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Scale Effect on Interface Reaction between PDMS-E Emulsion Droplets and Gelatin Cong Zhu, Jing Xu, Zhaosheng Hou, Suqing Liu, and Tian-Duo Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02532 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017
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Scale Effect on Interface Reaction between
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PDMS-E Emulsion Droplets and Gelatin
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Cong Zhu1, Jing Xu1,*, Zhaosheng Hou2, Suqing Liu3, and Tianduo Li1,*
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AUTHOR ADDRESS: 1. Key Laboratory of Fine Chemicals of Shandong Province, Qilu
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University of Technology, Jinan 250353, P. R. China
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2. College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal
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University, Jinan 250100, P. R. China
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3. Shandong Province Leather Industrial Research Institute，Jinan，250353，P. R. China
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KEYWORDS: Scale effect; Oil/water interface reaction; Monodisperse PDMS-E latex droplet;
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Zeta potential; Raman spectra
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ABSTRACT: In this study, scale effect on interface reaction between PDMS-E emulsion
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droplets
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α-[3-(2,3-epoxy-propoxy)propyl]-ω-butyl-polydimethysiloxanes (PDMS-E) emulsion droplets
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with different scales were prepared using Shirasu Porous Glass (SPG) membrane with 0.5-µm
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pore size. The results of Zeta potential showed that the surface charge density of PDMS-E
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droplets decreased with droplet scale, and the variation went through three stages, which were
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corresponding to the diameter range of 100-450 nm, 450-680 nm and 670-800 nm, respectively.
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The results of Raman spectra indicated that the distribution concentration of head groups in
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surfactants decreased but the polar epoxy groups tend to be exposed on the interface with
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increase in droplet scale. This was conducive to the nucleophilic attack of amino groups in
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gelatin on epoxy group. Thus, the conversion of amino groups was related to the scale of
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PDMS-E droplet. This study might provide a proper way to control the rate of interface reaction
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between immiscible macromolecule monomers.
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1.
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Copolymer that is derived from two typically heterogeneous macromolecular monomers often
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exhibits expected self-assembly property in some solution systems. These copolymers might
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spontaneously form spheres, cylinders and vesicles in solution systems. They also have some
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special application in drug delivery1-3, sensing4, medical imaging5-7, and so on. Understanding
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controlling the varied morphology of copolymer has been the central focus of copolymer-related
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research. Nevertheless, experimental studies still emphasize tuning the rate of heterogeneous
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reaction between macromolecular monomers, which is the key to determining the structure and
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self-assembly of copolymer8-10. In fact, it is a challenge to tune the rate of heterogeneous reaction
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between immiscible macromolecule monomers, which is limited by their compatibility or large
and
gelatin
was
studied
systematically.
The
monodisperse
INTRODUCTION
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molecule weight. This study might provide a proper way to control the rate of interface reaction
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between immiscible macromolecule monomers. Using monodisperse miniemulsions or
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microemulsion media might be an effective way to tune the rate of heterogeneous reaction11−19.
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When microemulsions are used as reaction media, their thermodynamic stability and
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microheterogeneous nature can induce drastic change in reagent concentration, on the basis of
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which the reaction rate can be tuned. Particularly, amphiphilic organic molecules can accumulate
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and orient at the oil-water interface, inducing regiospecificity in organic reactions20.
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In the past few decades, different methods, including seed emulsion polymerization21,22,
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micro-channel or micro-jet method23,24, ultrasonic dispersion method and membrane
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emulsification technology25, were used for preparing the monodisperse latex particles.
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Especially, cross-flow membrane emulsification has emerged as a promising technique for the
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large-scale production of particles with uniform size ranging from hundreds of nanometers to
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tens of micrometers26,27. Tubular Shirasu porous glass (SPG) membranes have been used in
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cross-flow membrane emulsification because of their chemical stability and tunable pore
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size28-36. The pore diameter of the membrane, the operation pressure and the concentration of the
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surfactant are critical factors determining the size of droplet37.
