Amino groups ionization

Electrostatic forces arise from the presence of charged groups on the particle surface or polymer molecule. For particles these may arise as a result of surface ionization (e.g a silica sol), unequal dissolution of ions of which the crystalline particles are composed (e.g., a Agl sol), or may be the result of ionic species produced by the initiator used in the preparation of the particle dispersion (e.g., polystyrene latex). For polymers the electrostatic charge is due to ionization of (typically) carboxyl, sulfonate, or amino groups. Ionization of these groups may or may not be pH dependent. 

Fig. 123. Difference spectra of 0.0003 M solutions of tryptophan at the temperatures indicated. The pH 1 vs. pH 6 and pH 13 vs. pH 6 data refer to the carboxyl and amino group ionizations, respectively. The reference pH is approximately 6 (Hermans e< of., 1960).

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

Typical values for pAlg are in the range of 9.0 to 9.8. At physiological pH, the a-carboxyl group of a simple amino acid (with no ionizable side chains) is completely dissociated, whereas the a-amino group has not really begun its dissociation. The titration curve for such an amino acid is shown in Figure 4.7. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

The titration curve of penicillamine hydrochloride at 25 °C revealed the presence of three ionizable groups with pKa values of 1.8 (carboxyl group), 7.9 (oc-amino group), and 10.5 (/J-thiol group). Recently, the ionization constants for the acidic functions of (D)-penicillamine were verified by pH titration at 37 °C and 0.15 M ionic strength [2], A 1% solution in water has a pH of 4.5-5.5 [3],... 

Figure 10.6 Dipolar or zwitterionic form of an amino acid. Amino acids exist in a charged form in aqueous solution, the carboxyl group being dissociated and the amino group associated. Some amino acids also have an extra ionizable group present in their side chain (R group). The ionization of each group is pH-dependent and for each amino acid there is a pH at which the charges are equal and opposite and the molecule bears no net charge. This is called the iso-ionic pH (p/).

In addition to the a-amino and a-carboxyl groups those amino acids with an extra ionizable group will also have a pvalue. Glutamic acid is an example of an amino acid with an extra acidic group (COOH) on the y-carbon, and lysine is an example of an amino acid with an extra amino group on the e-carbon atom. As a result they each have three ionizable groups and three pKa... 

The retention factor of an amino acid is given by the following equation, as the amino acid has two ionizable groups (a carboxyl group and an amino group, with Ka and Ka2, respectively) ... 

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