Simple Experiment to Determine Surfactant Critical Micelle

Laboratory Experiment pubs.acs.org/jchemeduc

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

Simple Experiment to Determine Surfactant Critical Micelle Concentrations Using Contact-Angle Measurements Mahmoud Y. Alkawareek, Boshra M. Akkelah, Sara M. Mansour, Hamza M. Amro, Samer R. Abulateefeh, and Alaaldin M. Alkilany* School of Pharmacy, The University of Jordan, Amman 11942, Jordan

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S Supporting Information *

ABSTRACT: A simple and reliable didactic laboratory has been developed to illustrate fundamental concepts of hydrophilicity, hydrophobicity, wetting, and spreading phenomena for undergraduate students. The described laboratory includes preparation of surfactant solutions at different concentrations and applying droplets of these solutions on a hydrophobic surface followed by imaging the applied droplets using a cost-effective USB digital microscope connected to a computer. Contact-angle values of droplets were measured using ImageJ, a freely available image-processing software. Students constructed surfactant-concentration−contact-angle plots to determine the surfactant’s critical-micelle-concentration value. The feasibility of the described didactic laboratory and the achievement of the intended learning outcomes (ILOs) were evaluated through complementary assessment tools, including lab reports, homework assignments, and students’ feedback. The described laboratory was found to be cost-effective, safe, reproducible, and successful in achieving the ILOs. KEYWORDS: Surface Science, Micelles, Physical Properties, Physical Chemistry, Upper-Division Undergraduate, Curriculum, Laboratory Instruction, Hands-On Learning/Manipulatives

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INTRODUCTION A surfactant is an amphiphilic molecule with a hydrophilic head and a hydrophobic tail,1,2 as demonstrated in Figure 1. At

planes tangent to the surfaces of the solid and liquid”, which is known as the contact angle.1 Upon reaching a certain concentration, known as the critical micelle concentration (CMC), the interface becomes saturated with surfactant molecules, directing additional ones to self-assemble in the bulk solution as submicrometer vesicles called micelles,3 as shown in Figure 2D,E. Accordingly, increasing the surfactant concentration above the CMC has no effect on the interfacial tension, and hence, the contact angle remains constant after this point. For further details on the topics of interfacial phenomena, contact angles, and CMC, readers are referred to references 1, 2, and 4. Surface phenomena such as surface tension, wettability, and spreading are becoming increasingly important for many areas of applied science and technology. During the last ten years, the Journal of Chemical Education published several papers highlighting fundamentals related to surface tension5,6 and describing the measurement of contact angles using in-house constructed contact-angle goniometers.7−9 Moreover, the Journal published various methods for the determination of CMC, including fluorescence-spectroscopy,10,11 electricalconductivity,11 UV-absorption-spectroscopy,11,12 and capillary-rise13 methods. However, there is a need to develop

Figure 1. Cartoon demonstrating the interaction between a surfactant molecule and its surrounding media. The hydrophilic head of the surfactant molecule interacts with the hydrophilic media, whereas the hydrophobic tail interacts with the hydrophobic media.

low concentrations, surfactants, because of their amphiphilic character, will be adsorbed at the interface between two immiscible phases, as shown in Figure 2. Adsorption of surfactant molecules at the interface decreases the interfacial tension and promotes the spreading of the droplet, which becomes more flattened as more surfactant is added to the system (see the progression in Figure 2A−C). Spreading of the droplet is associated with reducing “the angle formed between © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: April 17, 2018 Revised: September 2, 2018

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DOI: 10.1021/acs.jchemed.8b00276 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 2. Cartoon demonstrating the spreading of a liquid droplet on a solid substrate and the related decrease in its contact angle upon addition of a surfactant (A−C) because of the surfactant’s localization at the liquid−solid and liquid−gas interfaces until saturation (C). Further addition of surfactant molecules results in their assembly in the liquid bulk as micelles, and thus no further decrease in contact angle will be observed (D−E).

