Determination of the Isoelectric Point/Zeta Potential of Polymethylmethacrylate Surfaces

The charge profile of polymethylmethacrylate (PMMA) surfaces can be modified by a number of processes. Some of these processes are polishing with highly-engineered slurries, coating through deposition or chemically treating surfaces. One essential parameter that can offer information about changes in surface charge profile is the isoelectric point.

The isoelectric point is defined as the pH at which the surface has a neutral charge. This point can also be defined as the pH at which the zeta potential of the surface changes from positive to negative or from negative to positive.

Presently, there are very few tools that can study the isoelectric point of surfaces. The patented flat surface potential cell from DelsaNano allows to carry out these measurements efficiently and consistently.

This article discusses the methods involved in studying the isoelectric point of polymethylmethacrylate surfaces and shows representative data. The general approach and specific procedures presented can be applied to any surface of interest.

Material Requirements

• DelsaNano C, HC or Z (Beckman Coulter P/Ns A53878, A53879 or A53877).
• DelsaNano AT (Beckman Coulter P/N A53880).
• DelsaNano User’s Manual.
• Monitor Particles (Beckman Coulter P/N A54496).
• Flat Surface Cell Assembly (Beckman Coulter P/N A54117).
• Hydrochloric Acid (e.g., Mallinckrodt H613-05).
• Ammonium Hydroxide (e.g. Mallinckrodt 3256-05).
• DI water.
• Laboratory grade NaCl (e.g. Mallinckrodt 7581-12).
• Soap (e.g. Micro 90) (Beckman Coulter P/N 8304096).
• PMMA Standard (Beckman Coulter P/N A99370).
• Pipettes, beakers, volumetric cylinders.
• 30-60 mL syringe.
• 0.2 μm syringe filters.
• Vacuum trap with tubing to quickly dry parts (optional).

Reagents Preparation

Each burette is prepared and filled as explained below. After filling, the burettes are capped temporarily while the autotitrator is being set up for use.

Sample Tube: 10 mM NaCl = Dissolved 60 mg of NaCl in 100 mL of DI water filtered.

Burette 1 (use glass): 0.1M ACID = 25 mL DI water mixed with 0.2145 mL Hydrochloric Acid.

Burette 2 (use glass): 0.1M BASE = 25 mL DI water mixed with 0.163 mL Ammonium Hydroxide.

DelsaNano Autotitrator (DNAT) Preparation

The method of preparation is listed below:

1. Calibrate the pH meter following the procedure detailed in Appendix A (“Autotitrator”) of the DelsaNano User’s Manual under “Calibrating the pH electrode”.
2. Next, rinse the electrode with DI water.
3. Place the acid and base additives in glass burettes with their caps and connect them to the related tubing of the DNAT (Fig. 1, Table 1).
4. Prime the fluid-handling system following the procedure given in Appendix A (“Autotitrator”) of the DelsaNano User’s Manual (under “Priming”). The procedure is performed repeatedly for both the acid and base.

Figure 1. The DNAT. Image credit: Beckman Coulter

Table 1. DNAT components description.

Sample Cell Preparation

Refer to the DelsaNano User’s Manual Section 4.31-Flat Surface Cell for Zeta Potential (Fig. 2, Table 2). The method of sample cell preparation is listed below:

1. It is recommended to wear latex gloves during this procedure in order to avoid fingerprints on the quartz cells. The electrodes, quartz cell and cell tubing must be thoroughly cleaned with Micro 90 or another lab soap.
2. All the cell components must be thoroughly rinsed with DI water for approximately one minute.
3. All parts need to be thoroughly dried using a vacuum.
4. The quartz cell and the other parts of the sample cell must be assembled using the torque wrench to tighten the nuts. The top must be left off the sample cell. It is better not to over tighten as this may cause quartz cell cracking.
5. The PMMA flat surface standard is then opened and rinsed with DI water as per the instructions included.
6. The flat surface standard is placed in the sample cell.
7. A Teflon shim is placed on top of the flat surface standard.
8. The top is placed on the sample cell and the nuts are tightened with the torque wrench. It is important not to over tighten as this may cause cracking of the quartz cell.
9. The thumb and valve that secures the sample in place is tightened.

