Charge salts

Release of tetracycUne hydrochloride from PCL fibers was evaluated as a means of controlled administration to periodontal pockets (69). Only small amounts of the drug were released rapidly in vitro or in vivo, and poly(ethylene-co-vinyl acetate) gave superior results. Because Fickian diffusion of an ionic hydrochloride salt in a UpophiUc polymer is unlikely, and because PCL and EVA have essentially identical Fickian permeabilities, we attribute this result to leaching of the charged salt by a mechanism similar to release of proteins from EVA (73). Poly-e-caprolactone pellets have been found unsuitable for the release of methylene blue, another ionic species (74,75). In this case, blending PCL with polyvinyl alcohol (75% hydrolyzed) increased the release rate. 

Figure 8.8 Schematic representation of the total aqueous solubility of (a) an organic acid, and (b) an organic base as a function of pH. Note that for simplicity the same pKia values and maximum solubilities of the neutral and charged (salt) species have been assumed.

Electrostatic interactions involving permanent charges (salt bridges). Accord ing to the Bjerrum model the binding constant between two ions A+ and B can be described in terms of the product of the ionic charges zA-zB and the mean effective distance between the ions. These parameters along with the dielectric permittivity (e) determine the magnitude of the Bjerrum function Q(b). The... 

The dilute acid results in the formation of the charged, salt form of the amide. This is because the proton from the acid hydrogen-bonds to the lone-pair of electrons on the nitrogen atom, with the latter then becoming positively charged, and hence balanced by the anion of the acid. Snch polar molecnles are freely water-solnble and are easily and efficiently dissolved. The chloroform is nsed to remove any apolar material before basification, while addition of the sodinm bicarbonate results in the neutralization of the acid and formation of the free base form of the... 

A whole range of cations and anions in different combinations have been explored. The results are surprising. Measurements of coalescence rates for a range of typical electrolytes as a function of electrolyte concentration are shown in Fig. (3.5). There is a correlation between valency of the salt and transition concentration, defined as 50% bubble coalescence, with more highly charged salt effective at lower concentration. The effect is independent of gas flow rate. All the results scale with Debye length (ionic strength). Some salts and acids have no effect at all on bubble coalescence, a situation summarised in Table 3.1. 

In any determination of PZC, the ions that occupy the exchange sites and are subjected to measurement, called the index ions, are chosen based on their tendency to adsorb by essentially nonselective electrostatic bonds. These ions may not be in direct contact with the surface, being readily exchangeable by other ions of like charge. Salts of these ions (for example, NaCl, KNO3) are referred to as indifferent... 

In theory, one can calculate the distribution of electric potential within the double layer for any combination of colloid charge, salt concentration, counter-ion valence and interparticle distance. The Boltzmann equation (8.15) can then be used to calculate cation and anion distributions. From such distributions, cation exchange, colloid swelling, and anion repulsion can be inferred but the calculations are complex, tedious, and often only approximate. 

The stability of protein solutions is governed not only by the macromolec-ular net charge, salt concentration and valency, but also by the chemical nature of the dissolved ions. Traditionally, the latter falls under the category of Hofineister or ion-specific effects which in recent years has seen an appreciable renaissance — both firom experimental and theoretical perspectives. In Hofmeister s original studies, ions were arranged according to their ability to precipitate or salt-out egg white proteins. For the anions, which we focus on here, the following order was found ... 

When two different polymeis are grafted to a surface (mixed bmshes), the surface can adopt the properties of one or the other polymer, depending on external stimuli. Typically, one polymer is hydrophilic or can change the hydrophilicity with temperature (e.g PNIPAAm) and the other polymer is hydrophobic for example, PS. The wettability, charge, salt content, and so on can be changed by external stimuli. ... 

---