## 3 Results and Discussion
### Sensitivity of the Electrochemical Process to the Environment
(**a**) Aging effect in a CNT-K column used with DI-water as the mobile phase (1 mV/s); (**b**) the current differences of DI-water (0.1 mV/s) and 10 mM phosphate buffer (1 mV/s) determined in another column B; (**c**) the cyclic potential’s influence on pH and conductivity using DI-water in column B; (**d**) the cyclic potential’s influence on pH and conductivity using 10 mM phosphate buffer in column B. The potential gradient increases from 0 mV to +800 mV; then decreases to \(-\)800 mV; and increases again to +800 mV. The cycle is run three times and then stopped at 0 mV. The cation exchange membrane likely plays a role in the total system and interacts with the salt ions, too (for more details see Figure S15). However, the surface area of the membrane, and hence its number of binding sites, is less than the surface area of the electrode. If a buffer is used as a surrounding medium, the impact is clearly smaller and the signal output is more balanced because enough ions are available to dampen the concentration gradient of the mobile phase medium and the CNT-K surface (at least in the range of the chosen concentration profile). The regulation of the pH value is essential for biotechnological applications, the long-term target for our PCC process. Regarding this, the buffer system is more suitable for further application. Potential-Controlled Interaction between Electrode and a Charged Molecule: Influence on Retention and Elution  
The final goal of the PCC process is to separate molecules. To meet this objective, we studied the retention and elution of one model adsorbate, maleic acid, in the two media: DI-water and buffer. This molecule has two carboxyl groups and up to two negative charges at neutral pH. To analyze the potential influence on the column capacity, potential-step experiments were carried out. In the so-called adsorption phase, constant positive or negative potentials are applied to the CNT electrode, which induce a retention or a repulsion of the molecule. In the desorption phase, the potential is switched to \(-800\) mV, which leads to a desorption of the previously adsorbed molecules. The injection concentration of maleic acid was chosen to be as high as the column overload, to underscore the potential effect on the adsorption and to induce a first breakthrough in the adsorption phase. Figure 6a represents typical chromatograms for maleic acid in a DI-water system. As expected, the twice negatively charged molecule does not adsorb at higher negative potential, slightly adsorbs if no potential is applied and almost completely adsorbs if a positive potential is applied (no peak appears here). If the elution is enforced by the potential switch to \(-800\) mV, the amount of potential-dependent adsorbed maleic acid is visible in the elution peak, corresponding to the column capacity. The higher the applied positive potential, the more binding sites are on the CNT surface, and thus the higher the column capacity. However, due to the positive ocp of the CNT matrix, maleic acid is adsorbed without a potential being applied, and even \(-300\) mV cannot interrupt the interaction of the CNT-K and the adsorbate. If a \(10\) mM phosphate buffer is selected as the mobile phase (see Figure 6b), the adsorption capacity for maleic acid decreases, because of the competition between the adsorbate and buffer ions for binding sites. Although the electrical conductivity is improved in the electrochemical experiments in the presence of a buffer, the concurring buffer ions occupy and somehow create a barrier between the MWCNT surface and the adsorbate [19; 66]. Consequently, a buffer as a mobile phase, even if positive for the process, also has a limiting effect regarding the electrode capacity. Figure 6: Potential-step experiments to determine the retention of maleic acid in (**a**) DI-water and (**b**) 10 mM phosphate buffer (pH = 7, 50 \(\mu\)L, 1 mL/min, 216 nm).