Arlo McGinn and Dr. John D. Lamb, Chemistry and Biochemistry
Developing efficient means of making chemical separations of solutions is a problem that has long concerned chemists. This ability to separate a solution into its individual components or selectively remove elements of the solution is helpful in the treatment and refinement of nuclear waste and other applications. Common processes in use today are time-consuming and cumbersome to perform. Because of these problems chemists have been interested in developing new methods and procedures for making chemical separations of solutions.
Polymer inclusion membranes (PIMs) have been developed to assist in making chemical separations. PIMs are thin membranes made up of three major constituents: a polymer to add strength and durability to the membrane, a carrier molecule that selectively binds and transports a specific ion across the membrane, and a liquid plasticizer, acting as a solvent, that aids the diffusion of the carrier molecule across the membrane. This membrane is then placed between two chambers. One chamber contains an aqueous solution (the source phase) of several ions in varying concentrations and the other side contains a water receiving phase of variable pH. The carrier molecule incorporated into the membrane then transports the ion for which it is selective to the receiving phase where the ions can be collected in high purity.
The problem with PIMs at present is that they are commonly made with the polymer cellulose triacetate (CTA) which lacks stability to basic aqueous solutions. Other polymers have been studied to solve this problem and have been found to have excellent resistance to base but very low permeability, making them unacceptable for use in practical separations.
I studied the effectiveness of cellulose acetate/butyrate (CAB), a mixed polymer of cellulose triacetate (CTA) and cellulose tributyrate (CTB). CTA, as mentioned earlier, has been found to have high permeability but suffers only from its lack of resistance to base hydrolysis. CTB has been studied in our lab and has been found to be highly resistant to basic conditions but has extremely poor permeability. In deciding to study CAB as a candidate for a support polymer in PIMs we hoped that the combined polymer would be both resistant to base and highly permeable.
The present study was carried out in two parts. First, the CAB membranes were tested to determine their resistance to basic solutions. The membranes were made without the presence of a carrier molecule and were placed between an aqueous source and receiving phase. The source phase consisted of a solution of potassium hydroxide and the receiving phase consisted of an aqueous solution with a base indicator. Samples were taken from the receiving phase periodically until it was determined from the indicator that the membranes had decomposed. The samples were then analyzed for the presence of potassium using a Perkin Elmer Inductively Coupled Plasma Spectrometer (ICP). Determining the rate at which potassium leaked across the membrane allowed us to establish the rate of membrane decomposition.
Second, the CAB membranes were tested to determine their permeability. The membranes were made in the same way as for the base studies but a carrier molecule was added to the membrane to allow for transport of an ion across the membrane. The carrier molecule used was di-tertbutyl- dicyclohexano-18-crown-6 which selectively binds potassium. The source phase consisted of a solution of potassium and lithium nitrate. The lithium nitrate was present to determine that the potassium was being selectively removed from the source phase while the lithium was not being transported and to increase the ionic gradient to help drive the diffusion of potassium across the membrane. Samples were again taken from the receiving phase over a period of time usually lasting from one to two days, after which the samples were analyzed by ICP. From the data obtained membrane permeability and ion flux were calculated.
Through our experiments we were able to determine that CAB is a more viable candidate for use in PIMs than CTA. We determined that CAB is more resistant to base than CTA and it has comparable permeability. During our base resistance tests the CAB membranes consistently outlasted the CTA membranes, usually remaining intact for several days longer at concentrations of 0.1 M and at least a week longer at concentrations of 0.01 M.
Through our permeability comparison tests we were able to observe that the CAB membranes exhibited permeability that was not only competitive with the permeability usually observed with CTA membranes but also, in some cases, slightly higher.
The results that we obtained from these experiment have led to our increased interest in CAB as a support polymer in PIMs. The only disadvantage of CAB membranes was the difficulty in removing them from the molds in which they were made. The membranes made with CAB seemed to be stickier than those made with CTA and would adhere to the glass mold causing the membrane to tear while being removed. To overcome this problem I designed a new mold system that used Teflon coated plates instead of the regular glass plates used. The benefits of using the Teflon were soon realized as the membranes were easily removed and we were also able to prepare membranes with lower concentrations of polymer than we were previously able.
The benefit of using PIMs in making chemical separations has been realized in our laboratory and by others in the field of chemical separations. PIMs are effective for making both large and small scale separations and are also useful because of their simplicity in design and function. The data we have obtained regarding CAB membranes will help to make PIMs more diverse and useful under many varying circumstances. The further exploration into different polymers and other constituents of PIMs will help chemists to make more efficient and simple separations of chemical solutions.