Investigating the Phase Inversion of Pickering Emulsions: An

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Investigating the Phase Inversion of Pickering Emulsions: An Experiment To Explore Colloid and Interface Chemistry Concepts Dezhong Yin,* Jinhua Chen, Bin Gu, and Wangchang Geng School of Science, Northwestern Polytechnical University, Xi’an 710072, China S Supporting Information *

ABSTRACT: A new experiment based on phase inversion of Pickering emulsions was carried out in a physical chemistry course. The emulsions were stabilized by surfactant-modified SiO2 particles. As the concentration of surfactant increased, the emulsions changed from oil-in-water type (O/W) to water-in-oil type (W/O) type, and then reverted to be O/W type again. This double phase inversion of emulsions proved to be an ideal indicator for the adsorption of surfactant on particle surfaces. The students gained preliminary lab skills and improved their understanding of the basic concepts in colloid and interface chemistry. In addition, students were encouraged to work out a mechanism to explain the phase inversion; this practice helped strengthen and improve their reasoning abilities. KEYWORDS: Second-Year Undergraduate, Physical Chemistry, Colloids, Phases/Phase Transitions/Diagrams, Surface Science, Laboratory Instruction, Noncovalent Interactions, Learning Theories, Professional Development, Theoretical Chemistry

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BACKGROUND The term emulsion is defined to be a stable suspension of droplets of one liquid within a second, immiscible liquid.1,2 This characteristic makes an emulsion a good medium in a chemistry experiment, such as extraction3 or encapsulation4 of natural products, personal care products,5 and emulsion polymerization.6 Emulsion is related to many basic concepts in the physical chemistry curriculum, such as surface tension, interfacial adsorption, and emulsification. An experiment on emulsion is critical to make the related concepts intelligible for the students. A Pickering emulsion is an emulsion stabilized by colloidal particles rather than surfactants.7 The Pickering emulsion has been regarded as a useful methodology for the comprehension of colloid and interface chemistry.8,9 Hydrophilic particles are apt to stabilize as an oil-in-water (O/W) emulsion, while hydrophobic particles are apt to stabilize as a water-in-oil (W/ O) emulsion.10 Adsorption of modifiers on particles has been commonly employed to change the surface wettability of particles.11 This leads to a change of emulsion type, namely, a phase inversion of emulsion. If the emulsions change from O/ W to W/O and to O/W again, or from W/O to O/W and to W/O again, this is called a double phase inversion. Phase inversion of a Pickering emulsion is an ideal indication of surface adsorption of a modifier on particles and the surface wettability of particles. Therefore, a comprehensive experiment based on Pickering emulsion is beneficial for the students to understand the related concepts. Herein, a new experiment based on the double phase inversion of Pickering emulsions was carried out by second-year © XXXX American Chemical Society and Division of Chemical Education, Inc.

undergraduates in their physical chemistry curriculum. A cationic surfactant was adsorbed electrostatically on negatively charged SiO2 particles.12,13 Surface wettability of particles was adjusted to be hydrophobic or hydrophilic, as the amount of adsorbed surfactant on particles increased. As a result, a double phase inversion of emulsions was observed. The whole experiment included five sections: 1. Theoretical study 2. Preview of the experiment 3. Lab experiment 4. Laboratory report 5. Postlab questions During the lab experiment period, the students prepared related materials, made emulsions, tested zeta potential and conductivity, and decided the emulsion type. After the lab experiment, the laboratory report and postlab questions are required for every student to evaluate their performance. In their laboratory report, the students deduced the reason for phase inversion by considering the wettability of SiO2 particles, and then evaluated the adsorption pattern of surfactant molecules on SiO2 particles. The teacher worked throughout the process, correcting and improving the students’ understanding of related concepts. Through this experiment, the students gained a precise understanding of the related concepts and knowledge in the physical chemistry curriculum. Also, the Received: December 11, 2017 Revised: February 7, 2018

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DOI: 10.1021/acs.jchemed.7b00950 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

nearly neutral as the concentration of surfactant increases to 10.0 mM (Figure 1I). Also, the zeta potential has a significant

students acquired transferable reasoning ability through this problem-based learning (PBL) model.