Droplet Application and Imaging

modern, interactive, and simple educational materials for students to gain proper theoretical and practical knowledge regarding surface phenomena. Herein, we report a simple, safe, and inexpensive method to measure contact angles of aqueous droplets and to determine a surfactant’s CMC using a portable microscope and the freely available image-processing software ImageJ, unlike previously published related work.7−9 Because this experiment is mainly intended as an undergraduate didactic lab, its suitability for such a purpose was validated by allowing its actual execution by a sample of 12 undergraduate students. This was accompanied by a formative assessment confirming students’ achievement of the intended learning outcomes (ILOs) of this didactic lab. In this publication, we also present comprehensive student and instructor materials for this experiment (see the Supporting Information) which makes it readily applicable to undergraduate laboratory courses. The contact angle of a liquid droplet on a surface is a visual measure that helps students appreciate the surface tension between the liquid and the surface. High surface tension between a liquid and solid surface results in a high contact angle and vice versa. The addition of surfactants decreases the surface tension, thereby decreasing the contact angle. In this work, our students imaged sessile drops of surfactant solution with various concentrations on a hydrophobic solid substrate using a cost-effective microscope. Captured images were then processed using ImageJ to calculate contact-angle values at each concentration, and then the critical micelle concentration of the used surfactant was estimated from the contact-angle− surfactant-concentration plot.

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For each of the previously mentioned concentrations of CTAB, a 10 μL droplet was applied on a clean, dry Petri dish using a micropipette. After 30 s of application, an image for each droplet was taken using a USB digital microscope connected to a computer. Full experimental details are included in the instructor lab manual in the Supporting Information. Measuring the Contact Angle Using the Image-Processing Software ImageJ

The contact angle for each droplet was measured using ImageJ software. The Drop Analysis LB-ADSA plugin was used, and the contour for each droplet was adjusted manually for more accurate results (for step-by-step details, see Video S1 in the Supporting Information). Calculating the CMC of CTAB in Water

Using a spreadsheet software (i.e., Microsoft Excel), a scatter plot of CTAB concentration versus measured contact angle was constructed. The resulting scatter plot was divided into two segments; each was fitted using linear regression in Excel. The intersection point between the two fitted lines (which represents the CMC) was obtained using the equations of the obtained lines by manual calculation or by using an online app.14

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HAZARDS The described laboratory does not include the use of highly toxic or flammable materials. However, general lab safety rules were followed, and students wore gloves, goggles, and laboratory coats. Also, it is generally warned that powdered surfactants may pose an inhalation hazard as they can interfere with the function of lung surfactants.

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EXPERIMENT

Preparing the Aqueous Surfactant Solutions

Cetyltrimethylammonium bromide (CTAB) was used as a water-soluble surfactant. Different concentrations of aqueous CTAB solution (0.12, 0.17, 0.26, 0.39, 0.59, 0.88, 1.32, 1.98, 2.96, 4.44, 6.67 mM) were prepared using 1.5-fold serial dilutions of a stock solution (10 mM). The CMC of CTAB according to the supplier (0.92 mM) was within the range of prepared concentrations to show the behavior of the droplets before and after reaching the CMC.

RESULTS AND DISCUSSION

Laboratory Theoretical Introduction

The lab starts with a theoretical introduction (45 min) to explain the main concepts, including (1) surface tension, hydrophilicity, hydrophobicity, wetting, and spreading phenomena; (2) improved surface wettability upon addition of surfactant; (3) critical micelle concentration of a surfactant; B




Figure 3. (A) Top view of a Petri dish with applied droplets (10 μL each) of aqueous CTAB solutions with increasing concentrations. (B) Imaging setup using a cost-effective USB microscope showing the alignment of the objective lens to the applied droplet.

Figure 4. (A) Measuring the contact angle of an imaged droplet using ImageJ software. Arrows indicate software parameters. (B) Determination of CMC using the constructed CTAB-concentration−contact-angle plot. The plot is an example from a student’s report.

following practical part as evidenced from student feedback, which was collected through interviews with students and student responses to homework questions.

