The goal of the selective transport experiments was to demonstrate that ion selective transport through gold-coated NCAMs could be achieved by chemisorption of chloride ions. When diffusion occurs through a nonselective membrane, there is no unbalance of charge across the membrane, and the electrochemical potential across the membrane is zero. When diffusion occurs through a membrane that is permselective, a Donnan equilibrium potential is established assuming the electrochemical potentials across the interfaces are equal at equilibrium. The current experiment explored the permselectivity of gold-coated NCAMs for different ionic strengths. Electroless gold deposition was performed for five hours on a 30 nm PCTE membrane, with nanotubule pore sizes being reduced to approximately 1-2 nm [25]. The membranes were pretreated in potassium chloride solution to allow chloride ions to adsorb on the surface and pore wall of the membrane. Chemisorption of chloride ions on the gold-coated NCAM ensured a negative surface charge on the surfaces of the membrane. The membrane was therefore made permselective for potassium ions. The concentration on the feedside varied from 0.1-100 mM, while the permeate side of the membrane was kept constant at 0.1 mM. The experiments were performed using a concentrated cell, and the potential of the membrane was measured between two Ag/AgCl reference electrodes shown in figure 4.9.
Figure 4.9. Schematic of the concentrated membrane cell used to measure the membrane
potential. Magnetic stirrers were used to reduce the unstirred diffusion boundary layer.
The membrane potential was also calculated by using the Donnan equilibrium equation expressed as:
( ) ( ) (4.21),
where t+ and t- are the transference numbers of potassium chloride, and ah and al are the
activity coefficients of the feed and permeate concentrated cells. For an ideal cation selective membrane, Em would vary with log(ah/al), with an intercept of zero and a slope of
59.1 mV. Figure 4.10 shows the membrane potential as a function of log(ah/al). The plots
show that the permselectivity of the membrane is in good agreement with the theoretical value up to log(ah/al) ≈ 2. Above a ratio of 2 or a 10 mM feed concentration, both ionic
species are able to diffuse across the membrane. As the ionic strength is increased, the selectivity of the membrane decreases due to a decrease in the electrostatic interaction with the ions and pore walls of the NCAM. The deviation from cation selective membranes
Feed Cell Permeate Cell
Membrane
was demonstrated to be slightly lower than previous studies which demonstrated selectivity up to log(ah/al) ≈ 3 for similar deposition times [21]. The higher selectivity which was
shown in previous work can be attributed to a pH=11, which was used for the gold bath solution [25]. A higher pH level increases the kinetic reaction of the electroless deposition process with an increase of hydroxide ions, as demonstrated in Equation 4.20. Therefore, for the same deposition time, the gold nanotubule membranes radius will be smaller for a pH=11 compared to a gold bath solution with a pH=10. The electric double layer varies from 30-nm (lowest concentration) to 0.3 nm (highest concentration) during these experiments. This confirms that the permselectivity of a membrane is a strong function of the Debye length, which is in turn a function of the ionic strength of the solution.
Figure 4.10. Membrane cell potential measured as the concentration on the feedside
varied from 0.1 mM to 100 mM of potassium chloride.
0
50
100
150
200
0
1
2
3
E
m(mV)
log(a
h/a
l)
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