MaterialsViews Surface Functionalized Hydrophobic .
www.advmat.dewww.MaterialsViews.comAlireza Abbaspourrad, Nick J. Carroll, Shin-Hyun Kim, and David A. Weitz*Porous particles are attractive for applications in drugdelivery,[1–3] sensing,[4–6] and absorption of organic pollutants.[7,8]The removal of organic pollutants from undesirable sites, suchas the surface of water or a sub-surface aquifer remains a challenge; innovation of materials for effective organic contaminant removal is essential to minimize ecological damage.[9–11]Silica aerogels and core-shell nanoparticles have been usedto this end;[12–14] however, due to the hydrophobic nature oftheir surfaces, these particles are highly unstable in aqueousenvironments. As a result, the application of these particleshas been restricted to air-water interfaces. Effective remediation requires particles which can remain stable in a variety ofaqueous environments. A promising route to achieve water dispersion of particles with hydrophobic nature is to tailor the particle surface chemistry: for example, by constructing core-shellparticles with a hydrophobic porous oil-absorbing core and ahydrophilic surface to facilitate dispersal in water. Particles thatabsorb organic contaminants while remaining well dispersed inwater are potentially useful for water filtration or purificationapplications.In general, these particles must possess three critical features: they should be porous, hydrophobic and coated by ahydrophilic surface. There are various techniques to produceporous particles.[15–17] One flexible and convenient methodto form porous particles is the use of pore forming agentsknown as porogens.[18–20] A highly robust class of porogen processing exploits phase separation of two miscible liquids toproduce different domains.[21] By solidifying one of the liquidsand selective removal of the second liquid, a porous structureis formed. The porogen-template method is a facile route toproduce porous particles; however, the method must be combined with other techniques to produce porous particles withcontrollable core-shell structure and surface properties. Capillary microfluidic devices have been used to produce core-shellDr. A. Abbaspourrad, Dr. N. J. Carroll,Prof. D. A. WeitzHarvard UniversitySchool of Engineering and Applied Sciencesand Department of Physics29 Oxford Street, Cambridge, MA 02138, USAE-mail:weit z@seas.harvard.eduDr. S.-H. KimHarvard UniversitySchool of Engineering and Applied Sciences and Department of Physics29 Oxford Street, CambridgeMA 02138, USADepartment of Chemical and Biomolecular EngineeringKAIST, Daejeon, South KoreaDOI:10. 1002/adma.201300656Adv. Mater. 2013, 25, 3215–3221particles using double and single emulsion strategies;[22–28] forexample, by controlling the spreading of two immiscible oils ina continuous phase to form core-shell structures.[29] However,these methods have not yet been investigated for fabrication ofporous core-shell particles designed with controllable surfaceproperties. Thus, the fabrication of hydrophobic porous particles tailored for aqueous dispersion remains an important yetunmet need.In this paper, we introduce a new type of porous core-shellparticle designed to uptake organic contaminants in aqueoussolution. We use microfluidic and porogen templating techniques to fabricate particles with a hydrophobic porous coreand a hydrophilic surface. The hydrophilic surface enables theparticles to be dispersed in water, while the hydrophobic coreabsorbs organic molecules from the surrounding aqueousenvironment. We use a capillary microfluidic device to preparepaired droplets consisting of two photocurable monomer mixtures dispersed in an aqueous continuous phase. To lower interfacial energy, the less hydrophobic monomer mixture spontaneously spreads and engulfs the more hydrophobic mixture toform a non-centric core-shell droplet. The monomer mixturewhich spreads to form the shell contains hydrophilic silicananoparticles; the nanoparticles adsorb at the aqueous interfaceand confer hydrophilicity to the particle surface for dispersionin water. The inner mixture is a binary blend of photocurablemonomer and porogen PDMS template. We form porous particles by photopolymerizing the monomers and subsequentlyremoving the template porogen. These surface-modified particles demonstrate a good ability to absorb organic contaminantsfrom an aqueous environment while remaining well dispersedin the water phase.To synthesize