D along the Y-direction for the glycerol phase (B1, C1, D1) and dodecanol phase (B2, C2, D2) at a G-D volume fraction of 0% (black-solid), 1% (red-dashed), 5% (blue-short dashed) and 10% (greendotted). NPs: N=10, upper distribution of sulfonic acid groups, 40% sulfonation degree. The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively. 33Environment ACS Paragon Plus


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(A)

Figure 11. (A) Snapshot of the G (red), D (blue-pink) and G-D (bright green-bright blue) equilibrium distribution at the surface of PS-grafted NP (PS, green; SO3H, yellow) and 10% G-D concentration. Local concentration profiles (Z-averaged) of (B) G, (C) D and (D) G-D along the Y-direction for the glycerol phase (B1, C1, D1) and dodecanol phase (B2, C2, D2) at a G-D volume fraction of 0% (black-solid), 1% (red-dashed), 5% (blue-short dashed) and 10% (greendotted). NPs: N=30, upper distribution of sulfonic acid groups, 40% sulfonation degree. The surface density of PS brushes and SiOH groups were kept constant at 1.0 groups/nm2 and 2.8 groups/nm2, respectively.


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Table 1. Interaction parameters aij in kBT/rc units. The symbols G, Pe, Pol, SH, PS and S represent glycerol, propane, propanol, sulfonic acid groups, styrene and the silica surface, respectively.

G Pe Pol SH PS S

G

Pe

Pol

SH

25.00 71.69 28.38 28.89 59.81 25.00

25.00 49.37 47.34 27.32 67.70

25.00 25.02 39.79 27.95

25.00 38.24 28.40

PS

S

25.00 57.05 25.00



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Table 2. D/G and G/D molar ratios on the NP surface as a function of the PS chain length, sulfonation degree and distribution of sulfonic acid groups in the PS brushes. The surface density of PS brushes and SiOH groups was 1.0 groups/nm2 and 2.8 groups/nm2, respectively.

N (-)

10

30

Sulfonation degree (%)

Glycerol phase (D/G)x Surf upper uniform bottom

bulk

Dodecanol phase (G/D)x Surf upper uniform bottom

0%

2.59

2.59

2.59

0.20

0.20

0.20

20%

1.14

1.23

1.54

0.21

0.28

0.56

40%

0.84

2.10

1.40

2.29

4.19

0.30

60%

2.43

1.57

1.26

8.31

7.61

5.05

80%

1.43

2.18

1.00

8.23

0.59

0.56

100%

1.55

1.55

1.55

5.52

5.52

5.52

0%

1.85

1.85

1.85

0.22

0.22

0.22

20%

2.90

2.86

0.16

5.71

4.10

3.90

40%

0.05

0.09

0.10

0.47

0.42

3.66

60%

0.06

0.06

0.22

0.54

0.47

1.02

80%

0.59

0.40

1.21

13.5

7.37

9.20

100%

1.48

1.48

1.48

15.2

15.2

15.2

0-0.002

0-0.12


Bulk

0.13-0.68

0.062-0.22

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TOC GRAPHICS


Dodecanol Pickering Emulsions

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