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Specifically, the rate of heterogeneous reaction is related to the size of monodisperse latex
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particle. Suen and Morawetz reported the kinetics of reaction of monodisperse poly (vinylbenzyl
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chloride) latex with a reactive surface38. They observed a different effect for the rate of reaction
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with the neutral diethanolamine and with the negatively charged glycinate38. At surfactant-free
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system, the concentration of the reactive chloromethyl groups increases with increasing particle
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size, which is conducive to the nucleophilic attack on the chloromethyl groups. Particularly, poly
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(vinylbenzyl chloride) has high glass-transition temperature (Tg), which means that the move of
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polymer chains will be restricted at room temperature conditions. This means that the
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concentration of reactive chloromethyl groups is surely determined by the size of latex particle.
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fact, there is a simple linear relationship between the concentration of reactive chloromethyl
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groups and size of latex particle. Moreover, many studies also indicate that the exposure of
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reactive groups in interface is a key role in determining the reaction rate10,39. However, the added
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surfactant results in the increasing in surface charge density, which can shield the reactive
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chloromethyl groups from being attacked38. Up to now, this kind of polymer with high Tg has
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widely used for such heterogeneous reactions9,40,41. But the relationship among charge density,
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concentration of reactive groups and particle size has not been built in such reaction system.
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Different from high Tg, low Tg enables the polymer chains to freely move at room temperature,
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which means that the charge density or reactive groups of the surface of latex particle will
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response to surrounding condition, such particle size. In other words, the surface composition
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can be tuned when the latex particle is prepared using polymer with low Tg. This suggests that
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the oil/water interface reaction can be controlled by tuning the scale of latex particle.
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Nevertheless, studies on the effect of droplet scale on oil/water interface reaction rate are still
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scarce. In this study, monodisperse emulsion droplets were prepared with mono epoxy
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terminated polydimethylsiloxane (PDMS-E, Tg -127 °C), and the interface reaction between
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PDMS-E latex particles and gelatin was regulated by tuning the scale of PDMS-E latex particle.
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This study will contribute a proper way to control the rate of interface reaction between
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immiscible macromolecule monomers.
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2. EXPERIMENTAL SECTION
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2.1 Materials
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Sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) were purchased
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from Alfa Aesar and recrystallized from ethanol before use. The allyl glycidyl ether (AGE) and
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glacial acetic acid were purchased from Alfa Aesar. Hexamethylcyclotrisiloxane (D3),
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n–Butyllithium (C4H9Li) and chlorodimethylsilane (C2H7ClSi) were purchased from Sigma
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Aldrich. Benzene and tetrahydrofuran (THF) were purchased from China National Medicine
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Corporation and were purified and strictly dehydrated before use. SPG membrane with 0.5-µm
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pore size was purchased from China National Medicine Corporation.
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Type A gelatin from pigskin was purchased from China National Medicines Corporation and
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used after dialysis. Molecular weight of the gelatin was determined by Gel Permeation
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Chromatography (GPC, see Supporting Information, Table S1). Results indicated that Mw of the
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gelatin was about 1.40×105 g mol-1 and Mw/Mn was 1.43. The content of primary amino groups
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in the gelatin was determined by the Van Slyke method at 50 oC and was 4.95×10-4 g mol-1. Van
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Slyke method is a professional method to determine the content of amino groups in amino acid
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or protein molecules. When a protein such as amino acid or protein is added into nitrous acid, the
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nitrous acid begins to react with the free amino groups in the protein. This reaction is the basis of
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the Van Slyke method for the quantitative determination of free amino groups42,43. To improve
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the accuracy of testing, traditional Van Slyke method was modified as in our previous
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studies44,45, and the testing error of the content of free amino groups in gelatin was below 1%.
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2.2 Synthesis of PDMS-H and PDMS-E
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D3, C4H9Li and C2H7ClSi were used to synthesize polydimethylsiloxanes with Si-H group at
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one end (PDMS-H) through anionic polymerization. The molar ratios of D3:C4H9Li:C2H7ClSi
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were about 2:4:1. First, 10 mL of benzene was added to the flask, and then 24 mL of C4H9Li was
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added. After reducing pressure and ventilation with argon gas, 45.99 g of D3 that was resolved in
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40 mL of benzene was added to the flask. After reaction for 30 min, 50 mL of THF was added
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the flask for reacting for 8 h. Then, 11 mL of C2H7ClSi was injected into the flask for stopping
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reaction. Subsequently, the products were purified. 45.44 g of PDMS-H was obtained. The yield
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PDMS-H was 56.11%. Next, 8.51 g of AGE was added to the flask, then, argon was injected.