Figure 2. The Flat Surface Zeta Potential Cell. Image credit: Beckman Coulter

Table 2. Flat Surface Zeta Potential Cell components description.

Priming the Sample Cell

The method of priming the sample cell is listed below (Fig. 3):

1. The stir bar is placed in the sample burette and load into place in the autotitrator. Turn the stirrer on to full speed.
2. Around 30 mL of 10 mM NaCl using a 0.2-mm filter is filtered and poured into a clean burette.

   

   Figure 3. Priming the sample cell. To prime, send outlet tubing to waste. Image credit: Beckman Coulter

3. By using sample circulation dialog, approximately 5 mL of the NaCl solution is passed through the tubing and sent to a waste container. It is recommended to use approximately 40% pump duty to do so. In order to find the sample circulation dialog, choose “pH Maintenance” from the left-hand menu, then “Sample Circulation”. The dialog is shown in figure 4.

   

   Figure 4. The Sample Circulation Dialogue is found under the pH maintenance menu. Image credit: Beckman Coulter

4. The tubing is connected to the sample cell and approximately 5 mL more of the NaCl solution is circulated through the cell.
5. The outlet tubing is inserted back into the burette to set up the recirculation flow (Fig. 5).

   

   Figure 5. After priming, reinsert the outlet tubing to the sample burette. Image credit: Beckman Coulter

6. The tubing and sample cell must then be degassed using the procedure found in the DelsaNano User’s Manual (Appendix A “Autotitrator”-“Priming” -”Degassing the Tube and Degassing the Cell”).
7. After degassing, if no bubbles are observed then the sample cell must be again placed into the cell temperature control block in the DelsaNano (Fig. 6).
8. 200 μL of monitor beads (Beckman Coulter P/N A54496) are added to the sample burette.
9. The cell is primed with the monitor particles by setting the pump duty to 45% and allowing recirculation for 30 seconds.

   

   Figure 6. After priming and degassing, insert the flat surface cell into the temperature control block. Image credit: Beckman Coulter

Zeta Sop Designer Settings

Solutions for Polymer Analysis & Development eBook

Maximise your knowledge about polymer analysis and research.

Download a free copy

The Zeta Sop Designer Settings include the following (Fig. 7):

1. The parameters are set to the following:
   Measurement Type: Type 3.
   pH Values, increasing alkalinity: 3, 4, 5, 6, 7 and 8 or increasing acidity: 8, 7, 6, 5, 4 and 3.
   pH Tolerance: 0.1 tolerances.
   Accumulation Times: 10 accumulation times.
   Applied voltage: 60 V.
   Polarity: Auto.
   Cell positions: Default (0.8/0.6/0.3/0/-0.3/-0.6/-0.8).
2. “Measure Cell Center” is selected at the top of the Cell Parameters window. The wizard steps need to be followed placing new values into the Cell Position X and Cell Position Z values.
3. The Measurement Parameters are set to the following:
   Pinhole: 50 μm.
   Repetitions: 1.

   

   Figure 7. Your Zeta SOP settings should look similar to these. Image credit: Beckman Coulter

4. The Diluent Parameters are set to:
   Diluent: Water.
5. The Analysis Parameters are completed as desired, then all four parameter files are loaded into the SOP.
6. The new SOP file is right clicked and “Add to Measuring SOP” is chosen.
7. Go to the “Data Acquisition” screen and select “Start”.

Completing the Analysis

1. After completing the analysis, the sample is removed and the pH meter probe is washed with DI water.
2. If desired, the sections B-D can be repeated for additional samples.

Viewing Results

1. Select “Zeta Analysis” on the left-hand menu.
2. Select “pH Analysis” on the left-hand menu.
3. Select “Open” in the window that appears.

Conclusion

Results with the PMMA standard indicates that the surface isoelectric point can be determined with approximately 10% CV, if optimal practices are followed. Most surfaces are highly sensitive to pH changes. Hence running the sample from low to high pH may yield different results than operating in the opposite direction. It is more convenient to go from high to low pH as below the IP, the monitor beads may adhere to the sample as there is no charge repulsion. Though every individual data point exhibits some error, the point at which the zeta potential changes sign must be relatively consistent.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.