(4) contact-angle measurement and how it is related to surface tension and spreading and wetting phenomena. Moreover, during this theoretical session, the experimental procedure and an introduction to ImageJ software were also discussed. Brief instructions on the proper use of micropipettes and the USB digital microscope were also delivered prior to the experimental part. Students found the theoretical part essentially helpful for understanding and performing the

Surfactant-Solution Preparation and Droplet Application and Imaging

After the theoretical introduction, surfactant solutions with varying concentrations were prepared by the instructor to reduce the time of the lab and to minimize the variability C




among student groups. Preparation of surfactant solutions was straightforward and carried out in front of students with reminders on the correct use of balances, graduated cylinders, and micropipettes. Students then used polystyrene Petri dishes as the solid substrate for the aqueous surfactant droplets. The selection of Petri dishes was based on their hydrophobicity, affordable price, and availability as a disposable laboratory supply. We found that washing the surface of Petri dishes prior to use with distilled water followed by complete drying with a nonfibrous tissue is a good practice to maximize reproducibility because the presence of fibers or dust can significantly affect the contact-angle measurements. On a clean Petri dish, each student applied a series of sessile drops (10 μL each) from the aqueous surfactant solutions (stock and corresponding serial dilutions); this was followed by a waiting period of 30 s during which the droplets reached equilibrium, resulting in stable contact-angle values. It is worth noting that the height from which students apply the drops should be reasonable and consistent to minimize variation in results. Application of droplets on the Petri dish was an effective visual demonstration for students to appreciate how a surfactant decreases the surface tension, promotes wetting, and decreases the contact angle. Students clearly observed that the droplets became more flattened and spread as the concentration of CTAB increased in the droplet, as shown in Figure 3A. For the applied drops, images were captured using a USB digital microscope, as shown in Figure 3B. It is important to make sure that the microscope is well aligned with the drops to get a good focus (see Video S1). Instructors may help students by preparing the imaging setup by connecting the USB digital microscope to a computer and aligning the microscope to ensure that its lens is in close proximity to the drop, as shown in Figure 3B. Prior to image-capturing, the focus was adjusted manually following the microscope manufacturer’s instructions. Image capturing by students was smooth and fun for them without any noticeable difficulty or inconsistencies. It is worth mentioning that the used microscope is inexpensive (about $20−30) and was found to be suitable in this laboratory for capturing droplet images with good resolution.

FORMATIVE ASSESSMENT AND ACHIEVED LEARNING OUTCOMES Formative assessment was conducted on the 12 participating students using lab reports, homework exams, written feedback, and direct short interviews. For example, all students submitted their lab reports in which they summarized their analysis and results. Moreover, every student was instructed to submit a homework exam including their answers to three thematic questions that covered the most important concepts taught in this lab. Student feedback sheets, which included eight questions, were completed by each student to assess the achievement of the intended learning outcomes (ILOs) and the validity of this lab. The described formative assessments are available in the Supporting Information. Upon completion of the described laboratory, students are expected to achieve various intended learning outcomes that include (1) the ability to explain hydrophilicity and hydrophobicity, wetting and spreading phenomena, and the interplay between them; (2) the ability to recognize the effect of surfactants on the surface tension, contact angles, and wetting and spreading phenomena; (3) the ability to measure the contact angles of different droplets using the described imaging setup and image analysis; (4) the ability to analyze the experimental data and calculate the CMC of the tested surfactant. Collective analysis of the conducted formative assessment confirmed the achievement of the mentioned ILOs, as detailed in Table 1.