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molar ratios of PDMS-H:AGE were about 1.6:1. After 30 min, about 40 µL of isopropanol-Pt
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injected to the flask. Argon was continually injected, and the temperature of the flask was
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enhanced to 80 oC. PDMS-H started to add at the speed of 1 drop per 2 second. Then, the
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temperature of the flask continued to enhance to 110 oC. After reaction for 6 h, the products were
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purified. The yield of PDMS-E was about 84.32%. The obtained PDMS-H were used to o
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α-[3-(2,3-epoxy-propoxy)propyl]-ω-butyl-polydimethysiloxanes (PDMS-E, Tg -127
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preparation scheme saw Figure 1a). Weight-average molecular weight and relative
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molecular weight (Mw=1.14×103 g mol-1, Mw/Mn = 1.16, Table S1) were measured on Waters
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GPC equipped with three Ultrastyragel columns (500, 103, 104 Å) in series and refractory index
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detector (RI 2414) at 30 oC using monodisperse polystyrene as calibration standard. THF was
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as fluent at a flow rate of 1.0 mL min-1. 1H NMR spectra of PDMS-H and PDMS-E were shown
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Figure 1b.
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Figure 1. Scheme of PDMS-H and PDMS-E polymers synthesis (a). The 400 MHz 1H NMR spectra of
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PDMS-H and PDMS-E in CDCl3 at 25 oC (b).
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2.3 Preparation of PDMS-E emulsion droplets using SPG membrane
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PDMS-E, as a dispersed phase, was added to 200 mL of deionized (DI) water containing
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different amounts of SDS, SDBS and glacial acetic acid (about 0.05 mL) to form
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PDMS-E-in-water emulsion (Table S2). The total concentration of surfactants ranged from 0.25
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to 1.0 wt %. Dispersed phase (about 2.000 g) passed through the SPG membrane pores under
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various nitrogen pressures. The process was based on that in literature37. Thus, emulsion droplets
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with different sizes were obtained by changing the SDS/SDBS ratio (w/w).
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2.4 General procedure for preparing PDMS-E grafted gelatin
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All gelatin samples were prepared from a stock solution of gelatin to minimize the
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experimental errors. The stock solution was prepared by dissolving gelatin in distilled water (5
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wt %), and after 3 h the gelatin solution was heated to 50℃ to ensure the complete dissolution of
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gelatin. Subsequently, pH of each solution was adjusted to 10.0 using sodium hydroxide solution
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(NaOH, 2.0 mol L-1). Then, the above emulsion was added to the gelatin solution at a rate of 20
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drops min-1 with stirring at 50 ℃ until the predetermined PDMS-E/gelatin ratio was reached.
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The reaction was continued for 24 h. The content of free amino groups was determined by the
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Van Slyke method. Then, the difference in conversion rate of amino groups was analyzed as
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related to the difference in the size of emulsion droplets.
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2.5 Characterization
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The physical size, Zeta potential and polymer dispersity index (PDI) of emulsion droplets
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were measured using a laser particle analyzer (Zetasizer 2000, Malven Instruments, UK). The
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instrument, on the basis of Mie-scattering theory, could convert the diffraction patterns to
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particle-size distribution and use electrophoresis to measure the Zeta potential. Firstly, the
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emulsion was carefully put into a color matching test tube. Then, the tube was put into the
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ZetaSizer 2000 laser particle instrument to measure the PDI or electrophoretic mobility (Zeta
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potential).
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Raman spectra were obtained by LabRAM HR800 (Horlba JY) equipped with an 800-nm red
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diode laser and a 630-nm argon ion laser. Raman spectra obtained from various scales of the
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PDMS-E droplets.
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Optical Microscopic (OM) images were obtained by optical microscope (Leica Microsystems
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GmbH, Germany) equipped with a Lecia DFC 420C CCD image capturing system. The
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magnification was 400X.
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All samples were tested under the same conditions (50 ℃, the conversion rate of NaNO2 was 40 wt %) to minimize the experimental errors.
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3. RESULT AND DISCUSSION
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3.1 Preparation of monodisperse PDMS-E emulsion droplets
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Scheme S1 illustrated the cross-flow membrane emulsification using tubular SPG membranes.