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ASSIGNMENT OF EACH LAB PERIOD Didecyldimethylammonium bromide (DDAB, analytical grade), isooctane (analytical grade), and SiO2 gel (Ludox HS30, 30 wt %, pH 9.8), and other reagents, were obtained commercially and used without further purification. The students worked individually. The laboratory experiment was divided into two sections (2 h each section, typically). Detailed procedures are provided in the Supporting Information. In the first lab period, a surfactant-modified SiO2 dispersion was obtained, and the zeta potential was determined. SiO2 stock dispersion was prepared from Ludox SiO2 gel. Surfactant solutions with different concentrations were prepared by the diluting method. Surfactant-modified SiO2 dispersion was obtained by mixing the SiO2 stock dispersion and a surfactant solution. Zeta potential was determined by a Malvern Zetasizer Nano-ZS instrument at 25 °C. In the second lab period, emulsions were prepared by mixing isooctane (5 mL) and surfactant-modified SiO2 dispersion (5 mL) in a beaker and emulsifying with a homogenizer under 3000 r min−1 for 5 min. After emulsification, milky Pickering emulsions were transferred into a container. To judge the emulsion type, conductivity of the emulsions was determined immediately after emulsification using a digital conductivity meter. The surface wettability of the particles and the adsorption characteristic of the surfactant on SiO2 particles were deduced from the type of emulsions. After the lab experiment, every student was tasked with finishing the laboratory report. The teacher graded according to the grading system shown in the Supporting Information. The grading rubric includes four components: preparation for the experiment, performance of the experiment, the laboratory experiment report, and postlab questions.

Figure 1. (I) Zeta potential and (II) appearance of SiO2 modified under different surfactant concentration. Content of SiO2 gel was fixed to be 2.0 wt %. The pH value of SiO2 suspension was 9.8. For the zeta potential(I), three time determination was practiced for each sample and standard deviations were shown as error bars.

effect on the dispersion of particles in water. Highly charged particles are hydrophilic and disperse in water, while hydrophobic particles flocculate into large flocs (Figure 1II).

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HAZARDS AND SAFETY PRECAUTIONS All work should be done wearing appropriate personal protective equipment (chemical splash goggles, appropriate gloves, and lab coat). Handling of isooctane should be carried out in a ventilated hood. Besides common precautions for chemistry lab operation, students should operate the homogenizer according to the operating manual. Specifically, ensure no handling of the experimental material occurs while the head of the homogenizer is rotating.

Emulsions Stabilized by SiO2 Gel Particles under Different Surfactant Concentration

Emulsions were prepared under a fixed oil/water ratio of 1:1 (by volume) and different surfactant concentration. No emulsion was stabilized solely by SiO2 (first vial in Figure 2I). With surfactant-modified SiO2 as a stabilizer, three O/W emulsions, two W/O emulsions, and three O/W emulsions were stabilized as the concentration of surfactant increased. Besides the appearance, the conductivity of the emulsion is a precise parameter to judge the type of emulsion.14,15 O/W emulsion presents high conductivity, while the conductivity of a W/O emulsion is near zero. The conductivity of prepared emulsions is shown in Figure 2II. O/W emulsions were stabilized under low surfactant concentration. When surfactant concentration increased to 7.0 mM, the emulsions inverted from the O/W type to the W/O type (first phase inversion). Afterward, the emulsions inverted into the O/W type again when the concentration of surfactant increased to 20.0 mM.

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RESULTS AND DISCUSSION In the lab experiment period, every student was asked to prepare several series of emulsions. In each series, the SiO2 content was fixed (2 or 0.2 wt %) and the surfactant concentration changed (from 0, 0.1, 1.0, 5.0, 7.0, 10.0, 20.0, 50.0 to 100.0 mM). After the lab experiment, students were required to finish their laboratory reports and were encouraged to work out a phase inversion mechanism. Typical experimental results are described in the following three sections. All proposed results were proved to be highly reproducible.

