Coacervates of serum albumin and gum arabic are unstable with respect to size. Studies of these static coacervates, that is coacervates in which no chemical reactions are occurring, showed that the turbidity increased with time to a maximum, then after a period of time, the turbidity decreased.
The turbidity was dependent upon the initial concentration of the coacérvate forming system. Furthermore, based on light-scattering measurements, the coacervates were not stable during the period in which the turbidity measurements remained constant, because the coacervates coalesced into larger droplets (Gladilin, 1973).
The coacervation of albumin-acacia has been studied by Burgess etal. (1991a). Microelectrophoresis studies were used to determine the optimum pH condition for complex coacervation to occur. At pH 3.9 the polymers carry equal and opposite charges. The coacervate yield was maximum at an ionic strength of approximately 10 mM. The coacervate yield decreased at both lower and higher ionic strengths. High ionic strength affects the charge carried by the polymers through a screening effect of the counter ions; thus, the attraction of one polymer for the other is decreased. The decrease in coacervate yield with decreasing ionic strength is not predicted by a number of theories. This phenomenon can be explained when the configurations of the molecules are taken into account. Highly charged molecules may exist in a rod-like configuration, rather than a random coil, and thus act as specific sites in a distributive manner. The high viscosity of the coacervate phase at optimum pH values for maximum coacervation makes it difficult to disperse the coacervate which is required for the formation of small spherical microcapsules.
In a second paper Burgess etal. (1991b) further investigated the complex coacervation of bovine serum albumin and acacia. Maximum coacervation was predicted to occur at pH 3.9 where both polyions carry equal and opposite charges. The optimum ionic strength for maximum coacervation yield was 10 mM. At pH 3.9 the viscous coacervate phase could not be emulsified into the equilibrium phase; however, at pH 3.8 and 4.2, microcapsules could be successfully prepared. Particle size of the microcapsules decreases slightly as the stirring speed increases.
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