particles, we emulsify two different monomermixtures into single drops in an aqueous continuous phaseusing a glass capillary microfluidic device. The device is constructed from a theta (θ) shaped injection capillary which hastwo separate channels. We taper the theta (θ) shaped capillaryand insert it inside a cylindrical collection capillary to increasethe velocity of the continuous phase by confining the flow nearthe tip of the injection capillary. Both theta (θ) shaped andcylindrical capillaries are placed coaxially inside a square capillary whose inner dimension is the same as that of the outerdiameter of the theta shaped and cylindrical capillaries; a schematic of the device is shown in Figure 1a. We flow monomer ofethoxylated trimethylolpropane triacrylate (ETPTA) containing1 wt.% silica nanoparticles through one channel of the θ-shapedcapillary and isobornylmethaacrylate (IBMA) plus PDMS oiland a crosslinker (hexandiol dimethylacrylate) through thesecond channel, as shown in Figure 1b. We refer to the IBMA,PDMS and crosslinker as IBMA mixture. We introduce the continuous phase by pumping an aqueous surfactant solution of 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.comCOMMUNICATIONSurface Functionalized Hydrophobic Porous ParticlesToward Water Treatment Application3215
gure 1. a) Schematic illustration of the microfluidic device comprised of a tapered theta (θ)-shaped capillary for injection of two oil phases and acircular capillary for droplet collection. b) Optical microscope image showing droplet generation in dripping mode. c) Optical microscope image ofresultant multilayer particles following UV exposure.1 wt% ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (Pluronic F-108) through the interstices of the squareand cylindrical capillaries. We generate drops consisting of twodifferent monomer mixtures; the upper phase is ETPTA withdispersed silica particles while the lower phase is the IBMAmixture, as shown in Figure 1a. The ETPTA phase completelywets the surface of the IBMA mixture, forming a non-centriccore-shell drop as shown in Figure 1a and Movie S1. The complete coverage occurs because the spreading of ETPTA reducesthe interfacial area of high surface tension between the IBMAmixture and water. The ETPTA phase contains hydrophilic silicaparticles which spontaneously adsorb to the interface with water,where they minimize the total interfacial energy by reducing thecontact area between ETPTA and water. The particles remaintrapped at the ETPTA/water interface because the reduction ofinterfacial energy is much greater than thermal energy.Following droplet generation, ETPTA spreads on the surfaceand mixes with the IBMA and PDMS in the core. Due to the lowmiscibility of ETPTA and PDMS, a mixture-driven phase separation occurs within the droplet; thus, PDMS phase separates fromthe mixture resulting in formation of a PDMS-rich inner layersurrounded by a monomer-rich middle layer. Both layers consist of a binary blend of photocurable monomers and porogenPDMS. However, the PDMS-rich inner layer is distinguishedby a much larger ratio of porogen PDMS to monomer in comparison with the middle layer. The outermost layer of the drop iscomprised of silica nanoparticles dispersed within monomer mixture. A diagram illustrating the droplet layers is shown in the firststep of Figure 2a. We prepare porous particles by in situ photopolymerization of the multi-layered droplets containing porogenPDMS. The difference in porogen to monomer ratio within the3216wileyonlinelibrary.commonomer-rich and PDMS-rich layers results in the formation ofdissimilar polymeric structures within each layer. In the PDMSrich layer, the precipitation polymerization of monomers results ininter-connected polymeric particles whose sub-micron intersticesare filled with PDMS. By contrast, polymerization of the monomer-rich middle layer leads to the exclusion of PDMS to nanometer size domains dispersed within a continuous polymeric matrix.Polymerized regions are shown as amber color while dark browncolor represents porogen PDMS in the second step of Figure 2a.To replace liquid PDMS with air we wash the particles with isopropanol (IPA) and subsequently dry them. Air-filled pores are represented with white color as shown in the last step of Figure 2a.We cut the prepared multilayered particles in half and obtainSEM images to examine the surface and internal structure. Atthe surface, anchored silica particles form hexagonal arraysover