## 3 Results and Discussion
### Static Binding Capacity of CNT-K for Small Molecules  
Before the interaction between the mobile phase, target adsorbates and CNT-K were studied inside the chromatographic column (dynamic processing), it is helpful to analyze the adsorption  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **O/C** \\ \hline Untreated powder & 0.057 \\ Conditioned CNT-K & 0.069 \\ CNT-K packed bed column 1 & 0.165 \\ CNT-K packed bed column 2 & 0.138 \\ CNT-K packed bed column 3 & 0.142 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Determination of surface oxygen content using X-ray photoelectron spectroscopy for untreated (as provided by the producer), conditioned (acidic treatment, washed, stored as DI-water slurry and dried) and used CNT-K (applied in different packed beds). The spectra and further information are found in Figures S7 and S8. The material was washed several times with DI-water to avoid adsorbate residues.  
behavior of different model molecules on the particles under static conditions; i.e., to quantify this behavior. These so-called static binding capacity experiments were performed with maleic acid (MA) as the standard adsorbate and fumaric acid (FA) for comparison due to the similarity of its chemical structure (Figure 2a). Both carboxylic acids carry one or two negative charges in DI-water at neutral pH. Incubation of 24 h leads to an adsorption amount higher than 100 mg/g, probably caused by electrostatic and polar interactions between the negative molecules and the positive, hydrated nanotube's outer surface. Moreover, both the capillarity effect on the spongy agglomerates and the possibility of precipitation and crystallization of the organic molecules on the nanotubes further explain the high adsorption capacity [60]. In addition, \(\pi\)-\(\pi\) interactions between the carbon acids and CNT-K can occur. To gain further insight into the interaction of the CNT-K with small molecules, several other adsorbates with different properties were analyzed. The amino acids histidine (His), phenylalanine (Phe) and tryptophan (Trp) were selected as aromatic representatives, which suggest dominating \(\pi\)-\(\pi\) interactions, while alanine (Ala) was selected due to its aliphatic structure. All molecules were dissolved at their pIs, leading to net charges in solution. The isotherms in Figure 2b show that only Trp follows a typical saturation trend with increasing equilibrium concentration and a maximum of up to 75 mg/g. His and Phe adsorb exponentially after an overload, which indicates weak \(\pi\)-\(\pi\) interactions, whereas Ala shows a typical multi-layer behavior. All amino acids reach similar adsorbed amounts to the ones of the carbon acids around the equilibrium concentrations of 5 g/L, although their adsorption behaviors are different. The exception is alanine, which reveals clearly higher adsorbed amounts. Hence, CNT-K exhibit not only a high interaction capacity with the molecule selected, but also have different interaction mechanisms, depending on the target molecule properties. The first attribute, the adsorption capability of the CNT-K, is a critical requirement for the profitability of the PCC process, while the second attribute, the existence of various interaction mechanisms, is also an important feature necessary for triggering molecule adsorption and separation in the through-flow system. Thus, both points indicate that the CNT-K material is suitable for the development of a potential-controlled separation technology.