Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Janus Particles in a Nonpolar Solvent Joohyung Lee, Benjamin A Yezer, Dennis C Prieve, and Sven Holger Behrens Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04255 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
377x282mm (96 x 96 DPI)
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 11
Janus Particles in a Nonpolar Solvent Joohyung Lee,￥ Benjamin A. Yezer,§ Dennis C. Prieve§ and Sven Holger Behrens￥ ￥
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332, USA
§
Center for Complex Fluids Engineering and Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Supporting Information ABSTRACT: Amphiphilic Janus particles are currently receiving great attention as “solid surfactants”. Previous studies have introduced such particles with a variety of shapes and functions, but there has so far been a strong emphasis on water-dispersible particles that mimic the molecular surfactants soluble in polar solvents. Here we present an example of lipophilic Janus particles which are selectively dispersible in very nonpolar solvents such as alkanes. Interfacial tension measurements between the alkane dispersions and pure water indicate that these particles do have interfacial activity, and, like typical hydrophobic, nonionic surfactants, they do not partition to the aqueous bulk. We also show that the oil-borne particles, by retaining locally polar domains where charges can reside, generate electric conductivity in nonpolar liquids – another feature familiar from molecular surfactants, and one commonly exploited to mitigate explosion hazards due to flow electrification during petroleum pumping, and in the formulation of electronic inks.
Introduction Janus particles, named after the two-faced Roman god, are colloidal particles with two distinctly different sides or “faces”. Amphiphilic Janus particles, with one hydrophilic and one hydrophobic face, have attracted much attention, because they combine properties of ordinary colloid particles and of small molecular surfactants.1-3 Thanks to the asymmetric bi-compartmentalization of these particles, their amphiphilicity can be finely tuned by varying either the chemistry and wettability of two compartments4-5 or their geometry.6-9 The strategy here is analogous to the case of designing molecular amphiphiles, where the molecules’ affinity to water (or oil) is controlled by their hydrophiliclipophilic balance (HLB) or packing parameter.10 One should note that although the term “amphiphilic” suggests a simultaneous affinity for both polar and nonpolar phases, surfactants often show very selective partitioning between the two phases whose interface they populate, sometimes to the point of being soluble in one bulk phase only. For example, the molecular surfactants with high HLB number (or small packing parameter) are predominantly hydrophilic and used in applications where they are dissolved in aqueous phases. Examples include common water-soluble surfactants such as sodium dodecyl sulfate (SDS), alkyl benzene sulfonates, or highly ethoxylated sorbitan esters (Tweens).11 On the other hand, surfactants with low HLB number (or large packing parameter), such as sorbitan esters of the Span series or polyiso-
butylene succinimide dispersants (OLOA), are hydrophobic and lipophilic, and typically added to nonaqueous media, sometimes purely dispersive10 nonpolar liquids such as saturated hydrocarbons.11-12 When it comes to amphiphilic Janus particles, it is notable that selectively oil-dispersible particles are grossly underrepresented in the literature;13-14 the vast majority of amphiphilic Janus particles are either primarily water dispersible1-9 or dispersible both in water and oils.14 To our knowledge, it has not yet been established to what extent Janus particles can mimic the behavior of lipophilic molecular surfactants in nonpolar solvents. Such oil-borne molecular surfactants adsorb to the interface with immiscible polar solvents like water, thereby reducing the interfacial tension. By doing so, they are also capable of dispersing polar solvents in the primary nonpolar continuous phase, thus creating confined “polar pools” wherein (polar) materials can be solubilized and stored, or in which chemical reactions can be carried out.15 Moreover, the surfactant aggregates (inverse micelles) incorporating the polar pools can play a role as “electric charge carriers”, causing an otherwise non-conducting liquid phase (e. g. alkanes with the dielectric constant ≈2) to support mobile electric charges and, by extension, display increased conductivity.16-24 Such surfactant mediated charging phenomena in nonpolar oils have long been used in the petroleum industry to prevent the dangerous accumulation of static charge by flow electrification;23 more recently, they have been exploited in advanced technologies such as electrophoretic image displays16 or electrorheological fluids.17 In the present study, we discuss an example of lipophilic Janus particles that do, in many respects, mimic the behavior of oil-soluble molecular surfactants.