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In this work, a liquid of PDMS-E passed through the SPG membrane under a given pressure (P)
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into an aqueous solution (pH =3.75, adjusted by glacial acetic acid) containing SDS and SDBS
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surfactants. The total concentration of SDS and SDBS was set to 0.25 wt %, 0.50 wt % and 0.75
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wt %, respectively. In addition, the quality ratio of SDS to SDBS was set to 1:9, 2:8, 3:7, 4:6,
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5:5, 6:4, 7:3, 8:2 and 9:1 at each value of total concentration. Emulsion droplets were detached
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from the pore due to the shear force applied by the stirrer in the continuous phase. SDS and
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SDBS were rapidly adsorbed at the interface of the generated PDMS-E/water droplets to
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stabilize the emulsion droplets. The pore size of SPG membrane, pressure and surfactant
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concentration are the critical factors determining the diameter of emulsion droplets37. When the
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pore size and the pressure remain unchanged, surfactant concentration can determine the droplet
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diameter. As shown in Figures 2a, b and c, monodisperse PDMS-E emulsion droplets were
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successfully prepared using the SPG membranes with a mean pore size of 0.5 µm. The average
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diameters of emulsion droplets were 413 ± 70 nm, 616 ± 80 nm and 732 ± 65 nm at three
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different total concentration of SDS and SDBS, respectively. The average diameter of emulsion
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droplets was found to range from 190 ± 40 nm to 796 ± 50 nm when the SDS/SDBS ratio (w/w)
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was adjusted. All the droplets showed a narrow distribution of sizes, as indicated by the fact that
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the coefficient of variation (CV) values was less than 21%. Figure 2d presented the general trend
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of average diameter of droplets when the SDS/SDBS ratio (w/w) was changed. Clearly, the
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average diameter of droplets firstly increased with the SDS/SDBS ratio (w/w) and then decreased
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with the ratio.
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Figure 2. OM images of monodisperse PDMS-E droplets with an average diameter of 413 ± 70 nm (a), 616 ± 80
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nm (b) and 732 ± 65 nm (c) (Top-right inset: size distribution of monodisperse PDMS-E droplets). The average
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diameter of PDMS-E droplets changing with SDS/SDBS ratio (w/w) for total concentration of SDS and
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SDBS at 0.25, 0.50, 0.75 wt % (d).
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The effect of surfactant concentration in a single surfactant system on droplet diameter has
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been previously studied46. The values of the equilibrium “packing densities” of different
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surfactants at oil/water interface are almost equal when the critical micelle concentration (cmc) is
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reached. The mean size of emulsion droplets varies linearly with the interfacial tension (γ) of
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single surfactant system at concentration ranges near the cmc46. This rule is in agreement with
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laminar or turbulent viscous flow theory. When SDS concentration is low, γ value increases with
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the addition of SDS surfactant. Then γ value tends to remain constant when the cmc (0.236 wt
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%) of the SDS surfactant is reached47. As SDS concentration increases, γ value increases, which
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leads to increase in droplet size and finally leads to a higher critical concentration of SDS
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required for producing monodisperse emulsions48. However, in this work, γ value was adjusted
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by SDS/SDBS ratio (w/w, SDBS cmc: 0.052 wt %). SDS and SDBS had different head groups
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but same hydrocarbon tails. Firstly, γ value of the mixed surfactant system was measured (Figure
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S1). When the total concentration of SDS and SDBS was 0.25 wt %, γ value increased with the
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increasing SDS/SDBS ratio (w/w). The maximum γ value appeared when the SDS/SDBS ratio
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(w/w) was 6:4. At this ratio, the concentration of SDS was near to cmc. The increase in γ value
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resulted in the increase in droplet size. With further increase in the SDS/SDBS ratio (w/w), γ
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value gradually decreased, which was corresponding to the decrease in droplet size (Figure 2d).
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A similar trend was also observed in 0.50 wt % and 0.75 wt % mixed surfactant systems, the
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maximum γ value of which was obtained when SDS/SDBS ratio (w/w) was 4:6 (Figure 2d).
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In sum, monodisperse PDMS-E emulsion droplets were successfully prepared by using SPG
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membranes and the average diameter of droplets ranged from 190 nm to 796 nm by changing the
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SDS/SDBS ratio (w/w). The surface composition of droplet, including the surface charge density
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and reactive groups, was closely related to the droplet diameter, which was discussed next.