the entire exterior of the microparticle, as shown in theSEM image in Figure 2b. A high magnification SEM image ofthe protruding silica particles, 380 nm in diameter, is shownin the inset of Figure 2b. The outer layer is characterized by apolymer matrix enriched with dispersed hydrophilic silica particles as shown in Figure 2c. Functionalization with colloidalsilica confers surface hydrophilicity to the particles which facilitates dispersal in water. By contrast, the porous structure ofthe hydrophobic middle and inner layers is designed to absorborganic molecules. The middle layer is characterized by smallpores of tens of nanometers in size, as shown in Figure 2d; ahigh magnification SEM image of the middle layer polymericnanostructure is also shown in Figure S1 in the SupportingInformation. The inner layer is comprised of interconnectedIBMA spheres characterized by larger pores with dimensionsof hundreds of nanometers, as shown in Figure 2e. 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2013, 25, 3215–3221
gure 2. a) Schematic illustration describing the mechanism of mixing driven phase separation, precipitation polymerization and subsequent poreformation after removal of porogen PDMS oil. b-e) SEM images of the surface and the three different layers within the porous particle.The multilayered droplets which act as templates for our porous particles likely form byphase separation of the ETPTA, IBMA, andPDMS mixture. To test this hypothesis, we usebulk mixtures of these three fluids to determine their ternary phase diagram; the phaseboundary is shown by the dashed line withinthe phase diagram in Figure 3. For small concentrations of ETPTA and PDMS, the fluidsmix homogeneously, as indicated by the bluepoints in Figure 3. By contrast, for sufficientlylarge ETPTA and PDMS concentrations, thefluids do not mix homogeneously; instead,they phase-separate into PDMS-rich andmonomer-rich phases, as indicated by the redpoints in Figure 3. Clearly, to form multilayered droplets which act as templates for ourporous particles, the droplet fluid compositionmust fall within the two phase region; this isachieved by adjusting the relative flow ratesof ETPTA and IBMA/PDMS mixture. In thiscase, phase separation within the droplet initiates upon component mixing to form PDMSrich and monomer-rich layers; eventually,phase separation will proceed to completionAdv. Mater. 2013, 25, 3215–3221Figure 3. ETPTA, IBMA and PDMS Ternary phase diagram, the phase boundary is indicatedby the dashed line. 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com3217
sulting in a multi-layered equilibrium structure. The molarratio of porogen to monomer is constant as predetermined by theflow rates of fluids injected to form the droplet; however, as phaseseparation proceeds within the droplet, this ratio increases withinthe droplet inner layer while concurrently decreasing within thedroplet middle layer. Therefore, the time interval between dropletgeneration and UV exposure is critical; it determines the finalporogen to monomer ratio at the time of polymerization and thusthe corresponding polymeric nanostructures formed. In our case,we arrest phase separation a few seconds after droplet generationby in situ photopolymerization to obtain the desired microstructures; thus, adjusting the interval between droplet generation andUV exposure, could in principle, provide an additional level ofdesign control for these porous particles.To approximate the porogen to monomer ratio within thehighly porous inner layer at the time of in-situ photopolymerization, we prepare various combinations of IBMA monomerand porogen in bulk. We photopolymerize the mixtures, subsequently remove the porogen PDMS, and examine the resultantporous structures within the bulk polymeric matrix usingSEM. The pores within the polymer matrix become larger insize as the porogen to monomer ratio increases, as shown bythe SEM images in Figure S2. By comparing the structuresformed in bulk with that of the microparticles, we estimatethe ratio of porogen to monomer within the inner layer at thetime of in-situ polymerization as 1:1. Using our approach, theporogen to monomer ratio and corresponding particle internalmicrostructure can be controlled by adjusting mixture composition and time interval between droplet generation and UVexposure.Despite its applicability to tailoring porous structures by controlling the extent of phase separation, our technique is limitedto relatively