Experimental Particle Synthesis. We prepare the oil-dispersible “snowman”-like Janus particles following well-established procedures of two-step seeded emulsion polymerization6-9 with a small but crucial chemical modification. In the first step, an aqueous dispersion of linear polystyrene (PS) seed particles (Bangs Laboratories, Inc., Catalog #PS02N, LOT #9598), spherical latexes with 99 nm in diameter, with a particle concentration 9.2 wt. %, is mixed with a monomer mixture of styrene (St, Sigma Aldrich) and 3(timethoxysilyl)propyl acrylate (TMSPA, Sigma Aldrich) (84:16 in wt. %), and an initiator 2,2’-azobisisobutyronitrile (AIBN, Sigma Aldrich) (0.5 wt. % of the monomer mixture). The weight ratio of the monomer mixture to PS seed particles is 50:50. The PS seed particles are swollen with the monomers for 12 hours. The polymerization is then carried out at an elevated temperature (70 °C) for 24 hours, which results in spherical hydrophilic parti-
ACS Paragon Plus Environment
Page 3 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
cles with the surface consisting of poly(St-co-TMSPA) (Figure S1A, Supporting Information). In the second step, an aqueous dispersion of the spherical hydrophilic particles synthesized in the first step with a particle concentration 11.7 wt. % is mixed with an emulsion of a monomer mixture, consisting of styrene and isodecyl methacrylate (IDMA, Sigma Aldrich) (32:68 in wt. %), the initiator AIBN (0.5 wt. % of the monomer mixture), in an aqueous 1.6 wt. % solution of polyvinyl pyrrolidone (PVP, Mw ~40000 g/mol, Sigma Aldrich). The weight ratio of the monomer mixture to aqueous PVP solution is 12:88. The weight ratio of the monomer mixture to hydrophilic particles is 82:18. The hydrophilic particles are swollen with the monomers for 12 hrs. The polymerization is then carried out at elevated temperature (70 °C) for 24 hrs, which results in the second lipophilic bulbs consisting of poly(St- co-IDMA), partially engulfing the first hydrophilic bulbs. The synthesized particles are washed three times with methanol and three times with ethanol to remove the unreacted monomers and PVP, by centrifuging the suspensions, removing the supernatant, and redispersing the particles in the cleaning alcohol with the help of sonication. It is found that the colloidal stability of these hydrophobic particles in the polar alcohols significantly diminishes as the washing proceeds and the water/alcohol soluble stabilizer PVP is washed out, with macroscopic particle aggregates forming rapidly after the sonication (typically after the 2nd wash with methanol). The particles are finally washed five times with the nonpolar solvent hexane (> 98.5%, ≈1.89 VWR), in which the particles appear to disperse without difficulty. After the final washing with hexane, the particle dispersions are diluted to the respective target concentrations. Water-in-Oil (w/o) emulsion preparation. To prepare the colored w/o emulsion, we added 1 mL of DI water containing 0.01 wt. % Rhodamine B (Sigma Aldrich) and 2 mL of hexane containing 0.5 wt. % Janus particles in a glass vial, and homogenized at 30000 rpm for 1 min using a rotor-stator homogenizer (IKA UltraTurrax® T10 basic). To test the sensitivity of the particles’ emulsification performance to pH, 1 mL of aqueous phases with pH 2.03 and 11.33 and 1 mL of DI water were homogenized with 2 mL of hexane containing 0.5 wt. % Janus particles at 30000 rpm for 1 min., where the pH of the aqueous phase was adjusted with 1.0 M HCl and 100 mM NaOH solutions. Characterization. The morphology of the particles was characterized by scanning electron microscope (SEM), using an Ultra-60 Field Emission SEM (Carl Zeiss AG). The solvodynamic diameter of oil-borne particles in nonpolar dispersions was characterized by dynamic light scattering (DLS), using an ALV DLS/SLS5022F (ALV-Laser GmBH) standard goniometer system (for a diluted dispersion) and a 3D cross-correlation setup (LS Instruments AG) (for concentrated dispersions). The effective interfacial tensions of nonpolar dispersions with water were measured by drop shape analysis, using a Ramé-hart goniometer model-250. The electric conductivity of nonpolar dispersions was measured using the nonaqueous conductivity probe DT-700 (Dispersion Technology, Inc.). Electrochemical impedance spectroscopy (EIS) was performed for a concentrated nonpolar dispersion using a custom setup following a procedure described in the literature.22 The moisture content in nonpolar dispersions was determined by volumetric Karl Fischer titration using a TitroLine KF titrator (SCHOTT). The detailed experimental protocol can be found in Supporting Information.