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3.2 Scale structuring of PDMS-E emulsion droplets
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The basic theory of colloid science indicates that charged surfaces in aqueous solution attract
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counterions from the solution and electrostatic double layer is thus formed49. A special type of
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electrical double layer that exists between an electrode and an electrolyte solution is called Stern
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layer. Stern studied theoretically the distribution of ions in this special electrical double layer. As
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shown in Figure 3a, the ordinate represented the electrical potential with respect to the interior of
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the solution, and the abscissa represented the distance from boundary 0 denoting the electrode
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surface. On the electrode side of boundary 0, the excess of positive over negative electricity was
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ɳ0 per cm2, assumed to be concentrated at boundary 0. On the solution side, there was an
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adsorbed layer of ions, with net charge of ɳ1, assumed to be concentrated at boundary 1. The
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distance between boundaries 0 and 1 was δ. Besides, there was a diffuse layer with net charge of
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ɳ2, whose charge density decreased rapidly as the distance from boundary 1 increased. The
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electrical potentials were zero in the interior of the solution, Ψ1 at boundary 1, and Ψ0 at
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boundary 0. The distribution of counterions outside a charged particle is highly nonuniform
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which is owing to long-range electrostatic attractions9. Commonly, the density of the surface
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charge is reflected by Zeta potential at boundary 150.
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In this work, Zeta potential of the PDMS-E emulsion droplets at different surfactant
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concentrations was measured as function of droplet size. As shown in Figure 3b, the surface
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charge of monodisperse droplet which depended on the droplet diameter varied over a wide
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range. When the total concentration of SDS and SDBS was 0.25 and 0.75 wt %, the Zeta
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potential of droplet decreased with the increasing droplet diameter. Pelton found that the charge
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distribution was likely to fall between the following two extremes51. (1) Charge is uniformly
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distributed throughout the particle volume. When the diameter decreases at elevated
18
temperatures, the volumetric charge density increases. (2) All charge is distributed near the
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hydrodynamic surface of the particle. When the latex diameter decreases, the surface charge
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density increases. Above results implied that SO4- in SDS and SO3- in SDBS were tightly
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adsorbed at the hydrodynamic surface of the PDMS-E latex droplet. As the size of the PDMS-E
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emulsion droplet increased (at constant pH), decrease in the surface charged density led to a
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continuous decrease in the magnitude of the Zeta potential.
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On the basis of Stern theory, Ohshima et al derived the relationship between Zeta potential and charge density52.
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where σ is the surface charge density, εr is the relative permittivity of the solution (εr = 78.5, T =
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298 K), ε0 is the relative permittivity of a vacuum (ε0 = 8.854×10-12 F/m), ݇ is Boltzmann
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constant (݇ = 1.3806×10-23 J/K), χ is the Debye-Hückel parameter (χ = 0.392, T = 298 K), ݁ is
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elementary electric charge (݁ = 1.6×10-19 C), ߞ is the Zeta potential and a is the droplet radius.
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The calculation results were shown in Table S3. The results indicated that the charge density
9
decreased as the droplet diameter increased. The change of the surface charge density went
10
through three stages, which were corresponding to the diameter range of 100-450 nm, 450-680
11
nm and 670-800 nm, respectively (Figure 3c). The results indicated that the decrease in the
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charge intensity was only due to the increase in the scale of PDMS-E droplet. Xia et al reported
13
that a tight interface layer should be formed at high charge density53. Thus, when the size of
14
droplet was smaller than 450 nm, a tight interface layer would be formed and inhibit the
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exposure of epoxy groups on the droplet’s surface. With increase in droplet size, the interface
16
charged layer tended to be looser, which induced epoxy groups to move toward the surface of
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droplet.
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Figure 3. Diagram of the Stern double layer (a). Zeta potential of PDMS-E droplet changing with droplet
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diameter for total concentration of SDS and SDBS at 0.25, 0.50, 0.75 wt % (b). Charge density of PDMS-E
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droplet changing with droplet diameter for total concentration of SDS and SDBS at 0.25, 0.50, 0.75 wt % (c). The
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three stages of change were distinguished by the variation curvature of charge density.