short time intervals between drop generation andUV irradiation. We observe in the case of long time intervals,phase separation proceeds to completion and, due to the lowmonomer content within the PDMS-rich inner layer, no consolidated structure can be formed within the inner layer uponUV exposure. To substantiate this observation, we generatedrops and store them for 12 hours and subsequently photopolymerize them. Following washing steps, we image the particlesusing SEM. We observe a 45 µm diameter cavity at the particlesurface as evidenced by Figure S3a; a higher magnificationSEM image is shown in Figure S3b. This result illustrates theimportance of the time interval between droplet generation andphotopolymerization for fabricating particles with controlledmicrostructure using this technique. We also control the sizeof the porous inner layer by varying the relative volumetric flowrates of the ETPTA (Q1) and IBMA/PDMS (Q2) mixtures. Operating in the dripping mode, we adjust the size of the inner layerwith three different relative flow rates of Q1/Q2 0.6, Q1/Q2 1 and Q1/Q2 1.5, while maintaining a constant sum of Q1and Q2. Optical microscope images of the resultant microparticles are shown in the left and middle columns of Figures 4a–cwith SEM images in the right column. In the optical images,Figure 4. Optical microscope (left and middle column) and SEM images (right column) of particles formed using three different relative volumetricflow rate ratios of ETPTA (Q1) and IBMA mixture (Q2). a) Q1/Q2 0.6; b) Q1/Q2 1; c) Q1/Q2 1.5.3218wileyonlinelibrary.com 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2013, 25, 3215–3221
www.advmat.dewww.MaterialsViews.comAdv. Mater. 2013, 25, 3215–3221 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.comCOMMUNICATIONwe observe a distinct dark sphere inside theparticle, which is the porous inner layer;this high opacity is caused by strong lightscattering in the heterogeneous inner layerstructure. Using SEM, we are also ableto confirm that the size of the inner coreregion increases with increasing ratios of Q1/Q2. Adjusting the relative flow rate offers asimple and effective way to tailor the size ofthe hydrophobic porous inner layer.To demonstrate the effectiveness of ourporous particles to absorb organic contaminants, we immerse them in an aqueoussolution saturated with decane hydrocarbonoil and subsequently measure the amountof oil absorbed. We place dried porous particles (Q1/Q2 0.6) within a glass vial andset a paper filter on top of the particles toprevent them from rising. To prepare anaqueous solution saturated with organiccontaminants, we add a water layer to thebottom of the vial followed by gentle addition of a decane oil layer on top of the watersurface. After sealing the vial, we incubatethe samples at 65 C for predeterminedamounts of time. We then dry the particlesand subsequently weigh them to measurethe amount of hydrocarbon oil absorbed atdifferent intervals over a 52 h time period asshown in Figure 5a. The maximum amountof absorbed oil reaches 16 wt.% of the initialparticle sample weight at 48 hours; no significant increase in oil uptake is measuredafter 48 hours as evidenced by the blacksquares in Figure 5a. Remarkably, despite theintrinsic hydrophobic nature of the porousparticles, they remain stable and well dispersed in water throughout the experiment;we attribute the excellent dispersion of the Figure 5. a) The amount of oil absorbed as characterized by measuring particle weight overtime for porous particles (black squares) and non-porous particles (red circles). b) Opticalparticles to the designed hydrophilic surface.images series of particles absorbing oil and consequent increase of inner layer brightnessThis result illustrates the potential of surface- during 48 h time period.functionalized porous particles as effectiveorganic contaminant absorbers in aqueousenvironments. The significance of nanopores for effective conresulting in diminished scattering and increased transpartaminant absorption is substantiated by an implementation ofency. We image the particles using an optical microscope andthe preceding experiment using non-porous particles of theobserve a change in the brightness of the inner layer as timesame material; these particles are constructed using our fabproceeds. At 48 h core becomes transparent, after which norication method, the only difference being exclusion of PDMSsignificant changes are observed. Microscope