Aggregation in Emulsions

Aggregation is a normal physical phenomenon in emulsion formulations. Oxygen-transporting emulsions of perfluorodecalin needed stabilizing additives to prevent aggregation.722 A series of total parenteral nutrition admixtures exhibited changes in the droplet size during storage, which was detected by Coulter counter and laser diffractometry measurements (Fig. 183).723 The emulsion stability was dependent on the emulsion zeta potential and was predicted by the Deryaguin-Landau-Verey-Overbeek theory.724

Increasing the storage temperature from 25 to 40°C markedly reduced the stability of a clofibride emulsion for oral administration, whereas storage at 4°C caused rapid phase separation owing to decreased solubility.725 In this study, the physical stability of emulsions under stressed conditions was evaluated by subjecting them to ultracentrifugation (25,500 x g over 1 h), repeated freeze-thawcycles (16 h of freezing at -18°C and 8 h of thawing at 25°C), and excessive shaking (150 strokes/min at 25°C over 48 h).

Changes in the droplet size of emulsions with time at 4°C. (Reproduced from Ref. 723 with
Figure 184. Rheologic aging of a lotion formulation with time. (a) yield value; (b) dynamic yield value; (c) plastic viscosity; (d) thixotropic area. (Reproduced from Ref. 727 with permission.)

Photomicrographs were taken with a specialized low-temperature scanning electron microscope726 to assess the phase inversion of a cream formulation during long-term storage. Rheologic aging of lotion formulations was represented as a function of time using the following equation184:

In this equation, P represents a rheologic parameter, and a and b are constants. Figure 184 shows the plots obtained for four rheologic parameters.

Figure 185. Moisture adsorption by gelatin capsules. a, Monomolecular layer of water molecules bound to surface; c, multimolecular layers of water; d,adsorption from dried state; s, desorption from saturation. (Reproduced from Ref. 728 with permission.)

Figure 185. Moisture adsorption by gelatin capsules. a, Monomolecular layer of water molecules bound to surface; c, multimolecular layers of water; d,adsorption from dried state; s, desorption from saturation. (Reproduced from Ref. 728 with permission.)

Figure 186. Discoloration of parenteral ascorbic acid formulation plotted according to the Weibull equation. a: Decrease in percent transmittance. (Reproduced from Ref. 730 with permission.)

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