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In addition, the chemical composition of droplet surface was analyzed by Raman
8
spectroscopy. Normally, non-polar groups including alkyl chains in surfactant molecules and
9
siloxane chains in PDMS-E molecules should locate in the hydrophobic core of the emulsion
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droplets. Hydrophilic organic groups, including head groups in surfactant molecules and polar
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groups (including epoxy and ether groups), can accumulate and orient at the oil/water interface.
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Raman spectra obtained from various scales of the PDMS-E droplets, and the spectra from
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different parts of the samples could be obtained by adjusting the focus of the laser beam (the
14
focal volume)54. Focusing on a transparent object in a water solution is aided by the optical
15
display of an octagon when a surface or interface is in focus. Firstly, the droplets were detected
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by OM. Then, the laser beam was focused using a 50× water immersion objective for
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measurements on the inner wall of the capillary. The spectra from surface of the droplets could
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be obtained by adjusting the focus of the laser beam (the focal volume) when the PDMS-E
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droplet closed to the inner wall of the capillary. The test for the same sample was repeated ten
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times at different focus points on the inner wall of the capillary, which was for eliminating the
2
difference caused by the move of droplets. The curves with same level of peak intensity were
3
selected. Measurements of the total intensity made sure that any significant changes in the
4
spectra were from the different of chemical composition on the surface of PDMS-E emulsion
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droplet with the same probing volume.
6 7
Figure 4. Raman spectra of four samples with different scales (a). Molecular formula and chain length analysis
8
(b). Schematically illustration of the internal and external composition of PDMS-E droplet (c).
9 10
Figure 4a and Figure S2a showed the Raman spectra of PDMS-E droplets with the size of 188.9
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nm, 225.7 nm, 248.9 nm, 290.9 nm, 373.3 nm, 395.0 nm, 473.0 nm, 522.8 nm, 555.8 nm, 616.1
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nm, 687.8 nm, 732.4 nm and 796.8 nm, respectively. In these spectra, the SDS, SDBS and
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PDMS-E inclusions were clear55. The bands at 429 cm-1 and 604 cm-1 were associated with the
14
rocking vibration and bending vibration of -SO3- from SDS or SDBS, respectively. The band at
15
827 cm-1 was related to the stretching vibration of C-O-SO3 from SDS. At 707 cm-1, there was a
16
strong peak, which was because -CH2 and C-Si overlapped together. The peak at 860 cm-1 was
17
due to epoxy groups in PDMS-E chains. The peaks at 932 cm-1, 1074 cm-1 and 1132 cm-1 were
18
assigned to CH2-O-CH2 attached to epoxy groups. The calculation of length of molecule chains
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showed that these connected groups would locate in the interface (Figure 4b). This indicated that
2
the interface layer should include the head groups of surfactants, epoxy groups and the groups
3
connected with them (Figure 4c).
4
Furthermore, a clear trend was shown in Figure 4a: The intensity of the bands at 429 cm-1 and
5
604 cm-1 decreased with the increasing droplet scale, whereas the intensity of bands at 860 cm-1,
6
932 cm-1, 1074 cm-1 and 1132 cm-1 obviously increased with the increasing droplets scale. The
7
results indicated that the distribution density of head groups in surfactants decreased but the
8
polar epoxy group trend to locate in the interface as the droplet scale increased. The
9
accumulation of epoxy groups at interface was conducive to the nucleophilic attack on it.
10
3.3 Reaction with gelatin
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In short, the charge density and the concentration of epoxy groups, which were the key to
12
determining the reaction rate, were highly dependent on the droplet scale. Thus, it was important
13
to observe the variation of droplet size after the PDMS-E latex was placed for 1 h for the
14
following experiment procedure. OM recorded the morphology and size of the PDMS-E
15
emulsion droplets after they were placed at room temperature for 0, 0.5, 1.0, 3.0, 6.0 and 12 h.
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Figure 5 indicated that the morphology and size of the PDMS-E emulsion droplets changed little
17
after 0-3.0 h.
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Figure 5. OM images of PDMS-E droplets after they were placed for different time.
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According to literatures9,38, the heterogeneous reaction site is located at the interface between
3
the hydrophobic core of the emulsion droplets and the surrounding water, and the reaction rate
4
depends on the surface charge density as well as the local concentration of functional groups39.