ity. Another considerable advantage of these Janus particles is that they can be synthesized at high-yield via seeded emulsion polymerization.8 Scanning electron micrographs of our amphiphilic Janus snowman particle are shown in Figure 1A and Figure S1B (Supporting Information). The surface of the hydrophilic lobes consists of a copolymer of styrene (St) and 3(trimethoxysilyl)propyl acrylate (TMSPA), and the hydrophobic lobes consist of a styrene and isodecylmethacrylate (IDMA) copolymer. In previously reported similar procedures, pure PS was selected as the “hydrophobic” compartment of dimers.6-9 It should be noted, however, that even the pure PS particles without hydrophilic bulb are not dispersible in very nonpolar liquids without additives (Figure 1B). Therefore, the conventional Janus snowman particles or dumbbells generated by seeded emulsion polymerization with PS as hydrophobic compartment also cannot be dispersed in alkanes. By contrast, the Janus snowman particles prepared in the present study, with a hydrophobic compartment made of (random) copolymer of alkylmethacrylate and styrene, are found to have excellent oil-dispersibility (Figure 1C). We have measured the solvodynamic diameter of these particles in hexane, using dynamic light scattering (DLS). While a standard goniometer instrument for DLS was used to characterize dilute dispersions, we employed 3D cross-correlation DLS (3DDLS)25 to more concentrated dispersions, because this technique suppresses the contribution of multiply scattered light to the analyzed intensity decorrelation that could otherwise introduce artifacts in the particle size analysis. Interestingly, the measured size (~400nm, Table 1) is roughly consistent with two or few associated snowman particles (such as the particle pairs shown in the insets of Figure 1A). This size appears fairly independent of particle concentration, with no indication of the significantly larger aggregates commonly found in unstable dispersions, up to high concentrations (8600ppm) where the dispersion looks milky.
Figure 1. (A) Oil-dispersible Janus snowman particles consisting of poly(St-co-TMSPA) (hydrophilic) and poly(St-co-IDMA) (hydroResults and Discussion phobic) surfaces. The particles were deposited on the glass slide The oil-dispersible Janus particles prepared here are dimers of from a 20 ppm hexane dispersion and sputter-coated with small two partially merged polymeric spheres, a shape that can be degold/palladium particles for SEM imaging. The (larger) hydrophoscribed as “snowman” or “dumbbell”-like.6-9 This type of Janus bic lobes, which presumably swell in the alkane solution, are harder particles is considered attractive since it is possible to control to discern in the images because of their weak contrast. The insets independently the chemistry and size ratio of the two lobes, where show examples of two associated snowman particles with their hyboth parameters significantly influence the particles’ amphiphilicdrophilic heads in contact. The scale bar represents 100 nm. (B) Pure (linear) PS particles failing to disperse in hexane (C) Lipophilic Janus snowman particles dispersed in hexane (13434 ppm) ACS Paragon Plus(D) Environment Lipophilic Janus snowman particles failing to disperse in water. Particles contents in (B) and (D) are same as the one in (C)
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 11
Table 1. Solvodynamic diameter (nm) of the oil-borne particles in hexane measured by 3DDLS. The estimated uncertainty in the reported diameters based on 10-12 measurements for each sample is 10 nm. Particle Concentration (ppm)
Solvodynamic Diameter (nm)
Measurement Method
7
403
DLS
300
414
3DDLS
1500
400
3DDLS
8600
397
3DDLS
