Coacervates viscosity

Xylans from beech wood, corncobs, and the alkaline steeping liquor of the viscose process have been shown to be applicable as pharmaceutical auxiliaries [3]. Micro- and nanoparticles were prepared by a coacervation method from xylan isolated from corncobs [150]. The process is based on neutralization of an alkaline solution in the presence of surfactant, which was shown to influence both the particle size and morphology. They are aimed at applications in drug delivery systems. 

Cellulose acetate phthalate forms a pH-triggered phase transition system, which shows a very low viscosity up to pH 5. This system will coacervate in contact with the tear fluid (pH 7.4), forming a gel in few seconds and releasing the active ingredients over a prolonged period of time. The half-life of residence on the rabbit corneal surface was approximately 400 seconds compared to 40 seconds for saline. However, such systems are characterized by a high polymer concentration, and the low pH of the instilled solution may cause discomfort to the patient. 

The polymers were fractioned into 8-14 components by coacervate extraction from the benzene - methanol system. For fractions and nonfractioned polymers, characteristic viscosities [t ], were me-asured. Because that was the first example of studying conformations of macromolecules of this ty-pe in diluted solutions, authors of the work [56] paid much attention to selection of an equation, which would adequately describe hydrodynamic behavior of polymeric chains. Figure 10 shows de-pendencies of [q] on molecular mass (MM), represented in double logarithmic coordinates. Parame-ters of the Mark-Kuhn-Hauvink equation for toluene medium at 25°C were determined from the slo-pe and disposition of the straight lines. 

Viscosity. Since coacervates are heterogeneous liquid systems with a nonhomogeneous distribution of substances, the viscosity of a coacervate layer differs markedly from that of an equilibrium liquid. The viscosity of the equilibrium liquid is lower than the viscosity of the solutions from which the coacervate was obtained, whereas the viscosity of the coacervate layer or drops is higher than that of the initial solutions. The decrease of the viscosity of the equilibrium liquid results from the decrease in the total volume of the particles, since they concentrate into larger coacervate drops. Coacervates are formed most completely when the viscosity of the equilibrium liquid is lowest. 

The viscosity is influenced by the pH of the medium and the concentration and relative amounts of the initial solutions from which the coacervate is formed. The relationship between these factors is illustrated in Figure 3. The formation of coacervates is shown as a function of viscosity and concentrations of gelatin and gum arabic solutions at various pH values. 

Some papers60-61 have been devoted to phase separation of polyionic complexes from partially furated (PVA-S) and aminoacetylated (PVA-AAC)poly(vinyl alcohol) in aqueous salt solutions. The separation liquid-liquid or complex coacervation occurs at a definite value of the charge density on the macromolecule. From the concentration dependence of the reduced viscosity of the initial components PVA-S, PVA-AAc and their equivalent mixture in water it follows that the viscosity of the components noticeably increases with dilution, and the curve of the equivalent mixture is concentration independent. This fact confirms the formation of the neutral polymer salt, due to electrostatic interactions of PVA-S (strong polyadd) and PVA-AAc (weak polybase). 

Naturally, the aforementioned polymers may also be coacervated by desolvation upon adding a nonsolvent, such as hexane, heptane, liquid paraffin, or a vegetable oil. The advantage of using the coacervation by Polymer 2-Polymer 3 repulsion is that the viscosity and volume fraction of the coacervate phase and the stability of coacervated droplets can be controlled by... 

The particular feature of coacervation by Polymer 2-Polymer 3 repulsion is that phase separation occurs already after the addition of a minute volume fraction of Polymer 3, which is in contrast to the coacervation by polymer desolvation. In the very first step, a dispersion of Polymer 3-in-Polymer 2 phase is formed (Fig. 3). Further Polymer 3 addition produces a phase inversion, whereupon the Polymer 2 phase (coacervate droplets) is dispersed in the Polymer 3 phase. Upon further Polymer 3 addition, the solvent is partially extracted from the coacervate droplets thereby increasing their viscosity and physical stability against coalescence. Optimal coacervate stability is generally achieved within a certain range of Polymer 3 volume fraction. This stability window has been determined by various authors for different PLA and PLGA types.f ... 

The main advantage of coacervation by Polymer 2-Polymer 3 repulsion over polymer desolvation resides in the good control of the composition and viscosity of both the coacervate and dispersing phases. This, in turn, provides a means to control particle size and prevent undesired coalescence of the coacervate droplets. 

The polymer employed to prepare microspheres must be characterized in terms of molecular weight and purity,however this topic is beyond the scope of this article. Characterization of the materials may have implications for the formation of the microspheres. The viscosity and film-forming properties of the polymers used should be known. Viscosity can affect the tendency to form microspheres, their size, and even their shape. Burgess and coworkers have shown that albumin-acacia coacervates do not form microcapsules under certain conditions of pH and ionic strength, if the viscosity of the coacervate phase is too high. Burgess and Carless developed a method to predict the optimum conditions for complex coacer-vation based on the charge carried by the two polymers involved. 

Many salts reduce the viscosity of aqueous acacia solutions, while trivalent salts may initiate coagulation. Aqueous solutions carry a negative charge and will form coacervates with gelatin and other substances. In the preparation of emulsions, solutions of acacia are incompatible with soaps. 

Fig, 21. Colloid mixing ratio expressed as the NaCMC percentage (left ordinate) and specific viscosity T) p (right ordinate) vs pH of the NaCMC-gelatin coacervation mixtures [120]... 

Some factors which may retard the wetting and preclude attainment of equilibrium are a combination of low capillary pressures and high fluid viscosities, metastable equilibria, selective adsorption, and coacervation. 

An interesting feature in the course of the viscosity curves for such cases in which flocculation (or coacervation) accompanies the depression of the electroviscous effect, is the crowding together of these curves. As examples in Fig. 34 we give the influence of some indifferent salts on the relative viscosity of the sodium thymus nucleate sol and in Fig. 35 the same for the carrageen sol ... 

If the coacervate shows relatively little viscosity then an actual formation of floccules no longer takes place, since the coalescence of the cohering drops occurs fairly quickly. At each stage of the coarsening microscopic investigation then only shows detached coacervate drops of increasing size. (See Fig. 4). 

Coacervation can also occur with association colloids, for example, oleate-f-KCl. As a preliminary to the coacervation high viscous systems occur in a certain range of KCl concentrations, at higher KCl concentrations the viscosity falls and at still higher coacervation sets in. See p. 701 and 708 Ch. XIV 4 and 5. 

The coacervates, which have been investigated as regards their viscosity behaviour, behave as Newtonian liquids. See note 3 on page 245. 

From measurements of viscosity it was found, that jsp decreases considerably in this case also previous to or simultaneously with the production of coacervation, but increases again considerably at high resorcinol concentrations at which coacervation ceases again (see p. 201, Fig. 13 b). 

The combination gelatin — gum arabic can be considered as the most favourable object as yet for the study of coacervation. In this case the complex coacervate has relatively little viscosity and consequently readily fuses to a single transparent liquid layer whereby it becomes possible to take samples of coacervate layer and equilibrium liquid and investigate them as regards their composition. The two colloids can be kept in the dry state for unlimited times and show no denaturation phenomena in solution. The only factor to which one must pay attention is the temperature, since one otherwise obtains the complications mentioned above as a result of gelation. 

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