Contact-Angle Measurement and CMC Calculation

Observed Deviation and Difficulties

After imaging all droplets in the series, each student obtained the resulting images and performed image analysis using ImageJ software to measure the contact angle at each surfactant concentration. Figure 4A shows an example of measuring a contact angle using the software, which was found to be user-friendly. Measurements were reliable and relatively consistent; measured values of contact angles among students had an acceptable variability, with a relative standard deviation (RSD) of less than 13%. For step-by-step details regarding the imaging set up, droplet application and imaging, and image analysis, readers are referred to Video S1. To calculate the CMC, students then constructed a plot of measured contact-angle values against surfactant concentrations using Microsoft Excel, as shown in Figure 4B. In this plot, with increasing surfactant concentration, a decrease in the contact angle was observed until a certain point, after which the contact angle became constant. Linear regression was performed to obtain two intersecting lines, as shown in Figure 4B. The CMC is determined as the concentration at the intersection point of the two lines. Equations for each best fit line were obtained using Excel, and students calculated the

Measurements were found to be sensitive to vibrational levels caused by student crowding and movement around the experimental area, which may change the spreading and wetting of the applied droplets. Vibration-induced wetting is well documented in the literature,15 and thus students should be instructed to keep the movement around the experimental area minimal while applying and imaging droplets. Moreover, we observed that some droplets might not show regular spherical shapes upon manual application of droplets from pipettes by the students. Any droplet with the mentioned irregularities must be discarded and reapplied on a clean and completely dry surface to ensure accurate and consistent results. At high surfactant concentrations, the droplet may build up around the pipet tip instead of being suspended in the air (see Figure S2). To prevent this, students are instructed to wipe the tip gently with nonfibrous tissue before applying the droplet, as this simple step improved the measurement reproducibility significantly (RSD values for measurements with and without tip wiping were 0.08 and 4.3%, respectively. For details, see Table S3.)

CMC from the intersection point of the two lines. According to the results produced by students, the average CMC value of CTAB is 0.79 ± 0.13 mM, which is fairly close to that presented by the supplier (0.92 mM) and thus satisfactory for an undergraduate laboratory with such simple and costeffective tools.

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Journal of Chemical Education ILO achieved

ILO achieved

Grading of the question resulted in a score of 71%.

All the students measured the contact angles of the droplets successfully. All the students calculated the CMC correctly. Grading of the question resulted in a score of 94%.

Question 2 in the homework assignment: “How do surfactant molecules enhance the wetting of the droplets?”

In-lab and postlab evaluation of each student’s measurement of contact angles as part of the lab report

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Postlab evaluation of each student’s calculation of the CMC from the intersection point as part of the lab report Question 3 in the homework assignment: “Why did the contact angle remain almost constant after reaching the CMC?”

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00276.

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Instructor lab manual, template of student lab report, template of student homework, and template of student feedback sheet (PDF, DOCX) Video S1 with step-by-step details regarding the imaging set up, droplet application and imaging, and image analysis using ImageJ software (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mahmoud Y. Alkawareek: 0000-0002-4852-0617 Alaaldin M. Alkilany: 0000-0001-9004-7256

Carrying out the experiment with direct support and supervision from instructors

Theoretical-introduction session

ASSOCIATED CONTENT

S Supporting Information *

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge funding support from the Deanship of Academic Research at the University of Jordan.

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REFERENCES

(1) Tadros, T. Encyclopedia of Colloid and Interface Science; SpringerVerlag: Berlin, 2013. (2) Sinko, P. Interfacial Phenomena. In Martin’s Physical Pharmacy and Pharmaceutical Sciences; Sinko, P., Singh, Y., Eds.; Lippincott Williams & Wilkins: Philadelphia, PA, 2011; pp 355−385. (3) Cui, X.; Mao, S.; Liu, M.; Yuan, H.; Du, Y. Mechanism of Surfactant Micelle Formation. Langmuir 2008, 24, 10771−10775. (4) Yuan, Y.; Lee, T. R. Contact Angle and Wetting Properties. In Surface Science Techniques; Bracco, G., Holst, B., Eds.; Springer: Berlin, 2013; pp 3−34. (5) Huck-Iriart, C.; De-Candia, A.; Rodriguez, J.; Rinaldi, C. Determination of Surface Tension of Surfactant Solutions through Capillary Rise Measurements: An Image-Processing Undergraduate Laboratory Experiment. J. Chem. Educ. 2016, 93 (9), 1647−1651. (6) Chiu, Y.-C.; Jenks, M. A.; Richards-Babb, M.; Ratcliff, B. B.; Juvik, J. A.; Ku, K.-M. Demonstrating the Effect of Surfactant on Water Retention of Waxy Leaf Surfaces. J. Chem. Educ. 2017, 94 (2), 230−234. (7) Lamour, G.; Hamraoui, A.; Buvailo, A.; Xing, Y.; Keuleyan, S.; Prakash, V.; Eftekhari-Bafrooei, A.; Borguet, E. Contact Angle Measurements Using a Simplified Experimental Setup. J. Chem. Educ. 2010, 87 (12), 1403−1407.