images of a parporogen. In this case, the particles absorb only 1.7 wt% ofticle core increasing in brightness at different time intervals aretheir initial weight over 52 hours as evidenced by the red cirshown in Figure 5c.cles in Figure 5a; this considerable reduction in the absorptionIn addition to absorption, the desorption capacity of the particapacity illustrates the importance of the porous nanostructurescles is an important feature for practical use in water treatmentobtained by porogen templating. In the case of water-dispersedapplications. To demonstrate the recycling capability of our parporous particles, the air-filled pores result in an opaqueness ofticles, we remove the absorbed organic oil by immersing thethe inner layer as characterized by optical microscopy in transparticles in IPA solution at 60 C for ten minutes. Thereafter,mission mode. Mie scattering dominates because of the highthe particles are dried and reweighed; the weight of the recycontrast in refractive indices between air (nair 1) and IBMAcled particles is the same as the weight measured prior to theirparticles (nIBMA 1.47) within the inner layer. As decane oiluse in the preceding water treatment experiment. Moreover, wemolecules (ndecane 1.49) are absorbed, the contrast decreases,observe an opaque inner layer within the dried particles which3219
rther suggests effective removal of the organic contaminantfrom the pores. We then repeat the water treatment experiment using the recycled particles to measure their absorbance capacity. The amount of organic contaminants absorbedby the recycled particles after 52 h of exposure is 16 wt.% ofthe initial particle sample weight. This result demonstratessimple recycling of the particles with reasonable time scale forboth the contaminant absorption and desorption proces
Toward Water Treatment Application Alireza Abbaspourrad , Nick J. Carroll , Shin-Hyun Kim , and David A. Weitz * . The porogen-template method is a facile route to produce porous particles; however, the method must be com- . properties. Thus, the fabrication of hydrophobic porous parti-
The hydrophobic effect is associated with low enthalpy and high heat capacity Now that we have the basic molecular picture for the hydrophobic effect, lets see if we can also understand two more features of the hydrophobic effect as well. First, why is the enthalpic contribution of the hydrophobic
Cooperative Adsorption of Functionalized Nanoparticles onto Silica S. Smoukov et al., J. AM. CHEM. SOC. 2007, 129, 15623-15630 (2007) - Single functionalized particle-type will NOT assemble. - When functionalized NPs are mixed with oppositely charged functionalized NPs assembly DOES occur!
16) In a lipid bilayer the head is _ and the tail is _, composed of long carbon chains A) nonpolar, polar B) nonpolar, hydrophobic C) hydrophilic, hydrophobic hydrophilic polar. Hydrophobic nonpolar D) hydrophilic, polar E) hydrophobic, hydrophilic 17) Amines are derivatives of ammonia and
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Hydrophobic Interaction Chromatography: Fundamentals and Applications in . aromatic residues on a proteins surface). The hydrophobic interaction is directly proportional to the length of the alkyl chain. The most commonly used ligands in HIC resins . counter ion condensation theory (Manning, 1978), whereas hydrophobic interaction is .
presence of clay minerals. Instead, we find that clay surfaces have a significant hydrophobic character at the atomistic scale, as previously noted in the context of cation-exchange selectivity.42,43 This hydrophobic character is modulated by the presence of exchangeable cations and by the templating of interfacial water by the clay surface.
1 Deuterium Isotope Effects on Hydrophobic Interactions. The Importance of Dispersion Interactions in the Hydrophobic Phase Maciej Turowski,†,‡ Naoki Yamakawa,† Jaroslaw Meller,§ Kazuhiro Kimata,† Tohru Ikegami,† Ken Hosoya,† Nobuo Tanaka,*,† and Edward R. Thornton*, Contribution from the Department of Polymer Science and Engineering, Kyoto Institute of Technology,
Application of Silicon Carbide in Abrasive Water Jet Machining Ahsan Ali Khan and Mohammad Yeakub Ali International Islamic University Malaysia Malaysia 1. Introduction Silicon carbide (SiC) is a compound consisting of silicon and carbon. It is also known as carborundum. SiC is used as an abrasive ma terial after it was mass produced in 1893. The credit of mass production of SiC goes to Ed .
