Nonstoichiometric PECs

Fig. 10 Fragment of a nonstoichiometric PEC particle having hydrophilic as well as hydrophobic units. GPE guest PEL, HPE host PEL. Adapted from [82]...

The interaction between modified starch and sticky-containing wastewater was mentioned in Sect. 3.2. of this article [67, 68]. Mihai [141] investigated chitosan-based nonstoichiometric PECs (NPECs) as specialized flocculants. Such NPEC were more effective than chitosan in kaolin separation. Their main advantage is the increase in critical concentration for kaolin restabilization. The NPEC particles were adsorbed on the kaolin surface, protecting them more efficiently against re-dispersion. 

As already mentioned, with regard to the stoichiometry three things have to be determined the end point stoichiometry of the PECs, the degree of conversion (degree of release of counterions), and the overall composition f(X) of the complexes at nonstoichiometric mixing ratios. The methods used for the investigation of the PEC stoichiometry are not very sensitive to the level of aggregation i.e., they are practically the same for soluble PECs and colloidal PEC structures. 

Several methods revealed that PECs between strong polyelectrolytes have a 1 1 end point stoichiometry and also a 1 1 stoichiometry of ionic binding under full release of the low molecular counterions at nonstoichiometric mixing ratios. However, it remains an open question whether the major component in such systems is bound in excess in the PEC structures, giving them a net excess charge. To solve this problem, viscometry, analytical or preparative ultracentrifugation, and fractionation techniques in combination with analyzing methods can be employed. 

Summarizing our results on highly aggregated PECs, we can state that at nonstoichiometric mixing ratios electrostatically stabilized particles in the nm scale with controlled levels of aggregation and structural density can be prepared. These particles should be of interest as potential carrier systems for drugs and enzymes, because they offer an easy way to incorporate charged material by Coulomb interactions. 

There are materials, which can form varieties of nonstoichiometric compounds. For example, Pb02 can coexist as PbO where x can have values from 0 to 2. Among these oxides, most photoactive oxide is PbOo.8- If PbOo.8 oxide is to be prepared by anodic oxidation of lead, it is possible that surface of the lead film might contain other forms of oxides as well. Thus, a PEC cell prepared by anodization of lead may give a low photoresponse, not because PbOo.8 is an inappropriate material but because anodized surface may be contaminated with other forms of oxides as well. There seems to have no nondestructive technique available to confirm the uniformity of surface with photoactive species, especially of a large area electrode. Sharon and coworkers [111] has recently developed a laser scanning system to overcome this problem. A laser... 

Our work is mainly focused on colloidal PEC particles, which are prepared by mixing polycation and polyanion solutions in nonstoichiometric ratios [10-15], and on exploring their potential to interact in a useful manner with pharmaceutically and biomedically relevant compounds. The main issues of our research are reproducibility in the preparation protocol, uniformity of size and shape, conservation of colloidal stability after binding of compounds and the interaction with surfaces. In typical PEC systems, standard cationic and anionic PELs and PELs of natural origin (e.g., polypeptides, polysaccharides, and their modified analogues) are combined. 

Different modes of realizing nonstoichiometric mixing ratios are possible. Some authors always use equally concentrated solutions with respect to charge or monomer concentration and control the mixing ratio by the volumes of the PC and PA solutions (e.g., PEC-0.66 indicates 0.66 mL PA in 1 mL PC) [21, 42]. Others use differently concentrated PC and PA solutions and use equal volumes or even different volumes. 

Kabanov et al. [59] reported nonstoichiometric complexes between cationic poly (V-ethyl-4-vinylpyridinium) and anionic block copolymer poly(ethyleneoxide-co-methacrylate). This system revealed highly soluble stoichiometric PEC particles. 

PEC particles with a spherical shape are the most prominent and the possibility to influence their overall particle size has been extensively described in Sect. 3. Additional internal shaping aspects within spherical PECs can be seen in the ratio between the more flexible hydrophilic nonstoichiometric shell and the more compact hydrophobic stoichiometric core part of PEC particles, according to a widely claimed PEC model [85]. Such internal shaping presumably could also be influenced by stractural and media parameters. However, analytical access to the internal structure of water-rich and therefore emulsion-like rather than suspensionlike PEC particles is limited with the present analytical techniques. More information can be obtained on the overall external shape of PEC particles. 

However, when polyanions and polycations are mixed in nonstoichiometric ratios, soluble complexes can be formed. These polyelectrolyte complexes are found in the form of discrete soluble gel particles in the aqueous medium. For nonstoichiometric polyelectrolyte complexes the major component can also be described as host polyelectrolyte (HPE), whereas the oppositely charged minor component can be described as guest polyelectrolyte (GPE). The latter joins the repeat units of the HPE via electrostatic interactions, so that a network is formed. Eor the formation of soluble particles, it is beneficial if the HPE consists of high molecular weight material, whereas the GPE has a low molecular weight. Eurthermore, it is beneficial for a stable PEC if at least one of the polyelectrolytes has weak ionic groups. 

---