Our Janus snowman particles have poor water-dispersibility (Figure 1D), unlike many Janus particles that are either primarily water dispersible or dispersible both in water and oils. In this regard they resemble the selectively oil-soluble surfactants often used as dispersants in nonpolar solvents. We note that the poor water solubility of such surfactants and their consequent retention in nonpolar media upon contact with water can be advantageous in their practical applications, and that the strong bias for nonpolar phases does not prevent these surfactants from adsorbing at oilwater interfaces and reducing the interfacial tension.12 The question arises whether such interfacial activity can also be observed for our oil-borne, water-indispersible Janus snowman particles. To answer this question, we measured the interfacial tension of hexane-based snowman particle dispersions with water using drop shape analysis. Upon increasing the particle concentration, the (effective) interfacial tension between water and hexane dispersions steadily decreases from a value close to 50 mJ/m2 in the absence of particles to a plateau value of ~31 mJ/m2 for particle concentrations above 1000 ppm (Figure 2A, the time-dependent interfacial tension from which the plateau value was obtained is shown in Figure S2 of the Supporting Information). The snowman particles thus adsorb at the oil-water interface with acollective adsorption energy large enough to substantially reduce the overall interfacial energy. Given this interfacial activity,3 one might expect that the lipophilic Janus snowman particles can stabilize w/o Pickering emulsions,26 as is indeed confirmed by emulsification experiments illustrated in Figure 2B. Interestingly, no pH dependence is found in performing the emulsification with these oilborne Janus particles (Figure 2C). This is in contrast to the emulsification performance of many particulate emulsifiers primarily dispersed in water, which can be affected strongly by the pH of the aqueous phase.9,26 The ubiquity of electric charges at particlewater and oil-water interfaces27 can cause water-borne particles to be repelled electrostatically from the interface unless the pH is adjusted appropriately. However, with the particles dispersed in the nonpolar phase, where low dielectric permittivity disfavors surface charging,28 we observe no evidence for a pH-dependent electrostatic barrier to particle adsorption. The pH-insensitive emulsifier performance of the oil-dispersed snowman particles could be quite useful for applications in which pH-altering reactions are carried out inside the aqueous droplets. We also note that the presence of dispersed snowman particles slightly increases(!) the surface tension of hexane (by ~1 mJ/m2), indicating a negative surface excess or depletion of the particles from the airhexane interface, another characteristic reminiscent of oil-soluble surfactants.12 Accordingly, it is found that these oil-borne particles do not stabilize Pickering foams, in contrast to some waterdispersible particles.
Figure 2. (A) Equilibrium interfacial tension of water with hexane-based dispersions of Janus snowman particles, represented as a function of particle concentration (plateau values obtained from the time-dependent interfacial tension, Figure S2 in Supporting Information). Error bars correspond to the maximum deviation from the plateau values. (B) Formation of w/o emulsion using oilborne Janus snowman particles. Left: Water phase dyed with Rhodamine B (0.01 wt.%) and hexane based particle dispersion (white) prior to homogenization; Right: w/o emulsion with sedimented water droplets. (C) Stable emulsions produced at different pH with the Janus snowman particles. We have seen that in many respects our selectively oildispersible Janus snowman particles closely resemble oil-soluble surfactants. One last characteristic of many oil-soluble surfactants to test for is the ability to raise the conductivity of nonpolar liquids. The formation of stable mobile ions in such media is usually impeded by the low dielectric permittivity , as can be appreciated by considering the Bjerrum length ⁄4 , i.e. the distance at which the interaction energy of two ions with elementary charge equals the thermal energy unit (here is the vacuum permittivity, denotes the Boltzmann constant and the absolute temperature). While in water at room temperature the Bjerrum length is 0.7 nm, its value in hexane is 29 nm, much larger than the size of classical ions, and such ions would thus not be stable but recombine into neutral entities.28 Oil-soluble surfactants can mitigate this problem by forming inverse micellar aggregates, whose polar core typically contains some residual moisture and provides electric charges with an extended environment of locally elevated dielectric constant.18-22,24 According to classical fluctuation theory, a small fraction of inverse micelles will charge as a matter of equilibrium fluctuations around a zero mean charge,29-30 and this fluctuation charge has helped explain the conductivity of nonpolar micellar solutions18-20 and microemulsions.29-30 We find that our Janus snowman particles also increase the electric conductivity of hexane (Figure 3), and appear to do so with an almost linear dependence on the particle concentration that is reminiscent of the linear conductivity increase with surfactant concentration often observed in the micellar regime of nonpolar surfactant solutions.18-20,22 We checked
