Desalting: Intact SepCons
Introduction
One area of use to explore for the membranes is desalting, the removal of salts from solutions (such as those containing proteins). Ideal membranes have uniform pore distributions, high mechanical strength, tunable, sharp pore cutoffs, and high porosity. In this study intact samples from W565, which showed no air permeability but allowed rhodamine to pass and had high burst pressures were used in the SepCon format to characterize diffusive flux during desalting. Previously desalting using pnc-Si membranes with pinholes, samples with no membranes (positive control) and solid Si (negative control) were carried out, as well as evaluation of commercially available products with cellulose membranes.
Procedure
Single SepCons were assembled with single slits (100µm by 1000 µm) using intact, pinhole free samples from W565. 250 µL 4M NaCl was loaded into the SepCon and floated in 100mL deionized water under moderate stirring. A conductivity probe was used to measure elution of salt from the SepCon over time. Flux was then calculated during the linear range of salt diffusion.
Results
|
Trial |
Flux (Molarity/(mm^2*min)) |
Dilution |
# of Units |
Volume 4M NaCl/Unit |
|
Dialysis Cassette |
2.00E-06 |
400 |
1 |
500 µL/400 µL |
|
Mini Dialysis Unit |
3.00E-06 |
400 |
4 |
62.5 µL |
|
Broken Membrane |
1.16E-02 |
400 |
1 |
18.75 µL |
|
Pinholes Trial 1 |
6.00E-05 |
400 |
3 |
62.5 µL |
|
Pinholes Trial 2 |
1.00E-04 |
600 |
2 |
62.5 µL |
|
Pinholes Trial 3 |
2.00E-05 |
400 |
1 |
200 µL |
|
Intact Trial 1 |
3.00E-05 |
400 |
1 |
250 µL |
|
Intact Trial 2 |
2.00E-05 |
400 |
1 |
250 µL |
Interestingly, the flux through intact membranes is not that much slower than through pinholed samples. Also, it is a positive sign that the flux through intact membranes is greater than through either dialysis unit.
Future Work
I plan on running pinholed samples in SepCons with 250µL 4M NaCl to see if this changes the flux, as we discussed two weeks ago in the meeting that small volumes in the SepCon causes fast concentration gradient depletion, thereby increasing diffusion time. I also plan on running one more SepCon using an intact sample from W565. If I get the same result, I will then try oxidizing intact samples to see how this affects salt transport.
Pinholes are a much bigger deal in flow experiments or if both sides of the membrane were stirred well during dialysis. In your experiments, the area near the pinhole will deplete quickly and then the transport would slow. Obviously, it will be somewhat faster as more pinholes are added, but I would not expect an order of magnitude. The 2X-5X increase in your data is actually higher than I would expect – were these membranes Swiss cheese or was there just one or two pinholes?
The membranes used in pinhole trials had between 5-15 pinholes; they were Swiss cheese.