Carbon isoelectric point

Amino acids with two carboxylic groups or those with two amino groups behave slightly differently in that they are not entirely neutral, but may be acidic or basic. All amino acids therefore have a different isoelectric point (see Table 2.1.1). These differences in polarity form the basis of the separation of amino acids the neutral amino acids are in the middle part of the chromatogram and the dibasic amino acids elute late. In addition, the length of the aliphatic chain of the molecule makes the amino acid less polar, causing later elution (e.g., ornithine, which has five carbon atoms, elutes before its homolog lysine, which has six carbon atoms). 

In nitrogen heteroaromatics, upfield protonation shifts are found for carbons a to nitrogen, while those in / and y positions are deshielded on protonation [94, 99,100]. This is shown in Fig. 3.5 for quinoline [94]. The protonation shifts for C-/1 and C-y can be rationalized in terms of the cannonical formulae of protonated pyridine [73 d], while the upfield shifts for C-a are probably due to the lower n character of the N — C-a bond. The curves in Fig. 3.5 representing the pH dependence of 13C shifts resemble titration curves. pK values and, in the case of amino acids, the isoelectric points pi can be obtained from the point of inflection of the (5 versus pH plot for each individual carbon [84, 94, 98]. 

A persistent question regarding carbon capacitance is related to the relative contributions of Faradaic ( pseudocapacitance ) and non-Faradaic (i.e., double-layer) processes [85,87,95,187], A practical issue that may help resolve the uncertainties regarding DL- and pseudo-capacitance is the relationship between the PZC (or the point of zero potential) [150] and the point of zero charge (or isoelectric point) of carbons [4], The former corresponds to the electrode potential at which the surface charge density is zero. The latter is the pH value for which the zeta potential (or electrophoretic mobility) and the net surface charge is zero. At a more fundamental level (see Figure 5.6), the discussion here focuses on the coupling of an externally imposed double layer (an electrically polarized interface) and a double layer formed spontaneously by preferential adsorp-tion/desorption of ions (an electrically relaxed interface). This issue has been discussed extensively (and authoritatively ) by Lyklema and coworkers [188-191] for amphifunctionally electrified... 

TABLE 1 Differences Between Point of Zero Charge (pH,.z() and Isoelectric Point (pHup) Values for a Series of Chemically Treated Activated Carbons... 

Experimental carbons, both of which have an isoelectric point of ca. 2.0. 

In the virtually ignored study by Dai (with three nonself citations in five years) [521], the paper by Graham [451] is not cited either, but the key issue is both identified and clarified. Based on the results summarized in Fig. 21, the author concluded that electrostatic interaction between cationic dyes and the surface of activated carbon has a great effect on adsorption capacity. Below the isoelectric point of the activated carbon (when the positive zeta potential was above 60 mV), the capacity is significantly reduced due to electrostatic repulsion between cationic dyes and the carbon surface. In a follow-up study, while still failing to acknowledge earlier important contributions to the resolution of the key issues, Dai [522] reinforced and confirmed the electrostatic attraction vs. repulsion arguments. The author used anionic dyes (phenol red, carmine, and titan yellow) and... 

Wang et al. [607] studied the adsorption of dissolved organics from industrial effluents onto a commercial activated carbon. As illustrated in Table 20, they place emphasis on the pK, pK, or isoelectric point of the adsorbate and state that the pH effect upon the effectiveness of carbon adsorption mainly depends upon the nature of the adsorbed substance. Based on their own work and analysis of the literature, they postulate that maximum adsorption of organic acids and bases occurs around their respective pK , or pKh value, even though they acknowledge, at least as the ionic organic compounds become more complex, that electrostatic adsorption forces between the adsorbent and the ionic adsorbate appear to govern. ... 

Let us call this the donor-acceptor complex proposal, similar to that presented recently for adsorption of substituted nitrobenzenes and nitrophenols on mineral surfaces [739]. The experiments on which this proposal is based are (1) isotherms of phenol, nitrobenzene, and m- and / -nitrophenol on one commercial activated carbon at pH = 2-7 and very low solute concentrations ( <1.5% of the solubility limit of these species [6]) and (2) detailed infrared (internal reflection) spectroscopic analysis of the surface after adsorption of / -nitrophenol. Interestingly, neither in this study, nor in any subsequent study that supports this mechanism, has a similar analysis been performed with carbons containing varying concentrations of carbonyl surface groups. Also of interest is that the authors dismiss the electrostatic explanation of the reported pH effects by assuming that the isoelectric point of the carbon (which was dried at 200°C for 12-24 h) was ca. 2.4. 

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