ACS Paragon Plus Environment
Page 5 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
that the conductivity of the supernatant of a highly concentrated dispersion (15151 ppm) was only ~13 pS/m, which confirms that the conductivity increase is not mainly caused by any oil-soluble impurities residing in the continuous phase. A plausible explanation for the conductivity increase mediated by the snowman particles is that these, too, can solubilize small pools of water, located in all likelihood on the hydrophilic particle lobes or in the hydrophilic contact region of associated snowman particles. We verified the overall water content by Karl Fischer titration (Figure 4), and found that the addition of the Janus snowman particles indeed increases the total moisture content in the nonpolar phase. We also carried out electrochemical impedance spectroscopy for a dodecane sample containing the snowman particles (Figure S3, Supporting Information). The observed charge transport behavior is qualitatively similar to that previously reported for nonpolar fluids doped with the commercial charge control surfactant and engine oil additive OLOA 11000, where the measured electric current was attributed to the electrophoretic migration of charged inverse micelles and the formation of electrical double layers at the electrode surface.22 The magnitude of conductivity generated by our Janus snowman particles in the investigated range of particle concentrations is small, but reaches well above the minimum value of 50 pS/m recommended for hydrocarbon transport to prevent explosions as a result of flow electrification.23
Conclusion In summary, we have synthesized lipophilic Janus snowman polymer particles via seeded emulsion polymerization. These particles are not dispersible in aqueous media but well dispersible in pure alkanes, with a mean solvodynamic diameter consistent with small aggregates of snowman particles up to high concentrations where the dispersions appear milky. We have seen that these particles in many ways mimic the behavior of selectively oilsoluble surfactants. They readily adsorb to oil-water interfaces and lower their interfacial tension, but are depleted from the airoil interface. As a result, they can be used to stabilize water-in-oil emulsions (and do so with no apparent sensitivity to the pH of the aqueous phase), but they do not stabilize foams. Moreover, they can solubilize small amounts of water, and are seen to raise the conductivity of alkanes much like the oil-soluble surfactants commonly used to generate mobile charges in traditional petroleum handling, and to control particle charging in oil-based printing toners, electrorheological fluids, and in electrophoretic inks for electronic displays. Surfactant-mediating charging in nonpolar media, although long exploited in the industrial practice, is still poorly understood.22 We hope that the availability of Janus particles mimicking the behavior of molecular charge-control surfactants will open up a useful new avenue in the ongoing quest for a better understanding of electrical charging in nonpolar liquids with amphiphilic additives.
Figure 4. Moisture content of Janus snowman particle dispersions as a function of particle concentration. The red line indicates the moisture content in pure hexane.
ASSOCIATED CONTENT
Supporting Information Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION
Corresponding Author Sven H. Behrens (
[email protected])
Present Addresses School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332, USA
Author Contributions J. Lee designed and carried out all the experiments, and prepared the manuscript together with S. H. Behrens. B. A. Yezer and D. C. Prieve contributed the EIS measurement and helpful suggestions for the manuscript.
Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We sincerely thank J. S. Hyatt and A. Fernandez-Nieves for performing the 3DDLS measurement. This material is based upon work supported by the National Science Foundation under grant number 1160138. REFERENCES (1) Jiang, S.; Granick, S.; Schneider, H. J. Janus Particle Synthesis, SelfAssembly and Applications; Royal Society of Chemistry: London, 2012. (2) Walther, A.; M u ller, A. H. E. Janus Particles: Synthesis, SelfAssembly, Physical Properties, and Applications. Chem. Rev. 2013, 113, 5194-5261. (3) Kumar, A.; Park, B. J.; Tu, F.; Lee, D. Amphiphilic Janus Particles at Fluid Interfaces. Soft Matter 2013, 9, 6604-6617. (4) Rho, K. H.; Martin, D. C.; Lahann, J. Biophasic Janus Particles with Nanoscale Anisotropy. Nat. Mater. 2005, 4, 759-763.
Figure 3. Electric conductivity of hexane with dispersed Janus snowman particles as a function of particle concentration. The ACS Paragon Plus Environment conductivity of the pure solvent is 5-8 pS/m.
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(5) Suzuki, D.; Tsuji, S.; Kawaguchi, H. Janus Microgels Prepared by Surfactant-Free Pickering Emulsion-Based Modification and Their SelfAssembly. J. Am. Chem. Soc. 2007, 129, 8088-8089. (6) Mock, E. B.; Bruyn, H. D.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Synthesis of Anisotropic Nanoparticles by Seeded Emulsion Polymerization. Langmuir, 2006, 22, 4037-4043. (7) Kim, J.-W.; Lee, D.; Shum. H. C.; Weitz, D. A. Colloid Surfactants for Emulsion Stabilization. Adv. Mater. 2008, 20, 3239-3243. (8) Park, J.-G.; Forster, J. D.; Dufresne, E. R. High-Yield Synthesis of Monodisperse Dumbbell-Shaped Polymer Nanoparticles. J. Am. Chem. Soc. 2010, 132, 5960-5961. (9) Tu, F.; Lee, D.Shape-Changing and Amphiphilicity-Reversing Janus Particles with pH-Responsive Surfactant Properties. J. Am. Chem. Soc. 2014, 136, 9999-10006. (10) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press: Oxford, 2011. (11) Griffin, W. C. Classification of Surface-Active Agents by “HLB.” J. Soc. Cosmet. Chem. 1949, 1, 311-326. (12) Lee, J.; Zhou, Z.-L.; Behrens, S. H. Characterizing the Acid/Base Behavior of Oil-Soluble Surfactants at the Interface of Nonpolar Solvents with a Polar Phase. J. Phys. Chem. B 2015, 119, 6628-6637. (13) Adams, D. J.; Boker, A.; Krausch, G. Janus Particles at Liquid-Liquid Interfaces. Langmuir 2006, 22, 5227-5229. (14) Sun, Y.; Liang, F.; Qu, X.; Wang, Q.; Yang, Z. Robust Reactive Janus Composite Particles of Snowman Shape. Macromolecules 2015, 48, 2715-2722. (15) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. Interactions of Small Molecules with Reverse Micelles. Adv. Colloid Interface Sci. 1999, 82, 189-252. (16) Chen, Y.; Au, J.; Kazlas, P.; Ritenour, A.; Gets, H.; McCreary, M. Flexible Active-Matrix Electronic Ink Display. Nature 2003, 423, 136-136. (17) Hao, T. Electrorheological Fluids. Adv. Mat. 2001, 13, 1847-1857. (18) Hsu, M. F.; Dufresne, E. R.; and Weitz, D. Z. Charge Stabilization in Nonpolar Solvents. Langmuir 2005, 21, 4881-4887.