Emulsions Prepared under Different SiO2 Contents and pH Values

Zeta Potential of SiO2 Gel Particles

When the content or pH of the SiO2 gel changed, double phase inversion was still observed; however, the inversion point changed. The inversion point of emulsions under different SiO2 content and pH values is summarized in Table 1. Compared to emulsions stabilized by 2.0 wt % of SiO2 content (Figure 2I), the inversion point moved toward lower concentrations of

The zeta potential of SiO2 particles was measured to trace the adsorption of surfactant on the particles. Since the silanol group deprotonates to SiO− under pH 9.8, the original HS-30 gel particles are highly negatively charged (−36.1 mV; this can enhance the colloidal stability). The SiO2 particles became B




Instructor Notes in the Supporting Information). As a result, the concentration of surfactant for the first phase inversion point increased. The fixed second inversion point indicates that the second inversion is independent of the surface charge on SiO2 particles.

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PHASE INVERSION MECHANISM In the theoretical demonstration section, students studied general knowledge on colloid and interface chemistry. Then they took part in this experiment and worked out the results of the emulsification. On their laboratory reports, students were required to give a detailed explanation of their experimental results. Students were encouraged to work out a mechanism and explain the phase inversion by the proposed mechanism. In the next lab experiment period, the teacher introduced the exact mechanism to the students and discussed it briefly with them. The double phase inversion originates from the formation of a surfactant monolayer and bilayer under different concentrations of surfactant.16 Original HS-30 gel particles are too hydrophilic to stabilize emulsion. When interacting with a cationic surfactant electrostatically, SiO2 particles become moderately hydrophilic for O/W emulsion. As the concentration of surfactant increases, a dense monolayer of surfactant molecules can be self-assembled on the surface of SiO2 particles, which makes SiO2 particles hydrophobic. As a result, the emulsion inverts into a W/O type. When the surfactant concentration increases progressively, formation of a surfactant bilayer on particle surfaces renders the particles hydrophilic again, such that they prefer to stabilize as a O/W emulsion again. This mechanism gives an exact explanation of the double phase inversion of the Pickering emulsion.

Figure 2. Appearance (I) and conductivity (II) of O/W and W/O emulsions stabilized by SiO2 modified under different surfactant concentrations. The SiO2 content was fixed to be 2.0 wt %. The pH value of the SiO2 suspension was 9.8.

Table 1. Comparative Inversion Points of O/W and W/O Emulsions under Different Conditions Series No.

Content of SiO2 Gel, wt %

pH of SiO2 Gel

First Inversion Point, mM

Second Inversion Point, mM

Figure

1 2 3

2.0 0.2 2.0

9.8 9.8 7.0

7.0 1.0 10.0

20.0 5.0 20.0

2I 3I 3II

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SUMMARY In conclusion, an experiment based on double phase inversion of Pickering emulsions was carried out by second-year undergraduate students. This experiment serves as an ideal candidate for providing students with lab skills training. Also, this experiment has demonstrated that it is beneficial for students to understand basic concepts of colloid and surface science. Furthermore, the reasoning ability of the students develops through the problem-based learning model in this experiment.

surfactant when SiO2 content decreased to 0.2 wt % (Figure 3I). When the pH of the SiO2 gel was adjusted from 9.8 to 7.0, the O/W → W/O inversion point moved toward a higher concentrations of surfactant, and the W/O → O/W inversion point was still fixed at 20.0 mM (Figure 3II). Under lower pH values, SiO2 particles are less charged because of the lower dissociation degree of Si−OH groups on SiO2 particles (see the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00950.

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Student handout, instructor notes, and indications of student achievement (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dezhong Yin: 0000-0003-1122-0247

Figure 3. Double phase inversion of O/W and W/O emulsions stabilized by SiO2 under different surfactant concentrations. SiO2 content and pH values were (I) 0.2 wt %, pH 9.8, and (II) 2 wt %, pH 7.0, respectively.

Notes

The authors declare no competing financial interest. C



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ACKNOWLEDGMENTS The support from the Inquiry Curriculum Project of NPU (General Chemistry Experiment), English Taught Curriculum (Instrumental Analysis), Full English Teaching Program (Engineering Chemistry), and Natural Science Foundation of Shannxi Province (2017JM5080), and from the National Key Research and Development Program of China (No. 2016YFC0301302), is highly appreciated.

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REFERENCES

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Investigating the Phase Inversion of Pickering Emulsions: An

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