Complex coacervate water content

A wide variety of capsules loaded with water-immiscible or water-iasoluble materials have been prepared by complex coacervation. Capsule size typically ranges from 20—1000 p.m, but capsules outside this range can be prepared. Core contents usually are 80—95 wt %. Complex coacervation processes are adversely affected by active agents that have finite water solubiUty, are surface-active, or are unstable at pH values of 4.0—5.0. The shell of dry complex coacervate capsules is sensitive to variations ia atmospheric moisture content and becomes plasticized at elevated humidities. 

With regard to the water content one can in general say that it is large with unfavourable conditions for complex coacervation and decreases in proportion as these become more favourable. 

Thus the above stated fact is exhibited clearly in this figure the water content is a minimum at the most favourable condition for complex coacervation (here a definite pH at the given mixing proportion). On decrease or increase of the pH the water content increases. 

The existence of an optimum pH for complex coacervation (p. 360, Fig. 21) appears to speak in favour of a fairly considerable interpenetration of the clews of both kinds. The equivalent coacervate at this pH is poorer in water than equivalent coacervates at other pH values. With strong penetration the pairs of positive and negative charges are to be found more or less uniformly throughout the enclosed volume of a macromolecule and the mutual attraction of these dipoles (which leads to a state of contraction) depends then also on their number. This number would then be the greatest at the optimum pH so that here the water content becomes a minimum. 

The flocculi therefore consisted of a very viscous coacervate. As was to be expected, on the addition of an indifferent salt these flocculi are first transformed into thoroughly liquid coacervate drops and at still higher salt concentrations the coacervation is suppressed. The added salt increases the water content by weakening the complex relations (p. 364), whereby the coacervate becomes less viscous. As a result rapid complete fusion occurs of the innumerable very small coacervate drops, which in the flocculi were only superficially fused together here and there. 

Since however larger charge density (small equivalent weight), as is discussed below ( 2r) results in general in a smaller water content of the complex coacervate, the ultrami-croscopic coacervate drops in the combination with clupein are very viscous or possibly glass-like in nature, which greatly impedes the mutual fusion into larger drops. 

Further one sees that the maximum of the coacervate volume curve lies much lower for G + N than for G + A. This happens because the G + N coacervate is appreciably richer in colloid than the G + A coacervate (the G + N coacervate is also much more viscous). This relatively smaller water content of the G + N coacervate is also to be expected from the much stronger complex relations in this combination ( 2o, p. 370). 

Complex coacervates sometimes possess a relatively high water content and the diagrams of Fig. 48 (p. 410) invite one to make a comparison with the water content of the demixed salt-rich liquid. Naturally this latter water content depends on the temperature chosen but for a comparison with the water content of the complex coacervate gelatin (positive) 4- gum arabic (negative) it is indicated to choose a temperature sufficiently far below the critical solution point, for example 20°, so that the composition does not change much per degree fall in temperature. This because at temperatures at which the water content of the gelatin — gum arabic coacervate is known, one is far removed from a possible critical solution point and the water content at those temperatures is almost constant (p. 341). 

The water-poorest complex coacervate (pH == 3.5, mixing proportion 50%) only contains about 18% colloids, thus more than 80% water. From the dis rams of Fig. 48 we read off for the procaine salt-rich layer at 20° on the other hand a salt content of about 75—85% and this layer therefore contains only 15— 25% water. 

We now turn to the complex coacervate gelatin — gum arabic. Vacuolation occurs in this coacervate on cooling also but since the water content is independent of the temperature at higher temperatures (33—50°), this vacuolation (which occurs at about 28.5°) is a phenomenon accompanying the gelation itself (and not as in... 

Therefore, PEC act as a model material with the same local molecular structure of the complex, but have the advantage of a variable stoichiometry and known ion content. In PEC, the content of small cations and anions is known because it depends on the mixing ratio of the poly ions. Furthermore, systems with mainly one type of counterion can be prepared if excess salt is removed by dialysis. In this way, conductivity data in dependence of the composition can be related to the conductivity contribution of a single type of charge carrier [40, 41]. For this purpose, solid PEC complexes have to be prepared from complexes formed in aqueous solution. The broad composition range includes both water-soluble as well as insoluble complexes, i.e. complex coacervates. Both can be treated by drying and subsequently pressing the polymer material to form a dense solid [40]. 

---