Page 6 of 11
(19) Roberts, G. S.; Sanchez, R.; Kemp. R.; Wood. T.; Barlett, P. Electrostatic Charging of Nonpolar Colloids by Reverse Micelles. Langmuir 2008, 24, 6530-6541. (20) Guo, Q.; Singh, V.; Behrens, S. H. Electric Charging in Nonpolar Liquids Because of Nonionizable Surfactants. Langmuir 2010, 26, 32033207. (21) Lee, J.; Zhou, Z.-L.; Alas, G.; Behrens, S. H. Mechanisms of Particle Charging by Surfactants in Nonpolar Dispersions. Langmuir, 2015, 31, 11989-11999. (22) Yezer, B. A.; Khair, A. S.; Sides, P. J.; Prieve, D. C. Use of Electrochemical Impedance Spectroscopy to Determine Double-Layer Capacitance in Doped Nonpolar Liquids. J. Colloid Interface Sci. 2015, 449, 212. (23) Klinkenberg, A.; van der Minne, J. L. The Prevention of Explosion Hazards; Elsevier: New York, 1958. (24) Karvar, M.; Strubbe, F.; Beunis, F.; Kemp, R.; Smith, N.; Goulding, M.; Neyts, K. Charging Dynamics of Aerosol OT Inverse Micelles. Langmuir 2015, 31, 10939-10945. (25) Schatzel, K. Suppression of Multiple Scattering by Photon CrossCorrelation Techniques. J. Mod. Opt. 1991, 38, 1849-1865. (26) Wang, H.; Singh, V.; Behrens. S. H. Image Charge Effects on the Formation of Pickering Emulsions. J. Phys. Chem. Lett. 2012, 3, 29862990. (27) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Charging of Oil-Water Interfaces Due to Spontaneous Adsorption of Hydroxyl Ions. Langmuir 1996, 12, 2045-2051. (28) Van der Hoeven, P. C.; Lyklema, J. Electrostatic Stabilization in Non-Aqueous Media. Adv. Colloid Interface Sci. 1992, 42, 205-277. (29) Eicke, H. F.; Borkovec, M.; Das-Gupta, B. J. Conductivity of WaterIn-Oil Microemulsions: A Quantitative Charge Fluctuation Model. J. Phys. Chem. 1989, 93, 314-317. (30) Hall, D. G. Conductivity of Microemulsions: An Improved Charge Fluctuation Model J. Phys. Chem. 1990, 94, 429-430.
ACS Paragon Plus Environment
Page 7 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
.
Insert Table of Contents artwork here
ACS Paragon Plus Environment
6
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(A) Oil-dispersible Janus snowman particles consisting of poly(St-co-TMSPA) (hydrophilic) and poly(St-coIDMA) (hydrophobic) surfaces. The particles were deposited on the glass slide from a 20 ppm hexane dispersion and sputter-coated with small gold/palladium particles for SEM imaging. The (larger) hydrophobic lobes, which presumably swell in the alkane solution, are harder to discern in the images because of their weak contrast. The insets show examples of two associated snowman particles with their hydrophilic heads in contact. The scale bar represents 100 nm. (B) Pure (linear) PS particles failing to disperse in hexane. (C) Lipophilic Janus snowman particles dispersed in hexane. (D) Lipophilic Janus snowman particles failing to disperse in water. 257x276mm (150 x 150 DPI)
ACS Paragon Plus Environment
Page 8 of 11
Page 9 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Figure 2. (A) Equilibrium interfacial tension of water with hexane-based dispersions of Janus snowman particles, represented as a function of particle concentration (plateau values obtained from the timedependent interfacial tension, Figure S2 in Supporting Information). Error bars correspond to the maximum deviation from the plateau values. (B) Formation of w/o emulsion using oil-borne Janus snowman particles. Left: Water phase dyed with Rhodamine B (0.01 wt.%) and hexane based particle dispersion (white) prior to homogenization; Right: w/o emulsion with sedimented water droplets. (C) Stable emulsions produced at different pH with the Janus snowman particles. 250x275mm (150 x 150 DPI)
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 3. Electric conductivity of hexane with dispersed Janus snow-man particles as a function of particle concentration. The conductivity of the pure solvent is 5-8 pS/m. 273x209mm (300 x 300 DPI)
ACS Paragon Plus Environment
Page 10 of 11
Page 11 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Figure 4. Moisture content of Janus snowman particle dispersions as a function of particle concentration. The red line indicates the moisture content in pure hexane. 258x201mm (300 x 300 DPI)
ACS Paragon Plus Environment