Students will be able to measure the contact angles of different droplets using the described imaging setup and image analysis. Students will be able to analyze the experimental data and calculate the CMC of the tested surfactant.

Students will be able to recognize the effect of surfactants on the surface tension, contact angle, and wetting and spreading phenomena.

CONCLUSION The described didactic laboratory was found to be safe and cost-effective with low group-to-group variability and an overall duration of less than 3 h. Participating students found it exciting, useful, and enriching to their understanding of important concepts, including hydrophilicity, hydrophobicity, surface tension, wettability, spreading, contact angles, and CMC. The intended learning outcomes were satisfactorily achieved, as evidenced by the results of various assessments. Collectively, the described lab is suitable to be implemented into an undergraduate curriculum.

ILO achieved

ILO achieved Grading of the question resulted in a score of 90%. Question 1 in the homework assignment: “Why does water form droplets with a high contact angle (poor spreading) on the Petri dish (polystyrene) surface?” Students will be able to explain hydrophilicity and hydrophobicity, wetting and spreading phenomena, and the interplay between them.

Theoretical-introduction session Carrying out the experiment with direct support and supervision from instructors Theoretical-introduction session Applying different concentrations of surfactant solutions and observing the effects on wetting and spreading Theoretical-introduction session Demonstration using the software

ILO Status Results Assessment Methods Learning Methods ILO

Table 1. Comparative Analysis of the Achievement of the ILOs

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(8) Dionísio, M.; Sotomayor, J. A Surface Chemistry Experiment Using an Inexpensive Contact Angle Goniometer. J. Chem. Educ. 2000, 77 (1), 59−62. (9) Kabza, K.; Cochran, K. From Polarimeter to Contact Angle Goniometer-Inexpensive Conversion of Laboratory Equipment. J. Chem. Educ. 1997, 74 (3), 322−323. (10) Rujimethabhas, M.; Wilairat, P. Determination of Critical Micelle Concentration Using Acridine Orange Dye Probe. An Undergraduate Experiment. J. Chem. Educ. 1978, 55 (5), 342. (11) Domínguez, A.; Fernández, A.; González, N.; Iglesias, E.; Montenegro, L. Determination of Critical Micelle Concentration of Some Surfactants by Three Techniques. J. Chem. Educ. 1997, 74 (10), 1227−1231. (12) Huang, X.; Yang, J.; Zhang, W.; Zhang, Z.; An, Z. Determination of the Critical Micelle Concentration of Cationic Surfactants: An Undergraduate Experiment. J. Chem. Educ. 1999, 76 (1), 93. (13) Ritacco, H.; Kovensky, J.; Fernández-Cirelli, A.; Castro, M. J. L. A Simplified Method for the Determination of Critical Micelle Concentration. J. Chem. Educ. 2001, 78 (3), 347. (14) Wolfram Alpha. Wolfram Alpha Widgets. http://www. wolframalpha.com/widgets/view.jsp?id= 7b9037eb9f5f7493a73df97a38bc58e6 (accessed Aug 2018). (15) Manor, O.; Friend, J. R.; Yeo, L. Y. Vibration-Induced Wetting. In Encyclopedia of Surface and Colloid Science; Taylor & Francis: New York, 2013.

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Simple Experiment to Determine Surfactant Critical Micelle

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