Week 7 consisted continuing my work in lab, attending a presentation about the GRE test, and spending some time with my fellow REU students.
In lab this week, I got to begin the study of the cellulose-polyacrylic acid (PAA) membrane that I made last week. One parameter of this membrane that I got to look at was permeability, which is an indicator of how fast substances flow through the membrane. One interesting thing about this parameter for membranes containing PAA is that it should change as the pH of the inlet solution and membrane change. This is because the units of PAA's repeating structure become negatively charged (rather than neutral) at higher pH values, and thus, repel each other and cause swelling of PAA that constricts the pores. To test for this behavior in the cellulose-PAA membrane, I soaked the membrane in an acidic solution in an effort to reduce the swelling of PAA and measured the flow of a similar solution through the membrane afterwards; I also did the same thing with basic solutions to measure flow when the PAA was swollen in the pores. During this experiment, I noticed the same inverse relationship between permeability and pH that I did with the PVDF-PAA membrane. However, one difference that I noticed between the cellulose-PAA and PVDF-PAA membranes is that the cellulose-PAA membrane had a lower permeability at both pH values and an especially low permeability at the basic pH. This difference indicates that the pores of the cellulose-PAA membrane were likely smaller than those of the PVDF-PAA membrane and smaller at a basic pH. With this information, one of my lab mentors suggested that I measure how much the cellulose-PAA membrane rejects a molecule of known size at a basic pH in order to see if this membrane is capable of blocking small molecules, which could be useful in certain scenarios. So, I passed a solution of a molecule with a molecular weight of 5 kDa through the membrane and by analyzing samples of the inlet and outlet, I found that the cellulose-PAA membrane rejected about 44% of these molecules. Since this rejection is pretty low given the size of the molecule, this membrane is likely not capable of rejecting small enough molecules. However, my advisor mentioned that I could possibly improve selectivity if I created a membrane with a higher composition of cellulose.
Aside from studying cellulose-PAA membranes this week, I did two experiments on membranes made of PVDF. One of these experiments was the measurement of the surface charge of a bare PVDF membrane (no PAA) using an electrokinetic analyzer. Also, I tried to investigate the contact angle, a measure of hydrophilicity, of PVDF-PAA membranes at different pH values. Theoretically, the hydrophilicity of the membrane should increase with pH due to an increased number of charged units in PAA, which are more likely to interact with polar water molecules than the neutral units found more often at lower pH values. However, I was unable to observe this difference, so I will try to improve my technique before performing other contact angle measurements.
During the week, I also got to attend a panel about the GRE, a graduate school entrance exam, that was put on by some graduate students. As I knew little to nothing about this test, it was helpful to learn about its content, the fact that it is mostly administered electronically, and the time that I should take it if needed. Also, towards the end of the discussion, the graduate students talked about some aspects of the beginning of graduate school, such as different ways students are paired with their advisors and the classes that students have to take.
In addition to participating in REU-related activities, I got to spend some time with my fellow REU students. Over the weekend, a few of us visited the Reed Valley Orchard in Paris, KY to pick fresh blueberries. It was nice to get outside on a beautiful day and come home with a tasty treat. Also, the majority of us got together and enjoyed a dinner towards the end of the weekend.
In lab this week, I got to begin the study of the cellulose-polyacrylic acid (PAA) membrane that I made last week. One parameter of this membrane that I got to look at was permeability, which is an indicator of how fast substances flow through the membrane. One interesting thing about this parameter for membranes containing PAA is that it should change as the pH of the inlet solution and membrane change. This is because the units of PAA's repeating structure become negatively charged (rather than neutral) at higher pH values, and thus, repel each other and cause swelling of PAA that constricts the pores. To test for this behavior in the cellulose-PAA membrane, I soaked the membrane in an acidic solution in an effort to reduce the swelling of PAA and measured the flow of a similar solution through the membrane afterwards; I also did the same thing with basic solutions to measure flow when the PAA was swollen in the pores. During this experiment, I noticed the same inverse relationship between permeability and pH that I did with the PVDF-PAA membrane. However, one difference that I noticed between the cellulose-PAA and PVDF-PAA membranes is that the cellulose-PAA membrane had a lower permeability at both pH values and an especially low permeability at the basic pH. This difference indicates that the pores of the cellulose-PAA membrane were likely smaller than those of the PVDF-PAA membrane and smaller at a basic pH. With this information, one of my lab mentors suggested that I measure how much the cellulose-PAA membrane rejects a molecule of known size at a basic pH in order to see if this membrane is capable of blocking small molecules, which could be useful in certain scenarios. So, I passed a solution of a molecule with a molecular weight of 5 kDa through the membrane and by analyzing samples of the inlet and outlet, I found that the cellulose-PAA membrane rejected about 44% of these molecules. Since this rejection is pretty low given the size of the molecule, this membrane is likely not capable of rejecting small enough molecules. However, my advisor mentioned that I could possibly improve selectivity if I created a membrane with a higher composition of cellulose.
Aside from studying cellulose-PAA membranes this week, I did two experiments on membranes made of PVDF. One of these experiments was the measurement of the surface charge of a bare PVDF membrane (no PAA) using an electrokinetic analyzer. Also, I tried to investigate the contact angle, a measure of hydrophilicity, of PVDF-PAA membranes at different pH values. Theoretically, the hydrophilicity of the membrane should increase with pH due to an increased number of charged units in PAA, which are more likely to interact with polar water molecules than the neutral units found more often at lower pH values. However, I was unable to observe this difference, so I will try to improve my technique before performing other contact angle measurements.
In addition to participating in REU-related activities, I got to spend some time with my fellow REU students. Over the weekend, a few of us visited the Reed Valley Orchard in Paris, KY to pick fresh blueberries. It was nice to get outside on a beautiful day and come home with a tasty treat. Also, the majority of us got together and enjoyed a dinner towards the end of the weekend.
Source: http://www.reedvalleyorchard.com
Next week, I look forward to continuing my study of cellulose-PAA and PVDF-PAA membranes, attending some REU presentations that will be similar to the panel that I attended this week, and getting to see the Lexington Legends play on Thursday.
Comments
Post a Comment