5
In this work, the conversion of amino groups occurred essentially at the interface between the
6
PDMS-E emulsion droplets and the surrounding aqueous phase and the reaction rate was
7
strongly dependent on the local concentration of epoxy groups as well as the charge density in
8
this interfacial region. Suen and Morawetz38 found that the variation of reaction rate could be
9
attributed to three effects caused by the surfactant. First, it decreases the interfacial tension
10
between the nonpolar surface and the aqueous medium. Second, it increases the charge density of
11
the surface. Third, it shields the reactive groups from being attacked. The charge density and
12
chemical composition of PDMS-E latex droplet are determined by the droplet scale.
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Figure 6. Conversion rate of free amino groups changing with the scale of emulsion droplet under different
2
total concentrations of SDS and SDBS (0.25%, 0.50% and 0.75%).
3
Results indicated that relatively high conversion rates (21.5-25%) of amino groups were
4
observed in the larger scale region (Figure 6). The conversion rate of amino groups decreased to
5
16-19% under the same conditions as the droplet scale decreased to 450-650 nm, and further
6
decreased to 10-15% as the droplet scale decreased to 160-450 nm. Thus, the conversion rate
7
was related to the scale of PDMS-E droplet. The change of conversion rate also went through
8
three stages. The droplet with diameter smaller than 450 nm can be considered as a rigid ball
9
(Figure S3) that was protected by a tight interface layer (Figure 6a), which shielded the epoxy
10
groups. When the droplet diameter increased to a certain value, the interface of the droplet
11
became looser and epoxy groups were exposed on the surface (Figures 6b and c). This led to the
12
rapid increase in conversion rate. Thus, in a larger scale region, the conversion rate of free amino
13
groups increased more rapidly.
14
4. CONCLUSIONS
15
This study systematically discussed the scale effect on interface reaction between PDMS-E
16
emulsion droplets and gelatin. Zeta potential results indicated that charge accumulated on the
17
surface of PDMS-E droplet and the charge density varied with the droplet scale. Raman spectra
18
results showed that epoxy groups gradually migrated to the surface of droplet as the scale of
19
droplet increased. The charge density and the distribution concentration of epoxy groups of the
20
surface of PDMS-E emulsion droplets responded to the droplet scale, which could make the
21
reaction controllable. At smaller scale, the droplet was protected by a tight interface layer, which
22
resulted in the shield of epoxy groups. Thus, the conversion rate of amino groups was merely
23
10-15%. With the increase in droplet scale, the interface layer became looser and epoxy groups
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migrated to the surface of droplet, leading to the rapid increase in conversion rate. The
2
conversion rate of amino group increased to 16-19% under the same conditions as the droplet
3
scale increased to 450-650 nm, and further increased to 21-25% as the droplet scale increased to
4
670-800 nm. Thus, the conversion rate of amino groups was related to the scale of PDMS-E
5
droplet. This study might provide a proper way to control the rate of heterogeneous reaction
6
between immiscible macromolecule monomers.
7
ASSOCIATED CONTENT
8
Supporting Information. The molecular weight charaction of the gelatin by Gel Permeation
9
chromatography (GPC); the dosages of materials used for preparation of PDMS-E emulsion
10
droplets using SPG membrane; SPG membrane emulsification device; the calculation results of
11
charge density and measurement of the interface tension of the SDS and SDBS mixed surfactants
12
system and PDMS-E; Ramam spectra of PDMS-E droplets with different scales; Rheological
13
data of PDMS-E droplets with different scales. The supporting information is available free of
14
charge on the ACS Publications website at: http://pubs.acs.org
15
AUTHOR INFORMATION
16
Corresponding Author:
17
Dr. Jing Xu. E-mail:
[email protected];
18
Prof. Tianduo Li. E-mail:
[email protected];
19 20
Author Contributions: The manuscript was written through contributions of all authors. All authors have given
21
approval to the final version of the manuscript.
22
ACKNOWEMENTLEDG
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Page 20 of 27
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This work was supported by the National Natural Science Foundation of China (Grant Nos.
2
21606138 and 21376125), the National Natural Science Foundation of Shandong Province (No.
3
2015GGX108002) and Program for Scientific Research Innovation Team in Colleges and
4
Universities of Shandong Province.
5
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